By all accounts, Donald Dewar has been fortunate. His leaking aortic valve has been caught in the early stages, when the only sign is a 'minor irregularity', and long before it started to cause him physical problems.

He can be assured that his heart has not sustained serious or irreversible damage, and that once it is replaced by a perfectly working valve he can bounce back into normal robust health.

We have four heart valves, two on each side. They open and close as the heart contracts and relaxes, working together to ensure that blood is first sucked into the heart when it relaxes, and then is driven onwards when it beats. The valves are there to prevent any back-surge of blood in the wrong direction.

The aortic valve (the one that is leaking in Donald's case) makes sure that the blood entering the left side of the heart from the lungs passes on to the brain and body.

With each beat it opens, and with each relaxation it closes. But it has to close perfectly, so that no blood is sucked back into the heart from the main artery, the aorta.

It is a ring with two tough elastic-like cups, the edges meeting together when the heart relaxes, and flying apart when it beats. When it leaks the edges have crinkled or shrunk with infection or other disease, or the ring has expanded, so that the edges can't meet.

Either way, when the heart relaxes, some blood expelled at the last beat flows back into the heart, to mix with the blood flowing normally in from the lungs.

The heart then has extra blood to pump through the leaky valve with each beat - and the extra load causes strain.

The earliest sign of the strain is the odd irregularity seen in Donald's check up. If it goes unchecked, then the heart muscle over-grows, to make the beat stronger so that it can expel the extra blood.

Eventually the heart gets too big for its own blood supply - the coronary arteries - and can start to fail. So the leaking valve must be corrected before the process goes too far.

A generation ago the commonest cause of aortic valve leakage was rheumatic fever in childhood - it left the valves inflamed and distorted. It's more likely to be caused today by a condition in which the base of the aorta expands with age - so that the 'ring' that holds the cups of the valve widens.

The actual cups are not distorted - they just no longer meet in the middle. What is needed then is a wider, better fitting valve.

People in this group may have a 'connective tissue disease', the best known of which is Marfan's syndrome - Abraham Lincoln almost certainly had it.

Marfan's sufferers are tall men, with long fingers and toes and thin cavernous faces. Their aortic walls are weakened by their illness, and expand under pressure like an overblown balloon.

At operation, the old valve is replaced by an artificial valve of exactly the right size and shape - they range from 2.6 to 3.5 centimetres across. It is stitched into place while the heart is stopped and the circulation is taken over by machine.

Today's heart valve replacement teams are phenomenally efficient and the operation takes only two to three hours. The patient is up and about the next day to prevent complications such as clotting in the veins, and is usually back to work in two to three months.

Donald will have to take anti-clotting drugs permanently to prevent clots forming on the artificial surface, but most people who have had the operation do exceptionally well and live normal energetic lives.

And the ones I've had to deal with have gone back to work full of vim and vigour - so don't be looking for a replacement for our First Minister just yet.

Converted for the new archive on 30 June 2000. Some images and formatting may have been lost in the conversion.