Atrioventricular septal defect — structured oral (viva)

Branch 1 — Anatomy and classification

Examiner: "What is the most likely diagnosis, and what is the underlying anatomy?" Candidate: The combination of Down syndrome, an infant with heart failure, a pansystolic murmur of atrioventricular valve regurgitation, a loud pulmonary component suggesting pulmonary hypertension, and a superior QRS axis is a complete atrioventricular septal defect. The anatomy is a deficient atrioventricular septum producing a common atrioventricular junction at the crux, with a primum atrial septal defect, a large inlet ventricular septal defect, and a single common atrioventricular valve with bridging leaflets. [1] [2]

Examiner: "Classify the complete form for me." Candidate: The complete form is graded by the Rastelli classification of the superior bridging leaflet. Type A is divided and attached to the crest of the ventricular septum; type B attaches anomalously to a right ventricular papillary muscle; and type C is a free-floating undivided common leaflet. Rastelli type C is the form most often seen in Down syndrome and carries the highest risk of left atrioventricular valve regurgitation and a more complex repair. [2]

Branch 2 — The ECG and clinical reasoning

Examiner: "Why does the ECG show a superior axis, and why is it so important?" Candidate: The superior QRS axis, typically between minus thirty and minus one hundred and fifty degrees, arises because the deficient atrioventricular septum alters ventricular activation, producing abnormal inferior-to-superior depolarisation. It is the single bedside fingerprint of an AVSD and is present across the entire spectrum — partial, transitional, and complete. Its importance is that it instantly distinguishes a primum atrial septal defect from a secundum defect, which has a normal axis. A child with an atrial shunt and a superior axis has a primum defect until proven otherwise. [1]

Examiner: "The family asks why their daughter has this. What do you tell them about the Down syndrome link?" Candidate: Roughly half of all children with Down syndrome have a congenital heart defect, and the atrioventricular septal defect — particularly the complete form — is the lesion most strongly associated with trisomy 21. The link is so reliable that up to three-quarters of all complete canals occur in children with Down syndrome. The reassuring part is that surgical outcomes in Down syndrome are excellent and comparable to non-syndromic children when the repair is timely, so trisomy 21 is never a reason to defer surgery. [3]

Branch 3 — Management and timing

Examiner: "What is the definitive management, and when would you operate?" Candidate: The definitive management is surgical — primary repair closing both the primum atrial and the inlet ventricular defects and reconstructing a competent left atrioventricular valve. For the complete form, repair is performed at three to six months of age. The rationale is that the pulmonary vasculature in an unrepaired complete defect becomes irreversibly remodelled within months, faster still in Down syndrome where the pulmonary arteries are more reactive, so closing the defect before six months prevents the Eisenmenger physiology that would make surgery impossible. [2]

Examiner: "What is the single most important long-term problem after repair?" Candidate: Left atrioventricular valve regurgitation. Despite meticulous cleft closure, a proportion of patients develop progressive regurgitation of the reconstructed valve, and this is the leading cause of reoperation over the long term, occurring in roughly ten to fifteen percent. Patients require lifelong follow-up in a congenital cardiac service, with transition to an adult congenital programme in late adolescence for surveillance of valve function, arrhythmia, and pregnancy counselling. [1]