Anesthesia Management |
|Chapter 30:||Cardiovascular Monitoring|
Transducer Setup: Zeroing, Calibrating, and Leveling
Prior to initiating patient monitoring, the pressure transducer must be zeroed, calibrated, and leveled to the appropriate position on the patient. The initial step in this process is to expose the transducer to atmospheric pressure by opening the adjacent stopcock to air, pressing the zero pressure button on the monitor, and thus establishing the zero pressure reference value. The transducer now has a referenceambient atmospheric pressureagainst which all intravascular pressures are measured. This process underscores the fact that all pressures displayed on the monitor are referenced to atmospheric pressure, outside the body. Although clinicians generally refer to zeroing the transducer, actually the transducer is exposed to atmospheric pressure via an open stopcock affixed to the transducer. To be precise, it is this airfluid interface at the level of the stopcock that is the zero pressure locus. This point must be aligned with a specific position on the patient to ensure the correct transducer level.
When a significant change in pressure occurs, the zero reference value should be rechecked prior to initiating therapy. 55 This can be accomplished quickly, by opening the stopcock and exposing the transducer to atmospheric pressure. The monitor should be inspected to ensure that the pressure trace overlies the zero pressure line on the display screen and the digital pressure value equals zero. Note that checking the zero value is different from establishing the zero reference, which is done at the beginning of the monitoring procedure. 138 If the stopcock is exposed to atmospheric pressure and pressure is not equal to zero, baseline drift of the transducers electrical circuit may have occurred. This transducer drift is caused by problems with membrane dome coupling to the electronic pressure transducing elements, as well as other technical problems with the transducer, the attached electrical cable, or the monitor itself. 55, 139, 140 These technical problems appear to be uncommon now that high-quality disposable transducers are widely available. 141
Historically, transducer calibration was the next step following the zeroing procedure. Calibration is an adjustment of system gain to ensure the proper response to a known reference pressure value. Traditionally, this has been performed using a mercury manometer as the standard. 55 Currently, however, disposable pressure transducers meet or exceed accuracy standards established by the AAMI and the American National Standards Institute. 141 Transducer calibration at the bedside thus appears to be unnecessary. By avoiding daily calibration, attendant serious risks of arterial air embolism and infection may be reduced. 141 In general, if a pressure transducer or monitoring cable is faulty, the initial zero value cannot be established, and the monitoring system must be changed. Rarely, despite successful zeroing, the recorded pressure values appear erroneous, and a malfunctioning pressure transducer, cable, or monitor must be suspected and replaced. 139, 140, 142
The final step in transducer setup is leveling the pressure monitoring zero point to the appropriate position on the patient. In general, zeroing and leveling the transducer are accomplished at the same time, prior to initiating patient monitoring. However, these are two distinct procedures. Zeroing exposes the transducer to ambient, atmospheric pressure via an open stopcock. Leveling assigns this zero reference point to a specific position on the patients body.
In the supine patient, pressure transducers are leveled most often to the midchest position in the midaxillary line, 55 a site chosen because it is easy to estimate by eye and provides a reasonable approximation for the midpoint of the heart in the chest. Although precise location for the zero reference level is important for all pressure monitoring, it is critical for measurement of cardiac filling pressures. An error in arterial blood pressure measurement of 10 mm Hg is generally of minor clinical importance, but the same error in central venous pressure (CVP) or pulmonary artery wedge pressure (PAWP) may have major diagnostic implications.
Some investigators have questioned the use of the midchest position as an appropriate zero reference level. In addition, they suggest that experienced clinicians and researchers do not understand how hydrostatic pressure influences the measurement of pressure with fluid-filled catheter-transducer systems. 143 The only factor that contributes to measured hydrostatic pressure with a fluid-filled catheter-transducer system is the level of the transducer relative to the uppermost fluid level in the chamber in which pressure is being measured.143 It follows that to remove all such hydrostatic pressure influences, transducers should be leveled to the top of the fluid column in the chamber or vessel being measured. For example, errors in measurement of left ventricular filling pressure up to 7 mm Hg occur when transducers are leveled to the midchest rather than to the top of the left ventricle. 143 Consequently, transducers used to measure pressures in the cardiac chambers or pulmonary artery are best positioned 5 cm below the left sternal border at the fourth intercostal space, to obviate the confounding influence of hydrostatic pressure. Because many critically ill or anesthetized patients have direct arterial pressure meas-ured along with other direct intravascular cardiac filling pressures, it seems prudent to use the same zero reference level for all direct pressure measurements.
