Somatosensory evoked potentials as an objective assessment of the sensory median nerve blockade after infraclavicular block.

Purpose: Median nerve somatosensory evoked responses (MnSSER) alterations were compared to clinical tests (cold and pinprick) variations, in 20 ASA I adult patients following infraclavicular block obtained with 40 mL ropivacaine 0.5% to assess first, the difference of time course of the respective electrophysiological and clinical signs, and second, the objectivity and the reproducibility of MnSSER changes.

Clinical Features: Four MnSSER derivations (Erb's point; cutaneous projection of peripheral end of brachial plexus; posterior neck at C6 level, frontal and controlateral parietal scalp) were monitored and recorded for retrospective analysis. Continuous data acquisition were started before ropivacaine injection (baseline) and maintained for 30 min thereafter. Every three minutes after ropivacaine injection, cold and pinprick tests were performed in the hand median nerve cutaneous supply zone and were assessed using a sensory visual score (varying from 0-10). Data were compared using analysis of variance. Although MnSSER values were stable during baseline period, after ropivacaine administration, severe progressive amplitude depressions of selected MnSSER were detected in every patient. While clinical cold and pinprick tests became positive (score > 8) only 15.8 +/- 1.2 min and 20.1 +/- 1.8 min respectively after ropivacaine administration, the mean time to observe the earliest MnSSER 20% amplitude decrease at Erb's point derivation was reduced to 5.6 +/- 1.1 min (P < 0.01).

Conclusion: Selected MnSSER amplitude reduction indicates objectively the onset of median nerve anesthesia following infraclavicular brachial plexus block before the appearance of clinical signs.