CONCENTRATIONS OF DESFLURANE AND PROPOFOL THAT SUPPRESS RESPONSE TO COMMAND IN HUMANS

The anesthetic concentration just suppressing appropriate response to command (minimum alveolar anesthetic concentration awake [MAC-awake] for volatile anesthetics or plasma concentration to prevent a response in 50% of patients [Cp50]-awake for intravenous anesthetics) provides three important measures. First, along with pharmacokinetics, the ratio of the awakening concentration to the anesthetizing concentration (MAC-awake/MAC or Cp50-awake/Cp50) determines time to awakening. Second, a correlation between MAC-awake and the anesthetic concentration sufficient to prevent learning suggests MAC-awake provides a surrogate measure of amnestic potency. Third, population values for MAC-awake provide evidence for or against commonality in anesthetic mechanisms. We studied 22 male volunteers twice to determine both MAC-awake for desflurane (2.60% +/- 0.46%) and Cp50-awake for propofol (2.69 +/- 0.56 mu g/mL). Awakening with desflurane occurs at a concentration closer to its anesthetizing concentration (36% of MAC) than propofol (18% of Cp50); that is, 1) desflurane requires less of a decrement in anesthetic concentration at the effect site for arousal; and 2) if MAC-awake (Cp50-awake) values reflect the concentrations providing amnesia, propofol is a more potent amnestic. Of interest, the dose response curves of desflurane and propofol were equivalently steep, a finding consistent with a common mechanism of action. In contrast, sensitivity of each volunteer to desflurane did not correlate with sensitivity to propofol (r(2) < 0.01, P = 0.98) arguing against a common mechanism.