Complete transposition of the great arteries, also known as D-transposition of the great arteries (D-TGA), is a condition in which the atrioventricular connections are consistent and the ventricular great artery connections are not, i.e., the anatomical right ventricle is connected to the aorta and the anatomical left ventricle is connected to the pulmonary artery.D-TGA is a common form of cyanotic congenital heart disease, accounting for approximately It is the most common cyanotic congenital heart disease in the neonatal period, most prone to heart failure, and has the highest mortality rate. If the disease is not combined with pulmonary stenosis, most of the children lose the chance to be cured in the first 4 months of life; if left untreated, more than 90% of the children die within 1 year of age. A complete and accurate preoperative diagnosis is important because of the different surgical approaches for children of different ages and conditions [1,2,3]. Cardiac angiography is the gold standard for the diagnosis of congenital heart disease, but its clinical application is limited due to its invasive nature and inconvenient review and follow-up [4]. Echocardiography technique has the advantages of simplicity and noninvasiveness and has been widely used for the diagnosis of various congenital heart diseases [5]. In this paper, the application value of echocardiography in the diagnosis and treatment of D-TGA is reviewed as follows. I. Pathological basis of complete transposition of the great arteries The embryological formation mechanism of D-TGA is related to the abnormal separation and rotation of the conical arterial trunk [6]. Due to the appearance of a cone beneath the aorta and the disappearance of the cone beneath the pulmonary artery, the anterior-posterior relative relationship of the great vessels is reversed, i.e., the aorta shifts from its original posterior position to an anterior position, while the anterior aorta emanates from the right ventricle and the posterior pulmonary artery from the left ventricle. Normally, there is no cone structure under the aorta. After transposition of the great vessels, a myocardial cone appears under the aortic valve, which is not directly continuous with the tricuspid valve. In contrast, there is fibrous continuity between the pulmonary valve and the mitral valve. The right ventricular wall is thin before birth but grows rapidly after birth and can overtake the left ventricle. The most obvious feature of transposition of the great arteries is the abnormal relative position of the aorta and pulmonary arteries. It is common for the aorta to be anterior to the right of the pulmonary artery. It is also seen to be anterior and anterior to the left of the pulmonary artery, and very rarely located posterior to the right of the pulmonary artery. D-TGA can be classified according to whether it is combined with ventricular septal defect and left ventricular outflow tract obstruction. When transposition of the great arteries is not associated with other cardiovascular malformations (except for patent foramen ovale and patent ductus arteriosus), it is called simple transposition of the great arteries (about 50% of cases). Ventricular septal defect is present in about 40% to 45% of cases, and nearly 2/3 of them have small septal defects. Ventricular septal defects are commonly perimembranous defects and myocardial defects. Poorly aligned ventricular septal defects can occur when the conical (outflow tract) septum is shifted forward or backward. Forward deviation of the conical septum can lead to subaortic stenosis, which may be associated with aortic constriction or interruption of the aortic arch. When the poorly aligned conical septum deviates toward the right ventricle, it can cause part of the pulmonary valve to ride over the right ventricle, and in severe cases the septal defect lies directly under the pulmonary valve, forming a type of right ventricular double outlet, Taussing Bing malformation. About 10% to 25% of cases are combined with secondary foramen ovale septal defects. Combined with left ventricular outflow tract obstruction in about 25% of cases, obstruction can occur anywhere in the left ventricular outflow tract. When the septum is intact, the right ventricular outflow tract is parallel to the left ventricular outflow tract, and the septum looks like a flat plate, unlike the normal spatial curvature deflection. In this case, there is often a combination of mild to moderate power subpulmonary stenosis associated with septal deflection into the left ventricular cavity, anterior mitral valve motion during systole, and pulmonary valve jitter until mid-systolic closure (due to low pulmonary artery pressure resulting in low end-systolic left ventricular cavity volume). In cases of combined ventricular septal defect, in addition to pulmonary valve and subvalvular stenosis, there is also a posterior displacement of the conical septum, abnormal attachment of the mitral valve apparatus to the ventricular septum, and overgrowth of the tricuspid valve protruding through the ventricular