As an isolated lesion atrial septal defect is the 5th most common CHD (6% of all CHDs). However, it is seen in 33-50% of other congenital heart diseases. Secundum atrial septal defects are more common in females who tend to be tall and thin.

The two sinus horns continue to be a paired structures after fusion of the two endocardial tubes, later they fuse to give a transverse sinus venosus.  The entrance of the sinus venosus shifts rightwards to eventually enter into the right atrium exclusively and the veins draining into the left sinus venosus (left common cardinal, umbilical and vitelline veins) degenerate and the left sinus venosus will become smaller as it will only drains the venous circulation of the heart, becoming the coronary sinus. 

 The sinus venosus orifice into the right atrium is slit like and to the right of the yet to develop septum primum.  The sinus venosus now connecting to the right atrium will assume a more vertical position and the sino-atrial junction will become guarded by two valve like structures resulting from the invagination of atrial wall at the right and left sino-atrial junction. This orifice will eventually enlarge and the superior and inferior vena cava and the coronary sinus open separately and directly into the right atrium.  The right and left sinoatrial valves join at the top forming the septum spurium, this septum and the two sinoatrial valve like structure obliterate and are not appreciated in the mature heart.

 Atrial septation starts when the common atria becomes indented externally by the bulbus cordis and truncus arteriosus, this indentation will correspond internally with a thin sickle shaped membrane developing in the common atrium at day 35.  This membrane divides the atrium into right and left chambers.  It grows from the postero-superior wall and extends towards the endocardial cushion of the atrioventricular canal.  This septum is the septum primum.  The septum primum initially has a concave shaped edge growing towards the atrioventricular canal, this orifice connecting the two atria is called the ostium primum.  As the superior and inferior endocardial cushions fuse, thus dividing the atrioventricular canal into a right and left orifice, the concave lower edge of the septum primum fuse with it thus obliterating the ostium primum.  However, just before this happens fenestrations appear in the postero-superior part of the septum forming the ostium secundum, therefore maintaining a communication between the two atria.  The ostium secundum and superior vena cava later acquire a more anterosuperior position, although they maintain their relationship to each other.  This is achieved through the growth of the atria.

 These fenestrations then coalesce and form a larger fenestration.  Meanwhile, another sickle shaped membrane develop on the anterosuperior wall of the right atrium, just right to the septum primum and left to the sinus venosus valve.  It grows and covers the ostium secundum which continue to allow blood passage since the two membranes do not fuse.  The septum secundum grows toward the endocardial cushion until leaving only an area at the posterosuperior part of the interatrial septum where the septum primum continue to exist as the foramen ovale membrane.  The septum primum disappears from posterosuperior portion of interatrial septation and the edge of the septum secundum forms the rim of the fossa ovalis. This is completed around day 42 of development.

True anomalous pulmonary venous connection with the inferior vena cava type sinus atrial septal defect is seen in scimitar syndrome. Scimitar syndrome is when one or more of the right pulmonary veins drain into the IVC with hypoplasia of right lung with or without pulmonary blood supply of the affected lung tissue from the descending aorta instead of the pulmonary circulation, this may be associated with an atrial septal defect.

Rarely does anomalous pulmonary venous drainage occur with an intact atrial septum.

• Primum atrial septal defect is when the atrial septum does not attach to the atrio-ventricular valve apparatus leading to an inter-atrial communication just above the atrioventricular valve. This may be associated with atrioventricular valve defect or cleft of the mitral valve.
• Common atrium m is when the entire atrial septum is missing. Right to left shunting occur with this anomaly leading to cyanosis. This is associated with mitral valve prolapse(MVP).

 Shunting across an atrial septal defect is left to right causing increased pulmonary blood flow. The amount of left to right shunting is determined by the right ventricular compliance which is affected by the pulmonary vascular resistance (PVR). PVR is high in the newborn period and consequently there is less left to right shunting.

