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When Chambers and Valves are in Normal Sequence and Position
When Shunting is Predominant

Atrial septal defect

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.

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