Paramedics transport a 6-month-old male infant to the acute care pediatric triage unit aft… — USMLE Step 2 CK

Paramedics transport a $6$ -month-old male infant to the acute care pediatric triage unit after his parents reported sudden unresponsiveness following three days of nasal congestion and a mild cough. According to the mother, the infant was born prematurely at $34$ weeks gestation and had been struggling to feed for the past $24$ hours. Upon arrival at the facility, his temperature is $37.8^\circ\text{C}$ ( $100.0^\circ\text{F}$ ), pulse is $175/\text{min}$ , and respirations are recorded at $65/\text{min}$ . Pulse oximetry shows an oxygen saturation of $86\%$ on room air.

Initial physical assessment reveals an agitated infant demonstrating profound subcostal and intercostal retractions, head bobbing, and pronounced nasal flaring. Diffuse expiratory wheezing with scattered crackles is heard throughout all lung fields. The clinical team immediately initiates heated, humidified high-flow oxygen therapy.

Thirty minutes into the intervention, the infant's clinical status changes: he becomes flaccid and difficult to arouse. The respiratory rate drops to $24/\text{min}$ , and previously noted wheezing is now replaced by severely diminished breath sounds bilaterally. A capillary blood gas is obtained while on high-flow oxygen, revealing:

Parameter	Value	Reference Range
$\text{pH}$	$7.31$	$7.35 - 7.45$
$\text{pCO}_2$	$52\text{ mm Hg}$	$35 - 45\text{ mm Hg}$
$\text{pO}_2$	$58\text{ mm Hg}$	$60 - 80\text{ mm Hg (capillary)}$
$\text{HCO}_3^-$	$25\text{ mEq/L}$	$22 - 26\text{ mEq/L}$

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

A
Administer nebulized albuterol and intravenous methylprednisolone
B
Maintain current respiratory support and reassess in 1 hour, as the decreased respiratory rate indicates clinical improvement
Endotracheal intubation and mechanical ventilationCevap
D
Initiate empiric therapy with intravenous ceftriaxone and vancomycin
E
Administer nebulized racemic epinephrine and oral dexamethasone

Cevap

Endotracheal intubation and mechanical ventilation

The correct answer is to perform endotracheal intubation and mechanical ventilation. This patient's initial presentation is classic for severe viral bronchiolitis. Infants with significant lower airway obstruction must exert tremendous effort to breathe, initially presenting with tachypnea and marked retractions. This hyperventilation typically results in a respiratory alkalosis (low

\text{pCO}_2

). However, infant respiratory muscles fatigue rapidly. When a previously struggling infant suddenly becomes lethargic, demonstrates a decreased respiratory rate, and loses audible wheezing (the 'silent chest'), it is a dire sign of imminent respiratory arrest. The capillary blood gas confirms acute ventilatory failure, demonstrating a marked respiratory acidosis (

\text{pCO}_2

52\text{ mm Hg}

) in a patient who can no longer compensate. At this stage of impending arrest, non-invasive support or pharmacotherapy is insufficient, and securing the airway with mechanical ventilation is the definitive, life-saving next step.

Adım Adım Çözüm

Diagnose the underlying condition based on the initial presentation.

The infant's age, viral prodrome, prematurity, diffuse wheezing, and crackles strongly indicate severe viral bronchiolitis (most commonly due to RSV).

Establishing the etiology rules out therapies specifically targeted at asthma or croup, which are common diagnostic distractors.

Evaluate the sudden change in clinical status.

The transition from an agitated, tachypneic state to flaccidity, lethargy, a normalized respiratory rate (

24/\text{min}

), and a 'silent chest' indicates severe respiratory muscle fatigue.

Infants cannot sustain the massive work of breathing required during severe airway obstruction indefinitely. A dropping respiratory rate in this context is a harbinger of arrest, not recovery.

Interpret the capillary blood gas in the context of the physical exam.

The blood gas shows respiratory acidosis (

\text{pH}

7.31

\text{pCO}_2

52\text{ mm Hg}

An infant with severe respiratory distress should be hyperventilating, leading to a respiratory alkalosis (low

\text{pCO}_2

). A high

\text{pCO}_2

confirms that the infant's ventilation has failed.

Select the definitive intervention for impending respiratory arrest.

Endotracheal intubation and mechanical ventilation is selected.

Once an infant enters hypercapnic respiratory failure and loses the drive or mechanical ability to breathe, non-invasive measures and medications are insufficient. The airway must be secured immediately.

Anahtar Kavram

Recognition and definitive airway management of impending respiratory failure in severe infant bronchiolitis.

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Cevap

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