Effects of surgical stimulation on the apnoeic thresholds for carbon dioxide during anaesthesia with sevoflurane

Mechanical hyperventilation may produce hypocapnic apnoea below the carbon dioxide off-switch threshold whereas an increase in arterial PCO2 after post-hyperventilation apnoea causes reappearance of respiratory effort above the carbon dioxide on-switch threshold. To study the effects of surgical stimulation on these two thresholds, we have measured end-tidal PCO2 (PE'CO2) at the two thresholds, before and during surgical stimulation, in 14 patients undergoing mastectomy, anaesthetized with sevoflurane (1.2 MAC). Based on the reproducibility of the results, data from 11 patients were analysed and data from the three other patients were discarded. Before surgical stimulation, mean resting PE'CO2' off-switch threshold and on-switch threshold were 5.7 (SEM 0.2), 5.2 (0.2) and 6.1 (0.2) kPa, respectively. The off-switch threshold was significantly less than resting PE'CO2 (P < 0.01) but the on-switch threshold was significantly greater than resting PE'CO2 (P < 0.01). During surgical stimulation, resting PE'CO2' off-switch threshold and on-switch threshold were 4.8 (0.2), 4.1 (0.2) and 4.7 (0.2) kPa, respectively. Although the off-switch threshold was significantly less than resting PE'CO2 (P < 0.01), there were no significant differences between resting PE'CO2 and on-switch threshold. These results indicate that surgical stimulation does not affect equally the carbon dioxide on- and off-switch thresholds.