USMLE® Step 1 Question of the Day: Lung ratios

USMLE® Step 1 Question of the Day: Lung ratios

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Learn about the ventilation, perfusion, and V/Q ratio in a 47-year-old man who experienced shortness of breath after returning from Germany. Find out the implications of his medical history, symptoms, and diagnostic findings.

A 47-year-old man presents to the emergency department for evaluation of shortness of breath. The symptoms developed shortly after he returned from an overseas business trip to Germany. Medical history is notable for hypertension. The patient smokes a pack of cigarettes per day. Temperature is 37.0°C (98.6°F), blood pressure is 148/89 mmHg, and pulse is 110/min. Physical examination reveals clear, bilateral breath sounds. A D-dimer level returns elevated. Which of the following best characterizes the ventilation, perfusion, and V/Q ratio for the affected regions of this patient’s lungs?

A. 

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B.

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C. 

D. 

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E. 

Scroll down for the correct answer!

The correct answer to today’s USMLE® Step 1 Question is…

A. 

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Before we get to the Main Explanation, let’s look at the incorrect answer explanations. Skip to the bottom if you want to see the correct answer right away!

Incorrect answer explanations

B. 

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Incorrect: This patient most likely has a pulmonary embolism. The condition will reduce blood flow to affected portions of the lung, thereby causing the perfusion to decrease. Since perfusion decreases whereas ventilation remains largely unchanged, the ventilation/perfusion (V/Q) ratio will increase.

C. 

Incorrect: This patient most likely has a pulmonary embolism. The condition will reduce blood flow to affected portions of the lung, thereby causing a decrease in perfusion.

D. 

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Incorrect: Conditions such as foreign body aspiration can lead to decreased ventilation, or gas exchange, within alveoli distal to the obstruction site. As a result, ventilation will decrease, and so will the V/Q ratio. However, this patient’s clinical history is more concerning for pulmonary embolism.

E. 

Incorrect: This patient has chest pain and an elevated D-dimer in recent long-distance travel, which is suggestive of pulmonary embolism. The embolus occludes vessels within the lungs, leading to decreased perfusion of affected portions of the lungs.

Main Explanation

This patient has chest pain and an elevated D-dimer in the setting of recent long-distance travel, which is suggestive of pulmonary embolism. The blood clot occludes vessels within the lungs, leading to decreased perfusion and a reduced V/Q ratio within affected portions of the lungs.  

The V/Q ratio is the ratio of alveolar ventilation (V) to pulmonary blood flow (Q). Ventilation and perfusion matching are essential to achieving optimal gas exchange. Conditions that lead to airway obstruction (e.g., foreign body aspiration, bronchoconstriction) will decrease ventilation of the affected alveoli and lead to an increased V/Q ratio. In contrast, conditions that decrease blood flow (e.g. pulmonary embolism) will decrease perfusion and lead to an increased V/Q ratio

illustration of the V/Q ratio

Major Takeaway

The V/Q ratio is the alveolar ventilation (V) ratio to pulmonary blood flow (Q). Conditions that cause airway obstruction will decrease the V/Q ratio, whereas conditions that decrease blood flow will increase the V/Q ratio.

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The United States Medical Licensing Examination (USMLE®) is a joint program of the Federation of State Medical Boards (FSMB®) and National Board of Medical Examiners (NBME®). Osmosis is not affiliated with NBME nor FSMB. 


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