A 67-year-old man is brought to the emergency department from his home by his daughter due… — USMLE Step 2 CK

A 67-year-old man is brought to the emergency department from his home by his daughter due to progressive confusion, a productive cough with rust-colored sputum, and subjective fever for the past 3 days. He has a history of severe obesity hypoventilation syndrome, untreated obstructive sleep apnea, and chronic lower extremity edema.

Vital signs include a temperature of $38.8^\circ C$ ( $101.8^\circ F$ ), heart rate of $115$ /min, blood pressure of $105/65$ mmHg, respiratory rate of $24$ /min, and oxygen saturation of $82\%$ on room air.

Physical examination reveals a body mass index of $42$ kg/m $^2$ . The patient is lethargic but rousable. Jugular venous distention is present to the angle of the jaw. Chest auscultation reveals bronchial breath sounds and egophony in the right lower lobe, along with trace bilateral expiratory wheezing; no fine crackles are heard. Cardiac examination reveals a prominent $P_2$ and a right-sided $S_3$ . There is $3+$ pitting edema bilaterally up to the knees.

Laboratory results:
Leukocyte count: $16,500/\mu L$ (with $12\%$ bands)
Serum sodium: $138$ mEq/L
Serum potassium: $4.1$ mEq/L
Serum chloride: $92$ mEq/L
Serum bicarbonate: $16$ mEq/L
Blood urea nitrogen: $32$ mg/dL
Serum creatinine: $1.4$ mg/dL
Serum lactate: $4.2$ mmol/L

Arterial blood gas (on room air):
pH: $7.13$
PaO $_2$ : $50$ mm Hg
PaCO $_2$ : $50$ mm Hg

Chest radiograph shows a dense right lower lobe consolidation. There is prominent central pulmonary vasculature and right ventricular enlargement, but no pulmonary venous congestion or pleural effusion.

A rapid lower respiratory tract multiplex PCR panel is positive for *Streptococcus pneumoniae* and negative for all other bacterial and viral targets.

Based on the patient's presentation and diagnostic findings, which of the following statements are correct regarding his diagnosis and management? (Select all that apply)

The arterial blood gas analysis demonstrates a primary metabolic acidosis with a concurrent primary respiratory acidosis.Answer
B
The patient's arterial blood gas demonstrates a primary metabolic acidosis with appropriate and expected respiratory compensation.
The elevated jugular venous pressure and bilateral pitting edema represent right ventricular failure secondary to chronic hypoxic pulmonary vasoconstriction.Answer
D
Intravenous loop diuretics should be administered immediately to treat acute left ventricular failure and suspected pulmonary edema.
Antimicrobial therapy should consist of targeted coverage for Streptococcus pneumoniae, such as ceftriaxone, rather than empiric broad-spectrum agents.Answer
F
Antimicrobial therapy should include vancomycin and piperacillin-tazobactam to empirically cover for resistant nosocomial pathogens.

Answer

The correct statements are that the ABG shows a primary metabolic acidosis with a concurrent primary respiratory acidosis, the physical exam findings represent right ventricular failure (cor pulmonale) from chronic hypoxia, and antimicrobial therapy should be targeted for Streptococcus pneumoniae.

The correct statements accurately interpret three key domains of this patient's presentation. First, the ABG shows a pH of 7.13 and HCO3 of 16 mEq/L, indicating a metabolic acidosis (likely lactic acidosis given the lactate of 4.2). Using Winter's formula, the expected PaCO2 is (1.5 * 16) + 8 = 32 +/- 2 mm Hg. The patient's actual PaCO2 is 50 mm Hg, diagnosing a concurrent primary respiratory acidosis. Second, the clinical signs of JVD, right-sided S3, and edema, coupled with clear lungs (no crackles) and a chest X-ray showing RV enlargement without venous congestion, are diagnostic of Cor Pulmonale (right heart failure). Finally, the rapid PCR directly isolates S. pneumoniae, meaning targeted therapy is indicated over broad-spectrum empiric coverage.

Step-by-Step Solution

Analyze the arterial blood gas values to determine the primary and secondary acid-base disturbances.

The patient has a primary metabolic acidosis (pH 7.13, HCO3 16 mEq/L) with a concurrent primary respiratory acidosis (PaCO2 50 mm Hg).

Applying Winter's formula (Expected PaCO2 = 1.5 * HCO3 + 8 +/- 2) yields an expected PaCO2 of 30-34 mm Hg. Because the actual PaCO2 is 50 mm Hg, respiratory compensation is inadequate, indicating a mixed acid-base disorder.

Interpret the physical exam and imaging findings to differentiate the etiology of the heart failure signs.

The findings are consistent with isolated right heart failure (Cor Pulmonale).

The patient has jugular venous distention, peripheral edema, and a right-sided S3, but lacks fine crackles on exam and lacks pulmonary venous congestion on chest X-ray. This differentiates his chronic right heart strain (from obesity hypoventilation syndrome) from acute left heart failure.

Determine the appropriate antimicrobial regimen based on the clinical setting and microbiological data.

Targeted therapy with an agent like ceftriaxone is indicated.

The patient has community-acquired pneumonia, and a multiplex PCR specifically identified S. pneumoniae while ruling out MRSA and Pseudomonas. Initiating broad-spectrum agents in this scenario violates antibiotic stewardship principles.

Key Concept

Management of complex respiratory failure requires systematic interpretation of acid-base status using Winter's formula, clinical differentiation of right versus left heart failure, and adherence to antibiotic stewardship when microbiological data is available.

Estimated Time:2m 0s

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