The delay in onset of vasodilator flare in human skin at increasing distances from a localized noxious stimulus.

Flare was measured on the arm of human subjects at 8, 16, and 24 mm from localized areas of skin heating, using laser-Doppler flowmetry. Vasodilatation started after a delay that averaged 3.2 sec at 8 mm and increased significantly by 0.4 sec at 16 mm and by 1.1 sec at 24 mm. In contrast, there were no significant changes in onset delay associated with changes in the amplitude of the heat stimulus. Flare appears to spread more slowly than would be expected if rate of spread were determined only by conduction delays in unmyelinated nerve terminals. This finding is discussed in relation to models of flare that involve coupling between adjacent nerve terminals.