Pharmacokinetics of alfentanil after epidural administration. Investigation of systemic absorption kinetics with a stable isotope method.

Background: The effects of epidurally administered alfentanil may be due in part to its uptake into the systemic circulation. Therefore we examined the systemic absorption kinetics after epidural injection of alfentanil.

Methods: Pharmacokinetics were determined using a stable isotope method in ten patients, undergoing lower abdominal surgery under general anesthesia. After epidural injection of 0.68 mg deuterium-labeled alfentanil (alfentanil-d5), 1 mg unlabeled alfentanil was administered over 1 h by an intravenous infusion. Blood samples were collected for 12 h. Concentrations of alfentanil and alfentanil-d5 were measured by a combination of gas chromatography and mass fragmentography. The systemic absorption profiles of alfentanil-d5 were determined by deconvolution of the plasma alfentanil-d5 concentrations with the biexponential unit disposition functions, derived from the intravenous data. In addition, data were analyzed by moment analysis.

Results: The mean (+/- SD) steady-state volume of distribution, total plasma clearance, elimination half-life and mean residence time, derived from the unlabeled alfentanil concentration-time data, were 43.2 +/- 19.5 1,418 +/- 129 ml/min, 119 +/- 34 min, and 103 +/- 26 min, respectively. The absorption of alfentanil-d5 was monophasic in most patients. The mean systemic availability and mean absorption time derived from the deconvolution data were 100 +/- 17% and 114 +/- 24 min. The values determined by moment analysis were 107 +/- 18% and 112 +/- 36 min, respectively.

Conclusions: After epidural administration alfentanil is slowly absorbed into the general circulation. Resulting plasma concentrations are very low and do not contribute appreciably to the systemic opioid effect.