Validity of Humidity Ramp Protocols for Identifying Limits of Survivability in Heat-exposed Persons

NCT ID: NCT05963529

Last Updated: 2023-11-29

Basic Information

• Recruitment Status: COMPLETED
• Clinical Phase: NA
• Total Enrollment: 12 participants
• Study Classification: INTERVENTIONAL
• Study Start Date: 2023-07-14
• Study Completion Date: 2023-09-24

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.

Conditions

Heat Stress; Heat Exposure; Temperature Change, Body; Temperature; Extreme, Exposure

Keywords

Heat wave; Hot weather; Heat strain; Cardiovascular strain; Environmental limits for compensability; Extreme heat events; Heat survivability

Study Design

• Allocation Method: RANDOMIZED
• Intervention Model: CROSSOVER
• Primary Study Purpose: BASIC_SCIENCE
• Blinding Strategy: DOUBLE

Study Groups

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.

Group Type: OTHER

Humidity-ramp protocol

Intervention Type: OTHER

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 esophageal temperature (and rectal temperature and heart rate) inflect is subsequently determined.

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.

Group Type: EXPERIMENTAL

Above-inflection fixed-condition exposure

Intervention Type: OTHER

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' individual 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.

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.

Group Type: EXPERIMENTAL

Below-inflection fixed-condition exposure

Intervention Type: OTHER

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 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.

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.

Group Type: ACTIVE_COMPARATOR

Control fixed-condition exposure

Intervention Type: OTHER

Participants are exposed for 9-hours to 28°C with 35% relative humidity. Participant will be allowed to drink tap water ad libitum.

Interventions

Humidity-ramp protocol

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 esophageal temperature (and rectal temperature and heart rate) inflect is subsequently determined.

Intervention Type: OTHER

Above-inflection fixed-condition exposure

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' individual 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.

Intervention Type: OTHER

Below-inflection fixed-condition exposure

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 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.

Intervention Type: OTHER

Control fixed-condition exposure

Participants are exposed for 9-hours to 28°C with 35% relative humidity. Participant will be allowed to drink tap water ad libitum.

Intervention Type: OTHER

Eligibility Criteria

Inclusion Criteria

• Male or female adults.
• Aged 18-85 years.
• Non-smoking.
• English or French speaking.
• Ability to provide informed consent.

Exclusion Criteria

• 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.)

• Minimum Eligible Age: 18 Years
• Maximum Eligible Age: 85 Years
• Eligible Sex: ALL
• Accepts Healthy Volunteers: Yes

Sponsors

University of Ottawa

OTHER

Sponsor Role: lead

Responsible Party

Glen P. Kenny

Full Professor

Responsibility Role: PRINCIPAL_INVESTIGATOR

Principal Investigators

Robert D Meade, PhD, MPH

Role: STUDY_DIRECTOR

University of Ottawa

Glen P Kenny, PhD

Role: PRINCIPAL_INVESTIGATOR

University of Ottawa

Locations

University of Ottawa

Ottawa, Ontario, Canada

Countries

Canada

Other Identifiers

uORamp

Identifier Type: -

Identifier Source: org_study_id