Transposition of the Great Vessels

Overview

Definition

Transposition of the great vessels (TGV) is the switching of the origins of the great vessels whereby the aorta arises from the right ventricle and the pulmonary artery arises from the left ventricle.

Epidemiology

• 3% of all cases of congenital heart disease and 20% of cyanotic congenital heart disease
• Prevalence: approximately 4 in 10,000 live births
• Associated cardiac anomalies:
  • Ventricular septal defect (VSD): in 50% of cases
  • Left ventricle outflow tract obstruction: 
    • Noted in 25% of cases
    • Pulmonary stenosis, especially if a VSD is present
  • Coronary arteries: variable origins and short course
  • Mitral or tricuspid valve abnormalities

Etiology

• Not completely delineated
• Attributed to:
  •  Failed growth of the subpulmonary conus 
  •  Failure of absorption at the subaortic conus
• Increased risk in diabetic mothers
• No strong genetic component, may be seen in some cases of DiGeorge syndrome

Pathophysiology and Clinical Presentation

Pathophysiology

• 2 parallel circuits:
  • Deoxygenated blood returns to the right ventricle and is pumped back into the systemic circulation by the misplaced aorta.
  • Oxygenated blood returns to the left ventricle and is pumped into the pulmonary circulation by the misplaced pulmonary vein.
• Tolerated well in utero:
  • Oxygenated blood from umbilical vein → right atrium
  • Right atrium → fossa ovalis → left atrium → left ventricle→ pulmonary artery→ ductus arteriosus→ aorta
• After birth, complete separation of the circuits is not compatible with life.
• Viability depends on the intercirculatory mixing of blood:
  • Intracardiac: 
    • Patent foramen ovale (PFO)
    • Atrial septal defect (ASD)
    • VSD
  • Extracardiac: patent ductus arteriosus (PDA)
• Mixing of blood: 
  • Reduced concentration of oxygen in the systemic circulation causing cyanosis
  • Effective: between atria because of low-pressure gradient
  • Less effective: VSD or PDA—blood is preferentially shunted in 1 direction because of pressure gradient
• Preferential shunting in 1 direction → clinical deterioration:
  • Worsening hypoxemia/cyanosis
  • Heart failure

Clinical presentation

• Apparent in the neonatal phase (1st 30 days of life)
• Cyanosis:
  • Severe, with intact ventricular septum
  • Mild, with ASD, PFO, or large VSD
  • Not responsive to supplemental oxygen
  • Not affected by feeding/crying
  • Reverse differential cyanosis:
    • Higher post-ductal than pre-ductal saturations
    • If aortic coarctation, interrupted aortic arch, or pulmonary hypertension are also present with TGV
• Tachypnea
• Heart failure:
  • Usually if there is a concomitant large VSD
  • Respiratory distress by 3–4 weeks of life

Diagnosis

Physical examination

• Visible cyanosis 
• Respiratory rate > 60/min
• Failed pulse oximetry screening test
• Murmurs:
  • Pansystolic murmur of VSD 
  • Systolic ejection murmur if left ventricular outlet obstruction is present
• Diminished femoral pulses: if there is also aortic coarctation or arch interruption

Imaging

• Echocardiogram:
  • Prenatal: not always diagnostic
  • Postnatal echocardiogram is a main confirmatory test:
    • Evaluate the degree of atrial mixing.
    • VSD
    • Valve abnormalities
    • PDA
    • Coronary artery anatomy
• Chest X ray: classic “egg on a string” appearance

Other tests

• Electrocardiogram:
  • Generally not diagnostic
  • May show right axis deviation and right ventricular hypertrophy
• Cardiac catheterization (angiogram):
  • Can help assess origins/anatomy of coronary arteries prior to surgery
  • Can be therapeutic: balloon atrial septostomy to increase blood mixing

Management and Prognosis

Management

Medical:

• For initial stabilization and optimization prior to surgery
• Intravenous prostaglandin infusion to keep PDA open
• Balloon atrial septostomy (with cardiac catheterization)

Surgery:

• Definitive management
• Performed within 1st 2 weeks of life
• Most common: 
  • Arterial switch operation (ASO) + VSD repair if necessary
  • Rastelli procedure if left ventricular outflow obstruction is also present
• Post-surgical complications that require regular screening include:
  • Coronary artery stenosis, or insufficiency
  • Pulmonary artery stenosis
  • Aortic root dilation
  • Right ventricular failure
  • Arrhythmias (e.g., atrial flutter and fibrillation)

Prognosis

• If not treated: 
  • 90% of patients will die within the 1st year.
  • 30% of patients will die within the 1st week.
• If treated, high morbidity with continuous follow-up and decreased exercise capacity
• Higher chance of neurodevelopmental delay in infants

Differential Diagnosis

• Tetralogy of Fallot: a congenital heart disease characterized by the occurrence of 4 key cardinal  features: overriding aorta, VSD, RVOT obstruction, and right ventricular hypertrophy. Patients present with cyanosis and a history of tet spells. Diagnosis is confirmed by an echocardiogram and patients are surgically managed.
• Tricuspid atresia: congenital heart disease characterized by the lack of development of the tricuspid valve. Presents with cyanosis, labored breathing, and hypoxic spells. Holosystolic murmur and single S2 are present on exam. Diagnosis is made by echocardiogram and ECG.
• Truncus arteriosus: emergence of aorta and pulmonary artery from a common trunk overriding a VSD. Symptomatic from the 1st days of life, including cyanosis, respiratory distress, heart failure, poor feeding, and excessive sweating. Diagnosed by echocardiogram.
• Total anomalous pulmonary venous return: rare congenital cardiopathy in which pulmonary veins drain to sites other than the left atrium. Presents with cyanosis from birth, heart failure, and respiratory distress. Characterized by wide-split S2.
• Ebstein anomaly: congenital heart defect in which there is downward displacement of the tricuspid valve leaflets causing RVOT obstruction. Presents with cyanosis, arrhythmias, and failure to thrive. Diagnosed by echocardiogram.