UMEM Educational Pearls

Title: How do we gain and lose heat?

Keywords: hyperthermia, hypothermia, environmental, international (PubMed Search)

Posted: 8/12/2014 by Jon Mark Hirshon, PhD, MPH, MD (Updated: 8/20/2014)
Click here to contact Jon Mark Hirshon, PhD, MPH, MD

General Background:

Heat transfer is important to understand, especially when evaluating and treating someone who is hyperthermic or hypothermic. Are they really hot (or cold) from exposure, or is there an underlying metabolic or toxicological cause?

Mechanisms of Heat Transfer:

There are 4 main methods of heat transfer:

Radiation
- Transfer of thermal energy through space by electromagnetic waves
- Dependent on exposed surface area
- Normally about 60% of heat loss
Conduction
- Transfer of thermal energy through direct physical contact
- Depends on conductivity of the surface (stone floors > water > air)
- Can cause significant hypothermia for someone who is down for an extended period on a stone floor
Evaporation
- Transfer of thermal energy through converting liquid to a gas
- Occurs through perspiration and respiration
- Can lead to dehydration
Convection
- Transfer of thermal energy through movement of air or liquid across an object
- Rate of heat loss depends on temperature gradient, density and velocity of moving substance
- Can lead to extreme heat loss, especially when combined with evaporation (wet clothes on a cold, windy day)

Bottom line:

When evaluation someone for hyper- or hypothermia from a potential environmental exposure, be sure to obtain the history about where they were found and the circumstances in which they were found. This can help you develop your diagnostic differential.

References

Reference: Pathophysiology, management and complications of hypothermia. Lantry J, Dezman Z, Hirshon JM. Br J Hosp Med (Lond). 2012 Jan; 73(1):31-7.