Sudden cardiac arrest (SCA) is a major public health problem that accounts for 50% of all cardiovascular deaths. The cause of SCA in 80% of the cases is coronary heart disease. Of the remaining 20%, another 10–15% occur in subjects with cardiomyopathies. Finally, 5–10% of the cases are due to a structurally abnormal heart that can arise from congenital cardiac conditions.
The primary cause of cardiac arrest is ventricular fibrillation, which "is when the heart quivers instead of pumping due to disorganized electrical activity in the ventricles.... The ventricular muscle twitches randomly rather than contracting in a co-ordinated fashion (from the apex of the heart to the outflow of the ventricles), and so the ventricles fail to pump blood around the body." The treatment for cardiac arrest is CPR to try to keep the subject alive as long as possible by pushing the blood throughout the body by chest compressions, and defibrillation which uses an electric shock to try to get the heart to start beating in rhythm again.
For people who have suffered heart damage, one option is to have an implantable cardioverter defibrillator (ICD) inserted close to the heart. The ICD is a defibrillator that delivers the electric shock from the inside (directly in the heart) rather than from the outside (with the paddles), but the principle is the same. Damage to the heart muscles from a previous heart attack makes the heart more susceptible to ventricular fibrillation and cardiac arrest. In particular, myocardial infarction (heart attack) can cause the left ventricular ejection fraction (LVEF, measured by echocardiogram) to decrease substantially. The left ventricle is responsible for pumping blood throughout the body, and a decrease in its function is evidence of irreversible damage to the critical muscles in the heart.
In many cases, a heart attack is not the immediate precipitating event of cardiac arrest, and most heart attacks do not immediately lead to cardiac arrest. Yet, coronary heart disease is the primary cause of cardiac arrest and so there is an important relationship. The missing link was discovered a few years ago with a study that showed that many instances of cardiac arrest were linked to previous silent heart attacks (CNN):
"Many people who die of sudden cardiac arrest may have had a heart attack earlier in life without ever realizing it, according to a new study. In the study, almost half -- 42.4% -- of people who had no prior knowledge of coronary artery disease, but died of sudden cardiac arrest, showed signs of having had a prior silent heart attack. The study published Wednesday in the journal JAMA Cardiology."A silent heart attack is "a heart attack that has few, if any, symptoms." The subject may mistake the heart attack for "indigestion, nausea, muscle pain or a bad case of the flu." It should be noted that there could be mild symptoms from a large heart attack, and so a silent heart attack could cause extensive heart damage (Figure 1).
A silent heart attack can be diagnosed by multiple methods. An electrocardiogram (ECG) detects heart muscle damage from a decreased voltage signal. An echocardiogram, as described above, can show a reduced ventricular ejection fraction also indicative of a heart attack. Finally, the most sensitive method is a cardiac MRI, which can directly image heart muscle scarring.
A study using cardiac MRI showed that roughly 20% of the participants with type 2 diabetes had a silent heart attack while another 10% had a recognized heart attack. This is a startling finding suggesting that many may not be aware of the extent of their coronary heart disease and the damage already done to their heart. The danger is that these people are much more likely to have cardiac arrest. Given that coronary heart disease (CHD) is the predominant factor leading to cardiac arrest, it is not surprising that so many people who did not think that they had CHD and suffered cardiac arrest, did in fact show signs not only of CHD but of a silent heart attack.
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