An ostium secundum atrial septal defect is an abnormally large opening in the atrial septum at the site of the foramen ovale and the ostium secundum.
In the early embryo heart, the atria comprise a common chamber. As the atria enlarge, the septum primum forms and grows toward the developing atrioventricular canal area, which is later divided by the superior and inferior endocardial cushions. These cushions fuse and bend with their convexity toward the atria, thereby approaching the down-growing septum primum. This process continually narrows the passageway between the atria, which is then defined as the ostium primum. 
The ostium primum completely closes; however, before this occurs, a central perforation appears in septum primum, allowing continuous unrestricted flow from the right atrium to the left atrium. This perforation, the second opening in the septum primum, is called ostium secundum. As the atria expand to either side of the truncus arteriosus, a fold is produced within the atria just to the right of septum primum. This passively formed fold is septum secundum. The leading edge of septum secundum is concave in shape and is called the foramen ovale. It comes to overlay the ostium secundum but does not interfere with blood flow from right to left through ostium secundum. After birth, with onset of pulmonary blood flow and elevation of left atrial pressure, the septum primum is pushed against the septum secundum, effectively closing the ostium secundum.
Fusion of the septum primum and the septum secundum closes the foramen ovale. Complete closure occurs in most individuals. In 25-30% of normal adult hearts, however, a probe can be passed from the right atrium to the left atrium via the foramen ovale and ostium secundum. This patent foramen ovale allows a tiny left-to-right shunt that can be detected by sensitive techniques, such as color Doppler echocardiography, in 15-20% of adults.
A secundum atrial septal defect is usually bordered by the edge of the fossa ovalis and the exposed circumference of ostium secundum. The shape of the defect varies from circular to oval. Less often, strands of tissue cross the defect creating a fenestrated appearance that suggests multiple defects. Rarely, a defect can extend posteriorly and inferiorly, approaching the site of inferior vena cava entrance into the right atrium. 
A secundum atrial septal defect can result from inadequate formation of the septum secundum so that it does not completely cover the ostium secundum. More often, the ostium secundum is excessively large because of increased resorption so that septum secundum cannot cover it.
Although heart failure from secundum atrial septal defect rarely occurs in children, this complication can often occur in adults. Adults also demonstrate a propensity for atrial dysrhythmias, including atrial flutter and fibrillation, presumably caused by chronic right atrial dilation. These complications may not be reversible if closure is delayed. Data indicate that closure in persons older than 40 years does not reduce the risk of atrial dysrhythmia. Spontaneous closure of a small secundum atrial septal defect does occur and is usually documented when the initial diagnosis is made early in infancy. [3, 4] A moderate atrial septal defect may also decrease significantly in size or even close when the defect is diagnosed early in life.  However, defects diagnosed later in childhood seldom decrease in size and many significantly increase in size. 
The foramen ovale mechanism remains patent in at least 15% of adults (echocardiography can identify clinically insignificant shunts with this frequency). Some of these individuals could be classified as having a small secundum atrial septal defect. The presence of this phenomenon has been identified as a potential risk factor for stroke due to embolization into the systemic arterial circulation. 
Although unusual, obstructive pulmonary vascular disease may occur in adults with an atrial septal defect. 
The surgical mortality rate should be less than 0.1%. If defects are closed in children younger than 8 years, the prognosis is excellent, with rare short-term or long-term cardiac abnormalities. 
Patients with incomplete closure or closure at older age remain at risk for development of cardiac rhythm disturbances. Whether closure of a secundum atrial septal defect in a person older than 40 years alters the patient's risk of either cardiac disability or limitation of life remains controversial. 
An isolated secundum atrial septal defect very seldom causes significant symptoms in pediatric patients, regardless of defect size.
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Rarely, an infant may develop congestive heart failure in the presence of a secundum atrial septal defect. Whether the defect alone is responsible for causing heart failure is not well established, although it certainly adds to the patient's hemodynamic difficulties. Failure to thrive caused only by a secundum atrial septal defect is, similarly, a rare occurrence.
No data indicate that an uncomplicated secundum atrial septal defect can cause pulmonary vascular disease in pediatric patients.
A patient with primary pulmonary hypertension may demonstrate a secundum atrial septal defect, but no cause-and-effect relationship can be established.
Patients with a significant secundum atrial septal defect who live at an altitude higher than 4000 feet may incur a small risk of developing pulmonary vascular disease.
Death from an isolated secundum atrial septal defect does not occur in pediatric patients.
Surgical damage to the sinoatrial node occurs rarely when experienced individuals accomplish closure. Short and long-term postoperative rhythm disturbances can occur, however, particularly when closure of a large defect is delayed beyond early childhood (ie, >8 y).
Late complications of device closure can include rhythm disturbance, perforation of the heart, fracture of the supporting structures of the device, and residual shunts. Long-term late complications remain unknown.
No significant racial influences have been identified.
Secundum atrial septal defect occurs more commonly in females than males, with a female-to-male ratio of 2:1.
Secundum atrial septal defect is a congenital abnormality and, therefore, is present at birth. It may be diagnosed at any age, including early infancy, although the characteristic findings on physical examination often escape detection until the patient reaches a few years of age. An acquired defect occurs only iatrogenically.
Isolated secundum atrial septal defects account for approximately 7% of congenital cardiac defects. Congenital heart defects of significance occur in approximately 8 per 1000 live births. Therefore, 5-6 cases of secundum atrial septal defect occur per 10,000 live births. This number refers only to defects that are large enough to come to clinical attention. Many small defects that remain undetected occur in addition to numerous cases of patent foramen ovale, as mentioned above.
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