The Effect of Sex on the Minimum Local Analgesic Concentration of Ropivacaine for Caudal Anesthesia in Anorectal Surgery

BACKGROUND: Caudal anesthesia is routinely used in our hospital for most of ambulatory anorectal surgery; patients need to recover as quickly as possible. The dose of local anesthetic may be different for male and female patients. We designed this study to investigate the effect of sex on the minimum local anesthetic concentration (MLAC) 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 allocated them to 2 study groups according to their gender. 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 and laxity of the anal sphincter. The concentration change was 0.025%. The up-and-down sequences were analyzed using the Dixon and Massey method to quantify the caudal analgesic block effective concentrations in 50% of patients. RESULTS: The MLAC of ropivacaine for caudal analgesia was 0.296% (95% confidence interval, 0.286%-0.307%) in male patients and 0.389% (95% confidence interval, 0.372%-0.407%) in female patients (P < 0.01). CONCLUSIONS: We conclude that the ropivacaine MLAC for caudal anesthesia in female patients is 31% larger than in male patients. (Anesth Analg 2010;110:1490-3)