Atrioventricular canal defect (AVSD)

What is Atrioventricular Canal Defect?

Atrioventricular canal defect is a congenital heart defect that includes a large group of anomalies united by one anatomical sign: a deficiency of the muscular and membranous atrioventricular (AV) septa with pathophysiology from the atrial to the functionally single ventricle.

An atrioventricular canal defect, an atrioventricular septal defect (AVSD), an endocardial cushion defect are synonymous names for this disease.

Embryology

Normally, the common atrioventricular opening is divided into separate mitral and tricuspid valves. At the same time, the atrial septum closes.

The tissue of the endocardial cushions, the conical septum, and the muscular portion of the interventricular septum forms the interventricular septum. Violation of the development of endocardial cushions and AV-septum leads to AVSD of various types.

Anatomy

The main anatomical sign of atrioventricular canal defect is a more or less pronounced deficiency of the membranous and muscle AV-septums. Various forms of the defect have common features: AVSD with variable distribution to the adjacent atrial and interventricular septa, a common AV-opening or divided AV-valves, a shortened path of inflow of the left ventricle.

The aorta is not between the AV-valves, as it should be. It is shifted anterior and upward, which leads to a disproportion in the length of the inflow and outflow tracts.

Three types represent the partial form of AVSD, in which the rings of the mitral and tricuspid valves are separate:

1. primary ASD with cleavage of the anterior mitral valve leaflet;
2. primary ASD with wide diastasis of the anterior septal commissure of the tricuspid valve (the so-called splitting of the septal leaflet);
3. isolated defect of the AV-septum (oblique AV-channel).

The last two types from the hemodynamic point of view are left ventricular - right atrial connections.

The complete form of AVSD is characterized by the presence of a large septal defect consisting of the atrial and interventricular components and a common AV-valve connecting both atria to the ventricles. The interventricular component of the defect in the form of a recess shortens the length of the septum due to its inflow part.

Classification

1. Partial.

2. Intermediate.

The combination of a partial form (separately formed AV-valves) with a VSD (usually restrictive) under the valves.

3. Complete (by Rastelli).

The complete form is divided into 3 types, depending on the anatomy of the upper bridging cusp of the bridging valve and VSD:

• Type А. The superior bridging valve is divided along the crest of the interventricular septum. The chordal apparatus is changed, the VSD is often small.
• Type В. Almost always combined with an imbalance in the development of the ventricles and chordal stradling.
• Type С. The superior bridging valve is not divided along the crest of the interventricular septum. Often combined with Tetralogy of Fallot.

Hemodynamics

Hemodynamic disorders are determined by the presence of a shunt at atrial and ventricular levels, left ventricular –right atrial blood shunt and AV-valve regurgitation. Pulmonary hypertension can be a consequence of both constant hypervolemia and venous stasis in the presence of small-sized ASD, leading to increased pressure in the left atrium.

Diagnostics

• Echocardiography, CT. Visualization of the defect. 
• ECG. Due to the displacement of the His bundle downward due to the lack of a inflow part of the septum, the electrical axis of the heart is shifted to the left. Signs of right ventricular hypertrophy are especially pronounced with complete AVSD.
• Chest X-ray. Signs of increased pulmonary blood flow.
• Cardiac catheterization with aortography. It is used for suspected high pulmonary hypertension, in order to assess the degree of damage to the pulmonary vessels.

Clinical manifestations

The clinical manifestations of atrioventricular canal defect depend on the volume of pulmonary blood flow, the presence of pulmonary hypertension and the severity of valve insufficiency. In the first weeks of life, while pulmonary resistance remains elevated, the symptoms of the disease may be mild. However, later the usual signs of pulmonary hypertension and a large left-right blood shunt appear: difficulties in feeding, dyspnea, respiratory infection, sweating, lag in weight gain.

An earlier manifestation of symptoms and a severe course of the disease are observed among patients with underdevelopment of the left heart or obstruction of the excretory tract of the left ventricle.

Usually, the presence of this defect is diagnosed early. In the presence of severe mitral valve insufficiency, an increased apical impulse and systolic trembling are noted.



Auscultation: the II heart sound is split during inhalation and exhalation with primary ASD; in the presence of complete AVSD the II heart sound is usually split only during inhalation and is accentuated due to pulmonary hypertension. An increasing – decreasing systolic murmur, extending upwards, is heard on the upper left edge of the sternum. It is formed due to the relative narrowing of the pulmonary artery valve ring due to increased pulmonary blood flow. A holosystolic murmur of mitral regurgitation is heard at the apex and spreads to the left and down to the chest. A low-frequency mid-diastolic murmur at the apex or on the left edge of the sternum is often determined, formed due to increased blood flow through the AV-valve.

Treatment

Drug therapy is aimed at mitigation of heart failure. The diagnosis of AVSD is an indication for surgical treatment. The essence of the operation is to close the atrial and interventricular defect, and to form two atrioventricular valves. Surgical treatment is usually indicated in the first months of life. Often the first stage of surgery is banding of the pulmonary artery, an artificial narrowing performed to reduce the volume of blood flowing into the lungs. In the presence of hypoplasia of the left or right ventricle, staged hemodynamic correction is performed.