Ventilatory effects of halothane and enflurane in dogs

The ventilatory effects of halothane and enflurane were studied in permanently tracheostomized dogs at the same anesthetic depth of 1 MAC. Inspiratory and expiratory durations were longer and tidal volume greater during enflurane than during halothane anesthesia. Mean inspiratory flow rate and minute ventilation during enflurane anesthesia were less than those during halothane anesthesia. As a result, end-expiratory carbon-dioxide concentration was higher during enflurane than during halothane anesthesia. When end-expiratory carbon-dioxide concentration was held at 7.5%, tidal volume was not different between the two anesthetics, while the difference of other parameters still remained. In addition, the magnitude of Hering-Breuer reflex determined by end-expiratory airway occlusion was essentially identical between halothane and enflurane anesthesia. The present results indicate that 1) depressant effect of enflurane on respiratory drive is greater than that of halothane and 2) the two anesthetics act on the respiratory timing mechanism differently.