Rush Center for Congenital
and Structural Heart Disease

Definition

 

The pulmonary artery arises from the left ventricle, while the aorta arises from the right ventricle.  The 2 great vessels run parallel to each other, rather than crossing each other in the upper mediastinum.  This arrangement creates 2 parallel circulations, the poorly oxygenated blood returning from the body goes back to the body through the RV-aorta connection, while the highly oxygenated blood returning from through the pulmonary veins, return to the lungs through the LV-pulmonary arterial connection.  Left to right and right to left shunting at the PFO and PDA allow some improvement of systemic oxygen saturation.

 

Incidence

 

0.2-0.4/1,000 Live births. This is the second most common congenital heart disease encountered in early infancy. It is also the most common cause of transfer to the cardiac care in the neonatal and infancy.  

 

 

Embryology

 

There is no apparent genetic predisposition to this congenital heart disease. The incidence does not appear to be higher when there is a positive family history. It is uncommon in low birth weight and premature infants. It appears to have a higher incidence in rural areas and higher birth order as well as advanced maternal age. Mechanism of occurrence is not clear. It is typically not associated with other anomalies.  

 

 

Pathology

 

   

he aorta is anterior and slightly to the right of the main pulmonary artery. The aortic valve is to the right of the pulmonary valve but still anterior to it. The main pulmonary artery is in such a position with the left ventricle allowing direct flow into the right pulmonary artery and it will consequently become larger and more flow to the right pulmonary artery and right lung. Most infants have patent foramen ovale and a patent ductus arteriosus. Fifty per cent (50%) of patients with TGA have ventricular septal defects and 50% have an intact ventricular septum. Pulmonary stenosis is the most commonly associated cardiac defect with transposition of the great vessels. Next are anomalies of the tricuspid valve, then coarctation of the aorta. These anomalies are more commonly associated in transposition of the great vessels with ventricular septal defect and with an intact ventricular septum.

Pulmonary stenosis in patients with transposition of the great arteries particularly those with VSD is due to leftward bowing of the ventricular septum and close proximity of the mitral valve to the ventricular septum leading to dynamic obstruction. Anatomical valvar  PS is rare. One-third (1/3) of patients with d-TGA and VSD will have closure of the VSD in the first year of life. The VSD could be located anywhere in the septum with no particular location more frequently than in patients without d-TGA.

Coronary arteries 60% of patients have a right coronary artery emerging from a right coronary sinus and a left coronary artery which gives the circumflex and left anterior descending coming from the left coronary sinus. In 10% of patients the right coronary artery and the circumflex emerge from the right coronary sinus with the left anterior descending emerging from the left coronary sinus. In these cases the circumflex travels posterior to the pulmonary artery.

In another 10% of the patients the circumflex emerges from the right coronary sinus and the right coronary artery and left anterior descending emerge from the left coronary sinus. The right coronary artery will travel rightwards anterior to the aorta.

In another 10% of patients the right coronary artery emerges from the right coronary sinus and gives the right coronary artery and the left coronary artery. The left coronary artery will travel in between the aorta and the pulmonary artery and give this circumflex and the left anterior descending branches.

The drop in the pulmonary vascular resistance soon after birth will increase the pulmonary blood flow through the PDA and this will result in dilation of the left atrium and stretching of the patent foramen ovale with shunting of blood from the left atrium to the right atrium across the stretched patent foramen ovale. This will allow mixing of blood and better oxygen saturation. Patients with ventricular septal defects tend to have better mixing of the pulmonary and systemic circulations. Patients with a VSD may become more cyanotic as the VSD gets smaller. Patients with an intact ventricular septum tend to have dynamic LVOT obstruction usually reversible after arterial switch operation. Patients with an anatomic left ventricular outflow tract obstruction tend to have better left-to-right shunting in the VSD and less cyanosis than others. d-TGA with intact ventricular septum are best repaired after 1-2 weeks from birth because a drop in the pulmonary vascular resistance will cause deconditioning of the left ventricle making repair difficult at that point. Shunting at the ASD, VSD and PDA are typically as following:

ASD:  Left-to-right shunting is in systole and right-to-left in diastole.

