An electrocardiogram (ECG) is a test that records the electrical activity of the heart.
How electricity produced in your heart helps create an electrocardiogram (ECG).
How the Test is Performed
You will be asked to lie down. The health care provider will clean several areas on your arms, legs, and chest, and then will attach small patches called electrodes to those areas. It may be necessary to shave or clip some hair so the patches stick to the skin. The number of patches used may vary.
The patches are connected by wires to a machine that turns the heart's electrical signals into wavy lines, which are often printed on paper. The doctor reviews the test results.
You will need to remain still during the procedure. The provider may also ask you to hold your breath for a few seconds as the test is being done.
It is important to be relaxed and warm during an ECG recording because any movement, including shivering, can alter the results.
Sometimes this test is done while you are exercising or under light stress to look for changes in the heart. This type of ECG is often called a stress test.
How an ECG test detects the electrical activity of your heart.
How to Prepare for the Test
Make sure your provider knows about all the medicines you are taking. Some drugs can interfere with test results.
DO NOT exercise or drink cold water immediately before an ECG because these actions may cause false results.
If your heart has been beating too fast, or you’ve been having chest pain, both you and your doctor will want to find out what’s causing the problem so you can get it treated. One way to diagnose heart problems is with a test of the heart’s electrical activity, called an electrocardiogram or ECG, or EKG for short. Your heart is controlled by an electrical system, much like the electricity that powers the lights and appliances in your home. Electrical signals make your heart contract so that it can pump blood out to your body. Heart disease, abnormal heart rhythms, and other heart problems can affect those signals. Using an ECG, your doctor can identify problems in your heart’s electrical system and diagnose heart disease. So, how is an ECG done? First you’ll lie down on a table. You’ll have to lie very still while the test is done. Small patches, called electrodes, will be attached to several places on your arms, legs, and chest. The patches won’t hurt, but some of the hair in those areas may be shaved so the patches will stick to your skin. The patches are then attached to a machine. You’ll notice that when the machine is turned on, it produces wavy lines on a piece of paper. Those lines represent the electrical signals coming from your heart. If the test is normal, it should show that your heart is beating at an even rate of 60 to 100 beats per minute. Many different heart conditions can show up on an ECG, including a fast, slow, or abnormal heart rhythm, a heart defect, coronary artery disease, heart valve disease, or an enlarged heart. An abnormal ECG may also be a sign that you’ve had a heart attack in the past, or that you’re at risk for one in the near future. If you’re healthy and you don’t have any family or personal history of heart disease, you don’t need to have an ECG on a regular basis. But if you are having heart problems, your doctor may recommend getting this test. An ECG is pretty accurate at diagnosing many types of heart disease, although it doesn’t always pick up every heart problem. You may have a perfectly normal ECG, yet still have a heart condition. If your test is normal but your doctor suspects that you have a heart problem, he may recommend that you have another ECG, or a different type of test to find out for sure.
How the Test will Feel
An ECG is painless. No electricity is sent through the body. The electrodes may feel cold when first applied. In rare cases, some people may develop a rash or irritation where the patches were placed.
Why the Test is Performed
An ECG is used to measure:
- Any damage to the heart
- How fast your heart is beating and whether it is beating normally
- The effects of drugs or devices used to control the heart (such as a pacemaker)
- The size and position of your heart chambers
An ECG is often the first test done to determine whether a person has heart disease. Your provider may order this test if:
Normal test results most often include:
- Heart rate: 60 to 100 beats per minute
- Heart rhythm: Consistent and even
Overview of ECG test waves as they detect the electrical activity in your heart.
What Abnormal Results Mean
Abnormal ECG results may be a sign of:
- Damage or changes to the heart muscle
- Changes in the amount of the electrolytes (such as potassium and calcium) in the blood
- Congenital heart defect
- Enlargement of the heart
- Fluid or swelling in the sac around the heart
- Inflammation of the heart (myocarditis)
- Past or current heart attack
- Poor blood supply to the heart arteries
- Abnormal heart rhythms (arrhythmias)
Some heart problems that can lead to changes on an ECG test include:
How doctors use waves and other information provided by ECG and EKG recordings to help diagnose heart arrhythmia.
There are no risks.
The accuracy of the ECG depends on the condition being tested. A heart problem may not always show up on the ECG. Some heart conditions never produce any specific ECG changes.
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Ganz L. Electrocardiography. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 54.
Mirvis DM, Goldberger AL. Electrocardiography. In: Zipes DP, Libby P, Bonow RO, Mann DL, Tomaselli GF, Braunwald E, eds. Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine. 11th ed. Philadelphia, PA: Elsevier; 2019:chap 12.
Last reviewed on: 5/16/2018
Reviewed by: Michael A. Chen, MD, PhD, Associate Professor of Medicine, Division of Cardiology, Harborview Medical Center, University of Washington Medical School, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.