From the journal archives: Practical applicability of the epidural electrical stimulation test

Confirming identification of the epidural space: a systematic review of electric stimulation, pressure waveform analysis, and ultrasound and a meta-analysis of diagnostic accuracy in acute pain

To review the use of epidural electric stimulation test, pressure waveform analysis, and ultrasound assessment of injection as bedside methods for confirming identification of the epidural space in adults with acute pain, the PubMed database was searched for relevant reports between May and August 2022. Studies reporting diagnostic accuracy with conventional Touhy needles and epidural catheters were further selected for meta-analysis. Sensitivity and specificity were estimated using univariate logistic regression for electric stimulation and pressure analysis, and pooling of similar studies for ultrasound. Risk of bias and applicability was assessed using QUADAS-2. For electric stimulation, pressure waveform analysis, and ultrasound, respectively 35, 22, and 28 reports were included in the review and 9, 9, and 7 studies in the meta-analysis. Electric stimulation requires wire-reinforced catheters and an adequate nerve stimulator, does not reliably identify intravascular placement, and is affected by local anaesthetics. Sensitivity was 95% (95% CI 93-96%, N = 550) and specificity unknown (95% CI 33-94%, N = 44). Pressure waveform analysis is unaffected by local anaesthetics, but does not identify intravascular nor intrathecal catheters. Sensitivity was 90% (95% CI 72-97%, N = 694) and specificity 88% (95% CI 78-94%, N = 67). B-mode, M-mode and doppler ultrasound may be challenging, and data is still limited. Risk of bias was significant and accuracy estimates must be interpreted with caution. Electric stimulation and pressure waveform analysis seem clinically useful, although they must be interpreted cautiously. In the future, clinical trials in patients with difficult anatomy will likely be most useful. Ultrasound requires further investigation.

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).