Reactive hyperaemia is the transient increase in organ blood flow that occurs following a brief period of ischaemia.

Reactive hyperaemia often occurs as a consequence of Raynaud's phenomenon, where the vasospasm in the vasculature leads to ischaemia and necrosis of tissue and thus a subsequent increase in blood flow to remove the waste products and clear up cell debris.

Moreover, the affected leg will pass through the normal pink colour to a red-range colour (often known as sunset foot) due to the dilatation of the arterioles in their attempt to remove the metabolic waste that has built up, through the phenomenon of reactive hyperaemia.