ventilation-to-perfusion ratios
- The ventilation/perfusion (V/Q) ratio is an indication of how well alveolar ventilation matches pulmonary capillary perfusion.
- Due to gravitational forces, the V/Q ratio ranges regionally in the lung, from 3.0 at the apex to 0.6 at the base
- Clinicians and physiologists typically use the average value for the entire lung as a reference point.
- Healthy average alveolar ventilation rate is approximately 4 liters of air/minute
- Blood flow rate is approximately 5 liters of blood/minute.
- A healthy V/Q for the entire lung is 0.8.
V/Q mismatches, aka, defects, occur along a spectrum.
- Two extremes of mismatches are shunts and dead space:
- Shunts occur when the rate of alveolar ventilation is zero; V/Q = 0
- Dead spaces occur where the rate of blood flow is zero; V/Q = infinity.
- Notice that the normal V/Q of 0.8 is between the two extremes (a V/Q of 1 would be perfect, but we are using the average value).
- Mismatches between 0 and 0.8 reflect defects in alveolar airflow.
- Mismatches greater than 0.8 reflect defects in pulmonary capillary blood flow.