Development of a contraction-based pain index (CONPAIN) to evaluate labor epidural analgesia effectiveness: a pilot proof-of-concept study

Background: Assessment of labour pain is essential to guide neuraxial analgesia, yet current tools such as the Numeric Rating Scale (NRS) are influenced by subjective variability. We developed and tested the Contraction Pain Index (CONPAIN), calculated as the percentage ratio of self-reported pain duration to contraction duration measured on external cardiotocography. We hypothesised that CONPAIN would show an early relative change from baseline than NRS in the first 10 minutes after dosing. Methods: In this exploratory, single-center pilot study, CONPAIN and NRS were recorded in 27 nulliparous women in active labor before and after initiation of epidural analgesia. Data collection started five minutes before the initial epidural bolus and continued for 20 min thereafter. The primary outcome was the temporal trend of CONPAIN relative to baseline; secondary outcome was the comparison of percentage change from baseline between CONPAIN and NRS and their correlation over time. Results: Both metrics decreased within the first five minutes. Median percentage change at 0–5 minutes was −27.96 % for CONPAIN vs. −7.14 % for NRS (P < 0.001); at 6–10 minutes, −48.99 % vs. −40.00 % (P = 0.027). Differences were not significant after 10 minutes. Conclusions: In this proof-of-concept study, CONPAIN showed greater relative changes compared to NRS during the first 10 minutes following initiation of labour analgesia, suggesting potential value in characterising the early dynamics of analgesic response. While these findings offer preliminary support for its clinical relevance, validation in larger populations is required.