Media playback is unsupported on your device Media caption Fergus Walsh explains how the patient's own cells are put back into his heart

The biggest ever stem cell trial involving heart attack patients has got under way in London. The study, which will involve 3,000 patients in 11 European countries, should show whether the treatment can cut death rates and repair damaged tissue after a heart attack.

All the patients will have standard treatment to widen their narrowed arteries, which involves inserting a small tube called a stent. In addition, half the patients will have stem cells taken from their bone marrow and injected into their heart.

This will happen within days of them suffering a heart attack.

It's fantastic to be part of this trial Neal Grainger

"It's fantastic to be part of this," said Neal Grainger, 54, from Essex, who was the first patient in the UK to be treated.

UK's biggest killer

He had an infusion of his bone marrow stem cells at the London Chest Hospital just days after his heart attack last month.

"It's strange having something taken out of you and then put back, but I hope it helps me and a lot of others."

Cardiovascular disease is the biggest killer in the UK.

During a heart attack, a fatty plaque causes a blood clot inside an artery, starving heart muscle of oxygen and leaving scar tissue.

Although more and more patients are surviving heart attacks, they can be left considerably weaker because heart muscle has been permanently damaged.

Fluid build-up on the lungs is another problem and patients are often on medication for life.

There have been dozens of smaller trials using stem cells to treat heart attack patients.

'Definitive trial'

An analysis by the Cochrane Collaboration in 2012 suggested the treatment offered "modest improvement".

Image caption The patient's stem cells ready for use

But many trials involved just small numbers of patients.

"This is the definitive trial," said Prof Anthony Mathur, director of cardiology at Barts Health NHS Trust and chief investigator for the trial.

"After 15 years of research we will now have a clear answer. We hope to show that stem-cell injections can cut the number of people dying from heart attacks by 25%.

"If it works, it would open up a whole new branch of medicine, and give heart attack patients an entirely new treatment."

It is unclear exactly how a patient's own bone marrow stem cells might help repair their heart.

Donor adult stem cells have been used successfully for decades in bone marrow transplants, but in that situation it is a like-for-like replacement.

Expecting these cells to survive in the heart and transform into specialised heart cells is a huge challenge.

'Could save NHS money'

One theory is that they release chemical signals that enhance the activity of the heart's own stem cells.

University College Hospital in central London and King's College Hospital in south London are the two other centres in Britain taking part.

The trial includes hospitals in other major European cities such as Paris, Frankfurt, Barcelona, Rome and Copenhagen.

John Martin, professor of cardiovascular medicine at University College London and adjunct professor of medicine at Yale, said: "This trial does not have the backing of the pharmaceutical industry as there is no money in it for them. You can't patent a patient's own cells.

"So not only could this treatment save lives it could also save the NHS money."

The study, known as the BAMI (bone acute myocardial infarction), has received nearly £5m from the European Commission.

The results will be announced in five years.