[Local anesthesia after percutaneous administration. I]

Local anesthesia of the intact skin is difficult because of the skin barrier to epicutaneous penetration. Using solutions of local anesthetics in organic agents, which have the ability of penetrating the skin without causing irreversible damage and enhancing the percutaneous absorption of all materials dissolved therein, topical anesthesia seems to be attainable. A satisfactory method for determining pain threshold in uninjured skin of animals has been set up. Measurments of the pricking pain threshold have been made by exposing the skin of guinea-pigs to defined mechanical and electrical stimuli. A nociceptive muscle reflex (twitch) has been taken as the index of pain sensation. The suppression of this twitch has been used as an indicator of anesthetic potency. Two methods have been applied with varying parameters: stimulation at a fixed intensity until the pain threshold was reached (duration of anesthetic effect) and stimulation with increasing stimulus strenght until the cutaneous reflex was elicited (intensity or "depth" of anesthesia). The local anesthetic effects of lidocaine, fomocaine and procaine bases were studied, after dissolving them in dimethyl sulfoxide (DMSO), a wellknown carrier for transmembranal application. The duration of local anesthesia measured by electrical stimuli was longer than that obtained with mechanical ones. Differences are discussed. All results indicate that fomocaine (5%) has a greater local anesthetic potency than procaine (5%) but both are less active than lidocaine (5%). No effect could be seen after application of DMSO alone.