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Question:

A full-term newborn is evaluated in the delivery room due to mild tachypnea and cyanosis immediately after birth.  Fetal ultrasonography performed during pregnancy revealed transposition of the great arteries.  Heart rate and blood pressure are normal.  Oxygen saturation is 72%.  Cyanosis is apparent in the hands, feet, lips, and tongue.  Peripheral pulses are strong.  Bedside echocardiography confirms the prenatal diagnosis; there is also a small patent foramen ovale and a small patent ductus arteriosus.  Definitive surgical correction of the main congenital abnormality is planned.  Pending definitive repair, which of the following temporary measures would be most likely to improve this patient's condition?

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Explanation:

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Transposition of the great arteries

Anatomy

  • Parallel (separate) systemic & pulmonary circuits
    • Right ventricle → aorta
    • Left ventricle → pulmonary artery
  • Deoxygenated blood is delivered to the systemic circulation
  • Mixing shunt (eg, VSD, PDA) must be present for viability

Clinical features

  • Cyanosis at or shortly after birth
  • Tachypnea & tachycardia
  • Murmur typically absent at birth due to low velocity shunting

Diagnosis

  • Chest x-ray: narrow mediastinum ± cardiomegaly ("egg on a string")
  • Hyperoxia test: little to no ↑ in arterial oxygenation with 100% O2
  • Echocardiography typically confirmatory

Initial management

  • Promote mixing of oxygenated & deoxygenated blood
    • Prostaglandin infusion (maintains PDA)
    • Atrial septostomy (maintains PFO)

PDA = patent ductus arteriosus; PFO = patent foramen ovale; VSD = ventricular septal defect.

Transposition of the great arteries (TGA) is caused by impaired spiraling of the conotruncal septum (divides the truncus arteriosus into the aorta and the pulmonary artery), resulting in reversed connections between the ventricles and great arteries:

  • The right ventricle empties into the aorta, causing deoxygenated mixed venous blood to be pumped directly into the systemic circulation (bypassing the lungs), leading to neonatal cyanosis

  • The left ventricle ejects blood into the pulmonary artery, causing the pulmonary venous return to be pumped back into the lungs, trapping oxygenated blood in the pulmonary circulation

The parallel (ie, separate) circulations in TGA are incompatible with life unless a mixing shunt (eg, patent ductus arteriosus, patent foramen ovale, ventricular septal defect) is present to provide a path for partially oxygenated blood to enter the systemic circulation.  Therefore, initial management of TGA involves ensuring adequate mixing of oxygenated and deoxygenated blood.  For example, administration of prostaglandin E1 to maintain patency of the ductus arteriosus or surgical procedures to open or widen the foramen ovale.

(Choice A)  Sinoatrial node ablation is sometimes used for treating tachyarrhythmias involving the sinoatrial node.

(Choice B)  Because deoxygenated blood bypasses the pulmonary circulation and is delivered directly to the systemic circulation in TGA, supplemental oxygen administration does not significantly improve hypoxemia or cyanosis.

(Choice C)  Digoxin is a positive inotropic agent that can treat heart failure due to left ventricular systolic dysfunction (a finding not present on this patient's echocardiogram).  Increased myocyte contractility would not improve this patient's oxygenation.

(Choice E)  Ligating a patent ductus arteriosus in a patient with TGA would decrease mixing of blood between the parallel systemic and pulmonary circulations, worsening hypoxemia and cyanosis.

Educational objective:
Transposition of the great arteries is characterized by parallel circulations of deoxygenated and oxygenated blood.  Initial management includes maintaining a mixing shunt (eg, patent ductus arteriosus, patent foramen ovale) to provide the systemic circulation with partially oxygenated blood.