Fallot's tetralogy (also known as tetralogy of Fallot, or TOF) is an abnormality of the heart and major blood vessels, which may be found in babies.

It's one of the most complex heart problems, as there are actually four different abnormalities in Fallot's:

• A large ventricular septal defect - one of the more serious types of 'hole' in the heart, when there's a connection between the two main pumping chambers of the heart (ventricles)
• Narrowing of the pulmonary valve (pulmonary stenosis) - this means the heart has to work harder to pump blood in to the lungs to collect oxygen
• Right ventricular hypertrophy - thickening of the muscle wall of the right ventricle
• A displaced aorta (the major blood vessel taking blood out of the heart and directing it around the body)

Although these are the main problems, every child is different and there may be all sorts of other abnormalities.

What these complex abnormalities mean is that the blood pumped out from the heart around the body doesn't contain normal levels of oxygen.

Without adequate oxygen, the blood in the arteries isn't as bright red as usual and the child usually looks blue (or cyanosed - check lips and fingernails) and breathless (because the body senses the need for more oxygen), often even at rest.

If the baby cries or gets agitated, or simply exerts itself to feed or open its bowels, the cyanosis gets worse. This can interfere greatly with feeding.

The baby may also have 'hypoxic spells'. These occur when the brain isn't getting enough oxygen to work properly and the child may lose consciousness.

Other symptoms include slow development and failure to gain weight normally.

As the child grows, the cyanosis gets worse and more continuous, and activity becomes increasingly difficult. They often sit in a squatting position - with their knees up to their chest - after activity as this helps them to recover from the breathlessness.

The cause of Fallot's tetralogy isn't fully understood. While the baby is in the womb, something interferes with the development of the heart and major blood vessels.

Although no specific single genetic abnormality has yet been found to explain every case, genetics often do play a part in these types of malformations (known as conotruncal abnormalities). In some children, a particular genetic problem can be identified, such as DiGeorge syndrome.

Some researchers have suggested that Fallot's tetralogy is caused by an autosomal recessive gene that has yet to be identified and which has variable penetrance (that is, doesn't always cause disease).

However, this is far from proven and Fallot's tetralogy has also been linked to certain medications and also to alcohol taken by the mother while pregnant.

Whatever the cause, in those families who have a child with Fallot's the risk of a second child being born with the condition is only increased very slightly.

Almost 70 per cent of babies with Fallot's tetralogy will need complex heart surgery within the first year of life if they're to survive. Without treatment, about 30 per cent will die before they reach their first birthday.

Heart surgery and anaesthetic techniques (including heart bypass techniques) have changed dramatically in recent years, making it much easier to operate on the very young. A one-stage repair is often done early on in the baby's life before other complications set in.

Prior to surgery, extensive tests will be done to work out exactly the anatomy of each baby's heart and blood vessels, so surgeons can decide on the best way to operate.

Up to ten per cent of children will need a second operation, usually within a few years of the first.

If surgery is successful, a normal life can be anticipated for the child.