Biotransformation of Isoflurane

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Abstract
The serum and urinary concentrations of fluorinated metabolites of isoflurane after inhalation of three different concentrations of isoflurane were studied in 18 ASA physical status 1 or 2 patients, scheduled for orthopedic or otolaryngeal surgery. Isoflurane was administered for 60 min during fentanyl-nitrous oxide-oxygen, and its end-tidal concentration was maintained at 0.3, 0.6, or 1.15% (groups I, II, and III). The organic fluorine was determined by combustion and fluoride ions were analyzed by ion chromatography. The amounts were expressed in terms of fluoride ion. The concentrations of serum fluoride ion and organic fluorine increased significantly 15 min after the onset of inhalation of isoflurane. The mean peak values of fluoride ions were 3.8 .+-. 1.1, 3.9 .+-. 1.4, and 4.2 .+-. 0.9 .mu.mole/1 (M .+-. SD) in patients in groups I, II, and III, respectively. The half-lives of fluoride ion and of organic fluorine as metabolites of isoflurane, calculated from the amounts excreted in urine, were 36 h and 41 h, respectively. The cumulative amounts of fluoride ion and organic fluorine excreted up to the 6th postoperative day were 548 .+-. 230 and 785 .+-. 452 .mu.moles in group I, 594 .+-. 138 and 1,378 .mu.moles in group II, and 1,032 .+-. 496 and 728 .+-. 265 .mu.moles (M .+-. SD) in group III, respectively. The urinary excreted fluoride ion increased in proportion to the dose of isoflurane and approximately 1.3 mmol was excreted per 1 MAC .times. hours inhalation of isoflurane. The authors concluded that isoflurane might be biotransformed to a greater extent than reported previously, although the serum fluoride ion level was found to be low.