The ostium primum atrial septal defect consists of the absence of the interatrial septum in the atrioventricular canal portion, which causes interatrial left to right shunting. The malformation may occur by itself or in association with an inlet ventricular septal defect, as part of an atrioventricular septal defect. When the ventricular septal defect is absent, the atrioventricular valves are part of a single five-leaflet valve that adheres to the interventricular septum. The anterior leaflet of the mitral valve has a cleft as part of this malformation.
Symptoms of ostium primum atrial septal defect depend on the magnitude of the shunt and that of the associated mitral regurgitation. Patients may be asymptomatic or may exhibit tachypnea and tachycardia, exacerbated with exercise, but they may also be present at rest. An effort such as feeding leads to diaphoresis, dyspnea and increased work of breathing that ultimately results in failure to thrive, especially if congestive heart failure due to pulmonary congestion is present. The thorax appears hyperinflated, with prominent Harrison grooves.
Symptoms may not appear during infancy, but as the child grows older and consist of exertional dyspnea, palpitations due to atrial fibrillation or flutter, as the atria dilate  or to sinus tachycardia that is a compensatory mechanism of heart failure. Sometimes right heart failure signs appear first. Paradoxical embolism may also occur. Cyanosis only appears once Eisenmenger syndrome has developed especially if there is no concomitant pulmonary stenosis.
The clinical examination may show a displaced apical impulse, a palpable dilated pulmonary artery, a widely split second heart sound (S2) with a loud pulmonary component in the presence of pulmonary hypertension and a variety of murmurs: a diastolic rumble due to the large flow through the tricuspid valve, a murmur of tricuspid regurgitation due to the pulmonary hypertension or one of mitral regurgitation due to the anterior leaflet cleft. As left and right atrial pressures equalize, the jugular venous pressure reflects left atrial pressure.
As pulmonary hypertension sets in, the splitting of S2 becomes more narrow and eventually disappears, as does the diastolic tricuspid rumble; the tricuspid regurgitation becomes more audible and a Graham-Steel pulmonary regurgitation diastolic murmur appears.
Chest radiography reveals right heart enlargement and pulmonary congestion with increased pulmonary vascular markings; if the mitral regurgitation is severe, the left heart may also appear enlarged. The pulmonary trunk and proximal right pulmonary artery may be dilated .
Echocardiography is the main diagnostic method. The ostium primum atrial septal defect is seen in the apical four chamber view as a lack of substance in the inferior part of the atrial septum. Parasternal and subcostal short axis views are more suitable for the evaluation of the mitral valve cleft and other mitral abnormalities: double orifice or single papillary muscle. Left ventricle outflow tract is long and narrow and may be obstructed by the abnormal mitral valve.
Color Doppler evaluates the direction of the atrial shunt, mitral, tricuspid and pulmonary regurgitations and identifies associated interventricular or left ventricle to right atrium communications.
Pulsed Doppler helps to evaluate systolic pulmonary artery pressure, based on tricuspid regurgitation peak velocity, while continuous Doppler is useful in identifying left ventricular outflow tract obstruction if present.
Transesophageal echocardiography is seldom used in the preoperative evaluation, being reserved for poor acoustic window patients, but is valuable in the intraoperative setting to evaluate mitral stenosis, residual mitral and tricuspid regurgitation or left ventricular outflow tract obstruction. This method also better describes the margins of the defect, usually represented by the septum primum and the atrioventricular valve annulus .
The electrocardiogram may reveal sinus rhythm, atrial flutter or fibrillation   . The sinus node may be displaced, leading to negative P waves . The right atrium overload, pictured as peaked P wave in leads D II, D III, V1 and V3R and first-degree atrioventricular block may be present. Right ventricle hypertrophy markers or incomplete right bundle branch block may be present. If mitral valve regurgitation is severe, the left ventricle may be hypertrophied and the left atrium may be overloaded and enlarged leading to a P wave duration of more than 0,08 sec and/or inversion in leads V1 or V3.
Cardiac catheterization is seldom necessary in this disease and is reserved for the evaluation of pulmonary vascular obstructive disease and in cases where systemic/pulmonary debt ratio (Qp/Qs0 needs to be invasively calculated. This procedure allows the direct measurement of intracardiac and pulmonary artery pressures and the calculation of pulmonary resistance. In addition, cardiac anatomy and associated malformations such as additional atrial or ventricular septal defects, patent ductus arteriosus, coarctation of the aorta or anomalous pulmonary venous return are best described by this method.
Cardiac magnetic resonance imaging is another reliable method to assess right ventricular size and function and cardiac anatomy, in general .