Atrial
septal defect
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| Fig. 4 |
Defects in the atrial septum are traditionally divided into four
types according to their location: secundum, primum, sinus venosus,
and coronary sinus defects. The general location of these defects
is shown in Fig. 4. It is important to note the type of defect or
defects present as each type is associated with other anomalies
and may require different surgical approaches. As shall be noted
later, primum atrial septal defects are very complex and argument
exists whether they should continue to be classified as atrial septal
defects alone.
Any atrial septal defect results in shunting of blood from the left
atrium into the right atrium. When the defects are large and the
left-to-right atrial level shunting is significant, the right ventricle
and right atrium enlarge significantly because of the increased
volume.
 |
| Fig. 5 |
Frequently, the actual defects may be identified. Ostium secundum
atrial septal defects are best visualized from the subcostal position.
As seen in Fig.
5, secundum defects are seen to lie centrally within the atrial
septum, and are bound on all sides by atrial septal tissue.
 |
| Fig. 6 |
Note in Fig.
6 that the normal position of the atrioventricular valves
is characteristic in that the tricuspid valve is always located
closer to the ventricular apex when compared to the normal location
of the mitral valve. In this normal position of atrioventricular
valves, an area of septal tissue is seen between the left ventricle
and right atrium. This is known as the atrioventricular septum.
In this entity, the atrial septum close to the atrioventricular
valves (known as the primum septum) is usually intact. Similarly,
the septum shared by the left ventricle and right atrium is also
intact. Thus, adequate interrogation of the atrial septum requires
visualization of its midportion and also portions adjacent to the
atrioventricular valves.
Secundum defects are areas adjacent to the atrioventricular valves
are best imaged from the subcostal approach where all these structures
in the center of the heart are readily identified. Using the subcostal
approach, most, but not all, secundum defects may be imaged. The
apical four-chamber view is usually unreliable for imaging of secundum
defects as the atrial septum lies parallel to the transducer beam
and an absence of septum in this view may be due to a "drop-out"
of targets. Thus, the presence of an atrial septal defect should
never be determined only from the apical four-chamber view.
Ostium primum defects involve areas of the atrial septum adjacent
to the atrioventricular valves. It is important to recognize that
primum defects are not only defects in the atrial septum. Rather,
the defect also involves the common atrioventricular septum, and
the result is deformity of the alignment of the atrioventricular
valves and may be associated with defects of the adjacent interventricular
septum.
 |
| Fig. 7 |
Such defects are also best examined from the subcostal approach
as seen in Fig. 7. One diagnostic hallmark of the disorder is the
absence of the atrial septum adjacent to the atrioventricular valves.
In addition, since the defect extends into the atrioventricular
septum, the level of insertion of both the tricuspid and mitral
valves onto the crest of the ventricular septum are equal. Thus,
in this disorder there is invariably a defect of the shared septum
and some possible or real malformation of the atrioventricular valves.
In contrast to secundum defects, primum defects are reliably visualized
from the apical as well as the subcostal approach, with atrioventricular
valve tissue forming their lower margin, and the secundum septum
forming their upper margin.
 |
| Fig. 8 |
It is important to note that a primum defect may extend into one
of the other atrioventricular valve leaflet. Fig. 8 shows a parasternal
short axis where the defect extends into the anterior mitral leaflet
giving the appearance of two anterior mitral valve leaflets. Such
a mitral anomaly is commonly known as a "cleft" mitral valve. Such
a cleft may, or may not, result in mitral regurgitation. Multiple
arguments exists whether this anomaly should properly be referred
to as a "cleft" and are beyond the scope of this discussion. In
short, the mere presence of an impressive cleft does not always
imply the presence of severe mitral regurgitation. As a consequence,
not all such cleft valves are deserving of repair.
 |
| Fig. 9 |
Sinus venosus defects are surprisingly difficult to detect as they
are located superiorly on the interatrial septum near its junction
with the superior vena cave. Traditional subcostal views are frequently
unrewarding and other nonconventional views must be attempted. Fig.
