In this study, we tested the feasibility of heat therapy sessions in older adults. We also performed pilot analyses on outcomes associated with benefits of heat therapy on cardiovascular fitness. Additionally, we noninvasively measured hemodynamic variables in humans while submerged to shoulder height water in order to ascertain the effects of water pressure on the human venous system. Older adults were chosen for this study as this demographic may realize the most benefit from beneficial effects of heat therapy on endovascular and arterial function. Additionally, older adults may be a group that would not tolerate sequential hot tub therapy sessions of 45-minutes each.
We found that all 15 participants were able to tolerate the hot tub sessions and also complete the required pre- and post-intervention treadmill exercise testing. The protocol utilized in this study of 8–10 sessions within a 14-day period was developed in hopes of developing “chronic” cardiovascular benefits of heat therapy instead of acute benefits. We also found that all subjects were able to be successfully monitored with a fingertip blood pressure cuff with reliable readings of heart rate, blood pressure and calculations of SVR and cardiac index while in the hot tub for the entirety of each session. The technology utilized in this novel method of SVR and cardiac index calculation is performed through volume clamping technology in which equal pressures are dynamically provided on either side of the wall of the artery by clamping the artery to a certain constant volume. This information is applied to already practiced pulse contour technology to algorithmically calculate various hemodynamic variables.10
Not only were measurements of heart rate, blood pressure and cardiac index important to understand for feasibility purposes, but also in terms of the effects of the pressure of water on the human cardiovascular system. When patients are subjected to external water pressure, which exceeds venous pressure, blood is displaced upward into the great vessels of the chest cavity and into the heart. Central blood volume increases by approximately 0.7 liters during immersion to the neck, which results in concomitant changes to heart rate.11 Heart rate typically decreases in neutral temperature water by 12–15%, but increases significantly in warm water contributing to yet a further rise in cardiac output at high temperatures.12,13 Additionally, the increase in SVR is off-set by increasing temperature14 and must be accounted for by strict hot tub temperature management, volunteer rectal temperature monitoring and volunteer compliance. Due to the amalgam of these inter-related cardiovascular changes, we asked for participants to remain in each hot tub session for 45-minutes with hopes of finding a steady state for these effects of hot water submersion.
In this pilot study, there was no difference in the main outcome of cardiovascular function and resultant exercise tolerance as measured by VO2 max (Table 2, Fig. 1). Cardiac index did have a signal toward significance however, this may have been due to one outlying subject (Fig. 2). Additionally, the participants did have an increase in exercise duration of a mean of 20 seconds following the hot tub sessions. Other measures of cardiovascular fitness were unchanged by heat therapy intervention.
Healthy, distensible arteries allow smoothing of blood pressure variations and ensure blood flow in one direction to all vital organ systems. As arteries stiffen and lose elasticity, pulsatility is dampened causing wider blood pressure fluctuations within the micro- and microvasculature placing vital organs such as the heart, brain and kidneys at risk.15 Endothelial dysfunction coincides with a decreased production of endogenous vasodilatory mediators, such as nitric oxide, worsening the nondistensibility and lack of vasodilation. Ultimately this lack of elasticity leads to failure of regulation of blood flow and vital, end-organ damage results.
This study was undertaken due to previous work demonstrating that heat exposure can improve arterial endothelial function in both acute and chronic therapy studies.3–7 In acute, one-time passive heat versus exercise studies, hot-water immersion increased major artery blood flow and shear rate equivalent to exercising at a moderate intensity.6,7
In more chronic studies, heat therapy via water immersion (4–5 times per week for eight weeks) had beneficial effects on vascular dilatation and a reduction in arterial stiffness as measured by increased flow-mediated dilatation, improved arterial compliance, reduced aortic pulse wave velocity, decreased carotid intima media thickness and decreased blood pressure.5 Similarly, another previous study utilized infrared, dry sauna therapy daily for eight weeks in patients with cardiovascular risk. In patients with at least one coronary risk factor, flow-mediated dilatation was improved by 5.3%.4 A follow up study showed that in patients with congestive heart failure, heat therapy not only improved flow-mediated dilatation but also decreased plasma brain natriuretic peptide (BNP) levels indicating less congestive heart failure burden.16
These findings are important and can have have far reaching beneficial effects on not only individuals, but also society as more than one million hospitalizations for heart failure occur yearly.17 Improvements in vascular health of just 2% have been correlated with a 15% reduction in cardiovascular risk.5,9
Heat-associated mechanisms thought to be involved in the improvements in vascular tone with heat therapy include the expression of heat shock proteins (HSP).18 Heat shock proteins are cytoprotective molecular chaperones that accelerate cellular repair and block cell death.19 Heat shock proteins are also thought to be beneficial to arterial integrity in that there is association of anti-inflammatory and anti-oxidative effects with HSP response.20 Another HSP, Hsp72, is thought to be the HSP most important for thermotolerance by decreasing gut-associated endotoxin translocation and by reducing the body’s inflammatory response.21 Transgenic mice overexpressing Hsp72 display a 2-fold increase in running capacity relative to wildtype mice and exhibit a 50% increase in mitochondria.22 Increased expression of Hsp72 could therefore contribute to an increase in aerobic capacity following chronic heat therapy.
There are several possible reasons for the lack of change in many parameters of cardiovascular function in the present study. Firstly, it is possible that the quantity of heat therapy was not adequate to bring about chronic arterial endothelial changes. Other studies demonstrated changes after eight weeks of heat therapy. Perhaps 14 days is not sufficient for these changes, which is valuable information for future research. However, in acute, one-time heat versus exercise studies, the beneficial effects were noted immediately after intervention. We chose to not ascertain cardiovascular parameters immediately following heat therapy due to aforementioned reasons and hence, this could account for differences in our findings.
Secondly, as with any human subject study, numerous confounders in participants’ daily lives could not be controlled. Hydration and fatigue were not able to be adequately accounted for at each session. Thirdly, benefits of heat therapy may not be as evident in people with a high degree of pre-existing exercise tolerance as was the case with many of the volunteers in this study. Lastly, the benefits of heat therapy may have been missed without measurement of HSP’s and other biomarkers of arterial integrity and distensibility23 or ultrasonographic measurements of arterial blood flow. However, the pilot part of this study aimed to assess the translation of heat therapy to cardiovascular performance, not other markers of vascular health.
The present study identified that 8–10 hot tub sessions of 45 minutes each did not result in benefits of cardiovascular fitness as measured by oxygen consumption. Furthermore, this intervention did not change laboratory testing. However, older volunteers were able to comfortably undergo these sessions while having noninvasive hemodynamic monitoring recording of heart rate, blood pressure, systemic vascular resistance and cardiac index. With knowledge of the techniques and protocols utilized in this study, future studies may wish to examine the effects of heat therapy on patients who may more quickly realize the benefits, such as those with NYHA Class II -III heart failure. By identifying alternative mechanisms to improve endothelial function, more people can have improved cardiovascular function and potentially have improved fitness levels and freedom from hospitalizations. Utilizing the human body’s own molecular mechanisms to combat cardiovascular disease is the ideal prevention and therapeutic strategy.