IN RATS BREATHING FROM A NONREBREATHING SYSTEM, SUBSTITUTION OF DESFLURANE FOR ISOFLURANE TOWARD THE END OF ANESTHESIA INCOMPLETELY RESTORES THE TIME OF RECOVERY TOWARD THAT OF DESFLURANE

The lower solubility of desflurane allows a more rapid emergence from anesthesia than after anesthesia with the more soluble but less expens ive anesthetic, isoflurane. Some practitioners use isoflurane for main tenance of anesthesia, crossing over to desflurane later in maintenanc e in an attempt to combine the cost-effectiveness of isoflurane with t he rapid emergence from desflurane. We hypothesized that this maneuver would not accomplish its goals. Twenty-four male Sprague-Dawley rats received 1.2 minimum alveolar anesthetic concentration (MAC) of desflu rane for the final 15, 30, or 60 min of a 2-h, 1.2-MAC isoflurane anes thetic in a nonrebreathing anesthesia system. We measured the time fro m cessation of anesthetic administration to the time each rat righted himself twice. Immediately after righting for the second time, we test ed each rat's ability to remain atop a rotating rod (Rota-Rod) for 60 s continuously. Early (righting reflex) and late (Rota-Rod) recovery o ccurred more rapidly (P < 0.001) after 120 min of anesthesia with desf lurane alone than after 120 min of anesthesia with isoflurane alone. A cross-over period of 30 min or longer produced a righting reflex time that did not differ from that found with desflurane alone, but a 15-m in cross-over did not. Progressively longer cross-over periods led to proportionally better Rota-Rod performance, but no crossover duration produced the rapidity of recovery seen with desflurane alone. We concl uded that in a nonrebreathing system, switching to desflurane during t he last 30 min of anesthesia substantially improved early recovery but produced a much smaller improvement in later recovery. Implications: The newer inhaled anesthetics offer the advantage of lower solubility, and thus more rapid emergence from anesthesia, than do the older inha led anesthetics. However, they can be more expensive to use. This stud y demonstrates that substituting the newer anesthetic, desflurane, tow ard the end of anesthesia for an older anesthetic of greater solubilit y, isoflurane, does not produce recovery comparable to that of desflur ane alone. Furthermore, this technique can be more costly than using d esflurane throughout anesthesia.