Complete transposition of the great arteries (TGA) is a complex malformation of congenital heart disease. The main pathological feature is the abnormal position of the aorta and pulmonary artery, with the aorta located anteriorly on the right and connected to the right ventricle, while the pulmonary valve is located posteriorly on the left and connected to the left ventricle, resulting in a complete separation of the blood flow in the body and pulmonary circulation, which would prevent the child from surviving without the presence of a shunt at the atrial, ventricular or arterial level.
The incidence of TGA is 0.2‰ to 0.3‰. It accounts for about 5% to 7% of the total number of congenital heart disease, and ranks second in cyanotic precocious heart disease, and the ratio of male to female is 2 to 4.
1, the cause of the development of TGA is still unclear. Diabetic mothers, certain drug application mothers, in vitro fertilization, and environmental pollution will increase the incidence of TGA. If left untreated, about 90% of patients die within 1 year of age. Surgical operation is the only effective treatment.
2. Pathologic anatomy and pathophysiology Under normal conditions, the pulmonary artery is located anteriorly on the left with a myocardial cone under the valve; the aorta is posteriorly on the right, and the annulus is fibrously connected to the mitral annulus. In transposition of the aorta, the aorta is located anteriorly on the right with a subvalvular myocardial cone and a myocardial connection to the tricuspid valve; the pulmonary motion is located posteriorly on the left with no subvalvular cone present and a fibrous connection to the mitral valve. In mirror-right heart TGA, the position of the great arteries is mirrored to the left heart, with the aorta located anteriorly on the left and the pulmonary artery located posteriorly on the right. Common combined malformations include: atrial septal defect or patent foramen ovale, ventricular septal defect, patent ductus arteriosus, and pulmonary valve or subvalvular stenosis.
TGA causes complete independence of the body and pulmonary circulations due to the complete inversion of the connection of the two great arteries: venous blood returning from the superior and inferior vena cava returns to the right heart and re-enters the body circulation through the aorta. Oxygenated blood returning from the pulmonary veins returns to the left heart and enters the pulmonary circulation again through the pulmonary artery. If there is no intracardiac traffic or traffic at the arterial level, the oxygenated blood from the pulmonary circulation cannot enter the body circulation and the child will not survive due to hypoxia. Atrioventricular level shunts (patent foramen ovale, atrial septal defect, ventricular septal defect) or arterial level shunts (patent ductus arteriosus, collateral vessels) can ensure partial blood mixing in the body-pulmonary circulation and ensure oxygen supply to the lowest body circulatory system for the child to survive.
The hemodynamics of TGA depends on the degree of mixing of the right and left heart blood communication and the narrowing of the pulmonary artery opening. Complete transposition of the great arteries can be classified into three main categories according to whether it is combined with ventricular septal defect and pulmonary artery stenosis.
(1) Complete transposition of the great arteries with intact ventricular septum (TGA/IVS): the right ventricle is connected to the aorta and enlarges with increased afterload; the left ventricle is connected to the pulmonary artery and decreases with pulmonary vascular resistance, afterload decreases and low pressure in the ventricular cavity, and the left ventricle degenerates in a prolonged low pressure state. The ventricular septum is often biased toward the left ventricle. Survival of the child depends on atrial level shunts (foramen ovale or atrial septal defect) with limited shunt flow and bruising of the child.
(2) Complete transposition of the great arteries combined with ventricular septal defect (TGA/VSD): complete transposition of the great arteries with ventricular septal defect, with more mixing of blood flow between the right and left ventricles. A nonrestrictive ventricular septal defect (defect inner diameter close to or larger than the inner diameter of the aortic annulus) simultaneously brings about biventricular pressure balance, with left ventricular pressure close to the right ventricular pressure and no degeneration of the left ventricle. At the same time, the child is mildly hypoxic because of the high mixed biventricular blood flow. In restrictive ventricular septal defect, the defect is not sufficient to balance the biventricular pressure, the left ventricle remains in a low pressure state and left ventricular degeneration will occur over time. Combined with an arteriovenous ductus arteriosus, the hemodynamic effects are the same as in ventricular septal defects and can be combined in this category.
