Validity of Humidity Ramp Protocols for Identifying Limits of Survivability in Heat-exposed Persons
- Conditions
- Temperature Change, BodyHeat ExposureTemperature; Extreme, ExposureHeat Stress
- Interventions
- Other: Control fixed-condition exposureOther: Humidity-ramp protocolOther: Below-inflection fixed-condition exposureOther: Above-inflection fixed-condition exposure
- Registration Number
- NCT05963529
- Lead Sponsor
- University of Ottawa
- Brief Summary
The global populace is at growing risk of heat-related illness due to climate change and accompanying increases in the intensity and regularity of extremely hot temperatures. In heat-exposed persons, heat gain from the environment and metabolism initially exceeds the rate of heat dissipation from the skin. Heat is stored in the body, causing core and skin temperatures to rise, which in turn triggers autonomically mediated elevations in cutaneous blood flow and sweating to facilitate heat loss. If conditions are compensable, heat loss increases until it balances total heat gain. At this point, the rate of heat storage falls to zero (i.e., heat balance is achieved) and body temperature stabilizes, albeit at a level elevated from thermoneutral conditions. If, however, the maximal achievable rate of heat dissipation is insufficient to offset heat gain, conditions are uncompensable, and prolonged exposure will cause a continual rise in core temperature that can compromise health if left unchecked. The environmental limits of compensability (i.e., the temperatures/humidities above which heat balance can not be maintained) are therefore an important determinant of survival during prolonged heat exposure. Evaluating this limit and how it can be modified (e.g., by behavior or individual factors like age or sex) is an increasingly important and active field of study.
Contemporary evaluations of the environmental limits of compensability utilize "ramping protocols" in which participants are exposed to increasing levels of temperature or humidity (in 5-10 min stages) while core temperature is monitored. It is generally observed that core temperature is relatively stable (or rises slightly) in the early stages of exposure but undergoes an abrupt and rapid increase as heat stress becomes more severe. The conditions (e.g., wet-bulb temperature or wet-bulb globe temperature) at this "inflection point" are taken as the limits of compensability. That is, it is assumed that inflection corresponds to the demarcation point, below which core temperature would remain stable for prolonged periods (theoretically indefinitely if hydration is maintained) but above which heat loss is insufficient to offset heat gain, causing core temperature to rise continuously. Despite the increasing use of these protocols, no study has clearly demonstrated their validity for identifying the environmental limits of compensability. The goal of this project is therefore to assess the validity of ramping protocols for determining the ambient conditions above which thermal compensation is not possible.
Enrolled participants will complete four experimental trials in a climate-controlled chamber: one ramping protocol followed by three randomized fixed-condition exposures. In the ramping protocol, participants will rest in 42°C with 28% relative humidity (RH) for 70 min, after which RH will be increased 3% every 10 min until 70% RH is achieved. The core (esophageal) temperature inflection point will be determined. For the fixed-condition exposures, participants will rest in i) 42°C with RH \~5% below their individual inflection point (below-inflection condition), ii) 42°C with RH \~5% above their individual inflection point (above-inflection condition), and iii) 26°C with 45% RH (control condition). Comparing the rate of change in esophageal temperature between each fixed-condition exposure will provide important insight into the validity of ramping protocols for identifying the limits of compensability.
- Detailed Description
Not available
Recruitment & Eligibility
- Status
- COMPLETED
- Sex
- All
- Target Recruitment
- 12
- Male or female adults.
- Aged 18-85 years.
- Non-smoking.
- English or French speaking.
- Ability to provide informed consent.
- Physical restriction (e.g., due to disease: 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 (e.g., medications increasing risk of heat-related illness; beta blockers, anticholinergics, etc.)
Study & Design
- Study Type
- INTERVENTIONAL
- Study Design
- CROSSOVER
- Arm && Interventions
Group Intervention Description Control fixed-condition exposure Control fixed-condition exposure Healthy male and female volunteers. Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized. Humidity-ramp protocol Humidity-ramp protocol Healthy male and female volunteers. Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized. Below-inflection fixed-condition exposure Below-inflection fixed-condition exposure Healthy male and female volunteers. Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized. Above-inflection fixed-condition exposure Above-inflection fixed-condition exposure Healthy male and female volunteers. Participants will complete all exposures. The humidity-ramp protocol will necessarily be performed first. The order of the fixed-condition exposures will be randomized.
- Primary Outcome Measures
Name Time Method Esophageal temperature rate of change End of fixed-condition heat exposure (hour 9 or termination) Rate of change of esophageal temperature measured over the final 2-hours of heat exposure
Predicted time until 40.2°C esophageal temperature End of fixed-condition heat exposure (hour 9 or termination) Estimated time for esophageal temperature to reach 40.2°C, a commonly-cited criteria for severe heat-illness (e.g., heat stroke), calculated from the end-exposure esophageal temperature and its rate of change (assuming this rate of change is sustained)
Predicted time until 40.2°C rectal temperature 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
Rectal temperature rate of change 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
- Secondary Outcome Measures
Name Time Method Mean skin temperature rate of change 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
Thermal sensation 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).
Net fluid loss 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.
Heart rate rate of change 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
Esophageal temperature End of fixed-condition heat exposure (hour 9 or termination) Esophageal temperature at the end of the fixed-condition exposures
Heart rate End of fixed-condition heat exposure (hour 9 or termination) Heart rate at the end of the fixed-condition exposures
Sweat rate 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)
Number of participants unable to finish fixed-condition heat exposure 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
Mean skin temperature End of fixed-condition heat exposure (hour 9 or termination) Mean skin temperature at the end of the fixed-condition exposures
Rectal temperature End of fixed-condition heat exposure (hour 9 or termination) Rectal temperature at the end of the fixed-condition exposures
Trial Locations
- Locations (1)
University of Ottawa
🇨🇦Ottawa, Ontario, Canada