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Under normal conditions, ventilation-perfusion coupling keeps ventilation (V) at approximately 4 L/min and normal perfusion (Q) at approximately 5 L/min. Thus, at rest, a normal V/Q ratio is 0.8. [2] Any deviation from this value is considered a V/Q mismatch.
An area with ventilation but no perfusion (and thus a V/Q undefined though approaching infinity) is termed "dead space". [6] 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.
On average, 4 liters of oxygen (V) and 5 liters of blood (Q) enter the alveoli in a minute, thus the normal V/Q ratio is 0.8. [10] It is considered abnormal when the ratio is greater or smaller than 0.8 and is referred to as ventilation-perfusion mismatch(V/Q mismatch).
This is because although the oxygen partial pressure of alveolar gas has been changed by giving pure supplemental oxygen, the arterial gas oxygen concentration will not increase that much because the V/Q mismatch still exists and it will still add some de-oxygenated blood to the arterial system via the shunt. [13]
Hypoxemia is caused by five categories of etiologies: hypoventilation, ventilation/perfusion mismatch, right-to-left shunt, diffusion impairment, and low PO 2. Low PO 2 and hypoventilation are associated with a normal alveolar–arterial gradient (A-a gradient) whereas the other categories are associated with an increased A-a gradient.
The Shunt equation (also known as the Berggren equation) quantifies the extent to which venous blood bypasses oxygenation in the capillaries of the lung. “Shunt” and “ dead space “ are terms used to describe conditions where either blood flow or ventilation do not interact with each other in the lung, as they should for efficient gas ...
An abnormally increased A–a gradient suggests a defect in diffusion, V/Q mismatch, or right-to-left shunt. [5] The A-a gradient has clinical utility in patients with hypoxemia of undetermined etiology. The A-a gradient can be broken down categorically as either elevated or normal. Causes of hypoxemia will fall into either category.
A V/Q mismatch can also occur when the surface area available for gas exchange in the lungs is decreased. [8] Pulmonary shunt, in which blood passes from the right to the left side of the heart without being oxygenated.