Heat Exhaustion

Heat exhaustion is a moderate heat-related illness. Heat stress from the environment, from muscular work or from both, combined with water and salt loss through sweat, strains the circulation until the person can no longer continue activity safely. Mental status stays normal. Core temperature is usually normal or mildly raised. Health Canada's technical guide for health care workers sets a core temperature below 40 °C as a diagnostic criterion, and the Merck Manual notes that temperature, if raised, usually does not exceed 40 °C. Heat exhaustion sits on a spectrum: untreated, it can progress to heat stroke, which is a medical emergency. Mechanism. The hypothalamus holds core temperature near a set point of about 37 °C. When heat builds up, the body sends warm blood to the skin and produces sweat; evaporation removes the heat. Skin blood flow can take up to about 30% of cardiac output. This competes with blood flow to the muscles, brain and gut. Sweat losses shrink the circulating volume. Health Canada describes dehydration above about 2–3% of body weight as enough to reduce muscle blood...