How is sudden cardiac death different from a heart attack?

SCD often occurs at the time of a heart attack (myocardial infarction). However, these are two distinct pathological conditions with different underlying mechanisms:

Heart attacks occur when there is an obstruction in one or more of the arteries irrigating the heart, preventing the heart from receiving enough oxygen-rich blood. If the oxygen in the blood cannot reach the heart muscle, the heart becomes damaged.

In contrast, SCD occurs when the electrical system of the heart malfunctions and suddenly becomes very irregular, and the heart beats dangerously fast. The ventricles may quiver (ventricular fibrillation), and blood is not delivered to the body. In the first few seconds, the blood flow to the brain is reduced so drastically that a victim will lose consciousness. Death follows in minutes unless emergency treatment is begun immediately.

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What are the symptoms of sudden cardiac death?

Some people may experience a racing heartbeat or they may feel dizzy for a few seconds or minutes before passing out, alerting them that a potentially dangerous heart rhythm problem has started. In over half of the cases, however, sudden cardiac death occurs without prior symptoms.

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What causes sudden cardiac death?

Most heart diseases can lead to cardiac arrest and SCD. Most of the cardiac arrests that lead to sudden death occur when the electrical impulses in the diseased heart become rapid (ventricular tachycardia) or chaotic (ventricular fibrillation) or both. This irregular heart rhythm (arrhythmia) causes the heart to suddenly stop beating. Less commonly, cardiac arrests are due to extreme slowing of the heart. This is called bradycardia.

In 90 percent of adult victims of SCD, mulitple major coronary arteries are narrowed by fatty buildups. Scarring from a prior heart attack is found in two-thirds of victims. When sudden death occurs in young adults, other heart abnormalities are more likely causes. Adrenaline released during intense physical or athletic activity often acts as a trigger for sudden death when these abnormalities are present. Under certain conditions, various heart medications and other drugs - as well as illegal drug abuse - can lead to abnormal heart rhythms that cause sudden death.

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What are the risk factors of sudden cardiac death?

There are many factors that can increase a person’s risk of SCD, including the following:

• Coronary heart disease: 80 percent of SCD cases are linked with this disease. A previous heart attack is retrieved in 75 percent of SCD cases. A person’s risk of SCD is higher during the first six months after a heart attack. Risk factors for coronary artery disease include mainly smoking, high cholesterol, diabetes, hypertension, obesity and family history of coronary heart disease.
• Dilated cardiomyopathy (cause of SCD in about 10 percent of the cases): a decrease in the heart’s ability to pump blood due to an enlarged (dilated) and weakened left ventricle
• Hypertrophic cardiomyopathy: a thickened heart muscle that especially affects the ventricles
• Heart failure: a condition in which the heart’s pumping power is weaker than normal. Patients with heart failure are 6 to 9 times more likely than the general population to experience ventricular arrhythmias that can lead to SCD. An ejection fraction of less than 40 percent, combined with ventricular tachycardia, is a high-risk factor for SCD.
• Personal or family history of SCD
• Personal or family history of abnormal heart rhythms, including long QT syndrome, Wolff-Parkinson-White syndrome, extremely low heart rates, or heart block
• Ventricular tachycardia or ventricular fibrillation after a heart attack
• History of congenital heart defects or blood vessel abnormalities
• History of syncope (fainting episodes of unknown cause)
• Illegal drug abuse
• Taking drugs that are "pro-arrhythmic" may increase the risk for life-threatening    arrhythmias

However, most cases of SCD occur in the asymptomatic general population, with no or minor previously identified SCD risk factors. Improvement in individual risk stratification is needed.

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Can sudden cardiac death be prevented?

Yes, SCD can be prevented. If you have any of the above listed risk factors for SCD, it is important that you speak with your doctor about possible steps to reduce your risk.

Keeping regular follow-up appointments with your doctor, making certain lifestyle changes, taking medications as prescribed, and having interventional procedures or surgery (as recommended) are ways you can reduce your risk.

