Ostium Primum Atrial Septal Defects Workup

The following imaging studies may be indicated in patients with ostium primum atrial septal defect (ASD).

Chest radiography

With an isolated primum defect and no significant mitral regurgitation, findings of right heart enlargement and of a variable degree of pulmonary overcirculation are noted. In children with hemodynamically significant mitral regurgitation, radiographic evidence of left heart enlargement is also seen.

In patients with relatively small shunts, radiographic evidence of mild-to-moderate right atrial and right ventricular enlargement may not be seen. An increase in pulmonary vascular markings also may not be noted.

With larger left-to-right shunts, the heart size is enlarged, with a cardiothoracic ratio greater than 50%. Pulmonary vascular markings are increased in proportion to the pulmonary-to-systemic flow ratio (Qp:Qs). The pulmonary trunk and proximal right pulmonary artery are dilated, and the aortic knob appears proportionally small. The right atrium and right ventricle are significantly enlarged.

Superimposed mitral regurgitation results in left atrial and left ventricular dilatation, further enlarging the cardiac silhouette.

Echocardiography

Echocardiography confirms the diagnosis of a primum ASD or partial atrioventricular (AV) septal defect. The anatomy is delineated by two-dimensional imaging, and shunt flow and AV valve regurgitation are assessed by color, pulsed, and continuous wave Doppler. ^[18]

Two-dimensional imaging

The apical four-chamber view and subcostal imaging planes readily demonstrate a primum ASD, showing an area of "drop-out" in the inferior atrial septum. Use care to differentiate this from the drop-out noted in the region of the coronary sinus, which may be particularly difficult to do when the coronary sinus is dilated from drainage of a persistent left superior vena cava.

In the apical imaging plane in a normal heart, the tricuspid valve is more apically positioned than the mitral valve. In AV canal defects, both valves are visualized at the same horizontal level, and the crux of the heart is absent. In a partial canal defect, distinct left and right AV valves are identified. The leaflets are attached to the crest of the interventricular septum and no defect of the interventricular septum is visualized. A tricuspid valve cleft or other abnormalities of the valve leaflets also may be noted.

When present, a cleft is visualized in the anterior mitral valve leaflet pointing toward the interventricular septum. This is best seen in a parasternal or subcostal short-axis view. A double orifice mitral valve or single papillary muscle may be noted. Because the aortic root is not wedged between the mitral and tricuspid annuli, the aortic valve is anteriorly positioned, resulting in a long, narrow, left ventricular outflow tract with a so-called "gooseneck" appearance. Left ventricular outflow tract obstruction may occur from mitral valve tissue crossing the subaortic area.

Color Doppler ultrasonography

The direction of atrial level shunting is readily detected by color Doppler and is best seen from the long-axis subcostal imaging plane. AV valve regurgitation is quantifiable by color Doppler, particularly well seen in the apical four-chamber view but best evaluated in multiple planes. Tricuspid regurgitation typically is mild in the absence of pulmonary hypertension, whereas mitral regurgitation may range from trivial to severe. A left ventricle -to-right atrium shunt should be interrogated, and lack of interventricular shunting should be confirmed. Differentiation of a primum ASD from a dilated coronary sinus is also aided by color and spectral Doppler.

Pulsed and continuous wave Doppler ultrasonography

The presence and degree of left ventricular outflow tract obstruction as well as relative pulmonary stenosis from increased flow across the pulmonary valve may be detected. The peak velocity (v) of the tricuspid regurgitant jet can be used to estimate right ventricular/pulmonary arterial systolic pressure. In the absence of right ventricular outflow tract obstruction, the pulmonary artery systolic pressure will approximate (4 × v × v) + right atrial pressure. Pulmonary artery systolic pressure higher than 25 mm Hg indicates the presence of pulmonary hypertension. Estimation of the pulmonary artery pressure by continuous wave Doppler technique is not reliable in patients with left ventricle-to-right atrial shunting; this high velocity jet may be misinterpreted for the tricuspid regurgitant jet, resulting in an overestimation of the pulmonary artery pressure.

Three-dimensional echocardiography

Although rarely used, three-dimensional echocardiography may offer a more accurate reconstruction of the AV valve(s) and the atrial septum, providing a more complete preoperative picture for the surgeon.

Transesophageal echocardiography

Transesophageal echocardiography (TEE) is generally reserved for anatomic definition in older patients with poor acoustic windows and for intraoperative assessment during surgical repair. Postoperative assessment is particularly helpful while weaning from cardiopulmonary bypass. Mitral stenosis, residual AV valve regurgitation, left ventricular outflow tract obstruction and residual atrial level shunting may be identified. Pulmonary artery pressure may be estimated by the peak velocity of the tricuspid regurgitation jet. Optimally, residual problems can be identified and corrected before leaving the operating room.

