The Effect of Sex on median effective concentration of ropivacaine for ultrasound-assisted caudal block in the elderly undergoing Anorectal Surgery

BACKGROUND: Caudal block is widely employed in pediatric lower abdominal surgeries and adult anorectal surgical by its simplicity of operation, reliable efficacy, and high safety. Previous studies have found that the dose of ropivacaine for Caudal block in adults exhibits gender differences. However, it remains unclear whether such differences in the median effective concentration (EC50) of ropivacaine also exist in the elderly population . METHODS: This is a double-blind, prospective study, We enrolled patients aged 60-80 years with ASA physical status I-? who were scheduled for anorectal surgery under caudal anesthesia, and allocated them to 2 study groups according to their gender. Each participant received a single injection of 20mL ropivacaine. Using Dixons up-and-down sequential allocation, the initial concentration of ropivacaine was set at 0.35% and the subsequent concentrations were determined by the analgesic response of the previous patients to the pinprick testing. The concentration change was 0.025%. The EC50 of ropivacaine in each group was determined using the the up-and-down method and probit regression. The primary outcome was the EC50 (95% confidence interval [CI]) of the 2 groups. Data on the surgical time, analgesic duration, and adverse events during surgery were also recorded. RESULTS: This study included a total of 40 elderly patients (20 male and 20 female). The EC50 of ropivacaine for caudal block in elderly male patients was 0.263% (95% CI: 0.179%-0.311%), while that in elderly female patients was 0.281% (95% CI: 0.161%-0.353%). The EC50 of ropivacaine for caudal block in elderly female patients was approximately 6.4% higher than that in elderly male patients. CONCLUSIONS: There is a significant gender difference in the EC50 of ropivacaine for caudal block among the elderly, with elderly female requiring a higher EC50 than male.