septal defect to the lower part of the pulmonary artery. Transposition of the great arteries creates a circulatory system parallel to each other between the corporal and pulmonary circulations. In the fetus, the parallel body and pulmonary circulations do not interfere with normal development because of the presence of an unclosed foramen ovale and ductus arteriosus. In the state of parallel body and pulmonary circulation after birth, the oxygenated pulmonary venous blood still flows into the pulmonary circulation via the left ventricle, while the unoxygenated body circulation blood cannot enter the lungs for gas exchange. Survival can only be maintained by the presence of traffic between the two circulations, such as open foramen ovale, atrial septal defect, ventricular septal defect, and patent ductus arteriosus. The amount of effective blood flow exchanged between the two circulations depends on the site and size of the exchange. The fractional flow also depends on the ventricular compliance, ventricular pressure step difference, respiratory phase and vascular resistance of the circulation. Echocardiographic diagnosis of D-TGA (a) Echodiagnosis of complete transposition of the great arteries By sequential segmental echocardiography, the location of atria, ventricles and great arteries and their interconnections can be determined. D-TGA can be diagnosed based on the characteristics of consistent atrioventricular connections and inconsistent ventricular aortic connections. The subxiphoid view can simultaneously show the outflow tracts and aorta of both ventricles, which is particularly valuable for the diagnosis of D-TGA. The subxiphoid four-chamber view shows the anatomical left ventricle on the left and the anatomical right ventricle on the right, and the probe facing upward shows the common pulmonary artery trunk connected to the left ventricle, which is divided into the left and right pulmonary arteries. In D-TGA, the aorta and pulmonary artery are in parallel, whereas in a normal heart, the right ventricular outflow tract is right-to-left in front of the aorta, and the left ventricular outflow tract is left-to-right, and the aorta and pulmonary artery are in a spiral shape. Thus, the parasternal long-axis view shows both the aorta in the anterior aspect and the pulmonary artery in the posterior aspect, but the aortic valve is positioned higher than the pulmonary valve. A short-axis view of the parasternal aorta shows the aorta in relation to the root of the pulmonary artery. In the majority of cases, the aortic valve is located right anterior to the pulmonary valve. With the probe pointing slightly upward, the posteriorly located pulmonary artery bifurcation can be seen as the left and right pulmonary arteries. (ii) Ultrasonographic diagnosis of combined malformations The unclosed ductus arteriosus is easily revealed in the long-axis view of the suprasternal aortic arch. Due to the parallel spatial position of the great arteries, the ductus arteriosus can be seen in the section of the aortic arch. In addition, when showing the pulmonary artery, color flow imaging reveals shunted blood flow through the arterial duct. A subxiphoid four-chamber or two-atrial view is often used to examine for the presence of patent foramen ovale or atrial septal defect, and in combination with Doppler examination, to estimate shunt flow. Ventricular septal defects are commonly found in the outflow tract septum and perimembranous region. A parasternal left ventricular long-axis view is the best view to show poorly aligned outflow tract septal defects. The funicular septum is anteriorly displaced and poorly aligned with the trabecular septum, which can result in the pulmonary artery riding over the trabecular septum and right ventricular outflow tract stenosis. A subpulmonary septal defect with more than 50% pulmonary artery span resembles a right ventricular double outlet, but D-TGA still maintains the direct connection between the pulmonary valve and mitral valve. In parasternal short-axis views, the aorta and pulmonary artery are juxtaposed in right ventricular double outlet, whereas in D-TGA the aorta and pulmonary artery are more anteriorly and posteriorly positioned. Right ventricular outflow tract stenosis may also be associated with aortic arch obstruction. Apical and subxiphoid four-chamber views of subaortic stenosis or tricuspid valve malformation are helpful. Parasternal left ventricular long-axis and subxiphoid four-chamber views may reveal poorly aligned septal defects and subpulmonary stenosis. The perimembranous ventricular septal defect appears to be characterized by the absence of septal tissue between the mitral and tricuspid valves in the apical and subxiphoid four-chamber views. Zhang Yuqi et al [7] summarized 118 patients with surgically confirmed D-TGA, and found 112 cases (94.9%) with atrial orthotropia and right transposition of the right collaterals of the ventricle (S.D.D), and 6 cases (5.1%) with atrial inversion and left transposition of the left collaterals of the ventricle (I.L.L). 