Age influences the minimum infusion rate (ED50) for continuous infusions of Althesin and methohexitone

The influence of age has been studied on equipotent rates of continuous infusions of two intravenous hypnotic agents. Two end points have been defined; ED50: the rate needed to suppress the initial response to the surgical incision in 50% of patients, and ED95: the corresponding rate in 95% of patients receiving continuous infusion anaesthesia to supplement 67% nitrous oxide in oxygen. All 210 patients included in the study were premedicated with morphine 0.15 mg kg-1. For patients aged 20-40 years (young group), the ED50 values for Althesin (as alphaxalone) and methohexitone were 14.6 micrograms kg-1 min-1 and 59.9 micrograms kg-1 min-1. The ED95 values were 21.0 micrograms kg-1 min-1 and 92.1 micrograms kg-1 min-1. For patients aged 55-80 years (old group), the ED50 values for Althesin and methohexitone were 11.0 micrograms kg-1 min-1 and 44.2 micrograms kg-1 m in-1, while the corresponding ED95 values were 16.4 micrograms kg-1 min-1 and 76.2 micrograms kg-1 min-1.