Respiratory Effects of Nitrous Oxide during Enflurane Anesthesia in Humans

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Abstract
Resting ventilation, ventilatory response to added CO2, the VD/VT ratio, the rate of CO2 output, and PaCO2 [arterial partial pressure of CO2] were measured in 4 healthy volunteers, awake and anesthetized with enflurane 0.4 MAC [minimum anesthetic concentration] with N2O 70%, enflurane 1.1 MAC with N2O 70%, and enflurane 1.1 MAC alone. Enflurane 1.1 MAC reduced ventilation and the response to added CO2 markedly, increased the VD/VT ratio, reduced rate of CO2 output and elevated values of PaCO2 from 41 .+-. 1 to 65 .+-.3 mm Hg. Enflurane 1.1 MAC with N2O 70% had similar effects. Enflurane 0.4 MAC with N2O 70% caused much smaller changes in each measured respiratory variable, increasing PaCO2 values to only 49 .+-. 1 mm Hg. Evidently, enflurane 1.1 MAC alone is too potent a depressant of alveolar ventilation to permit spontaneous breathing, but the equi-anesthetic enflurane 0.4 MAC with N2O 70% may not be. The magnitude of the beneficial respiratory effects of substituting N2O for an equivalent amount of vapor is substantially greater with enflurane than with either halothane or isoflurane.

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