VSD:  Left-to-right shunting in diastole and right-to-left shunting in systole.

PDA:  Left-to-right shunting in diastole and right-to-left shunting in systole.

 

 

Pathophysiology

 

In a normal circulation, de-oxygenated blood from the body is routed through the right ventricle to the pulmonary circulation to become oxygenated.  The blood then returns from the lung to the left heart to be ejected to the body.  In d-TGA both de-oxygenated and oxygenated blood re-circulate in separate circuits.  Deoxygenated blood return from the body to the right ventricle, only to be re-pumped into the the aorta.  On the other hand, the well oxygenated blood returning from the lungs is pumped through the left ventricle into the pulmonary artery to go back again into the lungs.  The only way such children can survive after delivery is because of bi-directional shunting of blood at the atrial level (PFO or ASD) and at the arterial level (PDA).  This bi-directional shunting will cause the blood in the two separate circuits to mix allowing the deoxygenated blood destined to the body to be somewhat oxygenated.  Children who have a VSD as well will have more mixing of blood from the two circuits allowing for better oxygenated blood to go to the body.  

 

 

Clinical Manifestations

 

d-TGA occurs in males slightly more than in females (64% in males). Babies are typically full term. Babies with d-TGA and intact ventricular septum or very small VSD present early in the first day or two of life with cyanosis not improving with oxygen therapy. Babies with a VSD present with cyanosis and congestive heart failure typically at a later date. The patients are typically tachypnic and cyanotic. On auscultation there is a single second heart sound and a murmur is heard with patients with a VSD. In addition, left ventricular outflow tract obstruction may cause systolic ejection murmur.

Hemoglobin should be kept at normal to high levels to lessen the extent of congestive heart failure.

 

 

 

ECG

 

Early is within normal limits and later will show right ventricular hypertrophy. If left ventricular hypertrophy is present with left axis deviation then straddling of the tricuspid valve or overriding with right ventricular hyperplasia should be suspected.

 

 

CXR

 

Early on it is also within normal limits but later particularly in those with VSD will develop cardiomegaly and increased pulmonary blood flow. At a later age it will present in the typical (egg-on-side). This is because of biventricular dilatation and narrow mediastinum due to the positions of the great vessels. Right aortic arch is seen in 1% of patients with intact ventricular septum and in 3% of patients with VSD. Right aortic arch is more common (10%) in patients with d-TGA with pulmonary stenosis or pulmonary atresia.

 

 

Echocardiography

 

Echocardiography demonstrates the anatomy of the d-TGA. The particular features should be assessed:
The VSD, its site and size.
The patent foramen ovale or ASD.
Patent ductus arteriosus.
The anatomy of the coronary arteries.
The aortic arch and its orientation.
Left ventricular outflow tract and if there is any obstruction and whether it is dynamic or anatomic.
Pulmonary stenosis and pulmonary atresia.
Tricuspid valve especially when it is associated with endocardial cushion defect and the right ventricular size.
Cardiac catheterization is indicated in patients when the coronary artery anatomy is not clear, the LVOT obstruction is thought to be anatomic rather than dynamic and to perform a Rashkind atrial septostomy although this could be done under ECHO guidance.

 

 
Parasternal long axis view showing the 2 great vessels parallel to each other.  In a normal heart, the 2 great vessels should not appear together in any plane as they cross each other in their proximal course.  The aorta here is anterior, not posterior as it normally should be.
Parasternal long axis with a tilt of the probe to show the entire length of the abnormally situated anterior aorta.  The posteriorly situated pulmonary artery can be seen to bifurcate, a clue that the posterior vessel is not an aorta.
Parasternal short axis view showing the aortic valve anterior to the pulmonary valve.  During arterial switch procedure, the posteriorly positioned main pulmonary artery is dissected and the proximal portion brought anteriorly.  This causes stretch on the pulmonary arteries and may lead to branch pulmonary artery stenosis, a not so uncommon complication of this procedure.
Subcostal view showing the pulmonary artery coming from the left ventricle and bifurcating as it travels distally.

