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The ABCs of ECGs

In an age of high-tech medical tools that make looking inside a body as easy as looking through a window, it’s easy to forget tried-and-true medical tests. Here are answers to a few questions you may have about one of those classic tests—the electrocardiogram.

What does an electrocardiogram (ECG) measure?

The heart’s “natural pacemaker,” the sinoatrial node, triggers the electrical impulses that cause the heart’s contractions. An electrocardiogram is simply a recording of the patterns made by those electrical signals.

Why does the doctor attach so many wires?

The wires, or “leads,” are attached to small adhesive-backed clips: There’s a clip for each arm and one for each leg, with more placed across the chest. The leads, which do not themselves transmit electricity, are sensors that pick up and record the electrical activity of the heart on a printout.

What kinds of patterns appear on the ECG?

Each wave on the printout of an ECG is broken into segments designated by the letters P, Q, R, S and T. The P segment shows the electrical activity at the beginning of a heartbeat—when the sinoatrial node first fires, prompting the atria to contract. The atrial contraction then triggers the ventricles—the lower chambers of the heart—to contract, which is represented by the QRS segment. The T segment shows the relaxation phase that signals the end of one heartbeat.

What kinds of ECG results might point to problems?

A flattening of the Q waves may indicate a heart attack, while flattening of the ST segment may indicate coronary narrowing.

Who should get an ECG—and when?

In this country, the ECG is used as a general screening tool for just about every adult over the age of 40, and it is always administered whenever heart disease is suspected. Most doctors prefer to do the test at least once by age 40 to serve as a baseline reading.

Is there any pain involved?

Other than the need to lie quite still for a minute or two, there is little discomfort and no pain.

Is an ECG always done in a doctor’s office or hospital?

Sometimes a portable device called a Holter monitor is used to monitor heart rhythms over a 24-hour period. This gives a doctor the opportunity to compare electrical impulses during activities such as eating and walking, as well as during periods of minor stress—like getting caught in traffic.