USMLE Step 1 Question (Pulmonology)
Understanding the V/Q mismatch.
The following is a high-yield USMLE Step 1 pulmonology question.
It is followed by a deep dive into each answer choice—the answer can be found at the bottom of the post.
Good luck!
QUESTION:
A 68-year-old woman with a history of congestive heart failure presents to the emergency department with worsening dyspnea, orthopnea, and lower extremity edema. Her vital signs show a heart rate of 100/minute, respiratory rate of 22/minute, blood pressure of 145/95 mmHg, temperature of 98.6°F (37.0°C), and oxygen saturation of 90% on room air. Physical examination reveals bilateral crackles in the lower lung fields and pitting edema in both lower extremities. Arterial blood gas analysis shows a PaCO2 of 40 mmHg and PaO2 of 60 mmHg. Which of the following physiologic findings would result from this patient’s V/Q mismatch?
A. Decreased oxygenation of the blood
B. Increased carbon dioxide removal
C. Increased arterial oxygen tension
D. Decreased minute ventilation
E. Enhanced pulmonary vasodilation
F. Increased bronchodilation
G. Decreased work of breathing
To see the answer, scroll to the bottom of the page. See the DEEP-DIVE below if you’re unsure or want to learn more about the answer choices.
DEEP-DIVE:
In the context of a 68-year-old woman with congestive heart failure (CHF) presenting with classic signs of decompensated heart failure, including dyspnea, orthopnea, lower extremity edema, and findings of pulmonary congestion on physical examination, the scenario suggests a state of increased pulmonary capillary pressure leading to pulmonary edema. This condition often results in ventilation/perfusion (V/Q) mismatch, where parts of the lung are ventilated but not well perfused due to fluid accumulation.
A. Decreased oxygenation of the blood
V/Q mismatch, particularly in the setting of CHF, where fluid accumulates in the alveolar spaces, impairs blood oxygenation as oxygen has difficulty diffusing across the fluid-filled alveoli into the blood.
B. Increased carbon dioxide removal
V/Q mismatch does not typically lead to increased CO2 removal. CO2 is more soluble in blood than oxygen and can diffuse more efficiently, even in some lung pathologies.
C. Increased arterial oxygen tension
V/Q mismatch in the context of heart failure typically leads to decreased, not increased, arterial oxygen tension (PaO2).
D. Decreased minute ventilation
Minute ventilation may remain the same or increase as a compensatory mechanism to maintain oxygenation and CO2 removal in the face of impaired gas exchange.
E. Enhanced pulmonary vasodilation
Pulmonary vasodilation is not a typical direct consequence of V/Q mismatch; in fact, hypoxia in some regions of the lung can lead to vasoconstriction (hypoxic pulmonary vasoconstriction) to redirect blood flow to better-ventilated areas.
F. Increased bronchodilation
V/Q mismatch and CHF do not directly lead to increased bronchodilation.
G. Decreased work of breathing
The work of breathing typically increases in heart failure and pulmonary edema due to the need to overcome the presence of alveolar fluid.
VERDICT: Given these considerations, decreased blood oxygenation is the most likely direct consequence of this patient’s V/Q mismatch as it directly impacts gas exchange efficiency, particularly oxygen transfer from the alveoli to the bloodstream.
FINAL ANSWER: A) Decreased oxygenation of the blood
