Neuroselective sensory electrodiagnostic evaluation of 4% liposomal topical lidocaine.

UNLABELLED We used a neuroselective transcutaneous electrical stimulus to determine the onset time of cutaneous anesthesia with 4% liposomal lidocaine under occluded and nonoccluded conditions. The pain tolerance threshold (PTT) was used to atraumatically evaluate nociception. Twenty adult volunteers had liposomal lidocaine applied to the volar surface of each forearm for durations ranging from 0 through 30 min (at 5-min intervals) under occluded and nonoccluded conditions. The PTT was determined using three different frequencies (2000 Hz, 250 Hz, 5 Hz) stimulating A beta, A delta, and C fibers, respectively. The time to reach the maximum PTT achieved defined the anesthetic onset time for each frequency. A differential onset of cutaneous anesthesia among the three frequencies was clearly demonstrated, however there was no significant difference in onset time between occluded and nonoccluded conditions. Blockade of C fiber transmission occurred significantly earlier than that of A delta (P = 0.029), which occurred earlier than that of A beta (P = 0.001) as determined using the Wilcoxon's signed rank test. We conclude that a mean onset time of approximately 4 +/- 2 min for blockade of C fiber transmission and 6 +/- 4 min for A delta fiber transmission suggests that painful stimuli such as venipuncture may be attenuated as early as 7 min. IMPLICATIONS An average onset time of approximately 7 min for topical 4% liposomal lidocaine was determined using neuroselective neurostimulation. This suggests that procedural pain such as venipuncture may be attenuated in this time frame. This technique requires correlation with conventional sensory measurements.