A 64-year-old man is brought to the emergency department by his family due to progressive … — USMLE Step 2 CK

A 64-year-old man is brought to the emergency department by his family due to progressive confusion and severe shortness of breath. The family reports that he has had fever, chills, and a cough producing rust-colored sputum for the past 4 days. His medical history is significant for heart failure with preserved ejection fraction (HFpEF) and hypertension. His temperature is $39.1^\circ\text{C}$ ( $102.4^\circ\text{F}$ ), pulse is $128\text{/min}$ , respirations are $34\text{/min}$ , and blood pressure is $82/48\text{ mm Hg}$ . Pulse oximetry shows an oxygen saturation of $85\%$ on $15\text{ L/min}$ of oxygen via a non-rebreather mask. On physical examination, the patient is lethargic and arouses only to painful stimuli. He has marked suprasternal retractions and uses accessory muscles of respiration. Chest auscultation reveals dense crackles and egophony over the left lower lung field. The cardiovascular examination shows a rapid, regular rhythm with no jugular venous distention. Laboratory studies show a leukocyte count of $22,000/\mu\text{L}$ and a serum lactate of $7.2\text{ mmol/L}$ . An arterial blood gas analysis on $15\text{ L/min}$ of supplemental oxygen reveals the following:

$pH$ : $7.12$
$pCO_2$ : $35\text{ mm Hg}$
$pO_2$ : $54\text{ mm Hg}$
$HCO_3^-$ : $11\text{ mEq/L}$

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

Endotracheal intubation and mechanical ventilationAnswer
B
Intravenous sodium bicarbonate administration
C
Intravenous furosemide administration
D
Initiation of intravenous meropenem
E
Transport for computed tomography pulmonary angiography

Answer

The most appropriate next step is endotracheal intubation and mechanical ventilation.

This patient presents with septic shock and impending respiratory failure secondary to severe community-acquired pneumonia. The arterial blood gas (ABG) shows a primary metabolic acidosis (pH

7.12

HCO_3^-

11\text{ mEq/L}

), largely driven by severe lactic acidosis (

7.2\text{ mmol/L}

) from tissue hypoperfusion. To determine if the patient is adequately compensating, Winter's formula must be applied: Expected

pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2

. Using the patient's bicarbonate value, the expected

pCO_2

22.5

26.5\text{ mm Hg}

(

1.5 \times 11 + 8 = 24.5

).

The patient's actual

pCO_2

35\text{ mm Hg}

, which is significantly higher than expected. This relative hypercapnia in the setting of severe metabolic acidosis indicates a concomitant respiratory acidosis, demonstrating that the patient's respiratory muscles are failing to maintain the extreme minute ventilation required for compensation. Combined with his profound lethargy (indicating an inability to protect the airway) and severe hypoxemia despite maximal noninvasive oxygen therapy (

15\text{ L/min}

via non-rebreather mask), definitive airway management with endotracheal intubation is the most critical and life-saving next step.

Step-by-Step Solution

Analyze the patient's clinical presentation.

The patient is lethargic, hypotensive, and severely hypoxic on maximal noninvasive oxygen, indicating septic shock and impending respiratory failure.

Identifying altered mental status and hemodynamic instability establishes the immediate need for airway protection and resuscitation.

Evaluate the arterial blood gas using Winter's formula.

Expected

pCO_2 = 1.5 \times 11 + 8 = 24.5 \pm 2\text{ mm Hg}

. The actual

pCO_2

35\text{ mm Hg}

is significantly higher than expected.

Calculating the expected respiratory compensation determines if the patient's respiratory drive is adequate or if the respiratory muscles are fatiguing.

Synthesize findings to determine the immediate intervention.

The combination of lethargy, severe hypoxia, and a mixed metabolic/respiratory acidosis dictates that securing the airway is the top priority.

Airway and breathing (ABCs) must be secured before administering medications or transporting the patient.

Key Concept

Indications for Endotracheal Intubation in Critical Illness

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