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Sudden cardiac arrest (SCA) and sudden cardiac death (SCD) occur when the heart abruptly begins to beat in an abnormal or irregular rhythm (arrhythmia). Without

organized electrical activity in the heart muscle, there is no consistent contraction of the ventricles, which results in the hearts inability to generate an adequate cardiac output (forward pumping of blood from heart to rest of the body). There are many different types of arrhythmias, but the ones most frequently recorded in SCA and SCD are ventricular tachycardia (VT) or ventricular fibrillation (VF).

Sudden cardiac arrest can result from cardiac and non-cardiac causes including the following:

WORKING ADDITIONS

In 2006 the American College of Cardiology/ American Heart Association/ Hearth Rhythm Society presented the following definitions of sudden cardiac arrest and sudden cardiac death: "Cardiac arrest is the sudden cessation of cardiac activity so that the victim becomes unresponsive, with no normal breathing and no signs of circulation. If corrective measures are not taken rapidly, this condition progresses to sudden death. Cardiac arrest should be used to signify an event as described above, that is reversed, usually by CPR and/or defibrillation or cardioversion, or cardiac pacing. Sudden cardiac death should not be used to describe events that are not fatal".

Sudden cardiac arrest (SCA) and sudden cardiac death (SCD) occur when the heart abruptly begins to beat in an abnormal or irregular rhythm (arrhythmia). Without organized electrical activity in the heart muscle, there is no consistent contraction of the ventricles, which in turn results in the hearts inability to generate an adequate cardiac output (forward pumping of blood from heart to rest of the body). There are many different types of arrhythmias, but the ones most frequently recorded in SCA and SCD are ventricular tachycardia (VT) or ventricular fibrillation (VF). The arrhythmias that lead to sudden cardiac arrest or death can be a result of cardiac and non-cardiac causes.
 * Coronary heart disease (CHD) is responsible for 62 to 70 percent of all SCDs. CHD is a much less frequent cause of SCD in people under the age of 40. A 1999 review of SCDs in the United States found that CHD accounted for 24% of SCDs for those under 30 years, and 8% of SCDs in military recruits.


 * Other types of structural heart disease not related to CHD (i.e. hypertrophic cardiomyopathy, congenital coronary artery anomalies, myocarditis) account for 10 percent of all SCDs. A 1999 review of SCDs in the United States found that this accounted for over 30% of SCDs for those under 30 years, and over 40% of SCDs in military recruits.
 * Arrhythmias not due to structural heart disease (i.e. Long QT syndrome, Wolff-Parkinson-White Syndrome, Brugada Syndrome) account for 5 to 10% of SCDs.
 * SCA due to non-cardiac causes accounts for the remaining 15 to 25%. Non-cardiac causes include pulmonary embolism, intracranial hemorrhage, trauma, bleeding, drug induced, drowning, and central airway obstruction.

Implantable cardioverter defibrillators
An implantable cardioverter defibrillators (ICD) is battery powered device that monitors electrical activity in the heart and when an arrhythmia or asystole is detected is able to deliver an electrical shock to terminate the abnormal rhythm. ICDs are used to prevent sudden cardiac death (SCD) in those that have survived a prior episode of sudden cardiac arrest (SCA) due to ventricular fibrillation or sustained ventricular tachycardia (secondary prevention). ICDs are also used prophylactically to prevent sudden cardiac death in certain high risk patient populations (primary prevention).

Numerous studies have been conducted on the use of ICDs for the secondary prevention of SCD. These studies have shown improved survival with ICDs compared to the use of anti-arrhythmic drugs. ICD therapy is associated with a 50% relative risk reduction in death caused by an arrhythmia and a 25% relative risk reduction in all cause mortality.

Primary prevention of SCD with ICD therapy for high risk patient populations has similarly shown improved survival rates in a number of large studies. The high risk patient populations in these studies were defined as those with severe ischemic cardiomyopathy (determined by a reduced left ventricular ejection fraction (LVEF)). The LVEF criteria used in these trials ranged from less than or equal to 30% in MADIT-II to less than or equal to 40% in MUSTT.

Causes
Although the most frequent cause of SCD is ventricular fibrillation, other causes include the following:
 * Coronary heart disease
 * Physical stress
 * low levels of magnesium
 * low levels of potassium
 * major blood loss
 * lack of oxygen
 * severe physical activity which triggers adrenaline
 * Inherited disorders
 * Hypertrophic cardiomyopathy
 * Enlarged heart due to increased blood pressure
 * Commotio cordis

Coronary artery disease
Coronary artery disease is the leading cause of sudden cardiac arrest. Many other cardiac and non-cardiac conditions also increase one's risk.

Coronary artery disease often results in coronary ischemia and ventricular fibrillation (v-fib). Cases have shown that the most common finding at postmortem examination of sudden cardiac death (SCD) is chronic high-grade stenosis of at least one segment of a major coronary artery, the arteries that supply the heart muscle with its blood supply. Left ventricular hypertrophy is thought to be the leading cause of SCD in the adult population. This is most commonly the result of longstanding high blood pressure which has caused secondary damage to the wall of the main pumping chamber of the heart, the left ventricle.

