Hypothermia is when you get really cold. Generally, hypothermia is defined as a core temperature less than 35C (95F). The Swiss staging system is fairly commonly used for correlating core temperature and clinical signs:

• Stage I (35 – 32C; 90 – 95F): conscious, shivering
• Stage II (28 – 32C; 82 – 90F): altered mental status, not shivering
• Stage III (24 – 28C; 75 – 82F): unconscious, not shivering, vital signs present
• Stage IV (< 24C; < 75F): apparent death (but resuscitation possible!)
• Stage V (< 13.7C; < 58F): death due to irreversible hypothermia

Healthy people have fairly effective mechanisms for maintaining a normal temperature (37C; 99F) (e.g., shivering, peripheral vasoconstriction) but these can be overwhelmed by extreme cold (Primary Hypothermia). Some comorbid patients can develop hypothermia even in a warm environment (Secondary Hypothermia). For instance, severely-burned patients can have increased heat loss, while acute spinal cord injury can impair peripheral thermoregulation.

For the development of hypothermia, it is sometimes helpful to remember that heat can be lost by 4 physical mechanisms:

• Radiation: heat exchange by infrared electromagnetic radiation
• Evaporation: molecules converting from liquid to gas phase (e.g., sweat)
• Convection: heat exchange by air/fluid currents
• Conduction: heat exchange by direct contact with a colder surface

• Brown DJA, Brugger H, Boyd J, Paal P. 2012. Accidental hypothermia. NEJM; 367:1930.
• Zafran K, Mechem CC. 2017. Accidental hypothermia in adults. In: UpToDate.