Infusion of propofol to identify smallest effective doses for induction of anaesthesia in young and elderly patients.

We studied 110 patients older than 60 yr or aged 18-50 yr as separate groups in two stages to identify the smallest effective doses of propofol for induction of anaesthesia. In the elderly patients, in stage 1, at infusion rates of 25, 50 or 100 mg min-1 the mean (SD) doses administered were propofol 0.82 (0.14) mg kg-1, 1.22 (0.24) mg kg-1 and 1.65 (0.60) mg kg-1 and the induction times 140.1 (21.9) s, 103.2 (23.5) s and 69.4 (10.0) s, respectively. In stage 2, after induction with a fixed dose of 0.82 mg kg-1 as a bolus over 5 s or as an infusion at 25 mg min-1, the times for induction were 38.5 (14.0) s and 144.5 (36.6) s, respectively. In the young patients, at induction rates of 33.3, 50, 100 or 200 mg min-1, the doses administered were propofol 1.36 (0.28) mg kg-1, 1.46 (0.12) mg kg-1, 1.85 (0.43) mg kg-1 and 2.39 (0.50) mg kg-1 and the induction times 145.0 (25.4) s, 120.0 (18.4) s, 80.2 (19.2) s and 54.5 (10.4) s, respectively. In stage 2, a fixed induction dose of 1.46 mg kg-1 resulted in induction times of 35.0 (8.5) s and 134.0 (26.8) s, respectively. In stage 2 of each age group, induction was achieved with smaller doses than those recommended previously and there was no difference in the number of patients in whom induction of anaesthesia was successful or in the measured cardiorespiratory variables between the two induction regimens. This suggests the latter effects are caused by the dose administered and not the rate of administration.