Effect of local warming on forearm reactive hyperaemia

Summary. Measurement of minimal vascular resistance has proved useful in quantifying structural changes in regional circulations. Accurate measurement of minimal vascular resistance requires full relaxation of all resistance vessels within the region under examination. The usual procedure in humans involves the measurement of maximal forearm blood-flow following 6–10 min of forearm ischaemia. We conducted this study to find whether forearm skin was fully vasodilated by this procedure. Peak forearm blood-flow was measured by plethysmography in six healthy subjects following 10 min of ischaemia while the arm was at a neutral temperature (33°C) and while the arm was locally warmed to 42°C. Peak reactive hyperaemia blood-flow was significantly elevated by local heating (P<0·001) to 79·6 ml100ml^-1min^-1 from a value of 50·2 ml100ml^-1min^-1 during normothermia. Peak reactive hyperaemia blood-flow in the contralateral unheated forearm showed no significant change between the two periods of ischaemia (P>0·05). These findings were confirmed in four subjects by laser Doppler velocimetry, which gives a linear index of skin blood-flow. In normothermic conditions, this index rose to 0·89 V following 10 min of ischaemia and to 1·26 V with local warming to 42°C (P<0·001). Ischaemia plus local warming did not cause a further significant rise in this index of skin blood-flow (1·35 V, P>0·05). These data suggest that 10 min of ischaemia during normothermia is insufficient to relax fully cutaneous resistance vessels and that maximal forearm blood-flow is underestimated by this procedure.