An infant is born at 41 weeks and 3 days of gestation to a 32-year-old G2P1 via urgent ces… — USMLE Step 2 CK

An infant is born at 41 weeks and 3 days of gestation to a 32-year-old G2P1 via urgent cesarean delivery due to repetitive late decelerations. The amniotic fluid is heavily stained with thick meconium. Birth weight is 3,850 g (8.5 lb). Apgar scores are 4 at 1 minute and 7 at 5 minutes. At 2 hours of life, the neonate develops severe tachypnea, marked subcostal retractions, and cyanosis. Vital signs are temperature 36.6°C (97.9°F), heart rate 168/min, and respiratory rate 78/min.

Physical examination reveals a barrel-shaped chest and scattered yellow-white papules on an erythematous base localized to the trunk. A Wright stain of a pustule smear reveals numerous eosinophils.

The infant is placed on continuous positive airway pressure (CPAP) with 100% fraction of inspired oxygen ( $FiO_2$ ). Pulse oximetry demonstrates an oxygen saturation ( $SpO_2$ ) of 95% on the right hand and 82% on the left foot.

An arterial blood gas analysis drawn from an umbilical artery catheter reveals the following:

Parameter	Value	Reference Range
$pH$	7.22	7.35 - 7.45
$pCO_2$	55 mm Hg	35 - 45 mm Hg
$pO_2$	48 mm Hg	50 - 70 mm Hg
$HCO_3^-$	22 mEq/L	20 - 24 mEq/L

Chest radiograph displays hyperinflation, patchy bilateral alveolar infiltrates, and flattening of the hemidiaphragms.

Which of the following is the most appropriate next step in the management of this patient?

A
Initiate inhaled nitric oxide therapy
B
Administer exogenous surfactant via an endotracheal tube
Perform a transthoracic echocardiogramCevap
D
Initiate empirical intravenous vancomycin and cefepime
E
Apply topical mupirocin to the truncal lesions

Cevap

Perform a transthoracic echocardiogram

This post-term neonate born through meconium-stained fluid has meconium aspiration syndrome (MAS). The significant difference (>10%) between the pre-ductal (right hand) and post-ductal (left foot) oxygen saturations indicates a right-to-left shunt across the patent ductus arteriosus. This is the hallmark of persistent pulmonary hypertension of the newborn (PPHN), a common and severe complication of MAS. Before initiating specific pulmonary vasodilators like inhaled nitric oxide, a transthoracic echocardiogram is the essential next step. It confirms the diagnosis of PPHN by visualizing elevated right heart pressures and, crucially, definitively rules out structural cyanotic congenital heart disease (such as transposition of the great arteries or total anomalous pulmonary venous return). These structural defects can present identically with differential cyanosis but require entirely different, often surgical, management.

Adım Adım Çözüm

Identify the primary respiratory pathology based on the birth history and imaging.

A post-term infant born through meconium-stained fluid presenting with respiratory distress, hyperinflation, and patchy infiltrates on chest radiograph has Meconium Aspiration Syndrome (MAS).

MAS causes mechanical airway obstruction and chemical pneumonitis, frequently complicated by secondary conditions.

Evaluate the severe hypoxemia and differential oxygen saturations.

A pre-ductal (right hand)

SpO_2

of 95% and post-ductal (left foot)

SpO_2

of 82% reveals a right-to-left shunt across the patent ductus arteriosus.

This significant gradient (>10%) strongly suggests Persistent Pulmonary Hypertension of the Newborn (PPHN), a life-threatening complication of MAS.

Determine the most appropriate immediate diagnostic step.

A transthoracic echocardiogram must be ordered immediately.

Echocardiography is required to confirm the diagnosis of PPHN (by documenting elevated right-sided pressures) and to definitively rule out structural cyanotic congenital heart disease (which can present identically) before starting targeted therapies like inhaled nitric oxide.

Anahtar Kavram

In a neonate with suspected persistent pulmonary hypertension (indicated by a pre-ductal to post-ductal oxygen saturation gradient >10%), a transthoracic echocardiogram must be performed to confirm the diagnosis and rule out structural cyanotic congenital heart disease before initiating inhaled nitric oxide.

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