Dose response study of subarachnoid diamorphine for analgesia after elective caesarean section

Subarachnoid diamorphine provides excellent analgesia after elective caesarean section but the optimum dose is still uncertain. We therefore investigated the effects of three regimens of subarachnoid diamorphine. Forty parturients were assigned to one of four groups. A control group received no diamorphine in their subarachnoid bupivacaine and three study groups received 0.1 mg, 0.2 mg or 0.3 mg diamorphine added to 12.5 mg hyperbaric bupivacaine 0.5% in a semi-blind randomised design study. All women received a 100 mg diclofenac suppository at the end of the caesarean section and were provided with morphine patient controlled analgesia (PCA) postoperatively. The patients were assessed for pain, morphine usage and side-effects at 2, 4, 8 and 24 h after the subarachnoid injection. Postoperative visual analogue scores for pain and PCA morphine consumption were significantly lower, and mean time to first use of morphine was significantly longer in the 0.3 mg diamorphine group. The mean (SD) dose of PCA morphine used over 24 h was 39.4 (14.7), 25.6 (16.5), 21.6 (15.9) and 3.1 (3.6) mg, and mean time to first use of morphine was 1.6 (0.5), 3.0 (1.4), 3.4 (2.4) and 14.1 (9.4) h, in the 0, 0.1 mg, 0.2 mg and 0.3 mg groups respectively. Side-effects of pruritus, nausea and vomiting were dependent on the dose of spinal diamorphine but did not require treatment in any patients. We conclude that 0.3 mg subarachnoid diamorphine provides significantly better postoperative pain relief than the smaller doses with an acceptable increase in side-effects.