Acute measurement of Flow-Mediated Dilation following passive heating in adults: The confounding role of altered shear stress and baseline vasodilation

Changes in flow-mediated dilation (FMD) following acute heating are not well understood, appear protocol-specific, and may be better understood by additional measures of acute vasoactivity. This study investigated FMD responses before and after three different 30-min hot-water immersion conditions (40°C-Shoulder, 42°C-Waist, and 40°C-Waist) in 22 adults. Brachial artery diameter was recorded at baseline (Dbase), during the final 30 s of occlusion (Docc), and at peak post-occlusion (Dpeak). Allometrically scaled FMD%, and changes in diameter during occlusion (OIV), and from end-occlusion to peak diameter (FMDDocc) were calculated. Pre-occlusion shear rate was greater post-immersion in 40-Shoulder (p < 0.001) and 42-Waist (p < 0.001), but not 40-Waist (p = 0.13), with the largest increase observed in 40-Shoulder. Alongside this, Dbase increased (Δ0.4 ± 0.2 mm, p < 0.001) and FMD% decreased (Δ−3.9 ± 3.8%, p = 0.04) following immersion in 40°C-Shoulder only. Across all conditions, ΔFMD% was negatively associated with ΔDbase (rrm = −0.47, p = 0.001). OIV% was the only vasoactivity metric to statistically differentiate between all conditions post-immersion (40°C-Shoulder: −8.1 ± 4.9%. 42°C-Waist: −3.0 ± 5.3%. 40°C-Waist: 1.1 ± 4.1%. p < 0.001). Post-heating FMD is confounded by heat-induced increases in baseline diameter, even after allometric scaling, while OIV% may provide complementary insight into acute vasoactivity following passive heating.