Ventilatory Effects of Continuous Epidural Infusion of Fentanyl

The effects of a continuous epidural administration of fentanyl on pain and on ventilation were studied in eight patients scheduled for orthopedic surgery of the knee. In each subject, epidural fentanyl was given by a bolus dose of 1 microgram.kg-1, followed by a continuous infusion of 1 microgram.kg-1.h-1 over 18 hours. Ventilatory measurements were performed during quiet breathing and during CO2 stimulation tests before surgery. After surgery measurements were made before epidural administration of fentanyl; 1, 2, 5, 18 hours after the start of epidural fentanyl infusion; and 6 hours after its discontinuation. Adequate pain relief was achieved in all patients during fentanyl administration. No significant change in ventilation was noted during quiet breathing. The slope of the ventilatory response to CO2 (VE/PaCO2) decreased significantly from 1.46 +/- 0.2 to 0.75 +/- 0.1 L.min-1.mm Hg-1 (mean +/- SEM; P less than 0.05) one hour after the onset of fentanyl administration, and remained stable throughout the infusion. Eighteen hours after the onset of epidural fentanyl infusion, VE/PaCO2 was still 0.76 +/- 0.14 L.min-1.mm Hg-1. At the end of fentanyl administration, plasma fentanyl levels measured in six patients had progressively increased from 0.42 +/- 0.02 ng.ml one hour after the onset of the infusion to 1.54 +/- 0.19 ng.ml at the end of the infusion. These results suggest that a continuous epidural administration of fentanyl is a technique of analgesia that can provide adequate pain relief but which is associated with ventilatory depression. However, with the doses used in this study, the ventilatory depression remained moderate and of no demonstrable clinical consequence.