Heart abnormalities: what is Tetralogy of Fallot?

By Cristiano Antonino On Apr 26, 2023

Tetralogy of Fallot (TOF) is a cardiac anomaly that refers to a combination of four related heart defects that commonly occur together

The four defects are:

Ventricular septal defect (VSD) – a hole between the left and right pumping chambers of the heart
Aorta override – the aortic valve is enlarged and appears to originate from both the left and right ventricles instead of the left ventricle as in normal hearts
Pulmonary stenosis – narrowing of the pulmonary valve and outflow tract or area under the valve that creates an obstruction (blockage) of blood flow from the right ventricle to the pulmonary artery
Right ventricular hypertrophy – thickening of the muscle walls of the right ventricle, which occurs because the right ventricle is pumping at high pressure

A small percentage of children with tetralogy of Fallot may also have additional ventricular septal defects

Is an atrial septal defect (ASD) or abnormalities in the branching pattern of their coronary arteries.

Some patients with tetralogy of Fallot have complete obstruction to flow from the right ventricle or pulmonary atresia.

Tetralogy of Fallot may be associated with chromosomal abnormalities, such as 22q11 deletion syndrome.

The pulmonary stenosis and right ventricular outflow tract obstruction observed with tetralogy of Fallot usually restrict blood flow to the lungs.

When blood flow to the lungs is restricted, the combination of the ventricular septal defect and the overlying aorta allows oxygen-poor (‘blue’) blood returning to the right atrium and right ventricle to be pumped from the aorta to the body.

This ‘shunting’ of oxygen-poor blood from the right ventricle to the body results in a reduction of arterial oxygen saturation so that children appear cyanotic or blue.

Cyanosis occurs because the oxygen-poor blood is darker and has a blue colour, so that lips and skin appear blue.

The extent of cyanosis depends on the amount of narrowing of the pulmonary valve and the outflow tract of the right ventricle.

A narrower outflow tract from the right ventricle is more restrictive to blood flow to the lungs, which in turn lowers the arterial oxygen level as more oxygen-poor blood is diverted from the right ventricle to the aorta.

Signs and symptoms of tetralogy of Fallot

Tetralogy of Fallot is often diagnosed in the first few weeks of life due to a loud murmur or cyanosis.

Babies with tetralogy of Fallot usually have a pervious ductus arteriosus at birth that provides additional blood flow to the lungs, so severe cyanosis is rare immediately after birth.

When the ductus arteriosus closes, which typically happens in the first few days of life, cyanosis can develop.

The degree of cyanosis is proportional to the pulmonary blood flow and therefore depends on the degree of narrowing of the outflow tract to the pulmonary arteries.

Rapid breathing may occur in response to low oxygen levels and reduced pulmonary blood flow.

Heart murmurs, which are commonly loud and harsh, are often absent in the first few days of life.

The arterial oxygen saturation of children with tetralogy of Fallot can suddenly drop significantly.

This phenomenon, called ‘tetralogy spell’, usually results from a sudden increase in the constriction of the outflow tract to the lungs so that pulmonary blood flow is further restricted.

The lips and skin of children who have a sudden decrease in arterial oxygen levels will appear acutely bluer.

Children who have a tetralogy spell will initially become extremely irritable in response to critically low oxygen levels and may become sleepy or unresponsive if severe cyanosis persists.

A tetralogy spell can sometimes be treated by comforting the child and bending the knees forward and upwards. Very often, however, immediate medical assistance is required.

Diagnosis of tetralogy of Fallot

When an infant with significant cyanosis is first seen, it is often given supplemental oxygen.

Increased oxygen improves the baby’s oxygen levels in cases of lung disease, but breathing extra oxygen will have little effect on the oxygen levels of a baby with tetralogy of Fallot.

Failure to respond to this ‘hyperoxia test’ is often the first clue to suspect a cyanotic heart defect.

Infants with tetralogy of Fallot may have normal oxygen levels if the pulmonary stenosis is mild (referred to as ‘pink’ tetralogy of Fallot).

In these children, the first clue suggesting a heart defect is the detection of a loud murmur when the child is examined.

Once a congenital heart disease is suspected, echocardiography can quickly and accurately demonstrate the four related defects characteristic of tetralogy of Fallot.

Occasionally, cardiac catheterisation is necessary to assess the size and distribution of the pulmonary arteries and to clarify the branching patterns of the coronary arteries.

Catheterisation can also demonstrate whether patients have pulmonary blood flow supplied by an abnormal blood vessel from the aorta (aortopulmonary collateral).

Treatment for tetralogy of Fallot

Once tetralogy of Fallot has been diagnosed, immediate management focuses on determining whether the child’s oxygen levels are in a safe range.

If oxygen levels are critically low immediately after birth, an infusion of prostaglandins is usually started to keep the ductus arteriosus open, which will provide additional pulmonary blood flow and increase the baby’s oxygen level.