In some circumstances, the clinician may choose to level the arterial pressure transducer at a different position on the body. During neurosurgical operations, performed with the patient in a seated position, the pressure transducer is often aligned with the patients ear to approximate the level of the circle of Willis and better estimate cerebral perfusion pressure. The transducer should not need to be rezeroed by pushing the zero pressure button on the monitor, in that only the reference level has been altered. In fact, if zero is rechecked, it should be unchanged, regardless of transducer location, because atmospheric pressure changes little over the few inches of height alteration being considered in this situation. Arterial blood pressure now recorded at the level of the head will be lower than that recorded at the heart, the difference being precisely equal to the hydrostatic pressure difference between the head and the heart (Fig. 3017). 138
|FIGURE 3017 Effect of pressure transducer level on measurement of blood pressure. (A) In the supine patient, the same arterial blood pressure (ART) is measured at the level of the heart or the brain. (B) With the patient in the sitting position, blood pressure recorded from a transducer that remains at heart level (#1) will be unchanged, but blood pressure recorded from a transducer adjusted to the level of the brain (#2) will be lower by an amount equal to the hydrostatic pressure difference between these two transducer positions (20 cm H2O, or approximately 15 mm Hg). See text for greater detail. (From Mark JB: Atlas of Cardiovascular Monitoring. New York, Churchill Livingstone, 1998: Fig. 919.)|
Often, pressure transducers are attached to an intravenous (IV) pole, and patient position is altered by adjusting the height of the operating room table or intensive care bed. A sudden change in blood pressure is noted and is attributable entirely to the change in transducer level in relation to patient position. Raising the patient above the transducer will produce spuriously high pressures, whereas lowering the patient below the transducer will produce spuriously low pressures. Again, the error introduced is exactly equal to the hydrostatic pressure difference between patient and transducer. Although these leveling artifacts are generally small relative to arterial blood pressure, they are of critical importance when measuring CVP or pulmonary artery pressure (PAP). However, on occasion, a pressure transducer will fall from its normal position and come to rest near the floor. In this instance, the blood pressure may be falsely elevated by nearly 80 mm Hg. Taping pressure transducers directly to the patient can obviate these types of level artifacts. Without question, the most common major mistakes in pressure monitoring involve failure to establish zero, failure to recheck the zero value for transducer drift, and failure to relevel the transducer appropriately when changes in patient position occur. 55
A final example may help to understand the distinction between zeroing and leveling pressure transducers. Consider the difference between invasive blood pressure and NIBP measurement in a patient in the lateral position (Fig. 3018). While the patient is supine, blood pressure is 120/80 mm Hg in both arms, as measured by noninvasive cuffs and direct indwelling radial artery catheters. The patient is now placed in the right lateral decubitus position, the left arm is 20 cm above the heart, and the right arm is 20 cm below the heart. The invasive blood pressure transducers remain leveled to the heart at the midthoracic position. Blood pressure measured in the left arm by cuff will be lower, that is, 105/65 mm Hg, because the left arm is 20 cm above the level of the heart, whereas blood pressure measured in the right arm by cuff will be higher, 135/95 mm Hg. However, blood pressure measured from either the left or right radial catheters will remain unchanged, 120/80 mm Hg, given that the reference level for zero pressure remains fixed at the midchest position. Indeed, if the arterial pressure transducers were attached to the arms, the zero reference levels for the transducers would have changed as the patient assumed the lateral position, and pressures recorded from right and left radial artery catheters would equal those recorded by the ipsilateral noninvasive cuffs.
|FIGURE 3018 Effect of patient position on the relation between direct arterial blood pressure (ART) and indirect noninvasive blood pressure (NIBP) measurements. (A) In the supine patient, pressures measured from the right (R) or left (L) arms by either technique will be the same. (B) In the right lateral decubitus position, ART pressures recorded directly from the right and left radial arteries will remain unchanged so long as the respective pressure transducers remain at heart level. However, NIBP will be higher in the dependent right arm and lower in the nondependent left arm. Differences in NIBP are determined by the positions of the arms above and below the level of the heart and are equal to the hydrostatic pressure differences between the level of the heart and the respective arm. A 20-cm difference in height produces a 15-mm Hg difference in pressure. See text for greater detail. (From Mark JB: Atlas of Cardiovascular Monitoring. New York, Churchill Livingstone, 1998: Fig. 922.)|
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