A 12-day-old male infant is brought to the emergency department by his parents due to leth… — USMLE Step 2 CK

A $12$ -day-old male infant is brought to the emergency department by his parents due to lethargy, poor feeding, and rapid breathing that started earlier today. He was born at $39$ weeks gestation via an uncomplicated spontaneous vaginal delivery and was discharged on day $2$ of life. The prenatal course was unremarkable. His temperature is $36.8^\circ\text{C}$ ( $98.2^\circ\text{F}$ ), heart rate is $185/\text{min}$ , respiratory rate is $72/\text{min}$ , and oxygen saturation is $94\%$ on room air. Blood pressure is $75/45\text{ mmHg}$ in the right arm and $50/30\text{ mmHg}$ in the left leg. On physical examination, the infant appears pale and mottled. Capillary refill time is $4$ seconds. Brachial pulses are bounding, but femoral pulses are barely palpable bilaterally. A grade $2/6$ systolic ejection murmur is heard at the left sternal border. Lung auscultation reveals mild bilateral crackles. Arterial blood gas analysis shows a pH of $7.15$ , $p\text{CO}_2$ of $32\text{ mmHg}$ , and $\text{HCO}_3^-$ of $11\text{ mEq/L}$ . Which of the following is the most appropriate initial pharmacological intervention for this patient?

Continuous infusion of prostaglandin E1Answer
B
Intravenous ampicillin and cefotaxime
C
Intravenous furosemide
D
Nebulized albuterol and hypertonic saline
E
Intravenous sodium bicarbonate

Answer

Continuous infusion of prostaglandin E1

The correct intervention is the continuous infusion of prostaglandin E1. This patient is presenting with cardiogenic shock due to critical coarctation of the aorta, a condition where narrowing of the descending aorta severely restricts blood flow. In neonates, perfusion to the lower body often relies on a patent ductus arteriosus (PDA). As the PDA naturally closes around 1 to 2 weeks of life, the left ventricle faces a massive increase in afterload, leading to heart failure, pulmonary edema, and systemic hypoperfusion (manifested as diminished femoral pulses, lower extremity hypotension, and severe lactic acidosis). Prostaglandin E1 (alprostadil) is a life-saving medication that reopens the ductus arteriosus, allowing right ventricular output to bypass the obstruction and perfuse the systemic circulation until surgical correction can be performed.

Step-by-Step Solution

Analyze the vital signs and physical exam findings.

The infant has differential blood pressures (

75/45\text{ mmHg}

upper vs.

50/30\text{ mmHg}

lower) and diminished femoral pulses compared to bounding brachial pulses.

These findings are the hallmark of an aortic arch obstruction, most commonly coarctation of the aorta.

Correlate the timing of symptom onset with pathophysiology.

The acute collapse at

12

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

In severe neonatal coarctation, systemic blood flow to the lower body is entirely dependent on a right-to-left shunt through a patent ductus arteriosus (PDA).

Determine the most appropriate initial pharmacological intervention.

Administer an infusion of prostaglandin E1 (alprostadil).

Prostaglandin E1 maintains or restores the patency of the ductus arteriosus, bypassing the coarctation and immediately restoring perfusion to the lower extremities and vital organs.

Key Concept

Ductal-dependent systemic circulation (Severe Coarctation of the Aorta)

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