4 Hs

• 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.

• Hypothermia - 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."

4 Ts

• 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.

Diagnosis

Cardiac Arrest is an abrupt cessation of pump function (evidenced by absence of a palpable pulse) of the heart that with prompt intervention could be reversed, but without it will lead to death. In many cases, lack of carotid pulse is the gold standard for diagnosing cardiac arrest, but pulselessness (particularly in the peripheral pulses) may be a result of other conditions (i.e. shock, or other conditions leading to poor circulation)

In a hospital or ambulance, cardiac arrest is identified by the lack of a pulse (or lack of heartbeat if listened to through a stethoscope), and advanced life support is given.

Out of hospital, lay rescuers identify cardiac arrest in a number of ways. In the USA, most lay rescuers are trained to check for a carotid pulse, whereas in the UK, rescuers are taught that a lack of normal breathing is evidence of cardiac arrest, and they begin CPR without checking a pulse.

An ECG clarifies the exact diagnosis and guides treatment, but basic life support should begin without awaiting an ECG. The ECG may reveal:

• Asystole (known colloquially as a flatline) - a complete stoppage of the heart
• Pulseless electrical activity (formerly called electromechanical dissociation) - where the heart's electrical system is working normally but there is a problem with mechanical function (so the rhythm on the heart monitor appears normal, but there is no pulse)
• ventricular fibrillation - A quivering of the ventricles
• ventricular tachycardia - The ventricles contract so rapildy that they do not refill fully between beats, so they do not pump enough blood to maintain circulation.

Treatment

First aid

Other prehospital care

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.

Hospital treatment

Prognosis