Respiratory effects of nitrous during enflurane anesthesia in humans

The authors measured resting ventilation, the ventilatory response to added CO2, the VD/VT ratio, the rate of carbon dioxide output, and arterial PCO2 in four healthy volunteers, awake and anesthetized with, in order (I) enflurane 0.4 MAC with nitrous oxide 70 per cent, (II) enflurane 1.1 MAC with nitrous oxide 70 per cent, and (III) enflurane 1.1 MAC alone. Enflurane 1.1 MAC reduced ventilation and the response to added CO2 markedly, increased the VD/VT radio, reduced rate of CO2 output, and elevated values of PaCO2 from 41 +/- 1 to 65 +/- 3 mmHg (mean +/- SEM). Enflurane 1.1 MAC with nitrous oxide 70 per cent had similar effects. Enflurane 1.1 MAC with nitrous oxide 70 per cent had similar effects. Enflurane 0.4 MAC with nitrous oxide 70 per cent caused much smaller changes in each measured respiratory variable, increasing PaCO2 values to only 49 +/- 1 mmHg. The results indicate that enflurane 1.1 MAC alone is too potent a depressant of alveolar ventilation to permit spontaneous breathing, but that the "equi-anesthetic" enflurane 0.4 MAC with nitrous oxide 70 per cent may not be. The magnitude of the beneficial respiratory effects of substituting nitrous oxide for an equivalent amount of vapor is substantially greater with enflurane than with either halothane or isoflurane.