Pulmonary vascular obstructive disease (PVOD) rarely manifest in atrial septal defect before 20 years of age. It is seen in 5-10% of adults with unprepared atrial septal defects.

 Asymptomatic when atrial septal defect is small. Larger defects will cause pulmonary edema and congestive heart failure (CHF) causing easy fatigability and shortness of breath.

On examination there is a hyperactive precordium with a prominent RV impulse. Auscultation reveals a prominent first heart sound. Second heart sound is split wider than normal and there is no respiratory variation since the blood flow through the pulmonary valve is always increased due to the left to right shunting at the atrial level causing a delay in pulmonary valve closure throughout the respiratory cycle. A systolic (Crescendo-decrescendo) murmur is heard at the left upper sternal border, due to increase blood flow across the pulmonary valve and in larger atrial septal defect a diastolic early murmur at the left lower sternal border, due to increase blood flow across the tricuspid valve is heard.

The right atrium is enlarged due to volume overload. This will manifest as tall P waves (more than 2-3 mm). RVH is also noted, typically as rsR’ pattern of the QRS complexes in the right chest leads.

The atrial septal defect is seen well by 2D-echo. In the four chamber view the atrial septum is parallel to the ultrasound waves and a false "Drop out" in the thin septum primum which may simulate a secundum atrial septal defect. Therefore, it is best to confirm this in a view were the atrial septum is perpendicular to the sound waves, such as the subcostal view. Color Doppler shows left to right shunting across the atrial septal defect unless the pulmonary vascular resistance is high resulting in higher pressure in the right atrium compared to the left atrium leading to right to left shunting at the atrial level. The right atrium and right ventricle will appear dilated. Right ventricular dilation may lead to flattening of the interventricular septum. In adults with poor echogenic window TEE is useful to visualize the atrial septum.

 

Subxiphoid view showing secundum ASD

Not indicated for diagnostic purposes, unless the patient is an adult and elevated pulmonary vascular resistance (PVR) is suspected.  In the cath lab the PVR is measured and if elevated oxygen, NO, prostaglandin, adenosine or milrinone are given to assess the reversibility of the high PVR. Patients with high and irreversible PVR do not benefit from atrial septal defect closure, which may be dangerous since the atrial septal defect acts as a vent for the blood to shunt right to left when the PVR becomes excessively high as with straining or exercise.

On the other hand therapeutic closure of ASDs, particularly secundum ASD is currently the therapeutic method of choice.

 Atrial septal defects are typically asymptomatic.  Congestive heart failure (CHF) due to increase pulmonary blood flow may be presetn. particularly with larger defect.  This could be managed with anti-CHF medications such as digoxin,  and diuretics. Therefore atrial septal defects should be closed only when they are unlikely to close spontaneously. The size of the atrial septal defect and the age of the child should be considered when making such a decision. Prognosis of atrial septal defects assessed in the neonatal period:

• Atrial septal defects < 3 mm: 100% close by 18 months
• 3-5 mm: 80% close at 12 months
• 5-8 mm: 80% close at 15 months
• > 8 mm: Most probably will not close

All ASD patients with a Qp:Qs ratio of 1.5:1 or more should have their ASDs closed, particularly if they are three years or older. 

Morbidity and mortality of atrial septal defect surgical closure is almost zero, while 10-15% of urepaired atrial septal defects will cause pulmonary vascular obstructive disease causing death within few years. Therefore all atrial septal defects should be repaired if found in adults.

Subacute bacterial endocarditis (SBE) prophylaxis should be given to patients post-surgical repair of atrial septal defects for 6-9 months

Closure of atrial septal defect by a catheter delivered device continue to be in the experimental phase.

Complications of unrepaired atrial septal defect include:

• Pulmonary Vascular Obstructive Disease, if irreversible will cause right to left shunting at the atrial septal defect resulting in cyanosis.
• Atrial dilation and fibrosis, leading to arrhythmias, such as atrial flutter and fibrillation.
• Paradoxical embolization, leading to stroke