Deficiencies occur in a number of positions within the atrial septum and an understanding of the anatomy is important when considering closure and the potential effects of a device within the heart (Fig. 26.2a). The atrial septum is not symmetrical. When viewed from the right atrial side, a significant proportion of the atrial septum is made up of infoldings of the atrial wall, usually in association with other structures (Fig. 26.2b). From the right, postero-superiorly the septum consists of an infolding of the atrial wall between the superior caval vein and the insertion of the right pulmonary vein into the left atrium (as the heart sits within the chest). Anterior to this the rim of the atrial septum continues behind the aortic root. Inferiorly, the rim of the atrial septum is in contact with the inferior caval vein and anterior to this margin is the interface with the coronary sinus, which in turn is separated from the mouth of the inferior caval vein by the Eustachian ridge and valve (sinus septum). Deeper within the Eustachian ridge runs the tendon of Todaro, one of the three walls of the triangle of Koch, an area of the atrial septum containing several important structures including electrically active tissue (Fig. 26.2c).

When viewed through the left atrial wall, the anatomy is less complex. Only a small portion of the anterior region of the atrial wall is true septum, the rest representing overlap and fusion of the primary atrial septum (flap valve) with the anterior atrial wall (Fig. 26.2d).

The atrial septum is a curved structure and sits at an angle within the heart relative to the anteroposterior position of the thorax – this is a simple concept but one that poses challenges for the delivery of a device delivered through a sheath positioned along the natural line of the inferior vena cava. Defects in the secundum atrial septum are variable in both size and position. Although a proportion are truly central, eccentric holes with extension to any margin can occur. Secundum ASDs are not infrequently multiple and often sit within mobile or “aneurysmal” septal tissue. All of these anatomical variables must be defined prior to the delivery of a device.

Assuming that an ASD is of significant size, the magnitude of the shunt is determined by the relative resistance to filling of the ventricles. Right ventricular resistance is usually less than the left and therefore the overall shunt is left to right. Flow across an ASD is phasic occurring predominantly in late ventricular systole and early diastole. Abnormalities of ventricular diastolic function in either ventricle (e.g. systemic hypertension leading to left ventricular hypertrophy) will affect the direction and magnitude of the atrial shunt and are part of the reason why atrial septal shunts increase in significance with age.

The vast majority of infants and children with ASDs are asymptomatic, although many have tendency to recurrent chest infections and respiratory symptoms. Very occasionally an infant will be encountered in whom an atrial shunt is responsible for failure to thrive.

During adult life, symptoms are progressive. Exercise intolerance is a common feature. Studies consistently show that with each passing decade, an increasing proportion of patients with ASDs display one or more of the characteristic sequelae of an important atrial shunt: pulmonary hypertension, atrial dysrhythmia or clinical right heart failure.

Traditional indications for the closure of intracardiac shunts were based on invasive oximetry, with a shunt of >1.5:1 taken as significant. In the modern era, virtually all decisions on the significance of atrial shunts (and therefore the indications for closure) are made using non-invasive imaging prior to an attempt to close a defect.

There is an abundance of literature demonstrating that patients of any age with significant ASDs benefit from closure. In children, most units defer closure of ASDs until around the 3rd birthday at the earliest unless the clinical situation is atypical, e.g. failure to thrive or there are particularly frequent chest infections.

Although in years gone by there was an active debate about the need for ASD closure in older adults, the issue of benefit for these patients has been resolved by the publication of a number of studies. Established atrial dysrhythmia is rarely solved by closing an ASD in this age group, but shunt-related symptoms are improved, pulmonary hypertension is resolved, the right heart is usually remodelled and clinical right heart failure, if present, is easier to treat medically.

Contraindications to ASD closure include defects that are anatomically unsuited to a device such as those that are greater than 40 mm in diameter or with inadequate margins. In these cases, surgery should be offered. Very rarely, patients are encountered who are affected by both primary pulmonary hypertension and a coexistent ASD. Often these patients are younger women and almost always have clinical signs out of context with the ASD itself, e.g. cyanosis due to right-to-left shunting at atrial level. Closure of an ASD in this situation should be avoided.

If an ASD is significant, then the options for closure are either surgery or a transcatheter delivered device. It is important that patients and families are thoroughly counselled about the pros and cons of both procedures so that an informed decision can be made.

Most units rely on transthoracic echocardiography (TTE) for initial assessment. Clear evidence of right heart dilation on TTE is usually a marker of a significant atrial shunt. In children TTE can also reliably delineate the anatomy, the margins of the defect and the presence of associated abnormalities (e.g. anomalous pulmonary venous drainage or mitral valve disease). In adults, TTE sometimes does not provide the resolution to accurately define the anatomy and margins of the defect. In many cases of this sort, it is appropriate to move on to attempted closure with a careful trans-oesophageal echocardiography (TEE) assessment to check suitability for closure before catheterisation. In some cases a pre-procedural MRI can be helpful. MRI has the additional advantage of providing accurate volumetric analysis and relative pulmonary/systemic flows ratios if there is any doubt about the indication for closure. It also provides clear imaging of the pulmonary veins. Against this the spatial resolution of MRI means that imaging of the atrial septum itself is not always as accurate as with ultrasound.

An important part of the assessment of an adult patient with an ASD should be a full assessment of left ventricular function. Impaired systolic and diastolic function can be masked by an atrial shunt and closure of the defect in these patients can (rarely) precipitate pulmonary oedema. In older adults (>50 years), it is important to rule out and if necessary treat coexistent coronary abnormalities/disease.