Effective background infusion rate of ropivacaine 0.2% for patient-controlled interscalene brachial plexus analgesia after rotator cuff repair surgery

Background Continuous interscalene nerve block （CISB） is considered to be the most effective method for postoperative analgesia after shoulder surgery with prolonged severe pain.This study was performed to evaluate the minimum effective background infusion rate and the effective background infusion rate of ropivacaine 0.2％ for CISB after arthroscopic rotator cuff repair surgery in 95％ of patients.Methods Patients scheduled for arthroscopic rotator cuff repair surgery under general anesthesia at Peking University Third Hospital were prospectively enrolled from December 2011 to May 2012.Preoperatively,an interscalene catheter （ISC） was placed under the guidance of ultrasound and nerve stimulation in each patient.Consecutively,30 patients with successful nerve block were included.A continuous infusion of ropivacaine 0.2％ with a 5 ml patient-controlled bolus available hourly was started at postoperative anesthesia care unit （PACU） after completion of surgery.The initial background infusion rate was 6 ml/h,which was subsequently varied for each consecutive patient according to the analgesic effects of the previous one.The minimum effective background rate was determined using the Dixon and Massey up-and-down method.The effective background rate in 95％ of patients was calculated using isotonic analysis.Results The minimum effective background rate based on the Dixon and Massey up-and-down method was 2.8 ml/h （95％ CI,2.3-3.3 ml/h）.The effective background rate in 95％ of patients calculated with the isotonic regression analysis was 4.4 ml/h （95％ CI,3.8-6.5 ml/h）.Conclusion The effective background rate for patient-controlled interscalene brachial plexus analgesia after shoulder surgery in 50％ and 95％ of the patients was 2.8 and 4.4 ml/h,respectively.