A full-term male newborn, aged 4 hours, is evaluated in the neonatal care unit due to the … — USMLE Step 2 CK

A full-term male newborn, aged 4 hours, is evaluated in the neonatal care unit due to the sudden onset of a bluish discoloration of the lips and face. The pregnancy and vaginal delivery were entirely uncomplicated, and Apgar scores were normal. Current vital signs include a temperature of $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), a heart rate of $160/\text{min}$ , a respiratory rate of $62/\text{min}$ , and an oxygen saturation of $71\%$ on room air. The patient is placed in a headbox with $100\%$ fraction of inspired oxygen; a repeat saturation measurement after 15 minutes is $73\%$ . Physical examination reveals a tachypneic infant with profound central cyanosis but no nasal flaring or retractions. Cardiac auscultation identifies a loud, single second heart sound (S2) and a grade 2/6 holosystolic murmur heard best at the left lower sternal border. Palpation of the brachial and femoral pulses reveals symmetric, 2+ intensity with no pulse discrepancies. A portable chest radiograph displays a narrow mediastinal silhouette resembling an egg on a string, alongside normal pulmonary vascular markings. Which of the following is the most appropriate next step in the management of this patient?

A
Empiric administration of broad-spectrum systemic antimicrobials
B
Tracheal intubation and endotracheal surfactant instillation
Administration of intravenous alprostadilAnswer
D
Urgent transport to radiology for a computed tomography scan of the chest
E
Routine nursery care with scheduled reassessment of pulse oximetry

Answer

The correct approach is the administration of intravenous alprostadil (prostaglandin E1) to maintain ductal patency.

The patient's presentation of severe cyanosis that fails to improve with

100\%

oxygen (a failed hyperoxia test) indicates a right-to-left intracardiac shunt. The specific findings of a single S2, a holosystolic murmur (indicative of an associated ventricular septal defect), and a narrow mediastinum ('egg on a string' sign) on chest radiography are classic for Transposition of the Great Arteries (TGA). In TGA, the aorta arises from the right ventricle and the pulmonary artery from the left ventricle, resulting in parallel rather than series circulations. Survival depends entirely on the mixing of oxygenated and deoxygenated blood through a patent ductus arteriosus (PDA) or septal defect. Intravenous alprostadil (prostaglandin E1) must be initiated immediately to prevent closure of the ductus arteriosus and maintain systemic oxygenation until a definitive surgical repair (arterial switch operation) can be performed.

Step-by-Step Solution

Analyze the patient's response to the hyperoxia test.

The oxygen saturation failed to improve significantly on 100% oxygen, confirming a right-to-left cardiac shunt rather than a primary pulmonary pathology.

Differentiating between cardiac and pulmonary causes of neonatal cyanosis is the critical first step in management.

Correlate physical exam and radiographic findings to establish a likely diagnosis.

A single S2, a holosystolic murmur (suggesting a VSD), and an 'egg on a string' narrow mediastinum on chest x-ray point to Transposition of the Great Arteries (TGA).

Specific anatomical features in congenital heart defects produce characteristic clinical and radiographic signs that guide immediate interventions.

Determine the appropriate intervention based on the underlying pathophysiology.

TGA creates parallel circulations that are incompatible with life unless mixing occurs. Alprostadil is chosen to keep the ductus arteriosus patent and ensure systemic oxygenation.

Maintaining blood mixing is a life-saving priority before definitive surgical correction can be performed.

Key Concept

Transposition of the Great Arteries (TGA) presents with severe cyanosis unresponsive to oxygen and requires immediate administration of prostaglandin E1 (alprostadil) to maintain a patent ductus arteriosus for life-saving blood mixing.

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