For people whose risk factors put them at great risk for SCD, an implantable cardioverter-defibrillator (ICD) may be inserted as a preventive treatment. An ICD is a small machine similar to a pacemaker that is designed to correct arrhythmias. It detects and then corrects a fast heart rate. The ICD constantly monitors the heart rhythm. When it detects a very fast or slow heart rhythm, it delivers energy (a small, but powerful shock) to the heart muscle to cause the heart to beat in a normal rhythm again. The ICD also records the data of each abnormal heartbeat, which can be viewed by the doctor through a third part of the system kept at the hospital. The ICD may be used in patients who have survived sudden cardiac arrest and need their heart rhythms constantly monitored. It may also be combined with a pacemaker to treat other underlying irregular heart rhythms.

Interventional procedures or surgery. For patients with coronary artery disease, an interventional procedure such as angioplasty (blood vessel repair) or bypass surgery may be needed to improve blood flow to the heart muscle and reduce the risk of SCD. Furthermore, when a heart attack has occurred, the scarred tissue may increase the risk of ventricular tachycardia. The electrophysiologist (doctor specializing in electrical disorders of the heart) can determine the exact area causing the arrhythmia. In some cases, this area can be treated by ablation (high-frequency electrical energy) to "disconnect" abnormal electrical pathways within the heart. For patients with other conditions, such as hypertrophic cardiomyopathy or congenital heart defects, an interventional procedure or surgery may be needed to correct the problem. Other procedures may be used to treat abnormal heart rhythms, including electrical cardioversion and catheter ablation.

Finally, an important problem for prevention remains the difficulty with identifying the people at risk. Among people who suffer SCD, those with a previously identified risk-conferring condition are quite rare. Risk factors such as cholesterol, diabetes or hypertension are insufficiently predictive when applied to specific individuals. Consequently, prevention remains inapplicable in the general asymptomatic population, where SCD prevalence is in fact the highest. The identification of meaningful, low-cost, high throughput genetic markers indicating elevated risk of SCD in specific individuals could add much to stratification and prevention.

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Can sudden cardiac death be treated?

Yes, SCD can be treated and reversed, but emergency action must take place immediately, since brain death and permanent death start to occur in just four to six minutes after someone experiences cardiac arrest. Emergency treatment includes cardiopulmonary resuscitation (CPR) and defibrillation. CPR is a manual technique using repetitive pressing to the chest and breathing into the person's airways that keeps enough oxygen and blood flowing to the brain until the normal heart rhythm is restored with an electric shock to the chest, a procedure called defibrillation. Emergency squads use portable defibrillators and frequently there are public access defibrillators (AEDs, ambulatory external defibrillators) in public locations such as airports that are intended to be available for use by citizens who observe cardiac arrest.

Survival can be as high as 90% if treatment is initiated within the first minutes after SCD. The rate decreases by about 10% each minute longer it takes to initiate therapy. Few attempts at resuscitation succeed after 10 minutes. Those who survive have a good long-term outlook.

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What are the treatments for survivors?

If a cardiac arrest was due to ventricular tachycardia or ventricular fibrillation, survivors are at risk for another arrest, especially if they have underlying heart disease.
Survivors of cardiac arrest must have all causes corrected to prevent future episodes. Possible causes include myocardial ischemia (inadequate blood flow to the heart muscle), arrhythmia (abnormal heart rhythm), etc.
Possible tests and treatments include

• cardiac catheterization
• electrophysiologic tests
• coronary artery bypass surgery
• balloon angioplasty or PTCA
• antiarrhythmic medicine
• implantable cardioverter / defibrillator
• implantable pacemaker
• heart transplant

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What should I do if I witness sudden cardiac death?

If you witness someone experiencing SCD, dial your local emergency personnel immediately and initiate CPR. If done properly, CPR can save a person's life, as the procedure keeps blood and oxygen circulating through the body until help arrives.

If there is an ambulatory external defibrillator available, the best chance of rescuing the person includes defibrillation with that device. The shorter the time until defibrillation, the greater the chance the person will survive. It is CPR plus defibrillation that saves a person.

Once emergency personnel arrive, defibrillation can be used to restart the heart. This is done through an electric shock delivered to the heart through paddles placed on the chest.

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