Atrial septal defect
An atrial septal defect (ASD) is an opening in the atrial septum allowing blood to shunt
between left and right atria. When the defect involves tissue of the septum primum at or
around the area of the foramen ovale, it is a secundum ASD. Defects of the sinus venosus -
the embryological origin of the junction of superior and inferior vena cava into the right
atrium - are known as sinoseptal defects or sinus venosus ASDs. The latter defects are
often associated with partial anomalous pulmonary venous return. When the defect involves
endocardial cushion tissue, it is referred to as a primum ASD.
ASDs are a common type of congenital heart disease, though their precise incidence is not
well understood. An isolated ASD may present at any age, since the resulting murmur is
often overlooked. In children with congenital heart disease, it is estimated that as many
as 33-50% will have an associated ASD as part of their complex of disease. Interestingly,
the prevalence of ASDs increases in populations living at higher altitudes.
General comments
The patent foramen ovale is one end of the spectrum of ASDs while the common atrium is at
the other end. The most common defect is the secundum ASD which is located at or in the
region of the foramen ovale.
The size of the ASD is directly related to the clinical features in the patient. While
symptoms may occur in the neonate or infant, it is uncommon for ASDs to present in the
first year of life. Most commonly, these defects are first noted at the time of physical
examination in an otherwise asymptomatic well child. ASDs are said to be more common in
females than males.
Primum ASDs belong to the family of atrioventricular canal defects, also referred to as
endocardial cushion defects or atrioventricular septal defects (AVSDs). Those defects are
typically moderate to large in size and therefore allow significant shunting. Like the
complete atrioventricular septal defect, these defects are almost universally associated
with abnormalities of the left atrioventricular valve including clefts, mitral
regurgitation and other complicating features.
Sinoseptal defects involve the area of the atrial septum derived from the sinus venosus.
This includes defects at the junction of the superior vena cava and the right atrium,
although these defects may also occur at the junction of the inferior vena cava and the
right atrium. The right upper pulmonary veins typically enter the left atrium superiorly
just to the left of the atrial septum and sinus venosus region. When a defect of the
superior sinus venosus exists, the flow from these veins may be directed toward the right
atrium through the sinus venosus defect. Alternatively, these veins may truly be anomalous
in their drainage and enter the right atrium directly. These defects are known as sinus
venosus defects.
A third, and uncommon, type of sinoseptal defect is the unroofed coronary sinus. These
defects allow communication between left atrium and the coronary sinus. Depending on
relative pressures, these latter defects can allow blood to move from the left atrium to
the right atrium through the coronary sinus or vice versa.
Hemodynamics:
In normal cardiac development, the septum primum grows downward from the top of the atria
toward the central endocardial cushion tissue. As this process completes itself the primary
foramen becomes smaller and is eventually closed. Prior to its closure however,
perforations of the septum primum occur and these coalesce to form the ostium secundum in
the central portion of the septum primum. The septum secundum then begins to grow downward
to the right of the septum primum and toward the central portion of the heart. This growth
continues until the septum secundum covers the superior rim of the ostium secundum or
foramen ovale.
During fetal life, systemic venous return from the lower part of the body, including the
newly oxygenated blood returning from the placenta, is directed toward the foramen ovale by
a flap of tissue called the Eustachian valve. This allows the relatively well-oxygenated
blood to enter the left heart circulation so that it may be delivered to the coronary
arteries as well as those vessels supplying the developing central nervous system.
The foramen ovale is maintained patent in the fetus because right atrial pressure is higher
than the left atrial pressure. At birth, the organ of oxygenation is transferred from the
placenta, which is removed, to the lungs which become aerated. This results in a dramatic
increase in the volume of blood delivered to the lungs. As a result, the volume of blood
return from the lungs to the left atrium through the pulmonary veins increases, and this
leads to an increase in left atrial pressure which forces the septum primum against the
septum secundum, thus effectively closing the foramen ovale. This fusion is complete in
most infants by a month of age, although as many as 20% of adults may have a probe patent,
or intermittently patent foramen ovale.
In the transition from fetus to neonate, the relative compliance of the right ventricle and
left ventricle are similar. As a result, the right and left atrial pressures are relatively
equal and any flow that results from a communication between the right and left atrium may
be bidirectional, with a slight net left to right shunt predominately.
During infancy, the right ventricle gradually becomes more compliant than the left due to
the markedly lower pulmonary vascular resistance. This allows the more compliant right
ventricle to accept more volume than the left ventricle and there is a marked increase in
left to right shunting through any given atrial septal defect.
In fact, in patients with moderate to large atrial septal defects, the QP:QS ratio usually
exceeds 1.5:1 and may be as high as 2.5:1 in some patients. Factors that alter the
compliance of the right or left ventricle (i.e. right or left ventricular outflow
obstruction and hypertrophy) will impact the atrial shunting patterns in the patient that
has an atrial septal defect.
The degree of atrial shunting and thus the presentation of the patient, therefore depends
on the size of the defect, the cardiac hemodynamics and the resulting physiology.
Clinical:
Patients with an ASD are generally asymptomatic through infancy and childhood. Only rarely
are symptoms of pulmonary overcirculation, frequent respiratory infections and congestive
heart failure seen in infants who have no other cardiovascular abnormality. Patients
diagnosed later in childhood usually come to the cardiologist because of a systolic murmur
appreciated by the primary care provider. Symptoms typically are more apparent in the older
adolescent and young adult and these usually involve dyspnea on exertion, easy fatigue
compared to peers, palpitations or sustained atrial arrhythmias.
