Pioneering research to test if stem cells can repair heart muscle damage by a heart attack is beginning in the UK.
Blood clots are a common underlying cause of heart attacks
Sixty patients have volunteered to take part and will be injected with their own bone marrow stem cells during routine coronary bypass surgery.
The Bristol surgeons say this should reduce scarring in the heart, thereby improving its pumping action and guard against future heart complications.
Similar trials involving 100 patients are taking place in London.
In a heart attack, blocked blood vessels starve the heart's muscle of oxygen and cells in that part of the heart die, leaving scarring.
The scarred heart is then less able to pump blood around the body and can lead to heart failure.
While the blood supply to the heart can be improved with coronary bypass surgery or angioplasty (mechanical stretching of the heart blood vessels), these techniques do not restore the viability and function of the area already damaged.
And scarring can cause further complications - a fifth of patients develop a dangerous thinning of the walls of the heart within six months of bypass surgery.
Mr Raimondo Ascione and colleagues at the University of Bristol and the Bristol Heart Institute will begin the trial in August among patients who have had a heart attack within the last 10 days to three months - the ideal window for intervention, say the researchers.
Half of the 60 volunteers will receive heart muscle injections of their own stem cells, harvested from bone marrow, alongside their surgery, while the other half will have surgery and a dummy injection.
Neither the patients or the surgeons will be privy to this information, to make for an unbiased comparison over the following six months.
Mr Ascione, consultant cardiac surgeon, said: "We should know within six months of the surgery whether the stem-cell treatment has made a difference.
"If this trial is successful it could be a major breakthrough with major clinical implications. It could allow heart attack patients to have a good quality of life."
He said that using the patients' own stem cells should get round the risk of rejection and infection.
"It also gets around the ethical issues that would result from use of stem cells from embryonic or foetal tissue," he said.
Stem cells have the potential to turn into many different types of cell.
Professor Jeremy Pearson, of the British Heart Foundation, said: "We hope that this exciting Bristol project will provide information taking us a step nearer to the day when stem cells can be used routinely to help repair damaged hearts."
The British Heart Foundation has given a grant of £210,000 to the work.
Heart attacks are responsible for one in four deaths in men and one in six deaths in women.