Plasma level changes of fentanyl and midazolam after release of a prolonged thigh tourniquet.

In 14 elderly orthopedic patients undergoing total knee joint replacement, the influence of complete arterial occlusion of the limb on the course of plasma levels of fentanyl and midazolam was examined. The patients were premedicated with midazolam intramuscularly (0.05 mg/kg) and were then given neurolept anesthesia in dosages of 0.1 mg/kg midazolam and 0.01 mg/kg fentanyl intravenously prior to the placement of the tourniquet. Up to 4 h after the tourniquet was released, plasma levels of fentanyl and midazolam as well as pH value, PaCO2 and plasma lactate levels were measured. In 12 patients there was an increase in fentanyl and in 10 patients an increase in midazolam plasma levels after tourniquet release. The maximum increase varied between 1 min and 2 h after release. The plasma levels of midazolam after removal of the tourniquet varied greatly between individuals. Especially patients older than 70 years showed excessively high concentrations of midazolam. These results would indicate that there can be a clinically significant increase of fentanyl and midazolam levels due to initial reperfusion of the lower extremity following prolonged ischemia. Therefore a correspondingly extended period of postoperative surveillance is advisable.