Subspecialty Management |
|Chapter 48:||Anesthesia for Thoracic Surgery|
Indications for Separation of the Two Lungs
There are several absolute and relative indications for separation of the two lungs during thoracic operations or procedures (Table 48–10).
TABLE 48–10. Indications for Separation of the Two Lungs (Double-Lumen Tube Intubation) and/or One-Lung Ventilation
Separation of the two lungs for any of the absolute indications discussed here should be considered a lifesaving maneuver because failure to separate the lungs under any of these conditions could result in a life-threatening complication or situation. There are three general absolute indications for separating the lungs (see Table 48–10). First, separation of one lung from the other is absolutely necessary to prevent spillage of pus or blood from an infected (abscessed) lung or bleeding lung, respectively, to a noninvolved lung. Acute contamination of a lung with either blood or pus from the other lung usually results in severe massive (bilateral) atelectasis, pneumonia, and sepsis. Second, there are a number of unilateral lung problems that can prevent adequate ventilation of the noninvolved side. A large bronchopleural or bronchopleural-cutaneous fistula or a surgically opened conducting airway has such a low resistance to gas flow that a tidal inspiration delivered by positive pressure will exit via the low-resistance pathway, and it may become impossible to ventilate the other, more normal, lung adequately. A giant unilateral bulla or cyst may rupture if exposed to positive-pressure ventilation and result in a tension pneumothorax or pneumomediastinum. Very severe or life-threatening hypoxemia due to unilateral lung disease may require differential lung ventilation and PEEP. 239 Finally, positive-pressure ventilation of a lung with a tracheobronchial tree disruption can result in dissection of gas into the pulmonary interstitial space or mediastinum, causing a tension pneumomediastinum. Third, separation of the lungs is absolutely necessary to perform unilateral bronchopulmonary lavage in patients with pulmonary alveolar proteinosis (and rarely, asthma or cystic fibrosis).
There are a large number of relative indications for separation of the lungs, and they are all for the purpose of facilitating surgical exposure by collapsing the lung in the operative hemithorax. These relative indications can be divided into high-priority and low-priority categories (see Table 48–10). Of the relative indications, repair of a thoracic aortic aneurysm usually has the highest priority because it may require exposure of the thoracic aorta as it runs the entire length of the left hemithorax. A pneumonectomy, especially if performed through a median sternotomy, 240 is greatly aided by the wide exposure of the lung hilum that is afforded by collapse of the operative lung. Similarly, an upper lobectomy, which is technically the most difficult lobectomy, and many mediastinal exposures may be made much easier by eliminating ventilation to the lung on the side of the procedure. Examination of the pleural space (thoracoscopy) and pulmonary resections through a thoracoscope are considerably aided by collapse of the ipsilateral lung. The surgical items in the medium-priority category do not routinely require collapse of the lung on the operative side but still significantly aid surgical exposure and eliminate the need for the surgeon to handle (retract, compress, pack away) the operative lung. Severe intraoperative retraction of the lung on the operated side can traumatize the operative lung and impair gas exchange both intraoperatively 241, 242 and postoperatively. 243, 244 The lower-priority items consist of middle and lower lobectomies, less extensive pulmonary resections, thoracic spinal procedures that are approached anteriorly through the chest, and esophageal surgery. However, even relatively small operations such as wedge and segmental resections benefit by double-lumen tube insertion because of the ability to alternate easily and quickly between lung collapse and inflation, which is sometimes required to better visualize lung morphology and facilitate identification and separation of planes and fissures. Additionally, the separation of the lungs after removal of totally occluding and predominantly unilateral chronic pulmonary emboli (postcardiopulmonary bypass) can be very helpful because of the possibility of massive transudation of hemorrhagic fluid across the alveolar capillary membrane in the region of the lung supplied by the previously occluded vessel (reperfusion of a previously and chronically nonperfused vascular bed). Should significant and predominantly unilateral pulmonary edema occur following thromboembolectomy with cardiopulmonary bypass, the patient should be returned to cardiopulmonary bypass and a double-lumen endotracheal tube should be inserted so that differential lung ventilation may be used. Finally, significant hypoxemia due to unilateral lung disease may be more easily treated by differential lung ventilation and PEEP. 239
Techniques of Lung Separation
In general, three types of devices are available for providing one-lung ventilation during anesthesia: double-lumen endotracheal tubes (DLT), bronchial blockers, and endobronchial tubes. DLTs have come to be considered the lung separation technique of choice for the majority of thoracic surgery cases and are discussed later in detail. Bronchial blockade with the Univent tube and with independently passed bronchial blockers (Fogarty embolectomy catheter) in adults is greatly increasing in use and is also described at length below. Endobronchial tubes are not often used today and are only briefly described at the end of the chapter. The primary reason that DLTs are favored over bronchial blockers or endobronchial tubes for lung separation is that they are more versatile than the other two devices. The most important DLT function not available with a bronchial blocker is independent bilateral suctioning. In addition, it is easier to apply CPAP to the nonventilated operative lung with a DLT than with a bronchial blocker, and it is also easier to rapidly convert from two-lung to one-lung ventilation and vice versa with a DLT than with a bronchial blocker. Endobronchial tubes are very limited in function and only allow one-lung ventilation.
There are two firm disadvantages (contraindications) to the use of a DLT as compared with a bronchial blocker. First, very distorted tracheobronchial tree anatomy, including exophytic and stenotic lesions, as well as tortuosity, may preclude successful correct placement or positioning of a DLT. Second, changing from a DLT to a single-lumen tube during or at the end of an operation can be expected to be a difficult and/or risky procedure on occasion. Such a situation might occur in a patient with a relatively difficult airway before operation who undergoes a long operation requiring considerable intravenous fluids; one would expect the airway to be edematous and thus a postoperative tube change more hazardous in that setting.
There are two relatively minor disadvantages to DLTs, both related to the fact that the lumina of a DLT may be narrow. First, suctioning may be more difficult down a narrow lumen, but this is usually not a problem with the new disposable Robertshaw type of DLTs which have nonadhering suction catheters that slide easily down the lumina. Second, although airway resistance may be increased with a narrow lumen, the increased resistance can be easily overcome by positive-pressure ventilation. 245
Double-Lumen Endotracheal Tubes
Commonly Used Double-Lumen Endotracheal Tubes
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A DLT is essentially two catheters bonded together side by side, with each lumen intended to ventilate one of the two lungs. DLTs are made as left- and right-sided tubes. With a left-sided tube, the left lung catheter is placed into the left mainstem bronchus, whereas the right lung catheter ends in the trachea; therefore, for a left-sided tube, the left lung catheter is longer than the right lung catheter (Fig. 4814). With a right-sided tube the right lung catheter is placed into the right mainstem bronchus, whereas the left lung catheter ends in the trachea; therefore, for a right-sided tube, the right lung catheter is longer than the left lung catheter (see Fig. 4814). All DLTs have a proximal cuff for the trachea and a distal cuff for a mainstem bronchus; the endobronchial cuff causes separation and sealing off of the lungs from each other, and the tracheal cuff causes separation and sealing off of the lungs from the environment. The part of the right lung catheter of the right-sided DLT that is in the right mainstem bronchus must be slotted to allow ventilation of the right upper lobe (see Fig. 4814) because the right mainstem bronchus is too short to accommodate both the right lumen tip and the right endobronchial cuff. All double-lumen endotracheal tubes have two curves that lie in planes approximately 90 degrees apart from one another. The distal curve is designed to facilitate placement of the distal catheter tip into the appropriate mainstem bronchus, and the proximal curve is designed to approximate the oropharyngolaryngeal curve.
|FIGURE 4814 Schematic diagram depicting the essential features and parts of left-sided and right-sided double-lumen endotracheal tubes. RUL, right upper lobe; LUL, left upper lobe. (From Benumof604 )|
The DLTs that are now used for lung separation and one-lung ventilation are the Carlens and the Robertshaw. The Robertshaw type of tube is by far the more commonly used, and the disposable polyvinylchloride (PVC) Robertshaw tube has significantly replaced the red rubber Robertshaw tube (the former is easier to pass, is positioned more quickly, and causes less mucosal damage). 246 Consequently, the modern PVC tube will be described in great detail.
The left-sided Carlens tube (Fig. 4815) was the first DLT used for one-lung ventilation. 247 The tube has a carinal hook to aid in its proper placement and minimize tube movement after placement. Potential problems with carinal hooks include increased difficulty (more rotations) and laryngeal trauma during intubation, amputation of the hook during or after passage, malpositioning of the tube caused by the hook, and physical interference during pneumonectomy. 248 Therefore, some anesthesiologists prefer to use the tube with the hook removed. The tube is available in four sizes: 41, 39, 37, and 35 French (which correspond to an internal diameter of each lumen of approximately 6.5, 6.0, 5.5, and 5.0 mm, respectively). The cross-sectional shape of each lumen is oval, and this accounts for the occasional difficulty in passing a suction catheter down the lumen.
|FIGURE 4815 (A) Sketch of the red rubber (nondisposable) Carlens double-lumen endotracheal tube. (B) Close-up of the placement of the red rubber Carlens double-lumen endotracheal tube at the carina. Note that the left endobronchial lumen and carinal hook straddle the carina. (From Benumof604 )|
The original Robertshaw DLT, introduced in 1962, was made as a reusable red rubber tube (Fig. 4816). 249 This tube was designed to provide the largest possible lumen to decrease airway resistance and facilitate removal of secretions. The lumina are D-shaped and lie side by side, like those of the Carlens tube, but are larger in size. As with the other DLTs, it has two curves (in planes approximately 90 degrees apart), which facilitate intubation and proper endobronchial placement. Both a right- and a left-sided tube are available, and the absence of a carinal hook allows for easier tracheal intubation and perhaps correct positioning. The right-sided tube has a slotted endobronchial cuff to effect ventilation of the right upper lobe. The right upper lobe ventilation slot is relatively long, which facilitates ventilation of this lobe. However, the endobronchial cuff has an additional area of inflation on the nonslotted side above the slot to effect a more reliable seal (see Fig. 4816A). On the slotted side, inflation of the endobronchial cuff is restricted. However, the right endobronchial cuff design forces the right upper lobe slot to lie flat against the right upper lobe orifice, and if this slot is not perfectly aligned with the right upper lobe orifice, the ventilation slot will be blocked (obstructed) by the right mainstem bronchial wall (and vice versa). Nevertheless, because of its many good features, the original Robertshaw DLT rapidly gained wide popularity. 250
|FIGURE 4816 (A) Sketch of the left-sided red rubber Robertshaw double-lumen endotracheal tube. (B) Close-up of the placement of the left-sided Robertshaw double-lumen endotracheal tube at the carina. (C) Sketch of the right-sided Robertshaw double-lumen endotracheal tube. (D) Close-up of the placement of the right-sided Robertshaw double-lumen endotracheal tube at the carina. (From Benumof604 )|
The Robertshaw type of tube is now made of a clear nontoxic tissue-implantable plastic (denoted by the marking Z-79) and is disposable (see Fig. 4814). The tubes are made in sizes 41, 39, 37, 35, 28, and 26 French (internal diameter of each lumen is approximately 6.5, 6.0, 5.5, 5.0, 4.5, or 4.0 mm, respectively). The 26 and 28 French tubes are available only as left-sided models. These tubes are relatively easy to insert and have appropriate end-of-lumen and cuff arrangements that minimize lobar obstruction. The endobronchial cuff is colored brilliant blue, which is an important recognition feature when using a fiberoptic bronchoscope. The ends of both lumina have a black radiopaque line, which is an essential recognition marker when viewing a chest radiograph. The tubes have high-volume, low-pressure tracheal and endobronchial cuffs. The slanted doughnut-shaped endobronchial cuff on the Bronchocath model right-sided DLT allows the right upper lobe ventilation slot to ride off (away from) the right upper lobe orifice, which minimizes the chance of right upper lobe obstruction by the tube. The clear tubing is helpful because it permits continuous observation of the tidal movement of respiratory moisture as well as observation of secretions from each lung. The tubes are packaged with malleable stylets and are relatively easy to insert and position. These tubes have large internal to external diameter ratios and therefore are relatively easy to suction through, and they are packaged with their own nonadhering suction catheters. The large internal to external diameter ratio also provides a relatively low resistance to ventilation. For all these reasons, these disposable tubes are now considered the DLTs of choice by most anesthesiologists. Several companies are currently manufacturing them. Reviews of other DLTs are available. 251
A left-sided DLT should be used for right thoracotomies requiring collapse of the right lung and ventilation of the left lung (Fig. 4817). A left- or right-sided tube may be used for left thoracotomies requiring collapse of the left lung and ventilation of the right lung (see Fig. 4817). However, because the right upper lobe ventilation slot of a right-sided tube has to be closely apposed to the right upper lobe orifice to allow unobstructed right upper lobe ventilation and since there is considerable anatomic variation in the exact position of the right upper lobe orifice and therefore in the length of the right mainstem bronchus (in fact, it is well known that an anomalous right upper lobe can take off from the trachea), use of a right-sided tube for left lung collapse introduces the risk of inadequate right upper lobe ventilation. For this reason, a left-sided tube is preferable for most cases requiring one-lung ventilation. If clamping of the left mainstem bronchus is necessary, the tube can be withdrawn at that time into the trachea and then used in the same manner as a single-lumen endotracheal tube (ventilation of the right lung with both lumina) (see Fig. 4817). Contraindications to use of a left-sided DLT are carinal and proximal left mainstem bronchial lesions that could be traumatized by the passage of a left-sided tube. These lesions include strictures, endoluminal tumors, tracheobronchial disruptions, compression of the airway by an external mass, and tenting of the left mainstem bronchus so that the angle of the take-off from the trachea is approximately 90 degrees. The largest size of tube that can comfortably pass the glottis should be used, as a relatively small DLT may require excessive cuff volume for endobronchial cuff seal and may cause difficulty with suctioning secretions or ventilating the patient. In general, as height and weight increase, the appropriate DLT size (as defined above) increases, although height is much more important than weight. 252
|FIGURE 4817 Use of left-sided and right-sided double-lumen endotracheal tubes for left and right lung surgery (as indicated by the clamp). (A) When surgery is performed on the right lung, a left-sided double-lumen endotracheal tube should be used. (B) When surgery is performed on the left lung, a right-sided double-lumen endotracheal tube can be used. However, because of uncertainty about the alignment of the right upper lobe ventilation slot with the right upper lobe orifice, a left-sided double-lumen endotracheal tube can also be used for left lung surgery. (C) If the left lung surgery requires a clamp to be placed high on the left main stem bronchus, the left endobronchial cuff should be deflated, the left-sided double-lumen endotracheal tube pulled back into the trachea, and the right lung ventilated through both the lumina (use the double-lumen endotracheal tube as a singlelumen tube). (From Benumof604 )|
In summary, the plastic disposable Robertshaw-type tubes are by far the most commonly used DLTs. Because a right-sided tube incurs the risk of inadequate right upper lobe ventilation, left-sided tubes are used far more often. Consequently, the rest of this chapter emphasizes the insertion and precise positioning of the left-sided Robertshaw-type DLT.
