Atrioventricular septal defect — short-answer question

Part A — Anatomy, classification and ECG (10 marks)

a) Define an atrioventricular septal defect and list its structural components (4 marks)

An atrioventricular septal defect is a congenital malformation in which the atrial and ventricular septa fail to meet at the crux of the heart, producing a common atrioventricular junction with malformed bridging leaflets. The five structural components are a deficient atrioventricular septum, a common atrioventricular junction, a primum atrial septal defect, an inlet ventricular septal defect, and an abnormal left atrioventricular (mitral) valve with a cleft. ^[1]

b) Describe the three forms of AVSD and the Rastelli classification (4 marks)

The partial form has an ostium primum atrial septal defect, a cleft in the anterior mitral leaflet, two separate atrioventricular orifices, and no ventricular septal defect. The transitional form has a primum defect, two orifices with tethered leaflets, and a small restrictive inlet ventricular septal defect. The complete form has a large primum atrial defect, a large inlet ventricular defect, and a single common atrioventricular valve with bridging leaflets spanning both ventricles. ^[2]

The complete form is graded by the Rastelli classification of the superior bridging leaflet: type A is divided and attached to the septal crest; type B attaches anomalously to a right ventricular papillary muscle; type C is a free-floating undivided common leaflet, the form commonest in Down syndrome and carrying the highest risk of left atrioventricular valve regurgitation. ^[2]

c) What is the characteristic ECG finding, and why is it useful? (2 marks)

The signature finding is a superior QRS axis, typically between minus thirty and minus one hundred and fifty degrees, produced by abnormal inferior-to-superior ventricular activation through the deficient septum. It is present across the entire spectrum — partial, transitional, and complete — and is the single bedside feature that distinguishes a primum from a secundum atrial septal defect. ^[1]