AEMCA Practice Exam

In a person standing upright, what is the V/Q ratio like at the base of the lung?

1. High V/Q mismatch

2. Uniform V/Q ratio

3. Low V/Q mismatch due to gravity

4. Stable regardless of body position

The correct answer is: Low V/Q mismatch due to gravity

The V/Q ratio, or ventilation-perfusion ratio, assesses the efficiency and adequacy of gas exchange in the lungs. At the base of the lungs in an upright person, gravity has a significant impact on both ventilation and perfusion. In general, ventilation is slightly less effective at the base compared to the apex, and perfusion is greater due to the hydrostatic effects of gravity. Because the blood flow (perfusion) to the base of the lung is much higher than the ventilation there, this results in a lower V/Q ratio; thus it reflects a low ventilation relative to the amount of blood flow. This phenomenon occurs due to the gravitational gradient that causes blood to pool more in the lower parts of the lungs while ventilation may not increase proportionately. This lower V/Q ratio at the base is crucial for understanding how the lungs function in different body positions and is an important consideration in clinical assessments of lung and cardiac conditions. The areas of the lungs not affected by gravitational forces, such as in a supine position, could present different V/Q ratios; however, in an upright position, the effects of gravity clearly delineate the characteristics of V/Q matching at the lung’s base.