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 30 ml. Using the biased-coin design (BCD) sequential allocation method. The initial concentration of 30 ml 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 30 ml 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 30 ml 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).