Mountaineers were tested in the Himalayas

A lack of oxygen can trigger the heart and lung condition pulmonary hypertension.

But University of Giessen, Germany, researchers found Viagra prevents this.

The researchers say their study, in the Annals of Internal Medicine, shows Viagra could also help others who develop the condition.

With the drug, more blood was going through the lung vessels Dr Friedrich Grimminger, University of Geissen

In patients with pulmonary hypertension, the blood vessels in the lungs constrict, making the heart work harder.

This can eventually lead to heart damage, and even death. For many, the cause of the condition is unknown.

It is estimated that around 100,000 people are affected by the condition in the US and Europe.

Exercise capacity

The researchers used climbers as "guinea pigs" to test how effective Viagra was in relieving pulmonary hypertension.

Fourteen healthy male mountaineers were given either Viagra (sildenafil) or a dummy pill.

They were tested at both sea level and at high altitude - the Mount Everest base camp.

It was found sildenafil reduced high blood pressure and improved the transport of oxygen in the blood in both situations.

The authors say it is the first drug to be shown to increase the capacity to exercise at sea level, and when oxygen supply is affected at high altitude.

Sildenafil is used because there is a biochemical similarity between the penis and the lungs.

Both contain a large amount of the enzyme phospodiesterase, which limits men's erections and constricts blood vessels in the lungs.

Sildenafil stops the effect of this enzyme. The researches say this means it helps men maintain erections - and can also allow better circulation in the walls of the lungs.

Dr Friedrich Grimminger, who led the study, told the BBC the findings could help pulmonary hypertension patients: "They aren't able to do the smallest activity without complaining.

"With the drug, more blood was going through the lung vessels, the gas exchange [the exchange of oxygen and carbon dioxide between the air and the blood in the lungs] improved and the heart's pumping capacity could be raised."