American Board of Internal Medicine (ABIM) Certification Practice Exam

What do low ventilation-perfusion (V/Q) ratios indicate?

1. Normal gas exchange

2. A shunt in pulmonary circulation

3. Improved oxygenation

4. Obstructive sleep apnea

The correct answer is: A shunt in pulmonary circulation

Low ventilation-perfusion (V/Q) ratios are indicative of a mismatch between air coming into the lungs and blood flow in the pulmonary circulation. In this context, a low V/Q ratio typically suggests that the area of the lung is receiving blood flow but has reduced or inadequate ventilation. This phenomenon commonly occurs in conditions where there is a shunt, meaning that blood bypasses the ventilated areas of the lung due to obstruction or other pathological processes. In essence, a shunt in pulmonary circulation directs blood from the right side to the left side of the heart without participating in gas exchange adequately, leading to decreased oxygen levels and potential hypoxia. Therefore, low V/Q ratios are closely associated with shunting mechanisms and reflect an inefficiency in gas exchange across the alveolar-capillary membrane. Understanding this relationship is crucial in diagnosing and managing respiratory and circulatory conditions.