Comparison of thermoregulatory, cardiovascular, and immune responses to different passive heat therapy modalities

This study compared thermoregulatory, cardiovascular, and immune responses to acute hot water immersion (HWI), traditional sauna (TRAD), and far infrared sauna (FIR). Twenty healthy adults (10 F; age 24 ± 4 yr) completed three bouts of passive heating: HWI (45 min at 40.5°C), TRAD (3 × 10 min at 80°C), and FIR (45 min at 45-65°C) separated by at least 1 wk. Core temperature (T_c) was measured at baseline and every 5-min during heating. Cardiac output (Q_c) was measured before, halfway, and at the end of heating using the open-circuit acetylene wash-in method. Venous blood was collected at baseline and ∼20-min, 24-, and 48-h postheating for measurement of inflammatory cytokines and immune cell populations. The overall change in T_c from baseline to end of heating was greater in HWI [+1.1°C (0.9, 1.3)] vs. TRAD [+0.4°C (0.2, 0.6), <i>P</i> < 0.01], HWI vs. FIR [+0.0°C (-0.1, 0.2), <i>P</i> < 0.01], and TRAD vs. FIR (<i>P</i> = 0.02). Q_c increased to the greatest extent in HWI [+3.7 L/min (2.9, 4.4)] vs. TRAD [+2.3 L/min (1.8, 2.8), <i>P</i> < 0.01] and HWI vs. FIR [+1.6 L/min (1.2, 2.1), <i>P</i> < 0.01], with no difference between TRAD vs. FIR (<i>P</i> = 0.06). HWI was the only heating modality to elicit inflammatory and immune responses as evidenced by increased interleukin-6 (IL-6) concentration postheating [+0.386 pg/mL (0.093, 0.680), <i>P</i> = 0.04], NK cells 24-h postheating (<i>P</i> = 0.03), and CD8⁺ T cells at 24 (<i>P</i> = 0.02) and 48 h postheating (<i>P</i> = 0.03). A single bout of hot water immersion elicits the greatest thermoregulatory, cardiovascular, and immune responses compared with both sauna modalities.<b>NEW & NOTEWORTHY</b> Our goal was to compare three widely utilized modalities of passive heating in the manner that they are most commonly used for heat therapy. We have demonstrated that hot water immersion leads to the greatest thermoregulatory, cardiovascular, and immune responses compared with both traditional and far infrared saunas in young, healthy adults. Far infrared sauna heating was the least impactful on raising body core temperature and the resulting cardiovascular and immune responses.