END-TIDAL CONCENTRATIONS OF HALOTHANE AND ISOFLURANE DURING INDUCTION OF ANAESTHESIA IN YOUNG AND ELDERLY PATIENTS

Twenty-two young (18–32 yr) and 22 healthy elderly (60–80 yr) patients received either halothane or isoflurane for maintenance of anaesthesia during controlled ventilation. Endtidal fractional concentrations ( F E) of the agents were measured for 20 min after their introduction into inspired gas and the increase in end-tidal concentrations of the two agents was compared during induction of anaesthesia using the ratios of F E to the inspired fraction ( F 1). F E: F 1 ratios for isoflurane were higher than those for halothane in both young and elderly patients, confirming that equilibration of end-tidal with inspired concentration occurs more rapidly with isoflurane than with halothane in both age groups. F E: F 1 ratios for isoflurane became significantly lower in the elderly than in the young after 15 min administration of isoflurane. This suggests slower induction of anaesthesia in the elderly if equipotent concentrations of isoflurane are used; the clinical significance of this difference is probably small. Mean F E: F 1 ratios for halothane in elderly patients were similar to those in the young throughout induction of anaesthesia.