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 induced by trauma; the "lethal triad" in multisystem trauma is hypothermia, trauma induced coagulopathy and acidosis. Hypothermia may be deliberately induced to slow metabolic processes, as for surgery under cardiopulmonary bypass.
Physiology of hypothermia
Hypothermia changes all biochemical reactions in the body, with its effect specified in Q10: the increase or decrease of reaction rates caused by a 10 °C temperature change. A Q10 of 2, for example, means the rate will double with a 10 °C increase and halve with a 10 °C decrease. The Q10 of most body reactions, including total body oxygen consumption, is between 2 and 3.
As long as the water does not freeze, osmosis and water diffusion are effected, typically 3 percent per 10 °Cm vyr this is nonlinear and most dramatic near the dangerous freezing point. Chilling above that point will make they remaining fluid hyperosmolar. 
|Mild hypothermia (32-35°C)||Throughout the range, hyperventilation, tachypnea, tachycardia, and cold diuresis. Shivering begins between 34 and 34 degrees; altered mental status < 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
There are several current methods of rewarming, and the techniques continue to evolve.
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.
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.
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."
In 2005, the International Liaison Committee on Resuscitation and European Resuscitation Council recommended mild hypothermia, following out-of-hospital resuscitation following cardiac arrest. Benefit was seen in cardiac arrest for a wide range of causes, not limited to ventricular fibrillation. Field cooling techniques include intravenous cold (4°C) Lactated Ringer's injection at 30 ml/kg over 30 minutes after resuscitation, or traditional ice packs placed on the groin and armpits and around the neck and head. The study mentioned not only that lidocaine is ineffective in a hypothermic patient, but also amiodarone should not be used.  While Ringer's is not recommended in rewarming since a hypothermic liver cannot process lactate, this is the opposite situation.
The Nazi freezing experiments were conducted on unwilling prisoners, killing many and causing much suffering. Most Nazi medical experiments were badly designed and executed from a scientific standpoint, but apparently these experiments, which were adjudicated as war crimes, may have produced data that could benefit humanity, giving meaning to the deaths, by providing unobtainable data on hypothermia; this has been examined from the perspective of Jewish law.  A more common position, however, as described by Marcia Angell, is that the data are irrevocably tainted. While, she writes, it could be "tempting to overlook the ethical lapse. But to do so would be to regard the subjects as means to an end, albeit a worthy end,and judgments about the implications of research results would come to replace judgments about the study. She quotes the Massachusetts General Hospital Guiding Principles for Human Studies (1981), "a study is ethical or not at its inception. It does not become ethical because it succeeds in producing valuable data."
- Laurie K. Davis (1993), Hypothermia and Clinical Use, in Gravlee GP, Davis RF, Utley JF, Cardiopulmonary Bypass: Principles and Practice, Willians & Wilkins, ISBN 068303720X, p. 141
- Jamie Alison Edelstein et al. (29 October 2009), "Hypothermia: Treatment & Medication", Medscape
- Asher Hirshberg and Kenneth Mattox (2005), Top Knife: the Art & Craft of Trauma Surgery, TFM Publishing, ISBN 1093378222, p. 13
- Lance Becker (January 2011), "(prepublication abstract) State of the Art in Therapeutic Hypothermia", Annual Review of Medicine
- Arrich J, Holzer M, Herkner H, Müllner M (2009), "Hypothermia for Neuroprotection in Adults After Cardiopulmonary Resuscitation", Cochrane Database Syst Rev, CD004128
- Gerasimos Gavrielatos, Klaus Dieter Werner, Eftichios Voridis, Dimitrios T. Kremastinos, (2010), "Contemporary Practices in Postcardiac Arrest Syndrome: The Role of Mild Therapeutic Hypothermia", Ther Adv Cardiovasc Dis. 4: 325-333
- Eisha Christian, B.A.; Gabriel Zada, Gene Sung, Steven L. Giannotta (2008), "Selective Hypothermia in the Management of Traumatic Brain Injury", Neurosurgical Focus 25 (4): E9
- Baruch C. Cohen, "The Ethics Of Using Medical Data From Nazi Experiments", Jewish Law Articles
- Marcia Angell (1992), Editorial Responsibility: Protecting Human Rights by Restricting Publication of Unethical Research, in George J. Annas & Michael A. Griffin, The Nazi Doctors and the Nuremberg Code: Human Rights in Human Experimentation, Oxford University Press, p. 278