A 74-year-old man is evaluated in the surgical telemetry unit on postoperative day 4 follo… — USMLE Step 2 CK

A 74-year-old man is evaluated in the surgical telemetry unit on postoperative day 4 following an open cholecystectomy and common bile duct exploration. The nursing staff requested a review because his urine output has decreased to $250 \text{ mL}$ over the last 24 hours, and he has become increasingly confused and short of breath. His medical history includes hypertension, type 2 diabetes mellitus, and a 40-pack-year smoking history. Temperature is $37.6^\circ\text{C}$ ( $99.7^\circ\text{F}$ ), blood pressure is $155/92 \text{ mm Hg}$ , pulse is $112/\text{min}$ , and respirations are $26/\text{min}$ . Oxygen saturation is $88\%$ on room air. Physical examination shows jugular venous distention $10 \text{ cm}$ above the sternal angle. Auscultation of the lungs reveals bilateral basilar crackles. The abdomen is soft and non-tender, with intact surgical staples and no erythema. Bilateral $2+$ pitting edema is present in the lower extremities.

Laboratory studies show:
Serum sodium: $134 \text{ mEq/L}$
Serum potassium: $5.2 \text{ mEq/L}$
Serum chloride: $98 \text{ mEq/L}$
Serum bicarbonate: $14 \text{ mEq/L}$
Blood urea nitrogen: $68 \text{ mg/dL}$
Serum creatinine: $3.1 \text{ mg/dL}$ (baseline $1.1 \text{ mg/dL}$ )

Arterial blood gas (room air):
pH: $7.24$
PaCO2: $42 \text{ mm Hg}$
PaO2: $58 \text{ mm Hg}$

Which of the following is the most accurate characterization of this patient's acid-base status and the most appropriate next step in management?

Mixed high anion gap metabolic acidosis and respiratory acidosis; administer intravenous furosemideAnswer
B
Uncompensated primary high anion gap metabolic acidosis; initiate urgent hemodialysis
C
Mixed high anion gap metabolic acidosis and respiratory alkalosis; administer intravenous normal saline
D
Primary respiratory acidosis with metabolic compensation; administer nebulized albuterol and ipratropium
E
Mixed high anion gap metabolic acidosis and respiratory acidosis; administer intravenous piperacillin-tazobactam

Answer

Mixed high anion gap metabolic acidosis and respiratory acidosis; administer intravenous furosemide

This patient presents with acute kidney injury, severe volume overload, and a complex acid-base derangement. The low pH (

7.24

) and low serum bicarbonate (

14 \text{ mEq/L}

) establish a primary metabolic acidosis. The anion gap is elevated at

22

(

134 - (98 + 14)

), pointing to a high anion gap metabolic acidosis, likely from retention of unmeasured uremic acids due to his acute kidney injury. To determine if respiratory compensation is adequate, Winter's formula must be applied: Expected

\text{PaCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2

. For a bicarbonate of

14 \text{ mEq/L}

, the expected

\text{PaCO}_2

27

31 \text{ mm Hg}

. The patient's actual

\text{PaCO}_2

42 \text{ mm Hg}

, which is inappropriately high, indicating a concurrent primary respiratory acidosis. Clinically, this patient is in acute decompensated heart failure (ADHF) with pulmonary edema (crackles, hypoxia) and cardiorenal syndrome. The pulmonary edema impairs gas exchange, leading to

\text{CO}_2

retention and respiratory acidosis. The most appropriate immediate management for ADHF with pulmonary edema and cardiorenal acute kidney injury is diuresis with intravenous furosemide to decrease preload, relieve pulmonary congestion, and improve renal hemodynamics.

Step-by-Step Solution

Identify the primary acid-base disorder using pH and bicarbonate levels.

The pH is 7.24 (acidemia) and serum bicarbonate is 14 mEq/L (low), indicating a primary metabolic acidosis.

Establishing the primary disturbance is required to determine the direction of the expected compensatory response.

Calculate the serum anion gap.

Anion Gap = Na - (Cl + HCO3) = 134 - (98 + 14) = 22 mEq/L. This is a high anion gap metabolic acidosis.

Differentiating between normal and high anion gap acidosis narrows the differential diagnosis (in this case, uremia from acute kidney injury).

Calculate the expected respiratory compensation using Winter's formula.

Expected PaCO2 = (1.5 x 14) + 8 = 29 (range 27-31 mm Hg).

Winter's formula determines if the pulmonary response to metabolic acidosis is appropriate or if a secondary respiratory disorder exists.

Compare the actual PaCO2 to the expected PaCO2.

The actual PaCO2 is 42 mm Hg, which is significantly higher than the expected range (27-31 mm Hg), confirming a superimposed primary respiratory acidosis.

An inappropriately elevated PaCO2 in the setting of metabolic acidosis indicates a mixed disorder (metabolic acidosis + respiratory acidosis).

Integrate clinical findings to determine the underlying etiology and next best step.

The patient exhibits signs of severe volume overload (hypertension, JVD, crackles, edema) causing cardiorenal syndrome (AKI) and pulmonary edema (respiratory acidosis). The correct intervention is intravenous furosemide.

Loop diuretics decrease preload, relieve pulmonary congestion, and can improve renal hemodynamics in acute decompensated heart failure.

Key Concept

Diagnosis of mixed acid-base disorders using Winter's formula and management of cardiorenal syndrome.

Estimated Time:2m 0s

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