The effect of heat thermotherapy on cardiovascular function and cardiometabolic health

Price, Ben Sebastian ORCID: 0000-0002-0678-6302 (2025). The effect of heat thermotherapy on cardiovascular function and cardiometabolic health. University of Birmingham. Ph.D.

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Abstract

Heat thermotherapy (HT) is a promising strategy to improve cardiovascular (CV) function. HT modalities such as sauna bathing and hot water immersion (HWI) have been shown to induce positive acute CV responses and long-term CV adaptations, including artery structural remodelling, lower resting blood pressure (BP), and improved endothelial function; all associated with a lower risk of CV disease. Furthermore, HT could be used alongside high-intensity interval exercise (HIIEx) to augment positive CV adaptations associated with HIIEx alone. Unfortunately, substantial heterogeneity within the HT literature prevents evidence-based HT prescription for CV health. Additionally, common HT strategies can be prolonged and uncomfortable for participants. Therefore, this thesis investigated the efficacy of HT to improve CV function (meta-analysis, Chapter 2) and assessed different HT protocols and their acute cardiovascular response in young, healthy adults (Chapters 4 and 5).

The meta-analysis (Chapter 2) revealed HT improves CV function, as shown by the positive CV response after a single bout of HT (reduced diastolic blood pressure (DBP), mean arterial pressure (MAP) and increased flow-mediated dilation)) and the longer-term CV adaptations (reduced resting DBP and MAP) following multiple bouts of HT. Health status nor age did not affect these changes, demonstrating that HT can improve CV function in various population demographics. Chapter 4 investigated whether a short, hot, neck-level, 30-minute bathing session (HWI30) can elicit a similar acute CV response as the 60-minute, shoulder/waist level immersion bathing session (HWI60). Additionally, it was hypothesised that there would be no meaningful CV response (i.e. lower BP) in the thermoneutral water immersion condition (TWI). There was a condition-by-time interaction effect for DBP, MAP, HR and core body temperature (Tc). Despite the higher Tc achieved during and post-bathing for HWI60 than HWI30, the heart rate (HR) elevation and reduction in DBP and MAP were similar for HWI30 and HWI60. Meanwhile, there was a reduction in DBP and MAP during bathing for TWI; however, DBP and MAP values returned to pre-bathing values immediately post-bathing. This demonstrated that HWI30 was a more time-efficient approach that elicited the same BP response as HWI60.

Chapter 5 investigated the acute CV response of HIIEx followed by HWI30 (ExHWI) versus HIIEx followed by TWI (ExTWI). There was a condition-by-time interaction effect for DBP, MAP, HR and Tc. ExHWI led to a greater hyperthermic and CV response than ExTWI, as shown by the greater elevated Tc and HR during bathing and immediately post-intervention. DBP and MAP were similarly reduced during bathing for ExHWI and ExTWI while only ExHWI saw a reduction in DBP and MAP immediately post-intervention. Therefore, ExHWI led to a greater hyperthermic response and lower BP response (immediately post-intervention) than ExTWI. ExHWI, HWI30, and HWI60 caused a robust CV response, including a hypotensive effect during bathing and immediately post-intervention in young, healthy adults. However, this effect was short-lasting, as blood pressure was restored by one-hour post-intervention and did not change 24 hours post-baseline. The thesis has revealed that despite the popularity of researchers using HWI60 within the literature, shorter HT protocols such as HWI30 can result in a similar hypotensive response during and immediately after bathing in young, healthy adults. Yet whether HWI30 or ExHWI can lead to long-term CV adaptations in young, healthy adults is yet to be determined.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):