Which term best describes a mismatch between ventilation and perfusion in the lungs?

• A. Diffusion limitation
• B. Shunt
• C. Ventilation-perfusion mismatch
• D. Dead space ventilation

Answer: (C)

Ventilation-perfusion mismatch describes any imbalance between the air reaching the alveoli (ventilation) and the blood flowing through the pulmonary capillaries (perfusion) for gas exchange. When these two processes aren’t aligned, some alveoli receive air but not enough blood to exchange gases efficiently, while others receive blood but not enough air to oxygenate it properly. This mismatch is the broad term that encompasses the various patterns of abnormal gas exchange in the lungs. Shunt is a specific form where blood passes through the lungs without gas exchange because the alveoli are not ventilated, effectively a V/Q of zero. Dead space ventilation is the opposite pattern, where air reaches alveoli that aren’t perfused, giving a very high V/Q ratio. Diffusion limitation refers to a problem with the movement of gas across the alveolar-capillary membrane itself, not the balance between ventilation and perfusion. So the term that best describes a mismatch between ventilation and perfusion is ventilation-perfusion mismatch.