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This article originally appeared in the March 2008 Harvard Heart Letter and is provided courtesy of Harvard Health Publications.
Electrocardiogram: Visualizing the heart’s electrical signature
A recording of the heart’s electrical activity offers clues to its health and problems.
In 1887, British physiologist Augustus Desiré Waller used a device containing mercury and sulfur placed on the skin of the chest to record the heart’s electrical activity. More than a century later, the descendants of that contraption provide one of the most commonly used tests for heart disease — the electrocardiogram. The squiggle of lines it records yields valuable information about the health of the heart or problems in and around it.
What’s the procedure?
Having an electrocardiogram, sometimes called an ECG or EKG (after the German spelling), is simple, quick, and painless. You lie on a bed or table with part or all of your chest exposed. A doctor, nurse, or technician places small pads, called electrodes, on your chest, arms, and legs and then attaches wires from the ECG machine to each electrode. The electrodes capture the electrical signature of your heart as you relax, breathe normally, and avoid talking or moving. These signals are melded into a series of tracings on graph paper or a computer screen.
What it shows
An electrocardiogram shows a complete heartbeat as a tracing with three parts (see “Tracing a heartbeat”): a P wave, a QRS complex, and a T wave.
Doctors analyze these three elements and their beat-to-beat patterns for signs of trouble. They also look for subtle differences in the shape, height, or duration of different parts of the heart tracing. For example, an ST segment that fails to drop back to the baseline but hovers high over it is a key indicator of a serious heart attack.
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Special cases
Heart symptoms often come and go. Some people have chest pain only when they exercise or are under stress; others have palpitations every now and then. Variations on the standard ECG are used to identify such evanescent heart problems.
Stress test. Some cardiovascular problems are easier to diagnose when the heart is working harder than usual, either naturally with exercise or with the help of a medicine that makes the heart beat faster and harder. By comparing the electrocardiogram before, during, and after the stress, a doctor can detect narrowed coronary arteries or exercise-induced rhythm problems.
Holter monitor. Capturing erratic heartbeats during a two-minute electrocardiogram is like finding the proverbial needle in a haystack. Doctors sometimes suggest wearing a Holter monitor. This pager-sized device is essentially a portable ECG machine you wear on your belt. It records the heart’s electrical activity for 24 hours via electrodes placed on the chest.
Event recorder. These long-term recorders can be worn for days, weeks, and longer. Like Holter monitors, they continuously record the electrical activity of every heartbeat. But instead of storing all this information, the device keeps only the latest hour or so. When the wearer finally experiences symptoms such as fainting, dizziness, or light-headedness, he or she presses a button that freezes and stores the previous few minutes of the electrocardiogram so a physician can look for an arrhythmia. A newer version that is implanted under the skin can be used for up to three years