Conventional Double-Lumen Tube Intubation Procedure
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Before intubation with a double-lumen endotracheal tube, both cuffs and the lumen connections are checked. A 3-mL syringe with stopcock should be placed on the end of the bronchial cuff pilot tube, because proper bronchial cuff inflation rarely requires more than 1 to 2 mL of air; a 5- or 10-mL syringe with stopcock should also be placed on the tracheal cuff pilot tube. Because the high-volume, low-pressure cuffs can be easily torn by teeth, the distal tube is coated with a lubricating ointment (preferably containing a local anesthetic) to minimize this possibility. If a less than optimal view of the larynx is anticipated, the stylet that is packaged with the tube is lubricated, inserted into the left lumen, and appropriately curved. The patient is then anesthetized and paralyzed as described previously. A curved open-phalanged blade (MacIntosh) is usually preferred for laryngoscopy, because it approximates the curvature of the tube and therefore provides the largest possible area through which to pass the tube. However, a straight (Miller) blade may be a better choice in patients with overriding upper teeth or an excessively anterior larynx.
Double-lumen endotracheal tubes with carinal hooks are first inserted through the vocal cords with the hook facing posteriorly. When the tip of the tube has passed the vocal cords, the tube is rotated 180 degrees so that the hook passes anteriorly through the glottis. If this maneuver is not done, then one can damage the vocal apparatus by dislocating an arytenoid cartilage should the hook snare or hang up on the arytenoid cartilage. After the tube tip and hook pass the larynx, the tube is rotated 90 degrees so that the tube tip enters the appropriate bronchus.
The Robertshaw-type DLT is passed with the distal curvature initially concave anteriorly (Fig. 4818A). After the tube tip passes the larynx and while anterior force on the laryngoscope is continued, the stylet (if used) is removed, and the tube is carefully rotated 90 degrees (so that the distal curve is now concave toward the appropriate side and the proximal curve is concave anteriorly) to allow endobronchial intubation on the appropriate side (Fig. 4818B). Continued anterior force by the laryngoscope during tube rotation prevents hypopharyngeal structures from falling in around the tube and interfering with a free 90-degree distal tube tip rotation. Failure to obtain a near 90-degree rotation of the distal tube tip while the proximal end rotates 90 degrees will cause either a kink or a twist in the shaft of the tube and/or prevent the distal end of the lumen from lying free in the mainstem bronchus (i.e., not up against the bronchial wall). After rotation, the tube is advanced until most of it is inserted 252 (Fig. 4818C). When the proper depth of insertion has been achieved (defined as when the cephalad surface of the bronchial cuff is immediately below the carinal bifurcation), the average depth of insertion for both male and female patients 170 cm tall is 29 cm, and for each 10-cm increase or decrease in height, average placement depth is increased or decreased by 1 cm. 252 The correlation between depth of insertion and height is highly significant (P<.0001) for both male and female patients. Nevertheless, it should be understood that the depth of DLT insertion at any given height is still normally distributed, and correct DLT position should always be confirmed fiberoptically after initial placement. DLTs may also be passed successfully via tracheostomy, although it should be remembered that the tracheal cuff may be at the tracheal stoma or lie partly outside the trachea in this situation. 253, 254 Because of this, one may prefer to use a specially manufactured (i.e., short) nondisposable double-lumen endotracheal tube for these particular patients. 255
|FIGURE 4818 Schematic diagram depicting the passage of the left-sided double-lumen endotracheal tube in a supine patient. (A) The tube is held with the distal curvature concave anteriorly and the proximal curve concave to the right and in a plane parallel to the floor. The tube is then inserted through the vocal cords until the bronchial cuff passes the vocal cords. The stylet is then removed. (B) The tube is rotated 90 degrees counterclockwise so that the distal curvature is concave anteriorly and the proximal curvature is concave to the left and in a plane parallel to the floor. (C) The tube is inserted until either a mild resistance to further passage is encountered or the end of the common molding of the two lumina is at the teeth. Both cuffs are then inflated, and both lungs are ventilated. Finally, one side is clamped while the other side is ventilated and vice versa. (See text for further explanation.) (From Benumof604 )|
Once the tube tip is thought to be in an endobronchial position, the following checklist is used to ensure proper functioning of the tube. Inflate the tracheal and endobronchial cuffs until moderate tension is palpated in the external pilot balloons (the endobronchial cuff should not require more than 12 mL of air); deliver several positive-pressure ventilations, auscultate, and observe the chest bilaterally to determine that the trachea rather than the esophagus has been intubated and that both lungs are being ventilated (see Fig. 4818C). In addition to seeing the tube go through the vocal cords, check correct intubation position by feeling and observing the anesthesia reservoir bag to make sure it has the appropriate compliance and movement while maintaining normal pulse oximetry and ETCO2 values, and perhaps palpating the tracheal cuff in the neck. If only unilateral breath sounds or chest movement is present, it is likely that both lumina of the tube have entered a mainstem bronchus (if both lumina enter the left main stem bronchus, the findings may mimic an esophageal intubation, and vice versa). In this situation, quickly deflate the cuffs, withdraw the tube 1 to 2 cm at a time, inflate the cuffs, and reassess ventilation until bilateral breath sounds are heard. If bilateral breath sounds are not heard and the tube has been withdrawn a significant amount, the entire procedure must be repeated, beginning with establishing the airway and oxygen ventilation via mask, laryngoscopy, and reinsertion of the DLT through the vocal cords. If bilateral breath sounds are present, then one side is clamped and breath sounds and chest movement should disappear on the ipsilateral side and remain on the contralateral side. Breath sounds should be at least as audible anteriorly, at the apex of the lung, as they are along the lateral chest. If they are not heard clearly at the lung apex, it is likely that the tube has been advanced too far distally and that the upper lobe on that side is not being ventilated adequately. In that case the tube must be pulled back 1 cm at a time until apical breath sounds are clearly heard. In addition, when the lumen cap is opened proximal to the clamp, there should be no leakage of air out of that lumen, indicating that the bronchial cuff has effected a proper seal. Next, the clamped side should be unclamped and the lumen cap replaced, and the breath sounds and chest movement should reappear on that side. During unilateral clamping, the breath sounds on the ventilated side should be compared with and calibrated against unilateral chest wall movements and the inspiratory disappearance and expiratory appearance of respiratory gas moisture in the clear tubing of the ventilated side. In addition, the compliance of the lung should be gauged by using hand ventilation. The unilateral clamping and unclamping and removal of the lumen cap proximal to the clamp should then be repeated on the opposite side to ensure adequate lung separation and cuff seal.
In summary, when DLT position is correct, the breath sounds are normal and follow the expected unilateral pattern with unilateral clamping, the chest rises and falls in accordance with the breath sounds, the ventilated lung feels reasonably compliant, no leaks are present, and respiratory gas moisture appears and disappears with each tidal ventilation. Conversely, when the DLT is malpositioned, any or all of the following may occur: the breath sounds may be poor and correlate poorly with unilateral clamping, the chest movements may not follow the expected pattern, the ventilated lung may feel noncompliant, leaks may be present, or the respiratory gas moisture in the clear tubing may be relatively stationary. It is very important to realize, however, that even if the DLT is thought to be properly positioned by clinical signs, subsequent fiberoptic bronchoscopy may reveal an incidence of malpositioning that ranges from 38 to 78 percent. 256, 257
When it is believed, on the basis of clinical signs, that the DLT is malpositioned, it is theoretically possible to diagnose the malposition of the tube more precisely by a combination of several unilateral clamping, chest auscultation, and left endobronchial cuff inflation-deflation maneuvers (Fig. 4819). With reference to a left-sided DLT, there are three possible gross malpositions: in too far on the left (both lumina in left mainstem bronchus), out too far (both lumina in the trachea), and in or down the right mainstem bronchus (at least the left lumen in the right mainstem bronchus). When the right (tracheal) side is clamped and the tube is in too far on the left side, breath sounds are heard only on the left side. When the tube is out too far and the right side is clamped, breath sounds are heard bilaterally. When the tube is in or down the right side and the right side is clamped, breath sounds are heard only on the right side. When the left side is clamped and the left endobronchial cuff is inflated, the right lumen is blocked by the left cuff in all three malpositions. Consequently, with the left side clamped and the left cuff inflated, no or very diminished breath sounds are heard bilaterally in all three of the malpositions. When the left side is clamped and the left cuff is deflated, so that the right lumen is no longer blocked by the left cuff, breath sounds are heard only on the left side when the tube is in too far on the left, bilaterally when the tube is out too far, and only on the right side when the tube is in the right side. The left cuff inflation and deflation findings provide the key diagnostic data because they essentially define the position of the right tracheal lumen by blocking and unblocking it with the left cuff.
|FIGURE 4819 There are three major malpositions (involving a whole lung) of a left-sided double-lumen endotracheal tube. The tube can be in too far on the left (both lumina are in the left main stem bronchus), out too far (both lumina are in the trachea), or down the right main stem bronchus (at least the left lumen is in the right main stem bronchus). In each of these three malpositions the left cuff, when fully inflated, can completely block the right lumen. Inflation and deflation of the left cuff while the left lumen is clamped create a breath sound differential diagnosis of tube malposition. (See text for full explanation.) L, left; R, right; ¯, decreased. (From Benumof604 )|
There are, however, several situations in which these unilateral clamping, auscultation, and cuff inflation and deflation maneuvers for determining the integrity of lung separation are either unreliable or impossible. First, and most importantly, when the patient is in the LDP, has had a skin preparation, and is draped, access to the chest wall is impossible, and the anesthesiologist cannot listen to the chest. Second, the presence of unilateral or bilateral lung disease, either preexisting before anesthesia and surgery or anesthesia-induced, may markedly obscure the crispness of the chest auscultation end points. Third, the diagnosis of exactly where the DLT is located may be confused when the tube is just slightly malpositioned. Fourth, the tube may have moved as a result of some event, such as coughing, head flexion or extension while turning into the LDP, or tracheal manipulation and hilar retraction by the surgeon. Finally, some combination of these factors may culminate in uncertainty about where the DLT has located. The solution to any uncertainty about the exact position of the DLT is to determine the position by fiberoptic bronchoscopy.
Use of Fiberoptic Bronchoscope to Determine Precise Double-Lumen Tube Position
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As noted, even when a DLT is thought to be in proper position on the basis of clinical signs, subsequent fiberoptic bronchoscopy will reveal an incidence of malpositioning as high as 78 percent. 256 Indeed, when the position of the DLT is checked only by clinical signs, in up to 25 percent of cases there may be intraoperative problems with either deflating the nondependent lung, ventilating the dependent lung, or completely separating the two lungs. 256, 258 Given the high incidence of malpositioned DLTs when DLT position is determined by only auscultation (i.e., blindly) and the potentially serious consequences associated with a malpositioned DLT, it is only a matter of simple common sense to routinely use a fiberoptic bronchoscope to easily, quickly, and precisely determine the position of the DLT.
