A 72-year-old man is evaluated in the intensive care unit for progressive lethargy and wor… — USMLE Step 2 CK

A 72-year-old man is evaluated in the intensive care unit for progressive lethargy and worsening shortness of breath over the past 12 hours. The patient was admitted 5 days ago for an acute exacerbation of heart failure. His medical history is significant for ischemic cardiomyopathy with a left ventricular ejection fraction of $25\%$ and severe chronic obstructive pulmonary disease (COPD) with a baseline $\text{FEV}_1$ of $35\%$ of predicted. Since admission, he has been treated with a continuous intravenous furosemide infusion, resulting in a net negative fluid balance of 8 liters and resolution of his peripheral edema. Vital signs currently show a temperature of $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure of $100/60\text{ mm Hg}$ , heart rate of $105/\text{min}$ , and respiratory rate of $10/\text{min}$ . Oxygen saturation is $89\%$ on a $3\text{ L/min}$ nasal cannula. Physical examination reveals a somnolent but arousable patient. The jugular venous pressure is estimated at $4\text{ cm H}_2\text{O}$ . Pulmonary examination shows diminished breath sounds bilaterally with prolonged expiration and scattered end-expiratory wheezes, but no crackles are heard. Cardiovascular examination reveals distant heart sounds with a regular rate and rhythm.

Laboratory studies show the following:

Test	Result
Sodium	$135\text{ mEq/L}$
Potassium	$2.9\text{ mEq/L}$
Chloride	$85\text{ mEq/L}$
Bicarbonate	$40\text{ mEq/L}$
Blood urea nitrogen	$55\text{ mg/dL}$
Creatinine	$1.8\text{ mg/dL}$ (baseline $1.1\text{ mg/dL}$ )

Arterial blood gas analysis on

3\text{ L/min}

nasal cannula:

Parameter	Result
pH	$7.52$
$\text{pCO}_2$	$50\text{ mm Hg}$
$\text{pO}_2$	$58\text{ mm Hg}$

Which of the following is the most appropriate next step in the management of this patient's respiratory status?

Administer intravenous acetazolamide and potassium chlorideAnswer
B
Initiate noninvasive positive-pressure ventilation
C
Administer intravenous methylprednisolone and nebulized ipratropium-albuterol
D
Administer a rapid bolus of 0.9% normal saline
E
Administer intravenous levofloxacin and send sputum for culture

Answer

Administer intravenous acetazolamide and potassium chloride

This patient presents with a severe metabolic alkalosis (pH

7.52

, bicarbonate

40\text{ mEq/L}

) secondary to aggressive loop diuretic therapy, known as contraction alkalosis. The elevated

\text{pCO}_2

(

50\text{ mm Hg}

) is the expected physiological respiratory compensation for metabolic alkalosis (

\text{Expected pCO}_2 = 40 + 0.7 \times (\Delta\text{HCO}_3^-) = 40 + 0.7 \times 16 \approx 51\text{ mm Hg}

). The patient's hypoventilation (respiratory rate

10/\text{min}

) is a compensatory mechanism to retain

\text{CO}_2

, which in turn causes secondary hypoxemia and lethargy. To improve his respiratory status, the underlying alkalosis must be corrected. Isotonic saline is the standard treatment for saline-responsive contraction alkalosis; however, it is strictly contraindicated in this patient with a left ventricular ejection fraction of

25\%

who recently required

8\text{ L}

of diuresis, as it would precipitate flash pulmonary edema. Acetazolamide, a carbonic anhydrase inhibitor, promotes renal bicarbonate excretion to safely reverse the alkalosis, which will normalize his

\text{pH}

, restore normal respiratory drive, and resolve the hypoxemia. Concurrent potassium repletion is required because both the prior furosemide use and the new acetazolamide promote urinary potassium loss.

Step-by-Step Solution

Analyze the arterial blood gas values to identify the primary acid-base disorder.

The pH of 7.52 indicates alkalemia, and the elevated bicarbonate of 40 mEq/L confirms a primary metabolic alkalosis.

Establishing the primary disorder is essential to interpret the altered pCO2 and clinical presentation accurately.

Calculate the expected respiratory compensation for the metabolic alkalosis.

Expected pCO2 = 40 + 0.7 * (40 - 24) = 51.2 mm Hg. The patient's pCO2 of 50 mm Hg represents appropriate compensation.

This differentiates between an expected compensatory response and a superimposed primary respiratory disorder, such as an acute COPD exacerbation.

Correlate the acid-base derangement with the patient's clinical history and current symptoms.

Aggressive loop diuresis caused contraction alkalosis and severe hypokalemia. The patient is hypoventilating (RR 10/min) to retain CO2 as compensation, which leads to secondary hypoxemia and lethargy.

Understanding the physiologic mechanism allows for targeted, definitive therapy rather than symptomatic masking.

Select the most appropriate intervention that corrects the alkalosis without harming the patient.

Acetazolamide with potassium repletion is chosen. Isotonic saline is avoided.

While normal saline typically treats contraction alkalosis, it is absolutely contraindicated in this patient due to his recent severe acute decompensated heart failure and very low ejection fraction.

Key Concept

Patients with severe metabolic alkalosis compensate via alveolar hypoventilation, which can cause significant hypoxemia. In patients with concurrent severe heart failure where normal saline is contraindicated, acetazolamide is the treatment of choice to induce bicarbonate diuresis and reverse the alkalosis.

Estimated Time:2m 30s

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