Analgesia and ventilatory response to carbon dioxide after intramuscular and epidural alfentanil

The analgesic and ventilatory depressant effects of epidural and intramuscular alfentanil (15 micrograms/kg) were compared in two groups of seven healthy unpremedicated subjects. Fifteen minutes after IM injection, the slope of the ventilatory response to CO2 decreased significantly (from 2.72 +/- 0.34 to 1.8 +/- 0.20 L.min-1.mmHg-1) while assessment of periosteal analgesia showed no change. After epidural injection, the slope of the ventilatory response to CO2 decreased significantly (from 2.32 +/- 0.42 to 1.61 +/- 0.29, 1.51 +/- 0.29, and 1.53 +/- 0.21 L.min-1.mm Hg-1) at 15, 45, and 90 minutes (x +/- SD, P less than 0.05), and there was significant periosteal analgesia of the tibia (15 and 30 minutes after injection) and of the radius (30 to 90 minutes after injection). Throughout the study, plasma alfentanil levels were similar after intramuscular and epidural injection. These results suggest that epidural alfentanil induces ventilatory depression due to the rostral spread of the drug rather than to systemic absorption.