Expansion
A ratio of zero is shunt; a ratio of infinity is dead space
Expansion
V/Q = alveolar ventilation / pulmonary blood flow, overall about 0.8.
The two extremes:
- V/Q = 0: perfused but not ventilated, that is a shunt. Blood leaves with venous gas values
- V/Q = infinity: ventilated but not perfused, that is dead space. Gas leaves with inspired values
Real disease lies between: pneumonia and atelectasis tend towards shunt, pulmonary embolism and emphysema towards dead space.
Regional variation exists even in health. In the upright lung both ventilation and perfusion increase towards the base, but perfusion increases more, so:
- Apex: high V/Q, about 3. Higher oxygen, lower carbon dioxide
- Base: low V/Q, about 0.6. Lower oxygen, higher carbon dioxide
Mismatch lowers arterial oxygen more than it raises carbon dioxide, because the oxygen dissociation curve is flat at the top so well-ventilated regions cannot compensate for poorly ventilated ones, whereas the carbon dioxide curve is nearly linear and compensation is effective.