A 6-day-old male neonate is evaluated in the emergency department due to poor feeding, let… — USMLE Step 2 CK

A $6$ -day-old male neonate is evaluated in the emergency department due to poor feeding, lethargy, and increased work of breathing over the past $12$ hours. He was born at $39$ weeks gestation via an uncomplicated vaginal delivery and was discharged home at $48$ hours of life. Temperature is $36.4^\circ\text{C}$ ( $97.5^\circ\text{F}$ ), heart rate is $185$ /min, and respiratory rate is $68$ /min. Oxygen saturation is $94\%$ on room air in the right hand; a reliable plethysmographic waveform cannot be obtained in either foot. Physical examination reveals an ashen, mottled appearance with a prolonged capillary refill time of $5$ seconds. Marked subcostal retractions and grunting are present. The right brachial pulse is full and bounding, while bilateral femoral pulses are barely palpable. Blood pressure measured in the right arm is $84/45$ mmHg, and in the left leg is $46/24$ mmHg. Arterial blood gas analysis reveals a pH of $7.12$ , pCO $_2$ of $32$ mmHg, and pO $_2$ of $75$ mmHg, with a calculated bicarbonate of $10$ mEq/L. Which of the following is the most appropriate initial step in management?

Continuous intravenous infusion of alprostadilAnswer
B
Intravenous administration of furosemide
C
Intravenous administration of sodium bicarbonate
D
Nebulized administration of racemic epinephrine
E
Immediate transport for computed tomography angiography of the chest

Answer

Continuous intravenous infusion of alprostadil

The correct intervention is continuous intravenous infusion of alprostadil. This neonate presents with classic signs of cardiogenic shock secondary to severe coarctation of the aorta. In utero, the ductus arteriosus bypasses the aortic isthmus, allowing normal development. After birth, as the ductus arteriosus undergoes physiologic closure (typically between

2

and

7

days of life), the severe narrowing becomes fully obstructive. This isolates the descending aorta from systemic blood flow, leading to diminished femoral pulses, lower extremity hypotension, profound hypoperfusion, and lactic acidosis. The immediate, life-saving step is to administer alprostadil (prostaglandin E1) to maintain or reopen the ductus arteriosus, which restores a right-to-left shunt from the pulmonary artery to the descending aorta, ensuring systemic perfusion until surgical correction can be performed.

Step-by-Step Solution

Identify the primary clinical syndrome.

The neonate presents with severe cardiogenic shock, evidenced by tachycardia, ashen skin, prolonged capillary refill, and high-anion-gap metabolic acidosis.

Recognizing shock is the critical first step in an unstable patient.

Analyze the focal physical examination findings.

There is a profound discrepancy between upper and lower extremity pulses and blood pressures, along with an inability to obtain a pulse oximetry reading in the feet.

These findings localize the pathology to a severe left ventricular outflow tract obstruction, specifically distal to the right subclavian artery.

Correlate the patient's age with the likely pathophysiology.

The decompensation at

6

days of life coincides with the physiologic closure of the ductus arteriosus.

In severe coarctation of the aorta, systemic circulation to the lower body relies on a right-to-left shunt through a patent ductus arteriosus. Closure leads to rapid circulatory collapse.

Determine the most critical immediate intervention.

Administration of alprostadil (PGE1) to reopen the ductus arteriosus.

Re-establishing ductal patency bypasses the coarctation, restoring blood flow to the descending aorta and reversing the shock state prior to definitive surgical repair.

Key Concept

Neonates with severe coarctation of the aorta depend on a patent ductus arteriosus for systemic blood flow to the lower body. As the ductus closes (typically in the first week of life), they present with cardiogenic shock, requiring immediate prostaglandin E1 infusion.

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