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Page 4 of 43 Rao. Vessel Plus 2022;6:26 https://dx.doi.org/10.20517/2574-1209.2021.93

PAs or multiple sites may be involved. These obstructive sites should be identified by color, pulsed and
continuous wave (CW) Doppler examination. The pulmonary outflow tract obstruction produces elevated
Doppler flow velocity through these sites [Figure 4]. But it should be understood that: (1) the neonates
usually have high PA pressure and consequently, the pressure gradients by Doppler under-estimate the
magnitude of obstruction; and (2) in subjects with very severe pulmonary outflow tract narrowing, the
gradients by Doppler may not capture the severity of obstruction because of the limitations imposed by the
normal blood pressure and systemic vascular resistance. Careful evaluation of proximal coronary arteries
(CAs) must also be undertaken. Anomalous origin of left anterior descending CA from the right CA
traversing the RV outflow tract occurs in approximately 5% of patients with TOF . Other coronary
[9]
anomalies also exist. In cases where proximal CA structure cannot be demonstrated, other imaging
investigations, namely, magnetic resonance imaging (MRI), computed tomography (CT) or coronary
angiography, may be required prior to surgical correction.

Type II. TOF with pulmonary atresia
Inner heart structure is the same as that depicted above for classic tetralogy. However, the RV outflow
region is totally obstructed, i.e., pulmonary atresia, and, as a result, blood flow going from the RV to the PA
is not present. The site of atresia may be in the infundibulum, at the valve, or of the main PA, or a mixture
of the above. Infrequently, atresia confined only to the pulmonary valve is seen [Figure 5]. The branches of
the PAs are usually hypoplastic (or small) and rarely normal in size. These babies are obviously ductal-
dependent. In most of such babies, a patent ductus arteriosus (PDA) is present and the PDA is generally
long and tortuous.

On echocardiography, the findings of a large VSD, aortic override, and right to left shunt via the ventricular
defect [Figures 1 and 2] are the same as those described for classic TOF. The flow of blood through the RV
outflow tract is not seen. Valvar atresia may sometimes be demonstrated in echo studies [Figure 5]. The
diameters of the main and branch PAs may be demonstrated by 2D echo [Figures 5 and 6] and color flow
imaging [Figure 6A]; they are usually small and hypoplastic. In some patients, other imaging techniques
such as MRI, CT, or angiography are required to delineate the PA structure. The ductus arteriosus is usually
demonstrated by color Doppler studies, and as mentioned, it is usually long and tortuous.

Type III. TOF with multiple aorto-pulmonary collateral arteries
The pathologic structure of the heart is comparable to that portrayed above for tetralogy with pulmonary
atresia. There is no onward blood flow from the RV outflow tract into the PAs. The main PA is either very
small or absent. The PA branches are diminutive with many stenotic lesions. There is no continuity between
the right PA (RPA) and left PA (LPA), and different lobar branches may not be connected to each other.
Nevertheless, in some subjects with tetralogy and MAPCAs, hypoplastic central pulmonary arteries connect
to the RPA and LPA. However, in the majority of patients, the flow of blood to the lungs is mostly provided
[10]
by MAPCAs. The distribution of these vessels is variable and complex . They are abnormal and connect
the systemic arteries with PAs. The MAPCAs usually originate from the branches of the aorta. The origin of
these vessels may be either from above or below the diaphragm level. They frequently have circuitous
courses, cross the midline and supply the central, lobar, and segmental PAs, and may connect distal to
stenotic lesions or discontinuous vessels. The MAPCAs are usually tortuous and uneven in diameter, and
frequently have stenotic lesions within themselves. Despite having multiple vessels supplying the lungs,
pulmonary perfusion is disorderly and inefficient. The majority of babies have decreased blood flow to the
lungs with variable grades of hypoxemia. A few children, however, may have increased pulmonary blood
flow, though this is less common. Infrequently, a small PDA may be present, but in the classic type of
MAPCAs, PDA is not seen.