(3) Complete arterial transposition combined with ventricular septal defect and pulmonary stenosis: ventricular septal defect is mostly non-restrictive, ensuring sufficient blood mixing between the two ventricles. Pulmonary stenosis is mostly combined with subvalvular stenosis, which helps prevent the formation of severe pulmonary hypertension. The left ventricle has both a ventricular septal defect shunt and the presence of pulmonary stenosis afterload, which does not degenerate and can be selected to complete surgical correction after some older age.
Clinical features and treatment Clinical features are the early appearance of cyanosis. Most are present at birth. The cyanosis gradually worsens with age and increased activity. The cyanosis is generalized. The child has shortness of breath due to hypoxia, and in severe cases, hypoxic attacks or even death can occur.
The treatment of choice is surgical aortic reversal radical surgery (ASO). Current cardiac surgery techniques can ensure that ASO can be performed in the neonatal period with a low surgical mortality rate of 0-4.2%. Conservative medical treatment is not effective. TGA is now diagnosed by fetal echocardiography, and once diagnosed, ASO surgery should be performed as soon as possible after birth. Otherwise, the child may die at any time due to hypoxia, and after 20 days the child may have left ventricular degeneration, requiring staged surgery with significantly higher risk and mortality.
Echocardiography
(1) Purpose of examination
Echocardiography is the first and most important tool for the diagnosis of TGA. The diagnosis of TGA can be confirmed by echocardiography alone in newborns with TGA. Echocardiography also provides information on ventricular size and function, pulmonary artery pressure, valve function, and other co-morbidities, which can directly guide the choice of surgical treatment.
(2) Examination methods
TGA with echocardiographic manifestation is a complex precordial disease. In addition to conventional cross-sections, ultrasound examination also requires the flexible use of non-standard cross-sections to comprehensively understand each structure and blood flow characteristics from multiple angles.
The long-axis view of the parasternal aorta: the left atrium, left ventricular chamber and mitral valve structures are consistent with the normal heart, the physiological aortic position becomes the pulmonary artery, and the pulmonary valve is connected to the mitral annulus fibers. Tilting the probe anteriorly upward reveals that the aorta is located anteriorly emanating from the right ventricle. In the presence of a perimembranous ventricular septal defect, the defect is often adjacent to the inferior pulmonary valve, and adjustment of this cross-section can reveal the ventricular septal defect.
Color Doppler ultrasound can show a shunt from the right to the left ventricle of the ventricular septal defect. Continuous Doppler ultrasound measures the shunt velocity and, based on Bernoulli’s equation, calculates the shunt pressure difference, which allows determination of the pressure difference between the right and left ventricles. In combined pulmonary stenosis, color Doppler reveals accelerated flower-colored flow bundles in the pulmonary valve, and pulmonary valve flow velocity cannot be measured in this section because the flow is perpendicular to the direction of the acoustic beam.
M-mode ultrasound has no characteristic sign in this cross-section. after TGA/IVS type TGA ventricular degeneration, M-mode at ventricular level shows enlarged right ventricle, reduced left ventricle, septum shifted to left ventricular side, and asynchronous and isotropic motion with left ventricular wall motion.
Short-axis section of the great arteries: significantly different from the normal heart. Adjustment of the cross-section can show both short axes of the great arteries, with the aorta in the left anterior position. The pulmonary artery is located in the right posterior position. The long axis of the pulmonary artery cannot be displayed. The long axis of the main pulmonary artery can only be displayed by rotating the probe 90 degrees and adjusting the section close to the parasternal five-chamber heart section, while the long axis of the aorta can also be displayed in its right anterior position.
