Regional thermal hyperemia in the human leg: Evidence of the importance of thermosensitive mechanisms in the control of the peripheral circulation

Hyperthermia is thought to increase limb blood flow through the activation of thermosensitive mechanisms within the limb vasculature, but the precise vascular locus in which hyperthermia modulates perfusion remains elusive. We tested the hypothesis that local temperature‐sensitive mechanisms alter limb hemodynamics by regulating microvascular blood flow. Temperature and oxygenation profiles and leg hemodynamics of the common (CFA), superficial (SFA) and profunda (PFA) femoral arteries, and popliteal artery (POA) of the experimental and control legs were measured in healthy participants during: (1) 3 h of whole leg heating (WLH) followed by 3 h of recovery (n = 9); (2) 1 h of upper leg heating (ULH) followed by 30 min of cooling and 1 h ULH bout (n = 8); and (3) 1 h of lower leg heating (LLH) (n = 8). WLH increased experimental leg temperature by 4.2 ± 1.2ºC and blood flow in CFA, SFA, PFA, and POA by ≥3‐fold, while the core temperature essentially remained stable. Upper and lower leg blood flow increased exponentially in response to leg temperature and then declined during recovery. ULH and LLH similarly increased the corresponding segmental leg temperature, blood flow, and tissue oxygenation without affecting these responses in the non‐heated leg segment, or perfusion pressure and conduit artery diameter across all vessels. Findings demonstrate that whole leg hyperthermia induces profound and sustained elevations in upper and lower limb blood flow and that segmental hyperthermia matches the regional thermal hyperemia without causing thermal or hemodynamic alterations in the non‐heated limb segment. These observations support the notion that heat‐activated thermosensitive mechanisms in microcirculation regulate limb tissue perfusion during hyperthermia.

The present study investigated the effects of full and segmental leg heating on regional limb temperature, haemodynamics and tissue oxygenation. We found that prolonged whole leg hyperthermia induces a sustained and profound elevation in upper and lower leg blood flow, and that segmental leg hyperthermia matches the regional hyperaemia during whole leg hyperthermia, independent of changes in temperature, blood flow or tissue oxygenation of the non‐heated segment. Increased blood flow occurred without changing perfusion pressure or mean conduit artery diameter but were concurrent with elevations in local tissue oxygenation, blood velocity, flow rate, vascular conductance and shear rate, which were all positively related with the rise in local temperature. These observations support that local hyperthermia increases peripheral tissue blood flow through heat‐activated thermosensitive mechanisms in the microcirculation.