Sudden cardiac arrest

Sudden cardiac arrest is a medical emergency. If not treated immediately, it is fatal, resulting in sudden cardiac death. With fast, appropriate medical care, survival is possible. Administering cardiopulmonary resuscitation (CPR) — or even just rapid compressions to the chest — can improve the chances of survival until emergency personnel arrive.

Symptoms

Sudden cardiac arrest symptoms are sudden and drastic:

• Sudden collapse
• No pulse
• No breathing
• Loss of consciousness

Sometimes, other signs and symptoms precede sudden cardiac arrest. These may include fatigue, fainting, blackouts, dizziness, chest pain, shortness of breath, palpitations or vomiting. But sudden cardiac arrest often occurs with no warning.

Causes

The immediate cause of sudden cardiac arrest is usually an abnormality in your heart rhythm (arrhythmia), the result of a malfunction in your heart's electrical system.

Unlike other muscles in your body, which rely on nerve connections to receive the electrical stimulation they need to function, your heart has its own electrical stimulator — a specialized group of cells called the sinus node, located in the upper right chamber (right atrium) of your heart. The sinus node generates electrical impulses that flow in an orderly manner through your heart to synchronize heart rate and coordinate the pumping of blood from your heart to the rest of your body.

If something goes wrong with the sinus node or the flow of electric impulses through your heart, an arrhythmia can result, causing your heart to beat too fast, too slow or in an irregular fashion. Often, these interruptions in rhythm are momentary and harmless. But some types of arrhythmia can be serious and lead to a sudden stop in heart function (sudden cardiac arrest).

The most common cause of cardiac arrest is an arrhythmia called ventricular fibrillation — when rapid, erratic electrical impulses cause your ventricles to quiver uselessly instead of pumping blood. Without an effective heartbeat, your blood pressure plummets, cutting off blood supply to your vital organs.

Most of the time, cardiac-arrest-inducing arrhythmias don't occur on their own. In a person with a normal, healthy heart, a sustained arrhythmia is unlikely to develop without some outside trigger, such as an electrical shock, the use of illegal drugs, or trauma to the chest at just the wrong time of the heart's cycle (commotio cordis).

Heart conditions that can lead to sudden cardiac arrest

More often, a life-threatening arrhythmia develops in a person with a pre-existing heart condition, such as:

• Coronary artery disease. More than 80 percent of cases of sudden cardiac arrest occur in people who have coronary artery disease. In coronary artery disease, your arteries become clogged with cholesterol and other deposits, reducing blood flow to your heart. This can alter the ability of your heart to conduct electrical impulses smoothly.
• Heart attack. If a heart attack occurs, often as a result of severe coronary artery disease, it can trigger ventricular fibrillation and sudden cardiac arrest. In addition, a heart attack can leave behind areas of dead scar tissue. Electrical short circuits around the scar tissue can lead to abnormalities in your heart rhythm.
• Enlarged heart (cardiomyopathy). This occurs primarily when your heart's muscular walls stretch and enlarge or thicken. In both cases, your heart's muscle is abnormal, a condition that often leads to heart tissue damage and potential arrhythmias.
• Valvular heart disease. Leaking or narrowing of your heart valves can lead to stretching or thickening of your heart muscle, or both. When the chambers become enlarged or weakened because of stress caused by a tight or leaking valve, there's an increased risk of developing arrhythmia.
• Congenital heart disease. When sudden cardiac arrest occurs in children or adolescents, it's more often due to a heart condition that was present at birth (congenital heart disease). Hypertrophic cardiomyopathy is a genetic disorder and is the most common cause of sudden cardiac arrest in young athletes, triggered perhaps by vigorous exertion, although the process isn't completely understood. Even adults who've had corrective surgery for a congenital heart defect remain at increased risk of sudden cardiac arrest.
• Electrophysiological abnormalities. In some people, the problem is in the heart's electrical system itself, instead of a problem with the heart muscle or valves. These are called primary heart rhythm abnormalities. For example, some people have primary ventricular fibrillation, unrelated to another heart condition. Other examples include long QT syndrome, Wolff-Parkinson-White syndrome and Brugada syndrome. 

Risk factors

Because sudden cardiac arrest is so often linked with coronary artery disease, the same factors that put you at risk of coronary artery disease may also put you at risk of sudden cardiac arrest. These include:

• A family history of coronary artery disease
• Smoking
• High blood pressure
• High blood cholesterol
• Obesity
• Diabetes
• A sedentary lifestyle

Other factors that may increase your risk of sudden cardiac arrest include:

• Previous episode of cardiac arrest or a family history of cardiac arrest
• Previous heart attack
• A personal or family history of other forms of heart disease, such as heart rhythm disorders, congenital heart defects, heart failure and cardiomyopathy
• Age; the incidence of sudden cardiac arrest increases with age, especially after age 45 for men and age 55 for women
• Being male; men are two to three times more likely to experience sudden cardiac arrest
• Using illegal drugs, such as cocaine or amphetamines
• Lightning strike or another electrical shock
• Immersion in cold water

Complications

When sudden cardiac arrest occurs, your brain is the first part of your body to suffer because, unlike other organs, it doesn't have a reserve of oxygen-rich blood. It's completely dependent on an uninterrupted supply of blood. Reduced blood flow to your brain causes unconsciousness.

