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Page 2 of 43 Rao. Vessel Plus 2022;6:26 https://dx.doi.org/10.20517/2574-1209.2021.93

[1,2]
usually identified during prenatal echo studies, as reviewed elsewhere ; in this review, only post-natal
findings are reviewed.

TETRALOGY OF FALLOT
Tetralogy of Fallot (TOF) is the utmost frequent cyanotic CHD in patients who are more than one year of
[3-6]
age. The defect comprises 10% of all heart defects . TOF is a constellation of four abnormalities, namely,
(1) ventricular septal defect; (2) pulmonary stenosis; (3) hypertrophy of the right ventricle (RV); and (4)
[3-6]
dextroposition of the aorta . The ventricular septal defects (VSDs) typically abut directly on the
atrioventricular component of the membranous septum and are called as perimembranous VSDs and are
large and non-restrictive. At times the VSD encompasses the muscular portion of the ventricular septum.
Rarely, other defects in the muscular septum may coexist. Pulmonary stenosis (PS) varies both in regard to
the location of the narrowing and the magnitude of obstruction. The extent of narrowing may be noticeably
mild, not producing any decrease in arterial O saturation. Indeed, left to right shunting via the VSD may
2
occur. Or, the obstruction may be substantial, causing marked cyanosis even in the neonate. The location of
the narrowing may be situated in the infundibular region, at the valve itself, in a supra-valvar site, or at the
branch PAs; infundibular narrowing is the most common obstructive lesion among these. The narrowing
may be located at one of the above sites or may involve several locations. The infundibular narrowing in
TOF is considered to be caused by superior and anterior deviation of the conal (infundibular) ventricular
septum. The valve narrowing may be due to fusion of pulmonary valve commissures, hypoplasia of the
annulus of the pulmonary valve, or a mixture of the two. Sometimes, pulmonary valve leaflet dysplasia may
be present, causing the obstruction. Hypertrophy of the RV is seen in all patients and is generally severe in
degree. Aortic over-ride of the ventricular septum and rightward displacement (dextroposition) of the aorta
varies from one patient to the other. Dilatation of the aorta is seen, which is secondary to an abnormality of
development instead of the pathophysiology of tetralogy. The right-sided descending aorta is seen in 25% to
30% of tetralogy patients [5,7,8] . Atrial septal defects (ASDs) of significant size may be seen in some TOF
patients; in such instances, the condition is called pentalogy of Fallot. In one study, ASDs were seen in 27%
[8]
of autopsy specimens .

Because the ventricular defect is large and non-restrictive, the pressures in both ventricles are equal, and
both right and left ventricles function as a single unit. The relative quantities of blood flowing into the
pulmonary and systemic circulations depend on the resistances offered by the systemic circuit (systemic
vascular resistance) vs. degree of RV outflow tract narrowing. The greater the RV outflow obstruction, the
lesser is the blood flow into the lung. In a typical tetralogy patient, the resistance presented by the RV
outflow tract stenosis is greater than that of the systemic vascular resistance; consequently, right to left
shunt takes place through the ventricular defect.

Several types of TOF exist and each type has its own mode of presentation. Furthermore, the type of
[5,6]
management differs from one type to the other . On the basis of these considerations, we recommend the
following classification : (1) Type I, classic TOF; (2) Type II, TOF with pulmonary atresia; (3) Type III,
[5,6]
TOF with multiple aorto-pulmonary collateral arteries (MAPCAs); and (4) Type IV, TOF with the
syndrome of absent pulmonary valve. The echo features of the different types are reviewed one after the
other.

Type I, classic TOF
An echocardiographic examination is valuable in establishing the diagnosis. A large VSD is clearly imaged
along with an over-riding of the aorta [Figure 1]. A dilated and hypertrophied RV is seen. Usually, there is a
right to left shunt through the VSD [Figure 2] and this, along with RV outflow tract stenosis [Figure 3] can