Comparison of infusion rates of three i.v. anaesthetic agents for induction in elderly patients.

We have compared the effect of different rates of injection of 2.5% thiopentone, 0.5% methohexitone and 0.2% etomidate for induction of anaesthesia in 90 premedicated, elderly patients. The agents were administered by infusion pump at rates of 1200 ml h-1, 600 ml h-1 and 300 ml h-1, respectively until anaesthesia was induced as judged by loss of verbal contact with the patient. The times for induction were significantly greater with the slower infusion rates (thiopentone 41 s, 57 s and 91 s (P < 0.001); methohexitone 44 s, 62 s and 84 s (P < 0.01); etomidate 48 s, 59 s and 87 s (P < 0.001)). The doses were significantly smaller (P < 0.001) with the slower infusion rates for all three agents (thiopentone 5.0, 3.7 and 2.8 mg kg-1; methohexitone 1.00, 0.75 and 0.56 mg kg-1; etomidate 0.26, 0.15 and 0.11 mg kg-1). For each drug there was no significant difference in induction characteristics, oxygen saturation, heart rate or mean arterial pressure, at the different infusion rates.