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Page 2 of 19 Uppu. Vessel Plus 2021;6:21 https://dx.doi.org/10.20517/2574-1209.2021.101

[1]
accounting for about 15% and 35% of all congenital heart defects . In the absence of elevated pulmonary
vascular resistance (PVR), they result in left to right shunting, thus are part of acyanotic CHDs.
Hemodynamic manifestations are dependent upon the size, location, and number of defects. Relative
ventricular compliance drives the degree of shunting across an atrial septal defects (ASD), whereas the PVR
is crucial for shunting across the VSD . The pathophysiology of VSD is discussed elsewhere in this book,
[2-7]
and its general principles are applicable to ASD as well. In summary, hemodynamically significant ASDs
with pulmonary to systemic blood flow ratio (Qp/Qs) > 1.5 result in the right ventricular volume
[8]
[9]
overload , whereas the hemodynamically significant VSDs result in left heart volume overload . Patients
with smaller shunt lesions are usually asymptomatic and may incidentally be diagnosed later in life . Small
[10]
ASD and VSDs may spontaneously close in early life. Knowledge of the septal anatomy, identifying
associated lesions are crucial while planning their management [8,9,11] . This chapter will focus on noninvasive
imaging before, during, and post intervention.

ANATOMY OF ATRIAL SEPTAL DEFECTS
Atrial septum forms around the 4th week with septum primum growing towards the atrioventricular (AV)
canal from the roof of the common atrium; the resultant communication is ostium primum. As the
endocardial cushions fuse and join the septum primum, apoptosis at the superior margin of the septum
primum results in the formation of an ostium secundum atrial communication. A new septal infolding
appears on the right atrial aspect of the ostium secundum that ultimately becomes septum secundum. As
the septum secundum elongates, it becomes a stiff limbus of fossa ovalis. The septum primum remains thin
and has a flap valve mechanism that redirects oxygen-rich inferior vena caval blood towards the left atrium
across the patent foramen ovale. Postnatally increasing left atrial pressure closes the atrial septum [12,13] .
During the 5th week, the left vitelline veins regress along with the left horn of sinus venosus as the right
vitelline veins become dominant. By 10 weeks, the left cardinal vein regresses, and the left sinus horn is
represented postnatally by the coronary sinus and oblique vein of the left atrium that empty into the right
atrium [12,14,15] .

Atrial septal defects are classified into:

Ostium secundum atrial septal defect
Ostium secundum ASD (OS ASD) are the most common ASDs that result from a defect within the fossa
ovale or septum primum. As a result, they are centrally located within the atrial septum and are thus away
from vena cavae, right pulmonary veins, coronary sinus or the AV valves [Figures 1 and 2].

OS ASD can be single or multiple with fenestrations in the septum primum. Large OS ASDs can rarely
result in deficiency of limbus of the fossa ovalis, thus resulting in the superior location of the ASD closer to
the superior vena caval (SVC) entrance and are called high OS ASD. Inferior vena cava (IVC) confluent OS
ASD results in the deficiency of the septum primum all the way to the IVC entrance [2,7,16] [Figure 3].

Ostium primum atrial septal defect
Ostium primum ASD results from a defect within the endocardial cushion portion of the atrial septum at
the cardiac crux between the fossa ovale and the AV valve [Figure 4]. These defects account for about 10%
of the ASDs. It is a form of endocardial cushion defect and is often associated with AV valve
abnormalities .
[2,4]
Sinus venosus defect
Sinus venosus defects (SVD) are not true ASDs, and they account for 5%-10% of atrial septal