Critical Care Medicine |
|Chapter 72:||Respiratory Care|
Oxygen Delivery Systems
Outside the operating room setting, where gases are administered by rebreathing systems that include a CO2 absorber, oxygen therapy is provided by either partial rebreathing or nonrebreathing delivery systems that do not require a CO2 absorber. A partial rebreathing system allows for the initial portion of the expired gases containing little or no CO2 to be collected in a reservoir while the remaining expiratory gases are vented to the atmosphere. Most oxygen delivery systems are intended to function as nonrebreathing systems in which all exhaled CO2 is vented to the atmosphere and only fresh inspiratory gases are inhaled. Nonrebreathing systems are divided into high-flow (fixed performance) and low-flow (variable performance) systems.
A high-flow oxygen delivery system provides the total inspired atmosphere so that a premixed FIO2 is consistently and predictably delivered. To accomplish this goal, the inspiratory gas flows must be three to four times the measured minute ventilation. 4, 5, 6 So-called venturi devices (Fig. 722) are the most commonly used high-flow oxygen devices for nonintubated patients. The physical behavior of these air-entrainment devices is best explained by the principle of constant-pressure jet mixing, where viscous shearing forces distal to the orifice entrain room air in a specific ratio. 7 Variation in orifice or entrainment port size changes FIO2, whereas variation of the oxygen flow rate determines the total volume of gas provided by the device. FIO2 values of 0.24 to 0.40 are readily provided by air-entrainment mask systems, whereas higher FIO2 values are best provided by large-volume nebulizers with wide-bore tubing leading to a mask or endotracheal tube. The intubated patient should breathe close to 100 percent humidified 37°C gas, to avoid drying of the tracheobronchial mucosa. This is safely and economically accomplished by placing a reservoir tubing on the distal limb of the endotracheal T-connection so that inspiring room air is avoided.
|FIGURE 722 Principle of an air entrainment device. Pressurized oxygen is forced through a constricted orifice; the increased gas velocity distal to the orifice creates a shearing effect that causes room air to be entrained through the entrainment ports. The high flow of gas fills the mask; holes allow both exhaled and delivered gases to escape. A and B show that the size of the entrainment ports (EP) determines the amount of room air to be entrained. OS, oxygen source. Large ports (A) result in relatively low inspired oxygen fraction (FIO2); small ports (B) result in relatively higher FIO2. For any size EP, the FIO2 is stable; however, the total gas flow varies with the pressurized oxygen flow. (From Shapiro et al1 )|
A low-flow oxygen delivery system requires that the patient inspire some room air to meet inspiratory demands. These systems are popular because of their simplicity, patient comfort, and economics. The use of a low-flow oxygen delivery system does not imply delivery of low oxygen concentrations. As shown in Figure 723, the FIO2 is determined by the size of the oxygen reservoir, the oxygen flow rate, and the breathing pattern. For example, a nasal cannula at an oxygen flow rate greater than 6 L/min accomplishes minor increases in FIO2 because the nasopharyngeal reservoir is filled with 100 percent oxygen at a 6 L/min flow rate. An oxygen reservoir must be increased (placing a mask over the nose and mouth) to achieve an FIO2 greater than 0.40. Assuming a normal breathing pattern, Table 72–1 lists the approximate FIO2 at various flow rates for the common low-flow oxygen delivery systems. With abnormal ventilatory patterns (Table 72–2), the larger the tidal volume (V), or the faster the respiratory rate, the lower the FIO2.
TABLE 72–1. Guidelines for Estimating Inspired Oxygen Concentrations With Low-Flow Oxygen Devices
TABLE 72–2. Variability in Inspired Oxygen Concentrations With Low-Flow Oxygen-Delivery Systems and Variable Patterns of Ventilation
|FIGURE 723 Reservoirs in low-flow oxygen therapy. The anatomic reservoir consists of the nose, the nasopharynx, and the oropharynx. This reservoir is estimated to be approximately one-third of the anatomic dead space. The appliance reservoir consists of (1) the mask: 100- to 200-mL volume, depending on the appliance; and (2) the reservoir bag: 600 to 1,000 mL of added volume. (From Shapiro et al1 )|
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