Influence of arteriovenous sampling on remifentanil pharmacokinetics and pharmacodynamics

Remifentanil is a new, short-acting, rapidly metabolized opioid. Because remifentanil is metabolized in blood and tissues by nonspecific esterases, there is a substantial difference between arterial and venous remifentanil concentrations. This difference may greatly affect the estimation of pharmacokinetic and pharmacodynamic parameters.

To assess the effects of sampling site on the pharmacokinetic and pharmacodynamic characteristics of remifentanil.

Ten healthy female subjects received intravenous remifentanil at an infusion rate of 3 microg/kg/min for 10 minutes. Serial blood samples were collected during and after drug administration from the radial artery and antecubital vein. A spectral edge measure was derived from the processed electroencephalographic and used as a measure of opioid effect.

Venous concentrations were lower than arterial concentrations during the infusion of remifentanil. Pharmacokinetic parameters estimated from venous and arterial data differed significantly. When arterial concentrations were plotted against electroencephalographic effect, a classic counterclockwise hysteresis loop was observed, indicating a time-lag between changes in concentration and changes in effect. However, concentrations from venous blood produced a clockwise hysteresis loop that would classically suggest the development of acute tolerance.

If this study had been conducted with venous samples alone, inappropriate conclusions such as acute tolerance could have been inferred. When designing studies to measure the acute time course (ie, non-steady state) of concentration and effect, the potential effects of sampling site on pharmacokinetic and pharmacodynamic characteristics must be carefully considered, particularly when the arteriovenous drug concentration difference is large.