Definition The ventilation-perfusion ratio is the ratio of alveolar ventilation (V̇A) to alveolar blood flow/perfusion (Q̇) for a given alveolus or lung region, expressed as V̇A/Q̇. It is a quantitative concept used to assess the efficiency of gas exchange when ventilation and blood flow to different parts of the lung are mismatched (Guyton and Hall Textbook of Medical Physiology, 4th South Asia Ed., Ch. 40).

Normal value When both ventilation and blood flow to an alveolus are normal, V̇A/Q̇ is said to be normal, and alveolar PO2 (~104 mm Hg) and PCO2 (~40 mm Hg) lie between the values of inspired air (PO2 149 mm Hg, PCO2 0 mm Hg) and mixed venous blood (PO2 40 mm Hg, PCO2 45 mm Hg). Gas exchange under this condition is near optimal.

When V̇A/Q̇ = 0 (perfusion without ventilation) This occurs when an alveolus is perfused but not ventilated, as in bronchial obstruction. No fresh air reaches the alveolus, so the alveolar air equilibrates with venous blood — alveolar PO2 falls to 40 mm Hg and PCO2 rises to 45 mm Hg (same as venous blood). This represents a physiologic shunt, since blood passes through without being oxygenated.

When V̇A/Q̇ = infinity (ventilation without perfusion) This occurs when an alveolus is ventilated but not perfused, as in pulmonary embolism. Since there is no blood flow to remove O2 or deliver CO2, the alveolar gas composition equals that of humidified inspired air: PO2 149 mm Hg and PCO2 0 mm Hg. This represents physiologic (alveolar) dead space — ventilation is wasted because no gas exchange with blood occurs.

Clinical significance In many pulmonary diseases (chronic bronchitis, emphysema, pulmonary edema, embolism), different regions of the lung show varying degrees of V̇A/Q̇ mismatch rather than the pure extremes. Some units show low V̇A/Q̇ (shunt-like effect, causing hypoxemia) while others show high V̇A/Q̇ (dead-space-like effect, wasting ventilation). The overall efficiency of gas exchange is reduced, and the sum of these effects is described using the concepts of physiologic shunt (for low V̇A/Q̇ regions) and physiologic dead space (for high V̇A/Q̇ regions). This mismatch is a major cause of hypoxemia and hypercapnia in lung disease and is graphically represented by the PO2-PCO2, V̇A/Q̇ diagram.