Background: For nerve stimulator-guided regional anesthesia, one has to compromise between a presumed low success rate (using a high-current threshold) and a presumed increased risk of nerve damage (using a low-current threshold). We hypothesized that high-current thresholds in the range of 0.9 to 1.1 mA are not inferior with respect to the procedural and latency times compared with low threshold currents in the range of 0.3 to 0.5 mA for nerve stimulation in brachial plexus blocks.
Methods: Two hundred five patients scheduled for elective surgery were randomized to a low (0.3-0.5 mA, n = 103) or a high (0.9-1.1 mA, n = 102) stimulation current threshold for the axillary plexus block with 40 mL local anesthetic mixture (20 mL, each of prilocaine 1% and ropivacaine 0.75%). The primary end point was the time to complete sensory block. The secondary outcome measures were the time to readiness for surgery (defined as the time from the start of block procedure to complete sensory block) and the block performance time. The noninferiority margin was set at 5 minutes and was evaluated using the two-sided 95% bootstrap-confidence intervals ([CIs] 100,000 replications) for differences in means.
Results: The mean times to complete sensory block revealed a significant decrease with the low-current group (17.9 ± 12.1 (mean ± SD) versus 22.8 ± 12.4 minutes; 95% CI, 1.1 to 8.6; p = 0.012). The time to readiness for surgery was 30.3 ± 13.8 minutes in the low-current group and 31.7 ± 12.9 minutes in the high-current group (95% CI, -2.7 to 5.5; p = 0.49). The performance time was significantly shorter in the high-current threshold group (9.5 ± 4.7 versus 11.9 ± 5.7 minutes; 95% CI, -4 to 1.1; p = 0.001).
Conclusion: Noninferiority for the high-current threshold technique could neither be confirmed for the primary end point nor for secondary end points. However, we consider a difference in mean times of approximately 8.5 minutes to achieve readiness for surgery acceptable for clinical practice.
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