Ultrasound-guided interscalene needle placement produces successful anesthesia regardless of motor stimulation above or below 0.5 mA

BACKGROUND: We quantified the motor response after ultrasound (U-S)-guided needle placement for interscalene block (ISB). We then compared block characteristics based on motor response above or below 0.5 mA. METHODS: Sixty-one patients scheduled for ambulatory shoulder surgery under ISB and general anesthesia were included in this prospective, observational study. Preoperatively, an insulated needle was positioned by U-S in the interscalene groove. The lowest current producing motor response was determined, and 30 mL 0.5% bupivacaine with epinephrine was injected. Motor and sensory block were tested in the upper trunk distribution for 15 min until general anesthesia was induced. Postoperatively, the success of upper trunk block, pain score in the postanesthesia care unit and block duration, and analgesic tablet consumption overnight were recorded. Patients were divided a priori into Group A (current <= 0.5 mA) and Group B (current >0.5 mA), and results were compared between groups. RESULTS: The observed current range was 0.14-1.7 mA, with current <= 0.5 mA in 42% of patients (Group A). All patients had complete sensorimotor upper trunk block and none required narcotics in the postanesthesia care unit. Block duration (both groups: 17.8 +/- 4.9 h, mean +/-SD) and home analgesic use were equivalent. Sensory block onset was equivalent between groups, but incomplete motor block at 15 min was more likely in Group B: 37% vs 12% in Group A (P = 0.03). CONCLUSION: During U-S-guided ISB using nerve stimulation, the observed motor response below or above 0.5 mA had no impact on success or duration of upper trunk block.