Abstract: AIM: To investigate the effect of sex on the median effective concentration (EC₅₀) of ropivacaine for caudal anesthesia.METHODS: In this double-blind, prospective study, we enrolled 70 ASA physical status I patients (35 male and 35 female) who were scheduled for anorectal surgery under caudal anesthesia, and classified them into two study groups (Group M and Group F) on the basis of their gender difference. Each participant received a single injection of 20 mL ropivacaine through a caudal catheter. Using Dixon's up-and-down sequential allocation, the first participant received 0.2% and subsequent concentrations were determined by the analgesic response of the previous patients to the initial skin incision. The concentration change was 0.025%. The up-and-down sequences were analyzed using the Dixon and Probit methods to quantify the caudal analgesic block effective concentrations in 50% of patients.RESULTS: From Dixon method, The EC₅₀ values of ropivacaine for caudal analgesia were 0.296%(95% CI, 0.286%-0.307%) in male patients and 0.389% (95% CI, 0.372%-0.407%) in female patients. From Probit analysis, the EC₅₀ values ropivacaine in Group M and Group F were 0.295%(95% CI, 0.173%-0.325%), 0.382% (95% CI, 0.352%-0.405%) respectively and EC₉₉ was 0.364% (95% CI, 0.330%-1.313%), 0.422% (95% CI, 0.400%-0.646%) respectively. EC₅₀ values of ropivacaine was significantly higher in Group F as compare with Group M (P<0.01).CONCLUSION: We concluded that the ropivacaine EC₅₀ values for caudal anesthesia in female patients is 31% higher as compare to the male patients. The present findings provide suitable suggestion to anesthesiologists about ropivacaine concentration adjustment for caudal anesthesia based on gender difference.

Key words: caudal anesthesia, sex differences, ropivacaine, sequential allocation