The physical exam usually reflects the size of the ASD and the relative compliance of the
right and left ventricle. Small ASDs are extremely unlikely to be associated with any
historical or physical findings. If the defect is of moderate to large in size, the
precordium may demonstrate a hyperactive impulse. With inspection, there may be a
prominence of the left chest if the volume of shunt is large. On palpation of the chest
wall, there may be a ventricular lift, particularly apparent with expiration, in the
patient with a large volume shunt. The first heart sound may be accentuated or normal and
the second heart sound is widely split without significant variation with respiration. This
is in contrast to the normal child in whom the second heart sound splitting varies with
respiration. It is thought that the wide split in the patient with an ASD results from a
relative delay in emptying of the volume loaded right ventricle. The lack of variation of
the split most likely results from the free communication between the two atria, which
allows for equalization of the influence of respiration on both the right and left
ventricle.
There is often a systolic murmur (grade 3 or less) that has an outflow quality mimicking
that of pulmonary stenosis, and may in fact emanate from the turbulence across the
pulmonary valve resulting from the increased flow imposed by the ASD. The murmur is
typically loudest at the mid to upper left sternal border, and may radiate posteriorly.
With a large left to right shunt through an ASD, there may be an early to mid diastolic
rumble at the lower left sternal boarder, which results from turbulence across the
tricuspid valve.
Chest X-ray may demonstrate a normal cardiac size in the patient with a small or moderate
atrial septal defect. Pulmonary vascularity is likely to be increased in these patients.
With increasing size of the atrial shunt, the right atrium and right ventricle may appear
dilated. The former is best seen on the posteroanterior view, while the latter is most
evident on the lateral view with a noticeable reduction in the retrosternal clear space. As
the volume of left to right shunting increase, the pulmonary vascularity will become more
prominent and the main pulmonary artery segment will be enlarged.
ECG typically shows a sinus rhythm with a normal P wave axis in patients with a secundum
ASD. The P wave axis may be abnormal when there is a sinus venosus ASD. The normal
regression of relative right ventricular forces with age does not typically occur in
children with an ASD. There is usually right axis deviation and there is often an
incomplete right bundle branch block. In older children and adults, complete or incomplete
right bundle branch block pattern is seen in 90% of patients. Patients with a primum atrial
septal defect typically have a superior frontal QRS axis in addition to the findings noted
above.
Echo exam
Almost all atrial septal defects can be detected by the combination of two-dimensional
real-time echo and color Doppler flow mapping, although defects of the sinus venosus are
notoriously difficult in some patients. Conversely, secundum atrial defects are very well
profiled from the subcostal position, as the plane of ultrasound interrogation is
perpendicular to the plane of the atrial septum. From this view, atrial septal defects can
be sized and with the use of color Doppler flow mapping, a qualitative assessment of
shunting and the direction of shunting can be made.
The four-chamber apical view is a useful view to assess the hemodynamic effects of the ASD
imposed on the right ventricle, as its size and wall thickness can be evaluated. It is a
less reliable view for assessing the atrial defect itself since the ultrasound plane is
parallel to the atrial septum. Because of this, the images obtained may give a false
assessment as to the size and extent of the atrial defect, as the plane of the atrial
septum may move in and out of the plane of ultrasound.
The parasternal views often demonstrate the relative volume load imposed on the right
ventricle. In particular, the short axis view may demonstrate the rounded contour of the
interventricular septum during systole and the flattened septum during diastole resulting
from the right ventricular volume overload.
Because atrial septal defects can be associated with other types of congenital heart
disease, it is imperative that a complete and thorough echocardiographic examination be
performed.
Transesophageal Echocardiography (TEE) can be extremely useful in both the diagnosis and
management of the patient with atrial septal defect. In patients, particularly those that
are older with poor acoustic windows, TEE can provide extremely accurate assessment of the
anatomy and physiology of the atrial septum. TEE may also be the diagnostic study of choice
when the sinus venosus defect is suspected, but unable to be confirmed with transthoracic
echocardiography. Moreover, TEE may be important in identifying the pulmonary venous return
in such patients where as many as 90% may have anomalous return of the right pulmonary
veins.
Treatment:
Once an atrial septal defect has been detected, it should be closed at a convenient time
for the family. Usually this is prior to school age for young children. Exceptions to this
are the primum atrial septal defect and sinus venosus defects that may be associated with
morbidity by preschool age, and are thus usually closed in the first few years of life. The
primary indication to close an atrial septal defect is to prevent eventual right
ventricular dysfunction from chronic volume overload, risk of paradoxical thrombus and
pulmonary vascular obstructive disease. Ostium secundum defects in isolation do not require
antibiotic prophylaxis.
Two methods of closure currently exist for the common secundum atrial defect - surgical or
transcatheter closure. Surgical repair should carry an extremely low mortality and should
yield few incomplete closures. More recently, techniques employing transcatheter devices
have been developed. With this procedure, patients can undergo ASD closure on an outpatient
basis or with a short inpatient stay. Factors negating the transcatheter approach include
large atrial defects, close proximity of the defect to the atrioventricular valves and
sinoseptal defects. In addition, patients must be of sufficient size and weight to
accommodate the transvenous sheath delivery system.