 

 

Cardiac Catheterization

 

Rashkind atrial septostomy is needed if the atrial communication is restrictive.  This could be done with echo-guidance in the ICU or in the cardiac catheterization laboratory.  The cardiac anatomy is typically clear by echocardiography.  Coronary arterial anatomy is usually well demonstrated by echocardiography, however, occasionally clarification by angiography may be needed.  

 

 

Treatment

 

   

Left ventricle is better suited to be the systemic ventricle therefore arterial switch operation is best. This could be done in infants less than 2 weeks with an intact ventricular septum or infants within the first 2-3 months of age with a ventricular septal defect.

The left ventricle should be at or close to the systemic pressure to withstand becoming the systemic ventricle after the arterial switch operation. Patients with significant left ventricular outflow tract obstruction (anatomic) with VSD may be better candidates for a Rastelli procedure.

Patients with severe right ventricular hypoplasia secondary to AV canal defect and tricuspid valve anomalies will be better candidates for the Fontan procedure.

Main pulmonary artery banding is employed in patients with multiple VSD’s or to prepare the left ventricle in older children for the arterial switch operation.

The Mustard and Senning procedures were commonly performed ten years ago. In the Mustard procedure the baffling of the atrium is done with precordium or prosthetic material and the Senning procedure made of material are used to baffle the atrium.

Conversion of atrial switch procedure to arterial switch procedure will require that the MPA is banded first for a period of time until the left ventricle is able to tolerate the systemic pressure. In the past, palliative atrial switch procedure could be performed in patients with d-TGA with VSD and pulmonary vascular obstructive disease to lessen the extent of cyanosis.

Complications of atrial switch procedure:

1. Rhythm problems: All patients after 15 years have abnormal rhythms. Twenty-five per cent

(25%) have disorders of the SA node and 1% per year develop late sudden death.

Tricuspid regurgitation: Frequent with VSD closure done through the tricuspid valve. This is not well tolerated unlike VSD closure without d-TGA.

Right (systemic) ventricular failure: This occurs because the right ventricle is performing the job of the left ventricle in addition to possible tricuspid regurgitation.

Complete AV block: This would require a pacemaker.

CNS abnormalities is hard on those with prolonged cyanosis or IV manipulations, for example, cardiac catheterization which may result in emboli to the central nervous systemic from the venous circulation. As many as 10% of the population, particularly in the past developed a lower IQ and motor development delay.

Patients with d-TGA and intact ventricular septum with left ventricular outflow tract obstruction. The left ventricular outflow tract obstruction is usually dynamic and results after the arterial switch operation. Those with anatomical LVOT obstruction can benefit from atrial switch and LVOT resection.

d-TGA with coarctation of the aorta. This is rare and usually associated with patients who also have VSD. There will be increased pulmonary blood flow in these patients due to the high systemic vascular resistance secondary to the coarctation resulting in congestive heart failure. Management is staged usually by fixing the coarctation first and then performing an arterial switch operation. Occasionally both could be performed through an anterior mid-sternotomy.

d-TGA with pulmonary stenosis or pulmonary atresia. This pulmonary stenosis can be relieved without damage to the pulmonary valve in arterial switch operation is preferred. However, if the pulmonary stenosis is severe then if there is a VSD the Rastelli procedure could be performed and if there is no VSD or if the VSD is muscular than an atrial switch and relief pulmonary stenosis could be performed. Pulmonary atresia

 

 

 

Course and Prognosis

 

   

 

 

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