A 72-year-old man is brought to the emergency department by emergency medical services due… — USMLE Step 2 CK

A 72-year-old man is brought to the emergency department by emergency medical services due to severe, rapidly progressive shortness of breath that awoke him from sleep 2 hours ago. He has a history of ischemic cardiomyopathy with a left ventricular ejection fraction of 30%, chronic obstructive pulmonary disease (COPD), and chronic kidney disease. His home medications include lisinopril, metoprolol succinate, tiotropium, and as-needed albuterol. On arrival, he is in marked respiratory distress, speaking in short phrases and using accessory muscles of respiration. Vital signs are temperature $37.0^\circ\text{C}$ ( $98.6^\circ\text{F}$ ), blood pressure $178/96\text{ mmHg}$ , heart rate $112/\text{min}$ , respiratory rate $30/\text{min}$ , and oxygen saturation 85% on room air. Physical examination reveals an elevated jugular venous pressure at 12 cm $H_2O$ . Lung auscultation demonstrates diffuse expiratory wheezing and bilateral crackles extending to the mid-lung fields. Cardiac auscultation reveals a normal $S_1$ and $S_2$ with a prominent $S_3$ gallop. Bilateral 2+ pitting lower extremity edema is present.

Laboratory results:
Leukocyte count: $9,500/\text{mm}^3$
Thyroid-stimulating hormone (TSH): $1.1\ \mu\text{U/mL}$ (normal: $0.4-4.0\ \mu\text{U/mL}$ )
Free thyroxine ( $T_4$ ): $0.5\ \text{ng/dL}$ (normal: $0.9-1.7\ \text{ng/dL}$ )

Arterial blood gas (room air):
pH: $7.22$
$PaCO_2$ : $35\text{ mmHg}$
$PaO_2$ : $55\text{ mmHg}$
$HCO_3^-$ : $14\text{ mEq/L}$

Chest radiography reveals an enlarged cardiac silhouette, bilateral perihilar alveolar opacities, and prominent interlobular septal lines. Which of the following is the most appropriate initial step in the management of this patient?

Initiate noninvasive positive pressure ventilation and administer intravenous furosemideAnswer
B
Administer systemic glucocorticoids and scheduled nebulized short-acting bronchodilators
C
Administer intravenous sodium bicarbonate to correct the metabolic acidosis
D
Administer intravenous piperacillin-tazobactam and obtain blood cultures
E
Administer intravenous levothyroxine for presumed decompensated hypothyroidism

Answer

The most appropriate initial management is to initiate noninvasive positive pressure ventilation and administer intravenous furosemide.

The correct approach recognizes that this patient is suffering from acute decompensated heart failure (ADHF) complicated by mixed metabolic and respiratory acidosis. The presence of an elevated jugular venous pressure, an S3 gallop, and classic chest radiograph findings confirm cardiogenic pulmonary edema. Furthermore, calculating the expected respiratory compensation for his metabolic acidosis using Winter's formula (

PaCO_2 = 1.5 \times 14 + 8 = 29\text{ mmHg}

) reveals that his measured

PaCO_2

35\text{ mmHg}

is inappropriately high. This hidden respiratory acidosis signifies severe ventilatory fatigue. Noninvasive positive pressure ventilation (NIPPV) is the ideal first step because it improves oxygenation, decreases the work of breathing, and reduces both cardiac preload and afterload. Intravenous furosemide is concurrently required to diurese the volume overload.

Step-by-Step Solution

Analyze the physical exam and imaging to determine the primary etiology of the respiratory distress.

The combination of elevated jugular venous pressure, an S3 gallop, pitting edema, and a chest radiograph showing cardiomegaly with pulmonary edema firmly diagnoses acute decompensated heart failure (ADHF).

Differentiating ADHF from a COPD exacerbation is critical, as ADHF can present with wheezing ('cardiac asthma') due to peribronchial edema.

Interpret the arterial blood gas using acid-base compensation rules.

The patient has an acidemia (pH 7.22) with a low bicarbonate (

14\text{ mEq/L}

), indicating a primary metabolic acidosis. Using Winter's formula, the expected

PaCO_2

1.5 \times 14 + 8 = 29 \pm 2\text{ mmHg}

Calculating expected compensation is necessary to identify mixed acid-base disorders.

Compare the expected

PaCO_2

with the measured

PaCO_2

The measured

PaCO_2

(

35\text{ mmHg}

) is significantly higher than the expected range (

27-31\text{ mmHg}

). This indicates a concurrent, hidden respiratory acidosis.

Despite a 'normal-to-low' absolute

PaCO_2

value, the failure to compensate appropriately demonstrates impending respiratory failure due to severe pulmonary edema.

Select the appropriate evidence-based interventions for the clinical findings.

Noninvasive positive pressure ventilation (NIPPV) is selected to treat the respiratory failure and decrease cardiac preload/afterload, while intravenous furosemide is given to relieve volume overload.

These interventions directly target the pathophysiology of ADHF with respiratory failure, whereas treatments for COPD, infection, or isolated metabolic acidosis would be ineffective or harmful.

Key Concept

Management of acute decompensated heart failure and recognition of superimposed respiratory failure using Winter's formula.

Estimated Time:1m 30s

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