A 68-year-old male is evaluated in the intensive care unit for altered mental status. He w… — USMLE Step 2 CK

A $68$ -year-old male is evaluated in the intensive care unit for altered mental status. He was admitted $4$ days ago for an acute exacerbation of chronic obstructive pulmonary disease (COPD) and acute decompensated heart failure. His initial treatment included noninvasive positive pressure ventilation, intravenous furosemide, and a $5$ -day course of azithromycin and ceftriaxone, which he completed yesterday. He has a history of severe COPD, heart failure with preserved ejection fraction, and hypertension.

Over the past $24$ hours, he has become increasingly lethargic and is currently difficult to arouse. He is receiving oxygen at $2 \text{ L/min}$ via nasal cannula. Temperature is $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure is $105/60 \text{ mm Hg}$ , pulse is $108/\text{min}$ , and respirations are $9/\text{min}$ . Physical examination shows flat jugular veins when supine. Mucous membranes are dry. Cardiopulmonary examination reveals distant heart sounds and decreased breath sounds bilaterally without wheezing or crackles. There is no lower extremity edema.

Laboratory results are as follows:

Parameter	Result
Serum sodium	$135 \text{ mEq/L}$
Serum potassium	$2.6 \text{ mEq/L}$
Serum chloride	$82 \text{ mEq/L}$
Serum bicarbonate	$44 \text{ mEq/L}$
Blood urea nitrogen	$52 \text{ mg/dL}$
Serum creatinine	$1.1 \text{ mg/dL}$
Urine chloride	$8 \text{ mEq/L}$

Arterial blood gas analysis on $2 \text{ L/min}$ nasal cannula:

Parameter	Result
pH	$7.55$
PaCO $_2$	$54 \text{ mm Hg}$
PaO $_2$	$62 \text{ mm Hg}$

Which of the following is the most appropriate next step in the management of this patient?

Intravenous infusion of $0.9\%$ sodium chloride and potassium chlorideCevap
B
Initiation of noninvasive positive pressure ventilation
C
Intravenous administration of acetazolamide
D
Intravenous administration of furosemide
E
Initiation of empiric piperacillin-tazobactam and vancomycin

Cevap

Intravenous infusion of 0.9% sodium chloride and potassium chloride

The correct option is intravenous infusion of 0.9% sodium chloride and potassium chloride. This patient's lethargy and altered mental status are driven by a severe metabolic alkalosis (pH 7.55) and the resulting compensatory hypoventilation (RR 9/min, PaCO2 54 mm Hg). The primary disorder is established by the profound alkalemia and markedly elevated serum bicarbonate (44 mEq/L). The expected respiratory compensation for metabolic alkalosis is a 0.7 mm Hg increase in PaCO2 for every 1 mEq/L increase in HCO3-. Here, the expected PaCO2 is 40 + [0.7 × (44 - 24)] = 54 mm Hg, which perfectly matches the measured PaCO2. This rules out a superimposed primary respiratory disorder. The etiology is saline-responsive (contraction) metabolic alkalosis secondary to aggressive loop diuretic use, confirmed by physical signs of hypovolemia (flat jugular veins, dry mucous membranes), pre-renal azotemia (BUN/creatinine ratio of 47), hypokalemia, and importantly, a urine chloride of less than 20 mEq/L (8 mEq/L). The definitive treatment for saline-responsive metabolic alkalosis is volume expansion with isotonic saline and potassium repletion. This restores renal perfusion, enabling the excretion of excess bicarbonate, which will subsequently correct the pH and resolve the compensatory hypoventilation.

Adım Adım Çözüm

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

The pH is 7.55 (alkalemia). The elevated HCO3- (44 mEq/L) identifies the primary disorder as a metabolic alkalosis.

Establishing the primary acid-base derangement is the first step in diagnosing altered mental status in a patient with mixed pulmonary and renal issues.

Calculate the expected respiratory compensation for the metabolic alkalosis.

Expected PaCO2 = 40 + [0.7 × (44 - 24)] = 40 + 14 = 54 mm Hg. The measured PaCO2 is exactly 54 mm Hg.

This confirms a pure metabolic alkalosis with appropriate respiratory compensation. The hypercapnia is not a primary respiratory acidosis.

Assess the etiology of the metabolic alkalosis using volume status and urine chloride.

Flat neck veins, BUN/Cr ratio > 40, dry mucous membranes, and urine chloride < 20 mEq/L point to severe volume depletion (contraction alkalosis).

Aggressive loop diuretic therapy for decompensated heart failure caused profound hypovolemia, hypokalemia, and hypochloremia.

Select the appropriate targeted intervention.

Administer 0.9% sodium chloride and potassium chloride to restore intravascular volume and correct electrolyte deficits.

Correcting the volume and chloride deficit allows the kidneys to excrete bicarbonate, resolving the alkalemia and the compensatory hypoventilation.

Anahtar Kavram

Patients with severe metabolic alkalosis develop compensatory hypoventilation to retain CO2. If the metabolic alkalosis is saline-responsive (urine Cl < 20 mEq/L), the treatment is volume expansion with normal saline and potassium replacement, avoiding interventions that inappropriately increase ventilation.

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