USMLE PRACTICE QUESTION #56
TOPIC: Pulmonary Physiology
Today’s question will test your understanding of pulmonary physiology.
I’m Paul Ciurysek, MD, and this is The USMLE Times: An independent, subscriber-supported newsletter designed to help you survive medical school, pass your USMLE exams, and match into your dream residency program.
A 62-year-old man with a history of COPD presents to the emergency department with acute shortness of breath and cough. His vital signs are as follows: Heart rate 110/minute, respiratory rate 26/minute, blood pressure 135/85 mmHg, temperature 98.2°F (36.8°C), and oxygen saturation 88% on room air. Arterial blood gas analysis shows a PaCO2 of 60 mmHg and PaO2 of 55 mmHg. Which of the following is the most likely cause of this patient's increased physiologic dead space?
A) Increased alveolar ventilation
B) Decreased alveolar perfusion
C) Decreased respiratory rate
D) Increased pulmonary vascular resistance
E) Alveolar collapse
F) Pulmonary edema
G) Decreased tidal volume
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
The mental model used to answer this question comes from our detailed test-taking skills masterclass. Here’s how to think through this question:
Step 1. Read the last line to determine what the question is asking: What’s the cause of increased physiologic dead space in the patient?
Step 2: Is this a first-, second-, or third-order question?
Answer: 2nd order: 1st. Recognize the problem 2nd. Understand why it causes the physiologic change.
Step 3. Read the vignette to gather information.
Step 4. Consider the pathology and ask yourself how it could be causing an increase in physiologic dead space.
Step 5. Look at the answer choices and select the option most closely resembling your thoughts from “Step 4” above.
GENERAL ANALYSIS
This question involves a 62-year-old man with a history of COPD who presents with acute shortness of breath and hypoxemia. The arterial blood gas analysis indicates elevated PaCO2 and low PaO2, suggesting impaired gas exchange. The task is to identify the most likely cause of increased physiological dead space.
ANSWER CHOICES:
ANSWER CHOICE A: Increased alveolar ventilation
Explanation: Increased alveolar ventilation would typically improve gas exchange by enhancing the removal of CO2. This would reduce dead space.
ANSWER CHOICE B: Decreased alveolar perfusion
Explanation: Decreased perfusion means less blood flow to the alveoli, reducing the exchange of gases. This leads to a ventilation-perfusion mismatch, where alveoli are ventilated but not perfused, increasing physiological dead space.
ANSWER CHOICE C: Decreased respiratory rate
Explanation: A decreased respiratory rate would reduce overall ventilation but wouldn’t directly affect the physiological dead space.
ANSWER CHOICE D: Increased pulmonary vascular resistance
Explanation: This can lead to decreased perfusion of alveoli, and while it can contribute to decreased perfusion, the primary mechanism is the perfusion itself, not the resistance.
ANSWER CHOICE E: Alveolar collapse
Explanation: Collapsed alveoli do not participate in gas exchange. While this will affect gas exchange, it is more related to shunting than dead space.
ANSWER CHOICE F: Pulmonary edema
Explanation: Fluid in the alveoli impairs gas exchange, resulting in shunting rather than increased dead space.
ANSWER CHOICE G: Decreased tidal volume
Explanation: A lower tidal volume reduces the amount of air reaching the alveoli. This affects ventilation but is not directly related to increased dead space.
FINAL VERDICT…
Increased physiological dead space occurs when there is a mismatch between ventilation and perfusion, specifically when alveoli are ventilated but not adequately perfused.
In COPD, structural changes and emphysema can lead to areas of the lung where air reaches the alveoli, but blood flow is insufficient for adequate gas exchange. This increases dead space, as ventilated alveoli do not contribute to gas exchange due to lack of perfusion. This mismatch is a common feature in COPD and contributes to the inefficient gas exchange observed in these patients.
CORRECT ANSWER: B) Decreased alveolar perfusion.
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Thanks for checking out Q56!
See you next time 👋