NCT Number: NCT07032493
Climate change increases extreme heat events, elevating global heat-illness risk. Females have reduced heat loss capacity (~5%) compared to males, driven by differences in skin blood flow and sweating responses. While findings on sex-mediated mortality are mixed, some studies suggest older females (≥65 years), face higher heat-related mortality/morbidity risks, evidenced by disproportionate female deaths in the 2021 Western Heat Dome. The effects of extreme uncompensable heat on older females remain understudied.
Heat exposure initially causes net heat gain, raising core/skin temperatures and triggering heat-loss responses. Under compensable heat stress, heat loss balances gain, stabilizing core temperature. Uncompensable heat stress (exceeding maximal dissipation capacity) causes continuous core temperature rise, posing severe health risks. The specific temperature and relative humidity (RH) limits where compensability is lost are critical survival determinants, influenced by age and sex.
Ramping protocols identify these limits: participants face progressively increasing heat stress (e.g., staged humidity rises) while core temperature is monitored. Core temperature typically stabilizes initially, then exhibits an abrupt rapid increase at an inflection point, operationally defined as the limit of compensability. Despite increasing use, ramping protocol validity for accurately identifying this threshold remains unverified.
This project assesses ramping protocol validity for determining uncompensable conditions in older females and evaluates cumulative thermal and cardiovascular strain, as well as psychological and cognitive responses to both uncompensable and compensable heat. Participants will complete five trials. Trial 1 (Ramping): Rest at 42°C, 28% RH for 70min, then incremental RH increases (3% every 10min) to 70% RH. Individual core temperature (rectal) inflection points are identified from the ramping trial. Trials 2-5 (Fixed Conditions, Randomized): i) ~10% below inflection; ii) ~5% below inflection; iii) ~5% above inflection; iv) Thermo-neutral control (26°C, 45% RH). Comparing the rate of rectal temperature change and cumulative strain during prolonged fixed exposures (especially below vs. above inflection) will validate if the ramping inflection point represents the true limit of compensability for older females.
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Notify Me65 year–85 year
Female
Interventional
Not applicable
University of Ottawa, Ottawa, Ontario, Canada
Healthy volunteers accepted: Yes
Only the study team can determine whether someone qualifies for participation.
Inclusion criteria
Exclusion criteria
Participants are exposed to 42°C and 28% relative humidity for 70 minutes. Thereafter, humidity is increased 3% until an ambient humidity of 70% is achieved. The humidity at which rectal temperature inflect is subsequently determined.
After a 1 hour equilibrium at 42°C and 28% relative humidity, humidity will be increased 3% every 10 min until it is ~5% higher than the participants' rectal esophageal temperature inflection point identified in the humidity ramp protocol. These conditions will be held constant for the remainder of the 9-hour exposure period (starting from the beginning of equilibrium). Tap water will be provided at regular intervals to limit dehydration.
After a 1 hour equilibrium at 42°C and 28% relative humidity, humidity will be increased 3% every 10 min until it is ~5% lower than the participants' individual rectal temperature inflection point identified in the humidity ramp protocol. These conditions will be held constant for the remainder of the 9-hour exposure period (starting from the beginning of equilibrium). Tap water will be provided at regular intervals to limit dehydration.
After a 1 hour equilibrium at 42°C and 28% relative humidity, humidity will be increased 3% every 10 min until it is ~10% lower than the participants' individual rectal temperature inflection point identified in the humidity ramp protocol. These conditions will be held constant for the remainder of the 9-hour exposure period (starting from the beginning of equilibrium). Tap water will be provided at regular intervals to limit dehydration.
Participants are exposed for 9-hours to 28°C with 35% relative humidity. Participant will be allowed to drink tap water ad libitum.
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Rate of change of rectal temperature measured over the final 2-hours of heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Number of participants who could not complete each fixed-condition heat exposure due to participant withdrawal or achieving a core temperature of 39.5°C
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Rate of change of skin temperature measured over the final 2-hours of heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Area under the curve (AUC) of rectal temperature (in degree-hours) measured over the fixed-condition heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Estimated time for rectal temperature to reach 40.2°C, a commonly-cited criteria for severe heat-illness (e.g., heat stroke), calculated from the end-exposure rectal temperature and its rate of change (assuming this rate of change is sustained)
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Rate of change of heart rate measured over the final 2-hours of heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Area under the curve (AUC) of heart rate measured over the fixed-condition heat exposure
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Self-reported thermal sensation at the end of the fixed-condition exposures using and 8-point scale ranging from neutral (0) to extremely hot (8).
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Thermal comfort assessed via a visual analog scale ranging from extremely uncomfortable to extremely comfortable (midpoint: neutral).
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Sweat rate at the end of the fixed-condition exposures (calculated via change in body weight)
Time frame: End of fixed-condition heat exposure (hour 9 or termination)
Net fluid loss estimated as the percentage change in body mass over the fixed-condition exposures.
Time frame: Prior to (hour 0) and at the end of fixed-condition heat exposure (hour 9 or termination)
Potential changes in mood (7 subscales of mood: tension, anger, depression, fatigue, confusion, vigor and esteem-related affect). The POMS-40 is a validated, self-administered questionnaire that examines seven distinct aspects of mood state across two positive subscales (Esteem-Related Affect, and Vigor) and five negative subscales (Fatigue, Tension, Confusion, Anger, and Depression), which are described across 40 distinct adjectives (reference). For each individual item, participants were asked to describe "how you feel right now" by responding using a 5-point Likert scale (0 = "Not at all", 1 = "A little", 2 = "Moderately", 3 = "Quite a lot", or 4 = "Extremely"). The values of items associated with a specific subscale (e.g., Fatigue) were summed to calculate its score.
Time frame: Prior to (hour 0) and at the end of fixed-condition heat exposure (hour 9 or termination)
Self-reported environmental reactions and medical symptomatology associated with prolonged heat exposure. The ESQ-IV is a validated 68-item, self-administered questionnaire that has been used successfully in identifying symptomatology during exposure to a wide variety of environmental conditions, including heat exposure [24, 25]. Participants are asked to assess and described "how you have been feeling today" by responding to each item using a 6-point Likert scale (0 = "Not at all", 1 = "Slight", 2 = "Somewhat", 3 = "Moderate", 4 = "Quite a bit", or 5 = "Extreme"). Total Symptom Score was calculated from this data by taking the sum of the intensity ratings from all 68 individual items using reverse scores for the three positive items from the list ("I Felt Good", "I Felt Alert", and "I Felt Wide Awake").
Time frame: During the ramp protocol (up to 2.5 hours)
Inflection point for rectal temperature during the humidity-ramp protocol
Time frame: During the ramp protocol (up to 2.5 hours)
Inflection point for heart rate during the humidity-ramp protocol
University of Ottawa
Other
Identifying the Upper Environmental Limits of Thermal Compensation in Older Human Females Using a Rapid Humidity Ramp Protocol
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