DEFECTS OF THE
ATRIAL SEPTUM
 |
| ATRIAL SEPTAL DEFECTS |
The atrial septum normally consists of two overlapping, closely adjacent components. Each forms an incomplete partition. The right-side component, corresponding to the embryonic septum secundum, is muscular and firm and has a posteroinferior oval-shaped opening, the foramen ovale. The left-side component, derived from the embryonic septum primum, is fibrous and thin and has a somewhat round opening anterosuperiorly, the ostium secundum. Together, the two components act as a one-way flap valve, allowing the flow of blood from right to left (normal before birth) but not from left to right. After birth, with the establishment of pulmonary circulation, the increased amount of blood entering the left atrium elevates the pressure in that chamber, thereby closing the flap valve. In most cases, this functional closure is eventually followed by anatomic closure; that is, the two components of the septum fuse. In the minority of cases where fusion fails, an increase in the right atrial pressure due to congenital cardiac anomalies, or any other condition that elevates right ventricular and right atrial pressure, causes the right atrial blood to flow again into the left atrium. Such a probe-patent foramen ovale, however, should not be considered a form of atrial septal defect; it causes no hemodynamic abnormalities by itself. In ASD there is an abnormal opening in the atrial septum allowing blood to flow either way; a predominantly left-to-right shunt usually exists. With associated anomalies or other conditions tending to increase right atrial pressure, the shunt is always from right to left, as in tricuspid valve atresia, or an initially left-to-right shunt reverses, as occurs after pulmonary vascular changes with pulmonary hypertension.
Of the two main types of ASD, the secundum
type is more common and is one of the most frequently seen congenital
cardiac anomalies (see Plate 5-6). The
normal resorptive process that leads to the formation of the ostium secundum in
the embryo is exaggerated, and most of the septum primum disappears. Most secundum ASDs are large, and the resulting
left-to-right shunt is generally substantial, causing a several-fold increase in pulmonary blood flow. Both the
right atrium and the right ventricle dilate and hypertrophy, and the pulmonary
arteries enlarge considerably. Even though pulmonary venous return and
therefore blood flow in the left atrium are increased, the left atrium does not
enlarge because resistance to emptying into the left ventricle is higher than
to the right ventricle, and thus it can readily “bleed off” through the defect into the more compliant right atrium. Systemic
blood flow is generally at a low-normal rate or occasionally diminished.
The clinical features of ostium secundum ASD are not remarkable, considering the size of the defect and the magnitude of the shunt (see Plate 5-7). Only rarely are infants with ASD symptomatic; in fact, this anomaly is so well tolerated that disabling symptoms usually do not occur until adulthood, when it is the most common congenital cardiac defect. In children and young adults, the only symptoms are mild fatigability and dyspnea on exertion. Many patients are not even aware of the significance of ASDs, recognizing the defects only in retrospect after it has been corrected surgically and they have more energy and breathe more easily. Growth and development are generally normal. The heart is only slightly or moderately enlarged, and a thrill is extremely uncommon in isolated ASD. A left lower parasternal “heave” is often present, but a precordial bulge is seen only in patients with marked cardiomegaly. The murmur heard in ASD is not loud but rather systolic, medium pitched, and of the ejection type, best heard at the base to the left of the sternum. The ASD murmur is caused not by the left-to-right shunt itself but by the increased amount of blood passing through the otherwise-normal pulmonary valve. A similar mechanism is thought to cause the faint, short diastolic murmur heard in the tricuspid valve area (tricuspid flow murmur). Characteristically, the second sound at the upper left sternal border is split, and the splitting is fixed; unlike the variable splitting heard in normal children, the interval between the aortic (A2) and pulmonic (P2) components of S2 remains constant through all phases of respiration; P2 is often louder than A2. An ejection click is rare in children but may be present in adults, indicating the presence of pulmonary hypertension.
 |
| SURGERY FOR ATRIAL SEPTAL DEFECTS |
The classic chest radiograph features
of secundum ASD are mild to moderate cardiomegaly, prominent right-sided heart
border caused by right atrial enlargement, evidence of right ventricular (RV)
enlargement, prominent pulmonary artery segment at left upper heart border
caused by dilatation of the pulmonary trunk, and marked hypervascularity of the
lung fields. On fluoroscopy, distinct hilar pulsations can easily be seen,
called the “hilar dance.” The left atrium is never enlarged, and therefore the
esophagus is not displaced posteriorly.
