Post-exercise Hot Water Immersion to Improve Overnight Blood Pressure

In 2017, the American Heart Association updated standards for diagnosing hypertension, while emphasizing the need for lifestyle modification in the early stages of blood pressure elevation. Using these updated criteria, approximately 1/3 of U.S. adults have hypertension, but few are able to improve blood pressure through diet and exercise and often progress to more severe disease states. Novel interventions to promote blood pressure reduction in mild hypertension are needed to attenuate or prevent this progression to more severe hypertension. Heat therapy, the repeated application of passive heat through hot water immersion, has gained recent attention for promising blood pressure reduction and improvement in cardiovascular health. Preliminary work combining exercise and hot water leg immersion indicates improved overnight blood pressure in individuals with elevated blood pressure, but the magnitude and mechanisms of this improvement require further study.

It is unknown whether the decrease in blood pressure following post-exercise heat exposure results from changes in systemic regulation (autonomic nervous system function, measured through heart rate variability), local factors (improved blood vessel function, measured through flow-mediated dilation), or both. The goal of the proposed research is to examine the impact of exercise alone (30 minutes of treadmill walking), hot water immersion alone (45 minutes in a leg bath set to 42°C), and combined exercise + hot water immersion (30 minutes treadmill walking followed by 45 minutes in a 42°C leg bath) on nocturnal blood pressure, autonomic function, and blood vessel function in individuals with elevated blood pressure. This goal will be addressed with the following specific aims:

Aim 1. To examine the impact of exercise, hot water leg immersion, or combined exercise + hot water immersion on overnight blood pressure in individuals with elevated blood pressure. The researchers hypothesize that the combination of exercise and hot water immersion will result in reduced systolic and diastolic blood pressure through 12 hours post-exercise as compared to baseline or either standalone treatment.

Aim 2. To examine changes in autonomic function, as measured by heart rate variability (HRV) following exercise, leg heating, or combined therapies. The researchers hypothesize that combined exercise and hot water immersion will result in higher HRV (indicating improved autonomic balance) compared to baseline measures.

Aim 3. To examine changes in blood vessel (endothelial) function, measured using flow-mediated dilation, following exercise, leg heating, or combined therapies. The researchers hypothesize that combined exercise and hot water immersion will result in a higher flow-mediated dilation (indicating improved blood vessel relaxation) compared to baseline or either standalone intervention.

The results of this study have important health implications in the treatment of hypertension using lifestyle interventions for the millions of Americans with elevated blood pressure or hypertension. The findings have the potential to improve non-pharmacological treatment of hypertension by developing an exercise and heat exposure protocol that is cost-effective, easily implemented, and results in substantially reduced blood pressure and improved cardiovascular risk profile.

Inclusion criteria

• resting blood pressure >120/80 mmHg; measured at screening visit
• BMI between 18-39.9
• capable of walking 30 min at a moderate intensity on a treadmill

Exclusion criteria

• diagnosed hypertension
• taking antihypertensive medications
• history of heat injury or heat illness