Resistance of Isoflurane to Biotransformation in Man

Pulmonary and renal excretion of isoflurane and its metabolites was studied in nine surgical patients following administration of known quantitities of isoflurane. Uptake and pulmonary washout were predictable by a mathematical model for inert vapors. The agreement between predicted and experimental data supports the view that isoflurane is subject to little or no biotransformation. The average recovery in exhaled air was 95 per cent, SE 7 per cent. The postoperative increase of urinary excretion of fluoride and organic fluorine accounted for less than 0.2 per cent of fluorine administered as isoflurane. This small extent of biotransformation is probably biologically insignificant, but only after extensive clinical experience can the hazard of delayed toxic response be conclusively evaluated.