Tiny wire tubes used to prop open heart arteries after doctors clean out deposits of fat often become jammed with blood clots that close the arteries again, scientists reported Thursday.

The tubes, called stents, were first used experimentally five years ago by doctors seeking a way to keep coronary arteries open after they have been unblocked.Doctors routinely insert balloons into the arteries and inflate them to squeeze open blockages in a process called angioplasty. But once the balloons are removed, 25 percent to 45 percent of the arteries become blocked again.

A report in Thursday's New England Journal of Medicine said that in 105 cases where stents were used in arteries, 23 percent became blocked with blood clots within six months. In 8 percent of the cases, patients died within a year.

"The place of this form of treatment for coronary artery disease remains to be determined," wrote Dr. Patrick W. Serruys and others from Erasmus University in Rotterdam.

Researchers are at work on new kinds of stents they hope will keep arteries open without attracting blood clots.