Approximately 60–70% of SCD is related to coronary artery disease, also known as ischemic heart disease. Among adults, it is the predominant cause of arrest, with 30% of people at autopsy showing signs of recent myocardial infarction.

Non-ischemic heart disease
A number of non-ischemic cardiac abnormalities can increase the risk of SCD, including cardiomyopathy, cardiac rhythm disturbances, myocarditis, hypertensive heart disease, and congestive heart failure.

In a group of military recruits aged 18–35, cardiac anomalies accounted for 51% of cases of SCD, while in 35% of cases the cause remained unknown. Underlying pathology included coronary artery abnormalities (61%), myocarditis (20%), and hypertrophic cardiomyopathy (13%). Congestive heart failure increases the risk of SCD fivefold.

Many additional conduction abnormalities exist that place one at higher risk for cardiac arrest. For instance, long QT syndrome, a condition often mentioned in young people's deaths, occurs in one of every 5000 to 7000 newborns and is estimated to be responsible for 3000 deaths each year compared to the approximately 300,000 cardiac arrests seen by emergency services. These conditions are a fraction of the overall deaths related to cardiac arrest, but represent conditions which may be detected prior to arrest and may be treatable.

Non-cardiac causes
About 35% of SCDs are not caused by a heart condition. The most common non-cardiac causes are trauma, bleeding (such as gastrointestinal bleeding, aortic rupture, or intracranial hemorrhage), overdose, drowning and pulmonary embolism. Cardiac arrest can also be caused by poisoning (for example, by the stings of certain jellyfish).

Risk factors
The risk factors for SCD are similar to those of coronary artery disease and include age, cigarette smoking, hypertension, high cholesterol levels, lack of physical exercise, obesity, and diabetes, as well as family history. A prior episode of sudden cardiac arrest also increases the risk of future episodes.

Current cigarette smokers with coronary artery disease were found to have a two to threefold increase on the risk of sudden death between ages 30 and 59. Furthermore, it was found that former smokers risk was closer to that of those who had never smoked.

Mnemonic for reversible causes
"Hs and Ts" is the name for a mnemonic used to aid in remembering the possible treatable or reversible causes of cardiac arrest.
 * Hs
 * Hypovolemia – A lack of blood volume
 * Hypoxia – A lack of oxygen
 * Hydrogen ions (Acidosis) – An abnormal pH in the body
 * Hyperkalemia or Hypokalemia – Both excess and inadequate potassium can be life-threatening.
 * Hypothermia – A low core body temperature
 * Hypoglycemia or Hyperglycemia – Low or high blood glucose
 * Ts
 * Tablets or Toxins
 * Cardiac Tamponade – Fluid building around the heart
 * Tension pneumothorax – A collapsed lung
 * Thrombosis (Myocardial infarction) – Heart attack
 * Thromboembolism (Pulmonary embolism) – A blood clot in the lung
 * Traumatic cardiac arrest

Risk factors
The risk factors for SCD are similar to those of coronary artery disease and include smoking, lack of physical exercise, obesity, and diabetes, as well as family history. A prior episode of sudden cardiac arrest also increases the risk of future episodes.

Prognosis
The overall chance of survival among those who have cardiac arrest outside hospital is 10%. Among those who have an out-of-hospital cardiac arrest, 70% occur at home and have a survival rate of 6%. For those who have an in-hospital cardiac arrest, survival rate is estimated to be 24%. Among children rates of survival is 3 to 16% in North America. For in hospital cardiac arrest survival to discharge is around 22% with many having a good neurological outcome.

Prognosis is typically assessed 72 hours or more after cardiac arrest. Rates of survival are better in those who someone saw collapse, got bystander CPR, or had either ventricular tachycardia or ventricular fibrillation when assessed. Survival among those with Vfib or Vtach is 15 to 23%. Women are more likely to survive cardiac arrest and leave hospital than men.

A 1997 review into outcomes following in-hospital cardiac arrest found a survival to discharge of 14% although the range between different studies was 0-28%. In those over the age of 70 who have a cardiac arrest while in hospital, survival to hospital discharge is less than 20%. How well these individuals are able to manage after leaving hospital is not clear.

A study of survival rates from out-of-hospital cardiac arrest found that 14.6% of those who had received resuscitation by ambulance staff survived as far as admission to hospital. Of these, 59% died during admission, half of these within the first 24 hours, while 46% survived until discharge from hospital. This reflects an overall survival following cardiac arrest of 6.8%. Of these 89% had normal brain function or mild neurological disability, 8.5% had moderate impairment, and 2% had major neurological disability. Of those who were discharged from hospital, 70% were still alive four years later.

