NEW YORK (Reuters Health) - About one fifth of heart attacks among middle-aged and older adults may go unrecognized, according to new estimates from a US study.

But while that number may sound concerning, it marks an improvement over estimates from earlier studies, according to a report in a recent issue of the American Journal of Cardiology.

It is possible, the study authors explain, that greater public awareness of heart attack symptoms and more-sensitive cardiac tests have led to fewer unrecognized heart attacks in recent years.

Although most heart attacks involve chest pain or discomfort, some occur without these symptoms. In addition, milder chest discomfort and other heart attack symptoms such as dizziness or shortness of breath may not readily signal a heart attack to the patient, or even the doctor.

Previous research in the US and elsewhere has suggested that anywhere from 20% to 40% of heart attacks may go undetected. But these estimates are based on patients who had heart attacks prior to the late 1980s, according to the authors of the new report, led by Lori L. Boland of the University of Minnesota in Minneapolis.

Boland's team analyzed data on more than 12,800 men and women who had participated in a national study of cardiovascular health between 1987 and 1998. Participants were between the ages of 45 and 65, and free of coronary artery disease at the study's start. They were followed for 10 years, on average.

The researchers found that, based on periodic electrocardiograms taken as part of the study, 20% of the 500 heart attacks participants suffered went unrecognized. African Americans had a slightly higher percentage of undetected heart attacks than whites did--23%, compared with 19%.

Despite the commonness of undetected heart attacks among these patients, Boland and her colleagues point out that the percentage is "slightly lower than previous estimates from other populations."

Although they don't know for sure, the authors note that it's "plausible" that in the 1990s, Americans became more likely to seek medical care for possible heart attack symptoms. In addition, they write, recent advances in detection, such as cardiac enzyme tests, may have allowed doctors to spot more cases of mild heart attack.

Warning signs of a heart attack include chest discomfort--which in some cases feels more like pressure or fullness than acute pain--pain in other parts of the upper body, shortness of breath and light-headedness. Experts advise that anyone who thinks he or she might be having a heart attack call 911 immediately.

SOURCE: American Journal of Cardiology 2002;90:927-931.

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