and Structural Heart Disease
|Defect in the atrial septum creates an
opportunity of blood to shunt in a an abnormal fashion.
Type of ASD depends upon location of defect within the atrial
septum. ASDs may be associated with other congenital heart
|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 atria of the mature
heart are derived from more than one origin.
The trabeculated portions (appendages) of the right and left
atria are from the primitive atria, while the smooth walled
posterior portion of the left and right atria originate from
incorporation of venous blood vessels.
The posterior aspect of the left atrium is formed by the
incorporation of the pulmonary veins, while the posterior smooth
portion of the right atrium is derived from the sinus venosus.
he two sinus horns continue to be a paired structures after fusion of
the two endocardial tubes, later they fuse to give a transverse sinus
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.
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.
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.
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.
here are many types of atrial septal defects,
- Secundum atrial septal defect is due to a
defect in the foramen ovale membrane
- Sinoseptal defect is involves the atrial
septum between the sinus venosus component of the two atria, this
- Sinus venosus atrial septal defect, a
defect of the atrial septum adjacent to the superior vena cava
entrance into the right atrium. The right upper pulmonary vein
typically would drain into the right atrium. It may or may not be
aberrant in itís communication to the left atrium
- Inferior vena cava sinus
venosus atrial septal defect is when the defect is inferior &
closer to the right atrium-inferior vena cava junction
- Unroofed coronary sinus, this will lead
to a communication (and shunting) between the LA and the coronary
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).
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.
Factors causing increase pulmonary blood flow
(PBF), and consequently early presentation include:
- Additional shunts, e.g. VSD, PDA
- Myocardial dysfunction.
- Anatomically small LV
- Systemic hypertension
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.
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 rsRí pattern of the QRS complexes in the right
|The left to right shunting at the atrial
level will cause increase blood volume in the right atrium and right
ventricle resulting in cardiomegaly, in addition the increase pulmonary
blood flow causes the pulmonary vasculature to be more prominent.
|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.
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.
the other hand therapeutic closure of ASDs, particularly secundum ASD is
currently the therapeutic method of choice.
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.
The preferred method of
closure with secundum ASDs is with a device in the cardiac
catheterization laboratory. The Amplatz device is the easiest
and most effective at this time. It is also the only FDA
approved device. ASDs without an adequate rim for the closure
device to hold on to, such as in some large secundum ASDs, primum
ASDs and sinus venosus ASDs may not be amenable to closure with a
device. In such cases, if the Qp:Qs ratio is > 1.5, then
surgical closure should be considered.
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
- 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