Evaluation of a noninvasive method for monitoring percutaneous absorption of lidocaine in vivo.

The pharmacodynamic measurement of in vivo skin penetration of lidocaine was explored with an instrument used in dentistry to determine tooth pulp vitality. The instrument delivers a low-current, pulsatile electrical waveform of increasing intensity with time. The readings, which are reproducible, are in arbitrary units on a scale of 0-80. Testing of naive sites showed variation as a function of location, even over relatively small distances. The response at a marked site over a 12-hr period generally was consistent in five subjects. Following intradermal administration of 1 or 2% lidocaine hydrochloride injection in one subject, the instrument reached its maximum value within 2 min. This was followed by a sustained plateau and then a gradual falloff of the effect. Topical formulations containing 5% lidocaine base and corresponding blank formulations were applied under occlusion within Hilltop chambers to intact skin on the forearms of human volunteers for 3 hr. While the response to a 40% propylene glycol formulation was not significantly different from the corresponding control, a cream exhibited slow development of profound anesthesia that lasted for several hours following chamber removal.