These infants usually require surgery in the neonatal period. Infants with normal oxygen levels or only mild cyanosis are usually able to go home in the first week of life.

Complete repair is usually performed electively when infants are about 6 months of age, provided oxygen levels remain adequate.

Progressive or sudden decreases in oxygen saturation may require earlier corrective repair.

Surgical correction of the defect is always necessary.

Occasionally, patients require a palliative surgical procedure prior to final correction.

Corrective repair of tetralogy of Fallot involves closing the ventricular septal defect with a synthetic Dacron patch so that blood can flow normally from the left ventricle to the aorta.

The narrowing of the pulmonary valve and outflow tract of the right ventricle is then increased (enlarged) by a combination of cutting (resecting) obstructive muscle tissue in the right ventricle and widening the outflow pathway with a patch.

In some children, however, the coronary arteries branch through the outflow tract of the right ventricle where the patch would normally be placed.

In these children, an incision in this area to place the patch would damage the coronary artery and therefore cannot be performed safely.

When this occurs, a hole is drilled in the anterior surface of the right ventricle (avoiding the coronary artery) and a conduit (tube) is sewn from the right ventricle to the bifurcation of the pulmonary arteries to provide unobstructed blood flow from the right ventricle to the lungs.

Treatment for tetralogy of Fallot: results

The survival of children with tetralogy of Fallot has improved significantly in recent decades.

In the absence of confounding risk factors, more than 95 per cent of infants with tetralogy of Fallot undergo successful surgery in the first year of life.

Surgical repair is more difficult when the pulmonary arteries are extremely small or when pulmonary blood flow is predominantly provided by aortopulmonary collaterals.

Most children are quite sick in the first few days after surgery, as the right ventricle is ‘stiff’ due to previous hypertrophy (thickness) and as an incision is made in the ventricle muscle, making the muscle temporarily weaker.

This right ventricular dysfunction usually improves significantly in the days following surgery.

Patients may also experience rhythm problems after surgery.

An abnormally fast rhythm (called junctional tachycardia) may occur, which may require treatment with medication or the use of a temporary pacemaker.

This abnormal rhythm is usually temporary and the rhythm will generally return to normal when the right ventricle recovers.

Patients are also at risk of a slowed heart rate after surgery due to heart block.

Heart block can be caused by injury or inflammation of the heart’s conduction system.

In many patients, conduction improves and normal rhythm returns.

Rarely, a permanent pacemaker may be required.

Since the tetralogy of Fallot’s repair procedure produces normal circulation, long-term cardiac function is generally excellent.

However, the repair usually leaves the child with a leaky (insufficient) pulmonary valve.

In this situation, after the right ventricle has pumped blood into the pulmonary arteries, some of the blood will return to the right ventricle.

This creates extra volume in the right ventricle forcing it to work harder and dilate.

In a small percentage of children, this lung insufficiency can lead to reduced function of the right ventricle.

Symptoms of fatigue may develop, especially with exercise.

In these cases, pulmonary valve replacement is often recommended.

Patients who have had their tetralogy of Fallot repaired may also again develop narrowing in the outflow area or branch (left or right) of the pulmonary arteries, which will cause the right ventricle to pump at abnormally high pressures.

If these problems occur, surgery may be required to further widen the outflow tract or pulmonary arteries.

Narrowing of the pulmonary arteries can sometimes be treated without surgery, with balloon dilation of the vessels during cardiac catheterisation.

Long-term follow-up with a cardiologist is essential to detect recurring or new problems as soon as possible.

Follow-up visits to the cardiology clinic usually consist of a physical test, electrocardiogram and periodic echocardiography.

In addition, these visits will also include occasional cardiac MRI scans, stress stress tests and Holter evaluations when a child reaches adolescence and adulthood.

Adult and adolescent management

Most adult patients with tetralogy of Fallot were repaired during childhood.

They started life as a ‘blue child’ and surgery almost always made them pink.

The results of surgery in some patients are excellent and have no ongoing problems. Unfortunately, most have problems; the most important is the loss of the pulmonary valve or so-called ‘pulmonary regurgitation’.

This can enlarge the right heart chambers and lead to limitations in physical activity, as well as abnormalities in heart rhythm and occasionally sudden cardiac death.

Most patients with repaired tetralogy should undergo regular (usually annual) evaluation by a congenital cardiologist.

Pulmonary regurgitation can be treated with a tissue valve replacement.

This is usually a low-risk procedure that can allow the heart to shrink again and improve the patient’s quality of life and life expectancy.

These artificial pulmonary valves usually last many years in adult patients, but require expert supervision.

Fortunately, if these valves now fail, they can be replaced using valves inserted through catheters (e.g. the Melody valve) rather than reoperation.

Some adults with tetralogy have a genetic basis for the condition.

It is important that this is identified so that other manifestations of their genetic abnormality can be properly managed.

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