Low-dose intrathecal local anesthetic does not increase the threshold current for the epidural stimulation test: a prospective observational trial of neuraxial analgesia in labouring women.

PURPOSE

The purpose of this study was to investigate the ability of the electrical epidural stimulation test (EST) to determine the position of the epidural catheter during combined spinal-epidural (CSE) anesthesia for labour analgesia.

METHODS

This was a prospective observational trial of attempted EST during neuraxial analgesia in labouring women. Ten women received a double-segment CSE technique and one woman underwent continuous spinal analgesia following inadvertent dural puncture and deliberate placement of the catheter tip in the intrathecal space. In all CSE cases, the spinal injection was performed below the level of the epidural insertion. The motor threshold current (MTC) was determined by EST through the existing epidural/intrathecal catheter immediately following and at five, ten, and 15 mins after intrathecal injection of bupivacaine 1.75 mg and fentanyl 15 μg. Changes in the MTC were expressed as a percent change compared with baseline.

RESULTS

The MTC required to elicit muscle contractions in women with epidurally placed catheters was unaffected by the intrathecal injection of the analgesic mixture (P = 0.731). The MTC increased following an intrathecal injection of the same mixture in a woman who had the catheter placed intrathecally.

CONCLUSIONS

The intrathecal injection of a low dose of bupivacaine-fentanyl does not affect the MTC if the catheter is placed in the epidural space; however, it does affect the threshold if the catheter is placed intrathecally. We also confirm that the EST can help to determine the position of the epidural catheter prior to injection of the test dose. This trial was registered at ClinicalTrials.gov (NCT00464841).