Pulmonary atresia with ventricular septal defect (PA-VSD)

What is Pulmonary Artery Atresia with Ventricular Septal Defect (PA-VSD)?

Pulmonary artery atresia with ventricular septal defect (PA-VSD) is a complex congenital defect of the heart and pulmonary arteries, which is characterized by the absence of direct communication between the right ventricle and the pulmonary arteries, the presence of structural abnormalities of pulmonary arteries and extracardiac sources of pulmonary circulation.

Embryology

PA-VSD belongs to conotruncus defects in the embryo-anatomical relation. Conotruncus (the embryonic truncus) – is the term, which includes two closely related structures of the embryonic heart: the conus arteriosus and the arterial truncus, which are responsible for the formation of the aorta and the pulmonary artery, as well as the ventricular outflow tracts. The formation of the aorta and the pulmonary artery occurs as the result of the separation of the conotruncus and the aortic bulb by the atrial septum. The embryonic arterial trunk is divided into the left and right ventral aorta, which connect to form the dorsal (descending) aorta. Six pairs of aortic arches depart from the arterial trunk. Normally, I – V pairs become empty. The right and left pulmonary arteries are formed from the ventral part of the sixth pair of aortic arches.

Three independent anatomical segments are involved in the formation of the pulmonary arterial tree:

The aorta and the pulmonary artery are formed from the embryonic truncus when it is divided by the atrial septum. 

Pulmonary arteries are formed from the sixth pair of aortic arches.

Intrapulmonary vessels are formed from the pulmonary vascular plexus, which is directly connected with the dorsal (descending) aorta.

The pulmonary artery system is formed and differentiated from the 2nd month of embryonic development. The ingrowth of the endothelial processes of the pulmonary arteries into the lung bud and their contact with the primary vascular formations induces angiogenesis, which leads to the development of the circulatory system of the lung.

Growth retardation of certain elements of the pulmonary artery system and degeneration into fibrous tissue with the formation of atresia occurs because of disruption of the normal development of the pulmonary arterial system. For example, in case of a violation of the development of the VI pair of aortic arches, one or both of the central pulmonary arteries may be completely absent. In the presence of such congenital defect, persistent primitive intersegmental arteries, originating from the descending thoracic aorta, supply the pulmonary vascular plexus.

Anatomy

This CHD can exist both as a combination with ventricular septal defect (VSD), and with various CHD, such as the transposition of the great vessels (TGV), single ventricle, etc.

There are following anatomical criteria of PA-VSD:

1. the absence of PA at various levels;
2. the presence of a large VSD;
3. the presence of additional sources of collateral blood circulation of the lungs;
4. hypertrophy of the right ventricle;
5. aortic dextroposition.

Classification

The anatomy of the pulmonary arteries is polymorphic, there is no comprehensive classification of pulmonary arterial pathology. The individual characteristic of the defect, which is descriptive, prevails in surgical practice.

Among the many proposed classifications, the Somerville classification is the most convenient for clinical use and the most common, according to which four types of defects are distinguished:

1. I – atresia of the pulmonary valve. The trunk, right and left pulmonary arteries are fully formed and passable.
2. II – atresia of the valve and pulmonary artery trunk. Both pulmonary arteries are preserved and may have a common or separate onset.
3. IIIa – atresia of the valve, trunk and right pulmonary artery. The left pulmonary artery is formed and passable.
4. IIIb – atresia of the valve, trunk and left pulmonary artery. The right pulmonary artery is formed and passable.
5. IV – atresia of the valve, the trunk and both central branches of the pulmonary arteries. The blood flow in the lungs is due to the network of collateral vessels.

Hemodynamics

Central hemodynamics responds to the anatomical structure of the heart. In the presence of PA-VSD the lungs and the right ventricle are completely disconnected, therefore, venous blood does not fall into the pulmonary circulation. The prognosis of patients with pulmonary atresia depends entirely on the availability and function of alternative sources of pulmonary arterial perfusion. Free blood flow from the right ventricle to the aorta is carried out due to the presence of a large VSD. The right ventricle works under conditions of systemic pressure, which leads to its hypertrophy. The mixed arterialized blood enters the aorta and only then through the PDA or through the collateral vessels into the lungs. Blood oxygenation in the aorta, collateral arteries and the pulmonary artery is identical and reduced, because of the mixing of arterial and venous blood. Thus, all organs are in a state of chronic hypoxia.

