A 27-year-old woman is brought to the emergency department by her roommate due to severe, … — USMLE Step 2 CK

A 27-year-old woman is brought to the emergency department by her roommate due to severe, progressively worsening shortness of breath over the past 2 days. She has a history of severe persistent asthma and has been using her albuterol inhaler every 2 hours at home without relief. On arrival, the patient appears lethargic and speaks only in single words. Her temperature is 37.1°C (98.8°F), blood pressure is 112/68 mmHg, heart rate is 132/min, and respiratory rate is 34/min. Pulse oximetry shows an oxygen saturation of 88% on room air.

Physical examination demonstrates marked use of accessory muscles of respiration. Auscultation of the lungs reveals poor air movement bilaterally with very faint expiratory wheezing.

An arterial blood gas analysis on room air shows:

Parameter	Value
pH	7.18
$p\text{CO}_2$	42 mmHg
$p\text{O}_2$	58 mmHg
$\text{HCO}_3^-$	15 mEq/L

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

Perform endotracheal intubation and initiate mechanical ventilationAnswer
B
Initiate noninvasive positive pressure ventilation
C
Administer intravenous sodium bicarbonate
D
Administer intravenous furosemide
E
Administer intravenous piperacillin-tazobactam

Answer

Perform endotracheal intubation and initiate mechanical ventilation

The correct answer is to perform endotracheal intubation and mechanical ventilation. This patient is experiencing impending respiratory arrest secondary to a severe asthma exacerbation. Indications for immediate intubation in asthma include altered mental status (lethargy), a 'silent chest' (diminished breath sounds due to critically low airflow), and hypercapnia (or 'pseudonormalization' of

p\text{CO}_2

). During an acute asthma attack, patients typically hyperventilate, leading to a low

p\text{CO}_2

. As respiratory muscles fatigue, the

p\text{CO}_2

normalizes and then rises. In this patient, Winter's formula shows an expected

p\text{CO}_2

of approximately 30.5 mmHg. The actual

p\text{CO}_2

of 42 mmHg represents a severe superimposed respiratory acidosis, confirming ventilatory failure that must be managed with a definitive airway.

Step-by-Step Solution

Assess the clinical severity of the asthma exacerbation.

The patient exhibits lethargy, single-word speech, and a 'silent chest' (poor air movement with faint wheezing), which are cardinal signs of impending respiratory arrest.

Recognizing signs of severe respiratory muscle fatigue is critical to determining the need for advanced airway management.

Interpret the arterial blood gas (ABG) values.

The ABG reveals acidemia (pH 7.18) with a low bicarbonate (15 mEq/L) and a 'normal'

p\text{CO}_2

(42 mmHg). Using Winter's formula [Expected

p\text{CO}_2

= (1.5 × 15) + 8 ± 2 = 30.5 ± 2 mmHg], the actual

p\text{CO}_2

is significantly higher than expected.

Proper acid-base interpretation distinguishes between appropriate physiological compensation and pathophysiological ventilatory failure.

Synthesize clinical and laboratory findings to choose the intervention.

The combination of altered mental status, silent chest, and a mixed metabolic and uncompensated respiratory acidosis indicates catastrophic ventilatory failure, necessitating immediate endotracheal intubation.

Non-invasive measures and medications alone are insufficient and potentially unsafe for a patient who can no longer maintain their own airway or adequate alveolar ventilation.

Key Concept

In acute severe asthma, a 'normal' or elevated

p\text{CO}_2

combined with altered mental status and poor air movement indicates respiratory muscle fatigue and impending respiratory arrest, requiring immediate endotracheal intubation.

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