Emergency medical services bring a 59-year-old woman to the emergency department after her… — USMLE Step 2 CK

Emergency medical services bring a 59-year-old woman to the emergency department after her daughter found her unarousable at home. The daughter reports that the patient has had a productive cough, subjective fevers, and generalized weakness for the past three days. Medical history is significant for severe chronic obstructive pulmonary disease and type 2 diabetes mellitus. Her current temperature is 38.6°C (101.5°F), blood pressure is 88/54 mm Hg, pulse is 118/min, and respirations are 24/min and shallow. Pulse oximetry shows 84% on room air. On physical examination, the patient is obtunded and only withdraws to painful stimuli. Lung auscultation reveals diffuse coarse rhonchi bilaterally.

Laboratory test	Result
Arterial blood gas (room air)
pH	7.15
PaCO2	44 mm Hg
PaO2	52 mm Hg
Serum chemistry
Sodium	140 mEq/L
Potassium	4.8 mEq/L
Chloride	100 mEq/L
Bicarbonate	14 mEq/L
Blood urea nitrogen	32 mg/dL
Creatinine	1.8 mg/dL
Lactate	5.5 mmol/L

A peripheral intravenous line is established, and a bolus of isotonic crystalloid is initiated along with empiric ceftriaxone and azithromycin. Which of the following is the most appropriate next step in the management of this patient?

Endotracheal intubationAnswer
B
Trial of noninvasive positive-pressure ventilation
C
Intravenous sodium bicarbonate administration
D
Intravenous furosemide administration
E
Transition to piperacillin-tazobactam and vancomycin

Answer

Endotracheal intubation

The correct answer is endotracheal intubation. This patient presents with septic shock (hypotension, fever, elevated lactate) secondary to pneumonia, leading to a high anion gap metabolic acidosis. Applying Winter's formula (

PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2

), the expected

PaCO_2

for a bicarbonate of 14 mEq/L is 29 mm Hg (range 27-31). Her actual

PaCO_2

is 44 mm Hg, which indicates a concurrent respiratory acidosis (failure to mount the appropriate respiratory compensation, likely due to COPD and altered sensorium). While noninvasive positive-pressure ventilation (NIPPV) is typically the initial respiratory intervention for hypercapnic respiratory failure in COPD exacerbations, this patient is obtunded. Altered mental status is an absolute contraindication to NIPPV due to the inability to protect the airway and the consequent high risk of aspiration. Therefore, she requires immediate definitive airway protection with endotracheal intubation.

Step-by-Step Solution

Analyze the acid-base status to determine the primary and secondary disorders.

Anion Gap =

140 - (100 + 14) = 26

. Expected

PaCO_2 = 1.5 \times 14 + 8 = 29 \pm 2

mm Hg. Actual

PaCO_2

is 44 mm Hg.

Identifying the acid-base disturbances is essential for understanding the etiology of the patient's deterioration. The high anion gap indicates a primary metabolic acidosis (lactic acidosis from sepsis), while the inappropriately elevated PaCO2 indicates a concurrent primary respiratory acidosis (hypoventilation).

Assess the severity of the respiratory failure and the patient's neurologic status.

The patient is acidemic (pH 7.15), hypoxic (PaO2 52 mm Hg), and obtunded (withdraws only to pain).

To determine the appropriate modality for ventilatory support. An obtunded patient cannot protect their airway.

Select the appropriate intervention prioritizing the ABCs (Airway, Breathing, Circulation).

Endotracheal intubation is selected over noninvasive ventilation.

Although noninvasive positive-pressure ventilation (NIPPV) is standard for COPD exacerbations with hypercapnia, altered mental status is an absolute contraindication due to aspiration risk.

Key Concept

Patients with mixed metabolic and respiratory acidosis presenting with altered mental status and hypercapnic respiratory failure require immediate endotracheal intubation, as altered mental status is an absolute contraindication to noninvasive positive-pressure ventilation.

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