Nearly 80 percent of heart attacks in the United States go undiagnosed, according to the first nationwide study of its kind.

These “silent” heart attacks, often mistaken for indigestion or muscle pain, might be too small to draw attention, but they are much more common than cardiologists had previously thought, the study, released Sunday, found. And since they put people at greater risk of additional heart attacks later on — possibly big ones — experts say patients who feel any chest pain should seek medical care, even if their symptoms only seem like heartburn.

Heart attacks happen when arteries get blocked by plaques of cholesterol, making it impossible for oxygen-filled blood to nourish the heart. Without air, a part of the heart muscle dies, and scar tissue forms.

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Even the smallest heart attack leaves behind a scar, which is more fibrous than living muscle and won’t contract along with the rest of the organ. That makes the heart’s job harder and increases the risk of heart failure.

In the past, researchers have looked for signs of silent heart attacks using a test known as an electrocardiogram, which detects the electrical current that keeps the heart pumping. When scar tissue forms after a heart attack, the electrical signal changes, and doctors can see that abnormality on the ECG. But sometimes, with smaller scars, this test just isn’t precise enough to pick up on the difference.

Now, by turning to a newer scanning technology called magnetic resonance imaging, or MRI, researchers have shown just how much they had been undercounting silent heart attacks.

“It’s almost like having looked at a petri dish first with a magnifying glass, and then having the availability of a microscope. All of a sudden you can see things that you couldn’t see before,” said Dr. Richard Chazal, a cardiologist at the Lee Memorial Health System in Fort Myers, Fla., and president-elect of the American College of Cardiology, who was not involved in the study.

Even the smallest heart attack leaves behind a scar.

A team led by Dr. David Bluemke, director of radiology and imaging sciences at the National Institutes of Health Clinical Center in Bethesda, Md., studied older Americans, aged 45 through 84, from six US communities. Over the course of 10 years, the researchers examined any participant who complained of heart problems with clinical tests and ECGs. Then, in the 10th year, they administered cardiac MRIs to everyone to look for undetected attacks.

Of the 1,840 participants, 146 had experienced heart attacks — but most of these had been missed by doctors and patients alike. MRIs were needed to spot nearly 4 out of 5 of the heart attacks.

Compare these findings with data from 2002, and the difference is striking. When researchers were looking for silent heart attacks using ECGs, their findings showed that only 20 percent of heart attacks had gone undiagnosed.

The reason MRI is so much more sensitive is that it picks up changes in the architecture of the heart’s tissues. Before you go into an MRI machine, you are injected with a metallic liquid. That liquid pools differently in scarred heart tissue than it does in healthy heart tissue, which often makes the scar appear as a bright patch to the physician.

But don’t expect to get routine MRIs the way you might with colonoscopies or mammograms. “MRIs are costly, so we don’t anticipate screening people who are asymptomatic,” said Bluemke. “But we do know that it’s increasingly useful when there are unanswered questions about the dysfunction of the heart.”

Bluemke and his colleagues published their findings in the Journal of the American Medical Association.

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