Transposition of the great vessels

What is Transposition of the arteries (TGA)?

Transposition of the arteries (TGA) is a congenital heart disease congenital heart disease, which is characterized by the departure of the aorta from the right ventricle, and the pulmonary artery from the left ventricle. The aorta supplies venous blood to the tissues of the body, the pulmonary artery supplies arterial blood to the lungs.

Embryology

This CHD refers to anomalies of conotruncus. 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. Conotruncus is the outflow tract of the heart of a 2-3 week embryo. Normally, at the 4-5 week of fetal development, conotruncus is divided by a spiral septum into relatively equal in size large vessels: the aorta and the pulmonary trunk.

The classical theory of the development of this defect is a violation of the formation of the conotruncus spiral septum. An alternative theory is the underdevelopment of the subarterial cone. As a result, a fibrous connection occurs between the pulmonary and mitral valves (mitral-pulmonary contact), which is one of the signs of transposition.

Anatomy

Anatomically, in the normal heart, the aorta departs from the left ventricle, there is mitral-aortic fibrous contact and a subpulmonary cone. In the presence of classical TGA, pulmonary and systemic circulation are completely separate. The aorta, located anteriorly, departs from the right ventricle, and the pulmonary artery behind the aorta departs from the left ventricle. Desaturated blood circulates in a systemic circle, and oxygenated blood in a pulmonary circle.

For the patient's life, mixing of blood is necessary, which occurs at various levels - through ASD, VSD, and PDA. Often this defect is combined with other cardiac abnormalities. Other variants of this defect are described in the classification.

Classification

• D – TGA with intact ventricular septum – classic full TGA, the aorta is located in front and to the right of the pulmonary trunk.
• D – TGA with ventricular septal defect – classic full TGA, the aorta is located in front and to the right of the pulmonary trunk and ventricular septal defect.
• cc-TGA – congenitally corrected transposition of the great arteries, in which the morphological left and right ventricles with the corresponding atrioventricular valves are also displaced.

Hemodynamics

The blood flow is carried out in two divided circles of blood circulation. Venous blood from the right ventricle enters the aorta. Having passed a systemic circle of blood circulation, blood returns to the right heart. The left ventricle receives arterialized blood from the pulmonary veins and pumps it into the pulmonary artery. 

Mixing the blood of two circles is possible only if there are connections at various levels. The degree of arterial hypoxemia depends on the size of the connections and on the volume of mixing of pulmonary and systemic venous blood return.

In the presence of classical type of transposition, mixing of venous and arterial blood occurs only at the atrial level through the PFO and at the level of the main arteries through the PDA. Mixing is usually minimal, so severe hypoxemia occurs.

The blood circulation of the fetus does not suffer. However, immediately after birth, there is an acute threat to life due to low oxygen saturation of the systemic arterial blood. In cases where there are wide messages at different levels between the large and small circles of blood circulation, hypoxemia is not pronounced. In the presence of a small and medium VSD blood shunt from right to left occurs during systole, from left to right - during diastole. In the presence of large VSD with pulmonary hypertension due to rapidly developing obstructive pulmonary vascular disease, as well as with severe stenosis of the pulmonary artery, blood shunting from right to left into diastole, from left to right - into systole. If there are connections at different levels, the blood shunt is carried out in opposite directions.

In the presence of severe obstruction of the outflow tract of the right (systemic) ventricle, blood flow in a large circle is ductus-dependent. The closure of PDA is accompanied by a rapidly progressive deterioration in systemic perfusion.

Workup

• Echocardiography, CT. Visualization of the defect.
• ECG. The electrical axis of the heart is deflected to the right. Hypertrophy of the right ventricle (classical TGA). Hypertrophy of both ventricles can occur in the presence of concomitant VSD, PDA or obstructive pulmonary vascular disease.
• Chest X-ray. Cardiomegaly, an egg-shaped form of the heart shadow with a narrow upper mediastinum (the main vessels are located in the anteroposterior direction) are characteristic. Pulmonary pattern is intensified.
• Cardiac catheterization with aortography. It is used for balloon atrioseptostomy. Also used for diagnostic purposes to measure pressure in the left ventricle, clarify the details of the anatomy of the defect and the features of the passage of the coronary arteries.

Clinical presentation

Such clinical symptoms as cyanosis with congestive heart failure, dyspnea, and feeding difficulties during the neonatal period are characteristic for patients with TGA. Especially cyanotic newborns with a large body weight; tachypnea usually without retraction if there is no congestive heart failure. Cyanosis is typic for newborns with a large body weight. Severe arterial hypoxemia may be noted, and is not eliminated by oxygen inhalation.

Auscultation: II heart sound is loud, undifferentiated. If the interventricular septum is intact, there is no murmur. Systolic murmur is heard if there is a concomitant VSD. Sometimes a soft systolic murmur is heard in the presence of pulmonary artery stenosis.

Treatment

Surgical intervention is usually preceded by balloon atrioseptostomy (Rashkind operation). Patients who do not have a message at the atrial or ventricular level (ASD, VSD), balloon atrioseptostomy should be performed immediately after their admission to the cardiac surgery center. At the preoperative stage, it is necessary to correct metabolic acidosis, hypoglycemia and hypocalcemia, to start infusion of prostaglandin to improve the saturation of arterial blood with oxygen by maintaining the patency of the PDA. At the moment, there is an opinion that there is no need for prostaglandin infusion in the presence of a functioning ASD. If the patient has a sufficient ASD, the correction is performed without previous interventions. The choice of surgery depends on the presence or absence of concomitant heart defects, the anatomy of the coronary arteries, and the type of transposition (D / L - TMS). At the atrial level, redistribution of blood flows is performed using one of two options - Senning surgery (pulmonary and systemic venous return is redirected using atrial tissue) or Mustard surgery (using a pericardial patch).



Currently, these operations are used to treat corrected transposition of great vessels (as part of the Double Switch operation). The anatomical correction of the defect includes the operation of arterial switching. The arterial switch operation (Jatene) is performed in the neonatal period (3-5 days), while the left ventricle has not yet lost the ability to carry out systemic circulation, as in the intrauterine period. The essence of the operation is to move the aorta together with the coronary arteries to the site of the pulmonary trunk, and the trunk, respectively, to the site of the aorta. TMS with VSD and severe subaortic stenosis is corrected using the Damus-Kaye-Stansel operation (connect the proximal part of the pulmonary trunk with the ascending aorta, close the VSD and connect the right ventricle with the distal part of the pulmonary artery using synthetic or biological prostheses).