A cardiac 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.
The resulting lack of blood supply results in cell death from oxygen starvation. Cerebral hypoxia, or lack of oxygen supply to the brain, causes victims to lose consciousness and to stop breathing, which in turn causes the heart to stop. Brain damage is likely to occur after 3-4 minutes, except in cases of hypothermia. To improve survival and neurological recovery immediate response is paramount
Ventricular fibrillation (VF) constitutes the most common electrical mechanism in cardiac arrest, and is responsible for 65 to 80% of occurences. Another 20-30% is caused by severe bradyarrhythmias, pulseless electrical activity (PEA) and asystole. Other conditions are associated with impaired circulation due to a state of shock. 
Among adults ischemic heart disease is the predominant cause. At autopsy 30% of victims show signs of recent myocardial infarction. Other conditions include structural abnormalities, arrhythmias and cardiomyopathies. Secondary cardiac arrest may be elicited by non-cardiac conditions such as hypoxia from a variety of causes, overwhelming infection (sepsis), massive pulmonary embolus, arrhythmias, cardiac tamponade, shock, pneumothorax, ventricular rupture, as well as other conditions such as electrocution and near-drowning. Non-cardiac conditions constitute the principal cause of cardiac arrest in in-hospital patients.
Coronary heart disease (CHD) -also known as coronary artery disease, or (CAD)- is the predominant disease process associated with sudden cardiac death in the United States and elsewhere in the developed world. The incidence of CHD in individuals who suffer sudden cardiac death is between 64 and 90%.
In children, cardiac arrest is typically caused by hypoxia from other causes such as near-drowning. With prompt treatment survival rates are high.
There are 8 reversible causes of cardiac arrest, known as the "4Hs and 4Ts". They are looked for and treated by ambulance technicians/paramedics or by medical staff at the hospital while undertaking advanced life support, protocols for which will be used alongside any specific treatments for each of the causes. Lay rescuers performing basic life support can generally neither identify nor treat them (with the exception of hypoixa due to choking), and so can offer only supportive treatment pending the arrival of emergency medical services.
Hypoxia - A lack of oxygen to the heart, brain and other vital organs. This is treated by providing the patient with oxygen, either through a bag-valve-mask device, or through mechanical ventilation by inserting an endotracheal tube (intubation)
Hypovolemia - A lack of circulating body fluids, principally blood. This is usually (though not exclusively) caused by some form of bleeding. Peri-arrest treatment includes giving IV fluids and blood transfusions, and controlling the source of any bleeding - by direct pressure for external bleeding, or emergency surgical techniques such as esophagogastroduodenoscopy (i.e. esophageal varices) and thoracotomy for internal bleeding.
Hypo/Hyper-metabolic disorders - An abnormally high or low level of electrolytes such as potassium and calcium circulating the body. An arterial blood gas and blood electrolyte test are performed to find the problem, then IV crystalloids are given to correct it.
A low core body temperature, defined clinically as a temperature of less than 35 degrees Celsius. The patient is re-warmed either by using a cardiac bypass or by irrigation of the body cavities (such as thorax, peritoneum, bladder) with warm fluids; or warmed IV fluids. CPR only is given until the core body temperature reached 30 degrees Celsius, as defibrillation is ineffective at lower temperatures. Patients have been known to be successfully resuscitated after periods of hours in hypothermia and cardiac arrest, and this has given rise to the often-quoted medical truism, "You're not dead until you're warm and dead."
· Tension pneumothorax - A rush of air into one of the pleural cavities which is not able to escape compresses the lungs and causes the trachea to deviate away from the mid-line, often putting pressure on the heart so it is not able to beat effectively. This is relieved in an emergency by inserting a needle into the 2nd intercostal space at the mid-clavicular line, releasing the air and the pressure on the thoracic organs.
· Tamponade (Cardiac) - Blood or other fluids building up in the pericardium can put pressure on the heart so that it is not able to beat. This is treated in an emergency by inserting a needle into the pericardium to drain the fluid (pericardiocentesis), or if the fluid is too thick then an emergency thoracotomy is performed to cut the pericardium and release the fluid.
· Toxins - Toxic substances which have been ingested, injected, absorbed or inhaled into the body can lead to cardiac arrest. This may be evidenced by items found on or around the patient, the patient's medical history (i.e. drug abuse, medication) taken from family and friends, checking the medical records to make sure no interacting drugs were prescribed, or sending blood and urine samples to the toxicology lab for report. Treatment is mainly supportive, unless there is an antidote which can be administered.
· Thrombosis - Blood clots in the heart (myocardial infarction) or lungs (pulmonary embolism) are both well known causes of cardiac arrest. Treatment includes thrombolysis, and possibly surgical interventions such as percutaneous transluminal coronary angioplasty (PTCA), coronary bypass or surgical embolectomy.
· In addition to the specific treatments for the causes of cardiac arrest, full resuscitation (using advanced life support protocols) is offered to patients as soon as possible, and continues until the patient is either declared dead or regains a pulse and stable heart rhythm.
First aid treatment of cardiac arrest varies from country to country, but the general principles of the guidelines in all locales are to summon help (in the form of an ambulance) and then begin CPR.
Other Prehospital care
In many situations in the UK and USA, lay people are trained in the use of an automated external defibrillator, which analyzes the heart rhythm and delivers a controlled electric shock to the heart if indicated.
Jurisdictions are beginning to purchase automated CPR machines, such as AutoPulse, to assist first responders. Such machines are proving superior in cardiac arrest support over manual CPR, providing for greater circulation and, thus, lower rates of morbidity and mortality when used in a timely fashion.
Treatment within a hospital usually follows advanced life support protocols. Depending on the diagnosis, various treatments are offered, ranging from defibrillation (for ventricular fibrillation or ventricular tachycardia) to surgery (for cardiac arrest which can be reversed by surgery - see causes of arrest, above) to medication (for asystole and PEA). All will include CPR.
Cardiopulmonary resuscitation and advanced cardiac life support are not always in a person's best interest. This is particularly true in the case of terminal illnesses when resuscitation will not alter the outcome of the disease. Properly performed CPR often fractures the rib cage, especially in older patients or those suffering from osteoporosis. Defibrillation, especially repeated several times as called for by ACLS protocols, may also cause electrical burns. Internal cardiac massage, an ACLS procedure performed by emergency medicine physicians requires splitting open the rib cage, which is painful during the weeks of recovery. While such treatment is worthwhile when it saves a life, it is undignified and simply adds to the suffering of a victim with a terminal illness who wishes to die peacefully.
It is not surprising that some people with a terminal illness choose to avoid such "heroic" measures and die peacefully.
People with views on the treatment they wish to receive in the event of a cardiac arrest should discuss these views with both their doctor and with their family.
It is also important that these views are written down somewhere in the medical record. In the event of cardiac arrest, health professionals need to act quickly on the information that is available to them. As cardiac arrest often happens out of regular hours, the resuscitation team rarely includes anybody who actually knows the patient.
A patient may ask their doctor to record a do not resuscitate (DNR) order in the medical record. Alternatively, in many jurisdictions, a person may formally state their wishes in an "advance directive" or "advance health directive".