Effect of intrathecal diamorphine on block height during spinal anaesthesia for Caesarean section with bupivacaine

Background. Opioid analgesics are commonly added to intrathecal bupivacaine to improve patient comfort during Caesarean section under spinal anaesthesia, and provide post-operative pain relief. We sought to discover if the addition of diamorphine influenced block height when given with 0.5% w/v hyperbaric bupivacaine. Method. Eighty ASA I and II women of at least 37 weeks gestation and planned for elective Caesarean section under combined spinal-epidural anaesthesia were recruited. They were randomized into two groups to receive intrathecal hyperbaric bupivacaine 0.5% at an initial dose of 13 mg, with the next dose determined by the response of the previous patient (dose interval 1 mg). One group also received diamorphine 400 mu g intrathecally. If a block height of T5 to blunt light touch had been achieved after 20 min, the block was deemed effective. A difference in the ED50 for hyperbaric bupivacaine between the groups would indicate that diamorphine influenced block height. Intraoperative patient discomfort and need for analgesic supplementation was noted. Results. The median effective dose (ED50) to achieve a T5 block to light touch for Caesarean section using hyperbaric bupivacaine 0.5% was 9.95 mg [95% confidence interval (CI) 9.0-10.90] and with the addition of diamorphine it was 9.3 mg (95% CI 8.15-10.40), while the ED95 was 13.55 mg (95% CI 10.10-17.0) and 13.6 mg (95% CI 9.15-18.05), respectively. Five women who had received intrathecal diamorphine and 13 who had not received diamorphine needed intraoperative supplementation (not significant). Conclusion. The addition of intrathecal diamorphine does not appear to influence block height.