Atrioventricular Septal Defect (AVSD)

01Pathophysiology

AVSD results from incomplete fusion of the endocardial cushions, embryonic structures that normally give rise to the lower portion of the atrial septum, the inlet portion of the ventricular septum, and the septal leaflets of the mitral and tricuspid valves. When these cushions fail to fuse, the result is a combined defect involving the ostium primum atrial septum, the inlet ventricular septum, and the AV valves, which may form a single common valve rather than two distinct valves.

The hemodynamic consequence depends on the size of the defects. In the complete form, blood shunts freely between all four chambers. Initially, high pulmonary vascular resistance in the newborn limits the shunt, but as resistance falls over the first weeks of life, a large left-to-right shunt develops. This drives pulmonary overcirculation, leading to congestive heart failure with the classic feeding difficulties, sweating with feeds, tachypnea, and failure to thrive. The lungs become flooded, predisposing the infant to recurrent pneumonias.The cleft in the mitral valve causes mitral regurgitation, which worsens left atrial volume overload.

Untreated, the chronic high pulmonary blood flow causes pulmonary vascular remodeling, eventually producing irreversible pulmonary hypertension and Eisenmenger physiology with reversal of shunt direction. This progression is accelerated in patients with Down syndrome, who develop pulmonary vascular disease earlier and more severely than other children, which is why early surgical repair is essential.

The hallmark superior QRS axis on ECG reflects abnormal development of the conduction system: the AV node and bundle of His are displaced posteriorly and inferiorly because of the deficient inlet septum, producing leftward and superior depolarization.

02Classification of Clinical Manifestation

The Rastelli classification subdivides complete AVSD based on the morphology of the superior bridging leaflet:

• Type A: Superior bridging leaflet is divided and attached to the crest of the ventricular septum (most common)

• Type B: Superior bridging leaflet has anomalous papillary muscle attachment in the right ventricle (rare)

• Type C: Superior bridging leaflet is undivided and "free-floating," not attached to the septum (associated with complex disease and heterotaxy)

03Diagnostic Workup

The workup begins when an infant presents with feeding difficulties, failure to thrive, or a murmur, especially in a child with Down syndrome, in whom screening echocardiography is recommended at birth or within the first few weeks of life regardless of physical findings.

The best initial test is the electrocardiogram, which provides the most distinctive clue. A superior, leftward QRS axis (between -40 and -150 degrees) in an infant with heart failure features strongly suggests AVSD because most other causes of cyanotic or acyanotic congenital heart disease produce a rightward axis. The chest radiograph supports the impression by showing cardiomegaly and pulmonary plethora.

The most accurate test and gold standard is two-dimensional transthoracic echocardiography with color Doppler. This visualizes the primum atrial septal defect, the inlet ventricular septal defect, the common AV valve or cleft mitral valve, and quantifies the degree of regurgitation and the direction and magnitude of shunting. Echo also estimates pulmonary artery pressures, which guides timing of surgery.

Cardiac catheterization is not routinely needed for diagnosis but becomes important when pulmonary hypertension is suspected or when patients present late, because it measures pulmonary vascular resistance and determines whether the defect is still surgically repairable. A pulmonary vascular resistance index above 8 to 10 Wood units after vasodilator challenge generally indicates inoperability.

04Management & Treatment

Acute stabilization focuses on relieving congestive symptoms. Loop diuretics such as furosemide reduce pulmonary edema. Afterload reduction with an ACE inhibitor (captopril or enalapril) decreases the left-to-right shunt by lowering systemic vascular resistance relative to pulmonary resistance, although this requires close monitoring of blood pressure and renal function. Digoxin may be added for symptomatic relief. Nutritional optimization is essential because these infants have markedly elevated caloric requirements and often require fortified formula or nasogastric supplementation.

The definitive management is surgical repair. For complete AVSD, surgery is performed between 3 and 6 months of age, before irreversible pulmonary vascular disease develops. Operating earlier than this carries higher technical risk; waiting longer risks the development of fixed pulmonary hypertension, which is the principal reason these patients become inoperable. Surgical repair involves patching the atrial and ventricular septal defects and partitioning the common AV valve into competent left and right valves, often with closure of the mitral cleft. For partial AVSD, repair is typically scheduled in the toddler years (1 to 3 years) since heart failure is less severe.

Pulmonary artery banding was historically used as a palliative procedure to limit pulmonary blood flow when complete repair was not feasible, but with current surgical techniques, primary complete repair is preferred even in small infants.

The next best step in a vignette of an asymptomatic newborn with Down syndrome is screening echocardiography, regardless of physical examination findings, because up to half of these children have congenital heart disease and AVSD is the most common lesion.

Contraindications and special considerations: Patients who present late with established pulmonary vascular obstructive disease (Eisenmenger syndrome) are not candidates for repair; management becomes supportive with pulmonary vasodilators (sildenafil, bosentan) and eventual consideration of heart-lung transplantation. ACE inhibitors should be used cautiously in neonates due to risk of hypotension and renal impairment.

05Differential Diagnosis & Distractors

06Traps & High-Yield Pearls

The most common error students make is failing to consider AVSD in any infant with Down syndrome, even when the question emphasizes a different organ system. A vignette describing a 6-week-old with trisomy 21 who is feeding poorly and sweating during feeds is an AVSD question until proven otherwise, and the next best step is echocardiography, not a chest radiograph or "reassurance." The second classic trap is misreading the ECG: students often anchor on the murmur and forget that the superior (northwest) QRS axis is the single most distinguishing electrocardiographic feature, separating AVSD from a simple VSD or secundum ASD.

A third pitfall involves timing of surgery. Test-writers like to ask why surgery cannot be delayed in an asymptomatic Down syndrome infant with complete AVSD; the answer is early development of irreversible pulmonary vascular disease, which is accelerated in trisomy 21. A fourth trap is confusing partial AVSD, which presents later and resembles a secundum ASD clinically, with the complete form, which behaves like severe heart failure of infancy. Finally, watch for the cleft mitral valve detail: it is the structural reason for mitral regurgitation in AVSD and is often the giveaway in a vignette that mentions both an ASD and an apical holosystolic murmur in a young child.

The core competency being tested is pattern recognition of congenital heart disease in a syndromic infant, correct interpretation of ECG axis as a diagnostic shortcut, and understanding the urgency of repair before pulmonary vascular resistance becomes fixed. Master these three threads and AVSD becomes one of the most predictable diagnoses on the exam.