When performing a fetal echocardiogram for Tetralogy of Fallot (TOF), your assessment should focus on a comprehensive evaluation of the cardiac anatomy and blood flow dynamics, as well as the identification of associated anomalies and features that may impact prognosis. TOF is one of the most frequently diagnosed congenital heart lesions prenatally.
1. Fundamental Anatomical Features of Tetralogy of Fallot
TOF is classically defined by four main abnormalities that arise from a primary defect of the outflow septum:
Antero-cephalad (anterior and superior) deviation of the infundibular septum, which is considered the hallmark of the diagnosis.
A Ventricular Septal Defect (VSD).
Right Ventricular Outflow Tract (RVOT) obstruction.
Aortic override of the ventricular septum.
Right ventricular hypertrophy (RVH), which is a secondary consequence of the RVOT obstruction and VSD. Fetuses often have a "hypertrophied" RV because the RV is systemic during fetal life.
2. Detailed Echocardiographic Assessment Points
You should systematically evaluate these features using various echocardiographic views:
Ventricular Septal Defect (VSD):
Assess the size, position, and alignment of the VSD. It is typically a large perimembranous defect, often located subaortic and extending into the subpulmonary region.
Note if it's a doubly committed subarterial VSD, which extends towards the pulmonary valve.
Actively search for any additional muscular VSDs.
Right Ventricular Outflow Tract (RVOT) and Pulmonary Artery (PA):
Localize the site(s) of obstruction, which can be at the infundibular, valvular, and/or pulmonary artery levels. Infundibular stenosis is very common.
Evaluate the pulmonary valve morphology. Check for thickened, dysplastic, or bicuspid leaflets. Note if there are rudimentary valve remnants or if the valve is truly absent.
Measure the size of the main pulmonary artery (MPA) and its proximal branches. These can be hypoplastic.
In cases of Tetralogy of Fallot with absent pulmonary valve, look for massive aneurysmal dilatation of the PAs, which can be markedly pulsatile and cause bronchial compression.
Assess for stenosis at the origin of the branch pulmonary arteries, particularly the left pulmonary artery (LPA).
Aortic Valve and Aorta:
Assess the degree of aortic override of the ventricular septum.
Evaluate the aortic root and arch dimensions.
Check for the presence of a right aortic arch, which is seen in about 25% of TOF patients.
Ventricular Chambers:
Assess the relative sizes of the left and right ventricular cavities. Look for any imbalance in chamber sizes, which can be an important indicator of cardiac malformation. In TOF, the right ventricle may appear small or hypoplastic in some variants.
Note any abnormal septal motion.
Atrioventricular Valves:
Assess their morphology and function. In particular, determine their relationship to the VSD.
3. Doppler Echocardiography Assessment: Doppler is crucial for assessing blood flow and hemodynamics, and it provides important prognostic information:
Shunts and Gradients:
Localize and evaluate shunts, including the VSD flow direction and velocity.
Quantify the severity of valvular stenoses or regurgitations by measuring pressure gradients (ΔP = 4v²). This includes assessing forward flow velocity and any regurgitation in the RVOT and pulmonary artery. Gradient can be misleading in prenatal life when there is a large ductus and high PVR.
Assess for tricuspid regurgitation (TR), which can be used to estimate right ventricular pressure. Note if the TR is severe, as this can lead to right ventricular dilatation and potentially hydrops fetalis in cases of absent pulmonary valve.
Ductus Arteriosus (DA):
Evaluate the patency and flow direction in the ductus arteriosus.
Retrograde flow from the aorta to the pulmonary artery via the ductus signifies severe pulmonary stenosis or atresia.
Pulmonary Veins:
Use color Doppler to confirm all pulmonary veins are draining correctly into the left atrium. This is essential to rule out Total Anomalous Pulmonary Venous Connection (TAPVC).
4. Fetal Considerations and Prognostic Indicators
TOF can be diagnosed as early as 10-14 weeks' gestation.
Monitor the growth of the pulmonary artery and aorta throughout gestation, as their progression can influence outcomes.
In TOF with absent pulmonary valve, the combination of a dilated right ventricle, large pulmonary arteries, and marked pulmonary regurgitation is diagnostic. Severe pulmonary regurgitation can lead to hydrops fetalis.
An imbalance in the size of the cardiac cavities can be an early and important sign of a cardiac malformation in the fetus.
Doppler assessment is considered prognostic in fetal echocardiography, helping to predict the postnatal course and need for intervention.
References:
Alboliras, E. T., Hijazi, Z. M., Lopez, L., & Hagler, D. J. (Eds.). (2018). Visual Guide to Neonatal Cardiology. John Wiley & Sons Ltd.
Batisse, A. (n.d.). Échocardiographie Fœtale (4th ed.). Wolters Kluwer France.
Rudolph, A. M. (2009). Congenital Diseases of the Heart: Clinical-Physiological Considerations (3rd ed.). Wiley-Blackwell.
Acherman, R. J., & Evans, W. N. (n.d.). Chapter 16: Overview of Fetal Echocardiography. In Practical Neonatal Echocardiography. McGraw-Hill Medical.
View of the ascending aorta and overrinding aorta, with measurements on the stillframe image.
In this view, one may appreciate the overriding aorta. The ascending aorta is of adequate caliber with an opening aortic valve, which has thin leaflets.
View of the MPA and then ascending aorta.
The perimembranous VSD can be observed, There is a sweep in the long axis where we can observe the RVOT and MPA (anterior and coming out of the RV), and the overriding Aorta posterior to the pulmonary artery.
2D view and then with colour to appreciate the flow through the VSD, and anterograde from the RVOT to the MPA, as well as from the RV/LV to the Aorta.
In this short axis of the heart, we can appreciate the anterior right ventricle, RV outflow tract and main pulmonary artery. We can also appreciate the main pulmonary artery which is of adequate caliber bur smaller than the Aorta.
Flow is antegrade through the RVOT towards the Main Pulmonary Artery. There is no pulmonary atresia.
Short axis at the level of the Aortic Valve, Perimembranous VSD and RV outflow tract. One may even appreciate the anterior and cephalad deviation of the infendibular septum - with a zoom provided below in still frames. We can appreciate the leaflet of the pulmonary valve opening and closing.
Still frame to appreciate the VSD and the anterior/cephalad deviation of the infundibular septum.
Short axis at the level of the Aortic Valve, Perimembranous VSD and RV outflow tract. One may even appreciate the anterior and cephalad deviation of the infendibular septum - with a zoom provided below in still frames. We can appreciate the leaflet of the pulmonary valve opening and closing.
Diameter of the RPA and LPA which are here confluent and grew because of sufficient antegrade blood flow. RPA and LPA can also grow if there is enough flow coming through a ductus arteriosus, when present. The presence of a left to right or bidirectional duct indicate that the RVOT obstruction is of greater concern.
Measurement of the PV annulus in the short axis view (measurement #1).
Aortic valve annulus
MPA and PV annulus diameters.
Four chamber views with sweeps. We can appreciate the perimembranous VSD. The size of the RV and LV are appropriate. We can appreciate in 2D that the tricuspid and mitral valves are opening and closely appropriately.
There is flow through the VSD observed by colour Doppler. There is appropriate flow through the inflow of the RV and LV. We can also appreciate pulmonary veins draining into the left atrium.
RV-Tet view to observe the RVOT and pulmonary valve.