Validity of Humidity Ramp Protocols for Identifying Limits of Survivability in Heat-exposed Persons
Condition: Heat Stress; Heat Exposure; Temperature Change, Body; Temperature; Extreme, Exposure
MeSH terms: Body Temperature Changes
Study type: Interventional

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.
    


Intervention name(s): Humidity-ramp protocol; Above-inflection fixed-condition exposure; Below-inflection fixed-condition exposure; Control fixed-condition exposure
Intervention type(s): Other; Other; Other; Other
Intervention description(s): 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.; 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.; 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.; Participants are exposed for 9-hours to 28°C with 35% relative humidity. Participant will be allowed to drink tap water ad libitum.
Allocation: Randomized
Intervention model: Crossover Assignment
Intervention model description: Each participant will complete four trial conditions including one humidity-ramp protocol and three fixed-condition exposures. The humidity ramp protocol will always be completed first. Ambient temperature will remain constant at 42°C. After a 1-hour equilibrium period, relative humidity will be increased by 3% in 10-min increments, from 28% to 70%. The inflection point for core (esophageal) temperature will be subsequently determined.
On three separate days following the humidity ramp protocol, participants will complete three fixed-condition exposures in random order: 1) 42°C with relative humidity >5% below the estimate compensability limit (below-threshold condition); 2) 42°C with relative humidity >5% above the estimated compensability limit (above-threshold condition) and 3) 28°C with 35% relative humidity (control condition). Exposure will be terminated early if core temperature exceeds 39.2°C or if the participant requests to withdraw.
Masking: Double (Participant, Outcomes Assessor)
Primary purpose: Basic Science
Keywords: Heat wave; Hot weather; Heat strain; Cardiovascular strain; Environmental limits for compensability; Extreme heat events; Heat survivability