Hypothermia

Hypothermia is a condition in which the body's core temperature falls below 35°C (95°F). Core temperature refers to the temperature of the body's internal tissues and organs rather than the temperature of the skin. Hypothermia develops when heat loss exceeds the body's ability to produce and conserve enough heat to maintain its normal internal temperature .^1,2

When the body is exposed to cold, normal defenses include narrowing of peripheral blood vessels, which reduces heat loss from the skin, and shivering, which generates heat through repeated muscle contractions. If these responses cannot compensate for continued heat loss, core temperature falls and normal physiological functions become progressively impaired .¹

Hypothermia is commonly classified by core temperature, although exact staging systems can vary. Mild hypothermia is generally defined as 32 to below 35°C, moderate hypothermia as 28 to below 32°C, and severe hypothermia as below 28°C. Shivering and changes in coordination or thinking can occur early. With greater cooling, nervous-system activity, breathing, heart rate, and cardiac function can become increasingly depressed, and the risk of dangerous cardiac rhythm disturbances rises .^1,2

Although hypothermia is strongly associated with exposure to cold air or water, environmental cold is not its only possible cause. Conditions that reduce heat production, impair normal temperature regulation, increase heat loss, or interfere with behavioral responses to cold can also contribute. The term therefore describes the state of abnormally low core body temperature, not a particular cause of that cooling .¹