Background and Objectives We tested the hypothesis that the duration of cutaneous anesthesia elicited by the permanently charged compound N-phenylethyl lidocaine (tonicaine) would be longer than that elicited by its parent structure, lidocaine, and that it would be less affected by epinephrine (epi), after subcutaneous injection in rats, as a model for infiltration anesthesia.

Methods Subcutaneous injections were performed on the shaved dorsal skin of rats with either tonicaine or lidocaine (0.1% or 0.5%, n = 8 in each group) with and without epi (1:200,000). Inhibition of the cutaneous trunci muscle reflex was quantitatively evaluated by a blinded observer by the number of times pinpricks failed to elicit the nocifensive motor response out of a total of 6 pinpricks applied to the injected area.

Results Duration of complete nociceptive blockade in the 0.5% tonicaine and lidocaine groups was 619 ± 47 and 58 ± 2 minutes, respectively; duration of full recovery in these groups was 1,106 ± 19 and 86 ± 3 minutes, respectively. Epi increased the duration of complete block in the 0.5% tonicaine and lidocaine groups to 750 ± 13 and 97 ± 11 minutes, respectively, and the duration of full recovery to 1,185 ± 13 and 172 ± 6 minutes, respectively. Skin toxicity was seen only in the 0.5% tonicaine with epi group (3 of 8 rats).

Conclusions Tonicaine is a substantially longer lasting local anesthetic with a delayed onset of action compared with lidocaine and may be useful in situations where long duration of infiltration block is desirable.