CONGENITAL HEART DISEASE
ISOLATED ATRIAL SEPTAL DEFECT EMBRYOLOGY, CLINICAL
PRESENTATION, AND DIAGNOSIS
Dr. Muhammed Numan* Pediatric Cardiology Section,
Department of Cardiology and Cardiovascular Surgery,
Hamad Medical Corporation, Doha, Qatar
This review summarizes the embryological development
of the atrial septum and the abnormality resulting
from improper partitioning of the atrium leading
to a communication between the right and left
atria. The pathophysiology and clinical presentation
of isolated atrial septal defect are discussed.
Confirmatory diagnosis is based on echocardiography,
mainly two-dimensional and color Doppler echocardiography.
(Heart Views. 2002;3(2):63-67) © 2002 Gulf Heart
Association.
(Heart Views. 2002;3(2):55-60) © 2002 Gulf Heart
Association.
Key Words:
atrial septal defect
embryology
echocardiography
Congenital defects of the atrial septum are common
and they maybe located in different anatomic portions
of the atrial septum. Atrial septal defects maybe
solitary cardiac anomalies or they may be associated
with other congenital cardiac anomalies. The size
may vary greatly. The functional consequences
are related to the anatomic location of the defect,
the size of the defect, and the presence or absence
of other cardiac anomalies. The incidence of atrial
communication between the left and right sides
vary according to the age of the patient. In the
fetus, the incidence of atrial communication is
100% since the communication is necessary to keep
them alive. Patent foramen ovale is also common
in humans and remains through out life in 20%.
The true defects of the atrial septal wall constitute
6-10 % of all congenital heart defects (i.e. 1
in 1200 live births). Females have higher susceptibility
for these defects with female to male ratio of
2:1.
To understand the classification of atrial septal
defects (ASD), it is necessary to review the development
of the atrial septum. Figure 1 shows the stages
of the development of the atrial septation. r
Fig.1
Fig.1. Normal development of the
atrial septum. See text for details.
During the first 23 days of the
embryo’s life, the atrial chamber is a common
one and connected to the sinus venosus to drain
the blood coming from right and left vens horns.
After completion of cardiac looping by the third
week, atrial septation begins by developing the
septum primum partly from the endocardial cushion
tissue. It is important to note that there is
always atrial communication between the left and
right sides throughout all developmental stages
to maintain adequate right to left shunt and oxygenate
the developing brain tissue. Even before the completion
of the septum primum, fenestrations start to take
place in this septum to allow blood to flow freely
though it. By the early fifth week of fetal life,
the septum secondum starts to appear from the
superior-posterior aspect of the mid-atrial wall
and extends inferiorly and anteriorly, leaving
a flap of the septum primum in the mid-portion
to be uncovered. This flap is pushed by the blood
flow coming from the umbilical vein and remains
open till the end of pregnancy (Figures 2 and
3).
Fig. 2
Fig. 2. Fully developed atrial
septum in a 7-week fetus. (O1= ostium primum;
S1= septum primum; S2=septum secondum; FO= foramen
ovale).
Fig. 3
Fig. 3. Fetal echocardiogram showing
the patent foramen ovale.
Fig. 4
Fig. 4. Anatomic locations of different
types ASD in relation to other cardiac structures.
(IVC = inferior vena cavae; SVC = superior vena
cavae; TVL = tricuspid valve leaflets).
Fig. 5
Fig 5. Pathological specimen of
primum ASD
Fig. 6
Fig. 6. Pathological specimen of
secondum ASD.
Fig. 7
Fig. 7. Partial anomalous pulmonary
venous drainage with sinus venosus ASD.
Normally the left atrial pressure is slightly
higher than right atrial pressure by 2- 5 mmHg
due to less compliant left atrial wall compared
to the right. As a result the blood flows from
left to right across the ASD with consequent right
ventricular volume overload. The pulmonary flow
will increase more than systemic and takes more
time for the right ventricle (RV) to empty through
the pulmonary valve, resulting in delayed pulmonary
component of the second heart sound (wide split
2nd heart sound). Because of increased pulmonary
flow across the valve, a flow murmur (low frequency
ejection systolic murmur) will be heard over the
Fig. 8. Pathophysiological changes
of ASD.
left 3rd intercostal space, which is accentuated
with inspiration. The pulmonary vascular beds
initially will accommodate the extra flow with
no increase in pulmonary resistance. After long
standing increase in pulmonary flow, certain patients
(more in females) will have hyperplasia of the
pulmonary arterioles and smooth muscles resulting
in increased pulmonary artery pressure (pulmonary
hypertension) and right ventricular hypertrophy.
Rarely there will be paradoxical reversal of the
shunt across the atrial defect (i.e. right to
left shunt) in some patients during valsava maneuvers
or deep inspiration. If there is a small clot
in the systemic veins – which will normally be
filtered by the lungs – the danger of systemic
embolization to the brain and other vital organs
will be higher especially in presence of increased
right ventricular pressure. Figure 8 summarizes
these changes.
Usually atrial septal defects (ASD) are discovered
incidentally during routine physical examination.
