Association of obesity with ropivacaine and sufentanil EC50 in labor analgesia: a single-center prospective study

Objective: In part I, measure the EC50 of sufentanil in obese and non-obese parturients combined with 0.1% ropivacaine and compare the differences. Similarly, in part II, measure the EC50 of ropivacaine in obese and non-obese parturients combined with 0.5 µg/ml sufentanil and compare the differences.

Methods: This study comprises two parts, with an initial intention to enroll 120 full-term primiparous women who underwent vaginal delivery and sought epidural analgesia. Each part includes an obese group (OA group, Obese Adults, defined as prepartum BMI≥29 kg/m²) and a non-obese group (CON group, Control group, defined as 18.5<prepartum BMI<28 kg/m²), with 30 participants in each. Both parts, for both obese and non-obese women, utilized an initial concentration of 0.1% ropivacaine with 0.5 µg /ml sufentanil. The initial concentration of sufentanil is 0.5 µg/ml. When the NRS score is ≤3 within 30min after analgesia, it is considered effective analgesia, and the concentration of sufentanil in the next parturients decreases by 0.05 µg/ml. Otherwise, the concentration of sufentanil in the next parturient will increase by 0.05 µg/ml. The second part uses the same method to measure the EC50 of ropivacaine (increase or decrease by 0.01%) in the OA group and CON group combined with 0.5 µg/ml sufentanil, with 30 participants in each group. EC50 measurements were performed through up-and-down sequential allocation, with effective analgesia defined as an NRS score ≤3, 30 min post-analgesia.

Results: In part I, the EC50 of epidural sufentanil in the OA group was 0.090 µg/ml (95% CI, 0.061~0.115µg/ml), and in the CON group, it was 0.170µg/ml (95% CI, 0.117~0.219µg/ml). In part II, the EC50 of epidural ropivacaine in the OA group was 0.048% (95% CI, 0.041~0.053%), and in the CON group, it was 0.070% (95% CI, 0.064~0.075%). The secondary outcomes in both parts of the study showed no statistically significant differences.

Conclusion: Obese parturients exhibited significantly lower EC50 values for ropivacaine and sufentanil compared to non-obese parturients. Lower concentrations of both agents can be considered for labor analgesia in obese parturients.