CHANGING FROM ISOFLURANE TO DESFLURANE TOWARD THE END OF ANESTHESIA DOES NOT ACCELERATE RECOVERY IN HUMANS

Background: In an attempt to combine the advantage of the lower solubi lities of new inhaled anesthetics with the lesser cost of older anesth etics, some clinicians substitute the former for the latter toward the end of anesthesia. The authors tried to determine whether substitutin g desflurane for Isoflurane in the last 30 min of a 120-min anesthetic would accelerate recovery. Methods: Five volunteers were anesthetized three times for 2 h using a fresh gas inflow of 2 l/min: 1.25 minimum alveolar concentration (MAC) desflurane, 1.25 MAC isoflurane, and 1.2 5 MAC isoflurane for 30 min followed by 30 min of desflurane concentra tions sufficient to achieve a total of 1.25 MAC equivalent (''crossove r''). Recovery from anesthesia was assessed by the time to respond to commands, by orientation, and by tests of cognitive function. Results: Compared with isoflurane, the crossover technique did not accelerate early or late recovery (P > 0.05). Recovery from isoflurane or the cro ssover anesthetic was significantly longer than after desflurane (P < 0.05). Times to response to commands for isoflurane, the crossover ane sthetic, and desflurane were 23 +/- 5 min (mean +/- SD), 21 +/- 5 min, and 11 +/- 1 min, respectively, and to orientation the times were 27 +/- 7 min, 25 +/- 5 min, and 13 +/- 2 min, respectively. Cognitive tes t performance returned to reference values 15-30 min sooner after desf lurane than after isoflurane or the crossover anesthetic. Isoflurane c ognitive test performance did not differ from that with the crossover anesthetic at any time. Conclusions: Substituting desflurane for isofl urane during the latter part of anesthesia does not improve recovery, in part because partial rebreathing through a semiclosed circuit limit s elimination of isoflurane during the crossover period. Although high er fresh gas flow during the crossover period would speed isoflurane e limination, the amount of desflurane used and, therefore, the cost wou ld increase.