85 cases (72.0%) with D-TGA combined with atrial septal defect/unclosed foramen ovale and 68 cases (57.6%) with arteriovenous ductus arteriosus. 68 cases (57.6%), ventricular septal defect 35 cases (29.7%), pulmonary artery stenosis 29 cases (24.6%), and other combined malformations such as left superior vena cava remnant, left ventricular outflow tract obstruction, complete atrioventricular septal defect, aortic constriction, and interrupted aortic arch were basically consistent with the results of the literature [5]. Third, the diagnosis of D-TGA combined with coronary artery malformation Aortic reversal is the preferred surgical option for D-TGA, but coronary artery grafting is required. Since the coronary arteries in children are small, if they are combined with origin or alignment abnormalities at the same time, it is often difficult to operate or leads to intraoperative coronary artery injury, resulting in insufficient postoperative myocardial perfusion and cardiogenic death; therefore, it is important to correctly determine the type of coronary artery anatomy before surgery [3]. The application of two-dimensional echocardiography has been reported to clearly demonstrate the coronary arteries in approximately 95% of cases. A short-axis view of the parasternal aortic root can show the origin and proximal segment of the coronary artery, when the probe is rotated 10-30° clockwise to clearly show the left coronary artery and 10-30° counterclockwise to clearly show the right coronary artery. A long-axis view of the parasternal area with the probe facing the left shoulder will show the left coronary artery and its branches, while a long-axis view through the aortic root will show the right coronary artery. The subxiphoid and apical views are helpful in observing the abnormalities of the right coronary artery branches [8,9]. In 118 patients summarized by Zhang Yuqi et al [10], there were 104 normal coronary arteries (88.1%) and 14 abnormal coronary arteries (11.9%). The aorta was located on the right side combined with coronary artery malformation in 13 cases (92.9%); the aorta was anteriorly combined with coronary artery malformation in 1 case (7.1%). Among the 14 anomalous coronary arteries, echocardiographic examination was correct in 10 cases (71.4%); 3 cases were single coronary artery in the right coronary sinus, 3 cases were gyral branch from the right coronary artery, 2 cases were two coronary arteries in the right coronary sinus, and 2 cases were two coronary arteries in the left coronary sinus. 2 cases of coronary artery. In the remaining 4 patients, coronary artery “openings” were seen at the left and right coronary sinuses in 2 cases, which were mistaken for normal coronary arteries, with a misdiagnosis rate of 14.3%; in 2 cases, coronary artery openings were visible, but the proximal branches and their course were not clearly shown, and the diagnosis could not be clearly made, with a missed diagnosis rate of 14.3%. In this study, the results were the same as those of McMahon et al [11] in foreign countries, but the incidence was 11.9%, which was lower than that of 33.0% in foreign countries. A domestic study by Huang Meirong et al [12] concluded that the incidence of coronary artery anomalies was 25.5%, and the common types were single coronary artery in the right coronary sinus (7.3%) and two coronary arteries in the right coronary sinus (5.5%). The low incidence of coronary artery anomalies in this group may be related to the fact that all patients in this group were surgical patients and did not include those who abandoned treatment or died preoperatively. D-TGA is the result of paradoxical absorption of the embryonic inferior cones of the main and pulmonary arteries, and the difference in the degree of cone rotation leads to various different spatial positions of the semilunar valves in the two groups [6]. Due to the abnormal rotation, the connection between the spent sinus-derived coronary germ and the coronary vessel germ cells located in the interventricular sulcus and interatrial sulcus is abnormal, resulting in abnormal coronary artery origin and course. In our study, the incidence of coronary artery anomalies was significantly lower in those with anteriorly located aorta (1.9%) than in those with right anterior and right aorta (22.0%), while Eliiott et al [13] suggested that those with right anterior aorta were mostly typical coronary arteries and that gyral branches originating from the right coronary artery were mostly seen in those with right and left juxtaposition of the main and pulmonary arteries. This suggests that there may be a relationship between coronary artery anomalies and the angle of rotation of the main and pulmonary arteries. Other authors have suggested that the occurrence of coronary artery anomalies may be associated with ventricular septal defects. In our study, we found that the incidence of coronary artery anomalies was significantly higher in patients with combined ventricular septal defects (25.8%) than in those with intact ventricular septum (6.0%). This suggests that the occurrence of coronary artery anomalies