Ventilation-perfusion mismatch
In respiratory physiology, the ventilation/perfusion ratio (V̇/Q̇ ratio or V/Q ratio) is a ratio used to assess the efficiency and adequacy of the matching of two variables:
The V/Q ratio can therefore be defined as the ratio of the amount of air reaching the alveoli per minute to the amount of blood reaching the alveoli per minute—a ratio of volumetric flow rates. These two variables, V & Q, constitute the main determinants of the blood oxygen (O2) and carbon dioxide (CO2) concentration.
The V/Q ratio can be measured with a ventilation/perfusion scan.
A V/Q mismatch can cause a type 1 respiratory failure.
Ideally, the oxygen provided via ventilation would be just enough to saturate the blood fully. In the typical adult, 1 litre of blood can hold about 200 mL of oxygen; 1 litre of dry air has about 210 mL of oxygen. Therefore, under these conditions, the ideal ventilation perfusion ratio would be about 0.95. If one were to consider humidified air (with less oxygen), then the ideal v/q ratio would be in the vicinity of 1.0, thus leading to concept of ventilation-perfusion equality or ventilation-perfusion matching. This matching may be assessed in the lung as a whole, or in individual or in sub-groups of gas-exchanging units in the lung. On the other side Ventilation-perfusion mismatch is the term used when the ventilation and the perfusion of a gas exchanging unit are not matched.
The actual values in the lung vary depending on the position within the lung. If taken as a whole, the typical value is approximately 0.8.
Because the lung is centered vertically around the heart, part of the lung is superior to the heart, and part is inferior. This has a major impact on the V/Q ratio:
In a subject standing in orthostatic position (upright) the apex of the lung shows higher V/Q ratio, while at the base of the lung the ratio is lower but nearer to the optimal value for reaching adequate blood oxygen concentrations. The main reason for lower V/Q ratios at the base is that both ventilation and perfusion increase when going from the apex to the base, but Q does it more strongly thus lowering the V/Q ratio. The principal factor involved in the genesis of V/Q dishomogeneity between the apex and the base of the lung is gravity (this is why V/Q ratios change in positions other than the orthostatic one).
...
Wikipedia