From Richard N. Fogoros, M.D.,
Your Guide to Heart Disease.
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The ECG is the most commonly performed cardiac test. This is because the ECG is a useful screening tool for a variety of cardiac abnormalities; ECG machines are readily available in most medical facilities; and the test is simple to perform, risk-free and inexpensive.

How is the ECG performed?

The patient lies on an examination table, and 10 electrodes (or leads) are attached to the patient’s arms, legs, and chest. The electrodes detect the electrical impulses generated by the heart, and transmit them to the ECG machine. The ECG machine produces a graph (the ECG tracing) of those cardiac electrical impulses. The electrodes are then removed. The test takes less than 5 minutes to perform.

What does the ECG tracing look like?

Click here for an example of an ECG, and for an explanation of what those squiggly lines mean.

What information can be gained from the ECG?

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

• the heart rate
• the heart rhythm
• whether there are “conduction abnormalities” (abnormalities in how the electrical impulse spreads across the heart)
• whether there has been a prior heart attack
• whether there may be coronary artery disease
• whether the heart muscle has become abnormally thickened

All of these features are potentially important. If the ECG indicates a heart attack or possible coronary artery disease, further testing is often done to completely define the nature of the problem and decide on the optimal therapy. (These tests often include a stress test and/or cardiac catheterization.) If the heart muscle is thickened, an echocardiogram is often ordered to look for possible valvular heart disease or other structural abnormalities. Conduction abnormalities may be a clue to the diagnosis of syncope (fainting), or may indicate underlying cardiac disease.

What are the limitations of the ECG?

• The ECG reveals the heart rate and rhythm only during the time that the ECG is taken. If intermittent cardiac rhythm abnormalities are present, the ECG is likely to miss them. Ambulatory monitoring is needed to record transient arrhythmias.
• The ECG can often be normal or nearly normal in patients with undiagnosed coronary artery disease 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).