is indeed related to the presence of ventricular septal defect. Fourth, the role of echocardiography in determining the timing of rapid phase II aortic reversal surgery for D-TGA Aortic reversal surgery (ASO) is the best surgical option for correcting D-TGA. In patients with an intact ventricular septum or a combined small ventricular septal defect, age at the time of surgery is preferably less than 2 weeks. If the patient is older at the time of consultation, the left ventricle is degenerating and cannot take the afterload of the body circulation, the success rate of ASO is lower [14]. In these cases, intra-atrial transposition (Senning or Mustard procedure) was often chosen in the past, and although the success rate was around 95%, there were more long-term complications, such as right ventricular failure, tricuspid regurgitation and arrhythmias, which are now less commonly used in clinical practice. In 1989, Jonas et al [15] proposed the concept of rapid second-stage aortic reversal, and concluded that the left ventricular mass could rise by a factor of 1 in about 7 days after pulmonary artery annuloplasty, and it was safe to perform aortic reversal, but the indications for performing pulmonary artery annuloplasty and the indicators for judging the timing of the second operation were not clear. in 2001, Cedil et al [16] studied 22 patients with intact ventricular septum in D-TGA In 2001, Cedil et al [16] studied 22 patients with D-TGA with intact ventricular septum and concluded that if the child was older than 3 weeks at the time of surgery, the septum protruded to the left ventricular surface, and the left ventricular mass index was less than 35 g/m2, rapid second-stage aortic reversal should be performed; about 10 days after pulmonary artery annuloplasty and body-lung bypass, if the right and left ventricular pressure ratio was greater than 65% and the left ventricular mass index was greater than 50 g/m2, aortic reversal was feasible. According to this criterion, Zhang Yuqi et al [17] performed rapid second-stage great artery reversal in 13 patients with D-TGA with intact ventricular septum or combined with small ventricular septal defects, and the operation was successful in 10 cases and fatal in 3 cases. This is similar to the results of Cedil et al [16], indicating that rapid second-stage great artery reversal is feasible in D-TGA patients with intact septum or combined with small ventricular septal defects at an older age. The study by Zhang Yuqi et al. also found that the LV mass index was significantly higher than 50 g/m2 in all 13 patients, suggesting that LV mass index does not predict the outcome of second-stage surgery. Independent analysis of LVDD, LVPWT, and IVST in the calculation formula revealed that LVDD, LVPWT, and IVST were significantly increased in 10 patients with successful surgery and increased left ventricular mass; parasternal short-axis views of the left ventricle showed that the septum protruded toward the right ventricular surface. In contrast, none of the three patients who died had a significant increase in LVDD, and the left ventricular mass still increased due to the increase in LVPWT and IVST; however, the parasternal left ventricular short-axis view showed a flattened septum that failed to protrude significantly toward the right ventricular surface. This is similar to the results of Lyer et al [18], suggesting that LVDD and septal shape can be used to predict surgical outcome. In 13 patients with normal LV function before pulmonary artery annuloplasty, LV systolic function decreased significantly on day 1 after annuloplasty, which may be related to an acute increase in LV afterload and LV myocardial edema. In those with intact atrial septum, if the left ventricular afterload increases sharply and the ventricle is hypertrophied, this may lead to an increase in ventricular diastolic pressure, and it is known that ventricular systolic function will compensate according to the Frank-Starling curve [19,20]. In our patients, all of whom had combined atrial septal defect or patent foramen ovale and ventricular hypertrophy, ventricular diastolic pressure did not change significantly due to the presence of non-restrictive left-to-right shunt at the level of the atria during diastole, and compensatory mechanisms did not exist, then cardiac insufficiency occurred. Three to four days after annuloplasty, with the hypertrophy of the left ventricle, myocardial contractility rises, cardiomyocyte edema subsides, and left ventricular systolic function gradually returns to normal. The left ventricular ejection fraction and shortening fraction were normal before and after pulmonary artery annuloplasty in our three deceased patients, indicating that a single index of cardiac function cannot be used to predict the surgical outcome. In conclusion, we believe that echocardiography can diagnose D-TGA and its combined malformations more accurately, provide an important objective indicator for the judgment of the timing of surgery, and play a very important role in the diagnosis and treatment of precardiac disease.