VENTILATORY EFFECTS OF ISOFLURANE (FORANE) OR HALOTHANE WHEN COMBINED WITH MORPHINE, NITROUS OXIDE AND SURGERY

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Abstract
The effects on ventilation of isoflurane in the presence of 70% nitrous oxide (N2O) and following morphine premedication were determined before and during surgery in 16 healthy patients. An identical study was performed using halothane in a similar group of 16 patients. The results were compared after converting the dose of the agents to MAC multiples. Both isoflurane and halothane produced a dose-related depression of respiration which, at the 2.0 MAC level, resulted in a profound respiratory acidosis with no significant difference between the two agents. The minute volume during isoflurane anaesthesia was not maintained by an increase in respiratory frequency as the concentration was increased from 1.0 to 2.0 MAC. In contrast a significant increase in respiratory frequency was seen when halothane was increased from 1.0 to 2.0 MAC. Although surgical stimulation at the 1.0, 1.5 and 2.0 MAC levels produced an average increase in alveolar ventilation of 35%, the average reduction in Paco2 of 4 mm Hg was too small to be of clinical importance. A 25% increase in carbon dioxide (CO2) output occurred in association with surgical stimulation.

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