Stem cells harvested from a patient's own heart can be used to help repair muscle damaged during a heart attack, according to a preliminary study published online Monday in The Lancet. While it's too soon to know if the technique will help patients live longer, the study is the second small, promising study of cardiac stem cells in three months.

The new study involved 25 patients who had suffered very serious heart attacks; 24% of their heart's major pumping chamber had been replaced by scar tissue. One year later, doctors saw no improvement in those randomly assigned to get standard care. Among the 17 given stem cells, however, "we reversed about half the injury to the heart," said study author Eduardo Marban, director of the Cedars-Sinai Heart Institute in Los Angeles, in an e-mail. "We dissolved scar and replaced it with living heart muscle."

Warren Sherman, director of stem cell research and regenerative medicine at Columbia University Medical Center in New York, says the study was an important proof of the potential of stem cells - harvested from patients, grown in the lab, then injected back into patients' hearts.

Doctors don't yet know exactly how the stem cells reduce the size of the dead zone of scar tissue, says Kenneth Margulies, director of heart failure and transplant research at the University of Pennsylvania. And while the shrinking suggests that the stem cells are replacing dead cells with living ones, doctors can't definitely prove that without doing a biopsy of the actual cells, he says.

The new study's encouraging results seem to confirm the findings of another small study of heart stem cells, published in The Lancet in November, which also showed an improvement in heart-attack survivors who received the treatment, Margulies says. On the other hand, a third study, found no benefit from stem cells created from patients' own bone marrow.

Four stem-cell patients developed serious complications, compared to only one of the other patients, the study says. That suggests stem-cell therapy has a "satisfactory" safety record, but "is not risk-free," Margulies says.

The idea of regenerating heart tissue "was a pretty far-out idea" only 10 to 20 years ago, Margulies says. There's some evidence that heart tissue is capable of making some small repairs on its own, although not enough to help people who've had a heart attack.

Marban developed the process of growing heart stem cells while working at Johns Hopkins University, which has filed an application for a patent on the idea and licensed it to a company in which Marban has a financial interest. No money from that company was used to pay for the study, which was funded by Cedars-Sinai and the National Institutes of Health.

About 1.3 million Americans have a heart attack each year.