The exact position of a left-sided DLT can be ascertained at any time, in less than a minute, by simply passing a pediatric-size fiberoptic bronchoscope through the tracheal lumen of the DLT. It is rarely necessary also to have to pass the fiberoptic bronchoscope down the left endobronchial lumen. With reference to a left-sided DLT, looking down the right (tracheal) lumen the endoscopist should have a clear straight-ahead view of the tracheal carina, the left lumen going off to the left, and the upper surface of the blue left endobronchial balloon just below the tracheal carina (Fig. 4820). Because the distance between the right and left lumen tips for the clear plastic tube (69 mm) is longer than the length of the left mainstem bronchus (average 50 mm) in a typical patient and if the blue upper surface of the endobronchial balloon is not visible, it is possible for the right lumen to be above the tracheal carina while the left lumen tip obstructs the left upper lobe. 259 However, no matter which manufactured tube or size of tube is used and no matter how long or short the left mainstem bronchus is (within the range of extremes observed in extensive studies), 259 when the upper surface of the left endobronchial balloon is just below the tracheal carina, it is not possible for the left lumen tip to obstruct the left upper lobe or for the right (tracheal) lumen to be near a mainstem bronchus. It is important that the volume of air used to fill the left endobronchial cuff not cause the endobronchial cuff to herniate over the tracheal carina or cause the tracheal carina to deviate to the right (see Fig. 4820); both cuff herniation and carinal deviation can be readily appreciated by looking down the tracheal lumen. Looking down the left lumen (as is sometimes done when inserting a left-sided DLT with a fiberoptic bronchoscope [see the section immediately below] and in all cases of bronchopulmonary lavage where perfect tube position and tight cuff seal are extremely critical), the endoscopist should see a very slight narrowing of the left lumen (due to endobronchial cuff pressure) as well as the bronchial carina distal to the end of the tube (see Fig. 4820). The endoscopist should not see excessive left luminal narrowing (due to excessive left cuff pressure) (see Fig. 4820). Thus, aside from gross malposition, important undesirable findings on endoscopy are related to excessive left cuff inflation and pressure and consist of cuff herniation over the tracheal carina, carinal deviation to the right (both of which may block the right mainstem bronchial orifice and impair right lung ventilation), and excessive left lumen constriction (invagination), which may impair left lung ventilation 260 (see Fig. 4820). In addition, when an inappropriately undersized tube is used, the large endobronchial cuff volume required for endobronchial cuff seal tends to force the entire DLT cephalad, making a functional bronchial seal more difficult. 261
|FIGURE 4820 This schematic diagram depicts the complete fiberoptic bronchoscopy picture of left-sided double-lumen endotracheal tubes (both the desired view and the view to be avoided from both of the lumina). (A) When the bronchoscope is passed down the left lumen of the left-sided tube, the endoscopist should see a very slight left luminal narrowing and a clear straight-ahead view of the bronchial carina off in the distance. Excessive left luminal narrowing should be avoided. (B) When the bronchoscope is passed down the right lumen of the left-sided tube, the endoscopist should see a clear straight-ahead view of the tracheal carina and the upper surface of the blue left endobronchial cuff just below the tracheal carina. Excessive pressure in the endobronchial cuff, as manifested by tracheal carinal deviation to the right and herniation of the endobronchial cuff over the carina, should be avoided. (From Benumof604 )|
With reference to a right-sided DLT, looking down the left (tracheal) lumen, the endoscopist should see a clear straight-ahead view of the tracheal carina and the right lumen going off to the right (Fig. 4821A). The upper surface of the right endobronchial balloon may not be visible below the tracheal carina. Looking down the right lumen, the endoscopist should see a very slight narrowing of the right lumen as well as the right middle-lower lobe bronchial carina distal to the end of the tube. Most importantly, the endoscopist should locate the right upper lobe ventilation slot and be able to look directly into the right upper lobe orifice through the right upper lobe ventilation slot by simply flexing the tip of the fiberoptic bronchoscope superiorly and laterally (Fig. 4821B). There should be no overriding of the right upper lobe ventilation slot on the bronchial mucosa, and the bronchial mucosa should not be covering any of the right upper lobe ventilation slot.
|FIGURE 4821 This schematic diagram portrays use of a fiberoptic bronchoscope to determine precise right-sided double-lumen tube position. (A) When the fiberoptic bronchoscope is passed down the left (tracheal) lumen, the endoscopist should see a clear straight-ahead view of the tracheal carina and the right lumen going off into the right main stem bronchus. (B) When the fiberoptic bronchoscope is passed down the right (bronchial) lumen, the endoscopist should see the bronchial carina off in the distance; when the fiberoptic bronchoscope is flexed cephalad and passed through the right upper lobe ventilation slot, the right upper lobe bronchial orifice should be visualized. (From Benumof604 )|
In our experience, in 8 to 9 out of 10 cases, the clinical signs (breath sounds, chest movements, compliance of the lung[s], movement of respiratory gas moisture) indicate that the lungs are apparently clearly and without doubt completely separated when the DLT is first inserted with the patient in the supine position. However, in view of the finding that up to 78 percent of DLTs are malpositioned to some extent in the supine position, even though clinical signs indicate no problem, 256 it is strongly advisable to check the position of the tube with a fiberoptic bronchoscope in the supine position (especially in view of the fact that the procedure takes less than 1 minute). Even if no problem is identified, the procedure still allows the endoscopist to become familiar with the patients anatomy and facilitates the more important endoscopy after turning the patient into the LDP. In approximately 1 to 2 out of 10 cases, there is a definite doubt about tube location in the supine position, and in these patients the fiberoptic bronchoscope is always used to correct the DLT malposition. The fiberoptic bronchoscope is again used to confirm DLT position after the patient has been turned into the LDP. Of course, a determined effort is made to prevent dislodgement of the tube during turning by holding on to the tube at the level of the incisors and by keeping the head absolutely immobile in a neutral or slightly flexed position. Head extension can cause movement of the tube in a cephalad direction, which may result in bronchial decannulation; head flexion can cause movement of the tube in a caudad direction, which may result in an upper lobe obstruction or in both luminas being in a mainstem bronchus (see next section). 262, 263 Finally, the fiberoptic bronchoscope is used whenever during the case there is a question about DLT position. This is not an infrequent occurrence and is usually caused by surgical manipulation and traction on the hilum, carina, or trachea.
Use of Fiberoptic Bronchoscope to Insert the Double-Lumen Tube
The insertion of the bronchial lumen of a DLT into the appropriate mainstem bronchus may be aided by the use of a fiberoptic bronchoscope. This may be especially helpful if anatomic variation or a pathologic condition has caused carinal distortion. The DLT is first placed in the trachea in a conventional manner (laryngoscopy, manual tube insertion) until the tracheal cuff just passes the vocal cords, the tracheal cuff is inflated, and both lungs are ventilated through both lumina (the DLT should be used as if it were a single-lumen tube). A pediatric-sized fiberoptic bronchoscope can then be inserted into the bronchial lumen through a self-sealing diaphragm in the elbow connector to the bronchial lumen (which permits continued positive-pressure ventilation through that lumen around the fiberoptic bronchoscope) and passed into the appropriate mainstem bronchus. The tracheal cuff is then deflated and the bronchial lumen is passed over the fiberoptic bronchoscope stylet into the appropriate mainstem bronchus. The fiberoptic bronchoscope is then withdrawn from the bronchial lumen and passed down the tracheal lumen to determine the precise DLT position (see the preceding section).
Alternatively, once the DLT is in the trachea, the fiberoptic bronchoscope can be inserted into the tracheal lumen and passed just proximal to the tracheal carina. While the carina and the two mainstem bronchial orifices are in view, the DLT can be advanced and the degree of lateral rotation adjusted so that the appropriate lumen enters the appropriate mainstem bronchus. Final precise positioning (see the preceding section) can be done with the fiberoptic bronchoscope remaining in the tracheal lumen if a left-sided tube is used. If a right-sided tube is used, precise positioning must be confirmed with the bronchoscope passed through the bronchial lumen.
Relationship of Fiberoptic Bronchoscope Size to Double-Lumen Tube Size
The clear plastic disposable right and left double-lumen endotracheal tubes are manufactured in four sizes: 35, 37, 39, and 41 French. In addition, 26 and 28 Frenchsized tubes are available only as left-sided models. A 5.6-mm outside diameter diagnostic fiberoptic bronchoscope will not pass down the lumina of any size DLT. A 4.9-mm outside diameter fiberoptic bronchoscope passes easily through the lumina of the 41 French tube and moderately easily with lubrication through the 39 French tube; it causes a tight fit that needs a liberal amount of lubrication and a strong pushing force to pass through the lumen of the 37 French tube and does not pass through the lumen of the 35 French tube. A silicon-based fluid (such as that made by the American Cystoscope Co.) is the best lubricant for a fiberoptic bronchoscope because it does not dry out or crust and does not interfere with the view even if it coats the tip of the bronchoscope. Fortunately, from the point of view of using a 4.9-mm outside diameter fiberoptic bronchoscope, a 37 French tube or larger can be used in almost all adult females and 39 French tube or larger in almost all adult males. A 3.6- to 4.2-mm outside diameter (pediatric-sized) fiberoptic bronchoscope passes easily through the lumina of all adult-sized DLTs and because the bronchoscope has an increased amount of space, the maneuverability of the tip of the bronchoscope is greatly increased. Therefore, the 3.6- to 4.2-mm outside diameter bronchoscope is obviously the bronchoscope of choice for DLTs. Table 48–11 summarizes these fiberoptic bronchoscopeDLT relationships. Several companies (Olympus, Machida, Pentax) presently manufacture 4.9- and 3.6- to 4.2-mm outside diameter fiberoptic bronchoscopes that are of adequate length and have a suction channel.
TABLE 48–11. Relationship of Fiberoptic Bronchoscope Size to Adult Double-Lumen Endotracheal Tube Size
Use of Chest Radiograph to Determine Double-Lumen Tube Position
The chest radiograph can be used to determine DLT position. The chest radiograph may be more useful than conventional unilateral auscultation and clamping in some patients, but it is always less precise than fiberoptic bronchoscopy. To use the chest radiograph, the DLT must have radiopaque markers at the end of the right and left lumina. The key to discerning DLT position on the chest radiograph is seeing where the marker at the end of the tracheal lumen is in relation to the tracheal carina and whether the endobronchial lumen is located in the correct main stem bronchus. The end of the tracheal lumen marker must be above the tracheal carina; however, this does not guarantee correct position because this technique may not reveal a subtle obstruction of an upper lobe. If the tracheal carina cannot be seen (as sometimes happens with portable anteroposterior film), the chest radiograph method of determining DLT position is not usable. Furthermore, the chest radiograph method is time-consuming (for film transport and film development), costly, and awkward to perform and may dislodge the tube (the cassettes are often difficult to place under the operating room table and require moving the patient).
Other Methods to Determine Double-Lumen Tube Position
Three other methods may help to determine the position of a DLT. First, comparison of capnography (waveform and end-tidal CO2 pressure [PETCO2] value) from each lumen may reveal a marked discrepancy. For example, with all other conditions equal, one lung may be very poorly ventilated in relation to the other lung (high PETCO2), indicating obstruction to that lung; one lung may be very overventilated in relation to the other lung (low PETCO2), perhaps indicating ventilation of just a lobe of that lung; or the capnogram from one lung may have a much steeper slope to the alveolar plateau, indicating expiratory obstruction. 264, 265 Second, continuous spirometric data (Datex Capnomac Ultima) from both lungs and from each lung separately, such as pressure-volume or flow-volume loops, may be displayed and compared with a control loop that is stored in memory. 266 Third, the surgeon may be able to palpate the position of the DLT from within the chest and may be able to redirect or assist in changing its position (by deflecting the DLT away from the wrong lung, etc.). 267
Quantitative Determination of Cuff Seal Pressure Hold
The use of fiberoptic bronchoscopy to determine DLT position does not provide evidence or a guarantee that the lungs are functionally separated (i.e., against a fluid and/or air pressure gradient). There are times, such as during the performance of unilateral pulmonary lavage, when the anesthesiologist must be absolutely certain that functional separation has been achieved. Complete separation of the lungs by the left endobronchial cuff can be demonstrated in a left-sided tube by clamping the connecting tube to the right lung proximal to the right suction port and attaching a small tube (i.e., intravenous extension tubing) to the open right suction port (by appropriate adaptors) (Fig. 4822). The free end of this tube is submerged in a beaker of water. When the left lung is statically inflated to any pressure considered necessary and the left endobronchial cuff is not sealed, air will enter the left lung and will also escape from around the unsealed left cuff, move up the right lumen to the small connecting tube, and bubble through the beaker of water (Fig. 4822B). If the left endobronchial cuff is sealed, no bubbles should be observed passing through the beaker of water (Fig. 4822A). After demonstration of functional lung separation, the right connecting tube is unclamped, the right suction port closed, and ventilation to both lungs resumed. To test for lung separation with the pressure gradient across the endobronchial balloon reversed, the left airway connecting tube is clamped proximal to the left suction port, the left suction port opened to the beaker of water via the small tube, the right lung statically inflated to any desired pressure, and the absence or presence of air bubbles in the beaker of water noted. It should be remembered that even though the left endobronchial cuff may be adequately sealed, it is possible that during these maneuvers, compression of the nonventilated lung by the ventilated lung may initially cause some small amount of bubbling in the beaker, which will cease with repetitive inflation of the ventilated lung (no bubbles should be seen after several inflations). 251, 260 The absence of airflow from the nonventilated lung suction port is a very simple but sensitive indicator of functional separation of the two lungs.
|FIGURE 4822 Schematic diagram showing the air bubble detection method for checking adequacy of the seal of the left endobronchial cuff of a left-sided double-lumen tube. (A) When the left lung is selectively ventilated or exposed to any desired distending pressure and the left cuff is adequately sealed, no air will escape around the left cuff and out the open right suction port, and thus no bubbles will be observed passing through the beaker of water. (B) When the left lung is ventilated or exposed to any desired distending pressure and the left endobronchial cuff is not adequately sealed, air will escape around the left cuff and out the open right suction port, and thus air bubbles will be observed passing through the beaker of water. (From Benumof604 )|
Complications of Double-Lumen Endotracheal Tubes
In addition to the impediment to arterial oxygenation that is inherent in the use of DLTs for one-lung anesthesia, the tubes themselves occasionally cause other serious complications. These complications include tracheobronchial tree disruption (with the Carlens tube, 268 the red rubber Robertshaw tube, 269 the red rubber White tube, 270 and the disposable low-pressure cuff plastic tubes 271, 272) , traumatic laryngitis, 248 and suturing of a pulmonary vessel to the DLT. 273 With regard to tracheobronchial tree disruptions, a common thought in the reports cited is that excessive air volume and pressure in the bronchial balloon may be major factors in the genesis of these tears after DLT insertion. Recommendations to minimize tracheobronchial wall damage due to the cuffs include being particularly cautious in the use of DLTs in patients with bronchial wall abnormalities, choosing an appropriately sized clear plastic tube, 246 being certain the tube is not malpositioned, preventing overinflation of the endobronchial cuff, deflating the endobronchial cuff during turning, inflating the endobronchial cuff slowly, inflating the endobronchial cuff with inspired gases if nitrous oxide is used, and preventing the tube from moving during turning (Table 48–12).