Two-dimensional, apical, four-chamber echocardiogram of a partial atrioventricular (AV) septal defect. The asterisk (*) delineates an area of dropout in the inferior atrial septum at the site of the primum atrial septal defect. The AV valves are separate but aligned at the same horizontal level, consistent with a two-orifice common AV valve. In systole, the medial leaflets of the right- and left-sided AV valves demonstrate attachments to the crest of the interventricular septum, allowing no ventricular level shunting. LA = left atrium, LV = left ventricle, RA = right atrium, and RV = right ventricle.

View Media Gallery

Electrocardiography

Diagnosis of a primum atrial septal defect (ASD) or a partial atrioventricular (AV) septal defect can often be made based on physical examination findings and the electrocardiogram (ECG) alone. Left axis deviation is typically seen in most patients with ostium primum ASD and is helpful in differentiating from ostium secundum ASD. The ECG abnormalities (see the image below) are predominantly caused by abnormalities of the conduction system. Specifically, the AV node is displaced posteriorly and inferiorly, and atrial and/or AV nodal conduction is often delayed.

Electrocardiogram from a patient with a partial atrioventricular septal defect. The PR interval is mildly prolonged. Left axis deviation with Q waves in leads I and aVL are present, consistent with a counterclockwise loop in the frontal plane. Right atrial enlargement and an rsR' pattern in the right chest leads are also noted.

View Media Gallery

Note the following:

• Displacement of the AV node results in a counterclockwise loop in the frontal plane in 95% of cases. The QRS axis is outside the normal range for age, demonstrating either a left or far left axis, and Q waves are present in leads I and aVL.

• Delayed conduction through the atria or through the AV node may lead to prolongation of the PR interval (ie, a first-degree AV block).

• Abnormalities in the right precordial leads are similar to those in secundum-type ASDs. The QRS pattern is typically either an rSr' or an rsR' resulting from dilatation and hypertrophy of the right ventricular outflow tract caused by volume overload of the right heart.

• Right atrial enlargement is often detected, demonstrated by a peaked P wave measuring more than 2.5 mm (standard 10 mV/mm). It is best seen in leads II, III, V1, and V3R.

• With significant mitral regurgitation, left atrial enlargement may be present, demonstrated by a P-wave duration longer than 0.08 sec and/or terminal and deep inversion of the P wave in lead V1 or V3R.

Cardiac catherization

Because of the excellent information generally provided by echocardiography, cardiac catheterization is rarely warranted in the diagnosis and management of partial atrioventricular (AV) septal defects. The exception is a hemodynamic evaluation in a patient with suspected pulmonary vascular obstructive disease. Note the following:

• In the hemodynamic assessment, systemic and pulmonary venous saturations are generally normal but may be somewhat low in trisomy 21 patients secondary to upper airway obstruction. A step-up in oxygen saturation is noted at the right atrial level owing to left-to-right shunting across the atrial septum. A step-up in saturations at the right ventricular level should be notably absent. However, a step-up at the right ventricular level may be seen because of better mixing distally. A calculated pulmonary-to-systemic flow ratio (Qp:Qs) of 2:1 is hemodynamically significant.

• In the hemodynamic assessment, intracardiac and pulmonary artery pressures are directly measured. Pulmonary vascular resistance is calculated, but note that the results may be affected by the presence of upper airway obstruction or congestion. If significant upper airway obstruction resulting in carbon dioxide retention is present, placement of an airway or even intubation with mechanical ventilation may be required to get an accurate assessment of pulmonary vascular resistance. If pulmonary vascular disease is suspected or confirmed, administration of 100% oxygen and/or inhaled nitric oxide is warranted to assess pulmonary vascular reactivity.

Contrast injection

A left ventricular injection is performed to rule out ventricular level shunting, determine the degree of AV valve regurgitation, and assess the left ventricular outflow tract for evidence of obstruction. Goose-neck deformity of the left ventricular outflow tract is classic for endocardial septal defects, including ostium primum atrial septal defects (ASDs).

A right upper pulmonary vein contrast injection allows visualization of the ostium primum ASD and assessment for additional ASDs. A right ventricular injection delineates the right ventricular outflow tract and pulmonary arteries, but it is not necessary routinely.

Aortic or pulmonary artery injections may be warranted if patent ductus arteriosus, coarctation of the aorta, or anomalous pulmonary venous connections are suspected. Pulmonary arterial wedge angiography may be indicated in patients with suspected pulmonary vascular obstructive disease.