The ECG features are usually
unmistakable. Right-axis deviation is the rule, although the axis may be
normal or rarely even oriented to the left. Prominent, peaked P waves may be
seen in leads II and aVF and in the right
precordial leads. Most cases show an rSr′ or an rSR′ pattern over the right precordium, indicating
mild to moderate RV enlargement. An rR′, an Rs, and particularly a qR pattern, not
typically seen in children, indicate more severe RV hypertrophy, as seen with
the development of pulmonary vascular changes and hypertension.
At cardiac catheterization, it is
usually easy to enter the left atrium through the secundum defect, particularly
when this is carried out from the femoral venous circulation.
A distinct increase in oxygen content of the right atrium and an early
opacification of the right atrium, on selective left atrial angiocardiography,
demonstrate the presence of an ASD. RV and pulmonary artery pressures are
normal or only slightly elevated in most children and young adults. Pulmonary
hypertension may be present occasionally in early infancy or late in the
disease course. A slight (10-15 mm Hg) pressure gradient across the pulmonary
artery valve is common and does not, as a rule, indicate organic pulmonary valve stenosis, because it disappears after
surgical closure of the defect.
Generally the clinical, chest radiograph, eECG, and
cardiac ultrasound findings are so characteristic that many cardiologists do
not hesitate to refer ASD patients to a surgeon, without catheterization or
angiocardiographic studies. Medical treatment is often not required, but
symptoms. (e.g., arrhythmias) should be treated as in any other cardiac
condition. In children, cardiac failure rarely occurs, except in infants with
very large defects. Bacterial endocarditis, the bane of many types of CHD, is
extremely rare in uncomplicated ASD.
Surgical treatment ensures complete
cure and should always be advised because surgery is easily done, employing
cardiopulmonary bypass, and carries minimal risk. The ASD can usually be closed
by direct suture. Because the defect is so well tolerated, surgery can safely
be postponed until the child is 8 to 10 years or older. Percutaneous approaches
using a septal occluder device (e.g., Amplatzer) have now become the
transcatheter procedures of choice in patients, replacing the need for
cardiopulmonary bypass (see Plate 5-8).
 |
| SEPTAL OCCLUDER DEVICE |
Extremely large secundum defects,
involving practically all the septum and referred to as common atrium, are
seldom seen. The symptoms tend to be more pronounced, and slight arterial
desaturation may be present because of easy mixing of blood at the atrial
level. A prosthesis, consisting of a free pericardial graft, is usually necessary to close the defect (see
Plate 5-6).
In the sinus venosus type of ASD,
the region of the fossa ovalis is normal, the defect being located high
in the septum at the ostium of the superior vena cava, which tends to
straddle the defect (see Plate 5-6). Partial anomalous pulmonary venous return
is almost always present. Such anomalous veins usually drain the right upper lobe and the middle lobe. The embryology
of this much rarer kind of atrial septal defect is not clear. The clinical
picture and the radiographic and electrocardiographic findings are
similar to those described above.
Surgical correction of this defect
may require the use of a pericardial patch to reroute the anomalous
venous return to the left atrium and simultaneously close the ASD without
compromising the lumen of the superior vena cava or the pulmonary veins.
“Ostium Primum” Defect
A third variety of anomalous
interatrial communication is the “ostium primum” defect. Although some of its
clinical features resemble other types of ASD, the ostium primum ASD differs
significantly in other ways, and it is not truly a defect of the “atrial septum
proper.” The primum ASD is caused by a developmental anomaly of the embryonic
atrioventricular endocardial cushions
(see Plates 5-9 and 5-10).