TABLE 48–12. Endobronchial Cuff Considerations to Minimize Tracheobronchial Wall Damage (Disruption)
Relative Contraindications to Use of Double-Lumen Endotracheal Tubes
Lung separation by a DLT may be relatively contraindicated in several situations because insertion of the tube is either difficult or dangerous. These situations involve patients who have a full stomach (risk of aspiration); patients who have a lesion (airway stricture, 274 endoluminal tumor) that is present somewhere along the pathway of the DLT and thus could be traumatized; small patients for whom a 35 French tube is too large to fit comfortably through the larynx and for whom a 28 French tube is considered too small; patients whose upper airway anatomy precludes safe insertion of the tube (recessed jaw, prominent teeth, bull neck, anterior larynx); extremely critically ill patients who have a single-lumen tube already in place and who will not tolerate being taken off mechanical ventilation and PEEP even for a short time; and patients having some combination of these problems. Under these circumstances, it is still possible to separate the lungs safely and adequately by using a single-lumen tube and fiberoptic bronchoscopic placement of a bronchial blocker or by fiberoptic bronchoscopic placement of a single-lumen tube in a main stem bronchus.
Bronchial Blockers (With Single-Lumen Endotracheal Tubes)
Lung separation can be effectively achieved with the use of a single-lumen endotracheal tube and a fiberoptically placed bronchial blocker. This is often necessary in children because DLTs are too large to be used in them. The smallest DLT available is a left-sided 26 French tube, which may be used in patients 8 to 12 years old and weighing 25 to 35 kg. Bronchial blockers that are balloon-tipped luminal catheters have the advantage of allowing suctioning and injection of oxygen down the central lumen. The bronchial blocker most widely used for adults is the movable bronchial blocker that is contained in and is an integral part of the Univent single-lumen tube system (made by Fuji Systems Corp., Tokyo) 275, 276, 277, 278, 279, 280, 281 (Fig. 4823). The physiology of one-lung ventilation produced by bronchial blockade is identical to that produced by clamping one lumen of a DLT.
|FIGURE 4823 Single-lumen tube of Univent bronchial blocker system.|
Insertion of the Univent Tube and Positioning of the Bronchial Blocker
The tube is inserted in the following manner. First, the single-lumen tube along with the bronchial blocker (in the fully retracted position) is inserted as a unit into the trachea (Fig. 4824A). The cuff on the main endotracheal tube lumen is inflated, and the patient is ventilated and oxygenated (see Fig. 4824A). A fiberoptic bronchoscope is inserted through a self-sealing diaphragm in the elbow connector to a single-lumen tube while ventilation is maintained around the bronchoscope (but within the single-lumen tube) (Fig. 4824C). The right and left mainstem bronchi are identified (by noting the relationship of the main stem bronchi to the posterior membrane and the anterior cartilaginous rings [Fig. 4824B.]), and the tube of the bronchial blocker is located (by moving the bronchial blocker in and out just beyond the end of its own lumen and the main lumina of the Univent tube [Fig. 4824.]). The bronchial blocker cuff is colored blue and is easy to see. It will be seen that the bronchial blocker will usually (almost always) enter the right mainstem bronchus if it is simply pushed in (and the main single-lumen tube is not turned). If the left mainstem bronchus is to be blocked, the main single-lumen tube is turned 90 degrees to the left (counterclockwise) so that the concavity of the tube is facing toward the left side (Fig. 4824C) (and vice versa for the right side, if necessary). The bronchial blocker can also be rotated a slight amount at its distal end (obtain 13 mm of laterality) by twirling the proximal end in the fingers. The bronchial blocker is then advanced into the mainstem bronchus under direct vision (Fig. 4824D). Attempting to advance the bronchial blocker blindly into the appropriate mainstem bronchus (particularly the left) will be unsuccessful 87 percent of the time, and repeated attempts may cause excoriation of the tracheal mucosa. 227 In fact, blindly pushing the somewhat stiff bronchial blocker may result in perforation of the tracheobronchial tree and consequent tension pneumothorax. 278 The balloon is inflated until the cephalad surface of the balloon is just below the tracheal carina (Fig. 4824E) (so that the upper lobe of the blocked lung may also distend if CPAP is applied to the blocked lung [see below], and the fiberoptic bronchoscope is then withdrawn (Fig. 4824F).
|FIGURE 4824 The sequential steps of the fiberoptic-aided method of inserting and positioning the Univent bronchial blocker (BB) in the left mainstem bronchus are illustrated. One- or two-lung ventilation is achieved by simply inflating or deflating, respectively, the bronchial blocker balloon.|
Advantages and Noteworthy Positive Attributes of the Univent Bronchial Blocker Tube System
The Univent bronchial blocker tube has six important attributes that require special mention (Table 48–13). First and foremost, the degree of difficulty in inserting the Univent tube is equivalent to a standard single-lumen tube and therefore in many instances will be an easier and quicker way to separate the lungs to obtain simple one-lung ventilation (as compared with a DLT). 91, 93 Thus and as an example, the Univent tube may be preferable when difficult intubation is anticipated and when the patient has been treated with anticoagulants. Second, the patient can be continuously ventilated while the bronchial blocker is being placed into a mainstem bronchus, and the bronchial blocker can be placed into a mainstem bronchus just as easily in the LDP as in the supine position. Third, and provided that the postanesthesia care unit and intensive care unit personnel are instructed in the design and function of the Univent tube (particularly the ventilatory consequence of inflating the bronchial blocker cuff just distal to the main lumen [i.e., the main lumen will be obstructed]), the Univent tube may be left in situ for postoperative mechanical ventilation and the risk of a potentially difficult tube change (e.g., from a DLT to a single-lumen tube) thereby avoided. Fourth and similarly, the Univent tube may be left in situ if a patient is turned from the supine to the prone position midway through a surgical procedure (a common occurrence with surgery on the thoracic spine). Fifth, the unique characteristic of a movable endobronchial blocker permits the Univent endotracheal tube to create selective partial collapse (e.g., of a lobe) or total collapse of the targeted lung. 282 The capability of selectively blocking lung segments is extremely important in cases of isolated pulmonary hemorrhage. Partial versus total one-lung ventilation may allow for an improvement in PaO2 in cases of intraoperative hypoxemia during thoracic operations. Finally, although this is not a distinct advantage over a DLT, it should be noted that it is possible to apply CPAP to the nonventilated operative lung through the lumen of the bronchial blocker, 283 and therefore the Univent tube provides the same best solution to hypoxemia during one-lung ventilation as does a DLT. In fact, except for independent unilateral intermittent positive-pressure ventilation and suctioning, all selective differential lung functions possible with a DLT are possible with a Univent bronchial blocker tube.
TABLE 48–13. Advantage/Noteworthy Positive Attributes of the Univent Bronchial Blocker Tube (Relative to a Double-Lumen Tube and Other Bronchial Blockers)
Potential Limitations of the Univent Bronchial Blocker Tube System and Solutions to the Limitations
There are several distinct limitations to the Univent bronchial blocker tube system, but fortunately all have a relatively simple remedy (Table 48–14). First, the small lumen of the bronchial blocker results in slow inflation of the lung if gases are just insufflated (e.g., at a flow rate of 10 L/min, a lung will require at least 20 seconds to reach total lung capacity) or pushed in by conventional positive pressure. The operative lung may be made to expand rapidly if the bronchial blocker cuff is deflated (the operative lung will expand with one positive-pressure breath from the main single lumen) or if one very short (e.g., <0.5-sec) burst of wall oxygen-powered 20- to 30-psi jet ventilation (reduced from 50 psi) is administered. However, connection of the bronchial blocker lumen to a jet ventilator is potentially dangerous (i.e., it can cause barotrauma) because the lung can expand extremely rapidly, and it is of paramount importance that the anesthesiologist directly observe the lung and that the ventilation be very short or the pressure limited to 20 to 30 psi by an additional in-line regulator. Second, and also because the bronchial blocker lumen is small, the lung will deflate very slowly when blocked. This is easily remedied by deflating the bronchial blocker cuff (which reestablishes continuity between the operative lung and the main single lumen), disconnecting the patient from the ventilator, and leaving the endotracheal tube open to air while the surgeon gently compresses the lung to evacuate air from the operative lung through the main single lumen. After the lung is thus fully collapsed, the blocker balloon is inflated and ventilation resumed. 91, 92 Alternatively, the lumen of the bronchial blocker may be connected to the suction apparatus while the cuff is inflated; a normal amount of wall suction greatly facilitates lung collapse. Third, and also because the bronchial blocker lumen is small, the lumen is relatively easily blocked by blood and/or pus. High suction will occasionally clear the lumen of these materials and total blockage by inspissated secretions can be broken up by a wire stylet. Fourth, the Univent bronchial blocker behaves as a high-pressure cuff when intracuff volume is greater than 2 mL (the resting volume of the cuff) and may be expected to have an intracuff pressure between 150 and 250 mm Hg and a transmural pressure (intracuff pressure within the airway minus intracuff pressure outside the airway [free in the room]) between 50 and 60 mm Hg when intracuff volumes of 4 to 6 mL are used to seal 12- to 18-mm airways against the usual proximal airway pressure. 284, 285 Thus, the order of usual bronchial cuff pressures is left-sided PVC DLT < right-sided polyvinylchloride DLT < Univent Bronchial Blocker cuff < red rubber double-lumen tube. 286 These findings underscore the need to inflate the bronchial blocker cuff with a just-seal volume of air. Fifth, the Univent bronchial blocker has on occasion been reported to have a minor leak during surgery (25% in one series), 277 but this is not understandable in view of experiments showing that the Univent bronchial blocker cuff seals within normal-sized mainstem bronchi against proximal airway pressures as great as 100 cm H2 O with inflation volumes that are within the manufacturers recommendation. 285 Consequently, if an intraoperative leak occurs when less than a 6- to 7-mL intracuff volume has been used and the bronchial blocker cuff is completely subcarinal (as determined by fiberoptic bronchoscopy) and intact, the intracuff volume should be increased. If an intraoperative leak develops even though an adequate cuff inflation volume has been used (the bronchial blocker can be seen [fiberoptically] to fill the mainstem bronchus in question) and the bronchial blocker cuff is completely subcarinal (as determined fiberoptically) and intact, the relationship between the mainstem bronchus and the bronchial blocker cuff may no longer be a simple matter of a sphere or ellipsoid being inflated within a cylinder. Under these circumstances the surgeon may need to rearrange the surgical field so that the mainstem bronchus and bronchial blocker cuff are less distorted. Finally, the addition of the lumen for the bronchial blocker results in an endotracheal tube that has a large outside anteroposterior diameter relative to its inside diameter.
TABLE 48–14. Limitations to the Use of the Univent Bronchial Blocker Tube and Solutions to the Limitations
Methods to Obtain a Just-Seal Volume in the Bronchial Blocker Cuff
There are two methods to obtain a just-seal volume of air in the bronchial blocker cuff. The first method is the same as that already described for obtaining a just-seal volume of air in the endobronchial cuff of a DLT. It consists of pressurizing the main single lumen until air ceases to escape from the bronchial blocker lumen (detected by connecting the bronchial blocker lumen to a catheter that is submerged beneath the surface of a beaker of water; when air bubbles cease to come out, the bronchial blocker cuff has sealed) (see Fig. 4822).
