Anesthetic potencies of isoflurane, halothane, and diethyl ether for various end points of anesthesia

In experiments with rats, the authors compared potency ratios and slopes of dose-effect curves of isoflurane, halothane, and diethyl ether for three end points of anesthesia: loss of righting reflex (RR), abolition of purposeful movement (PM) response to painful stimuli, and abolition of heart rate (HR) response to painful stimuli. Determinations of potency were based on the direct measurement of brain concentrations of anesthetics with the use of gas chromatography. It was found that the ratio of the PM ED50 to RR ED50 was 2.41 for isoflurane, 1.74 for halothane, and 1.25 for diethyl ester. They were significantly different for all three agents. Differences between the slopes of the dose-effect curves for RR and PM were significant only with diethyl ether (7 vs. 28). The ratios of HR ED50 to PM ED50 were not significantly different for the studied agents and there were no differences found between the slopes of the dose-effect curves for PM and HR. The results suggest that heart rate response to a noxious stimuli in contrast to the righting reflex is depressed by inhalation anesthetics through a mechanism similar to that underlying the depression of purposeful movement response to a noxious stimuli. Heart rate response to a noxious stimuli might be used as an alternative index for the measurement of anesthetic potency.