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Back to tests and procedures

Electrocardiogram

(ECG, EKG, Resting ECG, Resting EKG)

Procedure overview

What is an electrocardiogram?

An electrocardiogram (ECG or EKG) is one of the simplest and fastest procedures used to evaluate the heart. Electrodes (small, plastic patches) are placed at certain locations on the chest, arms, and legs. When the electrodes are connected to an ECG machine by lead wires, the electrical activity of the heart is measured, interpreted, and printed out for the doctor's information and further interpretation.

Other related procedures that may be used to assess the heart include exercise electrocardiogram (ECG), Holter monitor, signal-averaged ECG, cardiac catheterization, chest X-ray, computed tomography (CT scan) of the chest, echocardiography, electrophysiological studies, magnetic resonance imaging (MRI) of the heart, myocardial perfusion scans, radionuclide angiography, and cardiac CT scan. Please see these procedures for additional information.

The heart's electrical conduction system

The heart is, in the simplest terms, a pump made up of muscle tissue. The heart's pumping action is regulated by an electrical conduction system that coordinates the contraction of the various chambers of the heart.

An electrical stimulus is generated by the sinus node (also called the sinoatrial node, or SA node), which is a small mass of specialized tissue located in the right atrium (right upper chamber) of the heart.

The sinus node generates an electrical stimulus regularly at 60 to 100 times per minute under normal conditions. This electrical stimulus travels down through the conduction pathways (similar to the way electricity flows through power lines from the power plant to your house) and causes the heart's lower chambers to contract and pump out blood. The right and left atria (the two upper chambers of the heart) are stimulated first and contract a short period of time before the right and left ventricles (the two lower chambers of the heart).

Cross section of the heart, showing the electrical pathways and the heart's normal rhythm
Click image to enlarge

The electrical impulse travels from the sinus node to the atrioventricular node (also called AV node), where impulses are slowed down for a very short period, then continue down the conduction pathway via the bundle of His into the ventricles. The bundle of His divides into right and left pathways to provide electrical stimulation to the right and left ventricles.

This electrical activity of the heart is measured by an electrocardiogram. By placing electrodes at specific locations on the body (chest, arms, and legs), a graphic representation, or tracing, of the electrical activity can be obtained. Changes in an ECG from the normal tracing may indicate one or more of several heart-related conditions.

Understanding ECG tracings

Almost everyone knows what a basic ECG tracing looks like. But what does it mean?

The first short upward notch of the ECG tracing is called the "P wave." The P wave indicates that the atria (the two upper chambers of the heart) are contracting to pump out blood.

The next part of the tracing is a short downward section connected to a tall upward section. This next part is called the "QRS complex." This part indicates that the ventricles (the two lower chambers of the heart) are contracting to pump out blood.

The PR interval corresponds to the time measured from the beginning of the P wave to the beginning of the R (or Q) wave. The PR interval indicates the amount of time the electrical impulses take to reach the ventricles from the sinus node. 

The next short upward segment is called the "ST segment." The ST segment indicates the amount of time from the end of the contraction of the ventricles to the beginning of the rest period before the ventricles begin to contract for the next beat.

The next upward curve is called the "T wave." The T wave indicates the resting period of the ventricles.

When the doctor views an ECG, he or she studies the size and length of each part of the ECG. Variations in size and length of the different parts of the tracing may be significant. The tracing for each lead of a 12-lead ECG will look different, but will have the same basic components as described above. Each lead of the 12-lead ECG is "looking" at a specific part of the heart, so variations in a lead may indicate a problem with the part of the heart associated with a particular lead.

Reasons for the procedure

Some reasons for your doctor to request an ECG include, but are not limited to, the following:

  • To determine the cause of chest pain

  • To evaluate other signs and symptoms which may be heart-related, such as fatigue, shortness of breath, dizziness, or fainting

  • To identify irregular heartbeats

  • To determine the status of the heart prior to procedures such as surgery and/or after treatment for conditions such as a heart attack (myocardial infarction, or MI), endocarditis (inflammation or infection of one or more of the heart valves), or after procedures such as heart surgery or cardiac catheterization

  • To assess the function of an implanted pacemaker

  • To determine the effectiveness of certain heart medications

  • To obtain a baseline tracing of the heart's function during a physical examination that may be used as a comparison with future ECGs, to determine if any changes have occurred

There may be other reasons for your doctor to recommend an ECG.

Risks of the procedure

An ECG is a quick, noninvasive method of assessing the heart’s function. Risks associated with ECG are minimal and rare.

Prolonged application of the adhesive electrode patches may cause tissue breakdown or skin irritation at the application site.

There may be other risks depending on your specific medical condition. Be sure to discuss any concerns with your doctor prior to the procedure.

Certain factors or conditions may interfere with or affect the results of the test. These include, but are not limited to, the following:

  • Obesity, pregnancy, or ascites (accumulation of fluid in the abdomen)

  • Anatomical considerations, such as the size of the chest and the location of the heart within the chest

  • Movement during the procedure

  • Exercise or smoking prior to the procedure

  • Certain medications

  • Electrolyte abnormalities, such as too much or too little potassium, magnesium, and/or calcium in the blood

Before the procedure:

  • Your doctor or the technician will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.

  • Generally, fasting is not required before the test.

  • Notify your doctor of all medications (prescribed and over-the-counter) and herbal supplements that you are taking.

  • Notify your doctor if you have a pacemaker.

  • Based on your medical condition, your doctor may request other specific preparation.

During the procedure

An ECG may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your doctor’s practices.

Generally, an ECG follows this process:

  1. You will be asked to remove any jewelry or other objects that may interfere with the procedure.

  2. You will be asked to remove clothing from the waist up. The technician will ensure your privacy by covering you with a sheet or gown and exposing only the necessary skin.

  3. You will lie flat on a table or bed for the procedure. It will be important for you to lie still and not talk during the procedure, so as not to interfere with the tracing.

  4. If your chest, arms, or legs are very hairy, the technician may shave or clip small patches of hair, as needed, so that the electrodes will stick closely to the skin.

  5. Electrodes will be attached to your chest, arms, and legs.

  6. The lead wires will be attached to the skin electrodes.

  7. Once the leads are attached, the technician may key in identifying information about you into the machine's computer.

  8. The ECG will be started. It will take only a short time for the tracing to be completed.

  9. Once the tracing is completed, the technician will disconnect the leads and remove the skin electrodes.

After the procedure

You should be able to resume your normal diet and activities, unless your doctor instructs you differently.

Generally, there is no special care following an ECG.

Notify your doctor if you develop any signs or symptoms you had prior to the test (for example, chest pain, shortness of breath, dizziness, or fainting).

Your doctor may give you additional or alternate instructions after the procedure, depending on your particular situation.

Online resources

The content provided here is for informational purposes only, and was not designed to diagnose or treat a health problem or disease, or replace the professional medical advice you receive from your doctor. Please consult your health care provider with any questions or concerns you may have regarding your condition.

This page contains links to other websites with information about this procedure and related health conditions. We hope you find these sites helpful, but please remember we do not control or endorse the information presented on these websites, nor do these sites endorse the information contained here.

American College of Cardiology

American Heart Association

Heart Rhythm Society

National Heart, Lung, and Blood Institute (NHLBI)

National Institutes of Health (NIH)

National Library of Medicine