ARTERIAL WASHIN OF HALOTHANE AND ISOFLURANE IN YOUNG AND ELDERLY ADULT PATIENTS

We have studied the effect of age on washin of isoflurane and halothane by comparing end-tidal (PE') and arterial (Pa) partial pressures of the agents in young (18-32 yr) and elderly (63-82 yr) healthy patients for 20 min after introduction of the agents, before surgery. PE' was measured by infra-red analysis and Pa by gas chromatography. Washin of isoflurane occurred at the same rate in the young and elderly, with no significant difference between young and elderly in PE' or Pa as proportions of the inspired partial pressure (Pl). After 20 min of isoflurane administration, mean Pa/Pl in the young was 0.57 (95% confidence limit (CL) 0.53-0.62) and 0.55 in the elderly (95% CL 0.51-0.59). Washin of halothane was slower in the elderly than in the young, with Pa/Pl significantly less in the elderly from 10 min after introduction of halothane. The difference between age groups, however, was small: mean Pa/Pl after 20 min of halothane administration 0.45 (95% CL 0.41-0.49) in the young and 0.38 (95% CL 0.35-0.41) in the elderly. Washin of isoflurane was significantly faster than that of halothane in both young and elderly subjects. For isoflurane, the PE'-Pa gradient was small relative to Pa and did not differ significantly between young and elderly. For halothane, PE'-Pa in the young did not differ significantly from that for isoflurane. In the elderly, PE'-Pa for halothane was significantly greater than in the young and than PE'-Pa for isoflurane.