Assessing bupivacaine 10 mg/fentanyl 20 microg as an intrathecal test dose

After ethics committee approval and verbal consent, women undergoing elective caesarean section given spinal anaesthesia with hyperbaric 0.5% bupivacaine 10 mg (2 mL) plus fentanyl 20 microg (spinal group, n = 20) and women requesting epidural analgesia in labour given the same drugs and doses epidurally, either in the same concentration (epidural small volume group, n = 10) or as 10 mL of 0.1% bupivacaine plus fentanyl 20 microg (epidural large volume group, n = 12) were recruited. The temperature of the great toes, sensory block on the outer ankle (S1 dermatome), motor block at the ankle and haemodynamic changes were recorded every 2 min for 10 min. There was a significant rise in foot temperature only in the spinal group. At four minutes a combination of warm toes and motor or sensory block, usually both, were seen only in the spinal groups. Haemodynamic changes were non-specific. We conclude that bupivacaine 10 mg with fentanyl 20 microg is a reliable agent to detect intrathecal placement by 4 min by which time a combination of motor and sensory block at the ankle and toe warming should be present.