Hypothermia: definition and grades

Hypothermia is defined, in adults, as a core temperature below 35°C. Core temperature is the temperature of the body's deep organs rather than of the skin, so it is measured with a core thermometer, not by a reading at the mouth. The usual normal range for an adult is about 36.5 to 37.5°C.

The hypothalamus (the brain's temperature control centre) keeps the core in that range using shivering (rapid muscle contractions that release heat as a by-product), vasoconstriction (the skin's blood vessels constricting, so less heat is lost) and other responses. Below 35°C the body is losing heat faster than it can make it. Enzymes (protein catalysts that drive the body's chemical reactions) work more slowly in the cold, nerves conduct more slowly and the brain becomes sluggish.

Hypothermia is then graded by temperature. Mild is about 32 to 35°C, moderate is about 28 to 32°C, and severe is below about 28°C, which is life-threatening. Severe is the lowest temperature band, not the start of hypothermia, which begins at 35°C. The lower the temperature falls, the worse the effects: shivering and confusion at first, then loss of shivering.

A grade only labels the temperature range, and it does not mean that the patient is well. An older person with mild hypothermia may still need urgent assessment and gradual rewarming. Older people are at particular risk because they often have a reduced sense of temperature, impaired mobility and less insulating fat under the skin. Hypothermia is a medical emergency, and the first priority is to prevent further heat loss by removing wet clothing and insulating the patient.