Comparison of Acute Physiological Responses to Different Passive Heating Modalities
Authors: Atencio Jessica , Reed Emma , Larson Emily , Gibson Brandon , Lucernoni Kathryn , Wiedenfeld Needham Karen , Comrada Lindan 
Published: May 2024

The repeated use of passive heating has gained popularity as a therapeutic approach toward promoting health and wellness. Passive heating elicits a rise in core body temperature and subsequent cardiovascular demand that can result in physiological adaptations beneficial to health. Hot water immersion, traditional sauna, and far-infrared sauna are three common modalities of passive heating; however, current literature lacks a direct comparison between these modalities. The purpose of this study is to characterize and compare the acute thermoregulatory and cardiovascular responses to a single bout of hot water immersion (HWI), traditional sauna (TRAD), and far-infrared sauna (FIR). We hypothesized that HWI will induce more pronounced thermoregulatory and cardiovascular responses when compared to both TRAD and FIR. In a randomized cross-over study design, nineteen healthy adults (9 F; age 24±4 years; body mass index 26.47±5 kg/m2) completed three bouts of acute passive heating: HWI (45 minutes at 40°C), TRAD (3x10 minutes at 80°C, separated by 5 min of thermoneutral rest), and FIR (45 minutes at 45-65°C) separated by at least 1 week. These temperatures and durations were chosen based on previous research and reflect typical use of these modalities. Measurements of arterial pressure, core temperature (Tc), and heart rate (HR) were collected at pre-heating baseline and at 5-min intervals during passive heating. Cardiac output (Q) was measured before, halfway, and at the end of heating using the open-circuit acetylene wash-in method. Mean arterial pressure (MAP) was calculated as 1/3 systolic + 2/3 diastolic and stroke volume (SV) and systemic vascular resistance (SVR) were calculated by Q/HR and MAP/Q. Data were analyzed using one-way ANOVAs to compare the changes from baseline to end of heating and are presented as mean with 95% confidence intervals. The change in Tc from baseline to end of heating was greater in HWI [+1.1°C (1.0, 1.3)] vs TRAD [+0.4°C (0.2, 0.6), p<0.0001], HWI vs FIR [+0.1°C (-0.1,0.2), P <0.0001], and TRAD vs FIR (P=0.0291). The increase in HR was greater in HWI [+39 bpm (35, 43)] vs. FIR [+26 bpm (21, 32), P=0.0017], with no differences in HWI vs. TRAD [+34 bpm (28, 40), P=0.4247] or TRAD vs. FIR (P=0.0793). Cardiac output increased to a greater extent in HWI [+3.9 L (3.3, 4.4)] vs. TRAD [+2.4 L (1.8, 2.9), P=0.0003)] and HWI vs. FIR [+1.7 L (1.2, 2.1), p<0.0001], with no difference between TRAD vs FIR (P=0.1379). MAP decreased in HWI [-14 mmHg (-20, -9)] vs TRAD [-3 mmHg (-8, 1), P=0.0058] and HWI vs FIR [-0.5 mmHg (-5, 4), P=0.0003], with no difference between TRAD vs FIR (P= 0.7506). Conclusion: A single bout of HWI elicits the greatest thermoregulatory challenge and cardiovascular strain compared to both TRAD and FIR. Funded by American Heart Association (19TPA34890033) and NIH (R01HL144128). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.