9 demonstrates a subcostal long axis of the inferior and superior
venae cavae where the defect is readily recognized. To reliably
obtain this view requires considerable practice and experience.
Supraclavicular and suprasternal views of this defect are also possible,
but less reliable. Because these defects are in an area so difficult
to access, they are frequently missed except by the most experienced
examiners.
Importantly these defects are frequently associated with anomalous
drainage of the right upper pulmonary vein to the right side of
the atrial septum. Careful examination of all views is required
to detect this complex anatomy.
Coronary sinus defects can be recognized by finding an interatrial
communication at the anticipated site for the coronary sinus. The
fossa ovalis may be intact, or there may be a secundum defect. Small
coronary sinus defects are easily missed, with the atrial septum
appearing intact in every view recorded. These defects are exceedingly
rare, difficult to detect with certainty and associated with anomalous
insertion of a left-sided superior vena cava into the coronary sinus.
It is important to precisely locate the position of the atrial septal
defect. Secundum defects are usually easy to repair. Primum defects
frequently include other structural anomalies of the atrioventricular
junction and valves, and therefore may require very complex preoperative
planning. Sinus venosus defects, located very high, require different
venous cannulation techniques than normally employed. The presence
of one atrial defect should alert the examiner to look carefully
for another, as two different types may be found in any one patient.
Indeed, echocardiographic location of atrial defects provides the
surgeon with detailed data by which to plan the operative approach.
 |
| Fig.10 |
The abnormal flow through the defects may be readily detected using
Doppler methods, particularly Doppler color flow imaging. If such
methods are not available, an echo contrast study can usually confirm
the diagnosis of atrial septal defect. To perform a contrast study,
3 to 10 ml of fluid (usually saline) is injected rapidly into an
antecubital vein. The amount of fluid injected depends on patient
size, with 10 ml the full adult volume. Mild agitation of the fluid
before injection may enhance the contrast. Rapid infusion through
a narrow canula lowers the pressure of the injectate so that some
dissolved gas comes out of the solution in the form of microbubbles.
These are carried to the heart by the bloodstream where they act
as strong ultrasound reflectors, causing a normally sonolucent,
blood-filled cavity to become opacified. This improves cavity delineation
and allows intracardiac shunts to be detected. The microbubbles
are fully absorbed in the lungs, so a peripheral venous injection
shows only a shunt with a right-to-left component. Fig. 10 shows
the presence of bi-directional shunting in a patient with an atrial
level communication.
The use of saline contrast for the identification of atrial level
shunting depends on the small degree of right-to-left shunting present
in all patients with interatrial flow communications. This shunting
occurs due to a short period of elevation of right atrial pressure
over left atrial pressure that occurs just after the onset of ventricular
systole. Saline contrast techniques are the most sensitive method
available for the detection of interatrial shunting and are more
reliable than angiography, green-dye, or oximetry. In fact, small
degrees of interatrial shunting, through presumed patent foramen
ovale may be seen in 12-to-17 percent of the otherwise normal population.
 |
| Fig.11 |
Continued experience with echocardiography indicates that other
anomalies of the interatrial septum may be identified. Normally,
the interatrial septum is relatively fixed in position and moves
passively with the movement of the entire heart. Occasionally, a
hypermobile septum may be seen (Fig. 11) and has been anatomically
linked with the presence of an aneurysm of the interatrial septum.
In this setting the septum is usually thin and may be fenestrated,
resulting in variable degrees of interatrial flow. In fact, when
such a finding is noted by echo, the majority of patients show small
degrees of interatrial shunting by saline microcavitation techniques.
The degree of shunting is rarely hemodynamically significant. It
should be noted, however, that recent evidence has indicated some
link between this disorder and the presence of embolic stroke in
older age population. The evidence is not strong enough as yet to
warrant operative intervention.
2004