Targeted temperature management
Cooling adults after cardiac arrest who have a return of spontaneous circulation (ROSC) but no return of consciousness improves outcomes. This procedure is called targeted temperature management (previously known as therapeutic hypothermia). People are typically cooled for a 24-hour period, with a target temperature of 32 - 36 C. There are a number of methods used to lower the body temperature, such as applying ice packs or cold-water circulating pads directly to the body, or infusing cold saline. This is followed by gradual rewarming over the next 12 to 24 hrs.

Recent meta-analysis found that the use of therapeutic hypothermia after out-of-hospital cardiac arrest is associated with improved survival rates and better neurological outcomes.

Causes
Sudden cardiac arrest (SCA) and sudden cardiac death (SCD) occur when the heart abruptly begins to beat in an abnormal or irregular rhythm (arrhythmia). Without organized electrical activity in the heart muscle, there is no consistent contraction of the ventricles, which results in the hearts inability to generate an adequate cardiac output (forward pumping of blood from heart to rest of the body). There are many different types of arrhythmias, but the ones most frequently recorded in SCA and SCD are ventricular tachycardia (VT) or ventricular fibrillation (VF).

Sudden cardiac arrest can result from cardiac and non-cardiac causes including the following:
 * Coronary artery disease (CAD)
 * Structural heart disease not related to CAD
 * cardiomyopathy
 * cardiac rhythm disturbances
 * myocarditis
 * hypertensive heart disease
 * congestive heart failure
 * Arrhythmias
 * Long QT Syndrome
 * Wolff-Parkinson-White Syndrome
 * Brugada Syndrome
 * Catecholaminergic polymorphic ventricular tachycardia
 * Non-cardiac causes
 * Trauma
 * Bleeding
 * intracranial hemorrhage
 * gastrointestinal bleeding
 * Overdose
 * Drowning
 * Pulmonary embolism
 * Reversible causes

Coronary artery disease
Coronary artery disease (CAD), also known as ischemic heart disease, is responsible for 62 to 70 percent of all SCDs. CAD is a much less frequent cause of SCD in people under the age of 40.

Cases have shown that the most common finding at postmortem examination of sudden cardiac death (SCD) is chronic high-grade stenosis of at least one segment of a major coronary artery, the arteries that supply the heart muscle with its blood supply.

Structural heart disease not related to CAD
Structural heart disease not related to CAD (i.e. hypertrophic cardiomyopathy, congenital coronary artery anomalies, myocarditis) account for 10% of all SCDs. Examples of these include: cardiomyopathy, cardiac rhythm disturbances, myocarditis, hypertensive heart disease, and congestive heart failure.

Left ventricular hypertrophy is thought to be a leading cause of SCD in the adult population. This is most commonly the result of longstanding high blood pressure which has caused secondary damage to the wall of the main pumping chamber of the heart, the left ventricle.

A 1999 review of SCDs in the United States found that this accounted for over 30% of SCDs for those under 30 years. A study of military recruits age 18-35 found that this accounted for over 40% of SCDs.

Congestive heart failure increases the risk of SCD fivefold.

Arrhythmias
Arrhythmias not due to structural heart disease account for 5 to 10% of SCDs.

Examples of arrhythmic syndromes associated with SCD include: Long QT syndrome, Wolff-Parkinson-White Syndrome, Brugada Syndrome, Catecholaminergic polymorphic ventricular tachycardia.

Long QT syndrome, a condition often mentioned in young people's deaths, occurs in one of every 5000 to 7000 newborns and is estimated to be responsible for 3000 deaths each year compared to the approximately 300,000 cardiac arrests seen by emergency services. These conditions are a fraction of the overall deaths related to cardiac arrest, but represent conditions which may be detected prior to arrest and may be treatable.

Non-cardiac causes
SCA due to non-cardiac causes accounts for the remaining 15 to 25%.

The most common non-cardiac causes are trauma, bleeding (such as gastrointestinal bleeding, aortic rupture, or intracranial hemorrhage), overdose, drowning and pulmonary embolism. Cardiac arrest can also be caused by poisoning (for example, by the stings of certain jellyfish).

Mnemonic for reversible causes
"Hs and Ts" is the name for a mnemonic used to aid in remembering the possible treatable or reversible causes of cardiac arrest.
 * Hs
 * Hypovolemia – A lack of blood volume
 * Hypoxia – A lack of oxygen
 * Hydrogen ions (Acidosis) – An abnormal pH in the body
 * Hyperkalemia or Hypokalemia – Both excess and inadequate potassium can be life-threatening.
 * Hypothermia – A low core body temperature
 * Hypoglycemia or Hyperglycemia – Low or high blood glucose
 * Ts
 * Tablets or Toxins
 * Cardiac Tamponade – Fluid building around the heart
 * Tension pneumothorax – A collapsed lung
 * Thrombosis (Myocardial infarction) – Heart attack
 * Thromboembolism (Pulmonary embolism) – A blood clot in the lung
 * Traumatic cardiac arrest