Repeated exposure to hot water immersion improves arterial function [1,2]. However, there is a lack of evidence supporting specific protocol selection. Acute elevations in shear rate and flow-mediated dilation (FMD) positively correlate to subsequent chronic adaptations with exercise [3] and therefore may provide evidence with hot water immersion to support a particular immersion protocol. Core and skin temperature have distinct contributions to acute vascular responses [4 meaning, different immersion protocols may lead to divergent responses. Accordingly, this study aimed to compare the effects of (i) immersion depth, (ii) water temperature, and (iii) rectal temperature on acute changes in brachial artery diameter, shear rate, and subsequent FMD. It was hypothesised that shear stress and arterial diameter would differ between conditions and that greater increases in diameter post-heating would result in larger reductions in FMD.

Twenty-two healthy young adults completed three thirty-minute bouts of hot water immersion in a randomised order consisting of (i) Shoulder-deep immersion in 40 °C water (40-Shoulder), (ii) Waist-deep immersion in 40 °C water (40-Waist), and (iii) Waist-deep immersion in 42 °C water (42-Waist). Rectal temperature and  vascular responses in the brachial artery (arterial diameter, blood flow, shear rate, FMD) were measured. Data were analysed using two-way repeated-measures ANOVAs, with the exception of FMD, which was allometrically scaled according to baseline diameter [5] and analysed using a generalised estimation equation. Significance was accepted at p < 0.05. Ethical approval was provided by the Coventry University (P146084) and conformed to the Declaration of Helsinki.

Rectal temperature increased less in the 40-Waist (Δ0.5 ± 0.1 °C) condition than either the 40-Shoulder (Δ0.9 ± 0.3 °C. p < 0.001), or 42-Waist condition (Δ0.9 ± 0.3 °C. p < 0.001), which were similar (p = 1.0). Shear rate differed between all conditions (p < 0.001. 40-Shoulder: Δ225 ± 131 1/s. 42-Waist: Δ105 ± 117 1/s 40-Waist: Δ-22 ± 66 1/s), whilst a greater increase in diameter was observed in the 40-Shoulder condition (Δ0.43 ± 0.22 mm), than either the 42-Waist (Δ0.11 ± 0.33 mm p < 0.001) or 40-Waist (Δ-0.02 ± 0.33 mm. p < 0.001) condition, which were similar (p = 0.14). Similarly, the 40-Shoulder condition demonstrated a larger decrease in FMD (Δ-3.9 ± 3.8%) than either the 42-Waist (Δ-1.7 ± 6.2%. p = 0.01) or 40-Waist (Δ0.2 ± 5.4%. p < 0.001) condition, which were similar (p = 0.14).

This study demonstrates that in response to a 30-minute hot water immersion bout, immersion depth has a larger influence than water temperature, or rectal temperature on arterial dilation, shear rate, and subsequent FMD. These findings provide novel evidence for the importance of heating larger body surface areas, over increases in rectal temperature, in the prescription of heating stimuli to elicit larger vasoactive and haemodynamic responses. In line with the stated hypothesis, larger reductions in FMD were observed in conditions that elicited greater brachial vasodilation. Future work is required to examine the physiological meaningfulness of this in terms of changes to endothelial function and potential inference for effects on subsequent adaptation.