Abstract
It is common practice to mix opioids with hyperbaric bupivacaine in a single syringe before intrathecal injection of the mixture. Mixing these drugs may alter the density of the hyperbaric solution, affecting the spread of local anaesthetic and opioid. Forty-eight women having elective caesarean section under spinal anaesthesia were recruited to this double-blind, randomised trial. Group M (n=24) received 2 ml of 0.5% hyperbaric bupivacaine plus morphine 100 μg plus fentanyl 15 μg, mixed in a syringe prior to administration. Group S (n=24) received 2 ml of 0.5% bupivacaine through one syringe, followed by morphine 100 μg plus fentanyl 15 μg through a separate syringe. All patients received patient-controlled intravenous morphine for 24 hours postoperatively. Block characteristics, postoperative pain scores and morphine use were noted. The patients in Group M had higher levels of sensory block to cold than those in Group S (median T2 vs T3)(P=0.003). Five patients in Group M and none in Group S had a block to cold ≥T1 (P=0.02). There was no difference between groups in the incidence of hypotension, need for vasopressor or side-effects. Morphine consumption was significantly higher in group M (13.3±11.2 vs 6.2±7.2 mg, P=0.015). Mixing of fentanyl and morphine with hyperbaric bupivacaine results in a higher level of sensory block than sequential administration of bupivacaine then opioid and may be associated with higher postoperative opioid requirement.