Induced hypothermia

Induced hypothermia (also known as therapeutic hypothermia) is the intentional induction of hypothermia for medical purposes. Such practice has been shown to reduce brain damage and increase survivability in certain cases of cardiac arrest. The method also has applications in cardiac surgery and stroke recovery.

Hypothermia may be a consequence of anesthetic induction drugs and occurs to some extent every time an anesthetic is given. Anesthetic drugs produce two different effects that lead to hypothermia during surgery: they increase the width of blood vessels, vasodilation, allowing blood to flow easily from the body's inner organs or core to the periphery or skin. This allows heat to be easily lost or radiated from the body. The temperature control center situated in hypothalamus is also affected by drugs causing the normal temperature control limits to be increased, compromising the body's normal cold response system.

In addition, induced hypothermia has been shown to improve neurologic outcomes after cardiac arrest. Survival post-arrest has been historically poor, as low as 4 percent in published literature. Patients who are successfully resuscitated after cardiac arrest (stopping of the heart), but remain in a comatose state, have greater chances survival and neurologic recovery if mildly cooled to about 33 degrees Celsius (91 °F). The strongest evidence of its utility is in patients who have arrested due to ventricular fibrillation or ventricular tachycardia, though its use has been expanded to other rhythms.

Pathophysiology
During the cardiac arrest there is inadequate perfusion of the brain. If cerebral perfusion is impaired for too long, neuronal cells begin to suffer hypoxic injury. If unchecked, hypoxic injury eventually leads to cell death. Though successful cardiopulmonary resuscitation may restore cerebral circulation, it is believed that the damage to the neurons is accelerated when oxygen and nutrients are reintroduced to the abnormal cellular environment too rapidly. This may result in reperfusion injury. Also, as brain cells begin to die, they may release various hormones signaling other nearby cells to begin the process of apoptosis (cell death). Hypothermia is thought to reduce the damage to brain cells by reducing the brain's metabolic activity.

Methods
Induced hypothermia is usually accomplished by a combination of several methods:


 * Rapid infusion of ice-cold intravenous fluids
 * Cooling of internal organs, such as with nasogastric lavage with ice-cold water
 * Evaporative cooling of the body surface
 * Intravascular cooling, using specialized vascular catheters
 * External cooling with ice packs or special cooling blankets
 * Prevention of excess heat generation (fevers, shivering) using medication

Induced hypothermia has been most effective when achieved within six hours of cardiac arrest. Reducing the delay between arrest and initiation of cooling may improve outcomes, but has not been systematically investigated. This therapy is increasingly being used in hospital emergency departments and intensive care units.