Atrial septal defect

What is Atrial Septal Defect (ASD)?

Atrial septal defect (ASD) – is a congenital heart defect, which is characterized by the presence of a hole between the right and left atrium.

Embryology

A median septum (septum primum) appears on the cranial wall of the atrium simultaneously with the formation of endocardial cushions and interventricular septum. Septum primum is a thin layer of slightly differentiated cardiac muscle covered by endothelium. It has the shape of a semicircular fold, with one of its arcs extending along the dorsal wall of the atrium, the other along the ventral wall, towards the atrioventricular canal, where they merge with the dorsal and ventral endocardial cushions respectively. This leads to the separation of the atria. However, therebetween remains an opening, which is called ostium primum (primary opening). At the same time, sinus venosus moves to the right and opens into the atrium on the right side of the interatrial septum.

The merger of the septum primum with the endocardial cushions leads to the closure of the primary opening. Immediately after that a new is formed. This opening appears in the cranial part of the septum primum, in the form of many small holes. They expand and form a hole, known as a secondary opening. The blood entering the right atrium enters the left atrium through this opening.

After the formation of a secondary opening in the septum primum, a septum secundum (secondary atrial septum) appears on the right side of it. It also has the shape of a semicircular fold. The open part of the semicircular fold of the secondary septum is directed toward the confluence of the sinus, which would later become the opening of the inferior vena cava in the right atrium. It is vital, because the opening in the secondary septum will remain in line with the secondary atrial opening in the septum primum. The hole in the septum secundum does not overgrow completely, although it decreases in the process of growth, and remains open as. Remnants of septum primum, covering foramen ovale in the secondary septum, form an effective valvular apparatus between the two atria.

Classification

1. Ostium primum ASD;
2. Ostium secundum ASD;
3. Sinus venosus type;
4. Coronary sinus ASD.

Anatomy

Ostium primum ASD is located directly above the atrioventricular valves, and its bottom edge is the septum between the valves. The extreme variant of the pathology is the common atrium. This type of defect often combined with the common atrioventricular canal and other cardiac anomalies. 

Ostium secundum ASD is the most frequent type of defect, characterized by the obligatory presence of the edge of the interatrial septum between the defect and the atrioventricular valves. In this case, the hole can be located in any part of the septum and have a different size. Defects can also be multiple or combined.

Sinus venosus type defect is an infrequent pathology, usually located in the upper-back part of the septum (near the confluence of the superior vena cava), but can be localized at the confluence of the inferior vena cava into the right atrium.

The defect in the area of the coronary sinus is found in the part of the interatrial septum, where the coronary sinus opens. Thus, the left atrium communicates with the right through the venous coronary sinus.

Hemodynamics

The basis of hemodynamic disturbances is blood shunting through a defect from one atrium to another. The direction and value of the shunt are determined by the pressure gradient between the atria, to a lesser extent by the size and location of the defect. Usually, the pressure in the left atrium is higher than in the right, which determines the left-right blood shunting, leading to diastolic overload of the right ventricle and hypervolemia of the pulmonary circulation. As the child grows, the general pulmonary resistance decreases, and the compliance of the right ventricle increases. 

At the same time, systemic vascular resistance and stiffness of the left ventricle increase. Shunting of blood through the ASD from left to right increases as the result of these changes. This direction remains dominant in subsequent years. 

Localization of the defect near vena cava ostium may be accompanied by a reflux of venous blood into the left atrium.

Considerable reserve capacity and low resistance of the pulmonary vessels allow for a long time to avoid the occurrence of high pulmonary hypertension, because the right ventricle is not required to develop high pressure to expel blood into the pulmonary artery.

Thus, the main consequences of ASD are volume overload of the right atrium and ventricle, as well as hypervolemia of the pulmonary circulation.

Workup

• Doppler echocardiography, transesophageal echocardiography. The main diagnostic feature is the visualization of the defect: size, location, number of defects, etc.
• Chest X-ray. Increased heart shadow, right ventricle or right atrium, increased vascular pattern of the lungs, pulmonary artery is dilated. Patients with small defects often have no changes.
• ECG. Changes appear in case of presence of large shunt: deviation of the electrical axis of the heart to the right, incomplete right bundle branch block, abnormal P-wave axis (sinus venous type), deviation of the electrical axis of the heart to the left against the background of right ventricular hypertrophy.
• CT, MRI. Useful in cases of complicated anatomy of the defect, when it is accompanied by other CHD.
• Catheterization of the cavities of the heart. This procedure is performed to determine pulmonary vascular resistance if there is a high pulmonary blood pressure (according to echocardiographic data).

Clinical presentation

Clinical manifestations depend on the type, size and location of the defect. ASD may exist with prolonged hemodynamic compensation. The only manifestation of ASD in the first month of life can be transient cyanosis during crying. Small ASDs do not cause characteristic complaints, growth and developmental delay and the first clinical signs of the defect can develop only in the second and third decades of life.

In cases of medium and large defects, clinical manifestations may appear in the first 3-4 months of life or by the end of the first year of life. Pallor of skin, tachycardia, moderate growth and developmental delay may be specific for these children. Repeated respiratory diseases are typical for children with ASD. They are caused by hypervolemia of the pulmonary circulation. 

Due to the insufficient blood flow to the periphery, children more often appear pale, moderately retarded in growth, body weight and physical development by 5-7 years of age.Typical auscultative signs: systolic murmur in the second and third intercostal space on the left sternal border (timbre is not coarse); strengthened second heart sound above the pulmonary artery; large defects may lead to occurrence of diastolic murmur above the tricuspid valve and the pulmonary artery; Koombs murmur – short mesodiastolic murmur of relative tricuspid stenosis, can be specific for older children; Graham Steel murmur - gentle protodiastolic murmur of the relative insufficiency of the pulmonary valve.

Treatment

Surgery is usually performed at the age of 2 years, with no indication for treatment at an earlier age. The presence of a defect larger than 5 mm or a Qp / Qs ratio of more than 1.5: 1 is an indication for surgery, according to some authors. Small defects tend to close spontaneously in the first two years of life. Closure of the atrial septal defect is possible by open and endovascular methods. Endovascular closure is used for secondary defects. There are limitations for this method due to the size and shape of the ASD.

For endovascular operations, an occluder is used, which is passed through the femoral vein and opens in the area of ​​the interatrial septum, closing it. 



Open interventions involve suturing (for small defects) or closing with patches, usually from the autopericardium.