[Plasma morphine levels during its continuous epidural infusion].

We evaluated plasma levels of morphine during its continuous epidural infusion for postoperative analgesia in nine adult patients. A bolus injection of 3 mg of morphine was administered epidurally 3 hours prior to the proposed end of the surgery, and thereafter continuous epidural infusion of morphine was continued at a rate of 0.167-0.042 mg.hr-1 with a pump (CADD-PCA, 5200P, Pharmacia) during and after the surgery until the 3rd postoperative day. The dose of morphine was gradually decreased to 0.021-0.042 mg.hr-1 without reducing the quality of postoperative analgesia. Plasma morphine levels were measured by gas chromatography-mass spectrometry method. Plasma concentrations of morphine were 4.6 +/- 0.7 (Mean +/- SE) ng.ml-1 at the end of surgery and they decreased thereafter to 0.7 +/- 0.1 ng.ml-1, 0.3 +/- 0.1 ng.ml-1 and 0.1 +/- 0.1 ng.ml-1 on the 1st, 2nd and 3rd postoperative days, respectively. Plasma levels of morphine decreased gradually correlating with reduction of infused morphine. Adequate postoperative pain relief was obtained throughout the procedure without any severe complications such as respiratory depression. The analgesic effect of epidural morphine did not parallel with that of plasma concentration. Plasma concentrations of morphine administered by continuous epidural infusion with a pump were estimated to be lower than the minimum analgesic plasma concentration (10-40 ng.ml-1), and the toxic levels of morphine during continuous epidural infusion were not detected by our method.