Anatomy and Physiology:
In D-TGA, there is discordant ventriculo-arterial connection. This means that the aorta is connected primarily to an anatomic right ventricle, and the pulmonary artery is connected to an anatomic left ventricle. The dextro-prefix indicates that the origin of the aorta is anterior and rightward to the origin of the pulmonary artery. This results in two parallel circulations. Deoxygenated blood returns from the body to the right atrium and then travels across the tricuspid valve to the right ventricle and back out the aorta to the body. Oxygenated blood returns from the lungs to the left atrium and then travels across the mitral valve to the left ventricle and back out the pulmonary arteries to the lungs.
Clinical Presentation Spectrum
The parallel circulation in D-TGA may lead to life threatening systemic desaturation. Survival depends on the presence of lesions that allow for mixing of blood between these two parallel circulations.
Unrestrictive atrial septum:
A large atrial septal defect allows for mixing of blood between the two parallel circulations such that the brain and heart can receive adequately oxygenated blood. The relatively lower pressure differences between the left and the right atrium allow for bidirectional flow across the ASD or PFO. Oxygenated blood returns from the pulmonary vasculature to the left atrium and can then travel left to right across the atrial septum to mix with deoxygenated blood returning from the systemic circulation. It then travels across the tricuspid valve to the right ventricle and out the aorta to the systemic circulation.
Restrictive/intact atrial septum:
Newborns with D-TGA/IVS with a restrictive or intact atrial septum can present with profound cyanosis as well as acidosis and hemodynamic instability. In such cases, maintaining an open PDA with prostaglandins will not provide adequate mixing of oxygenated and deoxygenated blood. Creation of an atrial level communication via a balloon atrial septostomy is warranted to allow for sufficient mixing. BAS may be performed in the ICU under echocardiographic guidance or in the cardiac cath lab under echo or fluoroscopic guidance.
General: Initial management of newborns with D-TGA/IVS is directed at stabilizing cardiac and respiratory status. Initiation of prostaglandins to maintain ductal patency as well as evaluation of atrial level mixing are important in ensuring adequate systemic oxygen delivery. With adequate intercirculatory mixing, most neonates can be extubated and begun on oral feeds.
Trial off PGE: With adequate mixing at the atrial level, it may be possible to discontinue PGE. Discontinuing PGE can prevent unwanted side effects, such as apnea, and can also allow for better identification of aortic arch obstruction. However, this is not always necessary or desirable.
Technique: The preferred surgical repair for D-TGA/IVS is the arterial switch operation (ASO). There are no specific contraindications to ASO, but unbalanced ventricles, prematurity, low birth weight, and comorbidities will all influence the decision to repair. In the ASO, the ascending aorta is transected, and the left and right coronaries are excised. The pulmonary artery is transected and rectangular flaps (or trapdoors) are developed where the coronary arteries will be implanted. The coronary buttons are anastomosed to these open flaps, and the distal aorta is moved posteriorly to lie behind the pulmonary artery. The aorta is connected to the proximal PA to create the left ventricular outflow tract, and the pulmonary artery is connected to the proximal aorta to create the right ventricular outflow tract.
Timing of Repair: In the newborn period, pulmonary vascular resistance is high. Hence, the pressure in the sub-pulmonary ventricle is high. Over the first few days of life as pulmonary vascular resistance falls, this ventricular pressure also falls. In D-TGA, this leads to a lower left ventricular pressure. This puts infants at risk for low cardiac output and left ventricular failure following ASO if surgery is delayed too long. Additionally, Anderson et al. identified that delaying surgery beyond 3 days was associated with increased morbidity and hospital costs.
Following an ASO, infants are at risk for low cardiac output. Preoperatively the left ventricle is pumping against low pulmonary vascular resistance and postoperatively this is transitioned to pumping against higher systemic vascular resistance. It may take time for the left ventricle to adapt to this, and hence, systemic cardiac output may be low during this transition period. Mean arterial pressure, left atrial pressure, and systemic perfusion should all be monitored closely. Given the need for coronary artery transfer, infants are also at risk for coronary arterial insufficiency, and close attention should be paid to monitoring for ST segment changes on ECG. Infants are also at risk for significant bleeding given the multiple suture lines.