Cardiopulmonary resuscitation (CPR) is an emergency medical procedure for a victim of cardiac arrest or, in some circumstances, respiratory arrest. CPR is performed in hospitals, or in the community by laypersons or by emergency response professionals.

For 50 years CPR has consisted of the combination of artificial blood circulation with artificial respiration i.e., chest compressions and lung ventilation. However, in March 2008 the American Heart Association and the European Resuscitation Council, in a historic reversal, endorsed the effectiveness of chest compressions alone--without artificial respiration--for adult victims who collapse suddenly in cardiac arrest (see Cardiocerebral Resuscitation below). CPR is generally continued, usually in the presence of advanced life support, until the patient regains a heart beat (called "return of spontaneous circulation" or "ROSC") or is declared dead.

CPR is unlikely to restart the heart, but rather its purpose is to maintain a flow of oxygenated blood to the brain and the heart, thereby delaying tissue death and extending the brief window of opportunity for a successful resuscitation without permanent brain damage. Defibrillation and advanced life support are usually needed to restart the heart.

A cardiac arrest, also known as cardiorespiratory arrest, cardiopulmonary arrest or circulatory arrest, is the abrupt cessation of normal circulation of the blood due to failure of the heart to contract effectively during systole. [http://books.mcgraw-hill.com/medical/harrisons/ Harrison's Principles of Internal Medicine] 16th Edition, The McGraw-Hill Companies, ISBN 0-07-140235-7

A cardiac arrest is different from (but may be caused by) a heart attack or myocardial infarction, where blood flow to the still-beating heart is interrupted.

"Arrested" blood circulation prevents delivery of oxygen to all parts of the body. Cerebral hypoxia, or lack of oxygen supply to the brain, causes victims to lose consciousness and to stop normal breathing, although agonal breathing may still occur. Brain injury is likely if cardiac arrest is untreated for more than 5 minutes, although new treatments such as induced hypothermia have begun to extend this time. To improve survival and neurological recovery immediate response is paramount. [http://www.lww.com/product/?0-7817-3548-3 Irwin and Rippe's Intensive Care Medicine] by Irwin and Rippe, Fifth Edition (2003), Lippincott Williams & Wilkins, ISBN 0-7817-3548-3

Cardiac arrest is a medical emergency that, in certain groups of patients, is potentially reversible if treated early enough (See "Reversible causes" below). When unexpected cardiac arrest leads to death this is called sudden cardiac death (SCD). The primary first-aid treatment for cardiac arrest is cardiopulmonary resuscitation (commonly known as CPR) which provides circulatory support until availability of definitive medical treatment, which will vary dependent on the rhythm the heart is exhibiting, but often requires defibrillation.