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."

There are several current methods of rewarming, and the techniques continue to evolve.

Prehospital treatment

Of greatest concern is that a hypothermic patient does not go into ventricular fibrillation. Lidocaine and cardiac pacing have not proven helpful in preventing it. While there is some evidence for the prophylactic value of bretylium, bretylium has not been manufactured in a number of years.

Nonpharmacologic measures are important. A patient in hypothermia must be handled gently, as physical stress can trigger arrythmia. Rewarming should be started with measures that are practical in the field, such as removing wet clothing, avoiding further chilling, and placing hot packs in the armpits and groin, and on the abdomen. If hotppacks are not available, as in wilderness situations, skin-to-skin contact with rescuers may be lifesaving.

Hospital-based

Once hospital facilities are available, internal rewarming using heated fluid techniques become possible. Normal saline should be used rather than Lactated Ringer's injection because a chilled liver cannot properly metabolize lactate. The temperature is controversial; 45 degrees is common but there are data suggesting 65 degrees may be more effective.

Complementary techniques include peritoneal and thoracic lavage with warmed fluids; warmed humidified oxygen is routine. There remains considerable argument as to the optimal rate of rewarming, and this will be apt to be decided by local experience and clinical response.

Cardiopulmonary bypass is appropriate in severe cases.

Traumatic hypothermia

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

Induced hypothermia

For many years, elective hypothermia has been induced prior to procedures, such as cardiopulmonary bypass, where it is desirable to reduce oxygen demand and otherwise reduce metabolism. Recently, induced hypothermia is being explored, with initially encouraging results, in other, less planned situations, such as pre-hospital treatment of post-cardiac arrest patients, as well as myocardial infarction without arrest, and in stroke. "Although the exact role of ischemia/reperfusion is unclear clinically, hypothermia holds significant promise for improving outcomes for patients suffering from reperfusion after ischemia. Research has elucidated two distinct windows of opportunity for clinical use of hypothermia. In the early intraischemia window, hypothermia modulates abnormal cellular free radical production, poor calcium management, and poor pH management. In the more delayed post-reperfusion window, hypothermia modulates the downstream necrotic, apoptotic, and inflammatory pathways that cause delayed cell death. Improved cooling and monitoring technologies are required to realize the full potential of this therapy."