Respiratory Variation Index as a marker of labour pain intensity with epidural analgesia: a pilot exploratory study

Pain assessment during labour analgesia mainly relies on patient self-report. However, this scoring method often delays pain data acquisition by anaesthesiologists, leaving parturients exposed to painful sensations for longer periods. This study aimed to validate an objective index for monitoring pain intensity during uterine contractions, to provide a reference for determining the timing of patient-controlled or manual rescue analgesia.

This prospective observational study was conducted between August and September 2025. Twenty-one labouring women were enrolled, and data including heart rate, Surgical Pleth Index, respiratory rate variability, respiratory amplitude variability and numerical rating scale scores were collected across 137 uterine contraction episodes. Spearman's rank correlation, receiver operating characteristic curve analysis and logistic regression were used to identify variables capable of differentiating no-to-mild pain from moderate-to-severe uterine contraction pain. Respiratory Variation Index was constructed using beta coefficients derived from logistic regression modelling.

Data from 137 uterine contraction episodes from labour women were analysed. Respiratory rate variability and respiratory amplitude variability correlated with uterine contraction pain and demonstrated good diagnostic performance for identifying moderate-to-severe contraction pain. Both variables were independently associated with moderate-to-severe pain (respiratory rate variability: odds ratio = 1.193, 95% confidence interval 1.100-1.293, β = 0.176, P < 0.001; respiratory amplitude variability: odds ratio = 1.067, 95% confidence interval 1.031-1.122, β = 0.073, P = 0.001). Respiratory Variation Index was calculated from these two variabilities. The area under the receiver operating characteristic curve for discriminating moderate-to-severe pain and severe uterine contraction pain was 0.853 (95% confidence interval 0.785-0.922, P < 0.001) and 0.748 (95% confidence interval 0.665-0.831, P < 0.001), respectively.

Respiratory Variation Index exhibits optimal diagnostic performance for stratifying moderate-to-severe or severe uterine contraction pain, and with refinements and further validation, may serve as a tool to guide the timing rescue epidural analgesia.