During morning multidisciplinary rounds in the intensive care unit, the team evaluates a 7… — USMLE Step 2 CK

During morning multidisciplinary rounds in the intensive care unit, the team evaluates a 73-year-old male who has been mechanically ventilated for the past $48$ hours. His initial presentation involved profound hypoxemia due to community-acquired pneumonia. The patient's medical history includes ischemic cardiomyopathy (left ventricular ejection fraction of $25\%$ ) and severe emphysema.

He was empirically started on intravenous vancomycin and piperacillin-tazobactam. The microbiology laboratory has just finalized the results of blood and bronchoalveolar lavage fluid cultures obtained at admission; both demonstrate heavy growth of *Streptococcus pneumoniae*. The susceptibility report confirms the isolate is sensitive to all tested antibiotics, including ceftriaxone and penicillin.

Vital signs reveal a heart rate of $112$ /min and a blood pressure of $118/72$ mm Hg. He is afebrile. Physical examination shows engorged neck veins when the head of the bed is elevated. Cardiac auscultation identifies a third heart sound (S3). Lung examination yields diffuse bilateral crepitations that extend to the mid-apices, accompanied by prolonged expiratory wheezing.

An arterial blood gas analysis on current ventilator settings reveals:
- pH: $7.18$
- pCO $_2$ : $56$ mm Hg
- pO $_2$ : $82$ mm Hg
- HCO $_3^-$ : $20$ mEq/L

Which of the following interventions are appropriate for this patient at this time? (Select all that apply)

Initiate intravenous loop diuretic therapyAnswer
Raise the set respiratory rate or tidal volume on the ventilatorAnswer
C
Maintain the existing empiric antimicrobial coverage with vancomycin and piperacillin-tazobactam
D
Administer systemic glucocorticoids and scheduled short-acting bronchodilators
E
Infuse intravenous sodium bicarbonate to correct the acidemia

Answer

The appropriate next steps are to initiate intravenous loop diuretic therapy and to raise the set respiratory rate or tidal volume on the ventilator.

The correct interventions address the patient's two most acute, life-threatening derangements. First, the patient presents with volume overload resulting in cardiogenic pulmonary edema, characterized by an S3 gallop, elevated jugular venous pressure, and widespread crackles with wheezing (so-called 'cardiac asthma'). Diuresis with an intravenous loop diuretic is the cornerstone of therapy for acute decompensated heart failure. Second, the arterial blood gas reveals a mixed acid-base disorder. With a bicarbonate of

20

mEq/L, the expected pCO

_2

calculated by Winter's formula is

38 \pm 2

mm Hg. The patient's actual pCO

_2

56

mm Hg, indicating a significant superimposed respiratory acidosis. Because the patient is mechanically ventilated, the primary treatment for this respiratory acidosis is to increase the minute ventilation by raising either the respiratory rate or the tidal volume.

Step-by-Step Solution

Evaluate the patient's current physical examination and vital signs.

The patient has elevated jugular venous pressure, an S3 gallop, and diffuse crackles with wheezing.

These are classic signs of volume overload and acute decompensated heart failure, which can present with wheezing (cardiac asthma), necessitating diuresis.

Review the microbiology results and current medications.

Cultures grew pan-sensitive Streptococcus pneumoniae, but the patient is still on vancomycin and piperacillin-tazobactam.

Antimicrobial stewardship requires de-escalating from broad-spectrum to targeted therapy (e.g., ceftriaxone) once susceptibilities are known.

Interpret the arterial blood gas using Winter's formula.

Expected pCO2 = 1.5 × 20 + 8 ± 2 = 38 ± 2 mm Hg. The actual pCO2 (56 mm Hg) is much higher than expected.

This confirms a superimposed respiratory acidosis compounding the mild metabolic acidosis, requiring an increase in minute ventilation (via rate or tidal volume) to correct.

Key Concept

Management of acute decompensated heart failure ('cardiac asthma') and mixed acid-base disorders in the ICU requires diuresis, targeted ventilation adjustments, and adherence to antimicrobial stewardship.

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