The second method appears promising and uses capnography. End-tidal CO2 analyzers draw gas samples from the anesthesia breathing circuit via tubing that terminates, at the patient end of the tubing, in a standard Luer lock male connector that inserts into a female port in the breathing circuit. The male connector also inserts into/attaches to the female port at the proximal end of the Univents bronchial blocker. The tracing from a gas analyzer, connected to the blocker with its cuff deflated, shows a typical respiratory waveform. As the cuff of the bronchial blocker is steadily inflated, a point is reached at which the respiratory waveform abruptly ceases, and a straight line is seen, indicating that lung isolation has occurred (Fig. 4825). The CO2 concentration remains near its end-tidal value until the blocked lung has collapsed and then rapidly decreases. The strengths of this method include simplicity, repeatability, and ability to ventilate the unblocked lung continuously throughout the procedure.
|FIGURE 4825 Capnogram tracing showing normal respiratory waveform changing to a straight line as bronchial seal occurs. (From Essig K and Freeman JA605 )|
Clinical Indications for Use of the Univent Bronchial Blocker System
There are several clinical situations in which use of the Univent bronchial blocker tube is relatively indicated. First, whenever it is anticipated that postoperative ventilation will be necessary (e.g., poor pulmonary function preoperatively, anticipated lung damage or massive fluid and/or blood infusion intraoperatively, anticipated very long case), use of the Univent bronchial blocker tube for lung separation may avoid a risky postoperative DLT to single-lumen tube change. Second and similarly, use of the Univent bronchial blocker tube will avoid a potentially dangerous DLT to single-lumen tube change in cases of surgery on the thoracic spine in which a thoracotomy in the supine or lateral decubitus position is followed by surgery in the prone position. Third, a very severely distorted airway may prevent successful placement of a DLT, whereas such distortion may have much less of an effect on the proper placement of the Univent tube. Finally, but least predictable or compelling, are situations in which both lungs may need to be blocked (e.g., bilateral operations, indecisive surgeons).
Bronchial Blockers That Are Independent of a Single-Lumen Tube
The bronchial blocker that is independent of the single-lumen tube most often used for adults is a Fogarty occlusion (embolectomy) catheter with a 3-mL balloon. 287 The Fogarty catheter includes a stylet so that it is possible to place a curvature at the distal tip to facilitate entry into the larynx and either mainstem bronchus (by twirling the proximal end). If no endotracheal tube is in place, the operator exposes the larynx and places a single-lumen tube with a high-volume cuff in the trachea. The Fogarty catheter is then placed either inside 288, 289 or alongside the single-lumen tube (Fig. 4826). In either case (bronchial blocker inside or outside the single-lumen tube), a fiberoptic bronchoscope is passed down to the end of the single-lumen tube through a self-sealing diaphragm in the elbow connector (which permits continued positive-pressure ventilation around the fiberoptic bronchoscope), and the Fogarty catheter is visualized below the tip of the single-lumen tube. The proximal end of the bronchial blocker is then twirled in the fingertips and advanced until the distal tip locates in the desired mainstem bronchus. The catheter balloon is then inflated under direct visualization and the fiberoptic bronchoscope withdrawn through the self-sealing diaphragm.
|FIGURE 4826 Lung separation with a single-lumen tube, fiberoptic bronchoscope, and right lung bronchial blocker. The sequence of events is as follows: (A) A single-lumen tube is inserted and the patient is ventilated. (B) A bronchial blocker is passed alongside the indwelling endotracheal tube. (C) A fiberoptic bronchoscope is passed through a self-sealing diaphragm in the elbow connecter to the endotracheal tube and is used to place the bronchial blocker into the right mainstem bronchus under direct vision. (D) The balloon on the bronchial blocker is also inflated under direct vision and is positioned just below the tracheal carina. (E) The fiberoptic bronchoscope is then removed (right lower diagram). During the lower panel sequence (insertion and use of fiberoptic bronchoscope, Figs. C to E) the self-sealing diaphragm allows the patient to continue to be ventilated with positive-pressure ventilation (around the fiberoptic bronchoscope, but within the lumina of the endotracheal tube). LL, left lung; RL, right lung. (From Benumof604 )|
Finally, other balloon-tipped luminal catheters (such as the Magill or Foley type) may be used as bronchial blockers.
For bronchial blockage in very small children (10 kg or less) a Fogarty embolectomy catheter with a balloon capacity of 0.5 mL or a Swan-Ganz catheter (1-mL balloon) should be used. 290 Of course, these catheters have to be positioned under direct vision; a fiberoptic bronchoscope method, as depicted in Figure 4826, is perfectly acceptable, except the fiberoptic bronchoscope outside diameter must be approximately 2 mm to fit inside the endotracheal tube (3-mm internal diameter or greater). Otherwise, the bronchial blocker must be situated with a rigid bronchoscope. Pediatric patients of intermediate size require intermediate size occlusion catheters and judgment on the mode of placement (i.e., via rigid versus fiberoptic bronchoscope).
Disadvantages of bronchial blockage with a blocker that is independent of the single-lumen tube as compared with DLT lung separation include inability to suction and/or to ventilate the lung distal to the blocker, increased placement time, and the definite need for a fiberoptic or rigid bronchoscope. In addition, if a mainstem bronchial blocker backs out into the trachea, the seal between the two lungs will be lost, and two catastrophic complications may occur. First, if the bronchial blocker was being used to seal off fluid (blood or pus) in one lung, then both lungs may become contaminated with the fluid. Second, the trachea will be at least partially obstructed by the blocker, and ventilation will be greatly impaired. Therefore, bronchial blockage requires that the anesthetist continuously and intensively monitor the compliance and breath sounds of the ventilated lung.
Endobronchial Intubation With Single-Lumen Tubes
In adults presenting with hemoptysis, endobronchial intubation with a single-lumen tube is often the easiest, quickest way of effectively separating the two lungs, especially if the left lung is bleeding, in which case one can simply take an uncut single-lumen endotracheal tube and advance it inward until moderate resistance is felt. In the vast majority of patients, the single-lumen tube will locate in the right mainstem bronchus, thereby blocking off the bleeding left lung and allowing selective ventilation of only the right lung. Under these circumstances it is highly possible that the right upper lobe bronchus will be blocked off as well, resulting in ventilation of only the right middle and lower lobes. Ventilation of only a soiled right lung or ventilation of only the right middle and lower lobes (even if they are unsoiled) incurs the risk of serious hypoxemia due to the very large transpulmonary shunt that is necessarily created by single-lung endobronchial intubation.
If the right lung is bleeding, there are two ways of selectively intubating the left mainstem bronchus. First, this may be done blindly with approximately a 92 percent success rate by turning the patients head to the right and passing the single-lumen tube with the concavity of the tube facing posterior (rotated 180 degrees from its normal position and relationship to the trachea). 291 The single-lumen tube enters the right or left mainstem bronchus when the concavity of the tube faces anteriorly or posteriorly, respectively, because the bevel is left- or right-facing, respectively (i.e., a left-facing bevel enters the right mainstem bronchus and a right-facing bevel enters the left mainstem bronchus). 292 Second, a fiberoptic bronchoscope can be passed through a self-sealing diaphragm in the single-lumen tube elbow connector and directed into the left mainstem bronchus. Persistent large, soft catheter suctioning of the carinal area through the single-lumen tube before use of the fiberoptic bronchoscope and suctioning through the fiberoptic bronchoscope (through the single-lumen tube) may be required to visualize the tracheal carina. The single-lumen tube can then be passed over the fiberoptic bronchoscope into the left mainstem bronchus, thereby isolating the bleeding right lung and allowing selective ventilation of the left lung. Passing the fiberoptic bronchoscope through a self-sealing diaphragm allows the continuance of positive-pressure ventilation and PEEP around the bronchoscope. However, it should be realized that visualization of the carina may not be possible when the bleeding is copious and that the only hope for the patient may lie in rapid thoracotomy and control of bleeding from within the chest. In addition, under these adverse conditions, conventional passage of a DLT tube may more rapidly and effectively separate the two lungs than visualization of the anatomy with a fiberoptic bronchoscope.
In summary, use of DLTs is the method of choice for separating the lungs in most adult patients. If there is any question, the precise location of a DLT can be determined by fiberoptic bronchoscopy at any time. There are a number of situations in which insertion of a DLT may be difficult and/or dangerous, and under these circumstances consideration should be given to separating the lungs with a single-lumen tube alone or in combination with a bronchial blocker (e.g., the Univent tube). However, when using a single-lumen tube in a mainstem bronchus or when using a bronchial blocker, ability to suction the operative site and control oxygen uptake (the blocked lung cannot be ventilated with oxygen at any time) is limited. In addition, placement of the single-lumen tube into one or the other mainstem bronchus and proper placement of a bronchial blocker usually require fiberoptic bronchoscopy. Therefore, no matter what method of separating the lungs is chosen, there is a real need for the immediate availability of a small-diameter fiberoptic bronchoscope (for checking the position of the DLT, placing a single-lumen tube in the left mainstem bronchus, and placing a bronchial blocker) that has a suction port to clear secretions and blood from the airway.
Physiology of One-Lung Ventilation
Comparison of Arterial Oxygenation and Carbon Dioxide Elimination During Two-Lung Versus One-Lung Ventilation
As discussed previously, the matching of ventilation and perfusion is impaired during two-lung ventilation in an anesthetized, paralyzed, open-chested patient in the LDP. The reason for the mismatching is relatively good ventilation but poor perfusion of the nondependent lung and poor ventilation and good perfusion of the dependent lung (see Figs. 4813 and 4826A). The blood flow distribution has been seen to be mainly and simply determined by gravitational effects. The relatively good ventilation of the nondependent lung has been seen to be caused, in part, by the open chest and paralysis. The relatively poor ventilation of the dependent lung has been seen to be caused, in part, by the loss of dependent lung volume with general anesthesia and by circumferential compression of the dependent lung by the mediastinum, abdominal contents, and suboptimal positioning effect. The compression of the dependent lung may cause the development of a shunt compartment in this lung (see Fig. 4813; Fig. 4827A). Consequently, two-lung ventilation under these circumstances may result in an increased P(A-a)O2 and impaired oxygenation.
|FIGURE 4813 Schematic summary of ventilation-perfusion relationships in the anesthetized patient in the lateral decubitus position who has an open chest and is paralyzed and suboptimally positioned. The nondependent lung is well ventilated (as indicated by the large dashed lines) but poorly perfused (small perfusion vessel); the dependent lung is poorly ventilated (small dashed lines) but well perfused (large perfusion vessel). In addition, the dependent lung may also develop an atelectatic shunt compartment (indicated on the left side of the lower lung) because of the circumferential compression of this lung. (See text for detailed explanation.) PAB, pressure of the abdominal contents. (Modified from Benumof604 )|
|FIGURE 4827 Schematic representation of two-lung ventilation versus one-lung ventilation. Typical values for fractional blood flow to the nondependent and dependent lungs as well as arterial oxygen tension (PaO2) and shunt (QS/QT) for the two conditions are shown. The QS/QT during two-lung ventilation is assumed to be distributed equally between the two lungs (5 percent to each lung). The essential difference between two-lung and one-lung ventilation is that during one-lung ventilation the nonventilated lung has some blood flow and therefore has an obligatory shunt, which is not present during two-lung ventilation. The 35 percent of total flow perfusing the nondependent lung, which was not shunt flow, was assumed to be able to reduce its blood flow by 50 percent by hypoxic pulmonary vasoconstriction. The increase in QS/QT from two-lung to one-lung ventilation is assumed to be solely due to the increase in shunt through the nonventilated, nondependent lung during one-lung ventilation. (From Benumof604 )|
If the nondependent lung is nonventilated, as during one-lung ventilation, then any blood flow to the nonventilated lung becomes shunt flow, in addition to whatever shunt flow might exist in the dependent lung (Fig. 4827B). Thus, one-lung ventilation creates an obligatory right-to-left transpulmonary shunt through the nonventilated nondependent lung, which is not present during two-lung ventilation. Consequently, it is not surprising to find that, given the same inspired oxygen concentration (FIO2) and hemodynamic and metabolic status, one-lung ventilation results in a much larger P(A-a)O2 and lower PaO2 than two-lung ventilation. This contention is best supported by one study that compared arterial oxygenation during two-lung and one-lung ventilation, wherein each patient served as his or her own control. 293
One-lung ventilation has much less of a steady-state effect on PaCO2 than on PaO2. Blood passing through underventilated alveoli retains more than a normal amount of carbon dioxide and does not take up a normal amount of oxygen; blood traversing overventilated alveoli gives off more than a normal amount of carbon dioxide but cannot take up a proportionately increased amount of oxygen because of the flatness of the top end of the oxyhemoglobin dissociation curve. Thus, during one-lung ventilation (the one-lung minute ventilation equals the two-lung minute ventilation) the ventilated lung can eliminate enough carbon dioxide to compensate for the nonventilated lung, and PACO2 to PaCO2 gradients are small; however, the ventilated lung cannot take up enough oxygen to compensate for the nonventilated lung, and PAO2 to PaO2 gradients are usually large. With a constant minute ventilation (two-lung ventilation as compared with one-lung ventilation), the retention of carbon dioxide by blood traversing the nonventilated lung usually slightly exceeds the increased elimination of carbon dioxide from blood traversing the ventilated lung, and the PaCO2 will usually slowly increase (along with the ETCO2).
