A comparison of motor stimulation threshold in ultrasound-guided interscalene brachial plexus block for arthroscopic shoulder surgery: a randomized trial

Background: As the use of ultrasound for regional anesthesia has increased, many studies have examined the distribution of local anesthetic and the location of the needle tip. Nevertheless, the relationship between motor stimulation threshold and distribution of local anesthetic is unclear. The aim of this study was to compare block onset time, distribution of local anesthetic, and location of the needle tip at two different motor stimulation thresholds, i.e., 0.2 and 0.5 mA, used in combination with ultrasound guidance. Methods: This study included 94 patients undergoing arthroscopic shoulder surgery with ultrasound-guided interscalene brachial plexus block (ISBPB) plus nerve stimulation. Patients were randomized into two groups for the ISBPB procedure, i.e., when an evoked motor response was obtained at a current intensity of either 0.2 mA (Group 0.2) or 0.5 mA (Group 0.5). Block onset time, location of the needle tip, and distribution of local anesthetic were assessed. Results: A response was elicited at the appropriate motor stimulation threshold in 88 patients (Group 0.2 = 43; Group 0.5 = 45). Block failure occurred in only three patients, all of whom were in Group 0.5. The mean [standard deviation (SD)] of block onset time was 8.0 (4.1) min in Group 0.2 and 11.4 (5.9) min in Group 0.5 [mean difference, 3.4 min; 95% confidence interval (CI), 1.2 to 5.9; P = 0.003]. The needle tip was located at a intraplexus position in 33 (77%) patients in Group 0.2 and in 15 (33%) patients in Group 0.5 (difference in proportion, 43%; 95% CI, 23 to 59; P < 0.001). The intramuscular spreading of local anesthetic occurred in 0 (0%) patients in Group 0.2 and in 8 (18%) patients in Group 0.5 (difference in proportion, 18%; 95% CI, 6 to 31; P = 0.007). Conclusion: The onset time of the block was significantly faster with a motor stimulation threshold of 0.2 mA than with a threshold of 0.5 mA.