Implications of chemical and physical properties of desflurane for longer surgery

Increased duration of anaesthetic administration has implications for recovery from anaesthesia, has cardiovascular effects, and potential for toxicity through metabolism and breakdown of the anaesthetics. Recovery of function after desflurane or sevoflurane anaesthesia, because of the low blood gas and tissue solubilities of these agents, is more rapid than after halothane, isoflurane or enflurane, with recovery being most rapid after desflurane. Increased duration of anaesthesia amplifies the differences in rate of recovery because of the additional anaesthetic (greater with more soluble agents) dissolved in tissues. Increased duration of anaesthesia lessens the cardiovascular depression associated with most halogenated inhaled anaesthetics including desflurane, but not isoflurane. Increased duration of anaesthesia allows for greater metabolism of anaesthetics and greater exposure to metabolites and potentially toxic breakdown products. Desflurane is the least metabolised of the available anaesthetics and is stable in soda lime and Baralyme. Thus, it has exceedingly low potential for toxicity. Sevoflurane undergoes considerable metabolism, producing free fluoride ion, with plasma concentrations proportional to dose and duration of anaesthesia exceeding 50 microM in approximately 7% of patients. In rats, the effects of a toxic breakdown product of sevoflurane, CF2 = C(CF3)OCH2F (compound A), are also dose- and duration-dependent, with lower concentrations producing toxic effects as duration of exposure increases. The clinical importance of the metabolism and in vitro breakdown of sevoflurane has still to be adequately tested.