If your heart rhythm doesn't rapidly return to its normal rhythm, brain damage occurs and death results. If sudden cardiac arrest lasts more than 10 minutes, the survival rate is less than 5 percent. Survivors of cardiac arrest may show signs of brain damage.

Diagnosis

If you experience an episode of sudden cardiac arrest without warning and survive, your doctor will want to investigate what caused the cardiac arrest. Identifying the underlying problem may help prevent future episodes of cardiac arrest.

Tests your doctor may recommend include:

Electrocardiogram

A test commonly given after cardiac arrest is an electrocardiogram (ECG). During an ECG, sensors (electrodes) that can detect the electrical activity of your heart are attached to your chest and sometimes to your limbs. An ECG measures the timing and duration of each electrical phase in your heartbeat and can reveal disturbances in heart rhythm. Because injured heart muscle doesn't conduct electrical impulses normally, the ECG may show that a heart attack has occurred.

Blood tests

Blood tests may include:

• Cardiac enzyme test. Certain heart enzymes leak into your blood if your heart has been damaged by a heart attack. Because a heart attack can trigger sudden cardiac arrest, it's important to know whether you've had a heart attack. Testing a blood sample for these enzymes may help indicate whether a heart attack has indeed occurred.
• Electrolyte test. A sample of your blood may also be tested for levels of electrolytes, such as potassium, calcium and magnesium. Electrolytes are minerals in your blood and body fluids that participate in the creation of electrical impulses. An imbalance in the levels of these substances can increase your risk of arrhythmia and sudden cardiac arrest.
• Drug test. Your doctor may check your blood for evidence of drugs that have the potential to induce arrhythmia, including certain prescription and over-the-counter drugs and illegal drugs.
• Hormone test. Testing for hyperthyroidism may indicate this condition as the trigger for your cardiac arrest.

Imaging tests

These may include:

• Chest X-ray. An X-ray image of your chest allows your doctor to check the size and shape of your heart and its blood vessels. It may also indicate whether you have congestive heart failure.
• Nuclear scan. This test, usually done along with a stress test, helps identify blood flow problems to your heart. Trace amounts of radioactive material, such as thallium, are injected into your bloodstream. Special cameras can detect the radioactive material as it flows through your heart and lungs.
• Echocardiogram. This test uses sound waves to produce an image of your heart. An echocardiogram can help identify whether an area of your heart has been damaged by a heart attack and isn't pumping normally or at peak capacity (ejection fraction), or whether there are valvular abnormalities.

Other tests

Other tests that are often done include:

• Electrophysiological testing and mapping. This test, if needed, is usually done later, after you've recovered and if an underlying explanation for your cardiac arrest hasn't been identified. With this type of test, your doctor may try to cause an arrhythmia while closely monitoring your heart. The test can help locate where in the heart the arrhythmia originates. During the test, thin, flexible tubes (catheters) tipped with electrodes are threaded through your blood vessels to a variety of spots within your heart. Once in place, the electrodes can precisely map the spread of electrical impulses through your heart. In addition, your cardiologist can use the electrodes to stimulate your heart to beat at rates that may trigger — or halt — an arrhythmia. This allows your doctor to observe the location of the arrhythmia.
• Ejection fraction testing. One of the most important predictors of your risk of sudden cardiac arrest is how well your heart is able to pump blood. Your doctor can determine your heart's pumping capacity by measuring what's called the ejection fraction. This refers to the percentage of blood that's pumped out of a filled ventricle with each heartbeat. A normal ejection fraction is 55 to 70 percent. An ejection fraction of less than 35 percent increases your risk of sudden cardiac arrest. Your doctor can measure ejection fraction in several ways, such as with an echocardiogram, magnetic resonance imaging (MRI) of your heart, a nuclear medicine scan (multiple gated acquisition, or MUGA) of your heart or a computerized tomography (CT) scan of your heart.
• Coronary catheterization (angiogram). This test can show if your coronary arteries are narrowed or blocked. Along with ejection fraction, the number of obstructed blood vessels is another important predictor of sudden cardiac arrest. During the procedure, a liquid dye is injected into the arteries of your heart through a long, thin tube (catheter) that's advanced through an artery, usually in your leg, to arteries in your heart. As the dye fills your arteries, the arteries become visible on X-ray and videotape, revealing areas of blockage. Also, while the catheter is in position, your doctor may treat a blockage by performing angioplasty and inserting a stent to hold the artery open. 

References

http://www.webmd.com/heart-disease/heart-failure/features/sudden-cardiac-arrest-why

http://emedicine.medscape.com/article/151907-overview

http://my.clevelandclinic.org/services/heart/disorders/arrhythmia/sudden-cardiac-death

http://circ.ahajournals.org/content/98/21/2334.long