Abstract: Dry-sauna is a strong thermal stimulus and is commonly used all over the world. The aim of this experiment was to comprehensively analyze cardiovascular and autonomic changes that result from an increase in core body temperature during sauna bath. The study included 9 healthy men with mean age 26.7 ± 3.0 years and comparable anthropomorphical characteristics. Each subject was exposed to one 15-minute session of dry-sauna treatment at 100°C and 30–40% humidity. The autonomic and baseline cardiovascular (i.e., hemodynamic and contractility) parameters were measured noninvasively with Task Force Monitor. Cardiovascular autonomic functions were assessed using baroreceptor reflex sensitivity (BRS) and spectral analysis of heart rate (HRV) and blood pressure (BPV) variability. Measurements were performed four times, at the following stages “before sauna,” “after sauna,” “sauna + 3 h,” and “sauna + 6 h.” The first recording constituted a baseline for the subsequent three measurements. The changes in core body temperature were determined with the Vital Sense telemetric measurement system. Results show that exposure to the extreme external environmental conditions of dry-sauna do not compromise homeostasis in healthy persons. The hemodynamic changes induced by heating are efficiently compensated by the cardiovascular system and do not exert negative effects upon its short-term regulatory potential.
Alex’s Notes: I admit that I am really only looking at this study for Carl. He loves his sauna, and although he trains inspiringly hard – it would be stupid to disregard any potential negatives of overheating the body for prolonged period of time. That said, I have no idea what type of saunas are common in North America, but traditionally the term “sauna” is derived from a Finnish steam bath with temperatures approaching 70-100°C and humidity of 10-20%. The duration is typically 5-15 minutes and immediately followed with a cold shower or bath. Most people will undergo 2-3 cycles of the heating and cooling process.
The aim of the study at hand was to examine the comprehensive cardiovascular and autonomic changes that result from being in a sauna. The subjects were nine healthy young males averaging 26 years of age and a BMI of 25 kg/m2 who were free of any disease or functional disorder. They sat within a 100°C sauna with 30-40% humidity for 15 minutes, after which they had testing done for another six hours. Unfortunately for them, there was no cold bath afterwards in order to remove any confounding variables, although I would imagine that the sudden shift from hot to cold would provide its own bodily challenges.
Regarding basic cardiovascular parameters, heart rate (HR) and baroreceptor reflex sensitivity (BRS; helps maintain blood pressure) were the only ones to show significant changes. HR increased from a pre-sauna value of 59 beats/min to a post-sauna 65 beats, where it remained until 6 hours post-sauna. Conversely, the BRS showed a sharp decline immediately post-sauna and gradually rose for the following 6 hours. Given that the core body temperature increased an average of 0.7°C and had not returned to baseline even after 6 hours, blood pressure changes are to be expected to maintain homeostasis.
Additionally, there was a decrease in the values of systolic index, end-diastolic index (EDI), and stroke volume immediately post-sauna, and a gradual increase for the 6 hours following.The changes of stroke volume probably resulted from an initial decrease in venous return caused by an enhanced perfusion of the skin during its heating.
Overall, while the current study did not consider the potential influence of respiratory rate on blood pressure and heart rate variability, and had a small sample size limited to healthy males, the study is somewhat applicable to my good friend Carl. As the authors conclude,
“This study has revealed that exposure to the extreme external environmental conditions of dry sauna does not compromise homeostasis in healthy persons. The hemodynamic changes induced by heating are efficiently compensated by the cardiovascular system and do not exert negative effects upon its short-term regulatory potential.”
And I would say that Carl is a healthy person.