Remifentanil and pulmonary extraction during and after cardiac anesthesia

We measured the apparent blood clearance and pulmonary extraction ratio of remifentanil in 10 adult patients undergoing elective myocardial revascularization for the first time with hypothermic cardiopulmonary bypass (CPB). Patients received continuous infusions of remifentanil 1.0, 1.5 or 2.0 microg x kg(-1) x min(-1). After surgery, remifentanil was infused at 1.0 microg x kg(-1) x min(-1) in all patients. Remifentanil concentrations were measured in pulmonary and radial artery blood by gas chromatography with high resolution mass spectrometry before and after CPB and 165 min (60 SD) after surgery. Cardiac output was measured by thermodilution at the time of blood sampling. The mean pulmonary extraction ratio of remifentanil was 5.7% (13.1% SD), which was not significantly different from zero. However, pulmonary extraction ratio was related inversely to the pulmonary artery hydrogen ion concentration and directly to the percent of nonionized form of the base in the pulmonary artery. Remifentanil concentrations in pulmonary and radial artery blood were related directly to infusion rate, but not to duration of infusion. There was no evidence of accumulation or sequestration. Mean apparent blood remifentanil clearance was 2.03 L/min (0.35 SD) and, in contrast to remifentanil pulmonary extraction ratio, was related directly to cardiac index and oxygen delivery. Increased tissue perfusion increased blood remifentanil clearance. We found predictable blood remifentanil levels with no evidence of accumulation or pulmonary extraction.