Transposition of great arteries

Conotruncal anomaly characterized by ventriculoarterial discordance.
Aorta arises from morphologic right ventricle (RV); pulmonary artery (PA) arises from morphologic left ventricle (LV).
Categorized into two primary types based on atrioventricular connections, ventricular looping, and spatial relationship of great arteries: Dextro-TGA (D-TGA) and Levo-TGA (L-TGA/CC-TGA).

Dextro-Transposition of the Great Arteries (D-TGA)

Morphology and Hemodynamics

Atrioventricular concordance (right atrium RA connects to RV; left atrium LA connects to LV).
Ventriculoarterial discordance (RV connects to aorta; LV connects to PA).
Aorta positioned anterior and rightward to PA.
Subaortic conus present; normally expected subpulmonic conus absent.
Parallel systemic and pulmonary circuits.
Survival dependent on mixing of oxygenated and deoxygenated blood via atrial, ventricular, or great vessel communications.

D-TGA Subtypes (Classification by Associated Lesions)

Subtype	Anatomic Features	Pathophysiology & Clinical Presentation
D-TGA with Intact Ventricular Septum (IVS)	Isolated/simple TGA; no VSD.	Hypoxemia within first days of life following ductus arteriosus closure. Survival entirely dependent on patent foramen ovale (PFO)/atrial septal defect (ASD) and patent ductus arteriosus (PDA) for mixing.
D-TGA with Ventricular Septal Defect (VSD)	VSD present, typically perimembranous.	Increased pulmonary blood flow. Severity of cyanosis determined by degree of mixing at ventricular level. Congestive heart failure develops around 4-10 weeks of age.
D-TGA with VSD and Pulmonary Stenosis (LVOTO)	Associated with subaortic stenosis or dynamic/acquired left ventricular outflow tract obstruction (LVOTO).	Mimics Tetralogy of Fallot physiology, but obstruction is in the LV. Results in left-to-right anatomic shunting with limited effective pulmonary blood flow.

Diagnosis

Prenatal / Fetal Echocardiography

Aneuploidy rarely associated; extracardiac anomalies generally absent except situs anomalies.
Four-chamber view often normal.
PA arises from LV centrally, bifurcates into branch pulmonary arteries.
Aorta arises from RV anteriorly.
Great vessels exhibit parallel orientation.
Abnormal "three-vessel view" demonstrates overlapping ductal and aortic arches ("I" sign or two-vessel view).
Restrictive atrial septum evaluation critical: identified by hypermobile septum, bowing >50%, lack of swinging motion, or pulmonary vein flow reversal during systole.
Restrictive atrial septum necessitates urgent postnatal balloon atrial septostomy.

Postnatal Evaluation

Chest Radiograph (CXR): Classic "egg on a string" appearance; narrow mediastinal silhouette; absent thymic shadow.
Electrocardiogram (ECG): Right axis deviation; right ventricular hypertrophy.
Echocardiography: Gold standard. Subcostal view confirms posterior PA arising from LV and anterior aorta arising from RV. Parasternal short-axis shows semilunar valves as two circles (aorta anterior/rightward). Evaluates mixing lesions (PFO, VSD, PDA) and coronary artery anatomy.

Surgical Management

Preoperative Palliation: Balloon atrial septostomy (Rashkind procedure) for restrictive atrial septum to improve mixing and reduce LA pressure.
D-TGA (IVS or small VSD): Arterial switch operation (ASO). Pulmonary artery and aorta transected and switched; coronary arteries transferred from aortic to pulmonary root.
D-TGA with VSD and LVOTO:
- Rastelli procedure: VSD patch connects LV to aorta; native PA transected/oversewn; RV-to-PA extracardiac conduit placed.
- Réparation à l'Etage Ventriculaire (REV): Infundibular septum resected; intraventricular baffle placed; native PA directly implanted to RV without conduit.
- Nikaidoh procedure: Aortic root and coronary arteries translocated entirely to LV; biventricular outflow tract reconstruction.
Historical/Alternative Operations: Mustard or Senning procedures (atrial switch) using pericardial or autologous atrial baffles to redirect venous flow. Used historically or if ASO contraindicated.

