DALLAS, Texas (Reuters) -- Heart attack survivors whose hearts were infused with stem cells from their own bone marrow showed nearly twice the improvement in the organ's pumping ability as patients given a placebo, according to a study presented Sunday.A further analysis of the data found that benefits to heart function seen four months after an attack appeared to be most pronounced in patients with more severe heart attacks that caused greater damage to the muscle, researchers said at the American Heart Association annual scientific meeting."The medications and interventional therapies available so far are intended only to limit further damage to the heart," said Andreas Zeiher, professor at J.W. Goethe University in Frankfurt, Germany, and a senior author of the study."In contrast, progenitor cell therapy has the potential not only to limit further damage, but to regenerate heart function," he said.Progenitor cells are immature cells that can become a variety of specialized cells."The concept that we can regrow heart muscle cells would be an extraordinary development in the treatment of heart disease," said Dr. Richard Stein, director of preventive cardiology at New York's Beth Israel Hospital, who is attending the meetings."It changes the entire game," he said of the prevailing thought that a person has a finite number of heart cells that cannot be regrown once they are destroyed.The primary goal of the 204-patient study was to show improvement in function of the left ventricle -- the pumping chamber of the heart -- which is considered a good gauge of a patient's prognosis following a heart attack.Both groups in the study had nearly identical left ventricular function going in, and both showed improvement after four months as expected, researchers said.But patients who received the bone marrow cell infusion saw an improvement in their left ventricular ejection fraction -- a measure of heart efficiency -- on average, of 5.5 percent. Those getting placebo saw a 3 percent improvement.Researchers said the compelling results stood in contrast to "widely mixed results" seen in prior smaller studies of the cell therapy.They were further encouraged by other results, including less heart enlargement seen in the bone marrow cell patients, and improved blood flow in the artery where the attack occurred, indicating the possibility that new blood vessels may have been created to nourish the damaged area.Heart enlargement, which often occurs after an attack as the heart tries compensate for reduced pumping ability, is a hallmark of a failing heart.The reduction in heart enlargement seen in the bone marrow cell patients appeared to lead to reduced incidence of new heart attacks, hospitalization due to heart failure and deaths, researchers said."The trial may be a landmark in helping to determine whether the concept of progenitor [cell] therapy will have a future for restoring heart function after" heart attacks, said lead investigator Volker Schachinger of Goethe University. Copyright 2005 Reuters. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.