The epidural electric stimulation test does not predict local anesthetic spread or consumption in labour epidural analgesia

The epidural electrical stimulation test (EEST) is a highly specific and sensitive test for confirming placement of the epidural catheter in the epidural space. The purpose of this study was to investigate if the EEST could predict the spread and consumption of local anesthetic solutions during labour epidural analgesia.

This observational study was conducted in labouring parturients requesting epidural analgesia. The EEST was performed after the epidural catheter placement (T0) and repeated five minutes after a test dose with 2% lidocaine 3 mL (T1). The minimum current required to elicit the motor response at each time point was recorded. A loading dose of 0.125% bupivacaine 10 mL and fentanyl 50 μg was administered and followed by patient-controlled epidural analgesia with 0.0625% bupivacaine and fentanyl 2 μg·mL⁻¹ (baseline infusion 10 mL·hr⁻¹, bolus dose 5 mL, lockout interval ten minutes, maximum dose 20 mL·hr⁻¹). The primary outcome was the correlation between the current required to elicit motor responses at T0 and T1 and the consumption of bupivacaine in the first two hours of epidural administration. The secondary outcomes included the muscle contraction patterns determined by the EEST and the incidence of failed, inadequate, or asymmetric blocks.

The study was conducted in 102 parturients. The mean electric current required to elicit muscle response was 4.43 mA (range 1-10 mA) at T0, 5.97 mA (range 1-14 mA) at T1, and the mean Δ (T1-T0) current was 1.54 mA (range 0-8 mA). There was no correlation between either the mean baseline current required or the Δ (T1-T0) current and the total bupivacaine consumption at two hours. The incidence of inadequate blocks at two hours was 18%; however, none of the catheters required replacement. Unilateral left (34%) or right (31%) leg contraction was the most frequent pattern elicited by the EEST.

The EEST shows a wide range of electrical current requirements and elicits a variety of muscle twitch patterns on the lower limbs. Although it confirms the epidural placement of the catheter, the EEST cannot be used to predict the spread or consumption of the local anesthetic solution during labour epidural analgesia.