Background: In labor analgesia, epidurals are the gold standard. However, inadequate analgesia in laboring patients with lumbar epidurals is between 1.7-19.8% [1]. Catheter tip location determines which region of the body is anesthetized. During early labor, epidural catheter placement near lumbar nerve roots provides excellent analgesic coverage while catheter placement near sacral nerve roots can lead to inadequate analgesia [2]. While commonly used to predict thoracic epidural placement, the noninvasive epidural stimulation test (EST) has not been tested for predicting lumbar versus sacral epidural placement [3]. In EST, muscle twitching indicates lumbar or sacral nerve root proximity. This study examines EST's ability to predict lumbar versus sacral epidural placement and its labor analgesia quality.

Methods: A single-center, observational study was performed after approval by Stanford University’s Institutional Review Board (reference ID 52856) and submission to the National Clinical Trials Registry (NCT 04109365). After obtaining consent, patients rated their pain on a 0-10 scale (0 = no pain) immediately preceding epidural catheter placement during stage 1 of labor. After epidural catheter placement, EST was performed. The twitching of muscles innervated by the nerve root adjacent to the catheter can be triggered by a typical threshold current of 1.0–9.9 milliamperes. Twitching in the quadriceps indicates lumbar catheter placement. Twitching in the gluteus, hamstring, calf, or foot suggests sacral placement. Analgesic medicine was then delivered. 3 hours later, a 0-10 pain score and loss of cold sensation to ice were recorded. The primary outcome is to identify the efficacy of the EST in determining the epidural catheter's position for labor analgesia in relation to the patient's loss of cold sensation to ice 3 hours later. The secondary outcome was catheter success, defined as self-reported pain score <3 at 3 hours. A two-proportion z-test was used to assess the relationship between activated muscle groups and catheter success.

Results: Primary outcome - Of the 21 patients who completed this study, 19 reported loss of sensation to ice. In all 19, muscle twitching was observed during EST. Secondary outcome - Of the 21 patients who completed the study, 18 experienced twitching in their quadriceps (lumbar plexus group) and 3 experienced twitching in their gluteus, hamstring, calf, or foot (sacral plexus group). In the lumbar plexus group, 16 patients reported a 3-hour pain score <3. In the sacral plexus group, 1 patient reported a 3-hour pain score <3. Using a two-proportion z-test, p=0.0232.

Conclusion: The epidural stimulation test is a valid noninvasive technique to confirm lumbar epidural placement in laboring patients. EST-induced quadricep twitching is associated with better labor analgesia compared to gluteus, hamstring, calf, or foot twitching.

References: [1] Carvalho, B. Failed epidural top-up for cesarean delivery for failure to progress in labor: the case against single-shot spinal anesthesia, Int J Obstet Anesth 2012; 21: 357–9.
[2] Arendt, K., et al. Why epidurals do not always work, Rev Obstet Gynecol 2008; 1(2): 49-55.
[3] Tsui, B., et al. Confirmation of epidural catheter placement using nerve stimulation, Can J Anaesth 1998; 45(7): 640-4.

Early prediction of sub-optimal labor epidural analgesia using electrical stimulation test

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Early prediction of sub-optimal labor epidural analgesia using electrical stimulation test

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