Antagonism of fentanyl and alfentanil by intravenous plus subcutaneous naloxone

Twenty patients undergoing microlaryngoscopy were anaesthetized with thiopentone. Half received fentanyl supplementation (about 8.5 micrograms/kg) and the other half alfentanil (about 65 micrograms/kg). Both groups were given naloxone 0.4 mg intravenously plus 0.4 mg subcutaneously shortly after the procedure which lasted some 12 minutes. The degree of ventilatory depression was assessed by a CO2 rebreathing test. The ventilation at an end-tidal PCO2 of 8.0 kPa (V8.0) was noted, and the findings related to a control value obtained on the day before anaesthesia. In the fentanyl group, V8.0 was significantly (p less than 0.05) less one hour after naloxone than 15 minutes after, and remained significantly below the control value for the first 8 hours after microlaryngoscopy. A second peak in plasma fentanyl concentration was observed four hours postoperatively in three patients. Respiratory depression in the alfentanil group was less pronounced and of shorter duration than in the fentanyl group. Postoperative plasma alfentanyl concentration decreased progressively with time in every patient.