Patients who got infusions of their own bone marrow stem cells after a heart attack experienced measurably better heart function, U.S. researchers reported on Monday.

The more cells the patients got, the better they did, privately held biotech company Amorcyte told a meeting of the American College of Cardiology.

"These results show that treatment with a patient's own bone marrow stem cells has the potential to reduce long-term complications after a heart attack," Dr. Arshed Quyyumi of Emory University in Atlanta and a paid consultant to Amorcyte said in a statement.

"We are encouraged by these results and are planning to conduct a more extensive study."

For years doctors have tried to use various forms of stem cells to treat damaged hearts, with varying results. Quyyumi's team focused on finding a dose-related effect, which would suggest the cells were in fact taking up residence in the heart and helping to heal it.

In the study, they used bone marrow stem cells that had started to become endothelial cells, which line blood vessels. Amorcyte calls this product AMR-001.

They treated 31 patients with angioplasty and stents to stretch and clear out a blocked coronary artery. Sixteen then had varying numbers of bone marrow cells infused into the affected artery.

Patients given higher doses of cells had greater improvement in blood flow within the heart than those patients treated with lower doses or those given drugs alone.

An estimated 1 million people have a heart attack in the United States each year, and about 20 percent of them are severe enough to damage the ventricles, the pumping chambers, of the heart, Amorcyte said.

"Having successfully confirmed a dose effect, Amorcyte looks forward to proceeding with a larger prospective randomized trial and furthering our clinical program with AMR-001," said Dr. Andrew Pecora, chairman of the Amorcyte board.

Earlier at the meeting, Dr. Douglas Losordo of Northwestern University in Chicago and his colleagues showed that patients with severe chest pain got some relief up to six months later after getting injections of their own bone marrow stem cells.

The patients were all on drug regimens but were not eligible for angioplasty, stents, or coronary artery bypass surgery.

Losordo's team used Baxter International's device for pulling out the desired stem cells from blood.

Patients who got the cells could walk a little longer without pain and had fewer episodes of chest pain than patients given saline alone, the researchers told the conference.