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Researchers reviewed data from more than 390,000 heart attack patients who underwent ECG, a diagnostic procedure that measures the heart's electrical activity. One group of patients, representing about 8 percent of the total, had a normal result on the initial test. About 35 percent had ECGs with a ''nonspecific'' result, suggesting that minimal heart damage had occurred but could not be specifically diagnosed. Fifty percent of the subjects had results that clearly indicated they had suffered a heart attack.

While patients whose ECGs were normal had a 41 percent lower risk of dying in the hospital compared with patients with abnormal results, they remained somewhat at risk. For instance, these patients had an almost 6 percent chance of dying during their hospital stay and a 19 percent risk of dying overall or developing a serious medical complication such as pulmonary edema, in which the lungs fill with fluid, or a dangerously rapid heartbeat.

In comparison, patients with nonspecific results had a 9 percent risk of dying in the hospital and a nearly 28 percent risk of dying or suffering another life-threatening event. Patients whose tests diagnosed an acute heart attack had an 11 percent risk of dying in the hospital and a 35 percent chance of developing a dangerous complication.

The unexpected finding of this study was that patients with an initially normal ECG had a substantial mortality rate according to Dr. Robert D. Welch from Wayne State University in Detroit, Michigan, and colleagues report in the October 24/31 issue of The Journal of the American Medical Association.

In an interview with Reuters Health, Welch said that while the findings suggest that some patients should receive additional tests such as repeat ECGs and blood tests, they do not negate the value of ECG as a diagnostic tool.

The researchers concluded that a normal ECG in patients who have suffered a confirmed heart attack does not necessarily confer a favorable prognosis, although these patients do fare better than those with ECGs that clearly indicate a heart attack, according to The Journal of the American Medical Association (2001;286:1977).