Until now Viagra (sildefanil) has been used mainly to correct the common problem of impotence in older men panting away in bed. But now it has a new medicinal purpose, for which it will be renamed Revatio. It will also acquire a new shape and colour and be prescribed in a different dose. For the drug, manufactured by Pfizer, has been found to have a dramatic effect in treating a rare but always debilitating and eventually fatal disease: pulmonary arterial hypertension.

Patients with pulmonary hypertension (PH) have an average age of 35 and are about six times more likely to be women than men. The illness leads eventually to right-sided heart failure. In the few years for which sufferers survive, it renders them unable to work or lead any form of normal existence.

At present medical treatment gives just short-term benefits; the only long-term hope for younger patients is a transplant.

The great maxim that helps doctors to make spot diagnoses in the middle of the night and allows them to return to bed reasonably happy that they haven’t missed something important is that common diseases occur commonly. When a birdwatcher spots a brown bird on the far side of a field, he assumes first that it is a thrush rather than a redwing — and usually he is right. But both doctors and ornithologists can make mistakes if they fail to consider rarities and, in the doctor ’s case, failure to recall a rarity can cost a life.

PH is easy to overlook because it affects only 4,000 people in Britain, and as a result is nearly always mistreated for at least two years. As well as their GP, the average PH patient sees four specialist doctors before finally one of them remembers a small paragraph in a textbook, or a similar case from 20 years ago.One type of PH is pulmonary arterial hypertension (PAH), or high blood pressure in the arteries of the lungs.

Patients are used to having their systemic blood pressure measured by their doctor. They hold out their arm, the sphygmomanometer cuff is wrapped around their biceps and the pressure in the brachial artery is taken. It is a simple procedure that gives an immediate result.

The problem with PAH is that the arteries in which blood pressure is high are concealed in the chest. They supply the lungs and are so inaccessible that the pressure in them is hard to measure.

There are no obvious signs or symptoms of pulmonary hypertension. Diagnosis depends on the clinical acumen of an experienced respiratory physician. He or she will have been alerted to the possibility by the patient’s history of progressive breathlessness that has failed to respond to treatment. The doctor will also have noticed subtle changes in heart sounds and possibly early evidence of right-sided heart failure on X-ray. More than 95 per cent of PH patients have increasing breathlessness on exertion as their initial symptom. Others may have fainted inexplicably or had chest pain after exercise.

PH may also be associated with Raynaud’s phenomenon — cold hands with characteristic white, blue and red colour-changes as the arterial spasm affects the arteries leading to the fingers — and arthritis.

Paul Corris, Professor of Thoracic Medicine at the University of Newcastle, says: “Options to treat PAH have previously been limited. Revatio offers an oral treatment that combines efficacy, tolerability and convenience.”

In future sildenafil, rather than being valued exclusively for its effect on the endothelium of the penile arteries, will be used to dilate the pulmonary arteries, reducing pressure in them and giving new hope to many young people.