An emergency department evaluation is requested for a 58-year-old male presenting with pro… — USMLE Step 2 CK

An emergency department evaluation is requested for a 58-year-old male presenting with progressively worsening shortness of breath and a cough producing frothy sputum over the past 48 hours. His medical history is significant for a 30-pack-year smoking habit and type 2 diabetes mellitus complicated by diabetic nephropathy. Upon arrival, he is in moderate respiratory distress.

Vital signs reveal a blood pressure of $185/110 \text{ mm Hg}$ , pulse of $112/\text{min}$ , and respiratory rate of $28/\text{min}$ . Oxygen saturation is 88% on room air. Pulmonary auscultation reveals bilateral crackles extending to the mid-lung fields along with diffuse expiratory wheezing. Examination of the lower extremities shows 2+ bilateral pitting edema.

Laboratory evaluation reveals the following:
Serum sodium: $138 \text{ mEq/L}$
Serum potassium: $5.9 \text{ mEq/L}$
Serum chloride: $102 \text{ mEq/L}$
Serum bicarbonate: $12 \text{ mEq/L}$
Blood urea nitrogen: $92 \text{ mg/dL}$
Serum creatinine: $7.4 \text{ mg/dL}$

Arterial blood gas (room air):
pH: $7.22$
$p\text{CO}_2$ : $36 \text{ mm Hg}$
$p\text{O}_2$ : $58 \text{ mm Hg}$

Based on the clinical and laboratory findings, which of the following best describes this patient's current acid-base disorder and the primary cause of his respiratory distress?

Metabolic acidosis with an elevated anion gap and a concurrent respiratory acidosis; distress is primarily due to acute volume overloadAnswer
B
Metabolic acidosis with an elevated anion gap and appropriate respiratory compensation; distress is primarily due to acute volume overload
C
Metabolic acidosis with an elevated anion gap and a concurrent respiratory acidosis; distress is primarily due to a chronic obstructive pulmonary disease exacerbation
D
Metabolic acidosis with an elevated anion gap and appropriate respiratory compensation; distress is primarily due to a chronic obstructive pulmonary disease exacerbation
E
Metabolic acidosis with an elevated anion gap and a concurrent respiratory alkalosis; distress is primarily due to acute volume overload

Answer

Metabolic acidosis with an elevated anion gap and a concurrent respiratory acidosis; distress is primarily due to acute volume overload

The patient has a metabolic acidosis with an elevated anion gap of 24, driven by his acute-on-chronic renal failure (uremia). Using Winter's formula (

1.5 \times [\text{HCO}_3^-] + 8 \pm 2

), the expected

p\text{CO}_2

24-28 \text{ mm Hg}

. The measured

p\text{CO}_2

36 \text{ mm Hg}

is substantially higher than this range, revealing a concurrent respiratory acidosis. The patient's profound hypertension, severe renal failure, crackles, pitting edema, and frothy sputum all indicate acute volume overload leading to pulmonary edema. The fluid accumulation in the lungs causes respiratory fatigue and

\text{CO}_2

retention, alongside 'cardiac asthma' (wheezing due to peribronchial edema).

Step-by-Step Solution

Calculate the serum anion gap to determine the primary metabolic disorder.

Anion gap =

138 - (102 + 12) = 24

Identifies the presence of a metabolic acidosis with an elevated anion gap, likely secondary to uremia given the markedly elevated BUN and creatinine.

Apply Winter's formula to determine the expected

p\text{CO}_2

for appropriate respiratory compensation.

Expected

p\text{CO}_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2 \text{ mm Hg}

Evaluates if a secondary acid-base disorder is present alongside the primary metabolic acidosis.

Compare the measured

p\text{CO}_2

to the calculated expected

p\text{CO}_2

The measured

p\text{CO}_2

36 \text{ mm Hg}

is significantly higher than the expected range (

24-28 \text{ mm Hg}

A higher-than-expected

p\text{CO}_2

indicates a concurrent respiratory acidosis due to inadequate ventilation.

Analyze the physical exam findings to identify the cause of respiratory distress and the respiratory acidosis.

Severe hypertension, frothy sputum, bilateral crackles, pitting edema, and severe renal failure point directly to acute pulmonary edema.

Volume overload leads to pulmonary edema, which impairs gas exchange and causes respiratory fatigue. This explains both the wheezing ('cardiac asthma') and the concurrent respiratory acidosis.

Key Concept

Mixed acid-base disorders and differentiation of acute pulmonary edema from primary obstructive lung disease.

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