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.