A 62-year-old woman is brought to the emergency department by her family due to a three-da… — USMLE Step 2 CK

A 62-year-old woman is brought to the emergency department by her family due to a three-day history of productive cough, fever, and progressively worsening shortness of breath. Her past medical history is significant for type 2 diabetes mellitus and essential hypertension. Upon arrival, she appears lethargic and is in marked respiratory distress. Vital signs show a temperature of $38.9^\circ\text{C}$ ( $102^\circ\text{F}$ ), blood pressure of $86/52$ mm Hg, pulse of $124$ /min, and respirations of $28$ /min. Pulse oximetry indicates an oxygen saturation of $87\%$ on room air. Physical examination demonstrates prominent use of accessory muscles of respiration and coarse crackles over the right lower lung field. Cardiac examination reveals tachycardia with a regular rhythm and no murmurs.

An arterial blood gas analysis on room air shows the following:

Parameter	Value	Reference Range
pH	$7.22$	$7.35-7.45$
$pCO_2$	$36$ mm Hg	$35-45$ mm Hg
$pO_2$	$56$ mm Hg	$75-105$ mm Hg
$HCO_3^-$	$14$ mEq/L	$22-28$ mEq/L

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

A
High-flow nasal cannula oxygen therapy
B
Intravenous furosemide therapy
C
Intravenous meropenem therapy
Endotracheal intubation and mechanical ventilationAnswer
E
Intravenous sodium bicarbonate infusion

Answer

The correct answer is endotracheal intubation and mechanical ventilation.

The correct answer is endotracheal intubation and mechanical ventilation. This patient presents with septic shock likely secondary to community-acquired pneumonia, leading to a profound primary metabolic acidosis (lactic acidosis). According to Winter's formula (

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

), the expected

pCO_2

for a bicarbonate of 14 mEq/L is

29 \pm 2

mm Hg (range 27-31 mm Hg). The patient's actual

pCO_2

is 36 mm Hg, which is inappropriately elevated for her degree of metabolic acidosis. This indicates a concurrent primary respiratory acidosis. Clinically, this manifests as a lethargic patient who is tiring out despite severe tachypnea and accessory muscle use. In the setting of hemodynamic instability and impending respiratory failure, securing a definitive airway with endotracheal intubation is the most critical next step.

Step-by-Step Solution

Interpret the primary acid-base disorder.

Primary metabolic acidosis.

The pH is acidemic (7.22) and the bicarbonate is low (14 mEq/L).

Calculate the expected respiratory compensation using Winter's formula.

Expected pCO2 is 27-31 mm Hg (

1.5 \times 14 + 8 \pm 2

To determine if a mixed acid-base disorder is present by evaluating the respiratory response.

Compare the expected pCO2 to the measured pCO2.

The measured pCO2 (36 mm Hg) is higher than the expected range.

This confirms the presence of a concurrent primary respiratory acidosis, indicating respiratory muscle fatigue.

Synthesize the clinical findings with the laboratory data to determine the next step.

The patient requires immediate endotracheal intubation.

A lethargic patient with severe sepsis, accessory muscle use, and a concurrent respiratory acidosis has impending respiratory failure requiring definitive airway management.

Key Concept

In patients with severe metabolic acidosis, a 'normal' pCO2 value often indicates respiratory muscle fatigue and impending respiratory failure, requiring immediate endotracheal intubation.

Estimated Time:1m 30s

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