EFFECTS OF SURGICAL STIMULATION ON THE APNEIC THRESHOLDS FOR CARBON-DIOXIDE DURING ANESTHESIA WITH SEVOFLURANE

Mechanical hyperventilation may produce hypocapnic apnoea below the ca rbon dioxide off-switch threshold whereas an increase in arterial Pco( 2) after post-hyperventilation apnoea causes reappearance of respirato ry effort above the carbon dioxide on switch threshold. To study the e ffects of surgical stimulation on these two thresholds, we have measur ed end-tidal Pco(2) (PE'(CO2)) at the two thresholds, before and durin g surgical stimulation, in 14 patients undergoing mastectomy, anaesthe tized with sevoflurane (1.2 MAC), Based on the reproducibility of the results, data from 11 patients were analysed and data from the three o ther patients were discarded. Before surgical stimulation, mean restin g 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 thres hold was significantly less than resting PE'(CO2) (P < 0.01) but the o n-switch threshold was significantly greater than resting PE'(CO2) (P < 0.01) During surgical stimulation, resting PE'(CO2), off-switch thre shold 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 diff erences between resting PE'(CO2) and on-switch threshold. These result s indicate that surgical stimulation does not affect equally the carbo n dioxide on- and off-switch thresholds.