An analysis of the ventilatory response to carbon dioxide during halothane, isoflurane, or enflurane anesthesia in humans

To analyze the effects of halothane, isoflurane, or enflurane on the ventilatory response to CO2, minute volume (VE), respiratory rate (f), occlusion pressure, mean inspiratory flow rate (VT/TI), and effective elastance (Pmax/VT) were measured in 26 patients during quiet breathing and when the respiratory system was stressed by CO2. Comparison was made at equipotent anesthetic concentration (1 MAC) and all measurements were made in the absence of surgical stimulation. Respiratory rate and Pmax/VT were unchanged in all groups after end-tidal CO2 was elevated. By contrast, VE, VT/TI, and occlusion pressure increased in all the groups when the respiratory system was stressed by CO2. Among the parameters tested delta VE/delta PaCO2 and delta VT/TI/delta PaCO2 were significantly (P less than 0.05) greater during halothane anesthesia than during isoflurane or enflurane anesthesia. We conclude that, at equipotent anesthetic concentrations, isoflurane and enflurane depress CO2 chemosensitivity more than halothane does, and that such effect is primarily related to the greater depressant effects of isoflurane and enflurane on the respiratory driving mechanisms.