Pharmacokinetics of alfentanil before and after cardiopulmonary bypass in pigs: Part II.

The pharmacokinetics of alfentanil before and after cardiopulmonary bypass (CPB) were investigated in six pigs undergoing mitral valve replacement. Before bypass, alfentanil, 100 micrograms/kg, was infused in 10 minutes and after bypass, alfentanil, 40 micrograms/kg, was infused in 10 minutes. Low inspiratory concentrations of halothane were given concomitantly. Arterial blood was obtained before and after CPB for determination of plasma alfentanil concentrations by gas chromatography. Bi-exponential functions were fitted to the plasma concentration-time data using weighted least-squares nonlinear regression analysis. The steady-state volume of distribution (Vss; 258 +/- 70 mL/kg), elimination clearance (Cle; 10.7 +/- 3.0 mL/kg/min), and distribution clearance (Cld; 6.8 +/- 3.3 mL/kg/min) before CPB were smaller than the Vss (1,107 +/- 373 mL/kg; P less than 0.01), Cle (20.0 +/- 3.0 mL/kg/min; P less than 0.002), and Cld (23.0 +/- 6.7 mL/kg/min; P less than 0.02) after CPB. The distribution half-life (t1/2 lambda 1; 2.8 +/- 0.8 minutes) was longer and the elimination half-life (t1/2 lambda 2; 36 +/- 8 minutes) was shorter before CPB than the t1/2 lambda 1 (1.7 +/- 0.2 minutes; P less than 0.05) and t1/2 lambda 2 (68 +/- 20 minutes; P less than 0.02) after CPB.