Injecting stemcells into the heartmuscle carries the risk of arrhythmia, said Techung Lee, associate professor of biochemistry at the State University New York at Buffalo.
About four months after each patient's original operation, when their hearts had stabilised, Dr Bolli used a catheter to deliver 1m of the newly bred stemcells to their damaged heartmuscle.
They found that the stemcells produced growth factors that traveled to the heart, in addition to stimulating the muscle itself to make growth factors that also improved cardiac function.
The long-term hope is to treat patients with some sort of stem cell that could turn into beating heartmuscle and replace dead cardiac tissue with fully functioning new cells.