During bedside rounds, the medical team evaluates a patient who was admitted four days ago… — USMLE Step 2 CK

During bedside rounds, the medical team evaluates a patient who was admitted four days ago for lower extremity edema and exertional dyspnea. The patient has a longstanding history of severe emphysema and right ventricular failure. Over the course of the hospitalization, aggressive intravenous furosemide has been administered, resulting in a $6\text{ kg}$ weight loss and significant symptomatic improvement. Current medications include inhaled tiotropium, oral amlodipine, and intravenous furosemide. Vital signs are within normal limits.

Laboratory studies are obtained:

Test	Value
Sodium	$137\text{ mEq/L}$
Potassium	$2.9\text{ mEq/L}$
Chloride	$91\text{ mEq/L}$
Bicarbonate	$39\text{ mEq/L}$

Arterial blood gas (room air):

Test	Value
pH	$7.37$
PaCO2	$66\text{ mm Hg}$
PaO2	$60\text{ mm Hg}$

Which of the following is the most accurate interpretation of this patient's current acid-base status?

Mixed chronic respiratory acidosis and metabolic alkalosisAnswer
B
Fully compensated chronic respiratory acidosis
C
Primary metabolic alkalosis with appropriate respiratory compensation
D
Uncompensated acute respiratory acidosis
E
Mixed acute respiratory acidosis and high anion gap metabolic acidosis

Answer

Mixed chronic respiratory acidosis and metabolic alkalosis

The correct answer accurately identifies two independent primary acid-base disorders based on a systematic calculation. The patient is acidemic (

pH = 7.37

), making the primary disorder an acidosis. The elevated

PaCO_2

(

66\text{ mm Hg}

) points to respiratory acidosis, which is clinically chronic due to severe emphysema. For chronic respiratory acidosis, bicarbonate is expected to rise by

3.5\text{ mEq/L}

per

10\text{ mm Hg}

increase in

PaCO_2

. Therefore, expected

HCO_3^- = 24 + 3.5 \times 2.6 = 33.1\text{ mEq/L}

. Because the patient's actual

HCO_3^-

(

39\text{ mEq/L}

) is greater than expected, there is an additional primary metabolic alkalosis, effectively resulting from recent aggressive loop diuresis (evidenced by weight loss and severe hypokalemia).

Step-by-Step Solution

Determine the primary acid-base disorder by evaluating the pH.

The pH is

7.37

, which represents acidemia (normal

7.40

). Because the

PaCO_2

is elevated (

66\text{ mm Hg}

), the primary disorder is a respiratory acidosis.

The primary disorder must always match the direction of the pH deviation.

Determine if the respiratory acidosis is acute or chronic.

The patient has a known history of severe emphysema and baseline cor pulmonale, strongly suggesting chronic carbon dioxide retention. This is classified as a chronic respiratory acidosis.

Clinical history dictates the baseline physiologic state prior to calculation.

Calculate the expected renal compensation for a chronic respiratory acidosis.

The rule for chronic respiratory acidosis is a

3.5\text{ mEq/L}

increase in

HCO_3^-

for every

10\text{ mm Hg}

increase in

PaCO_2

above

40\text{ mm Hg}

. Expected

\Delta HCO_3^- = 3.5 \times \frac{66 - 40}{10} = 3.5 \times 2.6 = 9.1

. Expected

HCO_3^- = 24 + 9.1 = 33.1\text{ mEq/L}

To detect hidden mixed disorders, the actual bicarbonate must be compared to the physiologically expected bicarbonate.

Compare the actual bicarbonate to the expected bicarbonate.

The actual

HCO_3^-

39\text{ mEq/L}

, which is significantly higher than the expected

33.1\text{ mEq/L}

. This excess bicarbonate represents an independent, concurrent primary metabolic alkalosis.

Aggressive loop diuretic use leads to volume contraction and hypokalemia (as seen with

K^+ = 2.9

), driving the generation and maintenance of a metabolic alkalosis.

Key Concept

Acid-base compensation calculation for chronic respiratory acidosis and identification of mixed disorders

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