Importance of volume and concentration for ropivacaine interscalene block in preventing recovery room pain and minimizing motor block after shoulder surgery.
This three-staged study estimated the volume and concentration of interscalene ropivacaine that would prevent recovery room pain after shoulder surgery under general anesthesia. Stages 1/2: Interscalene catheter administration of ropivacaine was by a 10% incremental up-down sequential manner depending on the presence of recovery room pain in the previous patient. Stage 1: Ropivacaine (0.5% volume) was varied from 30 ml. Stage 2: Ropivacaine (20 ml, the ED(volume)95 estimate from stage 1) concentration was varied from 0.45%. Stage 3: Subjects were randomly assigned to receive 30 ml of ropivacaine, 0.5% ("conventional dose"), or 20 ml of ropivacaine, 0.375% (the estimated ED(volume+concentration)95 from stages 1/2). A postoperative elastomeric infusion of 0.2% ropivacaine (2 ml/h) was administered. Grip strength was measured in the recovery room and time to first pain at 24 h. Stage 1 (n = 34): Ropivacaine 0.5% ED(volume)50/ED(volume)95 (95% CI) estimates were 2.7/20.5 ml (2.4-9.5/17.3-25.8). Stage 2 (n = 29): Ropivacaine 20 ml ED(concentration)50/ED(concentration)95 (95% CI) estimates were 0.15/0.34% (0.13-0.30/0.29-0.43). The ED(dose)50 was similar for stages 1/2 (13.5 vs. 30 mg), but the ED(dose)95 was higher for stage 1 (102.5 vs. 68 mg). Stage 3 (n = 40): Satisfaction (0-10) was modestly higher for the new/lower dose (median [interquartile range] = 10 [10-10] versus 9 [8-10], P = 0.007). Pooled data regression analysis showed that increasing ropivacaine concentration increased grip weakness but not block duration. Ropivacaine interscalene block requires a threshold volume and concentration, with concentration primarily determining motor block. When combined with continuous blockade, suprathreshold ropivacaine doses do not significantly prolong primary block duration but may compromise patient satisfaction.