NCT05963529

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

Completed NA Last updated 29 November 2023

What this trial tests

NA trial testing Humidity-ramp protocol in Heat Stress in 12 participants. Completed in 24 September 2023.

Timeline

14 July 2023

Primary endpoint
24 September 2023

24 September 2023

Quick facts

Lead sponsor	University of Ottawa
Phase	NA
Status	Completed
Study type	INTERVENTIONAL
Allocation	randomized
Design	crossover
Masking	double
Primary purpose	basic science
Enrollment	12
Start date	14 July 2023
Primary completion	24 September 2023
Estimated completion	24 September 2023
Sites	1 location across Canada

Drugs / interventions tested

• Humidity-ramp protocol
• Above-inflection fixed-condition exposure
• Below-inflection fixed-condition exposure
• Control fixed-condition exposure

Conditions studied

• Heat Stress — all drugs for Heat Stress →
• Heat Exposure — all drugs for Heat Exposure →
• Temperature Change, Body — all drugs for Temperature Change, Body →
• Temperature; Extreme, Exposure — all drugs for Temperature; Extreme, Exposure →

Sponsor

University of Ottawa

Who can join

Adults 18 to 85, any sex, with Heat Stress or Heat Exposure. Patients with the condition only — healthy volunteers not accepted.

Sponsor's own description

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.

Publications & conference data

1 peer-reviewed publication reference this trial (live from Europe PMC):

1. Validating new limits for human thermoregulation.
   Meade RD, O'Connor FK, Richards BJ, Tetzlaff EJ, et al · · 2025 · cited 4× · PMID 40163728 · DOI 10.1073/pnas.2421281122