Some like it hot: Cardiovascular health benefits of Finnish sauna

In Finland, sauna bathing has been practised for centuries, either for pleasure, but more importantly also for reasons of hygiene and maintenance of health. Many curative and magical effects have been attributed to its practice and seldom has it been thought to cause any disease.¹

In fact, in a general Finnish male population, sauna bathing has been shown to be a protective factor against the occurrence of sudden cardiac death, fatal coronary heart and cardiovascular disease, as well as all-cause mortality.² While the short-term cardiovascular and hormonal effects of Finnish sauna are quite well studied,³ the long-term mechanisms are still incompletely understood.

In contrast, another particular form of thermal therapy, developed in Japan and applied in the treatment of patients with cardiovascular disease for the past 20 years, is quite well studied: the so-called Waon therapy,⁴ ‘wa’ meaning soothing and ‘on’ meaning warmth. It is a dry sauna with a temperature maintained at 60℃ and defined as a warming of the entire body in a uniformly heated chamber (far-infrared ray) for 15 minutes, increasing the core temperature by 1.0–1.2℃, followed by a 30-minute rest outside the sauna. Waon therapy has been reported to improve the haemodynamics, cardiac function, ventricular arrhythmia, vascular endothelial function, neurohormonal factors, sympathetic nervous system function and symptoms in patients with chronic heart failure and peripheral artery disease.⁵ The molecular mechanism by which Waon therapy improves vascular flow and endothelial function involves the increased expression of endothelial nitric oxide synthase.

Due to the difference in temperature and humidity as well as the intermittent cooling during traditional Finnish sauna, the question of similar effects of this type of heat exposure compared with the milder form of Waon therapy remains. In this issue of the European Journal of Preventive Cardiology, Lee et al. now report about the effect of Finnish sauna on arterial compliance and the dynamics of blood flow and pressure.⁶ The experiment was conducted with a temperature of 73 ± 2° and a relative humidity of 10–20%. One single 30-minute session, interspersed with a short, 2-minute shower after 15 minutes was performed in 102 asymptomatic participants with a mean age of 51.9 years and at least one cardiovascular risk factor. The study showed an improvement of different parameters of arterial stiffness (pulse wave velocity, mean blood pressure, left ventricular ejection time and diastolic time) and hence similar potential effects for long-term protection of cardiovascular function as does Waon therapy.

Interestingly, these effects reported by heat exposure are the same as postulated by the practice of regular aerobic physical exercise,⁷ and one might suggest that sauna bathing could substitute for limited exercise options, e.g. during the winter months in Nordic countries or for exercise at all. To disentangle the potential interaction between exercise training and sauna practice, Kunutsor et al. evaluated 2277 men in a prospective study for the joint effects of cardiovascular fitness (low vs. high: median cutoffs) and frequency of sauna bathing (low vs. high: two or less and three to seven sessions/week) on the risk of fatal cardiovascular disease and all-cause mortality.⁸ During a median follow-up of 26.1 years, it turned out that cardiorespiratory fitness and frequency of sauna bathing are independently associated with a reduced mortality risk and that a combination of good cardiovascular fitness and frequent sauna bathing may confer additional survival benefits.

Regarding the recommendations to practise Finnish sauna in patients with cardiovascular disease, due to the alternation between heat exposure and cold water immersion, there still remains concerns related to haemodynamic tolerance, risk of arrhythmias, triggering of acute coronary syndromes or cardiac decompensation. As a response to heat exposure, cardiac output increases by 60–70%, almost exclusively due to an increase in heart rate, while cardiac stroke volume does not change. The increase in peripheral blood flow allows for greater heat exchange through the skin and diaphoresis, and consequently decreases circulation to muscles, kidneys and viscera. At the same time, the effect on blood pressure is variable: systolic pressure can remain unchanged, increase (9–21 mmHg) or decrease (8–31 mmHg), while diastolic pressure either remains unchanged or decreases (6–39 mmHg).³ Of note, frequent sauna bathing improves heat tolerance and reduces the magnitude of these changes. The sudden exposure to cold after sauna bathing stimulates the sympathetic nervous system with activation of cardiovascular changes that are opposite to those of heat exposure: heart rate decreases, while stroke volume, systolic and diastolic blood pressure increase. Within a few hours after sauna bathing or cold exposure these cardiovascular changes then normalise.

Without a doubt, the alternation of heat and cold exposure represents the most intensive strain on the cardiovascular system, especially when cold water immersion is practised. Patients early after an acute myocardial infarction or with severely reduced left ventricular function might therefore be particularly susceptible to develop ventricular arrhythmias.^9,10 Nevertheless, only very few sudden deaths have been reported during or within 24 hours of sauna bathing in countries that routinely use saunas (<3% of all sudden deaths),^11–13 and it is considered to be safe for patients with stable coronary artery disease. Also for heart failure patients, Finnish sauna and cold water immersion have been shown to be haemodynamically well tolerated without the occurrence of complex ventricular arrhythmias.¹⁴ Not to be neglected, however, is the effect of alcohol intake during sauna, which creates serious health risks and may contribute to myocardial infarction or accidental death (e.g. drowning).^15,16

In summary, the regular practice of Finnish sauna exerts a lot of positive health effects which also go beyond the cardiovascular system, and it is associated not only with a reduction of cardiovascular but also with all-cause mortality.² It can therefore be recommended to all stable cardiac patients as a lifestyle measure to improve prognosis. As with all newly started activities, it should be common sense to start with caution, to check the individual tolerance, increase the intensity and frequency of the activity slowly and to maintain it thereafter on a regular basis. Avoiding cold water immersion and using a shower for cooling might be prudent in patients with a particular risk of arrhythmias. Finally, combining sauna with regular physical activity seems to be a particularly attractive combination to boost the effects of these lifestyle measures.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.