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 S1 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 S1 motor block 10 min after intrathecal injection was bupivacaine 7 mg with fentanyl 14 mug (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 mug (95% CI, 8.7-11.4). Conclusion. We conclude that testing for S1 motor block 10 min after epidural injection of bupivacaine 10 mg is a reliable test to detect accidental intrathecal injection in the obstetric population.