Backgrounds and Objectives Chronaxie of the motor-neurons (A-α) is shorter than that of the sensory A-δ 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.