Transposition of the great vessels overview

# Overview
Transposition of the great arteries (TGA) is a pediatric cardiac congenital defect arising from an embryological discordance between the aorta and pulmonary trunk. During cardiac development, the conotruncal septum spirals toward the aortic sac thus dividing the truncus arteriosus into the pulmonary and aortic channels. These channels then become the pulmonary arteries and aorta, respectively. TGA occurs when the conotruncal septum fails to follow its spiral course and instead forms in a linear orientation. Consequently, the aorta arises from the right ventricle and the pulmonary trunk arises from the left ventricle. The TGA was first described in 1797 by Matthew Baillie as a "singular malformation". The word transposition was coined by Farre in 1814. Right atrium (RA) is connected to a morphologic right ventricle (RV). The morphologic left atrium (LA) is connected to the morphologic left ventricle (LV). This is called atrio-ventricular concordance. In a normal heart, the great arteries (aorta and pulmonary arteries) are concordant with the morphologic LV and RV. This is termed ventriculo-arterial concordance. In addition, the aorta and pulmonary trunk ascend in a spiral relationship. In the TGA the aorta arises from the morphologic right ventricle via a subaortic infundibulum and the pulmonary artery arises from the morphologic left ventricle, without a subpulmonary infundibulum. These ventriculoarterial connection is known as ventriculoarterial discordance. The abnormal origin of the great arteries results in an altered spiral relationship resulting in parallel circulation. Transposition of great vessels can be classified based on concordance of ventriclular positions into dextro-TGA and levo-TGA. The causes for transposition of the great arteries is unknown and is presumed to be multifactorial. The embryology likely involves abnormal persistence of the subaortic conus with resorption or underdevelopment of the subpulmonary conus (infundibulum). This abnormality aligns the aorta anterior and superior with the right ventricle during development. The prevalence of TPA is approximately 47 per 100,000 individuals worldwide. TGA accounts for 5-7 percent of all congenital heart disease and 20 percent of cyanotic heart disease. There is no racial predilection to transposition of great vessels. Transposition of the great arteries has a 60-70% male predominance. TGA is not known to be associated with any specific single gene defect, but some studies have shown possible genetic association in some cases of TGA, involving deletions of chromosome 22q11. Other risk factors in the mother that may increase the risk of this condition include age over 40, alcoholism, diabetes, prenatal malnutrition and rubella or other viral illness during pregnancy. Majority of the time, diagnosis can be made after 18 weeks gestation using an ultrasound. However, if it is not diagnosed in utero, cyanosis of the newborn should immediately direct towards diagnosis of TGA. If left untreated, over 50 percent of infants with transposition of the great arteries will die in the first month of life. 90 percent will die in the first year. Common complications of TGA include congestive heart failure, arrhythmia, pulmonary artery stenosis and aortic regurgitation. The prognosis for patients with TGA is generally excellent following surgical correction with survival rates greater than 90%. Without treatment, 30% of infants die within the first week of life, 50% will die in the first month, 70% will die in the first 6 months and 90% of infants will die before the end of the first year. The clinical features of D-TGA are solely dependent on the degree of mixing between the parallel circuits. Most patients present with signs and symptoms during the neonatal period. Symptoms of D-TGA present with cyanosis, tachypnea and murmurs. Patients with L-TGA present with symptoms of heart failure until later in life when the right ventricle can no longer compensate increased after load. Newborns with transposition of the great arteries are usually well developed, without dysmorphic features. Physical findings at presentation depend on the presence of associated lesions. In many cases, TGV is accompanied by other heart defects, the most common type being intracardiac shunts such as atrial septal defect (ASD), patent foramen ovale (PFO), ventricular septal defect (VSD), and patent ductus arteriosus (PDA). Stenosis, or other defects, of valves and/or vessels may also be present. An abnormal pulse oximetry findings with a discrepancy between the upper and lower extremities is a consistent finding with transposition of great vessels. D-TGA is difficult to detect on fetal ultrasound due to the absence of differences in ventricle size. There are no specific ECG findings associated with TGA. However, sometimes electrocardiogram may show right axis deviation and right ventricular hypertrophy. An x-ray may be helpful in the diagnosis of TGA. Findings on an x-ray suggestive of TGA include egg on a string appearance of the heart, increased pulmonary vascular markings, and cardiomegaly. Echocardiography and Doppler examination acts as the main diagnostic tool in transposition of great artyeries. Echocardiography demonstrates pulmonary arteries arising from the posterior left ventricle, and the aorta rising anteriorly from the right ventricle. Echocardiography also detects other associated anomalies like ventricular septal defect (VSD), patent ductus arteriosus and the coronary artery anatomy. Computed tomography can be helpful as a diagnostic tool in conditions where the echocardiographic findings are inconclusive. CT can done faster compared to MRI, thus avoiding the need for anesthesia in small children. Provides additional anatomic details compared to echocardiography. Magnetic resonance imaging (MRI) can be used as a diagnostic modality in congenital heart diseases. Helps in measuring heart volumes, blood flow and ventricular wall thickness. MRI can be used in cases where echocardiographic results are inconclusive. Cardiac catheterization is not frequently done to diagnose transposition of the great vessels, as it could be done confidently with echocardiography. It is in conditions when the echo findings are inconclusive. It can be used to determine the coronary anatomy. Cardiac catheterization can also be used as interventional while performing balloon atrial septostomy. This operation helps to increase mixing between the two circulatory systems. Surgery is the mainstay of treatment for TGA. However, infusion of PGE1 to a newborn diagnosed with TGA is recommended as it prevents the ductus arteriosus from closing, therefore providing an additional shunt through which to provide the systemic circulation with a higher level of oxygen. Surgical approach is the mainstay of treatment for transposition of great vessels. Type of surgery mainly depends on the age of the patient at presentation, the presence of associated congenital cardiac lesions, and the experience of the cardiothoracic surgeon with a given surgical technique. Most full-term neonates with uncomplicated transposition of the great arteries can undergo an arterial switch procedure in one operation, with minimal mortality. Recent advances in surgical correction of transposition of the great arteries have reduced the mortality drastically from 95% in uncorrected patients to 5% in corrected patients. Post-operative care is very similar to the palliative care received, with the exception that the patient no longer requires PGE or the surgical palliation procedures. Additionally, the patient is kept on a cooling blanket for a period of time to prevent fever, which could cause brain damage. The sternum is not closed immediately which allows extra space in the thoracic cavity, preventing excess pressure on the heart, which swells considerably following the surgery; the sternum and incision are closed after a few days, when swelling is sufficiently reduced.

