The Electrocardiogram (ECG)

What is it used for?

12-lead ECG
12-Lead ECG. Matt Meadows/Getty Images

The electrocardiogram, or ECG, is the most common test used by doctors to assess condition of the heart. The ECG is so widely used because it can screen for a variety of cardiac conditions, ECG machines are readily available in most medical facilities, and the test is simple to perform, is essentially risk-free, and is relatively inexpensive.

How is the ECG Done?

A person having a typical 12-lead ECG will lie down on an examination table with his or her chest exposed.

(Women can generally keep their bras on.) A total of ten electrodes (or leads) are attached - one on each arm and leg, and six on the chest. .

The electrodes are then used to record the electrical activity of the heart. These electrical signals are sent to the ECG machine, where they are processed, and printed out as an “ECG tracing.” The electrodes are then removed. The ECG test takes less than 5 minutes to perform.

What Does the ECG Tracing Look Like?

The picture on this page shows a typical, normal ECG. The electrical signals generated from the ten electrodes have been processed into 12 “views” of the heart’s electrical activity - the so-called 12-lead ECG. By examining any abnormalities on the ECG, and by observing which leads these abnormalities appear in, the doctor can get a lot of important clues about the status of the heart. 

What Information Can Be Learned From the ECG?

From the ECG tracing, the following information can be determined:

All of these features are potentially important. While the ECG can make a clear diagnosis for some cardiac conditions (such as a cardiac arrhythmia), it is more often useful as a screening test. Abnormalities seen on the ECG often need to be followed by a more definitive test in order to make a firm diagnosis. For instance, If the ECG indicates possible CAD, a stress test or cardiac catheterization might be needed. If ventricular hypertrophy is seen, an echocardiogram is often needed to check for valvular heart disease (such as aortic stenosis), or other structural abnormalities.

What are the Limitations of the ECG?

  • The ECG reveals the heart rate and rhythm only during the few seconds it takes to record the ECG. If an intermittent arrhythmia is suspected, ambulatory monitoring may be required. 
  • The ECG can often be normal or nearly normal in patients with undiagnosed CAD or other forms of heart disease (false negative results.)
  • Many "abnormalities" that appear on the ECG turn out to have no medical significance after a thorough evaluation is done (false positive results).

    When Should You Have an ECG?

    It is reasonable for your doctor to perform an ECG the first time he or she sees you, as a baseline study. This test can then be compared to a later test to see whether any change has occurred.

    It is also reasonable to perform an ECG as part of a yearly medical examination if you have had heart disease in the past, or if you have significant risk factors for cardiac disease. However, if you are entirely healthy and have no major risk factors, most experts no longer recommend “routine” yearly ECGs.


    Kligfield P, Gettes LS, Bailey JJ, et al. Recommendations for the standardization and interpretation of the electrocardiogram: part I: the electrocardiogram and its technology a scientific statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society endorsed by the International Society for Computerized Electrocardiology. J Am Coll Cardiol 2007; 49:1109.