Topical amethocaine gel in the newborn infant; how soon does it work and how long does it last?

Aim-To explore the time of onset and duration of action of topical amethoca ine gel in the newborn infant. Design-A randomised double blind placebo controlled trial. Subjects-Thirty six infants were studied after 30 minutes application and 3 6 after 60 minutes application. A total of 56 infants (gestation 27-42 week s, weight 0.79-4.1 kg) were studied in the first two weeks after delivery. Method-1.5 g amethocaine or placebo was applied to the dorsum of either foo t, occluded, and then left for 30 or 60 minutes. Local anaesthesia was asse ssed by observing the cutaneous withdrawal response to graded nylon filamen ts (von Prey hairs). If there was a difference between feet in filament thi ckness required to elicit a response, the infant was studied in an identica l manner at hourly intervals until the difference had disappeared. Results-Evidence of local anaesthetic action of amethocaine was seen in 23 of 36 (64%) infants after 30 minutes and 26 of 36 (72%) infants after 60 mi nutes application (no significant difference). Evidence of local anaestheti c action was independent of gestation and order of testing. Amethocaine res ponders showed a significantly deeper anaesthetic action than placebo respo nders. The median duration of action was 1.5 hours (range 0.5-3.5) after 30 minutes and three hours (range 1-5) after 60 minutes (p < 0.001). Conclusion-Topical amethocaine gel has a local anaesthetic action after 30 minutes application, but application for 60 minutes results in longer durat ion of action.