Impact of Metabolism

Saturation of the enzymes responsible for the metabolism of anesthetics may limit the ability of metabolism to significantly alter the rate at which the alveolar anesthetic partial pressure rises. This limitation does not exist on recovery, and metabolism may be an important determinant of the rate at which the alveolar anesthetic partial pressure declines. The importance of metabolism to recovery is implied by results from Munson and coworkers,^[51] who showed that the alveolar washout of halothane is more rapid than that of the less soluble enflurane, a result later confirmed by Carpenter and associates.^{[26] [28]} This agrees with the relative ease with which these two agents are metabolized: 15% to 20% of the halothane taken up during the course of an ordinary anesthetic can be recovered as urinary metabolites,^[94] whereas only 2% to 3% of enflurane can similarly be recovered.^[95] Halothane has two major routes of elimination: ventilation and hepatic metabolism. Enflurane elimination depends primarily on ventilation, which explains why Munson and colleagues^[51] found a more rapid fall in alveolar halothane. These results also apply to desflurane, isoflurane, and sevoflurane. Metabolism of desflurane and isoflurane is too small to accelerate their elimination, but metabolism of sevoflurane is sufficient to narrow the difference between its washout and that of desflurane.^[32]