Question
I have been looking for papers regarding turning off isoflurane and starting Desflurane at the end of a case regarding accelerating the wakeups. To me it makes sense, but someone told me this did not matter clinically. Do you know of such a paper describing this? Thank You,
Answer
Research articles on low solubility inhaled anesthetics are often written by investigators funded by Baxter (Desflurane) or Abbott (Sevoflurane) so as usual when reading the literature see who funded the study and how that might affect the findings. The classic paper on the switching (or crossover) technique you describe is attached.
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The lessons learned are that if one is to substitute desflurane for isoflurane at the end of a case one needs to keep in mind that partial rebreathing through a semiclosed circuit limits elimination of isoflurane more than you may think during the crossover or switching period. A higher fresh gas flow than expected during this crossover period is needed to speed the isoflurane elimination. Alternatively, start the crossover earlier than you might think you need if flows are low to ensure that isoflurane is gone by the end of the case.
Having used isoflurane exclusively for the first part of my career (I like sounding like a veteran of the field) I know I woke patients up just as fast as I do now. This was done by starting to reduce the inspired isoflurane concentration as the end of a case approached. Desflurane and sevovoflurane do provide the practitioner more margin for error, as turning the desflurane or sevoflurane vaporizer off at the very last minute of closure often yields a reasonably fast wakeup. Please keep in mind that studies that show a faster wakeup with desflurane or sevoflurane versus isoflurane use a protocol where the gas is left at the 1 MAC level, for example, until skin closure is finished, and only then is the vaporizer turned off. It is no surprise then that wakeup is faster the lower the blood gas solubility is. This protocol maynot reflect the actual practice by many anesthetists of titrating the inhaled anesthetic concentration down as case is coming to an end.