The pharmacokinetics of intradural morphine in major abdominal surgery

The pharmacokinetics of intradural morphine used for major abdominal surgery were evaluated. Lumbar spinal fluid and plasma concentrations were measured at intervals after morphine 0.05 mg/kg had been injected intradurally in 21 patients scheduled for elective abdominal aortic surgery. The CSF morphine concentrations were fitted by a biexponential function. A non-compartmental model based on statistical moment theory was used for calculating the intradural morphine disposition. Mean residence time was 137 +/- 54.9 minutes, mean initial volume of distribution 15 +/- 5.49 ml, mean volume of distribution at steady-state 42 +/- 18.25 ml and mean clearance 0.34 +/- 0.18 ml/min (0.02 +/- 0.01 L/h). The moments of the morphine concentration-time curves and the pharmacokinetic parameters varied between the patients. They were not significantly different with regard to morphine dosage, or patient sex or age. Free morphine could not be detected in plasma. Morphine-3-glucuronide appeared in plasma at 5 minutes, increased to a maximum at 240 minutes and fell below the detection limit at about 16 hours after morphine administration. Possible clinical causes of interindividual variations in the CSF morphine concentrations and the pharmacokinetics of intradural morphine are discussed.