Although cardiac arrest often strikes without warning, the event rarely takes place in a normal heart unless other factors (e.g., recreational drug use, trauma) are present. Otherwise, there is almost always an underlying heart disease (e.g., coronary artery disease) or heart damage (e.g., from a previous heart attack) that may progress to the point where a critical situation develops.

According to the American Heart Association (AHA), atherosclerosis in two or more coronary arteries is found in up to 90 percent of adults who succumb to cardiac arrest.  Moreover, heart attack survivors have up to six times the rate of sudden cardiac death compared to the general population. For example, people with coronary artery disease are at higher risk of ventricular tachycardiaventricular tachycardia, an abnormally fast heart rate originating from the ventricles. Ventricular tachycardia can worsen and become ventricular fibrillation, in which the ventricles quiver rather than pump, due to abnormal electrical activity. In fact, this is most common cause of cardiac arrest that leads to sudden cardiac death.

Ventricular fibrillation is life-threatening because it results in a heart rate of 350 beats per minute or higher. The heart cannot maintain that rate more than a few minutes without treatment (e.g., with a defibrillator). Unless the condition is treated within six to eight minutes, permanent brain damage and, possibly, death may result.   

However, sudden cardiac death caused by coronary artery disease is much less common among younger people, who generally have less severe coronary artery disease. In this population, a number of other conditions have been observed to contribute to sudden cardiac death, including: 

More rarely, other conditions known to cause cardiac arrest include:

• Certain types of congenital heart disease or heart defects.
  
  
• Embolism. The blockage of a blood vessel by a foreign substance, such as a blood clot.
  
  
• Kawasaki disease. A disease, usually seen in young children, which affects blood vessels and heart muscle.
  
  
• Severe aortic stenosis. Narrowing of the aortic valve of the heart.
  
  
• Coronary artery spasm. An abnormal constriction of the muscle fibers in a coronary artery.
  
  
• Bradycardia. Abnormally slow heart beat.
  
  
• Cardiac tamponade. A condition in which the pericardial space is filled with fluids to a degree that interferes with the filling of the heart and causes the cessation of blood flow from the heart. This may also occur with rupture of the aortic root with rapid accumulation of blood within the pericardial space.

One study linked the presence of “free” fatty acids in the blood to a 70 percent increase in sudden cardiac death among a group of middle-aged healthy men. Although the researchers could not determine why fatty acids in the blood increased the risk so significantly, they noted that high fatty acid levels in the blood have been known to cause abnormal heartbeats (arrhythmias) in people with heart disease.

Cardiac arrest can also be caused by non-disease events, such as choking, electrocution and physical trauma. Recreational drug use has also been known to cause cardiac arrest in otherwise healthy people.

Cardiac arrest is believed to run in families, especially among males. A man whose mother or father went into cardiac arrest has almost double the chance of such an incident as an unrelated person. A family history of sudden cardiac death is an independent predictor of SCD even after other cardiovascular risk factors had been taken into account. In 2003, an international panel of cardiologists recommended that people see a physician if a close relative has died from SCD.

Other causes of cardiac arrest and SCD may involve a genetic mutation. Researchers have identified a calcium-controlling gene that is linked to ventricular tachycardia. This gene produces a protein called RyR2, which in turn controls the release of calcium inside heart cells. When RyR2 malfunctions, there is a “calcium overload” that triggers the arrhythmia. A specific form of ventricular tachycardia - called catacholaminergic polymorphic ventricular tachycardia - that occurs with exercise may be seen due to this mutation. Such research may eventually lead to genetic testing for this and other underlying conditions that might lead to cardiac arrest and sudden cardiac death.