Paramedics arrive at the emergency department with a lethargic 65-year-old man in severe r… — USMLE Step 2 CK

Paramedics arrive at the emergency department with a lethargic 65-year-old man in severe respiratory distress. His wife reports that his breathing has grown progressively labored over the last two days, accompanied by a persistent cough. His medical background is significant for ischemic cardiomyopathy (ejection fraction $35\%$ ), severe chronic obstructive pulmonary disease (COPD), and hypothyroidism. Home medications include lisinopril, metoprolol, tiotropium, and levothyroxine.

Vital signs upon arrival show a temperature of $37.2^\circ \text{C}$ ( $99.0^\circ \text{F}$ ), blood pressure of $145/88 \text{ mm Hg}$ , heart rate of $118\text{/min}$ , and respiratory rate of $28\text{/min}$ . Oxygen saturation is $84\%$ on ambient air.

On physical examination, the patient is somnolent but awakens when his name is called loudly. His jugular venous pressure is normal at $7 \text{ cm H}_2\text{O}$ . Auscultation of the lungs reveals diminished air entry globally, a notably prolonged expiratory phase, and diffuse expiratory wheezing. No crackles or rales are heard. Cardiac auscultation demonstrates tachycardia with a regular rhythm and no $S_3$ gallop. The lower extremities are free of edema.

Initial laboratory results are as follows:

Parameter	Patient Value	Reference Range
$\text{pH}$	$7.21$	$7.35 - 7.45$
$\text{PaCO}_2$	$72 \text{ mm Hg}$	$35 - 45 \text{ mm Hg}$
$\text{PaO}_2$	$50 \text{ mm Hg}$	$75 - 105 \text{ mm Hg}$
$\text{HCO}_3^-$	$28 \text{ mEq/L}$	$22 - 28 \text{ mEq/L}$
Sodium	$138 \text{ mEq/L}$	$136 - 145 \text{ mEq/L}$
Potassium	$4.2 \text{ mEq/L}$	$3.5 - 5.0 \text{ mEq/L}$
Chloride	$100 \text{ mEq/L}$	$95 - 105 \text{ mEq/L}$
BUN	$18 \text{ mg/dL}$	$7 - 18 \text{ mg/dL}$
Creatinine	$0.9 \text{ mg/dL}$	$0.7 - 1.3 \text{ mg/dL}$
TSH	$2.5 \mu\text{U/mL}$	$0.4 - 4.0 \mu\text{U/mL}$

Chest radiograph reveals hyperinflated lungs with flattened hemidiaphragms; there are no focal consolidations, pleural effusions, or pulmonary vascular congestion.

Noninvasive positive pressure ventilation (NIPPV) is initiated, and the patient's respiratory effort improves slightly.

Which of the following represents the most accurate diagnostic assessment and most appropriate next step in pharmacologic management?

A
Myxedema coma causing primary hypoventilation; administer intravenous levothyroxine
B
Acute decompensated heart failure; administer intravenous furosemide and nitroglycerin
C
Severe pneumonia; administer intravenous meropenem and linezolid
COPD exacerbation with acute respiratory acidosis; administer systemic glucocorticoids and azithromycinAnswer
E
COPD exacerbation with superimposed metabolic acidosis; administer intravenous sodium bicarbonate

Answer

COPD exacerbation with acute respiratory acidosis; administer systemic glucocorticoids and azithromycin.

The correct answer is COPD exacerbation with acute respiratory acidosis; administer systemic glucocorticoids and azithromycin. This patient's presentation of worsening dyspnea, progressive lethargy, hyperinflated lungs on chest radiograph, and diffuse expiratory wheezing strongly points to an acute exacerbation of COPD. The arterial blood gas demonstrates a primary acidemia (

\text{pH} < 7.35

) driven by hypercapnia (

\text{PaCO}_2 > 45 \text{ mm Hg}

), indicating a primary respiratory acidosis. For an acute respiratory acidosis, the

\text{HCO}_3^-

is expected to rise by

1 \text{ mEq/L}

for every

10 \text{ mm Hg}

increase in

\text{PaCO}_2

. Here, the

\text{PaCO}_2

72 \text{ mm Hg}

(an increase of

\approx 32

from a normal baseline of

40

), so the expected

\text{HCO}_3^-

is around

27.2 \text{ mEq/L}

(normal

24 + 3.2

). The measured

\text{HCO}_3^-

28 \text{ mEq/L}

perfectly reflects an acute respiratory acidosis without a superimposed metabolic disturbance. Standard management of a severe COPD exacerbation requiring noninvasive positive pressure ventilation (NIPPV) includes short-acting bronchodilators, systemic glucocorticoids, and an antibiotic (such as a macrolide like azithromycin) to cover common respiratory pathogens, even if distinct focal infiltrates are absent.

Step-by-Step Solution

Evaluate the physical examination and radiographic findings to differentiate the cause of respiratory failure.

The presence of diffuse wheezing, a prolonged expiratory phase, hyperinflated lungs, and the notable absence of crackles, elevated JVP, or pulmonary edema indicate a primary COPD exacerbation rather than acute decompensated heart failure.

Patients with multiple cardiopulmonary comorbidities require careful symptom correlation to identify the precise driver of their acute decompensation.

Interpret the arterial blood gas (ABG) to determine the precise acid-base disturbance.

The ABG reveals a pH of 7.21 and PaCO2 of 72 mm Hg, indicating a primary respiratory acidosis. In an acute setting, HCO3- rises by 1 mEq/L for every 10 mm Hg increase in PaCO2. A PaCO2 increase of 32 mm Hg correlates to an expected HCO3- of approximately 27.2 mEq/L. The measured 28 mEq/L perfectly aligns with a pure acute respiratory acidosis.

Accurate calculation of expected compensation is mandatory to prevent misdiagnosing a mixed acid-base disorder, which would inappropriately alter management.

Select the appropriate pharmacologic interventions based on the diagnosis and severity.

In addition to NIPPV, standard management requires short-acting bronchodilators, systemic glucocorticoids, and an appropriate targeted antibiotic (e.g., azithromycin).

Current guidelines recommend antibiotics for COPD exacerbations if the patient requires noninvasive positive pressure ventilation, alongside systemic steroids. Broad-spectrum antibiotics are not indicated without specific risk factors.

Key Concept

Patients presenting with severe COPD exacerbations requiring noninvasive positive pressure ventilation should be treated with short-acting bronchodilators, systemic glucocorticoids, and targeted antibiotics (e.g., macrolides). Care must be taken to appropriately calculate acid-base compensation to avoid mistakenly treating a non-existent superimposed metabolic disturbance.

Estimated Time:1m 30s

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