The diaphragm-sparing effect of interscalene block with a low-volume of ropivacaine 0.1% vs. 0.5%

BACKGROUND Despite improvements, the incidence of hemidiaphragmatic paralysis following interscalene brachial plexus block (ISB) is still significant at 20–40%. OBJECTIVES We tested the hypothesis whereby the use of a low volume of ropivacaine 0.1% reduces the incidence of hemidiaphragmatic dysfunction (compared with the same volume of ropivacaine 0.5%), while providing sufficient analgesia following arthroscopic shoulder surgery. DESIGN Randomised controlled trial. SETTING A tertiary university hospital. PATIENTS Sixty patients undergoing shoulder arthroscopy under general anaesthesia. INTERVENTION Ultrasound-guided ISB with 7 ml of ropivacaine 0.1% or ropivacaine 0.5%. MAIN OUTCOME MEASURES The primary outcome was the incidence of hemidiaphragmatic dysfunction (a decrease in diaphragm excursion >25% of baseline) 30 min after ISB. The secondary outcomes included hemidiaphragmatic paralysis (a decrease in diaphragm excursion >75% of baseline), spirometry variables and postoperative analgesia. RESULTS Hemidiaphragmatic dysfunction occurred in 7/30 (23%) patients in the experimental group and in 29/30 (97%) patients in the control group, P < 0.001; relative risk (95% CI): 0.24 (0.13 to 0.46). Hemidiaphragmatic paralysis occurred in one patient in the experimental group and 25 of the patients in the control group ( P < 0.001). The median [IQR range] decrease (%) in forced vital capacity was −17 [−23 to −5] in the experimental group and −29 [−42 to −6] in the control group; P = 0.039). The difference in the median duration of effective analgesia between the experimental and control groups was not significant (9.8 h and 15.5 h, respectively; P = 0.23; Log-rank test; mean (95%CI) difference in restricted mean survival time: -3.2 h (-6.4 to 0.2); ( P = 0.061), but more patients required morphine postoperatively in the experimental group (10/30 vs. 3/30; P = 0.028). CONCLUSIONS ISB with 7 ml of ropivacaine 0.1% is associated with a lower incidence of hemidiaphragmatic dysfunction, when compared with the same volume of ropivacaine 0.5%, but at the cost of potentially less effective postoperative analgesia. TRIAL REGISTRATION https://clinicaltrials.gov/ct2/show/NCT04173364