Cardiac arrest is an event in which the heart abruptly stops or develops a very abnormal heart rhythm that prevents it from pumping blood. Although a heart attack may progress to cardiac arrest -- and cardiac arrest is sometimes referred to as a “massive heart attack” -- the two conditions are not the same. During a heart attack, the heart is still beating, while in cardiac arrest the heart has stopped beating. Once the patient has “gone into arrest,” he or she only has a few minutes to be treated. Otherwise, sudden cardiac death (SCD) will result.

SCD is death that results from cardiac arrest, usually as a result of ventricular fibrillation in the presence of existing heart disease, especially coronary artery disease. Some studies have shown that more than 80 percent of victims of SCD have coronary artery disease, and a majority have suffered from previous heart attacks. SCD can also be caused by severe and persistent bradyarrhythmias (e.g., slow heart rhythms), asystole and the failure of the heart to respond to normal electrical activity (pulseless electrical activity). Sustained ventricular tachycardia accompanied with low blood pressure (hypotension) is a less common cause. In rare cases, the heart muscle may rupture during or after a heart attack leading to sudden death.

Without immediate defibrillation, cardiac arrest will result in sudden cardiac death. SCD claims up to 330,000 American lives every year, accounting for about half of all cardiac deaths. Despite its name, however, people may survive sudden cardiac death. In that case, they are said to have experienced “aborted” sudden cardiac death.

If SCD occurs before six months of age, it is often referred to as sudden infant death syndrome (SIDS). After six months of age, SCD is rare among children and adolescents. After about 45, the incidence begins to climb again, as this population is more likely to suffer from accompanying forms of heart disease. Between the ages of 45 and 64, men are much more likely to suffer from SCD than women. After about 75, however, the rates even out. In general, the incidence rates of SCD closely correlate to the incidence rates of coronary artery disease. Higher rates were also found in the black population as compared to white, American Indian/Alaskan native and Asian/Pacific Islander populations. Hispanic populations appeared to have lower rates of SCD than non-Hispanic populations.

The CDC notes that the proportion of cardiac deaths classified as “sudden” remains high probably because of the unexpected nature of SCD and the failure to recognize early warning symptoms and signs of heart disease.