At the same time, there may be lung segments that are perfused only by the true pulmonary arteries or only by discrete aortic pulmonary collateral in isolated bronchopulmonary segments, and segments with double perfusion.

Hypertension and stenosis of the intrapulmonary arteries create a mosaic picture of the perfusion of certain lung segments. As the result, some of them may be in a state of pulmonary hypertension with severe sclerotic lesions of the vessels, whereas in other parts of the lungs such changes are not observed.

The threat of persistent pulmonary hypertension already exists after the neonatal period. Stenoses of collateral arteries create turbulence in the blood flow. In combination with polycythemia and increased blood viscosity it contributes to an increase in collateral obstruction, in some cases up to complete functional closure.

Workup

• Echocardiography, CT. Visualization of the PA-VSD. Assessment of the size of the right and left heart, the location and size of VSD, the degree of dextroposition of the aorta, the severity of the right ventricle hypertrophy.
• ECG. Hypertrophy of the right ventricle, deviation of the electrical axis of the heart to the right, incomplete right bundle branch block, signs of overload of the right atrium.
• Chest X-ray. Increased transparency of the pulmonary fields due to the reduced perfusion. If there are sufficiently large branches of the pulmonary artery, there is an increase in the pulmonary pattern associated with the presence of atypical shadows of the collateral vessels. In some cases, the asymmetry of the pulmonary pattern is revealed, when it is strengthened on one side and depleted on the other. 
• Cardiac catheterization with aortography. It is necessary for the determination of collateral vessels, their size, number and assessment of hemodynamic functions.

Clinical presentation

PA-VSD is a cyanotic CHD (the skin and visible mucous membranes are cyanotic). The intensity of cyanosis depends on the nature and development of collateral circulation. If collaterals create normal or excessive pulmonary circulation, systemic hypoxemia may be in a state of subcompensation for a long time. This provides satisfactory physical development for a certain time. However, even with excessive pulmonary perfusion, cyanosis results from the mixing of arterial and venous blood. 

If collaterals create normal or excessive pulmonary circulation, systemic hypoxemia can be in a state of subcompensation for a long time. This ensures satisfactory physical development for a certain period of time. Hypertensive collateral arteries, which are under systemic arterial pressure, have a serious impact on the natural course. 

This leads to the development of obstructive changes in the arterioles of the corresponding areas of the lung tissue.

The complete absence of blood ejection from the right ventricle into the pulmonary artery makes this defect "mute", however, in most cases, uncharacteristic systolic murmurs of varying intensity or continuous systolic-diastolic murmur of collaterals is heard over the heart. The clinical manifestations of PA-VSD for young children are influenced by the sources of pulmonary perfusion — the PDA, collateral arteries. 

Treatment

Most children are in need of intensive care. Prolonged intravenous supply of prostaglandin to premature babies with low weight and infants underdeveloped for their age improves arterial oxygenation and provides clinical improvement. However, some babies with complex forms of this defect may not have a PDA. There may be increased pulmonary blood flow and heart failure, requiring appropriate medical treatment. 

Surgical correction of the defect can be performed as a primary surgical procedure, usually in the first year of the patient’s life, or as the second stage of correction after palliative surgery. The goal of palliative surgery is to increase the volume of pulmonary blood flow and to prepare the patient for a radical correction of the defect.

It is recommended to perform the following types of palliative operations: creation of a systemic-pulmonary anastomosis; Reconstruction of the outflow pathways of the right ventricle without plastic surgery of VSD; creating a central anastomosis; stenting the ductus arteriosus, pulmonary artery branches. The purpose of staged surgical treatment is the surgery of VSD, the creation of adequate communication between the right ventricle and the pulmonary artery system, the elimination of extracardiac perfusion sources of the lungs, the recovery of true pulmonary perfusion using unifocalization methods. The first stage is usually the operation of reconstructing outflow tracts from the right ventricle without the surgery of VSD. All components of a multilevel stenosis are excised, plastic of the excretory tract of the right ventricle and the trunk of the pulmonary artery is performed by patch. 



In case of the II type of defect, an artificial pulmonary artery is implanted. The largest growth of the pulmonary arteries occurs for the first time 6 months after the surgery. All subsequent surgical procedures should be aimed at the complete elimination of extracardiac sources of lungs perfusion, restoring the most complete true pulmonary perfusion in the lobes and segments of the lung, increasing in underdeveloped segments or replenishment of missing segments of the central pulmonary arteries and creating a single source of pulmonary perfusion from the right ventricle.