A guide to understanding electrocardiograms

When you think of electricity, you probably think of the juice flowing from wall outlets that powers the TV. But a similar kind of electrical activity allows the heart to pump blood. An electrocardiogram (ECG) checks to see whether the heart's wiring is up to code by measuring the rhythm of its contractions. Here's what you need to know about it.

An ECG is an indispensable tool — if you have reason to have the test. But many people don't.

• The ECG, which records heart activity through electrodes that are attached to the skin, helps diagnose heart disease in patients who have chest pain or other symptoms.
• Some doctors, however, give all patients ECGs as part of routine physical exams starting at age 50, 40 or even earlier. Yet no major medical organization recommends routine ECGs for healthy people with a normal risk of heart attack.
• ECGs may be worthwhile in men over 45 and women over 55 who have a family history of cardiovascular disease or other factors that increase the risk of a heart attack, are about to start rigorous exercise plans or have jobs that affect public safety (such as airline pilots). But if you don't fall into one of these categories, you probably don't need an ECG.
• The test can identify some people at risk for heart disease, but it misses many others and causes a large number of false alarms.

An ECG can spot abnormalities such as arrhythmias, which may someday cause serious heart problems. It can also detect evidence that a person already has heart disease, a diagnosis that is typically confirmed with more tests.

• Unfortunately, there is little evidence that an ECG is a reliable method for spotting potential heart trouble in people who have no symptoms or major risk factors. False-positive results are common when ECGs are used as a routine screening test for heart disease.
• One study found that 71 percent of people who had abnormal test results on screening ECGs turned out not to have heart disease. A false-positive ECG could result in needless additional procedures, such as coronary angiography, which requires having a catheter inserted into a blood vessel.
• What's more, a resting ECG that turns up no signs of trouble is no cause for celebration; about 50 percent of people with heart disease have normal ECG results.
• Stress tests — ECGs conducted during vigorous exercise — are more accurate but still predict whether a person will have a heart attack only 40 to 62 percent of the time.

Among the countless differences between the sexes, here's one you may not know about: a woman's heartbeat "looks" different from a man's on an ECG.

• For instance, on average, a woman's heart beats slightly faster than a man's does. And certain phases of the female heartbeat last longer; they are depicted by greater intervals between the jagged peaks and valleys on an ECG.
• Many studies, dating back to 1920, have confirmed these sex-related differences in ECGs, yet doctors typically evaluate an ECG using criteria based on studies of men.
• As a result, ECGs may produce false-positive results in women 25 to 50 percent of the time. Fortunately, recent studies are providing doctors with new and better criteria for judging women's ECG results.

Despite the false-positive results, an electrocardiogram is an excellent way to spot heart problems early on. Keep this guide in mind and contact your doctor for more information.