Pulmonary hypertension, or high blood pressure in the arteries of the lungs, affects thousands of people in the UK.

It may be caused by heart conditions such as valve disease and by chronic lung conditions such as emphysema, bronchitis or clots in the pulmonary arteries and occasionally develops without an obvious cause.

It can be very disabling, severely limiting exercise capability, and shortening life expectancy.

The few treatments available are either difficult to administer or have unpleasant side effects.

However, researchers have found that Viagra may provide an effective alternative.

The work is a joint effort by scientists from Hammersmith Hospital and the National Center for Cardiology in Bishkek, Kyrgyzstan.

Low oxygen

The researchers studied healthy male volunteers who had breathed a low oxygen atmosphere for 30 minutes to induce pulmonary hypertension.

This low oxygen atmosphere caused a 56% increase in pulmonary artery pressure.

When the low oxygen test was repeated - this time after taking Viagra - their pulmonary artery pressure remained almost normal.

Viagra, technical name Sildenafil, appears to help blood vessels relax and prevent the constriction that normally accompanies lack of oxygen (hypoxia).

Laboratory studies on mice have confirmed these results and suggest the benefits of the drug are maintained with long-term administration.

Both normal mice and mice born with a genetic predisposition to pulmonary hypertension benefited from sildenafil administered through their drinking water when housed in a low oxygen atmosphere.

Important observation

Lead researcher Professor Martin Wilkins said: "This is an important observation which coupled with isolated case reports suggests that sildenafil may be a useful treatment for pulmonary hypertension.

"It must be stressed, however, that controlled studies in patients with the condition are necessary before it is used in this way."

Professor Sir Charles George, medical director at the British Heart Foundation said: "Pulmonary hypertension is relatively common in people with chronic lung disorders and also for some types of heart disease.

"As well as being life threatening it is highly debilitating because oxygen needs to be administered continuously.

"This exciting research points the way to a new treatment option for these patients and merits further investigation in suitable patients."

Dr John Harvey, of the British Thoracic Society (BTS), said: "This is an interesting new approach to the management of high blood pressure in the main lung arteries which can be caused by a variety of lung disorders.

"These results should stimulate much needed research in this area."

The research is published in Circulation, the journal of the American Heart Association.