Comparison of 3 Intensities of Stimulation Threshold for Brachial Plexus Blocks at the Midhumeral Level

We conducted this prospective randomized study to compare the success rate and the onset time between 3 intensities of stimulation threshold (0.5, 0.5-0.64, and 0.65-0.8 mA) when using a peripheral nerve stimulation at the midhumeral level.Sixty-nine adult patients undergoing elective hand surgery were studied. Blocks were performed using conventional nerve stimulation technique. Needle advance began at 2 mA (1 Hz, 0.1 millisecond). When motor response (MR) occurred at less than 0.5 mA, needle position was fixed for "group0.5 mA." For "group 0.5-0.64 mA," the needle was withdrawn until MR occurred at greater than 0.5 mA and disappeared at less than 0.5 mA. For "group0.65 mA," the needle was withdrawn until MR occurred at greater than 0.65 mA and disappeared at less than 0.65 mA. For each group, patients received 8 mL of ropivacaine 7.5 mg/mL on the 4 nerves (radial, median, ulnar, and musculocutaneous). Primary end point was the number of failed radial nerve sensory blocks at 30 mins.The time to perform the block was not different between the 3 groups (17 mins [SD, 7 mins] vs 13 mins [SD, 8 mins] and 13 mins [SD, 4 mins], respectively). The time required to obtain a complete sensory block was shorter for the 4 nerves in group0.5 mA, with a statistical significance for radial and musculocutaneous nerves in group0.5 mA versus group 0.5-0.64 mA and group0.65 mA. Patients in group0.5 mA had a greater success rate for complete sensory radial nerve compared with those of group 0.5-0.64 mA and group0.65 mA at any interval times between 5 and 30 mins (P = 0.0001). Supplemental local anesthesia was provided for the 3 groups more frequently for the median nerve, with no difference between groups. Group0.65 mA required 5 general anesthesias (20%) as compared with 1 (4%) in group0.5 mA (P0.05). No adverse event (dysesthesia) occurred after 48 hrs and 45 days.We conclude that intensity of stimulation influenced onset time and success rate.