Following a recent admission for a community-acquired lower respiratory tract infection, a… — USMLE Step 2 CK

Following a recent admission for a community-acquired lower respiratory tract infection, a hospitalized adult becomes abruptly less responsive to verbal commands. The clinical record confirms Streptococcus pneumoniae bacteremia, which has been managed with targeted intravenous ceftriaxone for the past 36 hours. Vital signs reveal a temperature of 38.2°C (100.8°F), a pulse of 122/min, a blood pressure of 85/52 mm Hg, and a respiratory rate of 28/min. Pulse oximetry shows an oxygen saturation of 88% on high-flow nasal oxygen. Physical examination demonstrates bronchial breath sounds over the left lung base, warm extremities, and absent jugular venous distention. The patient appears lethargic and only moans in response to pain.

An arterial blood gas analysis reveals:

Parameter	Value	Reference Range
pH	7.18	7.35–7.45
PaCO₂	42 mm Hg	35–45 mm Hg
PaO₂	58 mm Hg	75–105 mm Hg
HCO₃⁻	15 mEq/L	22–28 mEq/L

Concurrent serum chemistries show a sodium of 140 mEq/L and a chloride of 103 mEq/L.

Based on these findings, which of the following is the most appropriate next step in management?

A
Administer intravenous sodium bicarbonate
B
Escalate antimicrobial therapy to vancomycin and piperacillin-tazobactam
Perform endotracheal intubationAnswer
D
Administer intravenous furosemide
E
Transport the patient for a computed tomography pulmonary angiogram

Answer

Perform endotracheal intubation

The correct response is to secure the airway via endotracheal intubation. This patient presents with septic shock and worsening encephalopathy secondary to severe pneumococcal pneumonia. Her arterial blood gas shows a pH of 7.18 and a bicarbonate of 15 mEq/L, indicating a metabolic acidosis (likely lactic acidosis). Using Winter's formula (

PaCO_2 = 1.5 \times 15 + 8 \pm 2

), her expected

PaCO_2

should be between 28.5 and 32.5 mm Hg. However, her actual

PaCO_2

is 42 mm Hg. This indicates a mixed acid-base disorder: a primary metabolic acidosis and a concurrent respiratory acidosis. The lack of respiratory compensation, combined with lethargy and obtundation, signifies respiratory muscle fatigue and an inability to protect the airway, making immediate mechanical ventilation the critical next step.

Step-by-Step Solution

Calculate the anion gap to determine the primary metabolic disturbance.

The anion gap is

140 - (103 + 15) = 22

An elevated anion gap (

>12

) indicates a primary high anion gap metabolic acidosis, likely secondary to lactic acidosis from septic shock.

Apply Winter's formula to determine if the respiratory compensation is adequate.

Expected

PaCO_2 = 1.5 \times 15 + 8 \pm 2

, which equals

28.5

32.5

mm Hg.

This formula calculates the physiologic respiratory response to a metabolic acidosis. The actual

PaCO_2

42

mm Hg, which is significantly higher than expected.

Diagnose the complete acid-base disorder.

The patient has a mixed high anion gap metabolic acidosis and concurrent respiratory acidosis.

The failure to appropriately lower

PaCO_2

indicates that the patient's respiratory muscles are fatiguing, leading to hypercapnic respiratory failure.

Select the immediate life-saving intervention.

Endotracheal intubation and mechanical ventilation must be initiated.

The patient is obtunded (cannot protect her airway), in profound shock, and exhibiting signs of impending respiratory collapse.

Key Concept

Mixed metabolic acidosis and respiratory acidosis with impending respiratory failure

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