Tetralogy of Fallot (ToF) is one of the most common forms of cyanotic congenital heart disease in infants. ToF is a heart defect in infants consisting of 4 conditions, namely ventricular septal defect (VSD), right ventricular outflow tract obstruction (RVOTO), prominent aorta, and right ventricular hypertrophy. The appearance and severity of ToF are determined by the degree of RVOTO, the relative pressures in the right and left ventricles, and the proportion of the aorta that exceeds the VSD.
A. Ventricular septal defect (Ventricel Septum Defect)
This condition is a large non-restrictive defect. VSDs usually occur in the perimembranous and muscular ventricular septum, resulting in the absence of a partition between the ventricles, allowing for exchange of flow between the ventricles.
B. Right ventricular outflow tract obstruction (RVOTO)
Pulmonary blood flow obstruction at the right ventricular outflow tract is the cardinal feature of ToF. In the presence of a nonrestrictive VSD, worsening RVOTO will increase RV pressure, forcing a right-to-left shunt through the VSD, reducing pulmonary blood flow, and causing hypoxemia.
C. Aortic protrusion
Because of the misalignment of the outflow tract septum into the right ventricle, the aortic root displaces the ventricular septal muscle. In the setting of significant subpulmonary obstruction, the shunt crosses the interventricular space from right to left, which promotes the ejection of deoxygenated blood into the systemic circulation. The chronic volume load sustained by the main aorta is implicated in the aortic root dilation that commonly occurs in adults with tetralogy of Fallot.
D. Right ventricular hypertrophy
Right ventricular hypertrophy develops as a consequence of RVOTO because increased right ventricular pressure needs to be generated to maintain pulmonary blood flow. This condition also changes the size of the right ventricular cavity and the amount of muscle mass, which is an important issue after ToF repair.
ToF Classification
There are several subtypes of ToF which are distinguished based on the degree of cyanosis, namely:
- Pink Tetralogy of Fallot: no or minimal RVOTO with slight aortic protrusion
- Blue Tetralogy of Fallot: there is RVOTO and aortic protrusion
- Profound Cyanosis Tetralogy of Fallot : severe or complete RVOTO
Pathophysiology of ToF
The anatomy of ToF allows for mixing of blood between the pulmonary and systemic circulations. This mixing usually occurs in VSDs, where the right-to-left shunt adds low-oxygen (deoxygenated) blood to the systemic circulation and causes cyanosis.
Clinical Features of ToF
- Cyanosis (starts gradually and gets worse with age)
- Hypoxia/low SpO2 with little or no response to oxygen therapy
- Heart murmurs (pansystolic and ejection systolic)
- There are episodes of restless and depressed cyanosis.
- Arrhythmias (especially supraventricular and ventricular)
- Late symptoms appear such as: Poor exercise tolerance, Clubbing finger, Polycythemia, Delayed neurodevelopment, Heart failure, Recurrent respiratory tract infections, Brain abscess and Stroke.
Also read: Congenital Heart Defect: Recognize Heart Abnormalities in Babies
Reference :
- Bailliard F, Anderson RH. Tetralogy of Fallot. Orphanet J Rare Dis. 2009 Jan 13;4:2.
doi: 10.1186/1750-1172-4-2. PMID: 19144126; PMCID: PMC2651859. - Diaz-Frias J, Horenstein MS, Guillaume M. Tetralogi Fallot. [Diperbarui 2024 14 Februari]. Di: StatPearls [Internet]. Pulau Harta Karun (FL): Penerbitan StatPearls; 2024 Januari-. Tersedia dari: https://www.ncbi.nlm.nih.gov/books/NBK513288/
- Wilson R., Ross O., Griksaitis MJ. Tetralogy Fallot. BJA Educ. 2019 Nov; 19(11): 362–369. Published online 2019 Oct 14. doi: 10.1016/j.bjae.2019.07.003
