Does the duration of anesthetic administration affect the pharmacokinetics or metabolism of inhaled anesthetics in humans?

To define the effect of anesthetic duration on the pharmacokinetics of inhaled anesthetics, we determined the pharmacokinetics of isoflurane, enflurane, halothane, and methoxyflurane given simultaneously to seven healthy subjects for exactly 30 min and compared the results with data from a previous study in which these four anesthetics were administered for 120 min. End-tidal and mixed-expired anesthetic concentrations were measured during washin of anesthetic and for 3-9 days of washout. Multiexponential (multicompartment) models were fit by least squares to the alveolar washin and washout curves. We estimated the percentage of anesthetic that was metabolized from total uptake and recovery of anesthetic. Alveolar washout was more rapid after the shorter period of anesthetic administration. However, duration of administration did not affect the time constants determined, the number of compartments identified (i.e., five compartments were identified in both studies), or the percentages of anesthetic metabolized.