The initiation of one-lung ventilation has much more of an acute effect (first 5 minutes) on PETCO2. When one-lung ventilation is begun (keeping total tidal volume and respiratory rate constant), the ventilated lung is immediately hyperventilated in relation to its perfusion (i.e., has an increased V/Q ratio) and PETCO2 from this lung decreases in the first minute (e.g., by 5 mm Hg). 294
Over the next 5 minutes, HPV in the nonventilated lung shifts blood flow over to the ventilated lung, increases ventilated lung perfusion, decreases ventilated lung V/Q ratio and increases the PETCO2 back to the baseline two-lung ventilation value. 294 Thereafter, and as discussed previously, PETCO2 will slowly increase (along with PaCO2) because the same total minute ventilation to one lung is not as effective as when it is delivered to both lungs (i.e., there is an increased alveolar dead space within the one ventilated lung). 294
Blood Flow Distribution During One-Lung Ventilation
Blood Flow to the Nondependent, Nonventilated Lung
Fortunately, both passive mechanical and active vasoconstrictor mechanisms are usually operant during one-lung ventilation that minimize the blood flow to the nondependent, nonventilated lung and thereby prevent PaO2 from decreasing as much as might be expected on the basis of the distribution of blood flow during two-lung ventilation. The passive mechanical mechanisms that decrease blood flow to the nondependent lung are gravity, surgical interference with blood flow, and perhaps the extent of preexisting disease in the nondependent lung (Fig. 4828). Gravity causes a vertical gradient in the distribution of pulmonary blood flow in the LDP for the same reason that it does in the upright position (see Fig. 489). Consequently, blood flow to the nondependent lung is less than that to the dependent lung. The gravity component of blood flow reduction to the nondependent lung should be constant with respect to both time and magnitude.
|FIGURE 4828 Schematic diagram of the determinants of blood flow distribution during one-lung ventilation. The major determinants of blood flow to the nondependent lung are gravity, surgical interference with blood flow, amount of nondependent lung disease, and magnitude of nondependent lung hypoxic pulmonary vasoconstriction. The determinants of dependent lung blood flow are gravity, amount of dependent lung disease, and dependent lung hypoxic pulmonary vasoconstriction. (From Benumof604 )|
|FIGURE 489 Schematic representation of the effects of gravity on the distribution of pulmonary blood flow in the lateral decubitus position. The vertical gradient in the lateral decubitus position is less than in the upright position. Consequently, there is less zone 1 and more zone 2 and 3 blood flow in the lateral decubitus position than in the upright position. Nevertheless, pulmonary blood flow increases with lung dependency and is greater in the dependent lung than in the nondependent lung. PA, alveolar pressure; Ppa, pulmonary artery pressure; Ppv, pulmonary venous pressure. (From Benumof604 )|
Surgical compression (directly compressing lung vessels) and retraction (causing kinking and tortuosity of lung vessels) of the nondependent lung may further passively reduce nondependent lung blood flow. In addition, ligation of pulmonary vessels during pulmonary resection greatly decreases nondependent lung blood flow. The surgical interference component of blood flow reduction to the nondependent lung should be variable with respect to both time and magnitude.
The amount of disease in the nondependent lung is also a significant determinant of the amount of blood flow to the nondependent lung. If the nondependent lung is severely diseased, there may be a fixed reduction in blood flow to this lung preoperatively and collapse of such a diseased lung may not cause much of an increase in shunt. The notion that a diseased pulmonary vasculature might be incapable of HPV is supported by the observations that administration of sodium nitroprusside and nitroglycerin (which should abolish any preexisting HPV) to COPD patients (who have a fixed reduction in the cross-sectional area of their pulmonary vascular bed) does not cause an increase in shunt, 295 whereas these drugs do increase shunt in patients with acute regional lung disease who have an otherwise normal pulmonary vascular bed. 296 If the nondependent lung is normal and has a normal amount of blood flow preoperatively, collapse of such a normal lung may be associated with a higher nonventilated nondependent lung blood flow and shunt. A higher one-lung ventilation shunt through the nondependent lung may be more likely to occur in patients who require thoracotomy for nonpulmonary disease. 297 Two studies have systematically validated the inverse correlation between the amounts of nondependent lung disease and shunt during one-lung ventilation. 298, 299
The most significant reduction in blood flow to the nondependent lung is caused by an active vasoconstrictor mechanism. The normal response of the pulmonary vasculature to atelectasis is an increase in PVR (in just the atelectatic lung); this increase, thought to be due almost entirely to HPV, 153, 154, 155 diverts blood flow from the atelectatic lung toward the remaining normoxic or hyperoxic ventilated lung. The diversion of blood flow minimizes the amount of shunt flow that occurs through the hypoxic lung. Figure 486 shows the theoretically expected effect of HPV on arterial oxygen tension (PaO2) as the amount of lung that is made hypoxic increases. 209 When very little of the lung is hypoxic (near 0%) it does not matter, in terms of PaO2, whether the small amount of lung has HPV operating or not because in either case the shunt will be small. When most of the lung is hypoxic (near 100%) there is no significant normoxic region to which the hypoxic region can divert flow, and again it does not matter, in terms of PaO2, whether or not the hypoxic region has HPV operating. When the percentage of lung that is hypoxic is intermediate (between 30 and 70%), which is the amount of lung that is typically hypoxic during the one-lung ventilation-anesthesia condition, there is a large difference between the PaO2 expected with a normal amount of HPV (which is a 50% blood flow reduction for a single lung) 209 as compared with no HPV. In fact, in this range of hypoxic lung, HPV can increase PaO2 from hypoxemic levels to much higher and safer values. It is not surprising, then, that numerous clinical studies on one-lung ventilation have found that the shunt through the nonventilated lung is usually 20 to 30 percent of the cardiac output, as opposed to the 40 to 50 percent shunt that might be expected if there were no HPV operating in the nonventilated lung. 297, 300, 301, 302, 303, 304, 305 Thus, HPV is an autoregulatory mechanism that protects the PaO2 by decreasing the amount of shunt flow that can occur through hypoxic lung.
|FIGURE 486 Effect of hypoxic pulmonary vesoconstriction (HPV) on PaO2. As the amount of lung that is made hypoxic is increased (x axis), the arterial oxygen tension (PaO2) decreases (y axis). In the range of 30 to 70 percent of hypoxic lung, the normal expected amount of HPV increases PaO2 from arrhythmogenic levels to much higher and safer levels. Normal cardiac output, hemoglobin concentration, and mixed venous oxygen tension (Pv¯O2) are assumed. (Data from Marshall and Marshall209 )|
Figure 4829 outlines the major determinants of the amount of atelectatic lung HPV that might occur during anesthesia. In the following discussion, the HPV issues or considerations are numbered as they are in Figure 4829.
|FIGURE 4829 Listing of many of the components of the anesthetic experience that might determine the amount of regional hypoxic pulmonary vasoconstriction (HPV). The clockwise numbering of considerations corresponds to the order in which these considerations are discussed in the text. V/Q, ventilation/perfusion ratio; PVP, pulmonary vascular pressure; Pv¯O2, mixed venous oxygen tension; FIO2, inspired oxygen fraction; PACO2, alveolar carbon dioxide tension; PEEP, positive end-expiratory pressure. (From Benumof604 )|
The distribution of the alveolar hypoxia is probably not a determinant of the amount of HPV; all regions of the lung (either the basilar or dependent parts of the lungs [supine or upright] or discrete anatomic units such as a lobe or single lung) respond to alveolar hypoxia with vasoconstriction. 306 However, recent evidence suggests that on a sublobar level, collateral ventilation may be the first line and HPV the second line of defense against the development of arterial hypoxemia. 307
As with low V/Q ratios and nitrogen-ventilated lungs, it appears that the preponderance of blood flow reduction in acutely atelectatic lung is due to HPV and none of it to passive mechanical factors (such as vessel tortuosity). 153, 154, 155 This conclusion is based on the observation that reexpansion and ventilation of a collapsed lung with nitrogen (removing any mechanical factor) do not increase the blood flow to the lung, whereas ventilation with oxygen restores all the blood flow to precollapse values. This conclusion applies whether ventilation is spontaneous or due to positive pressure and whether the chest is open or closed. 154 In canines, a slight amount of further subacute (>30 min) decrease in blood flow to atelectatic lung may have been due to some mechanical effect of the atelectasis on lung blood vessels. 308 However, in humans a prolonged unilateral hypoxic challenge during anesthesia results in an immediate vasoconstrictor response with no further potentiation or diminution of the HPV response. 309
Most systemic vasodilator drugs either inhibit regional HPV directly or have an effect in a clinical situation that is consistent with inhibition of regional HPV (i.e., decreasing PaO2 and increasing shunt in patients with acute respiratory disease). The vasodilator drugs that have been shown to inhibit HPV or to have a clinical effect con-sistent with inhibition of HPV are nitroglycerin, 296, 310, 311, 312, 313, 314, 315, 316 nitroprusside, 296, 317, 318, 319, 320, 321, 322, 323 dobutamine, 324, 325 several calcium antagonists, 326, 327, 328, 329, 330, 331 and many b2 -agonists (isoproterenol,ritodrine, orciprenaline, salbutamol, ATP, and glucagon). 325, 332, 333, 334, 335, 336, 337, 338 Nitric oxide, a potent inhaled pulmonaryvasodilator, has been studied extensively and is considered to inhibit HPV. 339, 340, 341, 342, 343, 344 Aminophylline and hydralazine may not decrease HPV. 345
The effect of anesthetic drugs on regional HPV was previously discussed in the section Effect of Anesthetics on Hypoxic Pulmonary Vasoconstriction.
The HPV response is maximal at normal and decreased at either high or low pulmonary vascular pressure. The mechanism for high pulmonary vascular pressure inhibition of HPV whether the cardiac output is high or low 346, 347 is simple; the pulmonary circulation is poorly endowed with smooth muscle and cannot constrict against an increased vascular pressure. Furthermore, in the one-lung ventilation situation in the LDP, it is obvious, with all other factors remaining constant, that the fraction of the cardiac output perfusing the collapsed nondependent lung will increase with increasing pulmonary arterial pressure (i.e., the effect of gravity will be overcome). 348 The mechanism for low pulmonary vascular pressure inhibition of HPV is more complex. For this to occur, the hypoxic compartment must be atelectatic. Under these circumstances, when pulmonary vascular pressure decreases, it is possible for part of the ventilated lung (but not the atelectatic lung) to be in a zone 1 condition (alveolar pressure increases in relation to pulmonary artery pressure) and to experience a disproportionate increase in PVR, which would divert blood flow back over to the atelectatic lung, thereby inhibiting atelectatic lung HPV. 349
The HPV response also is maximal when Pv¯O2 is normal and is decreased by either high or low Pv¯O2. The mechanism for high Pv¯O2 inhibition of HPV is presumably due to reverse diffusion of oxygen, causing the oxygen tension of either the vessels, the interstitial or alveolar spaces, or all of these to be increased above the HPV threshold. 350 That is, if enough oxygen can reach some receptor in the small arteriole-capillary-alveolar area, then the vessels will not vasoconstrict. The mechanism for low Pv¯O2 inhibition of HPV is a result of the low Pv¯O2 decreasing alveolar oxygen tension in the normoxic compartment down to a level sufficient to induce HPV in the supposedly normoxic lung. 351 The HPV in the presumably normoxic lung competes against and offsets the HPV in the originally hypoxic lung and results in no blood flow diversion from the more obviously hypoxic lung.
Selectively decreasing the FIO2 in the normoxic compartment (from 1.0 to 0.5 to 0.3) causes an increase in normoxic lung vascular tone, thereby decreasing blood flow diversion from hypoxic to normoxic lung. 351 Indeed, with unilateral lung injury, ventilation of both lungs with a hypoxic gas mixture (FIO2 of 0.12) induces much more vasoconstriction in the normal, previously nonconstricted lung than in the injured and already hypoxically constricted lung, which redirects blood flow to, increases the shunt through, and increases edema in the injured lung. 352 In addition, the development of systemic hypoxemia, either when bilateral hypoxic ventilation is used or when there is a very largehypoxic compartment and a small normoxic compartment, may indirectly inhibit regional HPV by stimulation of arterial chemoreceptors. 353 At the other extreme, prolonged exposure to hyperoxia (FIO2 of 1.0) for 68 hours blunts a subsequent whole-lung HPV response. 354
Older studies have suggested that the vasoconstrictor drugs (dopamine, epinephrine, phenylephrine) constrict normoxic lung vessels preferentially, thereby disproportionately increasing normoxic lung 325, 329, 335 PVR. The increase in normoxic lung PVR would be expected to decrease normoxic lung blood flow and increase atelectatic lung blood flow. The HPV-inhibiting effect of vasoconstrictor drugs is similar to that of decreasing normoxic lung FIO2 (see item 7). In recent years, dopamine has been extensively studied. Although one reasonably straightforward study 355 agrees well with previous studies, 325, 329, 335, 347 most recent studies 356, 357, 358 have shown no significant effect of dopamine on HPV and/or arterial oxygenation. On the basis of these latter more recent studies, dopamine appears to be a reasonable cardiovascular stimulant to use in patients with lung disease provided that arterial oxygenation is monitored.
