Infusing stem cells into the arteries of heart attack patients can heal damaging scars, according to new research, a feat previously thought impossible.

Stem cells - cells that form different tissue of in the body - helped half of tested heart attack patients recover from their scars over a six-month period, according to the study. The control group did not see any additional recovery in their hearts.

The researchers recommended the experimental therapy expand into clinical trials beyond the 17 patients who received the original treatment.

This has never been accomplished before, despite a decade of cell therapy trials for patients with heart attacks. Now we have done it, Eduardo Marban, director of the Cedars-Sinai Heart Institute and one of the study's co-authors, said in a statement. The effects are substantial.

The research included a group of 25 patients who had suffered from heart attacks caused by a blockage in an artery. The online version of The Lancet published the research on Valentine's Day.

The year-long study tested the effects of the stem cells on 17 patients, compared to eight control patients who received standard treatments of medication along with diet and exercise recommendations.

For each test patient, clinicians created a stock of stem cells from a heart sample smaller than a raisin. The researchers then injected the stem cells back into an artery damaged from the heart attack.

The authors reported no deaths or major side effects in either group. However, four patients in the stem cell group showed adverse reactions to the treatment whereas only one control showed complications. Adverse reactions included problems that required implantation of a defibrillator, according to the study.

These results signal an approaching paradigm shift in the care of heart attack patients, Shlomo Melmed, dean of the Cedars-Sinai Heart Institute and a study co-author, said in a statement. In the past, all we could do was to try to minimize heart damage by promptly opening up an occluded artery. Now, this study shows there is a regenerative therapy that may actually reverse the damage caused by a heart attack.

Other doctors expressed cautious optimism based on the results of the trial therapy.

By preventing the consequences of a heart attack you may be able to prevent further down the heart failure that happens in [many of these] patients, Dr. Sonia Skarlatos, deputy director of the division of cardiovascular sciences at the NIH's National Heart, Lung, and Blood Institute, told CNN.

The researchers from the Cedars-Sinai Heart Institute initially set out to determine if the use of stem cells in heart attack patients was safe, and said they were surprised and excited to see the reduction in heart scarring and increase in healthy muscle tissue.

Marbán said the study will revolutionize how heart attacks are treated. This discovery challenges the conventional wisdom that, once established, scar is permanent and that, once lost, healthy heart muscle cannot be restored.