This is a medical question of type : exams and tests 
 How to diagnose Sudden Cardiac Arrest ? 
 Sudden cardiac arrest (SCA) happens without warning and requires emergency treatment. Doctors rarely diagnose SCA with medical tests as it's happening. Instead, SCA often is diagnosed after it happens. Doctors do this by ruling out other causes of a person's sudden collapse.
                
Specialists Involved
                
If you're at high risk for SCA, your doctor may refer you to a cardiologist. This is a doctor who specializes in diagnosing and treating heart diseases and conditions. Your cardiologist will work with you to decide whether you need treatment to prevent SCA.
                
Some cardiologists specialize in problems with the heart's electrical system. These specialists are called cardiac electrophysiologists.
                
Diagnostic Tests and Procedures
                
Doctors use several tests to help detect the factors that put people at risk for SCA.
                
EKG (Electrocardiogram)
                
An EKG is a simple, painless test that detects and records the heart's electrical activity. The test shows how fast the heart is beating and its rhythm (steady or irregular). An EKG also records the strength and timing of electrical signals as they pass through each part of the heart.
                
An EKG can show evidence of heart damage due to coronary heart disease (CHD). The test also can show signs of a previous or current heart attack.
                
Echocardiography
                
Echocardiography, or echo, is a painless test that uses sound waves to create pictures of your heart. The test shows the size and shape of your heart and how well your heart chambers and valves are working.
                
Echo also can identify areas of poor blood flow to the heart, areas of heart muscle that aren't contracting normally, and previous injury to the heart muscle caused by poor blood flow.
                
There are several types of echo, including stress echo. This test is done both before and after a cardiac stress test. During this test, you exercise (or are given medicine if you're unable to exercise) to make your heart work hard and beat fast.
                
Stress echo shows whether you have decreased blood flow to your heart (a sign of CHD).
                
MUGA Test or Cardiac MRI
                
A MUGA (multiple gated acquisition) test shows how well your heart is pumping blood. For this test, a small amount of radioactive substance is injected into a vein and travels to your heart.
                
The substance releases energy, which special cameras outside of your body can detect. The cameras use the energy to create pictures of many parts of your heart.
                
Cardiac MRI (magnetic resonance imaging) is a safe procedure that uses radio waves and magnets to create detailed pictures of your heart. The test creates still and moving pictures of your heart and major blood vessels.
                
Doctors use cardiac MRI to get pictures of the beating heart and to look at the structure and function of the heart.
                
Cardiac Catheterization
                
Cardiac catheterization is a procedure used to diagnose and treat certain heart conditions. A long, thin, flexible tube called a catheter is put into a blood vessel in your arm, groin (upper thigh), or neck and threaded to your heart. Through the catheter, your doctor can do diagnostic tests and treatments on your heart.
                
Sometimes dye is put into the catheter. The dye will flow through your bloodstream to your heart. The dye makes your coronary (heart) arteries visible on x-ray pictures. The dye can show whether plaque has narrowed or blocked any of your coronary arteries.
                
Electrophysiology Study
                
For an electrophysiology study, doctors use cardiac catheterization to record how your heart's electrical system responds to certain medicines and electrical stimulation. This helps your doctor find where the heart's electrical system is damaged.
                
Blood Tests
                
Your doctor may recommend blood tests to check the levels of potassium, magnesium, and other chemicals in your blood. These chemicals play an important role in your heart's electrical signaling.