Author(s): SU, Y., MARTINKOVA, A., ODONNELL, E., LEICHT, C., Institution: LOUGHBOROUGH UNIVERSITY , Country: UNITED KINGDOM, Abstract-ID: 1821

INTRODUCTION:
Heat therapy can promote cardiovascular health by lowering cardiovascular disease risk and mortality, potentially by reducing inflammation and altering arterial stiffness. While most research on heat therapy has focused on continuous heat exposure, traditional sauna practice involves alternating between heating and cooling down in 2-3 cycles. This study compares two methods: 60 min of continuous heating (CH) and intermittent heating (IH) consisting of three 20-min sessions with breaks.
METHODS:
Healthy young adults (n=21, 25.4±3.3 yrs, body mass index (BMI) 22.0±2.0 kg•m-2) underwent two trials in randomised order: CH exposure (air temperature: 71.8°C, humidity 7.6%) and IH exposure (72.0°C, 6.7%), separated by 15-min breaks (21.9°C, 41.8%). Blood samples to determine IL-6, IL-10 and IL-1ra plasma concentration were collected pre-, post- and 1 h post-intervention; brachial pulse pressure, central pulse pressure, augmentation index (AIx), rectal and skin temperature, basic affect and thermal comfort were assessed throughout the intervention.
RESULTS:
Rectal temperature was higher after CH compared with IH (P≤0.01, CH: 38.2±0.3°C, IH: 37.6±0.2°C). Plasma IL-6, IL-10 and IL-1ra concentrations did not differ between conditions (main effects: P≥0.383, interaction effects: P ≥0.057). An increase of IL-6 (P<0.001; pre to post, CH: 0.9±0.8 to 1.5±0.8 pg·ml−1, IH: 1.0±1.0 to 1.5±1.3 pg·ml−1) and IL-1ra was observed (P=0.008; pre to post, CH: 48.8±10.7 to 52.4±11.8 pg·ml−1, IH: 50.7±12.6 to 53.6±15.3 pg·ml−1). Post heat exposure, brachial and central pulse pressures were higher in CH than in IH (P≤0.002). In both conditions, AIx increased following 20 min of heat exposure (P≤0.042) but was lower in CH than IH at 40 and 60 min of heat exposure (P≤0.009). Post heat exposure, the mean skin temperature for CH was lower than for IH (CH: 38.1±1.5°C, IH: 38.7±1.0°C, P<0.001), and thermal discomfort was more pronounced for CH than IH (CH: 2.0±1.3, IH: 1.0±1.1, P=0.003).
CONCLUSION:
Despite CH leading to a higher rectal temperature than IH, CH and IH similarly raised the inflammatory markers IL-6 and IL-1ra, whilst the AIx response was blunted in IH following an initial increase. Both the inflammatory response as well as the temporary increase in AIx, indicating increased arterial stiffness, are comparable to the acute exercise response. Importantly, regular exercise can reduce resting inflammatory marker concentrations and improve arterial function and reduce stiffness, lowering AIx over time. It may hence be speculated that these observed acute changes during heat exposure may have long-term benefits of heat therapy, similar to regular exercise. Finally, although IH resulted in a higher mean skin temperature, this protocol was rated as more comfortable. These findings imply that for the inflammatory variables, the overall duration of passive heating might be more critical than a pronounced elevation of core temperature, which should be considered when designing effective heat therapy protocols.