90% minimum effective concentration of ropivacaine for ultrasound-guided supra-inguinal fascia iliaca compartment block for postoperative analgesia following total hip arthroplasty： a dose-finding study

Background The ultrasound-guided supra-inguinal fascia iliaca compartment block (S-FICB) is a widely utilized regional anesthesia technique for postoperative pain management following total hip arthroplasty. Current evidence suggests that effective analgesia with S-FICB necessitates the administration of relatively high volumes of local anesthetic, which may consequently elevate the risk of local anesthetic toxicity and contribute to postoperative lower limb motor block. Methods Fifty-four patients with total hip arthroplasty were enrolled. Ultrasound-guided S-FICB was applied, the volume of the local anesthetic ropivacaine was set to 30ml. Using the biased-coin design (BCD) sequential allocation method. The initial concentration of 30ml ropivacaine was 0.25% and the interval concentration at 0.025%. Positive postoperative analgesia following S-FICB was defined by the simultaneous fulfillment of the following criteria: (1) The S-FICB block was successful. (2) The Visual Analogue Scale (VAS) scores maintained﹤4 at 6, 12, 24 h postoperatively. (3) The effective compression times of Patient-Controlled Intravenous Analgesia (PCIA) ﹤5 within 24 h postoperatively. Failure to meet any one of these criteria was classified as negative postoperative analgesia. If the previous patient had a positive postoperative analgesia, the next patient was randomized to a lower concentration (defined as the previous concentration with a decrement of 0.025%), with an 11% probability (b = 0.11), or it would be administered the same concentration, with an 89% probability (1-b = 0.89). If a negative postoperative analgesia occurred, the next patient received a higher concentration (defined as the previous concentration with a increment of 0.025%). The study was terminated when 45 positive postoperative analgesia were achieved. Results Forty-five patients (83%) were positive postoperative analgesia. The 90% minimum effective concentration (MEC90) of 30ml ropivacaine for ultrasound guided S-FICB for postoperative analgesia following total hip arthroplasty was 0.286 (95% CI, 0.242 ~ 0.338) and the 95% minimum effective concentration (MEC95) was 0.314 (95% CI, 0.241 ~ 0.410). Conclusion The MEC90 and MEC95 of 30ml ropivacaine for ultrasound guided S-FICB for postoperative analgesia were 0.286% and 0.314%, respectively. Trial registration The trial was registered at the Chinese Clinical Trial Registry (registration number: ChiCTR2400092413, registration date: 2024 November 15).