Temperature and blood flow distribution in the human leg during passive heat stress

Abstract

The influence of temperature on the hemodynamic adjustments to direct passive heat stress within the leg's major arterial and venous vessels and compartments remains unclear. Fifteen healthy young males were tested during exposure to either passive whole body heat stress to levels approaching thermal tolerance [core temperature (Tc) + 2°C; study 1; n = 8] or single leg heat stress (Tc + 0°C; study 2; n = 7). Whole body heat stress increased perfusion and decreased oscillatory shear index in relation to the rise in leg temperature (Tleg) in all three major arteries supplying the leg, plateauing in the common and superficial femoral arteries before reaching severe heat stress levels. Isolated leg heat stress increased arterial blood flows and shear patterns to a level similar to that obtained during moderate core hyperthermia (Tc + 1°C). Despite modest increases in great saphenous venous (GSV) blood flow (0.2 l/min), the deep venous system accounted for the majority of returning flow (common femoral vein 0.7 l/min) during intense to severe levels of heat stress. Rapid cooling of a single leg during severe whole body heat stress resulted in an equivalent blood flow reduction in the major artery supplying the thigh deep tissues only, suggesting central temperature-sensitive mechanisms contribute to skin blood flow alone. These findings further our knowledge of leg hemodynamic responses during direct heat stress and provide evidence of potentially beneficial vascular alterations during isolated limb heat stress that are equivalent to those experienced during exposure to moderate levels of whole body hyperthermia.

Keywords: heat stress; hemodynamics; leg blood flow.

Fig. 1.

Sequence of the experimental protocols and schematic of the leg major vessels and anatomical sections. A: in study 1, following baseline measurements, participants were heated to core temperature (T_c) + 2°C with measurements (denoted by arrows) taken during mild (+0.5°C), moderate (+1°C), intense (+1.5°C), and severe (+2°C) heat stress, both before and after arterial cuff occlusions (denoted by C) at the level of the knee. Following the whole body heat stress protocol, the left leg was rapidly cooled and further measurements taken as before. B: participants in study 2 rested in ambient thermoneutral conditions while a single leg was heated for a duration of 1 h. C: illustration of the leg's major supplying [common, superficial, and profunda femoral arteries (CFA, SFA, and PFA)] and draining vessels [common femoral vein (CFV) and great saphenous vein (GSV)] and the anatomical differentiation between the thigh and the lower leg.

Fig. 2.

Temperature responses to passive whole body heat stress followed by single leg cooling and single leg heat stress. A: representative trace from a single participant showing typical core, muscle, subcutaneous, and skin temperature responses during progressive whole body heat stress followed by single leg cooling (indicated by dashed vertical line) (study 1). B: representative trace from a single participant showing typical core, muscle, and skin temperature responses during single leg heat stress (study 2).

Fig. 3.

Leg blood flow distribution in the thigh and leg during passive whole body heat stress and leg occlusion. Blood flow distribution to each of the major conduit arteries during passive heat stress. Thigh blood flow is represented by CFA blood flow following a cuff occlusion at the level of the knee, whereas lower leg blood flow is the difference between thigh blood flow and whole leg blood flow. A: CFA. B: SFA. C: PFA. Data are mean ± SE for 8 subjects. *P < 0.05, significantly higher than previous condition.

Fig. 4.

Relationship between leg hemodynamic responses and mean leg temperature during passive whole body heat stress followed by single leg cooling. CFA, SFA, and PFA blood flows vs. mean leg temperature (A) and GSV and CFV blood flow (B) during passive heat stress and rapid single leg cooling. Data are mean ± SE for 8 subjects. *Significantly higher than previous condition. #P < 0.05, significantly lower than severe heat stress.

Fig. 5.

CFA blood flow and oscillatory shear index responses to single-leg heat stress and moderate whole body heat stress. Leg blood flow (A) and oscillatory shear index (B) in the CFA during single leg heat stress and moderate whole body heat stress (T_c + 1°C). Data are mean ± SE. *Significant change from baseline.

Fig. 6.

Arterial blood flows vs. mean leg temperature during passive whole body and single leg heat stress. Data are represented as mean ± SE (large black squares) for baseline (thermoneutral), moderate (T_c + 1°C) and severe (T_c + 2°C) whole body heat stress. Single leg baseline and heat stress values are also displayed (large white squares). A: CFA. B: SFA. C: PFA. *Significantly higher than equivalent single leg heat stress value. Individual data points are also displayed (small circles); n = 8 for whole body heat stress and n = 7 for single leg heat stress.

Fig. 7.

Relationships between local blood flow and temperatures during isolated single leg cooling following whole body heat stress. A: relationship between profunda femoral arterial flow (predominantly supplying deep tissues of the thigh) and local deep tissue temperature. Data are mean ± SE for 8 subjects. B: relationship between GSV blood flow (predominantly draining superficial tissues of the thigh) and local subcutaneous temperature during both severe whole body passive heat stress and subsequent local thigh cooling with arterial cuff occlusion. *Significantly higher than previous measurement. #Significantly lower than severe heat stress.