Passive hyperthermia increases blood circulation in specific regions, largely independent of conduit artery mechanics and cardiac performance

Passive hyperthermia increases net peripheral and systemic blood flow in humans and other animals, yet the underlying haemodynamic forces that selectively accelerate blood movement remain incompletely characterized. Wave intensity analysis offers insight into the respective contributions of the heart and the vascular system to changes in blood circulation during physiological stress; however, the specific impact of hyperthermia on wave intensity metrics has not been elucidated comprehensively. To address this, we investigated wave speed and wave intensity parameters in the common carotid artery, along with local arterial distensibility in the internal carotid, brachial and common femoral arteries, in addition to total arterial compliance, in eight healthy males across four protocols: (1) 3 h of control measurements in normothermic conditions; (2) 3 h of one-leg heating; (3) 3 h of two-leg heating; and (4) 2.5 h of whole-body heating. Forward compression (1.5-fold; P = 0.041) and forward expansion (5.2-fold; P < 0.0001) waves in the common carotid artery (indices of ventricular contractility and late-systolic blood flow deceleration, respectively) increased exclusively during whole-body heating. In contrast, backward compression waves, wave speed, distensibility and reflection index remained unaltered across all conditions. Notably, distensibility in the major conduit arteries perfusing the brain (internal carotid artery), forearm (brachial artery) and leg (common femoral artery), in addition to total arterial compliance, remained unchanged across all conditions. Collectively, these findings suggest that increases in blood circulation within specific regions of the human body during passive hyperthermia are largely independent of conduit artery mechanics and cardiac performance.