Dose-response relationships for infusions of Althesin or methohexitone

A technique is described for the determination of equipotent rates of infusion of intravenous hypnotic agents. Two end points have been defined; the rates needed to suppress the initial response to the surgical incision in 50% (ED50) and 95% (ED95) of patients receiving continuous infusion anaesthesia to supplement 67% nitrous oxide in oxygen. For patients aged 20-60 years, and premedicated with morphine 0.15 mg/kg intramuscularly, the ED50 values for Althesin (as alphaxalone) and methohexitone by infusion were 13.7 and 48.8 micrograms/kg/minute respectively. The ED95 values were 18.1 and 75.9 micrograms/kg/minute. For patients of a similar age premedicated with diazepam 10 mg orally, the ED50 values for Althesin and methohexitone were 18.5 and 66.0 micrograms/kg/minute respectively; while the ED95 values were 24.2 and 80.8 micrograms/kg/minute.