Doctors have discovered a potential problem involving implanted heart valves that hundreds of thousands of people have received. The valves do not always open and close properly, possibly because a blood clot has formed that could raise the risk of stroke.
Experts stressed that not enough is known about the situation to change practice now, and officials from the Food and Drug Administration say the valves still seem safe and worth the risk.
The problem involves aortic valves made from cow, pig or human tissue rather than mechanical ones made from synthetic materials. A study published Monday by the New England Journal of Medicine suggests that these valves may not work right in up to 15 percent of cases.
The FDA urged doctors and patients to report any suspected problems.