Differences between Induction and Recovery

Recovery differs from induction in three crucial ways. First, on induction, the effect of solubility to hinder the rise in alveolar anesthetic concentration can be overcome by increasing the inspired anesthetic concentration (i.e., by applying overpressure). No such luxury is available during recovery; the inspired concentration cannot be reduced below zero. Second, on induction, all tissues initially have the same anesthetic partial pressure: zero. On recovery, the tissue partial pressures vary. The VRG has a pressure that usually equals that required for anesthesia; the VRG has come to equilibrium with the alveolar anesthetic partial pressure. The MG may or may not have the same partial pressure as that found in the alveoli. A longer anesthesia duration (2 to 4 hours) may permit equilibrium, but a shorter case would not. The high capacity of fat for all anesthetics except nitrous oxide precludes equilibration of the FG with the alveolar anesthetic partial pressure with hours or even days of anesthesia.

If the muscle and fat have not equilibrated with the alveolar anesthetic partial pressure, these tissues initially cannot contribute to the transfer of anesthetic back to the lungs. As long as an anesthetic partial pressure gradient exists between arterial blood and tissue, the tissue will continue to take up anesthetic. For the first several hours of recovery from halothane anesthesia, fat continues to take up halothane, and by so doing, it accelerates the rate of recovery. Only after the alveolar (which equals arterial) anesthetic partial pressure falls below that in a tissue can the tissue contribute anesthetic to the alveoli.

The failure of several tissues to reach equilibration with the alveolar anesthetic partial pressure means that the rate of decrease of alveolar anesthetic on recovery is more rapid than its rate of increase on induction and that recovery depends in part on the duration of anesthesia^{[85] [86]} (see Fig. 5-18 ). A longer anesthesia duration puts more anesthetic into the slowly filling muscle and fat depots. These reservoirs can supply more anesthetic to the blood returning to the lungs when they are filled

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Figure 5-18 Both solubility and duration of anesthesia affect the fall of the alveolar concentration (FA) from its value immediately preceding the cessation of anesthetic administration (FA₀ ). A longer anesthetic slows the fall, as does a greater solubility, and the effects are shown for the increasingly soluble desflurane, sevoflurane, and isoflurane (A to C). The horizontal lines designated (a), (b), and (c) indicate 80%, 90%, and 95% decreases, respectively, in the alveolar concentration from the concentration at the end of anesthesia. (Data from Yasuda N, Lockhart SH, Eger EI II, et al: Kinetics of desflurane, isoflurane, and halothane in humans. Anesthesiology 74:489–498, 1991, and from Yasuda N, Lockhart SH, Eger EI II, et al: Comparison of kinetics of sevoflurane and isoflurane in humans. Anesth Analg 72:316–324, 1991.)

Solubility influences the effect of duration of anesthesia on the rate at which the alveolar anesthetic partial pressure declines.^[86] The decline of the partial pressure of a poorly soluble agent such as desflurane is rapid in any case, and the acceleration imparted by a less than complete tissue equilibration minimally alters the rate of recovery. The approach to equilibration becomes more important with sevoflurane and even more important with isoflurane (see Fig. 5-18 ). Recovery may be rapid after a short isoflurane anesthetic but may be slow after a prolonged anesthetic. The recovery from anesthesia with desflurane and sevoflurane is quicker than with more soluble agents, such as isoflurane and halothane.^[84]

The importance of solubility and of duration of anesthesia to the rate of recovery may be appreciated by the use of context-dependent times to reach particular levels of washout.^[87] Regardless of the duration of anesthesia, the alveolar concentrations of poorly soluble anesthetics (e.g., nitrous oxide, desflurane, sevoflurane) and moderately soluble anesthetics (e.g., isoflurane, halothane) decrease by 50% in roughly the same period. If recovery were reached at a 50% decrease, the choice of anesthetic would matter little to the time to recovery from anesthesia. The impact of anesthetic solubility and duration becomes evident if greater levels of washout are required to achieve recovery. If 80% washout is required, increasing duration of anesthesia markedly affects recovery from isoflurane but little affects recovery from desflurane and sevoflurane; notice the differences in concentration intersecting the horizontal line at F_A /F_A0 of 0.2, as in line (a) in Figure 5-18A, B, and C . If 90% washout is required, as illustrated by F_A /F_A0 of 0.1 in line (b), or 95% washout is required, as illustrated by F_A /F_A0 of 0.05 in line (c), increasing duration of anesthesia affects the more soluble anesthetics much more than less soluble anesthetics. After 2, 4, or 8 hours of anesthesia with 1.25 MAC of sevoflurane compared with desflurane, initial ( Fig. 5-19 ) and later ( Fig. 5-20 ) recovery is nearly twice as rapid with desflurane, and the difference between the recovery times appears to increase with increasing duration of anesthesia.^[32]

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Figure 5-19 Different times to awakening occur after 2, 4, or 8 hours of anesthesia at a 1.25 minimum alveolar concentration (MAC) for desflurane compared with sevoflurane. Awakening to response to command or to orientation is almost twice as rapid after anesthesia with the less soluble desflurane. (Adapted from Eger EI II, Gong D, Koblin DD, et al: Effect of anesthetic duration on kinetic and recovery characteristics of desflurane vs. sevoflurane (plus compound A) in volunteers. Anesth Analg 86:414–421, 1998.)

The third factor affecting recovery is pharmacodynamic rather than kinetic. Anesthesia requires the provision of immobility. If this is accomplished with an inhaled anesthetic, this means that the anesthetist seeks a concentration exceeding MAC. However, recovery sets a different goal: awakening. One measure of this is the return of the capacity to respond to command. MACawake, the concentration of anesthetic allowing appropriate

Figure 5-20 The differential suggested by Figure 5-19 extends to measures of more subtle degrees of awakening, such as the digit symbol substitution test (DSST), a measure of judgment and cognition. Notice that there appears to be a greater spread between the results after 2 versus 8 hours of anesthesia. MAC is the minimum alveolar concentration. (Adapted from Eger EI II, Gong D, Koblin DD, et al: Effect of anesthetic duration on kinetic and recovery characteristics of desflurane vs. sevoflurane (plus compound A) in volunteers. Anesth Analg 86:414–421, 1998.)

Figure 5-21 As a fraction of the minimum alveolar concentration (MAC), MACawake is greatest for nitrous oxide^[89] ; less for desflurane,^[92] isoflurane,^[89] and sevoflurane^{[90] [91]} ; and least for propofol.^[92]