Aging is associated with declines in cardiovascular health and cognitive function as well as chronic inflammation and the development of neurodegenerative conditions such as Alzheimer's and Parkinson's disease. Additionally, ageing results in a loss of muscle size and strength, commonly referred to as sarcopenia, leading to increased risk of falls or adverse exercise-related events. It is well understood that frequent exercise improves cardiovascular and metabolic health whilst lowering the risk of disease and all-cause mortality, acting as a treatment for many different health conditions. Despite the evidence of its efficacy, many individuals struggle with adherence to exercise programmes, particularly older adults and clinical populations, with many individuals within these populations unable to perform exercise due to physical limitations or medical contraindications. With one in six people expected to be over the age of 65 years by 2050 and corresponding increases in non-communicable diseases (e.g., cancer, diabetes, cardiovascular disease) anticipated from this aging population, the development of adjunct treatments or alternatives to exercise to improve general health in older populations is needed.

Recent work has shown one such potential solution could be the use of passive heat therapy (PHT) (e.g., hot water immersion, sauna), which can achieve health benefits comparable to exercise. Acute PHT induces vascular improvements including increased blood flow, reduced blood pressure. and macro- and microvascular dilator function in various populations. Acute PHT has also been shown to increase reaction time, while mixed results have been seen in its effect on executive function in both young and older populations. The mechanisms for these improvements, however, are currently unclear, and may be related to increases in nitric oxide activity or reductions in biomarkers of inflammation.

One potential mechanism for the physiological benefits obtained from PHT is the increase of cytoprotective heat shock proteins (HSP) (specifically HSP27, HSP60, HSP70 and HSP90) intracellularly (i) and extracellularly (e) in response to increases in deep body temperature. Increases in the expression of these HSPs have been shown to play a role in reducing inflammation and protecting the vasculature via increases in endothelial nitric oxide synthase activity. Increased HSP concentrations may also prevent the accumulation of harmful biomarkers in the brain that play a role in cognitive decline and development of neurodegenerative diseases. There is also evidence that increased preoperative levels of both iHSP70 and eHSP70 are correlated with improved postoperative outcomes following cardiac surgery. Aging, however, is associated with a reduction in HSP transcription activity in various tissues, resulting in an impaired heat shock response and development of a pro-inflammatory state.

Previous work using acute hot water immersion has shown mixed results, with various studies showing increases in eHSP70, iHSP70 and iHSP90, while others have reported no change in expression. Heating stimuli applied in these studies has varied in magnitude, duration and whole body or not, and measurements of HSPs have taken place at various time points before, during and after PHT. Therefore, it is possible that upregulation of HSPs may have been missed, as no time course relationship exists between a single bout of PHT and upregulation of HSPs. Additionally, the duration for acute vascular and cognitive benefits from PHT and when function is returned to baseline is yet to be determined. A time course relationship study to determine the peak HSP response to acute passive heat therapy in older adults would provide insight into the relationship between acute changes in vascular or cognitive function and concurrent upregulation of both intracellular and extracellular HSPs, as well as its potential implications for improved surgical outcomes.