Paramedics bring a 9-year-old girl to the emergency department due to acute-onset, severe … — USMLE Step 2 CK

Paramedics bring a $9$ -year-old girl to the emergency department due to acute-onset, severe shortness of breath. She has a history of moderate persistent asthma but exhausted her supply of inhaled fluticasone-salmeterol two months ago. Her symptoms began abruptly after she visited a relative's house that has multiple indoor cats. In the emergency department, she receives three continuous nebulized treatments of albuterol and ipratropium, along with a dose of intravenous methylprednisolone. Reassessment $45$ minutes later reveals a temperature of $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), heart rate of $148/\text{min}$ , and respiratory rate of $38/\text{min}$ . Pulse oximetry is $91\%$ on a $4\text{ L/min}$ nasal cannula. On examination, the patient appears exhausted and somnolent. She is sitting in a tripod position and using prominent suprasternal and intercostal accessory muscles to breathe. Chest auscultation reveals a "silent chest" with markedly diminished air entry bilaterally and no audible wheezing. Arterial blood gas analysis on supplemental oxygen reveals a $\text{pH}$ of $7.31$ , $\text{PaCO}_2$ of $46 \text{ mmHg}$ , and $\text{PaO}_2$ of $62 \text{ mmHg}$ . Which of the following is the most appropriate next step in management?

Endotracheal intubation and mechanical ventilationAnswer
B
Initiation of noninvasive positive pressure ventilation
C
Administration of intravenous magnesium sulfate
D
Administration of intravenous broad-spectrum antibiotics
E
Administration of nebulized hypertonic saline

Answer

Endotracheal intubation and mechanical ventilation

This patient is experiencing a severe asthma exacerbation that has rapidly progressed to impending respiratory failure. Key clinical indicators include her exhausted and somnolent mental status, a paradoxically 'silent chest' (indicating airflow is too poor to generate wheezing), and a

\text{PaCO}_2

46 \text{ mmHg}

. During an acute asthma attack, patients typically hyperventilate, resulting in a low

\text{PaCO}_2

(respiratory alkalosis). A normal or elevated

\text{PaCO}_2

in a tachypneic, distressed asthmatic patient is a classic sign of severe respiratory muscle fatigue and inadequate alveolar ventilation. When a patient exhibits these signs of impending respiratory failure along with altered mental status, the most appropriate next step is securing the airway with endotracheal intubation and initiating mechanical ventilation.

Step-by-Step Solution

Assess the clinical severity of the asthma exacerbation.

The patient demonstrates exhaustion, somnolence, tripod positioning, and a 'silent chest' after initial maximal medical therapy.

Identifying signs of severe fatigue and poor air entry is critical for recognizing impending respiratory failure.

Interpret the arterial blood gas (ABG) results.

The ABG shows respiratory acidosis with a normal-to-elevated PaCO2 (46 mmHg).

In an acute asthma exacerbation, tachypnea usually causes a low PaCO2 (respiratory alkalosis). A normal or high PaCO2 indicates severe respiratory muscle fatigue and failure to ventilate adequately.

Determine the most appropriate immediate intervention.

Select endotracheal intubation.

Patients with impending respiratory failure and altered mental status require immediate airway security and mechanical ventilation to prevent cardiopulmonary arrest.

Key Concept

Patients with acute asthma exacerbations typically hyperventilate and have a low PaCO2; a normal or elevated PaCO2, especially when accompanied by somnolence or a 'silent chest', indicates impending respiratory failure requiring immediate endotracheal intubation.

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