Variable effects of anaesthetics on the ventilatory response to hypoxaemia in man

We compared the ventilatory response to isocapnic hypoxaemia with a standard response to hyperoxic hypercarbia in human subjects sedated with methoxyflurane, diethyl ether or nitrous oxide, or anaesthetized with methoxyflurane. Compared to the awake state, methoxyflurane 0.1 MAC sedation did not alter either response significantly; methoxyflurane 1.1 MAC anaesthesia depressed both, with a somewhat greater effect on the hypoxaemic response. Diethyl ether 0.1 MAC sedation reduced only the hypoxaemic response. Nitrous oxide 0.1 MAC reduced both hypoxaemic and carbon dioxide responses in parallel. The evidence suggests that all three agents - like thiopentone, halothane and enflurane - can impair the ventilatory response to isocapnic hypoxaemia in man, but that in relation to the carbon dioxide responses, the magnitude of this depressive effect varies. Halothane and enflurane are the most depressant, nitrous oxide and thiopentone the least, with methoxyflurane and diethyl ether appearing to be intermediate in effect.