Health reporter, BBC News
A heart attack deprives the heart of oxygen causing permanent damage
Temporarily stopping blood flow in the arm prevents damage in people having a heart attack, a Danish study has shown.
It is thought the procedure "kick-starts" natural mechanisms in the heart to counter the lack of oxygen.
In a study of around 150 patients, those who were treated this way in the ambulance on the way to hospital sustained less damage to heart tissue.
The British Heart Foundation said The Lancet study was "interesting" but more research was needed.
The Danish researchers said they had carried out the experiment in patients who were having a large heart attack because their coronary artery was blocked, preventing blood reaching the heart.
These patients are transferred directly to large specialist hospitals where a balloon can be quickly inserted into the artery to clear the blockage and restore blood flow.
In the study, paramedics used a standard blood pressure cuff to cut off the blood supply in a patient's arm for five minutes. They repeated this procedure four times with a five-minute break in between each time.
Once at hospital, the patient was injected with a radioactive material which allows doctors to see which parts of the heart had sustained damage from lack of oxygen.
The researchers took a final look at damage to heart tissue 30 days after the operation to restore heart function.
They found that those whose blood supply had been articificially restricted as part of this study ended up with less tissue damage to the heart.
Study leader Professor Hans Botker, from Aarhus University Hospital, said it had already been shown that you could "warm up" the heart by depriving it of oxygen for short periods to enable it to resist damage.
But what this study suggests is the effect can be reproduced by depriving a "distant organ", such as the arm, of oxygen.
"This seems to kick-start the organs' own protective mechanisms," he said.
"Stopping blood flow to the arm suggests to the heart that there is something wrong and that seems to cause some extra protection."
He added that more research is needed before the technique can be used widely in patients.
"We need to clarify whether there is a clinical benefit to the patient in reducing deaths and the incidence of heart failure."
Fotini Rozakeas, cardiac nurse at the British Heart Foundation, said the phenomenon had been shown to protect the heart muscle from damage when applied shortly before planned heart surgery.
"This interesting, yet small, study suggests that is also effective in reducing damage to heart muscle in patients who have had a suspected heart attack and are about to be treated with angioplasty.
"More research is now needed to find out if this treatment results in better recovery and ongoing clinical benefit.
"If so, it will pave the way for this simple, cheap and effective technique to be adopted more widely, in ambulances or in hospital."