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

Background. Opioid analgesics are commonly added to intrathecalbupivacaine to improve patient comfort during Caesarean sectionunder spinal anaesthesia, and provide post-operative pain relief.We sought to discover if the addition of diamorphine influencedblock height when given with 0.5% w/v hyperbaric bupivacaine. Method. Eighty ASA I and II women of at least 37 weeks gestationand planned for elective Caesarean section under combined spinal–epiduralanaesthesia were recruited. They were randomized into two groupsto receive intrathecal hyperbaric bupivacaine 0.5% at an initialdose of 13 mg, with the next dose determined by the responseof the previous patient (dose interval 1 mg). One group alsoreceived diamorphine 400 µg intrathecally. If a blockheight of T5 to blunt light touch had been achieved after 20min, the block was deemed effective. A difference in the ED₅₀for hyperbaric bupivacaine between the groups would indicatethat diamorphine influenced block height. Intraoperative patientdiscomfort and need for analgesic supplementation was noted. Results. The median effective dose (ED₅₀) to achieve a T5 blockto light touch for Caesarean section using hyperbaric bupivacaine0.5% 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 13.6mg (95% CI 9.15–18.05), respectively. Five women who hadreceived intrathecal diamorphine and 13 who had not receiveddiamorphine needed intraoperative supplementation (not significant). Conclusion. The addition of intrathecal diamorphine does notappear to influence block height.