Aging, Heat Exposure, Heat Stress, Hyperthermia, Thermoregulation
Conditions
Keywords
Thermoregulation, Warm-water immersion, Heat acclimation, Aging
Brief summary
Aging is associated with impairments in heat loss responses of skin blood flow and sweating leading to reductions in whole-body heat loss. Consequently, older adults store more body heat and experience greater elevations in core temperature during heat exposure at rest and during exercise. This maladaptive response occurs in adults as young as 40 years of age. Recently, heat acclimation associated with repeated bouts of exercise in the heat performed over 7 successive days has been shown to enhance whole-body heat loss in older adults, leading to a reduction in body heat storage. However, performing exercise in the heat may not be well tolerated or feasible for many older adults. Passive heat acclimation, such as the use of warm-water immersion may be an effective, alternative method to enhance heat-loss capacity in older adults. Thus, the following study aims to assess the effectiveness of a 7-day warm-water immersion (\ 40°C) protocol in enhancing whole-body heat loss in older adults. Warm-water immersion will consist of a one-hour immersion in warm water with core temperature clamped at 38.5°C. Improvements in whole-body heat loss will be assessed during an incremental exercise protocol performed in dry heat (i.e., 40°C, \ 15% relative humidity) prior to and following the 7-day passive heat acclimation protocol. The incremental exercise protocol will consist of three 30 minute exercise bouts performed at increasing fixed rates of metabolic heat production (i.e., 150, 200, and 250 W/m2), each separated by 15-minutes of recovery, with exception final recovery will be 1-hour in duration) performed in a direct calorimeter (a device that provides a precise measurement of the heat dissipated by the human body).
Interventions
OTHERHeat acclimation
Participants will complete a 7-day passive heat acclimation protocol consisting of immersion in warm water (\ 40°C) for 1 hour over 7 consecutive days.
Sponsors
University of Ottawa
Study design
Allocation
NA
Intervention model
SINGLE_GROUP
Primary purpose
BASIC_SCIENCE
Masking
NONE
Masking description
Data will be blinded prior to analysis.
Intervention model description
Each participant will complete seven consecutive experimental sessions consisting of a passive heat acclimation involving a one hour immersion in warm water (\ 40°C) with core temperature clamped at 38.5°C. Prior to and following the passive heat acclimation protocol, study participants will perform an exercise-heat stress test (Days 0 and 8) to assess whole-body heat loss in a hot-dry environment (40°C, 15% relative humidity). The exercise protocol will consist of three successive 30-minute bouts of exercise performed at increasing rates of metabolic heat production (i.e., 150, 200 and 250 W/m2), each separated by a 15-minute rest break, with the final recovery 1-hour in duration.
Eligibility
Sex/Gender
MALE
Age
60 Years to 80 Years
Healthy volunteers
Yes
Inclusion criteria
* Elderly (60-80 years), non-smoking adults. * Not engaged in regular physical activity (\>2 sessions/week for ≥20 minutes per session). * Willing to provide informed consent. * Healthy, no diagnosed health conditions. * Body Mass index (BMI) \<35 kg/m2.
Exclusion criteria
* Heat adapted due to repeated exposure to hot environments within the last 3 weeks (use sauna, recent travel to hot climates, other).
Design outcomes
Primary
| Measure | Time frame | Description |
|---|---|---|
| Evaporative heat loss | End of 30 minute exercise bout 1 (average of last 5 minutes). | Evaporative heat loss as assessed using a direct air calorimeter. |
| Whole-body heat loss | End of 30 minute exercise bout 1 (average of last 5 minutes). | The sum of evaporative and dry heat exchange will be quantified during each exercise period (i.e., 150, 200 and 250 W/m2). |
Secondary
| Measure | Time frame | Description |
|---|---|---|
| Body heat content | Change from start to end of 30 minute exercise bout 2 | Cumulative amount of heat stored within the body calculated as the temporal summation of metabolic heat production and net heat loss. |
| Cumulative body heat storage | Change from baseline resting to end of exercise bout 3 | Cumulative amount of heat stored within the body calculated as the temporal summation of metabolic heat production and net heat loss. |
| Dry heat loss | End of 30 minute exercise bout 1 (average of last 5 minutes). | Dry heat loss as assessed using a direct air calorimeter. |
| Change in core temperature | Change from baseline resting to end of end of 30 minute exercise bout 3. | Core temperature indexed by rectal/visceral temperature |
| Skin temperature | End of 30 minute exercise bout 1 (average of last 5 minutes). | Skin temperature measured at 4-sites (chest, upper arm, thigh, calf) with mean value calculated as weighted value of 4 sites - upper arm, 30%; chest, 30%; thigh, 20%; and calf, 20%. |
| Change in skin temperature | Change from baseline resting to end of end of 30 minute exercise bout 3. | Skin temperature measured at 4-sites (chest, upper arm, thigh, calf) with mean value calculated as weighted value of 4 sites - upper arm, 30%; chest, 30%; thigh, 20%; and calf, 20%. |
| Core temperature | End of 30 minute exercise bout 1 (average of last 5 minutes). | Core temperature indexed by rectal/visceral temperature |
| Heart rate variability | End of 30 minute exercise bout 1 (average of last 5 minutes). | Measures of variability computed from the time, frequency, time-frequency, scale-invariant, entropy, and other nonlinear domains |
| Thermal comfort scale | End of 30 minute exercise bout 1 (average of last 5 minutes). | Thermal comfort assessed via a questionnaire using the ASHRAE 4-point scale (1=comfortable to 4=very uncomfortable) |
| Thermal sensation scale | End of 30 minute exercise bout 1 (average of last 5 minutes). | Thermal sensation will be assessed using the ASHRAE 7-point scale (0=neutral to 7=extremely hot) |
| Thirst sensation scale | End of 30 minute exercise bout 1 (average of last 5 minutes). | Thirst sensation will be quantified using a 9-point scale (1=not thirsty at all to 9=very, very thirsty) |
| Rating of perceived exertion | End of 30 minute exercise bout 1 (average of last 5 minutes). | Rating of perceived exertion will be quantified using a 14-point scale (6=no exertion at all to 20=maximal exertion) |
| Heart rate | End of 30 minute exercise bout 1 (average of last 5 minutes). | Measured continuously with chest-band device |
| Body heat storage | Change from start to end of 30 minute exercise bout 1 | Cumulative amount of heat stored within the body calculated as the temporal summation of metabolic heat production and net heat loss. |
Countries
Canada
Outcome results
None listed