Surgical stimulation does not enhance ventilatory chemoreflexes during enflurane anaesthesia in man

To assess the impact of surgical stimulation on regulation of ventilation in anaesthetized man, we measured ventilation and the ventilatory responses to either hyperoxic hypercapnia or to isocapnic hypoxaemia in fifteen subjects anaesthetized with enflurane 1.1 MAC, just prior to and then during a surgical procedure. Anaesthesia alone reduced ventilation, increased PaCO2, decreased the response to carbon dioxide and virtually abolished the response to hypoxaemia. The addition of operation at the same level of anaesthesia augmented ventilation and reduced PaCO2, but did not improve the anaesthesia-induced impairment of the responses to hypercarbia and hypoxaemia. Over the range of PCO2 and PO2 values studied, the effects of surgery were constant and independent of chemical drive.