Reactive hyperaemia in skin of the human foot measured by laser Doppler flowmetry: effects of duration of ischaemia and local heating.

The effect of duration of ischaemia and of local heating on reactive hyperaemia in dorsal foot skin was determined in normal subjects using laser Doppler flowmetry. In the nine subjects studied, peak post-ischaemic blood flow continued to increase significantly with up to 10 min ischaemia and mean peak response was positively correlated with duration of ischaemia (r = 0.997, p less than 0.0002). This is in contrast to whole limb plethysmographic studies which have reported maximal peak flow responses after 3-5 min arterial occlusion. Resting blood flow was found to increase rapidly at skin temperatures above 33 degrees C. In ten subjects, local conductive heating from a median (range) skin temperature of 29.8(26.5-31.0) to 32.5(32.2-33.1) degrees C increased peak flow response after 4 min ischaemia from a mean (95% confidence interval) of 0.778(0.630-0.926) to 0.965(0.788-1.142) arbitrary units, p less than 0.001. The time course of the hyperaemic response was not altered. Local skin warming reduced the within subject coefficient of variation for peak response, calculated from 10 paired recordings, in two out of three subjects. These results show that the duration of ischaemia required to produce a maximal post-ischaemic peak flow response is longer for foot skin than for the whole limb and are consistent with temperature differences being the cause of this. A standard skin temperature (32-33 degrees C) may be useful for studying cutaneous blood flow responses.