Comparison of the respiratory depressant effects of halothane and isoflurane in routine surgery

Respiratory depression has been studied in 21 patients anaesthetized with either halothane and nitrous oxide, or isoflurane and nitrous oxide while undergoing routine surgery. Spontaneous ventilation was preserved and the end-tidal concentrations of the two volatile anaesthetics were at approximately equal MAC-multiples. The minute volume of pulmonary ventilation was not significantly different for the two anaesthetics, but tidal volume was significantly greater and frequency significantly lower with isoflurane. Arterial and end-tidal Pco₂ were consistently lower in the patients receiving isoflurane, the difference being significant in many of the sub-groups. There was no evidence of a progressive increase in Pco₂ with either anaesthetic during surgery lasting up to 225 min. Mean values for Pco₂′ however, tended to be well above the. normal reference range, with mean arterial Pco₂ in the range 7.2-8.6 kPa.