Tetralogy of Fallot (ToF)

Apologies for the delay in uploads. I performed an unintentional experiment on my laptop and it did not go well. 16oz of water+laptop=laptop doesn’t work. But let’s get into some medicine!

Tetralogy of Fallot (ToF), named after the French physician Arthur Fallot, is the most common cyanotic heart defect. It is also one of the primary causes of “blue baby syndrome”. It is a complex condition caused when the following four defects are all present at birth:

  1. Overriding aorta (on top of both ventricles)
  2. Ventricular septal defect (VSD)
  3. Right ventricular hypertrophy (RVH)
  4. Pulmonary stenosis

Let’s take a quick peek at the healthy heart before we dive in :

Now let’s take a look at the heart of a ToF patient:

The overriding aorta of this heart is positioned closer to the mid-septum. This pushes the pulmonary artery over, giving it less space and decreasing its diameter (pulmonary stenosis). We know that a narrower artery means increased pressure, but the RV is not built to pump against higher pressure. All that hard work causes the muscles around the RV to thicken and enlarge (Right Ventricular Hypertrophy). While all of this is happening the hole created by the ventricular septal defect is allowing deoxygenated blood to freely flow from the right ventricle into the left (known as right-to-left shunting), where is can then be pumped through the aorta.

As we know from previous posts, fluids like taking the path of least resistance, so if the pulmonary vascular resistance is greater than the systemic vascular resistance, then most of the blood will travel across the VSD and into the aorta. To further complicate things, there are three different types of severity.

Mild Pulmonary Resistance

This is sometimes called “Pink Tetralogy of Fallot”. This is the least severe type as there is a minimal amount of right ventricular outflow obstruction and malalignment of the conal septum. The patient is born “pink” and but will become cyanotic over time. The pulmonary resistance is less than the systemic resistance which causes more blood to enter the pulmonary artery. And because of the ventricular defect, the oxygenated blood from the left side of the heart feels left out and some of it flows over to the right side to be pushed through the pulmonary artery. So both oxygenated and deoxygenated blood will flow into the pulmonary artery. It should be mentioned that these patients can develop CHF due to excess amount of pulmonary blood flow.

I did not go to art school and there is not a single artistic bone in my body but here is my picture depicting what is happening:

Moderate Pulmonary Resistance

These patients will be cyanotic at birth. There is now a moderate amount of pulmonary stenosis noted and the pulmonary valve/vein is moderately compressed. Because the pulmonary artery is compressed, the pressure will increase. Now the systemic vascular resistance is less than the pulmonary vascular resistance so both deoxygenated and oxygenated blood will try to flow through the aorta. Mixing deoxygenated blood with oxygenated blood will cause the patient to become hypoxic. Because of the obstruction and turbulence of the blood near the pulmonary artery, you can auscultate a harsh systolic murmur.

Severe Pulmonary Resistance

In this type, the pulmonary artery may be completely obstructed and the baby will be cyanotic at birth. These patients may also have Pulmonary atresia which is a birth defect where the pulmonic valve does not form. This means that all of the blood going to the RV has nowhere to go right? Well when we are born, there is a small hole called the foramen ovale that separates the two atria. So blood is able to flow from the right side of the heart over to the left and allows the blood to be pumped into the aorta. The foramen ovale closes shortly after birth. There are two ways these patients are able to get deoxygenated blood to their lungs. One way is through the Patent Ductus Arteriosus (PDA). The PDA connects the aorta and the pulmonary artery.

The RV pushes blood to the aorta where it can cross the PDA and go to the lungs where the blood can be oxygenated. Unfortunately, the PDA does close shortly after birth.

The second way is the aorta to pulmonary artery collateral.

ECG Findings

ECG findings that can indicate ToF include:

  1. Right axis deviation
  2. RVH
  3. RBBB
  4. Sometimes widened QRS which correlates to worse ventricular dysfunction

Testing for ToF

A high level of suspicion should be had for newborns with cyanosis. But here are some tests that can help with the diagnosis:

  1. ECG
  2. Chest X-ray
  3. Echocardiogram
  4. Heart MRI

In patients who have a mild degree of ToF, there are a few signs and symptoms to raise your index of suspicion which include:

  1. Increased respiratory rate, dyspnea, and cyanosis during physical activity (This is called a “Tet Spell”)
  2. Child “squats” during activity
  3. Fatigue and loss of consciousness
  4. Difficulty feeding due to hypoxia
  5. Inability to gain weight and grow
  6. Fingernail clubbing due to chronic hypoxia (this usually occurs after 6 months)

Treatment

The treatment for ToF varies on what degree of severity they have. All of these patients will need some form of surgery. And all medications given should be those that decrease pulmonary vascular resistance and increase systemic vascular resistance if needed. A temporary fix is placing a shunt or stent to open the pulmonary artery until a complete repair can be done with a patch on the septum.

You can also administer:

  1. Beta blockers to reduce spasm that occur in the infundibular region of the RV. Spasms in this region can cause a decrease in the amount of blood to go into the pulmonary artery.
  2. Prostaglandin E1 (only for newborns). This medication temporarily maintains the patency of the ductus arteriosus and is great for patients with pulmonary atresia
  3. Knee to chest position during a Tet Spell

This is also simulated when the patient “squats” as discussed earlier in the post. If the patient is in a Tet Spell, squatting and knees to chest position causes an increase in the systemic vascular resistance which forces more blood to enter the pulmonary artery. This occurs because the difference in pressures in the pulmonary artery and systemically has decreased,

5. Alpha drugs to increase systemic vascular resistance

Check out this post to learn a bit more about the different types of receptors:

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This site is meant to be used for educational use only. We strive to push evidence based medicine with no bias to help you obtain all the important information. You should always follow your protocols that have been set in place.

-Scopeducation Team (Matt)

References

Beerman, L. (2020, December). Tetralogy of FALLOT – Pediatrics. Retrieved April 04, 2021, from https://www.merckmanuals.com/professional/pediatrics/congenital-cardiovascular-anomalies/tetralogy-of-fallot

The Children’s Hospital of Philadelphia. (2014, March 26). Tetralogy of fallot (tof). Retrieved April 04, 2021, from https://www.chop.edu/conditions-diseases/tetralogy-fallot

Diaz-Frias, J., & Guillaume, M. (2020, November 20). Tetralogy of Fallot. Retrieved April 04, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK513288/

Gibson, C., MD, & Feeney, K. (n.d.). Tetralogy of fallot echocardiography. Retrieved April 04, 2021, from https://www.wikidoc.org/index.php/Tetralogy_of_fallot_echocardiography

Pettersen, M. D. (2020, December 22). Tetralogy of Fallot with Pulmonary Atresia Medication: Prostaglandins, diuretic agents, Inotropic agents. Retrieved April 04, 2021, from https://emedicine.medscape.com/article/899368-medication#:~:text=Alprostadil%20is%20first%2Dline%20palliative,oxygenation%20and%20lower%20body%20perfusion.

Sanandajifar, H., MD. (n.d.). Tetralogy of FALLOT: Pediatric Echocardiography. Retrieved April 04, 2021, from https://pedecho.org/library/chd/tetralogy-fallot

Winn, K., & Hutchins, G. (1973, October). The pathogenesis of tetralogy of Fallot. Retrieved April 04, 2021, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1904044/

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