Abstract
The relative potencies of propofol and thiopentone were assessed using different indicators of induction of anaesthesia: abolition of the response to verbal commands and eyelash stimulation. Log-probit dose-response curves for these end-points were determined 30, 60 and 90 s after induction in 96 unpremedicated ASA group I patients. For propofol, ED50 values for abolition of the response to verbal commands and eyelash stimulation at different time intervals were in the ranges 1.16-1.42 and 1.23-1.72 mg kg-1, respectively; corresponding ED95 values were 2.18-2.67 and 2.42-3.27 mg kg-1, respectively. For thiopentone, the calculated ED50 values for verbal commands and eyelash stimulation at the same time intervals were 1.81-2.23 and 3.55-3.40 mg kg-1; corresponding ED95 values were 5.11-6.29 and 6.41-6.70 mg kg-1, respectively. The potency ratio of propofol to thiopentone observed in this study varied from 1:1.27 to 1:2.88. It is concluded that a dose-response curve reflecting one end-point of anaesthesia cannot be used to define another end-point of anaesthesia.