Ropivacaine-dexmedetomidine PIEB optimal volume for labor analgesia estimated at **11.0 mL** for 90% efficacy.

Effective labor analgesia is crucial for maternal comfort and safety, with epidural analgesia being a gold standard. The programmed intermittent epidural bolus (PIEB) technique has emerged as a superior method to continuous epidural infusion, offering improved analgesia and reduced motor block. While combinations of local anesthetics like ropivacaine with adjuvants such as dexmedetomidine are widely used to enhance efficacy and reduce side effects, the precise optimal PIEB volume for this specific drug combination remains undefined. This knowledge gap can lead to suboptimal dosing, affecting both analgesic efficacy and obstetric outcomes.

This randomized dose-response study enrolled 80 parturients requesting labor epidural analgesia. All participants received a 20 mL loading dose of 0.0625% ropivacaine mixed with 0.4 μg/mL dexmedetomidine via PIEB. One hour post-loading, parturients were randomized into four groups receiving maintenance PIEB volumes of 6, 8, 10, or 12 mL of the same formulation, administered at 45-minute intervals. The primary endpoint was effective epidural labor analgesia, defined as the absence of need for patient-controlled or manual analgesic supplementation within 6 hours after the loading dose or until full cervical dilation. Probit regression was used to estimate effective volumes.

Analysis included 79 parturients. The effective analgesia rates significantly increased with higher PIEB volumes: 50% in the 6 mL group, 63.2% in the 8 mL group, 75% in the 10 mL group, and 100% in the 12 mL group. Probit regression estimated the effective volume for 50% of parturients (EV50) at 6.5 mL (95% CI: 3.7-7.7), and the effective volume for 90% of parturients (EV90) at 11.0 mL (95% CI: 9.7-14.8).