Effect of impulse duration on patients' perception of electrical stimulation and block effectiveness during axillary block in unsedated ambulatory patients

Backgrounds and Objectives: Chronaxie of the motor-neurons (A-alpha) is shorter than that of the sensory A-delta and C neurons. Therefore, a short current impulse should elicit a painless muscle twitch. This randomized, double-blind study of patients having ambulatory axillary block by multiple neurostimulations compared patients' perception of electrical stimulation, latency, and quality of analgesia and the incidence of adverse effects. Methods: In group S (short impulse, n = 44) 0.1-ms-current impulses were used and in group L (long impulse, n = 43) 0.3-ms impulses were used. Initial amplitude was 2 mA. Local anesthetic was injected near the 4 terminal nerves (musculocutaneous, median, ulnar, radial) after reaching a target amplitude between 0.1 and 0.5 mA. Patients were specifically requested to categorize sensation of electrical stimulation "electric shocks" as follows: no discomfort, discomfort, pain. Pain was then quantified on a visual analog scale (VAS). Surgically ineffective blocks were supplemented after 30 minutes. A patient was defined as ready for surgery (complete block) when analgesia was present in all areas distal to the elbow. Results: There were no significant differences between groups in quali- and quantitative assessments of electrical stimulation. Eight patients (18%) in either group described the sensation as "strange or funny." Eight patients in group S and 13 (30%) in group L reported discomfort during stimulation. Twenty-eight patients (64%) in S group and 22 (52%) in L group experienced pain. Median VAS (0 to 100) of this pain was 21 and 24, respectively. Block performance time was 9 minutes in L group and 11 minutes in S group (P < .001), but the latency of analgesia was 23 minutes for both groups and the times to achieve complete block were, therefore, similar: 32 minutes in L group and 34 minutes in S group (not significant [NS]). Nine group S and 8 group L patients required supplementary blocks (NS). The incidence of vessel punctures and accidental intravascular injections were also similar in both groups. Conclusions: This study did not confirm our hypothesis that short-current impulses (0.1 ms) make neurostimulation of peripheral nerves painless, by selectively depolarizing motor-neurons. Longer impulses (0.3 ms) shorten block performance time, probably by easier location of the nerves, but the clinical relevance of this finding is doubtful.