Effect of topically applied local anaesthesia on histamine flare in man measured by laser Doppler velocimetry.

The direct and indirect effects of histamine on the cutaneous microvasculature were measured by laser Doppler velocimetry. Histamine (6.51 x 10(-4) M) was injected intradermally into the forearms of eight healthy subjects following treatment with a topically applied local anaesthetic cream (EMLA) or equivalent placebo. The blood flow at the injection site (0 cm) and at 1 and 2 cm proximally was measured by laser Doppler velocimetry over 80 minutes. Analysis of the changes in magnitude of the hyperaemic responses with time showed no difference at the 0 and 1 cm sites, but a marked reduction was found at the 2 cm site following EMLA treatment (p less than 0.05). At all three sites the decay of the histamine-induced hyperaemia was faster following EMLA treatment than with placebo (p less than 0.04). The experiments showed that the indirect effect of histamine on the cutaneous microvasculature in the peripheral flare around the injection site was greatly diminished by prior application of EMLA cream and this supports the neurogenic hypothesis: pre-treatment with EMLA cream did not affect the development of hyperaemia and oedema at the site of histamine injection where the mediator acts directly on the cutaneous microvasculature.