Gerbode Defect

Gerbode defect is a communication between the left ventricle and the right atrium. It passes either on the atrial side of the septal tricuspid leaflet (direct, supravalvular) or through a defect in the membranous septum with a perforated septal tricuspid leaflet (indirect, infravalvular).

It causes a left-to-right shunt at ventricular pressure into the right atrium.

Congenital cases are rare, under 1% of congenital heart disease. Acquired defects are now more common, and follow infective endocarditis (usually of the aortic valve), cardiac surgery (aortic, mitral or tricuspid valve replacement, subaortic resection), myocardial infarction, chest trauma, transcatheter aortic valve implantation, and device or surgical closure of a perimembranous ventricular septal defect.

Presentation ranges from an incidental murmur to heart failure and pulmonary hypertension, and acquired cases may present with sepsis and acute valve destruction.

Pathophysiology

The septum between the left ventricle and right atrium is short and membranous. Defects lie at the atrioventricular portion, immediately beneath the aortic valve and on the atrial side of the tricuspid valve.

Left ventricular systolic pressure drives a high velocity jet into a low pressure right atrium. The result is right atrial and right ventricular volume overload (increased flow through the tricuspid valve), with pulmonary overcirculation and, in time, pulmonary hypertension and left ventricular volume load.

In endocarditis, the jet can seed the right atrial endocardium and tricuspid valve, giving a chain of infection.

Many patients are asymptomatic.

Others have exertional dyspnoea, palpitations, fatigue and recurrent chest infections.

Signs are a harsh high pitched systolic murmur at the lower left sternal edge, often with a thrill, which can be mistaken for tricuspid regurgitation or a ventricular septal defect, giant v waves in the jugular venous pulse and right heart failure signs in large shunts.

Fever, embolic phenomena and rapid haemodynamic deterioration suggest endocarditis with perforation. Heart block follows invasive closure.

• Echocardiography (transthoracic and transoesophageal, with colour and continuous wave Doppler): systolic jet from the left ventricular outflow tract or subvalvular region into the right atrium. A high velocity jet of 4 to 5 m/s into the right atrium with a normal right ventricular pressure points to the defect, since a tricuspid regurgitant jet of this velocity would imply severe pulmonary hypertension. The tricuspid regurgitation derived pulmonary pressure may therefore be overestimated.
• Look for the cause: aortic valve vegetation or aneurysm of the membranous septum, prosthetic valve, previous surgery, tricuspid leaflet perforation, associated ventricular septal defect.
• Right heart enlargement, shunt fraction (pulmonary to systemic flow ratio, Qp:Qs), pulmonary pressure and left ventricular function. Three dimensional echocardiography defines size and location. Cardiac MRI or CT supports imaging when windows are poor.
• Catheterisation: oxygen step up in the right atrium, left ventriculogram with contrast entering the right atrium, and pulmonary vascular resistance.
• Blood cultures (three sets before antibiotics), C-reactive protein, full blood count and creatinine when infection is suspected. Daily ECG for PR interval in endocarditis.

Differential diagnosis.

• Tricuspid regurgitation with pulmonary hypertension and perimembranous ventricular septal defect with tricuspid pouch.
• Ruptured sinus of Valsalva aneurysm and aorto right atrial fistula.
• Atrioventricular septal defect and coronary arteriovenous fistula to the right atrium.
• Endocarditis with abscess and tricuspid vegetation.

Small, haemodynamically insignificant defect (pulmonary to systemic flow ratio under 1.5, no chamber enlargement, no endocarditis): observation with echocardiography every 6 to 12 months, and dental prophylaxis only when previous endocarditis or prosthetic material is present. Spontaneous closure by tricuspid leaflet tissue occurs in some small congenital defects.

Indications for closure. Pulmonary to systemic flow ratio of 1.5 or more, right atrial or ventricular dilatation, heart failure, pulmonary hypertension, recurrent endocarditis, or an acquired defect after endocarditis with destruction of valve tissue.

Closure.

• Surgical patch closure through the right atrium (with tricuspid valve repair or replacement when leaflets are destroyed) is the usual method, especially for acquired defects, endocarditis and associated valve disease.
• Transcatheter closure with a perimembranous or muscular ventricular septal defect occluder is an option for suitable congenital or post-surgical defects with adequate rims. Risks include complete heart block (about 3 to 5%), tricuspid valve impingement and device embolisation. Give cefazolin 1 g intravenously (children 25 mg/kg, maximum 1 g) before the procedure and heparin 100 units/kg (activated clotting time over 200 seconds). Aspirin 75 to 100 mg orally once daily for 6 months after device placement (children 3 to 5 mg/kg daily).
• Permanent pacing for persistent high grade atrioventricular block.

Acquired defect from infective endocarditis.

• Take cultures before antibiotics. Empirical therapy for native valve disease: ampicillin 2 g intravenously every 4 hours, flucloxacillin 2 g every 4 hours and gentamicin 3 mg/kg once daily. With severe penicillin allergy or MRSA risk: vancomycin 25 mg/kg loading dose, then 30 to 60 mg/kg daily in 2 to 3 doses (trough 15 to 20 mg/L), with gentamicin.
• Directed therapy, counted from the first negative blood culture: penicillin susceptible streptococci, benzylpenicillin 12 to 18 million units daily in 4 to 6 doses or ceftriaxone 2 g once daily for 4 weeks; methicillin susceptible Staphylococcus aureus, flucloxacillin 2 g every 4 hours (12 g daily) for 6 weeks.
• Monitoring: gentamicin trough under 1 mg/L and creatinine two to three times weekly, vancomycin trough, full blood count weekly, and PR interval daily.
• Early surgery for heart failure, abscess, uncontrolled infection, persistent bacteraemia after 7 to 10 days, and for any large perforation with haemodynamic effect.

Heart failure therapy before repair. Furosemide 20 to 80 mg orally once or twice daily (children 1 mg/kg every 12 hours, maximum 6 mg/kg daily), with electrolytes and creatinine within 1 week of changes. Ramipril 2.5 mg daily, titrated to 10 mg daily, for left ventricular dilatation (children enalapril 0.05 to 0.1 mg/kg daily, up to 0.5 mg/kg daily), with potassium and creatinine at 1 to 2 weeks.

Prevention of endocarditis. Dental prophylaxis after previous endocarditis, for 6 months after prosthetic material placement, and with residual shunts next to prosthetic material: amoxicillin 2 g orally (children 50 mg/kg, maximum 2 g) 30 to 60 minutes before the procedure; azithromycin 500 mg or doxycycline 100 mg with penicillin allergy.

Monitoring. Echocardiography at discharge, 1, 6 and 12 months, then yearly. ECG for conduction at each visit and ambulatory ECG for arrhythmia. Surveillance for residual shunt and tricuspid function.

Referral and escalation. Refer every confirmed defect to a congenital or valve heart team. Urgent referral for fever with a new shunt murmur, heart failure, heart block or haemodynamic compromise.

Special groups.

• Pregnancy: small defects are tolerated; large shunts, pulmonary hypertension or heart failure raise risk. Stop ACE inhibitors before conception.
• Children: congenital defects often close spontaneously when small; repair when they cause volume overload.
• Intravenous drug use: endocarditis recurs, so addiction treatment is part of care.
• Region specific: rheumatic and degenerative valve surgery and endocarditis are the usual causes, so access to cardiac surgery decides outcome.