Ventilatory response to carbon dioxide during extradural anaesthesia with lignocaine and fentanyl

Twenty-seven patients undergoing extracorporeal shock-wave lithotripsy or knee arthroscopy received extradural anaesthesia with 2% lignocaine plus adrenaline 1 in 200,000. They were allocated randomly to three groups, one receiving no fentanyl (n = 6), the two others receiving fentanyl 50 micrograms either extradurally (n = 15) or i.v. (n = 6). Three tests of sensitivity to carbon dioxide (Read's method) were performed successively on each patient: before operation and at 1 and 2 h after the extradural injection. Whereas lignocaine and adrenaline alone had no significant effects on basal ventilation and the ventilatory response to carbon dioxide, extradural fentanyl caused a slight reduction in resting ventilatory rate and ventilation at 1 and 2 h with no change in resting end-tidal carbon dioxide concentration. In addition, the slope of the ventilatory response to carbon dioxide was reduced slightly at 1 h and ventilation at end-tidal PCO2 of 7.3 kPa was reduced also at 1 and 2 h. Conversely, the same dose of fentanyl i.v. had lesser and shorter effects on ventilation at rest and during carbon dioxide rebreathing. Our results show that fentanyl 50 micrograms given extradurally caused slight ventilatory depression which is probably clinically unimportant.