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Sascha Ketelhut and Reinhard Ketelhut of Martin Luther University Halle-Wittenberg and the Medical Center Berlin published this study in Complementary Therapies in Medicine in 2019. 19 healthy volunteers completed two conditions: a 25-minute sauna session and a test on a cycle ergometer. Each person was compared with themselves across both conditions rather than with a separate control group.
Compared with the cycle ergometer test in the same participants, blood pressure and heart rate during the 25-minute sauna session climbed to a level similar to about 100 watts of submaximal exercise. The values fell below the starting level only well after the session had ended. The authors conclude that, in cardiovascular terms, a sauna session should be classed as a submaximal load.
The sample is small and consisted of healthy participants, not a population at risk. The measurements were acute and say nothing about long-term adaptation. How people with heart disease or low blood pressure respond was not examined.
The finding corrects a widely repeated assumption: that a sauna session lowers blood pressure while it is under way. In these participants the opposite happened, and the drop came afterwards. The study describes the acute load of one session; it does not set a threshold of fitness for sauna use and does not replace an individual medical assessment.