The research, published in the New England Journal of Medicine, found the device could help those with moderate to severe heart failure, and those who have a specific additional heart condition.

Heart failure leads to shortness of breath, low energy and mobility problems.

It is responsible for more hospitalisations than all forms of cancer combined.

Even if the early promise holds up, this is a treatment which is expensive, technically very demanding and applicable to a minority of patients

Professor John McMurray, Western Infirmary, Glasgow

It affects over 800,000 people in the UK.

The US study found cardiac resynchronisation (CR) therapy could improve the condition of many people with moderate-to-severe heart failure.

They fared better than those patients who only received medication, and had a reduced risk of being admitted to hospital, or of seeing their condition worsen.

The therapy delivers electrical impulses to both sides of the heart to co-ordinate the contractions of the heart's ventricles and improve its efficiency to increase blood flow to the body.

In the treatment, a pacemaker is placed under the skin in the chest.

It has three electrodes connecting it to the right atrium and a second to the right ventricle - like a normal pacemaker.

The third lead is connected to the left ventricle.

Quality of life

The MIRACLE study (Multicenter InSync Randomised Clinical Evaluation) looked at data for 453 patients at 45 centres across America, comparing people who had been given the pacemaker to those who had not.

It looked at how far people could walk in six minutes, assessed their quality of life - such as whether they were able to look after themselves and - and how many times they had to be admitted to hospital.

After six months, 67% of patients with the device were considered improved compared to 39% of those who were not.

Those on the therapy were able to exercise more, and had 50% fewer hospital admissions and spent 77% fewer days in hospital related to their heart failure.

Researchers say in addition to helping patients moderately to severely affected, it could also help those who have ventricular dysynchrony, a condition which occurs when there is a time delay between the beating of the heart's two lower chambers, or ventricles.

Early optimism 'misleading'

Dr Cliff Bucknall, consultant cardiologist at Guy's and St Thomas's Hospital, London, said: "This is very exciting.

"What it's done is to demonstrate that there is a significant improvement in wellbeing in what is a very difficult group of patients to help."

But John McMurray, professor of medical cardiology at Glasgow's Western Infirmary, told BBC News Online: "There are a few small trials such as MIRACLE which are encouraging.

"But we have been misled by early optimistic findings in short term studies in modest numbers of patients on many previous occasions when the treatments subsequently turned out to be of no benefit or even to be harmful.

"We need bigger and longer trials and several of these are already underway.

"Even if the early promise holds up, this is a treatment which is expensive, technically very demanding and applicable to a minority of patients."