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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.
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. While both ventilation and perfusion increase going from the apex to the base, perfusion increases to a greater degree than ventilation, lowering the V/Q ratio at the base of the lungs. The principal factor involved in the creation of this V/Q gradient between the apex and the base of the lung is gravity (this is why V/Q ratios change in positions other than the orthostatic position).
Gravity and the weight of the lung act on ventilation by increasing pleural pressure at the base (making it less negative) and thus reducing the alveolar volume. The lowest part of the lung in relation to gravity is called the dependent region. In the dependent region smaller alveolar volumes mean the alveoli are more compliant (more distensible) and so capable of more oxygen exchange. The apex, though showing a higher oxygen partial pressure, ventilates less efficiently since its compliance is lower and so smaller volumes are exchanged.
The impact of gravity on pulmonary perfusion expresses itself as the hydrostatic pressure of the blood passing through the branches of the pulmonary artery in order to reach the apical and basal areas of the lungs, acting respectively against or synergistically with the pressure developed by the right ventricle. Thus at the apex of the lung the resulting pressure can be insufficient for developing a flow (which can be sustained only by the negative pressure generated by venous flow towards the left atrium) or even for preventing the collapse of the vascular structures surrounding the alveoli, while the base of the lung shows an intense flow due to the higher resulting pressure.
Of note, few conditions constitute "pure" shunt or dead space as they would be incompatible with life, and thus the term V/Q mismatch is more appropriate for conditions in between these two extremes.