What Is an Electrocardiogram?

An electrocardiogram (e-lek-tro-KAR-de-o-gram), or EKG, is a simple, painless test that records the heart’s electrical activity. To understand this test, it helps to understand how the heart works.

With each heartbeat, an electrical signal spreads from the top of the heart to the bottom. As it travels, the signal causes the heart to contract and pump blood. The process repeats with each new heartbeat. The heart’s electrical signals set the rhythm of the heartbeat.

For more detailed information, see the Diseases and Conditions Index article on How the Heart Works.

An EKG shows:

• How fast your heart is beating
• Whether the rhythm of your heartbeat is steady or irregular
• The strength and timing of electrical signals as they pass through each part of your heart

This test is used to detect and evaluate many heart problems, such as heart attack, arrhythmia (ah-RITH-me-ah), and heart failure. EKG results also can suggest other disorders that affect heart function.

EKGs also are used to monitor how the heart is working. This article focuses on how EKGs are used for testing purposes.

Other Names for Electrocardiogram

An electrocardiogram also is called EKG or ECG. Sometimes the test is called a 12-lead EKG or 12-lead ECG because the electrical activity of the heart is most often recorded from 12 different places on the body at the same time.

Who Needs an Electrocardiogram?

Your doctor may recommend an electrocardiogram (EKG) if you have signs or symptoms that suggest a heart problem. Examples of such signs and symptoms include:

• Chest pain
• Heart pounding, racing, or fluttering, or the sense that your heart is beating unevenly
• Problems breathing
• Feeling tired and weak
• Unusual heart sounds when your doctor listens to your heartbeat

You may need to have more than one EKG so your doctor can diagnose certain heart conditions.

An EKG may be done as part of a routine health exam. The test can screen for early heart disease that has no symptoms. Your doctor is more likely to look for early heart disease if your mother, father, brother, or sister had heart disease—especially if it developed early.

You may have an EKG so your doctor can check how well heart medicine or a medical device, such as a pacemaker, is working. The test also may be used for routine screening before major surgery.

Your doctor may use EKG results to help plan your treatment for a heart condition.

What To Expect Before an Electrocardiogram

No special preparation is needed for an electrocardiogram (EKG). Before the test, let your doctor or doctor’s office know what medicines you’re taking. Some medicines can affect EKG results.

What Happens During an Electrocardiogram?

An electrocardiogram (EKG) is painless and harmless. A technician attaches soft, sticky patches called electrodes to the skin of your chest, arms, and legs. The patches are about the size of a quarter.

Typically, 12 patches are attached to detect your heart’s electrical activity from many angles. To help the patches stick, the technician may have to shave areas of your skin.

After the patches are placed on your skin, you lie still on a table while the patches detect your heart’s electrical signals. A machine records these signals on graph paper or displays them on a screen.

The entire test takes about 10 minutes.

EKG

The illustration shows the standard setup for an EKG. In figure A, a normal heart rhythm recording shows the electrical pattern of a regular heartbeat. In figure B, a patient lies in a bed with EKG electrodes attached to his chest, upper arms, and legs. A nurse watches the painless procedure.

Special Types of Electrocardiogram

The standard EKG described above, called a resting 12-lead EKG, records only seconds of heart activity at a time. It will show a heart problem only if the problem is present during the time that the test is run.

Many heart problems are present all the time, and a resting 12-lead EKG will detect them. But some heart problems, like those related to an irregular heartbeat, can come and go. They may occur for only a few minutes out of the day or only while you exercise.

Special EKGs, such as stress tests and Holter and event monitors, are used to help diagnose these kinds of problems.

Stress Test

Some heart problems are easier to diagnose when your heart is working hard and beating fast. During stress testing, you exercise to make your heart work hard and beat fast while your heart’s electrical activity is recorded. If you’re not able to exercise, you’re given medicine to make your heart work hard and beat fast.

For more information, see the Diseases and Conditions Index article on Stress Testing.

