Effect of Local Heat Stress Via Leg Bath on Artery Function (HEAT)

Arteries are flexible in their structure and function and change in response to a variety of factors. Endothelial cells, are the skin type cells that form a layer inside arteries, and they are very important for arterial health. When they work properly, they produce substances that make the artery enlarge and support a healthy environment; but when they do not work properly, they produce substances that make the artery constrict and lead to a harmful environment. We can measure the function of these cells using an ultrasound machine and a quick test. The application of heat has been shown to change the structure and function of arteries, but we need to understand how these changes happen before we can effectively use heat as a therapy. Limb heating protocols (e.g., leg bath), in contrast with whole-body heating modes (e.g., sauna), are appealing as therapies because they generally cost less and are easier to use; but evidence is needed to demonstrate that they work before widespread use can be considered. Imagine soaking your legs in a bathtub filled with warm water becoming part of the options you have for staying healthy. Clearly, there is a lot of potential for heat to be used as a therapy, particularly for people who are unable to, or have difficulty participating in other health interventions that are known to improve arterial, but we need to do this type of research before we are able to recommend warm foot baths for therapeutic purposes.

A within-subjects interventional design will be used in a cohort of 16 young, healthy males and females between the ages of 18-35 years old. Interested individuals will be invited to the lab for a screening and familiarization visit, in which the brachial artery (BA) and superficial femoral artery (SFA) will be scanned to vet for ultrasound image quality and BA and SFA flow-mediated dilation tests will be performed to ensure participant tolerance during collection visits. According to current FMD guidelines, all qualified participants will be instructed to refrain from vigorous physical activity > 24 hours, alcohol and caffeine > 8 hours, and food > 6 hours prior to each of the two experimental visits. Participants will also be asked to ingest a wireless telemetry pill 2 hours prior to each of the two experimental visits. Upon arrival, basic anthropometric measures will be collected and hematocrit will be measured in duplicate with a finger prick blood sample equivalent to two 70 µl capillary tubes. Experimental visits will begin with 10 minutes of supine rest, while the participant is instrumented with skin temperature probes on the foot, calf, thigh, chest, and arm for measurement of skin temperature; single-lead ECG (Powerlab model ML795, ADInstruments, Colorado Springs, CO, USA) and non-invasive finger cuff (Finometer MIDI, Finapres, The Netherlands) for continuous heart rate and blood pressure monitoring. Following rest, BA and SFA FMD tests will be conducted to assess baseline endothelial function. Afterwards, single-point laser Doppler flowmetry focused on the forearm will be used to track skin blood flow continuously throughout heating. The heating protocol will involve participants immersing their lower limbs either up to the ankles or the knees in a 42 °C water bath for 45 minutes, in separate conditions. The order by which participants undergo these interventions will be randomized. Five minutes after the conclusion of the intervention, BA and SFA FMD tests will be conducted to assess endothelial function in the acute phase. BA and SFA blood velocities, blood flow turbulence, endothelial shear rate, and oscillatory shear index will be assessed before and during each of the interventions; and arterial stiffness defined as carotid-femoral pulse wave velocity (PWV) and common carotid artery (CCA) distensibility will be assessed before and after each of the interventions as additional outcomes. Participants will provide written informed consent prior to beginning any portion of the study.

Participants will undergo two interventions in separate visits: lower limb hot water (42 deg C) immersion up to the (1) ankles or (2) knees.

Calculated using blood velocity and arterial diameter obtained from vascular ultrasound, and blood density and viscosity calculated using hematocrit

Collected using ultrasound imaging and applanation tonometry, and analyzed using semi-automated edge tracking software and data acqusition software

Inclusion Criteria: Apparently health (i.e., no cardiovascular, musculoskeletal, or metabolic disease), 18-35 years old 18-35 years old Recreationally active (exercising 2-3 times per week) Exclusion Criteria: History of cardiovascular, musculoskeletal, or metabolic disease

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