Acute heating-induced flow-mediated dilation: Exploring sustained shear stress responses in the brachial and femoral arteries of healthy adults

Flow-mediated dilation (FMD) is widely used as a non-invasive marker of endothelial function and vascular health (Green et al., Citation2017). Conventional FMD uses reactive hyperaemia following cuff-induced ischaemia (RH-FMD), which produces a brief shear stress stimulus. However, this transient stimulus may not fully capture endothelial responses to sustained physiological haemodynamic stress (Flammer et al., Citation2012). Acute heating may provide alternative stimulus by inducing prolonged elevations in shear stress, potentially providing a more ecologically valid assessment of endothelial function (Tremblay & Pyke, Citation2017). However, the reliability of heating-induced sustained shear stress FMD (SS-FMD) remains insufficiently described.

This study evaluated the test – retest reliability of brachial and femoral SS-FMD, compared SS-FMD with RH-FMD, and characterised the temporal profile of SS-FMD following acute heating. The study received institutional ethical approval (MMU Ethos: 78,720). Eight participants completed three laboratory visits separated by ≥48 hours in a repeated-measures design. Each visit involved full-body hot-water immersion (40°C, 20 min). RH-FMD and brachial and femoral artery diameter and blood flow were assessed by ultrasound before, immediately after, and 30 min after heating. Reliability was assessed using intraclass correlation coefficients (ICC), and physiological responses were quantified using heart rate (HR) and diastolic blood pressure (DBP).

Heating elicited haemodynamic responses (ΔHR 39 ± 9 bpm; ΔDBP −18 ± 4 mmHg). SS-FMD immediately post-heating demonstrated good reliability (ICC = 0.733, 95% CI 0.371–0.932), whereas SS-FMD at 30 min showed poor reliability (ICC = 0.044, 95% CI − 0.311–0.599). Baseline RH-FMD responses were moderately reliable (ICC = 0.568, 95% CI 0.118–0.880). SS-FMD was greatest immediately post-heating (22.6 ± 8.9%) and declined by 30 min (7.1 ± 3.7%; p < 0.001, d = 2.04). SS-FMD immediately post-heating was significantly greater than baseline RH-FMD (7.9 ± 2.5%; p < 0.001, d = 1.97). Participants with RH-FMD responses tended to exhibit larger SS-FMD responses (r = 0.658, p = 0.076).

These preliminary findings suggest acute heating elicits large and reproducible endothelial responses immediately post-stimulus, supporting its potential as a novel method for assessing sustained shear stress – mediated endothelial function. Ongoing recruitment and femoral analyses will further clarify the physiological relevance of SS-FMD.