Pharmacokinetics of alfentanil in chronic renal failure

The pharmacokinetics of alfentanil were studied during general anesthesia in nine patients with renal failure and in ten patients with normal renal function. All patients received 0.05 mg/kg alfentanil as an intravenous bolus injection. Plasma concentrations were measured at intervals up to 8 hr, using a specific radioimmunoassay technique. Protein binding was measured by equilibrium dialysis. Elimination half-life and plasma clearance were similar in both groups. The volume of distribution at steady state was greater (P less than 0.02) in patients with renal failure (405 +/- 86 ml/kg) than in patients with normal renal function (281 +/- 97 ml/kg). Patients with renal failure had a higher (P less than 0.01) alfentanil plasma free fraction (0.19 +/- 0.06) than patients with normal renal function (0.11 +/- 0.03). When kinetic parameters were corrected for protein binding, the unbound volume of distribution and the free drug clearance were unchanged in patients with renal failure. These results suggest that the modification of alfentanil free fraction in renal failure does not induce any change in elimination but may influence the distribution of alfentanil.