Identifying the Limits of Survivability in Heat-exposed Older Females

University of Ottawa

Status: Enrolling

Conditions: Aging; Heat Exposure; Thermoregulation; Temperature Change, Body; Heat Stress

Treatments: Other: Humidity-Ramp Protocol; Other: Below-inflection fixed-condition exposure (10%); Other: Control fixed-condition exposure; Other: Above-inflection fixed-condition exposure (5%); Other: Below-inflection fixed-condition exposure (5%)

Study type: Interventional

Funder types: Other

Identifiers

NCT07032493

HEPRU-2025-05-A

Details and patient eligibility

About

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.

Enrollment: 12 estimated patients

Sex: Female

Ages: 65 to 85 years old

Volunteers: Accepts Healthy Volunteers

Inclusion criteria

• Non-smoking.
• English or French speaking.
• Ability to provide informed consent.
• With or without a) chronic hypertension (elevated resting blood pressure; as defined by Heart and Stroke Canada and Hypertension Canada), b) type 2 diabetes as defined by Diabetes Canada, with at least 5 years having elapsed since time of diagnosis

Exclusion criteria

• Episode(s) of severe hypoglycemia (requiring the assistance of another person) within the previous year, or inability to sense hypoglycemia (hypoglycemia unawareness).
• Serious complications related to diabetes (gastroparesis, renal disease, uncontrolled hypertension, severe autonomic neuropathy).
• Uncontrolled hypertension - BP >150 mmHg systolic or >95 mmHg diastolic in a sitting position.
• Restrictions in physical activity due to disease (e.g. intermittent claudication, renal impairment, active proliferative retinopathy, unstable cardiac or pulmonary disease, disabling stroke, severe arthritis, etc.).
• Use of or changes in medication judged by the patient or investigators to make participation in this study inadvisable.
• Cardiac abnormalities identified during screening

Trial design

Primary purpose: Basic Science

Allocation: Randomized

Interventional model: Crossover Assignment

Masking: Double Blind

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12 participants in 5 patient groups

Humidity-ramp protocol

Other group

Description:

The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.

Treatment:

Other: Humidity-Ramp Protocol

Below-inflection fixed-condition heat exposure (5%)

Experimental group

Description:

Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.

Treatment:

Other: Below-inflection fixed-condition exposure (5%)

Control fixed-condition heat exposure

Active Comparator group

Description:

Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.

Treatment:

Other: Control fixed-condition exposure

Below-inflection fixed-condition heat exposure (10%)

Experimental group

Description:

Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.

Treatment:

Other: Below-inflection fixed-condition exposure (10%)

Above-inflection fixed-condition heat exposure (5%)

Experimental group

Description:

Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.

Treatment:

Other: Above-inflection fixed-condition exposure (5%)