A 71-year-old man is evaluated in the intensive care unit for worsening shortness of breat… — USMLE Step 2 CK

A $71$ -year-old man is evaluated in the intensive care unit for worsening shortness of breath and increasing somnolence. He was admitted $2$ days ago for an acute exacerbation of chronic obstructive pulmonary disease (COPD) and suspected pneumonia. His admission medications include intravenous piperacillin-tazobactam, vancomycin, methylprednisolone, and scheduled albuterol-ipratropium nebulizers. Medical history is significant for severe COPD (FEV $_1$ $45\%$ of predicted), ischemic cardiomyopathy (left ventricular ejection fraction $30\%$ ), and stage $3$ chronic kidney disease.

Temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), pulse is $112\text{/min}$ , blood pressure is $160/90\text{ mm Hg}$ , and respirations are $28\text{/min}$ . Oxygen saturation is $86\%$ on a $4\text{ L/min}$ nasal cannula. Physical examination reveals a lethargic but arousable patient in respiratory distress. Jugular venous pressure is estimated at $11\text{ cm H}_2\text{O}$ . Lung auscultation reveals bilateral basilar crackles and diffuse expiratory wheezes. Cardiac examination shows a regular, rapid rhythm with a prominent S $_3$ gallop. There is $2+$ bilateral pitting edema extending to the knees.

Laboratory results are as follows:

Test	Result
Leukocyte count	$11,000/\text{mm}^3$
$\text{Na}^+$	$136\text{ mEq/L}$
$\text{K}^+$	$4.2\text{ mEq/L}$
$\text{Cl}^-$	$102\text{ mEq/L}$
$\text{HCO}_3^-$	$15\text{ mEq/L}$
Blood urea nitrogen	$48\text{ mg/dL}$
Creatinine	$2.3\text{ mg/dL}$ (baseline $1.6$ )

An arterial blood gas on the current oxygen setting shows:
pH: $7.26$
PaO $_2$ : $56\text{ mm Hg}$
PaCO $_2$ : $36\text{ mm Hg}$
$\text{HCO}_3^-$ : $15\text{ mEq/L}$

A sputum culture obtained on admission has returned positive for Streptococcus pneumoniae that is susceptible to ceftriaxone, azithromycin, and levofloxacin.

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

Initiate noninvasive positive-pressure ventilation and intravenous furosemideAnswer
B
Administer intravenous sodium bicarbonate
C
Escalate systemic glucocorticoids and administer continuous nebulized bronchodilators
D
Continue empiric intravenous piperacillin-tazobactam and vancomycin
E
Administer a $1$ -Liter intravenous normal saline bolus

Answer

Initiate noninvasive positive-pressure ventilation and intravenous furosemide

The correct action is to initiate noninvasive positive-pressure ventilation (NIPPV) and intravenous furosemide. This patient is experiencing acute decompensated heart failure (ADHF), evidenced by an S3 gallop, elevated jugular venous pressure, basilar crackles, and peripheral edema. The diffuse wheezing is secondary to peribronchial edema ('cardiac asthma'). Furthermore, the patient has a severe primary metabolic acidosis (pH

7.26

, HCO

_3^-

15\text{ mEq/L}

). According to Winter's formula (Expected PaCO

_2

1.5 \times \text{HCO}_3^- + 8

), the patient's expected PaCO

_2

30.5 \pm 2\text{ mm Hg}

. The actual PaCO

_2

36\text{ mm Hg}

is higher than the expected range, diagnosing a superimposed respiratory acidosis. This indicates that the patient's respiratory drive is fatiguing and failing to compensate for the metabolic demand. NIPPV is indicated to reduce the work of breathing and improve oxygenation, while furosemide is required to offload the severe hypervolemia.

Step-by-Step Solution

Analyze the physical examination and clinical presentation.

Identifies clear signs of volume overload and left-sided heart failure: orthopnea, elevated jugular venous pressure, S3 gallop, crackles, and peripheral edema.

Differentiates primary acute decompensated heart failure (ADHF) from a pure COPD exacerbation, recognizing that wheezing in this context represents 'cardiac asthma'.

Interpret the arterial blood gas utilizing Winter's formula.

Expected PaCO2 =

1.5 \times 15 + 8 = 30.5 \pm 2\text{ mm Hg}

. The actual PaCO2 of

36\text{ mm Hg}

is significantly higher than expected.

Diagnoses a combined metabolic acidosis and superimposed respiratory acidosis, which signifies respiratory muscle fatigue and impending ventilatory failure.

Determine the most appropriate immediate intervention.

Selects noninvasive positive-pressure ventilation (NIPPV) and intravenous furosemide.

NIPPV provides vital respiratory support to decrease the work of breathing, while furosemide reduces preload to treat the underlying cardiogenic pulmonary edema.

Key Concept

Identifying acute decompensated heart failure presenting as 'cardiac asthma' and utilizing Winter's formula to uncover impending respiratory failure in mixed acid-base disorders.

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