Passive heat therapy: the next hot thing for cardiovascular health!

Cardiovascular disease is currently the leading cause of death in developed countries across the globe, with myriad risk factors that modern life has only exacerbated. As such, treatments and lifestyle changes that alleviate the risk of cardiovascular disease are of great relevance. While factors like exercise and good diet are known to have such ameliorative effects, patients at high risk of developing cardiovascular diseases are often elderly or obese, and thus may have difficulty exercising on a regular basis. Furthermore, individuals with lower incomes often cannot afford to eat healthily, or such a diet is logistically impossible for them. These factors will only be exacerbated by rapidly rising food prices and increasing populations aggravating socioeconomic phenomena such as food deserts, residential areas devoid of groceries or healthy food options (Rehm et al. 2015). Thus, medically subsidized alternatives to these options are extremely important to explore for those whom a good diet or frequent exercise may be difficult or infeasible.

In recent years, several studies have examined thermal therapy as one such alternative, primarily through methods such as saunas and bathing (Imamura et al. 2001; Hu et al. 2012). While the efficacy of thermal therapy on cardiovascular health has at this point been demonstrated by multiple studies, the underlying mechanisms are less understood. However, a recent publication in The Journal of Physiology (Brunt et al. 2016) has helped to elucidate some of these mechanisms. In the paper, Brunt et al. (2016) performed an 8 week study on a cohort of 20 subjects in which the subjects either underwent heat therapy 4–5 times a week for a total of 90 min or were immersed in thermoneutral water as an osmotic and hydrostatic control. The subjects assigned to heat therapy were immersed in 40.5°C water, a temperature sufficient to maintain rectal temperature greater than 38.5°C for 60 min. In contrast, the rectal temperature of the control group remained within 0.2°C of the original temperature. Subjects in both groups were matched for sex, age, height, body mass index and weight, ensuring a representative control group. The authors of the study took a multi‐pronged approach in determining the mechanism of action of heat therapy by measuring a variety of processes that they hypothesized could contribute: flow‐mediated dilatation, superficial femoral dynamic arterial compliance, aortic pulse wave velocity, carotid intima media thickness, and mean arterial blood pressure. These measurements were taken at the start of the study and every 2 weeks thereafter.

The literature has suggested that part of the mechanism by which heat therapy ameliorates cardiovascular health is through alleviation of arterial stiffness (Hu et al. 2012). Consequently, Brunt et al. (2016) hypothesized that flow‐mediated dilatation and superficial femoral dynamic compliance would increase with regular heat therapy. Indeed, flow‐mediated dilatation saw an immediate and significant increase with heat therapy when compared to the control group. While it appeared to drop temporarily during week 4, when corrected for shear stress, this outlier returned to the logarithmic pattern shown earlier. The trend is enormously encouraging as it implies that while increased use of heat therapy will show greater returns, only a brief initial treatment is needed for significant benefits. As the article mentions, increases in flow‐mediated dilatation as small as 2% have shown a 15% reduction in risk for cardiovascular disease, and an increase greater than 2% was shown after only 2 weeks, demonstrating the immediate clinical relevance of heat therapy. While carotid arterial compliance did not show a similar response to heat therapy, being statistically unchanged from the control, superficial femoral dynamic arterial compliance did display a significant increase also following a logarithmic pattern after heat therapy, reinforcing the results of earlier studies showing decreases in arterial stiffness (Hu et al. 2012; Brunt et al. 2016).

The authors also investigated other outcome measures to assess risk for cardiovascular disease such as pulse wave velocity, arterial wall thickness in both the carotid and femoral arteries, and blood pressure. A significant downwards trend was observed in pulse wave velocity when compared to week 0, although not compared to the control group, showing that further investigation is required. However, other results were more promising; while arterial wall thickness in the femoral arteries showed no significant difference, carotid wall thickness showed significant decreases, possibly due to the breakdown of atherosclerotic plaque, a finding of clinical relevance. While more detailed studies of the mechanism of this effect are required, this is an enormously promising result. Furthermore, the study also showed significant reductions in both mean and diastolic blood pressure. While these reductions were only 4 mmHg, the duration of the study was fairly short by the standards of a preventative measure, and the results feasibly could increase with time. Finally, as the article states, this reduction is found in healthy individuals; thus, it is plausible that there would be a greater drop from the elevated blood pressure seen in individuals with cardiovascular disease if heat therapy was used as a treatment rather than a preventative measure (Brunt et al. 2016).

