There are many different types of arrhythmias, but the ones most frequently recorded in sudden cardiac arrest are ventricular tachycardia and ventricular fibrillation. Less common causes of dysrhythmias in cardiac arrest include pulseless electrical activity (PEA), bradyarrhythmias or asystole. Such rhythms are seen when there is prolonged cardiac arrest, progression of ventricular fibrillation, or efforts such as defibrillation executed to resuscitate the person. The rhythm changes also appear to have a correlation to the underlying cause of cardiac injury when present (ischemic vs. nonischemic causes). : 831 Sudden cardiac arrest can result from cardiac and non-cardiac causes including the following:

Cardiac causes
Coronary artery disease
Coronary artery disease (CAD), also known as ischemic heart disease, is responsible for 62 to 70 percent of all sudden cardiac deaths. CAD is a much less frequent cause of sudden cardiac death in people under the age of 40. Cases have shown that the most common finding at postmortem examination of sudden cardiac death 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. This stenosis is often the result of narrowing and hardening of the arteries following deposition of cholesterol plaques and inflammation over several years. This accumulation and remodeling of the coronary vessels along with other systemic blood vessels characterizes the progression of Atherosclerotic Cardiovascular Disease. When a stable plaque ruptures, it can block the flow of blood and oxygen through small arteries resulting in ischemic injury as a result.
A statistical analysis of many of these risk factors determined that approximately 50% of all cardiac arrests occur in 10% of the population perceived to be at greatest risk due to aggregate harm of multiple risk factors demonstrating that cumulative risk of multiple comorbidities exceeds the sum of each risk individually.Previous adverse cardiac events, non-sustained ventricular tachycardia (NSVT), syncope, and left ventricular hypertrophy (LVT) have been shown to predict sudden cardiac death in children. Current cigarette smokers with coronary artery disease were found to have a two to threefold increase in 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.Functional changes in the heart such as reduced ejection fraction or cardiac arrhythmia have been shown to increase the risk of cardiac arrest and act independently from the risk factors previously mentioned. Conditions that produce these functional changes can be acquired following previous cardiac injury, or inherited through familial history of arrhythmogenic disorders. : 828

Causes and mechanisms
Sudden cardiac arrest (SCA), or 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).