Determination of the EC_(50) of epidural ropivacaine for analgesia in labor

Objective The aim of this study was to determine the effective concentration of epidural ropivacaine for analgesia in 50% of parturients (EC50) in the first stage of labor. Methods Thirty-three ASA Ⅰ - Ⅱ nulliparous women with 2-3 cm cervical dilatation who requested epidural analgesia were enrolled in the study. Epidural catheter was placed at L3-4 and advanced for 3-4 cm in the epidural space in a cephalad direction. 20 ml of ropivacaine solution of which the EC50 was being tested was given through the catheter into epidural space. The EC50 was determined by up-and-down sequential experiment. The initial concentration of the epidural ropivacaine being tested was decided to be 0.15% . If effective the next parturient received ropivacaine of a lower concentration; if ineffective the concentration of ropivacaine was increased in the next parturient. Each time the concentration of epidural ropivacaine increased/decreased by 0.01% . Analgesia was assessed by VAS score (1-10) . If VAS score was less than 3 within 30 min of ropivacaine administration, analgesia was defined as effective.Results Among the 33 parturients enrolled, three were excluded from the study because of uncertain analgesic efficacy. The results showed that the EC50 of epidural ropivacaine for analgesia in the first stage of labor was 0.063% [95% confidence interval (CI) was 0.05972%-0.06688%] .Conclusion The EC50 epidural ropivacaine for analgesia in the first stage of labor is 0.063 % determined by sequential experiment method.