Overview of the D-Loop
The bend of the normal primitive heart tube protrudes to the right, hence the name right flank (D-Loop), so that the right ventricle is located anteriorly on the right side and the left ventricle is located posteriorly on the left side. If during development the heart tube bends not to the right but to the left, i.e., called the left loop (L-Loop), the anatomic right ventricle is located posteriorly to the left and becomes the ventricle of the arterial system, whereas the anatomic left ventricle is located anteriorly to the right and becomes the ventricle of the venous system. At this point, the arterial trunks are still separated and rotated, but their positions are reversed, with the main pulmonary artery located right posteriorly and the ascending aorta left anteriorly. Despite the reversal of the position of the great vessels, the pulmonary artery is still connected to the venous ventricle and the aorta to the arterial ventricle, and the direction of the blood is physiologically corrected, so this is referred to as corrected transposition of the great arteries. The left and right branches of the atrioventricular conduction bundles are still distributed to the corresponding anatomic ventricles, but because the ventricles are in opposite positions, the conduction bundles are also opposite.
Etiology
This is a rare congenital heart disease in which the center canal bends to the left instead of the right during embryonic development, resulting in the anatomical right ventricle being located to the left posteriorly as the ventricle of the arterial system and the anatomical left ventricle being located to the right anteriorly as the ventricle of the venous system, where the arterial trunks are still rotated in isolation, but the position is reversed, and the direction of the blood is physiologically still normal, although the left and right branches of the AV conduction bundle are still distributed to the corresponding anatomical ventricles. However, although the left and right branches of the atrioventricular conduction bundle are still distributed to the corresponding anatomic ventricles, the conduction bundles are reversed because of the opposite position of the ventricles.
Symptoms
Depending on the combined cardiac anomalies, the following symptoms and signs are common in different anomalies:
1. Left-to-right shunt with ventricular septal defect, often with shortness of breath, decreased activity, weight loss, pulmonary infection and heart failure, signs similar to ventricular septal defect.
2. Right-to-left shunt with pulmonary stenosis, cyanosis and hypoxic episodes are common.
3. Incomplete closure of the left atrioventricular valve with an intact ventricular septum, the murmur is loudest in the parasternal 4th intercostal space and not in the apical region, and arrhythmias are common.
Blood from the superior and inferior vena cava returns to the normal right atrium, enters the right-positioned ventricle through the two-leaflet atrioventricular valve, and is then sent to the pulmonary artery. Blood from the pulmonary veins returns to the normal left atrium, enters the left-positioned ventricle through the three-leaflet atrioventricular valve, and is then sent to the aorta. The intraventricular surface of the right-positioned ventricle is structurally identical to the normal left ventricle, whereas the intraventricular surface of the left-positioned ventricle is structurally identical to the normal right ventricle, even though the positions of the left and right ventricles are reversed. Although the positions of the left and right ventricles are reversed, the direction of blood flow is still in accordance with the requirements of the normal body and pulmonary circulation because the aorta is also misplaced, and the condition can be asymptomatic if it is not accompanied by other malformations. However, most of the clinical cases are combined with other malformations, and the common ones are ventricular septal defect, pulmonary stenosis, left atrioventricular valve closure insufficiency, aortic stenosis, preexcitation syndrome, atrioventricular block and paroxysmal tachycardia.
Examination
1. Chest X-ray
There is a slightly oblique shadow of the ascending aorta on the left upper edge of the heart, absence of the shadow of the common pulmonary artery, and abnormal position of the heart, such as a right heart with normal visceral position or a middle heart or a left heart with visceral transposition.
2. Electrocardiography
Atrial arrhythmia and abnormal direction of ventricular depolarization are the main features of atrial arrhythmia, atrioventricular block accounts for about 1/3 of cases, and atrial separation, junctional arrhythmia, paroxysmal tachycardia and premature ventricular contractions are also seen.
3. Echocardiography
Echocardiography and Doppler examination can clarify the relationship between the atrial and ventricular arteries and the morphology and function of the atrioventricular valve. The position of the great arteries is abnormal, and the aorta is often located in front of the left side of the pulmonary artery. Primarily, the ventricles are interchanged in position, with the functional left ventricle having the morphologic features of the anatomic right ventricle and the functional right ventricle having the morphologic features of the anatomic left ventricle. The left atrioventricular valve is structurally characterized as a tricuspid valve, and the right atrioventricular valve is structurally characterized as a mitral valve. Echocardiography provides detailed information about whether the atrioventricular valves of the heart ride up or down and the function of the atrioventricular valves. In addition, the different combined malformations will have corresponding manifestations.
4. Right heart catheterization and cardiovascular angiography
It can show the contour, position and interrelationship of the ventricles and the aorta, and the shunt flow situation to make a clear diagnosis.
Diagnosis
Diagnosis can be made on the basis of etiology, clinical manifestations and laboratory tests.
Treatment
1. Internal medicine treatment
Mainly control heart failure and arrhythmia, but the effect is not great.
2.Surgical treatment
Depending on the deformity, including ventricular septal defect repair, if there is severe cyanosis, hypoxic episodes, body-lung shunt can be performed. Simple left atrioventricular valve insufficiency can be treated with valve repair or prosthetic valve replacement, and permanent artificial pacemakers are needed for second-degree atrioventricular block.
The surgical treatment of corrected transposition of the great arteries includes traditional surgery and anatomically corrected surgery. In recent years, anatomical corrective surgery has received increasing attention.
Currently, the most commonly performed procedure is double transposition, including intra-atrial transposition and aortic transposition or Rastelli procedure. Intra-atrial reversal is often performed using the Senning procedure or modified Senning procedure; aortic reversal is performed in patients with normal pulmonary valves and no subvalvular muscular stenosis, and the Rastelli procedure should be performed in patients with combined larger VSDs and pulmonary artery stenosis. Dual transposition is expected to become the procedure of choice for the treatment of corrected transposition of the great arteries.