What is the ventilation-perfusion ratio?
An important determinant of arterial oxygen tension is the effectiveness of coupling of lung ventilation to lung perfusion. Not all parts of the lung are equally ventilated or perfused. The relationship between ventilation and perfusion in a lung region is expressed as the ventilation-perfusion ratio (V/Q). The modest imbalance between ventilation and perfusion in normal individuals accounts for the small alveolar-arterial oxygen gradient routinely measured with arterial blood gas testing.
Equal ventilation and perfusion
When breathing room air at an FIO2 of 0.21, an alveolus with one unit of ventilation and one unit of perfusion has a ventilation-perfusion ratio of one, a PAO2 of 100 mmHg, and a PACO2 of 40 mmHg.
Perfused, not ventilated
In one extreme of ventilation-perfusion mismatch, an alveolus is perfused, but not ventilated; in other words, it has a ventilation-perfusion ratio of zero. Since no air enters the alveolus as alveolar gas equilibrates with mixed venous blood returning to the lungs, the alveolar gas tensions are those of mixed venous blood: PAO2 of 40 mmHg and PACO2 of 45 mmHg.
Ventilated, not perfused
In another extreme case of ventilation-perfusion mismatch, the alveolus is ventilated, but not perfused; in other words, the ventilation-perfusion ratio is infinity. In the absence of blood flow to the unit, the alveolar gas tensions are those of inspired air: PAO2 of about 150 mmHg and PACO2 of nearly 0 mmHg.
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There actually is a spectrum of ventilation-perfusion relationships throughout the lung, created by normal physiologic relationships that dictate regional perfusion and ventilation
In the upright lung, more ventilation goes to the lung base than the lung apex. This arises because there are more alveoli at the larger bases. In addition, the basilar alveoli are less stretched than the apical ones and can “give more” with inflation (i.e., they are more compliant).
In the upright lung, more perfusion goes to the lung base than the lung apex because there are more alveoli and pulmonary blood vessels in the larger bases, and because gravitational effects on pulmonary blood flow favor perfusion to the bases.
Although the apical-basal gradients for ventilation and perfusion are in the same direction, the magnitudes of changes in each from apex to base are different. The slope of the perfusion curve is steeper than that for ventilation. As a result, the ventilation-perfusion ratio decreases from apex to base.
In disease states, ventilation-perfusion relationships throughout the lung are altered, creating abnormal gas exchange, especially for oxygen. In particular, regions of the lung characterized by ventilation-perfusion ratios of less than one contributes to hypoxemia and widening of the alveolar-arterial oxygen gradient.
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