What Happens During an Electrocardiogram?

An electrocardiogram (EKG) is painless and harmless. A technician first attaches 12 soft patches called electrodes to the skin of the chest, arms, and legs. These electrodes are about the size of a quarter. To help an electrode stick to the skin, the technician may have to shave a patch of hair where the electrode will be attached.

After the electrodes are placed on the skin, the patient lies still on a table for a few minutes while the electrodes detect the electrical signals of the heart. A machine then records these signals on graph paper or displays them on a screen.

The entire test takes about 10 minutes. After the test, the electrodes are removed from the skin and discarded.

Special EKGs

The common EKG described above, called a resting 12-lead EKG, records only a few minutes of heart signals at a time. It will show a heart problem only if the problem is present during the few minutes that the test is being run. Many heart problems are present all the time and will be found by a resting 12-lead EKG. But some heart problems, like those related to irregular heartbeat, can come and go. They may be present for only a few minutes out of the day or only while exercising.

Special types of EKGs are used to help diagnose those kinds of problems. Three of these special EKGs are:

• Stress test
• Holter monitor
• Event monitor

Stress Test

Some heart problems are easier to diagnose when your heart is working harder and beating faster than when it's at rest. During stress testing, you exercise (or are given medicine if you are unable to exercise) to make your heart work harder and beat faster while heart tests are performed.

During exercise stress testing, your blood pressure and EKG readings are monitored while you walk or run on a treadmill or pedal a bicycle. Other heart tests, such as nuclear heart scanning or echocardiography, also can be done at the same time. These would be ordered if your doctor needs more information than the exercise stress test can provide about how well your heart is working.

If you are unable to exercise, a medicine can be injected through an intravenous line (IV) into your bloodstream to make your heart work harder and beat faster, as if you are exercising on a treadmill or bicycle. Nuclear heart scanning or echocardiography is then usually done.

During nuclear heart scanning, radioactive tracer is injected into your bloodstream, and a special camera shows the flow of blood through your heart and arteries. Echocardiography uses sound waves to show blood flow through the chambers and valves of your heart and to show the strength of your heart muscle.

Your doctor also may order two newer tests along with stress testing if more information is needed about how well your heart works. These new tests are magnetic resonance imaging (MRI) and positron emission tomography (PET) scanning of the heart. MRI shows detailed images of the structures and beating of your heart, which may help your doctor better assess if parts of your heart are weak or damaged. PET scanning shows the level of chemical activity in different areas of your heart. This can help your doctor determine if enough blood is flowing to the areas of your heart. A PET scan can show decreased blood flow caused by disease or damaged muscles that may not be detected by other scanning methods.