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Thousands of patients who had potentially defective heart valves implanted in the 1970s and 1980s can now undergo X-ray exams that will identify many - but not all - of the valves that are about to fail, suggests a study of 315 valve recipients.

The Bjork-Shiley convexo-concave metal valves were implanted in about 86,000 patients worldwide between 1979 and 1986, and because of a manufacturing problem, they have had a high rate of mechanical failure, which is often fatal.Patients and their doctors have faced an excruciating dilemma: The risk of valve fracture is between 0.5 percent and 1 percent a year, but removing and replacing the valve with open-heart surgery carries a death rate of 3 to 5 percent. Thus far, 564 valves have failed, causing death two-thirds of the time.

In this week's issue of the New England Journal of Medicine, researchers from William Beaumont Hospital in Royal Oak, Mich., report that with the aid of high-resolution X-ray images taken from two different angles, specialists spotted 11 valves in the 315 patients that looked as if they were about to fail. All 11 patients underwent operations: In 10 patients, the valves had cracked and were replaced, though one patient died during surgery. The 11th valve was found intact.

- Richard Saltus