A 26-year-old man is evaluated in the emergency department for severe shortness of breath … — USMLE Step 2 CK

A 26-year-old man is evaluated in the emergency department for severe shortness of breath that has progressively worsened over the past 48 hours. He has a lifelong history of asthma and uses a short-acting bronchodilator as needed. Yesterday, he moved into an apartment with a roommate who owns a cat. On arrival, the patient is fully alert but speaks in short, two-word sentences. Initial vital signs include a temperature of 37.1°C ( $98.8$ °F), blood pressure of 135/82 mmHg, pulse of 122/min, and respiratory rate of 30/min. Pulse oximetry shows an oxygen saturation of 93% on room air. Physical examination demonstrates prominent accessory muscle use and loud, diffuse expiratory wheezing.

He is immediately treated with continuous nebulized albuterol, ipratropium, and intravenous methylprednisolone. Two hours later, the nurse urgently alerts the physician because the patient has become drowsy and is difficult to rouse. Repeat vital signs show a blood pressure of 108/65 mmHg, pulse of 138/min, and a respiratory rate of 12/min. Oxygen saturation is 87% on 4 L/min via nasal cannula. Repeat chest auscultation reveals markedly diminished breath sounds bilaterally with absent wheezing.

An arterial blood gas analysis on the current oxygen setting shows:

Laboratory	Result
$pH$	$7.24$
$PaCO_2$	$60$ mmHg
$PaO_2$	$56$ mmHg
$HCO_3^-$	$25$ mEq/L

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

Perform endotracheal intubationAnswer
B
Administer intravenous sodium bicarbonate
C
Administer intravenous furosemide
D
Begin continuous intravenous piperacillin-tazobactam
E
Initiate bilevel positive airway pressure therapy

Answer

Perform endotracheal intubation

The correct intervention is endotracheal intubation. This patient is experiencing severe status asthmaticus complicated by impending respiratory failure. Initially, acute asthma exacerbations typically present with hyperventilation and a respiratory alkalosis (low

PaCO_2

). However, as the respiratory muscles fatigue against severe airway resistance, minute ventilation drops, leading to carbon dioxide retention (hypercapnia) and acute respiratory acidosis. The triad of altered mental status (drowsiness), a decreasing respiratory rate (from 30/min to 12/min), and a 'silent chest' (markedly diminished breath sounds due to critical airflow limitation) are hallmark signs that the patient can no longer maintain independent respiration. Immediate definitive airway protection via endotracheal intubation and mechanical ventilation is necessary to ensure adequate gas exchange and prevent anoxic brain injury or cardiac arrest.

Step-by-Step Solution

Analyze the clinical progression and physical examination findings.

The patient transitioned from being alert with loud wheezing and tachypnea to being lethargic with a 'silent chest' and bradypnea.

Tracking these changes identifies the transition from an acute asthma attack to severe respiratory muscle fatigue.

Interpret the arterial blood gas (ABG) values.

The ABG demonstrates a low pH (

7.24

) and a significantly elevated

PaCO_2

(

60

mmHg).

This confirms acute, uncompensated respiratory acidosis, verifying that alveolar hypoventilation has occurred.

Select the appropriate intervention for impending respiratory failure.

Endotracheal intubation and mechanical ventilation are required.

Lethargy, hypercapnia, and hypoventilation in the setting of status asthmaticus necessitate immediate definitive airway protection and ventilatory support to prevent cardiac arrest.

Key Concept

Impending respiratory failure in severe asthma exacerbations is clinically characterized by a normalizing or elevated

PaCO_2

, altered mental status, and a 'silent chest', requiring immediate endotracheal intubation and mechanical ventilation.

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