Ostium Primum Atrial Septal Defect

Ostium primum atrial septal defect is a partial atrioventricular septal defect. There is a defect in the lower atrial septum next to the atrioventricular valves, with separate right and left atrioventricular valve annuli and a cleft in the anterior leaflet of the left atrioventricular valve, which often causes regurgitation.

It accounts for about 3 to 5% of congenital heart disease. It is strongly associated with Down syndrome (about a third of partial atrioventricular septal defects occur in Down syndrome, which more often has the complete form) and with heterotaxy and 3p deletion.

Most children are asymptomatic or mildly symptomatic early on. Presentation in infancy occurs with significant left atrioventricular valve regurgitation. Untreated adults develop right heart enlargement, worsening regurgitation, atrial arrhythmia and, uncommonly, pulmonary hypertension.

Pathophysiology.

Failure of the endocardial cushions to fuse leaves a defect in the lower atrial septum, with the atrioventricular valves at the same level and a displaced, elongated left ventricular outflow tract (the gooseneck deformity). The left atrioventricular valve has three leaflets, with the cleft in the anterior leaflet directed toward the septum.

The shunt is left-to-right at atrial level. Regurgitation of the cleft valve adds left ventricular volume load, and may shunt blood from the left ventricle to the right atrium. Abnormal conduction tissue displacement causes a superior axis and risk of heart block after surgery.

Pulmonary vascular disease is less frequent than in complete defects, but it develops earlier in Down syndrome.

Children may have mild exertional breathlessness, recurrent chest infections or poor weight gain. Those with severe regurgitation show tachypnoea, sweating on feeds and failure to thrive. Adults present with dyspnoea, fatigue, palpitations and atrial arrhythmia.

Signs are a wide split second heart sound, a pulmonary flow murmur, an apical holosystolic murmur of regurgitation radiating to the axilla and a right ventricular heave.

• ECG: superior (left) axis deviation (about minus 30 to minus 120 degrees), prolonged PR interval, incomplete right bundle branch block, and atrial arrhythmia. A superior axis separates this lesion from secundum defects.
• Echocardiography: primum defect, both atrioventricular valves at the same level on the four chamber view, cleft of the anterior left atrioventricular valve leaflet, degree and direction of regurgitation, left ventricular outflow tract elongation and obstruction (chordal attachments, subaortic ridge), right heart size and pulmonary pressure. Transoesophageal or three dimensional imaging defines the cleft.
• Catheterisation or MRI for shunt quantification and pulmonary vascular resistance when pulmonary hypertension is suspected, especially in Down syndrome.

Differential diagnosis.

• Secundum and sinus venosus atrial septal defects.
• Isolated cleft mitral valve and primary mitral regurgitation.
• Complete atrioventricular septal defect and inlet ventricular septal defect.
• Left ventricular to right atrial shunt (Gerbode defect).

Indications for surgery. Right ventricular volume overload with a pulmonary to systemic flow ratio of 1.5 or more, significant left atrioventricular valve regurgitation, symptoms, or failure to thrive. Elective repair is usually performed between 3 months and 2 years of age, earlier for heart failure or severe regurgitation. Repair is delayed or avoided when pulmonary vascular resistance is over 5 Wood units without a net left-to-right shunt.

Repair. Patch closure of the primum defect with suture of the cleft (not complete obliteration of the valve), taking care to avoid stenosis and to avoid heart block (suture line on the right atrial side of the conduction tissue). Operative mortality is under 1% in experienced centres.

• Late complications: left atrioventricular valve regurgitation (moderate or more in 10 to 25%, reoperation in 5 to 10%), left ventricular outflow tract obstruction (2 to 4%), heart block needing a pacemaker (1 to 2%), residual shunt and atrial arrhythmia.

Medication.

• Heart failure in infants and children: furosemide 1 mg/kg orally every 12 hours (maximum 6 mg/kg daily), with electrolytes and creatinine checks within 1 week of changes; reduce frequency in renal impairment. Enalapril 0.05 to 0.1 mg/kg orally once daily, increased gradually to a maximum of 0.5 mg/kg daily (potassium and creatinine at 1 to 2 weeks; avoid in renal artery stenosis and with outflow obstruction).
• Adults with regurgitation and ventricular dilatation: ramipril 2.5 mg orally daily, titrated to 10 mg daily (potassium and creatinine at 1 to 2 weeks; avoid with outflow obstruction); furosemide 20 to 80 mg daily for congestion.
• Aspirin 3 to 5 mg/kg orally once daily (adults 75 to 100 mg) for 3 to 6 months after patch repair.
• Atrial fibrillation or flutter: apixaban 5 mg twice daily (2.5 mg twice daily if two of age 80 or over, weight 60 kg or under, creatinine 133 micromol/L or over), and rate control with metoprolol succinate 25 to 200 mg daily (maximum 400 mg daily; reduce in severe hepatic impairment). Warfarin to INR 2.0 to 3.0 is used instead when rheumatic or mechanical valve disease coexists.
• Endocarditis prophylaxis for 6 months after repair, and lifelong after previous endocarditis or with a residual defect next to prosthetic material: amoxicillin 2 g orally (children 50 mg/kg, maximum 2 g) 30 to 60 minutes before dental procedures; with penicillin allergy azithromycin 500 mg (children 15 mg/kg, maximum 500 mg) or doxycycline 100 mg (adults).
• Heart block: permanent pacing for symptomatic or high grade block.

Valve reoperation. Re-repair of the left atrioventricular valve for severe regurgitation with symptoms, ventricular dilatation (left ventricular end systolic diameter over 45 mm) or declining ejection fraction (under 60%); replacement when repair fails. Subaortic stenosis resection for a gradient of 50 mmHg or more or progressive aortic regurgitation.

Monitoring. Echocardiography at discharge, 6 months, 12 months, then every 1 to 2 years (yearly with moderate regurgitation). ECG and ambulatory ECG yearly. Lifelong specialist follow up.

Referral and escalation. Refer every confirmed case to a congenital heart service. Urgent referral for heart failure, heart block, progressive regurgitation, outflow obstruction or pulmonary hypertension.

Special groups.

• Pregnancy: well tolerated after successful repair with mild regurgitation and normal pressures. Moderate or severe regurgitation, pulmonary hypertension or heart failure raises risk; avoid ACE inhibitors and plan care in a cardio obstetric centre.
• Down syndrome: pulmonary vascular disease develops earlier, so repair should not be delayed.
• Older adults: surgery remains effective, but atrial fibrillation often persists.
• Region specific: late diagnosis with established pulmonary hypertension is common, so confirm resistance before surgery.