Thousands of people nationwide have experienced malfunctions in implanted devices that help the heart maintain a normal rhythm, problems that sometimes have resulted in death.

While the rate of pacemaker replacements due to malfunction has significantly declined, the replacement rate for implantable cardioverter-defibrillators, or ICDs, has been higher and more variable, said Dr. William Maisel, an assistant professor at the Harvard Medical School and a consultant for the U.S. Food and Drug Administration. But ICD malfunctions have declined in recent years, he added.

The findings were presented in two studies published today in the Journal of the American Medical Association. Some of the findings were presented earlier to a group of heart specialists.

The Heart Rhythm Society, a group of medical experts, has been working to develop guidelines for reporting device malfunctions with input from the FDA, manufacturers and patient advocates, said Dr. Anne Curtis, the group's president and chief of cardiology at the University of South Florida. A draft of the proposed guidelines was to be made public today.

Doctors said the studies provide important information about pacemakers, which prompt hearts that may beat too slowly to maintain a normal rhythm, and ICDs, which deliver a correcting shock to hearts that beat too fast.

While manufacturers publish product-performance reports, the reports typically have not contained comprehensive information about the number of device malfunctions or the rate or reasons for malfunction, Dr. Maisel and other researchers said.

The FDA agrees "it has been difficult to get a clear picture of device reliability," said Megan Moynahan, a study co-author and FDA branch chief, adding the findings provide more detailed information about recent trends.

The issue of device performance was highlighted last year when some companies, including Indianapolis-based Guidant Corp., recalled some models. The New York Times said that Guidant sold a defibrillator model that it knew had a small risk of malfunctioning, but didn't warn about it for three years.

In response, scores of patients at the University of Pittsburgh Medical Center had their devices changed, said Dr. Samir Saba, director of the UPMC cardiac electrophysiology laboratory.

But he noted that even without malfunctions or the threat of problems, patients need to have the devices removed when the batteries run low, typically every four to five years.

Dr. Saba also emphasized that the implanted devices can be lifesaving for many patients at risk of developing arrhythmias, or abnormal heart rhythms, that can quickly be fatal.

Their value was underscored earlier this month in the death of Maggie Dixon, the Army women's basketball coach and sister of Pitt men's coach Jamie Dixon. An autopsy indicated she had an enlarged heart that could have put her at risk for developing a potentially fatal arrhythmia, a problem that might have been corrected by an ICD.

In one study, Dr. Maisel and FDA researchers analyzed reports that pacemaker and ICD manufacturers were required to submit to the federal agency between 1990 and 2002.

During that period, more than 17,000 malfunctions resulted in removal and replacement with a new device, researchers found. Battery, capacitor or electrical problems accounted for half the failures. Thirty deaths were attributable to pacemaker malfunction and 31 deaths to malfunctions in ICDs.

The annual replacement rate for pacemaker malfunctions decreased during the study period, from 9 per 1,000 implants in 1993 to 1.4 in 2002. But the ICD replacement rate, after decreasing from 38.6 in 1993 to 7.9 in 1996, increased in the latter half of the study, peaking in 2001 at 36.4.

The researchers said that because ICDs are more sophisticated than pacemakers and are still evolving in technology, it is not surprising that they have a higher malfunction rate.

Dr. John R. Ward, chief of cardiology at Mercy Hospital's Heart Institute, said patients shouldn't be concerned as long as they have their devices checked regularly as their physicians direct.