Hypocapnia has been thought to directly inhibit and hypercapnia to directly enhance regional HPV. 346, 359 In addition, during one-lung ventilation conditions, hypocapnia can be produced only by hyperventilation of the one lung. The hyperventilation requires an increased ventilated lung airway pressure, which may cause increased ventilated lung PVR, which in turn may divert blood flow back into thehypoxic lung. Hypercapnia during one-lung ventilation seems to act as a vasoconstrictor by selectively increasing ventilated lung PVR (which would divert blood flow back to the nonventilated lung). In addition, hypercapnia is ordinarily caused by hypoventilation of the ventilated lung, which greatly increases the risk of developing low V/Q and atelectatic regions in the dependent lung. However, it should be noted as a theoretical possibility that if hypoventilation of the dependent lung is associated with decreased ventilated lung airway pressure, ventilated lung pulmonary vascular resistance may be decreased, thus in turn promoting or enhancing HPV in the nonventilated lung.
The effects of changes in airway pressure due to PEEP and tidal volume changes are discussed in detail in the sections Dependent Lung PEEP and Selective Dependent Lung PEEP. In brief, selective application of PEEP to only normoxic ventilated lung will selectively increase PVR in the ventilated lung and shunt blood flow back into thehypoxic nonventilated lung (i.e., decrease nonventilated lung HPV). 360, 361 On the other hand, high-frequency ventilation of the gas-exchanging lung is associated with low airway pressure and enhancement of HPV in the collapsed lung. 362
Finally, there is some evidence that certain types of infections (which may cause atelectasis), particularly granulomatous and pneumococcal infections, may inhibit HPV. 363, 364
Blood Flow to the Dependent Ventilated Lung
The dependent lung usually has an increased amount of blood flow due to both passive gravitational effects and active nondependent lung vasoconstrictor effects. However, the dependent lung may also have a hypoxic compartment (area of low V/Q ratio and atelectasis) that was present preoperatively or that developed intraoperatively. This hypoxic compartment may develop intraoperatively for several reasons. First, in the LDP the ventilated dependent lung usually has a reduced lung volume resulting from the combined factors of induction of general anesthesia and circumferential (and perhaps severe) compression by the mediastinum from above, by the abdominal contents pressing against the diaphragm from the caudad side, and by suboptimal positioning effects (rolls, packs, chest supports) pushing in from the dependent side and axilla 207, 231, 334, 365 (see Fig. 4813). Second, absorption atelectasis can also occur in regions of the dependent lung that have low V/Q ratios when they are exposed to high inspired oxygen concentration. 366, 367 Third, difficulty in secretion removal may cause the development of poorly ventilated and atelectatic areas in the dependent lung. Finally, maintaining the LDP for prolonged periods may cause fluid to transude into the dependent lung (which may be vertically below the left atrium) and cause further decrease in lung volume and increase in airway closure in the dependent lung. 368
The development of low V/Q ratio and/or atelectatic areas in the dependent lung increases vascular resistance in the dependent lung 365, 369 (because of dependent lung HPV), 306 thereby decreasing dependent lung blood flow and increasing nondependent lung blood flow. 370 Stated differently, the PVR in the ventilated compartment of the lung determines the ability of the ventilated, and supposedly normoxic, lung to accept redistributed blood flow from the hypoxic lung. Clinical conditions that are independent of specific dependent lung disease but that may still increase dependent lung vascular resistance in a dose-dependent manner are a decreasing inspired oxygen tension in the dependent lung (from 1.0 to 0.5 to 0.3) 351, 370 and decreasing temperature (from 40 to 30°C). 371
Miscellaneous Causes of Hypoxemia During One-Lung Ventilation
Still other factors may contribute to hypoxemia during one-lung ventilation. Hypoxemia due to mechanical failure of the oxygen supply system or the anesthesia machine is a recognized hazard of any kind of anesthesia. Gross hypoventilation of the dependent lung can be a major cause of hypoxemia. Malfunction of the dependent lung airway lumen (blockage by secretions) and malposition of the DLT are, in our experience, frequent causes of increased P(A-a)O2 and hypoxemia. Resorption of residual oxygen from the nonventilated lung is time-dependent and accounts for a gradual increase in shunt and decrease in PaO2 after one-lung ventilation is initiated. 369 With all other anesthetic and surgical factors constant, anything that decreases the Pv¯O2 (decreased cardiac output, increased oxygen consumption [excessive sympathetic nervous system stimulation, hyperthermia, shivering]) causes increased P(A-a)O2. 372, 373 Finally, transfusion of blood may cause pulmonary dysfunction, and the dysfunction has been attributed to the action of isoantibodies against leukocytes, which causes cellular aggregation, microvascular occlusion, and capillary leakage. Indeed, such a reaction has been described during prolonged one-lung ventilation. 374 Interestingly, the noncollapsed lung was preferentially injured, and the collapsed lung showed only minimal radiologic signs of edema after reexpansion. 374
Conventional Management of One-Lung Ventilation
The proper initial conventional management of one-lung ventilation is logically based on the preceding determinants of blood flow distribution during one-lung ventilation. In view of the fact that one-lung ventilation incurs a definite risk of causing systemic hypoxemia, it is extremely important that dependent lung ventilation, as it affects these determinants, be optimally managed. This section considers the usual management of one-lung ventilation in terms of the most appropriate FIO2, tidal volume, and respiratory rate (Table 48–15).
TABLE 48–15. Initial Conventional Ventilatory Management of One-Lung Anesthesia
Inspired Oxygen Concentration
Although the theoretical possibilities of absorption atelectasis and oxygen toxicity exist, the benefits of ventilating the dependent lung with 100 percent oxygen far exceed the risks. A high FIO2 in the single ventilated lung may critically increase PaO2 from arrhythmogenic and life-threatening levels to safer levels.
In addition, a high FIO2 in the dependent lung causes vasodilation, thereby increasing the dependent lung capability of accepting blood flow redistribution due to nondependent lung HPV. Direct chemical 100 percent oxygen toxicity does not occur during the operative period, 375 and absorption atelectasis in the dependent lung 375 is unlikely to occur in view of the remaining one-lung ventilation management characteristics (moderately large tidal volumes with intermittent positive-pressure, low-level PEEP). If the anesthesiologist is concerned about and wishes to limit the FIO2 in patients treated with either bleomycin or mitomycin, it is possible to carefully increase or decrease FIO2 to both lungs independently in an attempt to have the lowest FIO2 to both lungs (even when CPAP is administered to the nonventilated lung). 376
The dependent lung should be ventilated with a tidal volume of approximately 10 mL/kg. A much smaller tidal volume might promote dependent-lung atelectasis; a much greater tidal volume might excessively increase dependent lung airway pressure and vascular resistance 369 and thereby increase nondependent lung blood flow (decrease nondependent lung HPV). 360, 377, 378 If a tidal volume of 10 mL/kg causes excessive airway pressure, it should be lowered (after mechanical causes [i.e., tube malfunction] have been ruled out) and the respiratory rate increased (see later).
A dependent-lung tidal volume of 10 mL/kg is in the middle of a range of tidal volumes (815 mL/kg) that have been found in one study not to affect arterial oxygenation greatly during one-lung ventilation. 302 In that study, changes in PaO2 with alterations in tidal volume (during stable one-lung ventilation conditions) in individual patients were variable and unpredictable in both degree and direction (although the mean value for the group did not change). Thus, it appears that changing the tidal volume from 15 to 8 mL/kg during one-lung ventilation has an unpredictable but usually not great impact on arterial oxygenation. 302
Dependent Lung PEEP
No, or just a very low level of, dependent lung PEEP (<5 cm H2 O) should be used initially because of concern of unnecessarily increasing dependent lung PVR. Although this is unlikely to occur when dependent lung PEEP is less than 5 cm H2 O, 379 the presence of intrinsic PEEP during one-lung ventilation may make the total PEEP excessive 380 (see the section, Selective Dependent-Lung PEEP).
The respiratory rate should be set so that the PaCO2 remains at 40 mm Hg. Because a dependent lung tidal volume of 10 mL/kg represents a 20 percent decrease from the usual two-lung tidal volume of 12 mL/kg, the respiratory rate usually has to be increased by 20 to 30 percent to maintain carbon dioxide hemostasis. The trade-off between decreased tidal volume and increased respiratory rate usually results in a constant minute ventilation; although ventilation and perfusion are considerably mismatched during one-lung ventilation, an unchanged minute ventilation during one-lung ventilation (as compared with two-lung ventilation) can continue to eliminate a normal amount of carbon dioxide because of the high diffusibility of carbon dioxide. 369, 381, 382, 383 Hypocapnia should be prevented because use of the airway pressure in the dependent lung necessary to produce systemic hypocapnia may excessively increase dependent lung vascular resistance. Furthermore, hypocapnia may directly inhibit HPV in the nondependent lung. 346, 359
In summary, at the commencement of one-lung ventilation, 100 percent oxygen, a tidal volume of 10 mL/kg, and a 20 percent increase in respiratory rate are used as initial ventilation settings (see Table 48–15). Ventilation and arterial oxygenation are monitored by use of arterial blood gases, end-tidal carbon dioxide concentration, and pulse oximetry. If there is a problem with either ventilation or arterial oxygenation, one or more of the differential lung management techniques described next are used.
Differential Lung Management of One-Lung Ventilation
Intermittent Inflation of the Nondependent Operative Lung
Intermittent inflation with oxygen of the collapsed lung during one-lung ventilation may be expected to increase PaO2 for a variable period of time. In a group of thoracic surgery patients undergoing one-lung ventilation with an inspired nitrous oxide fraction (FIN2O) of 0.5 and an FIO2 of 0.5, the collapsed lung was manually inflated with a breath every 5 minutes with 2 L of oxygen and the lung was then allowed to collapse again; PaO2 increased by more than 28 mm Hg following each inflation. 384 The beneficial effect of each inflation persisted to a large extent to the next breath even if at a gradually decreasing level. Although PaO2 decreased between inflations, it never reached the level observed in controls (no lung inflation) during 19 minutes of one-lung ventilation.
Selective Dependent Lung PEEP
Because the ventilated dependent lung often has decreased lung volume during one-lung ventilation (see Figs. 4813, 4828, and 4830), it is not surprising that several attempts have been made to improve oxygenation by selectively treating the ventilated lung with PEEP. 207, 237, 304, 360, 377, 378 An accepted risk of selective dependent lung PEEP is that the PEEP-induced increase in lung volume can cause compression of the small dependent lung intra-alveolar vessels and increase dependent lung PVR; this diverts blood flow from the ventilated lung to the nonventilated lung (see Fig. 4830, upper right panel), increasing the shunt and decreasing the PaO2. That increases in both PEEP and tidal volume in the dependent ventilated lung have an additive effect in decreasing PaO2 during one-lung ventilation greatly supports the one-ventilated lung volume versus vascular resistance hypothesis. 377 Therefore, the effect of dependent lung PEEP on arterial oxygenation is a trade-off between the positive effect of increasing dependent lung FRC and V/Q ratio and the negative effect of increasing dependent lung PVR and shunting blood flow to the nonventilated lung. Not surprisingly, then, the various one-lung ventilationPEEP studies have had patients who have had an increase, 304, 377, 385 no change, 377, 385, 386, 387 or a decrease 304, 377, 385, 388, 389, 390 in oxygenation. It may be expected that in patients with a very diseased dependent lung (low lung volume and low V/Q ratio), the positive effects of selective dependent lung PEEP (increased lung volume and increased V/Q ratio) might outweigh the negative effects (shunting of blood flow to the nonventilated, nondependent lung), whereas in patients with a normal dependent lung, the negative effects would outweigh the benefits. Indeed, in one study in which 10 cm H2 O PEEP was selectively applied to the dependent lung, PaO2 increased in those patients with an initial PaO2 of less than 80 mm Hg (FIO2 of 0.5), whereas PaO2 decreased or remained constant in patients with an initial PaO2 higher than 80 mm Hg (FIO2 of 0.5). 385, 390 Presumably, in the patients with PaO2 lower than 80 mm Hg, the dependent lung had a low FRC (low V/Q ratio and atelectatic regions); therefore, the positive effect of increased dependent lung volume predominated over the negative effect of shunting blood flow to the nonventilated lung. Conversely, the patients with the higher PaO2 presumably had a dependent lung with an adequate FRC and V/Q ratio, and the negative effect of shunting blood flow to the nonventilated lung predominated over the positive effect of increased dependent lung volume. Although a dose (ventilated lung PEEP) versus response (PaO2, QS/QT value) relationship has been poorly described, it seems reasonable to postulate on the basis of these results 304, 377, 385, 386, 387, 388, 389, 390 that the therapeutic margin of using PEEP to increase PaO2 during one-lung ventilation is quite narrow. PEEP only to the dependent ventilated lung may be delivered by the same anesthesia apparatus that is ordinarily used to deliver PEEP to the whole lung. Other studies have shown that high tidal volumes, 301 variations in the inspiratory/expiratory ratio, 386 and intermittent manual hyperventilation of the lower lung are not beneficial in increasing PaO2 during one-lung ventilation. 386
|FIGURE 4830 Four-part schematic diagram showing the effects of various differential lung management approaches. (A) The one-lung ventilation situation. The DOWN (dependent) lung is ventilated (VENT) but is compressed by the weight of the mediastinum (M) from above, the pressure of the abdominal contents against the diaphragm (D), and by positioning effects of rolls, packs, and shoulder supports (P). The UP (nondependent) lung is nonventilated (NONVENT), and blood flow through this lung is shunt flow. (B) The dependent lung has been selectively treated with PEEP, which improves V/Q relationships in the dependent lung but also increases dependent lung vascular resistance; this diverts blood to, and thereby increases shunt flow through, the nonventilated lung. (C) Selective application of CPAP to the nondependent lung permits oxygen uptake from this lung; even if the CPAP causes an increase in vascular resistance and diverts blood flow to the dependent lung, the diverted blood flow can still participate in gas exchange in the ventilated dependent lung. Consequently, selective nondependent lung CPAP can greatly increase PaO2. (D) With differential lung CPAP (nondependent lung)/PEEP (dependent lung), it does not matter where the blood flow goes, since both lungs can participate in O2 uptake. With this latter one-lung ventilation pattern, PaO2 can be restored to levels near those achieved by two-lung ventilation.|
Selective Nondependent Lung CPAP
Low levels of positive pressure can be selectively and statically applied to only the nonventilated, nondependent lung. Because under these conditions the nonventilated lung is only slightly but constantly distended by oxygen, an appropriate term for this ventilatory arrangement is nonventilated lung CPAP. The application of CPAP (without tidal ventilation) to only the nonventilated lung significantly increases oxygenation. 388, 391 Also, the institution of 10 cm H2 O nondependent lung CPAP in patients has no significant hemodynamic effect. 385, 389 Low levels of CPAP simply maintain the patency of nondependent lung airways, allowing some oxygen distention of the gas exchanging alveolar space in the nondependent lung (Fig. 4830C) without significantly affecting the pulmonary vasculature. In all clinical studies, 385, 388, 389, 392 the application of 5 to 10 cm H2 O CPAP has not interfered with the performance of surgery and may, in fact, facilitate intralobar dissection. This is not surprising in view of the fact that the initial compliance of a collapsed lung is only 10 mL/cm H2 O, and 5 to 10 cm H2 O CPAP should create only a slightly distended lung that occupies a volume of 50 to 100 mL, which is hardly or not at all noticed by the surgeon.
