A neonate born at 36 weeks of gestation is currently 72 hours old and is being monitored i… — USMLE Step 2 CK

A neonate born at 36 weeks of gestation is currently 72 hours old and is being monitored in the intermediate care nursery. The nursing staff alerts the physician after noting a sudden decrease in the infant's responsiveness and failure to take the last two scheduled bottle feeds. He was delivered vaginally with a birth weight of 2,600 g. Apgar scores were 8 and 9 at 1 and 5 minutes, respectively. Maternal group B *Streptococcus* screening was reportedly negative, but the mother had limited prenatal care.

Upon examination, the infant is lethargic with mottled, cool skin. A generalized eruption consisting of erythematous macules with central yellow pustules is noted over the trunk. He exhibits significantly increased work of breathing with nasal flaring and expiratory grunting.

Vital signs show:
- Core temperature: 35.4°C (95.7°F)
- Heart rate: 182/min
- Respiratory rate: 78/min
- Blood pressure: 42/22 mm Hg
- Oxygen saturation: 85% on room air

An arterial blood gas is drawn, and results are as follows:

Parameter	Value	Reference Range
pH	7.15	7.35 – 7.45
PaCO₂	48 mm Hg	35 – 45 mm Hg
PaO₂	52 mm Hg	60 – 80 mm Hg
HCO₃⁻	16 mEq/L	22 – 28 mEq/L
Lactate	6.2 mmol/L	0.5 – 2.2 mmol/L

A rapid point-of-care blood multiplex polymerase chain reaction (PCR) assay is positive for *Streptococcus agalactiae*.

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

Endotracheal intubation and mechanical ventilationAnswer
B
Initiation of continuous positive airway pressure (CPAP)
C
Intravenous administration of sodium bicarbonate
D
Intravenous administration of acyclovir
E
Empiric administration of intravenous vancomycin and meropenem

Answer

The correct answer is endotracheal intubation and mechanical ventilation.

The correct intervention is endotracheal intubation and mechanical ventilation. This patient is presenting with septic shock (evidenced by hypothermia, tachycardia, hypotension, and mottling) and acute respiratory failure. The arterial blood gas demonstrates a primary metabolic (lactic) acidosis with a pH of 7.15 and bicarbonate of 16 mEq/L. According to Winter's formula, the expected

PaCO_2

(1.5 \times 16) + 8 = 32 \pm 2

mm Hg. The patient's actual

PaCO_2

is 48 mm Hg, which is significantly higher than expected. This indicates a superimposed, severe respiratory acidosis due to complete failure of respiratory compensation, which is a life-threatening emergency. In a lethargic neonate with septic shock and impending respiratory arrest, the immediate priority according to standard resuscitation guidelines (ABCs) is to secure the airway and provide adequate ventilation. Therefore, endotracheal intubation and mechanical ventilation is the most appropriate next step before initiating fluid resuscitation or antibiotic therapy.

Step-by-Step Solution

Evaluate the patient's vital signs and clinical presentation.

The neonate is lethargic, hypotensive, hypothermic, and tachypneic with mottled skin, which indicates severe septic shock.

To recognize the severity of the illness and prioritize resuscitation according to the ABCs (Airway, Breathing, Circulation).

Analyze the arterial blood gas using Winter's formula to assess respiratory compensation.

The expected

PaCO_2

(1.5 \times 16) + 8 = 32 \pm 2

mm Hg. The actual

PaCO_2

48

mm Hg, revealing a severe superimposed respiratory acidosis.

To determine if the infant is adequately ventilating in response to the profound metabolic (lactic) acidosis.

Identify the immediate next step in management based on physiological status.

Endotracheal intubation and mechanical ventilation must be performed immediately.

The infant is lethargic with acute respiratory failure and shock; securing the airway and ensuring ventilation is the highest life-saving priority.

Evaluate secondary findings (rash and pathogen) to plan subsequent care.

The rash is benign erythema toxicum neonatorum. The identified pathogen is Group B Streptococcus.

To avoid unnecessary antivirals and to select targeted narrow-spectrum antibiotics (e.g., ampicillin) once the airway is secured, adhering to antibiotic stewardship.

Key Concept

In neonatal septic shock complicated by severe respiratory failure (identified via uncompensated metabolic acidosis), securing the airway and providing mechanical ventilation is the immediate priority before fluid resuscitation or targeted antibiotic therapy.

Estimated Time:2m 0s

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