Comparison Of Acute Cardiovascular And Thermoregulatory Responses To Differentpassive Heating Modalities

The repeated use of passive heating has gained popularity as a therapeutic approach toward promoting health and wellness. Passive heating elicits a rise of core body temperature and cardiovascular demands 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. PURPOSE: The purpose of this study is to characterize and compare the acute thermoregulatory and hemodynamic responses to a single bout of hot water immersion (HWI), traditional sauna (TRAD), and far-infrared sauna (FIR). METHODS: In a randomized cross-over study design, six healthy adults (1 female; age 24 ± 2.8 years; body mass index 22.8 ± 11.6 kg/m2) completed three sessions of acute passive heating: HWI (45 minutes at 40 °C), TRAD (3x10 minutes at 80 °C, separated by 5 min of rest in normal room temperature), and FIR (45 minutes at 45-65 °C) on separate days. Measurements of systolic and diastolic arterial pressure and core temperature (Tc) were collected at baseline and at 5-min intervals during passive heating. Cardiac output (Q) was measured before and at the end of heating using the open-circuit acetylene wash-in method. One-way ANOVAs were performed to compare the changes in Tc, Q, and calculated mean arterial pressure (MAP) from baseline to end of heating. Data are presented as mean or percent change from baseline with 95% confidence intervals. RESULTS: The change in Tc from baseline to end of heating was greater in HWI [+1.3 °C (0.8, 1.7)] vs TRAD [+0.1 °C (-0.1, 0.4), P = 0.0002] and HWI vs IR [+0.1 °C (-0.4,0.5), P < 0.0001]. Cardiac output increased in HWI [+78.2% (65.8, 90.5)] vs TRAD [+50% (39.5, 60.6), P = 0.0069)] and HWI vs IR [+30.1% (12.3, 48.0), P < 0.0001). MAP decreased in HWI [-13 mmHg (-29, 2)], whereas MAP increased in TRAD [+7 mmHg (1, 12), P = 0.0204)] and IR [+3 mmHg (-8, 14), P = 0.0675]. There were no differences in TRAD vs IR for Tc (P = 0.9841), Q (P = 0.0598), or MAP (P = 0.9133). CONCLUSION: Preliminary data suggest that HWI elicits the greatest thermoregulatory challenge and cardiovascular strain compared to both TRAD and IR. This implies that HWI may elicit the greatest physiological adaptations with repeated passive heat therapy.