What question did this study set out to answer?

This research aims to understand how acute heating affects flow-mediated dilation by examining specific vasoactivity metrics.

February 19, 2026Open Access

Acute measurement of flow‐mediated dilation following passive heating in adults: The confounding role of altered shear stress and baseline vasodilation

Key Result

Passive heating at 40°C Shoulder increased baseline brachial artery diameter by 0.4 mm (p<0.001) and decreased FMD% by 3.9% (p=0.04), confounding post-heating FMD measures.

Key Points

This research aims to understand how acute heating affects flow-mediated dilation by examining specific vasoactivity metrics.
Conducted three hot-water immersion protocols (40°C-Shoulder, 42°C-Waist, 40°C-Waist) for 30 minutes each.
Measured brachial artery diameter at baseline, during occlusion, and post-immersion.
Calculated flow-mediated dilation percentage and changes in diameter during occlusion.
Post-immersion shear rate increased in 40°C-Shoulder and 42°C-Waist.
Diameter increased and FMD percentage decreased significantly only in the 40°C-Shoulder condition.
OIV% was significantly different among all conditions post-immersion.

Structured PICO

Does passive heating via hot-water immersion alter flow-mediated dilation and acute vasoactivity in adults?

Population

22 adults

Intervention

Three different 30-minute hot-water immersion conditions (40°C-Shoulder, 42°C-Waist, and 40°C-Waist)

Comparator

Baseline measurements (before immersion) and comparison between the three immersion conditions

Outcome

Flow-mediated dilation (FMD) responses, including allometrically scaled FMD%, changes in diameter during occlusion (OIV), and from end-occlusion to peak diameter (FMD Docc)surrogate

Post-heating flow-mediated dilation measurements are confounded by heat-induced increases in baseline arterial diameter, suggesting occlusion-induced vasodilation (OIV%) may be a better complementary metric for assessing acute vasoactivity.

Main Result

Absolute Event Rate: 0% vs 0%

Abstract

Abstract 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 (D base ), during the final 30 s of occlusion (D occ ), and at peak post‐occlusion (D peak ). Allometrically scaled FMD%, and changes in diameter during occlusion (OIV), and from end‐occlusion to peak diameter (FMD Docc ) 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, D base 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 ΔD base ( r rm = −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.