Holter and Event Monitors

Holter and event monitors are small, portable devices. They record your heart’s activity while you do your normal daily activities. A Holter monitor records the heart’s electrical activity for a full 24-hour period or longer.

An event monitor only records your heart’s electrical activity at certain times while you’re wearing it. For many event monitors, you push a button to start the monitor when you feel symptoms. Other event monitors start automatically when they sense abnormal heart rhythms.

For more information, see the Diseases and Conditions Index article on Holter and Event Monitors.

What To Expect After an Electrocardiogram

After an electrocardiogram (EKG), the electrodes (soft patches) are removed from your skin. You may get a rash or redness where the EKG patches were attached. This mild rash usually goes away without treatment.

You usually can go back to your normal daily routine after an EKG.

What Does an Electrocardiogram Show?

Many heart problems change the heart’s electrical activity in distinct ways. An electrocardiogram (EKG) can help detect a number of heart problems.

EKG recordings can help doctors diagnose a heart attack that’s happening now or has happened in the past. This is especially true if doctors can compare a current EKG recording to an older one.

An EKG also can show:

• Lack of blood flow to the heart muscle
• A heart that’s beating too fast, too slow, or with an irregular rhythm (arrhythmia)
• A heart that doesn’t pump forcefully enough (heart failure)
• Heart muscle that’s too thick or parts of the heart that are too big
• Birth defects in the heart (congenital heart defects)
• Problems with the heart valves (heart valve disease)
• Inflammation of the sac that surrounds the heart (pericarditis)

An EKG also can reveal whether the heartbeat starts at the top right part of the heart like it should. The test shows how long it takes for the electrical signals to travel through the heart. Delays in signal travel time may suggest heart block or long QT syndrome.

What Are the Risks of an Electrocardiogram?

An electrocardiogram (EKG) has no serious risks. It’s a harmless, painless test that detects the heart’s electrical activity. EKGs don’t give off electrical charges, such as shocks.

You may have a mild rash where the electrodes (soft patches) were attached. This rash usually goes away without treatment.

Key Points

• An electrocardiogram (EKG) is a simple, painless test that records the heart’s electrical activity.
• An EKG shows:
  • How fast your heart is beating
  • Whether the rhythm of your heart is steady or irregular
  • The strength and timing of electrical signals as they pass through each part of your heart
• EKGs are used to detect and evaluate many heart problems, such as heart attack, arrhythmia, and heart failure.
• Your doctor may recommend an EKG if you have signs or symptoms that suggest a heart problem. An EKG also may be done as part of a routine health exam, to check how medicine or a medical device is working, or for routine screening before major surgery. Your doctor may use EKG results to help plan your treatment for a heart condition.
• No special preparation is needed for an EKG. Before the test, let your doctor or doctor’s office know what medicines you’re taking. Some medicines can affect EKG results.
• For an EKG, soft, sticky patches called electrodes are attached to the skin of your chest, arms, and legs. You lie still on a table while the patches detect your heart’s electrical signals. A machine records these signals on graph paper or displays them on a screen. The entire test takes about 10 minutes.
• If you have a heart problem that isn’t present all of the time, you may need a special type of EKG, such as a stress test or Holter or event monitor.
• You usually can go back to your normal daily routine after an EKG.
• Many heart problems change the heart’s electrical activity in distinct ways. An EKG can help detect a number of heart problems. Results from an EKG can suggest problems such as lack of blood flow to the heart muscle, problems with heart rhythm or the heart’s pumping action, birth defects, problems with heart muscle or heart valves, and inflammation of the heart.
• An EKG has no serious risks. EKGs don’t give off electrical charges, such as shocks. You may have a mild rash where the electrodes were attached. This rash usually goes away without treatment.

Links to Other Information About Electrocardiogram

NHLBI Resources

• Holter and Event Monitors (Diseases and Conditions Index)
• Stress Testing (Diseases and Conditions Index)

Non-NHLBI Resources

• ECG (MedlinePlus)

Clinical Trials

• Current Research (ClinicalTrials.gov)