Congenitally Corrected Transposition of the Great Arteries (CC-TGA / L-TGA)

Morphology and Hemodynamics

Atrioventricular discordance (RA connects to morphologic LV; LA connects to morphologic RV).
Ventriculoarterial discordance (morphologic LV connects to PA; morphologic RV connects to aorta).
Aorta positioned anterior and leftward to PA (Levo-transposition).
Systemic venous blood routed: RA -> LV -> PA -> Lungs.
Pulmonary venous blood routed: LA -> RV -> Aorta -> Body.
Physiologically "corrected" circulation; presentation dependent on associated anomalies.
Morphologic RV functions as systemic ventricle, prone to eventual failure under systemic pressures.

CC-TGA Subtypes (Classification by Associated Lesions)

Isolated CC-TGA occurs in only 10-20% of cases. 80-90% present with associated structural defects:

Associated Lesion	Incidence	Pathophysiologic Impact
Ventricular Septal Defect (VSD)	60-80%	Typically perimembranous. Large VSDs lead to systemic-level pressures in both ventricles and congestive heart failure.
Pulmonary Outflow Obstruction	50%	Variable severity (mild to pulmonary atresia). Co-existing VSD and pulmonary stenosis mimics Tetralogy of Fallot physiology.
Left-sided Tricuspid Valve (TV) Anomalies	90%	Ebstein-like anomaly of the systemic TV; apical displacement of septal leaflet; dysplasia. Results in progressive systemic TV regurgitation, LA dilation, and heart failure.
Conduction System Abnormalities	~40% AV Block	Displaced, unstable AV node. Up to 40% born with or develop complete AV block; risk increases ~2% annually.

Diagnosis

Chest Radiograph (CXR): Straight left upper border of cardiac silhouette formed by ascending aorta. Levocardia without cardiomegaly typical in isolated/mild disease; progressive cardiomegaly correlates with severe TV regurgitation.
Electrocardiogram (ECG): Ventricular inversion reflected by absent Q waves in precordial lead V6; abnormal Q waves in III, aVR, aVF, V1; upright T waves across precordium. Frequently shows AV block.
Echocardiography: Demonstrates AV and VA discordance. Parasternal short-axis verifies anterior and levo-positioned (leftward) aortic valve. Long-axis views essential for sub-pulmonary obstruction evaluation.
Cardiac Magnetic Resonance (CMR) / Angiography: Flat, sagittally positioned interventricular septum separating right-sided LV and left-sided RV. CMR indicated for quantification of systemic RV size, volume, and function, including late gadolinium enhancement (LGE) for fibrosis.

Surgical Management

Indications: Declining systemic RV function, worsening systemic TV regurgitation, or symptomatic heart failure. Symptomatic CC-TGA with severe TV insufficiency requires TV replacement.
Conventional (Physiologic) Repair: Closure of VSD or relief of pulmonary stenosis without altering the morphologic RV's role as the systemic pump. High risk of postoperative complete AV block.
Double Switch Operation: Anatomic repair to restore morphologic LV as systemic ventricle. Includes atrial switch (Senning/Mustard) plus arterial switch (ASO) or Rastelli procedure (if VSD/pulmonary stenosis present).

Comparative Summary: D-TGA vs. L-TGA

Feature	D-TGA (Complete TGA)	L-TGA (CC-TGA)
Atrioventricular Connection	Concordant	Discordant (Ventricular Inversion)
Ventriculoarterial Connection	Discordant	Discordant
Aortic Position	Anterior, Rightward (D-loop)	Anterior, Leftward (L-loop)
Systemic Ventricle	Morphologic Right Ventricle (Pre-repair)	Morphologic Right Ventricle
Pulmonary Circulation	Parallel, isolated circuits	"Corrected", in-series physiological flow
Neonatal Presentation	Severe cyanosis, ductal/PFO dependent	Often asymptomatic unless associated lesions present
Primary Surgical Strategy	Arterial Switch Operation (ASO)	Double Switch Operation or Conventional Repair
Conduction System	Normal AV node position	Unstable AV node; high risk of complete AV block
Common Associated Valve Lesion	Cleft Mitral Valve	Dysplastic, Ebstein-like systemic Tricuspid Valve