Influence of halothane-diethyl-ether azeotrope and isoflurane on ventilation. Re-evaluation of an obsolete drug

The azeotropic mixture of halothane and diethyl-ether has been claimed to be a suitable anaesthetic agent for use during difficult conditions because of its negligible effect on circulation and ventilation. The purpose was to evaluate the effect of halothane-diethyl-ether azeotrope (HE) and isoflurane (ISO) on ventilation. 12 patients scheduled to undergo minor orthopaedic surgery and belonging to ASA 1, were randomly allocated to the HE group or the ISO group. Evaluation of resting ventilation and ventilation stimulated by hypercarbia and hypoxaemia was done on three occasions: (A) before anaesthesia, (B) after inhalational induction of anaesthesia and intubation without muscle relaxants when the level of anaesthesia was 1 MAC and (C) half an hour after operation and during recovery. Resting ventilation and the ventilatory response to hypercarbia during anaesthesia were maintained in the HE group but not in the ISO group, whereas the ventilatory response to hypoxaemia during anaesthesia was absent in both groups. The responses had returned to normal values in both groups during recovery. We conclude that halothane-diethyl-ether azeotrope is comparatively safe during anaesthesia with spontaneous breathing provided arterial oxygenation is adequate. This makes this azeotrope suitable for use by anaesthetists with limited experience and during difficult conditions such as civil disaster or war.