The indices of potency for intravenous anaesthetics

In rat experiments, the relative potency an safety of thiopentone, diazepam and etomidate were assessed using different indices of anaesthesia - loss of righting reflex, prevention of movement and heart rate responses to an noxious stimulus. Log-probit dose-response curves for these end-points and for lethal effect were determined. Etomidate proved to be more potent than thiopentone or diazepam; its relative potency figures varied from 5 to 18 with the use of different end-points of anaesthesia. According to the ED50 level of response for the loss of righting reflex and increased of heart rate, diazepam was more potent than thiopentone. At the same time it was less potent in the prevention of movement response. Etomidate has an extremely large standard safety margin (SSM) regarding the loss of righting reflex; with the use of movement response or heart rate response, the SSM for etomidate was close to that of thiopentone. With diazepam, SSM for movement response and heart rate response was negative (anaesthetic dose-response curve partially overlaps the lethal curve); it was positive for the loss of righting reflex. The extreme variability in assessment of relative potency and safety with different end-points of anaesthesia probably indicates that the indices used reflected various components of anaesthesia. It seems likely that for the proper assessment of the potency of intravenous anaesthetics, one index of potency is not sufficient. Several indices of potency corresponding to different components on general anaesthesia must be used.