Patient-controlled analgesia with extradural morphine or pethidine.

Two groups of patients were allowed to self-administer morphine (n = 17) or pethidine (n = 15) extradurally after abdominal surgery, for a mean period of 16 h. Bolus increments of morphine 1 mg or pethidine 20 mg were administered by programmable pump. Pain relief from extradural patient-controlled analgesia (PCA) was excellent in all but two patients in the morphine group. Pain relief was not qualitatively different between the two groups. No clinical respiratory depression was seen. The average consumption of extradural morphine was 0.52 +/- 0.29 mg h-1 (range 0.19-1.04 mg h-1) and of pethidine 18.0 +/- 8.1 mg h-1 (5.8-35.4 mg h-1). This yields an equianalgesic dose relationship of 1:35. Morphine consumption was more irregular than pethidine consumption. Morphine and pethidine plasma concentrations measured during PCA were well below the reported minimum analgesic plasma concentrations in most cases. Several patients, particularly in the pethidine group, tended to increase their opioid consumption during PCA. This could be explained by an increasingly smaller fraction of the pethidine bolus being absorbed to the subarachnoid space during frequent repetitive dosing. The large inter-individual variation in consumption makes it impossible to recommend a standard dose of extradural morphine or pethidine for analgesia of predictable duration and with a minimum of adverse effects.