Brunt et al. (2016) opened up a variety of avenues for further in‐depth studies, such as investigations into the sources of arterial wall thickness reduction as mentioned above, by studying the mechanisms by which heat therapy ameliorates risk for cardiovascular disease in breadth. However, even without such investigation, the results of the research article are remarkably useful with regards to preventative measures against cardiovascular illness. The logarithmic increase in flow‐mediated dilatation found by the authors suggest this treatment is useful even in the short term for those with elevated risk factors. Moreover, a treatment with the potential for non‐invasive reduction of atherosclerotic plaque could be a powerful tool to dramatically reduce risk of myocardial infarction, although the risk of embolism would also need to be evaluated. Furthermore, heat therapy may also be useful in the long term in healthy patients, both to reduce the risk factors already described and potentially to lower pulse rate, a trend the authors described, but which did not reach full significance during the time constraints of the study. Lowering blood pressure in the long term could also help to reduce arterial strain and atherosclerotic buildup, as well as many other factors, leading to a healthier long‐term cardiovascular system. Thus, it is clear that results of the study could be of clinical use both in the short and long term as a preventive measure.

However, additional investigations are warranted. Because the study in question only addressed the effects of heat therapy on young, healthy patients, the therapeutic value of the treatment in patients with significant risk factors for cardiovascular illness such as severely elevated blood pressure, atherosclerosis, or obesity is unknown. It also would have been worthwhile to comment on the potential dangers of heat therapy for certain patients, particularly those with aortic stenosis, unstable angina, severe orthostatic hypotension, pregnancy, or any history of recent myocardial infarction, as these adverse effects have been shown in some studies in the literature. Particularly, persons prone to orthostatic hypotension should avoid heat therapy because of the significant decrease in blood pressure, which usually occurs immediately after heat therapy (Laukkanen et al. 2015). Thus, heat therapy requires significantly more research before being of use as a preventative method or treatment for patients with cardiovascular disease, rather than to solely prevent its incidence. Finally, the study that inspired Brunt et al. (2016) showed diminishing results from heat therapy in older patients, meaning that it may not be as effective as a preventive treatment in elderly patients (Hu et al. 2012), who also have elevated risks for cardiovascular illness. This phenomenon should be studied in greater depth to verify it because the study in question only investigates the effects of footbaths on cardiovascular health. Therefore, this result may have been a consequence of diminished peripheral circulatory function in the elderly. Furthermore, the study in question showed that the use of footbaths only raised tympanic temperature to slightly over 37°C, whereas Brunt et al. (2016) raised rectal temperature above 38.5°C, a difference in temperature that may increase the efficacy of heat therapy in the elderly. Nonetheless, if valid, the mechanisms contributing to this reduction in efficacy should also be studied to confirm its incidence, and the effectiveness of heat therapy in the elderly should be examined regardless due to their high incidence of cardiovascular complications.

With the growing incidence of cardiovascular disease and an ageing global population, finding viable treatments and preventative measures for patients is becoming increasingly important. Having a large array of possible options for treating and preventing cardiovascular disease is especially crucial in treating a diverse patient population, some of whom may be incapable of exercise or have other circumstances preventing ideal cardiovascular health. This study helps to expand this array by delving into the mechanisms by which heat therapy improves cardiovascular health. It is heartening that cardiology research is finding solutions to an increasingly significant health problem. Yet there is still much to be discovered, and more clinical trials involving heat therapy need to be performed to understand the treatment in‐depth and add to our current knowledge.

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Competing interests

None declared.