Hypothermia

In its most basic definition, hypothermia is a mammalian body temperature significantly below normal for the species. Perhaps the most common cause is exposure to cold weather. It can also result from metabolic abnormalities, especially trauma. Hypothermia may be deliberately induced to slow metabolic processes, as for surgery under cardiopulmonary bypass.

Category	Effects
Mild hypothermia (32-35°C)	Throughout the range, hyperventilation, tachypnea, tachycardia, and cold diuresis. Shivering begins between 34 and 34 degree; altered mental staus < 34, ataxia and apathy < 33
Moderate hypothermia (28-32°C)	hypoventilation, hyporeflexia, decreased renal flow, and paradoxical undressing; stupor <32, shivering stops < 31, risk of arrythmia < 30, pupils fixed and dilated in lowest range
Severe hypothermia (<28°C)	Pulmonary edema, oliguria, coma, hypotension, rigidity, apnea, pulselessness, areflexia, unresponsiveness, fixed pupils, and decreased or absent activity on EEG; High risk of ventricular fibrillation; most patients comatose < 27

Exposure to cold

Cold can be lethal, but cold also can be lifesaving. An axiom of emergency medicine, when presented with a hypothermic patient in cardiac arrest, is "you're not dead until you're warm and dead."

Traumatic hypothermia

A core body temperature of 32 degrees Celsius, during a trauma laparotomy, is invariably lethal.