Tests and diagnosis

By Mayo Clinic Staff

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. An ECG can detect abnormal electrical patterns, such as a prolonged QT interval, that increase your risk of sudden death.

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 help create 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 heart failure.
  • 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.
  • 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 50 to 70 percent. An ejection fraction of less than 40 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), a nuclear medicine scan (multiple gated acquisition, or MUGA), a computerized tomography (CT) scan or a cardiac catheterization

  • Nuclear scan. This test, usually done along with a stress test, helps identify blood flow problems to your heart. Tiny 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.

Other tests

Other tests that are often done include:

  • Electrical system (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 found. 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 starts.

    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.

  • 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.

Nov. 15, 2012