Recognizing hypothermia in the field

Hypothermia is defined in medical references as a body core temperature below 35.0 °C (95.0 °F) in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe hypothermia, hallucinations and paradoxical undressing — the victim removing their own clothing — may appear, along with an increased risk of cardiac arrest.

The condition is described as having two main mechanisms: classical exposure to cold weather or cold-water immersion, and any condition that decreases heat production or increases heat loss. Common contributing factors include alcohol intoxication, low blood sugar, anorexia, and advanced age. Body temperature is normally maintained near 36.5–37.5 °C (97.7–99.5 °F) through thermoregulation.

In field practice, references describe assessment by symptoms in the presence of risk factors, since accurate core-temperature measurement requires a low-reading thermometer not commonly carried. The Swiss staging system is described as preferred when accurate core temperature cannot be obtained, and grades severity from clear consciousness with shivering through unconsciousness without vital signs. Heat is lost much more quickly to water than to air, so water temperatures that would be comfortable as outdoor air temperatures can produce immersion hypothermia rapidly. Alcohol consumption — present in 33 to 73 percent of hypothermia deaths in published series — increases risk through vasodilation and impairment of shivering and thermoregulation.