The wide split second heart sound and pulmonary
flow murmur are usually noted by most primary
care physicians and cardiac referral and evaluation
will reveal the defect. A mid-diastolic murmur
at the left lower sternal border indicates a large
size ASD. Isolated ASDs of moderate and large
size do not cause major symptoms in most cases
during infancy and early childhood. Failure to
thrive on the basis of the ASD alone is rare.
Occasionally some patients may present with easy
fatigue or palpitations. Others may remain asymptomatic
for several decades. Left-to-right shunting tends
to increase with age in many patients. The large
shunts present in many older patients cause stretching
of the atria, which predisposes them to atrial
arrhythmias such as atrial fibrillation, atrial
flutter and tachycardia. These arrhythmias increase
in incidence with age and are a major cause of
morbidity and mortality. Pulmonary hypertension
is not uncommon in large and moderate defects.
Usually this complication occurs in the second
decade of life and more in females. Pulmonary
hypertension rarely occurs in infants with large
ASD.
Chest X-Ray
The chest X-ray in patients
with atrial septal defect and sizable left-to-right
shunts generally shows cardiac enlargement and
increased pulmonary vascularity with a dilated
pulmonary trunk and central branches ((Figure
9).
Fig.9. Chest x-ray of a patient
with ASD, showing enlarged right atrium, prominent
pulmonary conus and increased pulmonary vascular
markings.
The electrocardiogram (ECG) may be normal with
an uncomplicated ASD and small shunt. However,
incomplete right bundle (with a rSR’ or rsR’ pattern
in the right precordial leads), right axis deviation,
and right ventricular hypertrophy may be present,
and atrial arrhythmias, particularly atrial fibrillation,
are commonly seen. A 12- Lead ECG with significant
ASD will usually show right axis deviation, right
atrial enlargement and right ventricular hypertrophy
(Figure 10).
Fig.10. 12 lead ECG of an ASD patient
showing right ward axis, right atrial enlargement
and right ventricular hypertrophy.
Confirmatory diagnosis usually will
be obtained by transthoracic echocardiogram and
occasionally by transesophageal echo. Rarely angiograms
will be needed to confirm the diagnosis. Figures
11- 14 show these diagnostic modalities. The M-mode
echocardiogram in patients with classic secundum
ASD usually shows right ventricular enlargement,
and paradoxical motion of the interventricular
septum is present in many cases. Transthoracic
two-dimensional echocardiography (2DE) can provide
direct noninvasive visualization of all types
of atrial septal defect. The reliability of 2DE
in demonstrating the characteristic dropout in
the atrial septum is best when the axis of the
echo beam is perpendicular to the atrial septum.
For most defects the subcostal approach provides
such a perpendicular angle. In addition to direct
visualization of the atrial septum, 2DE often
demonstrates enlargement of the right atrium,
right ventricle, and pulmonary arteries. Color
Doppler also provides useful diagnostic information
in these patients. It allows for the direct visualization
of the flow across the ASD. If additional confirmation
of diagnosis is desired, peripheral contrast echocardiography
using agitated saline is effective in demonstrating
right to left shunting across the ASD. A right-to-left
shunt can be detected by direct visualization
of microcavitation bubbles in the left atrium
and left ventricle (Figure 14). A left-to-right
shunt can be detected as a negative contrast washout
effect in the right atrium if good opacification
of the atrium is achieved. Evaluation of the atrial
septum with transthoracic echocardiography may
be suboptimal in older patients with poor echocardiographic
windows. In such patients, transesophageal echocardiography
can provide excellent definition of the atrial
septum. In addition, it is helpful in the sizing
of defects, the diagnosis of sinus venosus defects,
and the assessment of associated congenital anomalies
or other abnormalities such mitral valve prolapse
or partial anomalous pulmonary venous drainage.
Transesophageal echocardiography can help to guide
proper device placement during catheter atrial
septal defect occlusion procedures. Transesophageal
echocardiography can be successfully performed
in most adults with light sedation. However, infants,
children, and adolescents do not tolerate the
procedure without heavy sedation or general anesthesia.
Fig.11
Fig.11. Transthoracic echo of Primum
ASD, note the defect in the lower Portion of atrial
septum.
Fig.12
Fig. 12. Trans esophageal echo
shows secondum ASD
Cardiac catheterization is not essential
for diagnosis when noninvasive tests clearly demonstrate
the presence of an ASD. However, when pulmonary
vascular disease is suspected, determination of
pulmonary vascular resistance by catheterization
is indicated.
Most children and adolescents with an ASD are
asymptomatic even in the presence of large shunts.
While the defect is often asymptomatic until adulthood,
complications of an undetected lesion include
irreversible pulmonary hypertension, right ventricular
failure, atrial arrhythmias and paradoxical embolism.
Transthoracic imaging and an M-mode echocardiogram
frequently provide the first confirmation of the
diagnosis of ASD. The
Fig. 13
Fig. 13. Left atrial angiogram
showing the ASD pointed to by the arrowhead.
Fig. 14
Fig.14. Bubble agitated saline
study using Transesophageal echo showing crossing
of the saline bubbles to the left atrium confirming
right to left atrial shunt.
transesophageal approach is superior
to transthoracic echocardiography in its ability
to image the interatrial septum and is extremely
accurate in the diagnosis of all three types of
septal defects.
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