Considered the first diagnostic tool for heart disease, an electrocardiogram (ECG) assesses the heart's electrical activity using electrodes that are attached to the skin of the chest, arms, and legs. As recorded by the ECG, the heart's electrical activity is represented by a series of waves. The shape and size of the waves, time in between the waves, and rate and regularity of the waves provide information about the heart's rhythm, and can indicate one or more of several heart-related conditions, such as arrhythmias, insufficient blood flow to the heart, damage to the heart due to infarct (heart attack), valve disorders, an enlarged heart, and others.

To perform an ECG, a technician, nurse, or physician will place 12 separate electrodes (small plastic patches) at specific locations on your chest, arms, and legs, while you are lying down. The electrode patches will be connected to leads (wires) that are connected to the ECG device. You will need to lie very still and not talk during the ECG procedure, as movement or talking may interfere with the tracing. The technician, nurse, or physician will start the recording, which will take approximately one minute. You will not feel anything during the recording.

Other ECG Procedures

There are additional ECG procedures that are more involved than the standard ECG. These procedures include the following:

• Exercise ECG, or stress test. In this test, the patient is walking on a treadmill or pedaling a stationary bicycle while the ECG is recorded. This test is done to assess changes in the ECG during physical activity, such as exercise, and to evaluate patients for coronary artery disease or arrhythmias.
  
  
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• Signal-averaged ECG. This procedure is performed in the same manner as a standard ECG, except that the heart's electrical activity is recorded over a longer period of time, usually 15 to 20 minutes. A computer processes the information so that slight abnormalities that would not be seen in a standard ECG might be detected. Signal-averaged ECG is used to detect conditions that pose a risk for arrhythmias.
• Holter Monitor. A Holter monitor is a small, portable electrocardiograph machine that can record the heart's rhythm over a longer period of time. Three small electrodes are attached to the patient's chest and connected to a small portable ECG recorder by wires. The patient goes about his/her usual daily activities during this procedure – except for activities such as taking a shower, swimming, or any activity causing an excessive amount of sweating which would cause the electrodes to become loose or fall off.

  Holter monitoring may be used when arrhythmia is suspected but not seen on a standard or signal-average ECG, since arrhythmias may be transient in nature and not seen during the shorter recording times of the standard ECG.

  
  
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• Event/loop recording. With these methods the ECG is recorded and the information saved only when the patient feels symptoms and activates the recording. Especially useful in patients whose symptoms occur infrequently, event monitoring uses a Holter monitor-type device that the patient can wear for weeks and is activated by the patient only when symptoms are felt.
  
  

Why is an ECG performed?

Because the ECG is a fast, simple, painless, and relatively inexpensive test, it may be used as a part of an initial examination to help the physician narrow the scope of the diagnostic process. ECGs are also done with routine physical examinations so that comparisons can be made with previous ECGs to determine if a hidden or undetected condition might be causing changes in the ECG. Some conditions that may cause changes in the ECG pattern can include, but are not limited to, the following:

• Ischemia – which is decreased flow of oxygenated blood to an organ due to obstruction in an artery.
• Heart attack – also called myocardial infarction – which is caused by damage to the heart muscle due to insufficient blood supply.
• Conduction disorders – a dysfunction in the heart's electrical conduction system, which can make the heartbeat too fast, too slow, or at an uneven rate.
• Electrolyte disturbances – an imbalance in the level of electrolytes, or chemicals, in the blood, such as potassium, magnesium, or calcium.
• Pericarditis – an inflammation of the sac (thin covering) that surrounds the heart.
• Valvular heart disease – which occurs when one or more of the heart's four valves becomes defective, or may be congenitally malformed.
• Enlarged heart – a condition in which the heart is abnormally large. This condition can be caused by various factors, such as valve disorders, high blood pressure, congestive heart failure, myocardial infarction, etc.
• Chest trauma – blunt trauma to the chest, such as a motorist hitting the steering wheel in an automobile accident.

An ECG may also be done for the following reasons:

• to obtain a baseline tracing of the heart's function (during a physical examination). This baseline tracing may be used later as a comparison with future ECG's, to see if any changes have occurred.
• as part of an evaluation prior to a procedure (such as surgery) to make sure a heart condition does not exist that might cause complications during or after the procedure
• to check the function of an implanted pacemaker
• to check the effectiveness of certain heart medications
• to check the heart's status after a myocardial infarction, or after a heart-related procedure such as a cardiac catheterization, heart surgery, electrophysiological studies, etc.