# Historical Perspective
The TGA was first described in 1797 by Matthew Baillie as a "singular malformation". The word transposition was coined by Farre in 1814.

# Classification
Transposition of great vessels can be classified based on concordance of ventriclular positions into dextro-TGA and levo-TGA.

# Pathophysiology
Right atrium (RA) is connected to a morphologic right ventricle (RV). The morphologic left atrium (LA) is connected to the morphologic left ventricle (LV). This is called atrio-ventricular concordance. In a normal heart, the great arteries (aorta and pulmonary arteries) are concordant with the morphologic LV and RV. This is termed ventriculo-arterial concordance. In addition, the aorta and pulmonary trunk ascend in a spiral relationship. In the TGA the aorta arises from the morphologic right ventricle via a subaortic infundibulum and the pulmonary artery arises from the morphologic left ventricle, without a subpulmonary infundibulum. These ventriculoarterial connection is known as ventriculoarterial discordance. The abnormal origin of the great arteries results in an altered spiral relationship resulting in parallel circulation.

# Causes
The causes for transposition of the great arteries is unknown and is presumed to be multifactorial. The embryology likely involves abnormal persistence of the subaortic conus with resorption or underdevelopment of the subpulmonary conus (infundibulum). This abnormality aligns the aorta anterior and superior with the right ventricle during development.

# Differentiating  Transposition of Great Vessels from Other Diseases

# Epidemiology and Demographics
The prevalence of TPA is approximately 47 per 100,000 individuals worldwide. TGA accounts for 5-7 percent of all congenital heart disease and 20 percent of cyanotic heart disease. There is no racial predilection to transposition of great vessels. Transposition of the great arteries has a 60-70% male predominance.

# Risk Factors
TGA is not known to be associated with any specific single gene defect, but some studies have shown possible genetic association in some cases of TGA, involving deletions of chromosome 22q11. Other risk factors in the mother that may increase the risk of this condition include age over 40, alcoholism, diabetes, prenatal malnutrition and rubella or other viral illness during pregnancy.

# Screening
Majority of the time, diagnosis can be made after 18 weeks gestation using an ultrasound. However, if it is not diagnosed in utero, cyanosis of the newborn should immediately direct towards diagnosis of TGA.

