Evaluation of S1 motor block to determine a safe, reliable test dose for epidural analgesia.

BACKGROUND Accidental intrathecal injection of bupivacaine during epidural analgesia in labour remains a hazard, with the potential to cause total spinal anaesthesia and maternal collapse. Sacral block appears early after intrathecal injections compared with epidural ones, and we therefore used SI motor block to determine a safe and reliable test dose for epidural catheter misplacement. METHODS Mothers booked for elective Caesarean section were given various intrathecal doses of bupivacaine with fentanyl during routine combined spinal-epidural anaesthesia. RESULTS Using sequential allocation we found that the ED50 for SI motor block 10 min after intrathecal injection was bupivacaine 7 mg with fentanyl 14 micrograms (95% CI, 6.2-7.8 mg). We then used intrathecal bupivacaine 13 mg to look for the ED95. We found the calculated ED97.5 to be bupivacaine 9.7 mg with fentanyl 19.4 micrograms (95% CI, 8.7-11.4). CONCLUSION We conclude that testing for SI motor block 10 min after epidural injection of bupivacaine 10 mg is a reliable test to detect accidental intrathecal injection in the obstetric population.