On the other hand, in a canine study, 391 15 cm H2 O of nondependent lung CPAP caused changes in PaO2 and shunt similar to those of 5 to 10 cm H2 O of nondependent lung CPAP, whereas blood flow to the nonventilated nondependent lung decreased significantly. Therefore, high levels of nonventilated lung CPAP act by permitting oxygen uptake in the nonventilated lung as well as by causing blood flow diversion to the ventilated lung, where both oxygen and carbon dioxide exchange can take place (see Fig. 4830C). Because low levels of nonventilated lung CPAP are as efficacious as high levels and have less surgical interference and hemodynamic implications, it is logical to use low levels of nonventilated CPAP first.
In all patients in all clinical studies to date, 5 to 10 cm H2 O of nondependent lung CPAP has significantly increased PaO2 during one-lung ventilation. 385, 388, 389, 392, 393, 394, 395 It should be concluded that the single most efficacious maneuver to increase PaO2 during one-lung ventilation is to apply 5 to 10 cm H2 O of CPAP to the nondependent lung. In our experience, low levels of nonventilated lung CPAP have corrected severe hypoxemia (PaO2<50 mm Hg) more than 95 percent of the time, provided that the DLT was correctly positioned. However, the nondependent lung CPAP must be applied during the deflation phase of a large tidal volume so that the deflating lung can lock into a CPAP level with uniform expansion and obviate the need to overcome critical opening pressures of airways and alveoli.
In both the human 388 and canine 391 studies, oxygen insufflation at zero airway pressure did not significantly improve PaO2 and shunt, and this result was probably due to the inability of zero transbronchial airway pressure to maintain airway patency and overcome critical alveolar opening pressures. Although one study in patients has concluded that insufflation of O2 at zero airway pressure does increase PaO2, the study is difficult to interpret, because the patients did not serve as their own controls. 396
Several selective nondependent lung CPAP systems that are easy to assemble have been described. 389, 393, 394, 395 All these nondependent lung CPAP systems have three features in common (Fig. 4831). First, there must be a source of oxygen to flow into the nonventilated lung. Second, there must be some sort of restrictive mechanism (hand-screw valve, pop-off valve, weight-loaded valve) to retard the egress of oxygen from the nonventilated lung so that the nonventilated lung may become distended. Thus, oxygen from a free-flowing pressurized source flows into a lung, but the escape of the oxygen is restricted; the unrestricted flow in and the restricted flow out create a constant distending pressure. Third, the distending pressure must be measured by a manometer. In practice, it is often simplest to keep the restrictive mechanism constant and adjust the distending pressure with a relatively fine sensitivity by changing the oxygen flow rate. If the nondependent lung CPAP system includes a reservoir bag (which is highly desirable), the reservoir bag will reflect the amount of CPAP (by distention), and the nondependent lung may also be ventilated with intermittent positive pressure whenever desired.
|FIGURE 4831 The three essential components of a nondependent lung CPAP system consist of (1) an oxygen source, (2) a pressure relief valve, and (3) a pressure manometer to measure the CPAP. The CPAP is created by the free flow of oxygen into the lung versus the restricted outflow of oxygen from the lung by the pressure relief valve. ZEEP, zero end-expiratory pressure. (From Benumof604 )|
The availability of a commercial nondependent nonventilated lung CPAP device (Mallinckrodt Medical, Inc.) replaces the need for all homemade devices. The Broncho-Cath CPAP System (Fig. 4832) is similar in concept to the system shown in Figure 4831 except that the restrictive mechanism is a continuously vented slot (opening); the larger the vent slot, the lower the CPAP level delivered, and the smaller the vent slot, the higher the CPAP level. The level of CPAP is selected by simply turning an outer calibrated cylinder (calibrated for an oxygen flow rate of 5 L/min), which progressively uncovers the inner (pop-off) slot. Because the dial is calibrated and marked with the CPAP level selected, a pressure manometer is unnecessary. The rest of the CPAP system consists of the other components shown in Figure 4831 (oxygen tubing, reservoir bag). A unique feature of the Broncho-Cath CPAP System is the fact that the valve is continuously vented. Because the vent slot cannot be totally occluded, the danger of overpressurization of the nondependent lung is minimized. In addition, the CPAP system is calibrated from 1 to 10 cm H2 O pressure, which permits accurate delivery of very low levels of CPAP. This may be desirable with extremely compliant lungs or where maximum surgical exposure is required. This small, lightweight CPAP device can be included within the DLT package (the DLT is available with or without the CPAP device), is extremely easy to use, and is preassembled, cost-effective, sterile, reliable, and disposable. Perhaps its most important attribute is that a preassembled device is obviously more readily available than devices that require a search for spare parts, and the commercial product has built-in quality assurance that cannot be duplicated by individual remedies.
|FIGURE 4832 The Mallinckrodt Broncho-Cath CPAP System. (A) Photograph of entire CPAP system connected to a double-lumen tube. (B) Schematic of the entire CPAP system. (C) Close-up of the CPAP-generating restrictive mechanism. (Photography courtesy of Mallinckrodt Medical, Inc., St. Louis, MO.)|
Differential Lung PEEP/CPAP
In theory and from the preceding considerations, it appears that the ideal way to improve oxygenation during one-lung ventilation is the application of differential lung PEEP/CPAP (see Fig. 4830D) (see the following section for the step-by-step approach). In this situation, the ventilated (dependent) lung is given PEEP in the usual conventional manner in an effort to improve ventilated lung volume and V/Q relationships. Simultaneously, the nonventilated (nondependent) lung receives CPAP in an attempt to improve oxygenation of the blood perfusing this lung. Therefore, with differential lung PEEP/CPAP, it does not matter where the blood flow goes nearly as much as during simple one-lung ventilation, because wherever it goes (to either ventilated or nonventilated lung), it has at least some chance to participate in gas exchange with alveoli that are expanded with oxygen. In indirect support of this contention, arterial oxygenation has been increased significantly in patients during thoracotomy in the LDP (using two-lung ventilation) when PEEP has been added to the ventilated dependent lung, while the nondependent lung was also able to participate in gas exchange by virtue of being ventilated at zero end-expiratory pressure (ZEEP). 237 In direct support of this contention, in patients undergoing thoracotomy and one-lung ventilation, arterial oxygenation was unchanged by the application of 10 cm H2 O dependent lung PEEP alone (consistent with an equal positive/negative effect trade-off), was significantly improved by 10 cm H2 O nondependent lung CPAP alone, and was further and even more significantly increased by use of 10 cm H2 O nondependent lung CPAP and 10 cm H2 O dependent lung PEEP together (differential lung PEEP/CPAP ventilation). 385, 389 The use of 10 cm H2 O nondependent lung CPAP together with 10 cm H2 O dependent lung PEEP in patients caused only small, clinically insignificant hemodynamic effects. 385, 389
There are multiple reports of significant increases in oxygenation obtained with the application of differential lung ventilation and PEEP (either PEEP/PEEP, PEEP/CPAP, or CPAP/CPAP) through DLTs to intensive care unit patients with acute respiratory failure due to predominantly unilateral lung disease. 397 In all cases conventional two-lung therapy (mechanical ventilation, PEEP, CPAP) had been administered via a standard single-lumen tube and either failed to improve or actually decreased oxygenation. In these patients the single-lumen tube was replaced with a DLT. In most cases the amount of PEEP initially administered to each lung was inversely proportional to the compliance of each lung; ideally, this PEEP arrangement should result in equal FRC in each lung. In some cases, the amount of PEEP that each lung received was later adjusted and titrated in an effort to find a differential lung PEEP combination that resulted in the lowest right-to-left transpulmonary shunt. This treatment modality for severe unilateral lung disease is discussed more fully in Chapter 72.
Recommended Combined Conventional and Differential Lung Management of One-Lung Ventilation
Figure 4833 summarizes the recommended plan for obtaining satisfactory arterial oxygenation during one-lung anesthesia. Two-lung ventilation is maintained for as long as possible (usually until the pleura is opened). When one-lung ventilation is commenced, a tidal volume of 10 mL/kg is used, and the respiratory rate is adjusted so that PaCO2 equals 40 mm Hg. A high inspired oxygen concentration (FIO2 of 0.81.0) should be used, and SaO2 should be monitored continuously.
|FIGURE 4833 An overall one-lung ventilation plan. FIO2, inspired oxygen concentration; TV, tidal volume; RR, respiratory rate; PEEP, positive end-expiratory pressure; CPAP, continuous positive airway pressure; ASAP, as soon as possible.|
If hypoxemia is present after this initial conventional approach, two major causes of hypoxemia, namely malposition of the DLT and poor hemodynamic status, must be ruled out. Proper tube position should be confirmed via fiberoptic bronchoscopy. If the DLT is correctly positioned and the hemodynamic status is satisfactory, simple tidal volume and respiratory rate adjustments should be made. 302 For example, if the tidal ventilation is thought to be too high, it should be decreased, and if the tidal ventilation is thought to be too low, it should be increased. If these simple maneuvers do not quickly resolve the problem, the studies of selective nondependent lung CPAP 385, 388, 389, 390, 391, 392, 393, 394, 395 and differential lung PEEP 397 dictate that the next treatment should be to apply 5 to 10 cm H2 O of CPAP to the nondependent lung. Nondependent lung CPAP should be applied during the deflation phase of a large tidal volume breath to overcome critical opening pressures in the atelectatic lung. If oxygenation does not improve with nondependent lung CPAP (as it does in the large majority of cases), 5 to 10 cm H2 O of PEEP should then be applied to the ventilated dependent lung. If dependent lung PEEP does not improve oxygenation, nondependent lung CPAP should be increased to 10 to 15 cm H2 O while the dependent lung is maintained at 5 to 10 cm H2 O of PEEP. If arterial oxygenation is still not satisfactory, the nondependent lung CPAP level should be matched with an equal amount of dependent lung PEEP. In this way, a differential lung PEEP/CPAP search for the maximum compliance and a minimum right-to-left transpulmonary shunt are made in an attempt to find the optimal end-expiratory pressure for each lung and the patient as a whole.
If severe hypoxemia persists after the application of differential lung PEEP/CPAP (which would be extremely rare), it should be remembered that the nondependent lung may be intermittently ventilated with positive pressure with oxygen (see Fig. 4833). Finally, most of the V/Q imbalance is eliminated during a pneumonectomy by tightening a ligature around the nonventilated lung pulmonary artery as early as possible, thus directly eliminating all shunt flow through the nonventilated lung (see Fig. 4833). Indeed, clamping the pulmonary artery to a collapsed lung functionally resects the entire lung, and the PaO2 is restored to a level not significantly different from a two-lung ventilation or postpneumonectomy one-lung ventilation value.
Because nondependent lung CPAP has been shown to relieve hypoxemia consistently and reliably during one-lung ventilation, 388, 389 its routine use to prevent hypoxemia during thoracic surgery using DLTs should be considered. Low levels of CPAP used in this manner have not compromised surgical conditions and have occasionally improved surgical exposure by facilitating the identification of intralobar planes. 385, 388, 389, 395
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