Mild hypothermia (32-35°C)
Between 34°C and 35°C, most people shiver vigorously, usually in all extremities. As the temperature drops below 34°C, a patient may develop altered judgment, amnesia, and dysarthria. Respiratory rate may increase. At approximately 33°C, ataxia and apathy may be seen. Patients generally are stable hemodynamically and able to compensate for the symptoms. In this temperature range, the following may also be observed: hyperventilation, tachypnea, tachycardia, and cold diuresis as renal concentrating ability is compromised.
Moderate hypothermia (28-32°C)
Oxygen consumption decreases, and the CNS depresses further; hypoventilation, hyporeflexia, decreased renal flow, and paradoxical undressing may be noted. Most patients with temperatures of 32°C or lower present in stupor. As the core reaches temperatures of 31°C or below, the body loses its ability to generate heat by shivering. At 30°C, patients develop a higher risk for arrhythmias. Atrial fibrillation and other atrial and ventricular rhythms become more likely. The pulse continues to slow progressively, and cardiac output is reduced. J wave may be seen on ECG in moderate hypothermia. Between 28°C and 30°C, pupils may become markedly dilated and minimally responsive to light, a condition that can mimic brain death.
Severe hypothermia (< 28°C)
At 28°C, the body becomes markedly susceptible to ventricular fibrillation and further depression of myocardial contractility. Below 27°C, 83% of patients are comatose. Pulmonary edema, oliguria, coma, hypotension, rigidity, apnea, pulselessness, areflexia, unresponsiveness, fixed pupils, and decreased or absent activity on EEG are all seen.