Low-Flow Anesthetic Delivery

Low-flow anesthetic delivery produces more stability than closed-circuit delivery. Low-flow delivery can provide most of the advantages of closed systems, decreasing instability, while retaining considerable advantages over open systems in economy, maintenance of humidification and temperature, and atmospheric pollution. Relative to closed-circuit delivery, low-flow delivery produces a constancy of oxygen and anesthetic levels and provides greater elimination of carbon monoxide and toxic anesthetic breakdown products.

In a low-flow delivery system, two factors control the relationship between the concentration delivered from a vaporizer (FD) and that in the alveoli (FA), a relationship that may best be described by the ratio (FD/FA) of the two variables. First, we have already seen that uptake governs this ratio in a closed system (see Fig. 5-17A ). FD/FA is higher for more soluble anesthetics, and regardless of solubility, the ratio is highest early in anesthetic administration and decreases rapidly in the first 5 to 10 minutes of anesthesia (as uptake by the VRG of tissues decreases to the point of near-equilibration) and more slowly there-after (as uptake by MG, FG, and the fourth compartment decreases).

Second, inflow rate also governs FD/FA. The relationship is inverse: The higher the inflow rate, the lower is the ratio (compare Fig. 5-17B to D with Fig. 5-17A and notice the large scaling differences associated with increasing inflow rate). An increase in inflow rate decreases FD/FA by decreasing rebreathing. Rebreathing is important because uptake of anesthetic depletes the anesthetic concentration in rebreathed gases, and the concentration in FD must be sufficient to compensate for this depletion. The higher the inflow rate, the less compensation is required because rebreathing is reduced.

However, increases in inflow rate do not proportionally decrease FD/FA (see Fig. 5-17A to D ). The greatest reduction in FD/FA comes with only modest increases in inflow rate. A large decrease occurs when the inflow rate is changed from that needed for a closed circuit to an inflow rate of 1 L/min, whereas only a small decrease occurs when the inflow rate is increased from 2 to 4 L/min. After the inflow rate exceeds minute ventilation (i.e., a non-rebreathing system exists), further increases in inflow rate have no effect on FD/FA, and FD/FA is the same as the ratio of FI to FD (i.e., FD/FA = FI/FA).

I use the term anesthetic tether as a metaphor for the FD/FA ratio.^[82] A large ratio equates to a long tether, one permitting considerable freedom or variability in the alveolar concentration. With a long tether, changes in uptake caused by changes in physiologic variables (e.g., an increase in cardiac output consequent to surgical stimulation) can appreciably alter the alveolar concentration and the level of anesthesia. In the example just cited, there is positive feedback because, by increasing uptake, surgical stimulation decreases the alveolar concentration and thereby increases the perception of that stimulation. Most anesthetists prefer a short anesthetic tether because a short tether provides a tighter control over the level of anesthesia. The use of less soluble anesthetics and higher inflow rates shortens the tether.

Another benefit to a short tether accrues to the anesthetist who does not use an agent-specific analyzer. In the absence of such an analyzer, some anesthetists rely on the dial setting of the vaporizer to indicate the concentration of anesthetic in the patient's lungs (i.e., assume that FD equals FA). The vaporizer setting may correlate poorly with the concentration in the lungs under three circumstances: early in anesthesia for all agents, later in anesthesia for closed circuits or very low inflow rates, and later in anesthesia for more soluble agents, such as isoflurane, even at higher inflow rates. The vaporizer setting may correlate well with poorly soluble anesthetics such as sevoflurane and desflurane 30 to 60 minutes after the inception of anesthetic administration. At this time, the delivered concentration from the vaporizer may be less than 20% greater than that in the alveoli (i.e., the FD/FA ratio is 1.2), even at inflow rates of 1 to 2 L/min (see Fig. 5-17B and C ).

Economic concerns increasingly dictate the practice of anesthesia, and anesthesiologists may wish to appreciate the differences in anesthetic consumption as a function of the choice of inflow rate, the duration of anesthesia, and the choice of anesthetic. The reports by Yasuda and associates^{[11] [12]} provide constants that can be used to estimate the uptake of commonly available potent inhaled anesthetics. By using the gas laws and published values for specific gravities, the values for uptake of vapor may be converted to milliliters of liquid taken up. Combining this information with a knowledge of the function of circuit rebreathing systems^[74] allows an estimate of the amounts of liquid in milliliters that must be delivered at various inflow rates to provide a constant alveolar concentration equal to the MAC^[83] ( Table 5-4 ). The relative costs of anesthesia may be estimated by applying the price of the anesthetic of interest to the number of milliliters needed to sustain anesthesia.

If economy and a low FD/FA ratio are desirable, the aforementioned considerations suggest that a good compromise is the use of a low-flow delivery system after an initial period of higher flows. Higher flows (4 to 6 L/min) may be applied early in anesthesia (i.e., at the times of highest uptake) and then decreased progressively as uptake decreases. Flows of 2 to 4 L/min might be given for the period from 5 to 15 minutes after inducing anesthesia, and flows of 1 L/min may be given thereafter. If the average inflow rate were 1 L/min, 1 hour of anesthesia with the four potent anesthetics listed in Table 5-4 would require administration of 6.5 (halothane) to 26.1 (desflurane) mL of liquid. This fourfold range of values is smaller than the eightfold range of potency (MAC) values because the amount of anesthetic delivered must account for more than potency. The amount delivered also must compensate for uptake and losses of anesthetic through the overflow valve. The relatively smaller uptake and

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