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 microg 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.