Tetralogy of Fallot:

Tetralogy of Fallot is the most common cyanotic heart defect. It is a congenital heart defect which involves four anatomical abnormalities of the heart. It is also the most common cause of blue baby syndrome.

Signs and symptoms:

Tetralogy of Fallot results in low oxygenation of blood. This occurs because of the ventricular septal defect (VSD), which is one of the heart defects seen in Tetralogy of Fallot. VSD results in mixing of oxygenated and deoxygenated blood in the left ventricle, and since there is an obstruction to flow through the pulmonary valve, this low oxygenated blood leaves the heart via the aorta. This is known as a right-to-left shunt.
The primary symptom is low blood oxygen saturation with or without cyanosis from birth or developing in the first year of life. Other symptoms include a heart murmur which may range from almost imperceptible to very loud, difficulty in feeding, failure to gain weight, retarded growth and physical development, dyspnea on exertion, clubbing of the fingers and toes, and polycythemia.

Children with tetralogy of Fallot may develop “tet spells.” The precise mechanism of these episodes is in doubt, but presumably results from a transient increase in resistance to blood flow to the lungs with increased preferential flow of desaturated blood to the body. Tet spells are characterized by a sudden, marked increase in cyanosis followed by syncope, and may result in hypoxic brain injury and death. Older children will often squat during a tet spell, which increases systemic vascular resistance and allows for the right-to-left shunt to temporary reverse back into the right ventricle and out in to the lungs.

Pathophysiology of Tetralogy of Fallot:

“Tetralogy” means a four-part phenomenon in various fields, including literature, and the four parts the syndrome’s name implies are its four signs.
Tetralogy of Fallot involves four heart defects:

• Pulmonary valve stenosis
• An overriding aorta
• Ventricular septal defect (VSD)
• Right ventricular hypertrophy

A) Pulmonary Valve Stenosis:
This defect involves narrowing of the pulmonary valve and the passage from the right ventricle to the pulmonary artery. Normally, oxygen-poor blood from the right ventricle flows through the pulmonary valve and into the pulmonary artery. From there, the blood travels to the lungs to pick up oxygen. In pulmonary stenosis, the pulmonary valve cannot fully open. Thus, the heart has to work harder to pump blood through the valve. As a result, not enough blood reaches the lungs.

B) Overriding Aorta:

Heart defects of Tetralogy of Fallot ©wikipedia

This defect occurs in the aorta, the main artery that carries oxygen-rich blood from the heart to the body. In a healthy heart, the aorta is attached to the left ventricle. This allows only oxygen-rich blood to flow to the body. In tetralogy of Fallot, the aorta is located between the left and right ventricles, directly over the VSD. As a result, oxygen-poor blood from the right ventricle flows directly into the aorta instead of into the pulmonary artery.

C) Ventricular Septal Defect:
The heart has an inner wall that separates the two chambers on its left side from the two chambers on its right side. This wall is called a septum. The septum prevents blood from mixing between the two sides of the heart. A VSD is a hole in the septum between the heart’s two lower chambers, the ventricles. The hole allows oxygen-rich blood from the left ventricle to mix with oxygen-poor blood from the right ventricle.

D) Right Ventricular Hypertrophy:
With this defect, the muscle of the right ventricle is thicker than usual. This occurs because the heart has to work harder than normal to move blood through the narrowed pulmonary valve.

Diagnosis of Tetralogy of Fallot:

Echocardiogram:
Congenital heart defects are now diagnosed with echocardiography, which is a painless test that uses sound waves to create a moving picture of the heart. During the test, the sound waves (called ultrasound) bounce off the structures of the heart. A computer converts the sound waves into pictures on a screen.

Echo allows the doctor to clearly see any problem with the way the heart is formed or the way it’s working. Echo is an important test for diagnosing tetralogy of Fallot because it shows the four heart defects and how the heart is responding to them. This test helps the cardiologist decide when to repair the defects and what type of surgery to use. Echo also is used to check a child’s condition over time, after the defects have been repaired.

Electrocardiogram (EKG):
An EKG is a simple, painless test that records the heart’s electrical activity. The test shows how fast the heart is beating and its rhythm (steady or irregular). An EKG also records the strength and timing of electrical signals as they pass through the heart. This test can help the doctor find out whether your child’s right ventricle is enlarged (ventricular hypertrophy).

Chest X Ray:
A chest x ray is a painless test that creates pictures of the structures in the chest, such as the heart and lungs. This test can show whether the heart is enlarged or whether the lungs have extra blood flow or extra fluid, a sign of heart failure.

Pulse Oximetry:
For this test, a small sensor is attached to a finger or toe (like an adhesive bandage). The sensor gives an estimate of how much oxygen is in the blood.

Treatment for Tetralogy of Fallot:

Emergency management of tet spells
Prior to corrective surgery, children with tetralogy of Fallot may be prone to consequential acute hypoxia (tet spells), characterized by sudden cyanosis and syncope. These may be treated with beta-blockers such as propranolol, but acute episodes may require rapid intervention with morphine or intranasal fentanyl to reduce ventilatory drive and a vasopressor such as epinephrine, phenylephrine, or norepinephrine to increase systemic vascular resistance. Oxygen (100%) may be effective in treating spells because it is a potent pulmonary vasodilator and systemic vasoconstrictor. This allows more blood flow to the lungs by decreasing shunting of deoxygenated blood from the right to left ventricle through the VSD. There are also simple procedures such as squatting and the knee chest position which also increases systemic vascular resistance, and will also decrease the right to left shunting of deoxygenated blood into the systemic circulation.

Total surgical repair:

Surgery to repair tetralogy of Fallot improves blood flow to the lungs. Surgery also ensures that oxygen-rich and oxygen-poor blood flow to the right places.

The surgeon will:

• Widen the narrowed pulmonary blood vessels. The pulmonary valve is widened or replaced. Also, the passage from the right ventricle to the pulmonary artery is enlarged. These procedures improve blood flow to the lungs. This allows the blood to get enough oxygen to meet the body’s needs.
• Repair the ventricular septal defect (VSD). A patch is used to cover the hole in the septum. This patch stops oxygen-rich and oxygen-poor blood from mixing between the ventricles.

Fixing these two defects resolves problems caused by the other two defects. When the right ventricle no longer has to work so hard to pump blood to the lungs, it will return to a normal thickness. Fixing the VSD means that only oxygen-rich blood will flow out of the left ventricle into the aorta.