# Natural History, Complications, and Prognosis
If left untreated, over 50 percent of infants with transposition of the great arteries will die in the first month of life. 90 percent will die in the first year. Common complications of TGA include congestive heart failure, arrhythmia, pulmonary artery stenosis and aortic regurgitation. The prognosis for patients with TGA is generally excellent following surgical correction with survival rates greater than 90%. Without treatment, 30% of infants die within the first week of life, 50% will die in the first month, 70% will die in the first 6 months and 90% of infants will die before the end of the first year.

# Diagnosis

## History and Symptoms
The clinical features of D-TGA are solely dependent on the degree of mixing between the parallel circuits. Most patients present with signs and symptoms during the neonatal period. Symptoms of D-TGA present with cyanosis, tachypnea and murmurs. Patients with L-TGA present with symptoms of heart failure until later in life when the right ventricle can no longer compensate increased after load.

## Physical Examination
Newborns with transposition of the great arteries are usually well developed, without dysmorphic features. Physical findings at presentation depend on the presence of associated lesions. In many cases, TGV is accompanied by other heart defects, the most common type being intracardiac shunts such as atrial septal defect (ASD), patent foramen ovale (PFO), ventricular septal defect (VSD), and patent ductus arteriosus (PDA). Stenosis, or other defects, of valves and/or vessels may also be present.

## Laboratory Findings
An abnormal pulse oximetry findings with a discrepancy between the upper and lower extremities is a consistent finding with transposition of great vessels. D-TGA is difficult to detect on fetal ultrasound due to the absence of differences in ventricle size.

## Electrocardiogram
There are no specific ECG findings associated with TGA. However, sometimes electrocardiogram may show right axis deviation and right ventricular hypertrophy.

## X-ray
An x-ray may be helpful in the diagnosis of TGA. Findings on an x-ray suggestive of TGA include egg on a string appearance of the heart, increased pulmonary vascular markings, and cardiomegaly.

## Echocardiography and Ultrasound
Echocardiography and Doppler examination acts as the main diagnostic tool in transposition of great artyeries. Echocardiography demonstrates pulmonary arteries arising from the posterior left ventricle, and the aorta rising anteriorly from the right ventricle. Echocardiography also detects other associated anomalies like ventricular septal defect (VSD), patent ductus arteriosus and the coronary artery anatomy.

## CT scan
Computed tomography can be helpful as a diagnostic tool in conditions where the echocardiographic findings are inconclusive. CT can done faster compared to MRI, thus avoiding the need for anesthesia in small children. Provides additional anatomic details compared to echocardiography.

## MRI
Magnetic resonance imaging (MRI) can be used as a diagnostic modality in congenital heart diseases. Helps in measuring heart volumes, blood flow and ventricular wall thickness. MRI can be used in cases where echocardiographic results are inconclusive.

## Other Diagnostic Studies
Cardiac catheterization is not frequently done to diagnose transposition of the great vessels, as it could be done confidently with echocardiography. It is in conditions when the echo findings are inconclusive. It can be used to determine the coronary anatomy. Cardiac catheterization can also be used as interventional while performing balloon atrial septostomy. This operation helps to increase mixing between the two circulatory systems.

# Treatment

## Medical Therapy
Surgery is the mainstay of treatment for TGA. However, infusion of PGE1 to a newborn diagnosed with TGA is recommended as it prevents the ductus arteriosus from closing, therefore providing an additional shunt through which to provide the systemic circulation with a higher level of oxygen.

## Surgery
Surgical approach is the mainstay of treatment for transposition of great vessels. Type of surgery mainly depends on the age of the patient at presentation, the presence of associated congenital cardiac lesions, and the experience of the cardiothoracic surgeon with a given surgical technique. Most full-term neonates with uncomplicated transposition of the great arteries can undergo an arterial switch procedure in one operation, with minimal mortality. Recent advances in surgical correction of transposition of the great arteries have reduced the mortality drastically from 95% in uncorrected patients to 5% in corrected patients.
Post-operative care is very similar to the palliative care received, with the exception that the patient no longer requires PGE or the surgical palliation procedures. Additionally, the patient is kept on a cooling blanket for a period of time to prevent fever, which could cause brain damage. The sternum is not closed immediately which allows extra space in the thoracic cavity, preventing excess pressure on the heart, which swells considerably following the surgery; the sternum and incision are closed after a few days, when swelling is sufficiently reduced.

## Primary Prevention
The development of a fetal heart starts during the first trimester of pregnancy. Thus, many a times the fetal heart has already developed, by the time the female becomes aware of being pregnant. There are some risk factors that if avoided before and during pregnancy can decrease the occurrence of congenital heart diseases.