Apical five-chamber heart section: it can show the pulmonary arteries emanating from the left ventricular outflow tract and bifurcating upward into the right and left pulmonary arteries. Tilting the probe anteriorly to the right reveals the aorta emanating from the right ventricle. In the case of combined ventricular septal defects, the defect can be visualized below the pulmonary valve.
Color Doppler: A combined ventricular septal defect may reveal a right-to-left shunt. Stenosis or regurgitation of the pulmonary valve can also be clearly demonstrated in this cross-section. Continuous Doppler can accurately measure the antegrade flow velocity of the pulmonary valve orifice and determine the degree of stenosis.
In TGA/VSD, the left ventricular wall is not degenerating, the size and function of both ventricles remain normal, and the septum is not shifted, and the motion of the left ventricle is normal and cooperative; in TGA/IVS, when the left ventricle is degenerating, the septum is shifted to the left ventricular side, which is not consistent with the motion of the left ventricular wall and changes to cooperative motion of the right ventricular wall. In combination with pulmonary valve or subvalvular stenosis, left ventricular hypertension is partially preserved and left ventricular function is similar to the TGA/VSD type. When the VSD is a small restrictive defect, it is not sufficient to preserve left ventricular pressure, and the left ventricle also degenerates, similar to TGA/IVS. Combined with an atrial septal defect, adjustment of the cross-section can show the edges of the defect.
Color Doppler ultrasound can show atrial shunts, and in TGA the shunt is mostly left-to-right.
Subxiphoid cross-section: A double atrial cross-section can clearly show foramen ovale separation or atrial septal defect. Color Doppler ultrasound can also clearly show atrial shunts. Aortic long-axis views can show the long axis of both aorta, with the aorta originating from the right ventricle and the pulmonary artery originating from the left ventricle.
TGA can be combined with narrowing of the descending arch, interrupted arch and other vascular malformations. The ductus arteriosus can be clearly shown in this section in the presence of an unclosed ductus arteriosus. Color Doppler ultrasound can show catheter shunts. Pulmonary artery pressure can be determined from the shunt velocity and pressure difference, which can be used to determine left ventricular pressure.
In children with TGA beyond the neonatal period, the function of the left ventricle needs to be determined in order to decide whether ASO can be performed. Left ventricular degeneration is determined primarily by assessing left ventricular pressure. ASO surgery should be considered only when the left ventricular pressure reaches 80% or more of the right ventricular pressure. Ventricular shunt pressure difference, arteriovenous shunt pressure difference, and mitral regurgitation pressure difference can all be used to estimate LV pressure difference. Measurement of the pulmonary orifice pressure difference in the presence of pulmonary valve or subvalvular stenosis can also determine LV pressure difference. In the absence of either shunt or regurgitation, it is difficult to determine left ventricular pressure. The surgeon will choose to open the chest to measure the left ventricular pressure, and perform ASO when the pressure meets the standard, or switch to left ventricular training when the pressure is insufficient, i.e. pulmonary artery annuloplasty with body-pulmonary shunt.
(3) Ultrasound differential diagnosis
Anomalous origin of the right pulmonary artery from the aorta: The ascending aorta emits the right pulmonary artery which is easily treated as a bifurcation of the main pulmonary artery into the right and left pulmonary arteries. The main pulmonary artery continues only as the left pulmonary artery and is easily treated as the aorta. This is the most likely congenital malformation to be misdiagnosed as TGA, in which the aorta is located right anterior to the pulmonary artery. In TGA, the aorta is located on the right side of the pulmonary artery. The pulmonary artery, which is easily treated as the aorta in the anomalous origin of the right pulmonary artery, is located anteriorly on the left side.
Double outlet of the right ventricle: identification of double outlet of the right ventricle. In the abnormal aortic relationship type, the relationship between the two aorta is also reversed and needs to be distinguished from TGA. In TGA, the aorta is completely from the right ventricle and the pulmonary artery is completely from the left ventricle.