Cardioprotective Benefit of Passive Heat Therapy in Higher-Level Spinal Cord Injury: Safety and Proof of Concept

The risk of cardiovascular diseases for people with a spinal cord injury (SCI) is approximately 3x greater than the general population, and Cardiovascular (CV) diseases account for ~40% of deaths in SCI. Traditional CV risk factors are unable to fully explain these outcomes. This group also has other secondary complications that are concerning, including susceptibility to infections and metabolic health issues. Some of the increased risk in SCI is due to a relatively sedentary lifestyle and insufficient exercise capability. This is particularly insidious in those living with tetraplegia with poor cardiovascular responses to exercise or even the inability to exercise due to upper limb paralysis.

Heat therapy in the form of hot baths and saunas has been utilized for centuries, with common reports of improved quality of life and overall improved well-being. A 20-year prospective study on 2,315 middle-aged men (at enrolment) in Finland reported that regular sauna bathing was associated with reduced risk of sudden cardiac death, coronary heart disease, CV disease, and all-cause mortality. Further, more frequent sauna bathing sessions were related to a considerably decreased risk of fatal cardiac and CV events, as well as all-cause mortality independently from conventional risk factors, activity levels, and diet. The mechanisms which underlie these improvements remain largely unknown. It has been proposed that CV benefits of exercise training are partly mediated by repeated increases in core temperature. Exercise and heat stress elicit many common physiological responses, including increases in core temperature, heart rate and contractility, redistribution of blood flow, and changes in conduit vessel endothelial shear stress, all of which are believed to improve CV health. Additionally, heat exposure and exercise induce the expression of heat shock proteins, which stabilize a variety of other proteins that reduce vascular inflammation and oxidative stress, improve NO bioavailability, promote angiogenesis, and improve glucose signaling.

SCI results in a host of physiological changes that greatly increase CV risk and vascular dysfunction following injury. Increased arterial stiffness and vascular resistance are common features in SCI, likely secondary to the loss of sympathetic innervation below the lesion, increased circulating vasoconstrictors, and subsequent oxidative stress and vascular inflammation. Heat exposure is a stress to the CV system, like exercise, which may result in numerous chronic adaptations that improve vascular and metabolic function, resulting in a profoundly reduced risk of CV disease.

Passive heat therapy may provide a novel and practical approach to help these individuals. There have been observed profound improvements in vascular and metabolic function as well as indications of benefits to the immune system and reductions in inflammation in healthy but sedentary able-bodied people after long-term passive heat exposure. Similar benefits were observed in obese women with polycystic ovary syndrome. Recent studies in SCI with lower limb and full-body (only in low level SCI) immersion have demonstrated the potential of hot water immersion to be safe, with the potential to stimulate a cardio-protective response. The proposed pilot study is the first step in developing a unique approach for combating CV dysfunction in the SCI population.

Specifically, the goal of this pilot study is to build on the current literature and demonstrate the safety, tolerance, and effectiveness (providing a mild/moderate exercise response) of a ~ 45-minute full-body hot water immersion in those with higher level SCI (T7-C4). Ultimately, the investigators will develop a protocol for translating these and future findings to the home environment and use of a typical bathtub, thereby demonstrating that passive heat therapy is a viable approach to improve health in the SCI population.