USMLE PRACTICE QUESTION #107

TOPIC: Respiratory Pathophysiology

Dec 16, 2024

In USMLE practice question #107, we’ll challenge our understanding of the highly tested concepts of respiratory pathophysiology.

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A 35-year-old man is brought to the emergency department by his roommate after he was found unconscious at home. The roommate reports that the patient has been complaining of severe fatigue, frequent headaches, and a persistent dry cough over the past week. The roommate found him on the floor this morning, unresponsive, with shallow breathing. On physical examination, the patient appears somnolent with labored breathing. He has a respiratory rate of 10/minute, pulse of 100/minute, and blood pressure of 105/70 mmHg. His skin is cool and clammy, and he has pinpoint pupils. Arterial blood gas (ABG) results are as follows:

pH: 7.20
PaCO₂: 55 mmHg
PaO₂: 60 mmHg
HCO₃⁻: 24 mEq/L

Which of the following best explains the underlying physiologic mechanism for this patient’s ABG findings?

A) Increased dead space ventilation due to pulmonary embolism
B) Impaired alveolar ventilation due to central nervous system depression
C) Decreased functional residual capacity due to pleural effusion
D) Diffusion impairment due to interstitial lung disease
E) Right-to-left shunt due to congenital heart defect

Detailed Breakdown of Answers + Correct Answer Below ⏬

ANSWER + QUESTION BREAKDOWN

It’s important to adopt the correct MENTAL MODEL when answering USMLE questions; it saves time and increases accuracy. The mental model outlined below is a foundational component of our test-taking skills masterclass (check it out if you want to elevate your skills). Here’s how to think through this question:

Step 1. Read the last line to get to the heart of the question: “Which of the following best explains the underlying physiologic mechanism for this patient’s ABG findings?”

Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order. 1st: Identify the metabolic disturbance; 2nd: Identify the physiology responsible for the disturbance.

Step 3: Read the vignette carefully and ask yourself: “What’s the most likely cause of the metabolic disturbance?”

Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.

GENERAL ANALYSIS

These findings suggest respiratory acidosis with hypoxemia, indicating impaired ventilation. The patient's pinpoint pupils and altered mental status suggest possible opioid intoxication, which can cause central nervous system depression, leading to hypoventilation.

ANSWER CHOICES:

CHOICE A: Increased dead space ventilation due to pulmonary embolism

Explanation: A pulmonary embolism can cause increased dead space ventilation, leading to hypoxemia without hypercapnia. The ABG typically shows respiratory alkalosis (low PaCO₂) due to hyperventilation. This patient has respiratory acidosis (elevated PaCO₂), which is not consistent with pulmonary embolism.

CHOICE B: Impaired alveolar ventilation due to central nervous system depression

Explanation: Central nervous system depression, such as that caused by opioid intoxication, can lead to hypoventilation. This decreases alveolar ventilation, causing CO₂ retention (hypercapnia), respiratory acidosis, and hypoxemia.

CHOICE C: Decreased functional residual capacity due to pleural effusion

Explanation: Pleural effusion can reduce lung volumes, including functional residual capacity, leading to hypoxemia. However, it typically does not cause significant hypercapnia unless the effusion is massive. While pleural effusion can cause hypoxemia, it is unlikely to cause the severe hypercapnia seen in this patient.

CHOICE D: Diffusion impairment due to interstitial lung disease

Explanation: Interstitial lung disease impairs gas exchange by thickening the alveolar-capillary membrane, leading to hypoxemia. However, it typically does not cause significant hypercapnia unless the disease is very advanced.

CHOICE E: Right-to-left shunt due to congenital heart defect

Explanation: A right-to-left shunt allows deoxygenated blood to bypass the lungs and enter systemic circulation, causing hypoxemia that does not improve with oxygen therapy. While a right-to-left shunt can cause hypoxemia, it does not typically cause hypercapnia or respiratory acidosis unless there is also significant ventilatory impairment.

FINAL VERDICT…

The patient's presentation—somnolence, shallow breathing (hypoventilation), pinpoint pupils, and respiratory acidosis—strongly suggests opioid intoxication as the underlying cause of his symptoms. Opioids depress the central nervous system, leading to impaired alveolar ventilation, which results in CO₂ retention (hypercapnia), respiratory acidosis (low pH), and hypoxemia (low PaO₂). This mechanism best explains the patient's ABG findings and clinical presentation.

CORRECT ANSWER: B) Impaired alveolar ventilation due to central nervous system depression