transposition of the great arteries

**Definition/Anatomy:** In d-TGA, there is ventriculoarterial discordance. The aorta arises anteriorly from the morphologic right ventricle, and the pulmonary artery arises posteriorly from the morphologic left ventricle. Systemic venous blood (deoxygenated) returns to the RA → RV → aorta, and pulmonary venous blood (oxygenated) returns to LA → LV → pulmonary artery. The result is two parallel circuits, which is incompatible with life unless mixing occurs.

**Embryology/Pathophysiology:** Failure of the aorticopulmonary septum to spiral during truncal septation. The septum runs straight rather than spiraling. Fetal life is tolerated because of the placenta and fetal shunts (PFO, PDA). After birth, the PDA closes and cyanosis becomes severe. Mixing can occur at the atrial level (PFO/ASD), ventricular level (VSD), or arterial level (PDA). Patients with an intact ventricular septum (about 50-60%) present earliest and most severely. Those with a VSD may present later, sometimes with heart failure.

**Risk factors/Associations:** Maternal diabetes (classic), male sex, and maternal alcohol use. It is the most common cyanotic congenital heart lesion presenting in the newborn period. It is not typically associated with chromosomal abnormalities such as trisomy 21, and it is less often syndromic than many other lesions. Associated lesions include VSD, pulmonary stenosis (LVOT obstruction), PDA, and coronary artery anomalies.

**Clinical presentation:** Cyanosis within hours to days of birth, often in a large, otherwise healthy-appearing term male infant. Cyanosis does not improve with 100% oxygen (hyperoxia test is negative). The single S2 is loud, because the anterior aortic valve is close to the chest wall. There may be no murmur if the septum is intact.

**Diagnosis:** Echocardiography is diagnostic. CXR shows an "egg on a string" appearance (a narrow mediastinum from the anteroposterior relationship of the great vessels, with increased pulmonary vascular markings). ECG may show right axis deviation and RVH.

**Management:** 1. Prostaglandin E1 to keep the ductus arteriosus open and increase mixing. 2. Balloon atrial septostomy (Rashkind procedure) if mixing is inadequate. 3. Definitive treatment: arterial switch (Jatene) procedure within the first 1-2 weeks of life, with reimplantation of the coronary arteries. Coronary artery transfer is the most technically challenging step, and coronary complications are the main cause of early postoperative mortality. 4. Older atrial switch procedures (Mustard, Senning) leave the RV as the systemic ventricle, which can lead to late RV failure, baroreceptor problems, and atrial arrhythmias.

**Do not confuse with:** - **Congenitally corrected TGA (l-TGA):** both atrioventricular and ventriculoarterial discordance, so blood flow is physiologically normal. It is often asymptomatic early, but patients may develop complete heart block and systemic RV failure later. - **Tetralogy of Fallot:** the other main cause of cyanosis, with a "boot-shaped heart" on CXR. - **Truncus arteriosus:** failure of the truncus to divide, leading to a single vessel. - **Total anomalous pulmonary venous return:** "snowman" sign.

**High-yield buzzwords:** cyanosis at birth not responsive to O2, "egg on a string," maternal diabetes, failure of the aorticopulmonary septum to spiral, parallel circulations, PGE1, arterial switch.

**Ancillary board point:** Do not give indomethacin (which would close the PDA) in this setting. A common vignette is a newborn with cyanosis who gets worse as the ductus closes.