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Any deviation from this value is considered a V/Q mismatch. Maintenance of the V/Q ratio is crucial for preservation of effective pulmonary gas exchange and maintenance of oxygenation levels. A mismatch can contribute to hypoxemia and often signifies the presence or worsening of an underlying pulmonary condition. [3]
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.
Some references refer to “shunt-effect” or “dead space-effect” to designate the ventilation/perfusion mismatch states that are less extreme than absolute shunt or dead space. The following equation relates the percentage of blood flow that is not exposed to inhaled gas, called the shunt fraction Q s / Q t {\displaystyle Q_{s}/Q_{t ...
A pulmonary ventilation-perfusion scan (lung V/Q scan) can be used to diagnose the V/Q mismatch. A ventilation scan is used to measure airflow spread and a perfusion scan for blood flow distribution in the lungs. A radioactive tracer is used to scan the whole lung and the ventilation and perfusion function. [21]
A pulmonary shunt occurs as a result of blood flowing right-to-left through cardiac openings or in pulmonary arteriovenous malformations. [clarification needed] The shunt which means V/Q = 0 for that particular part of the lung field under consideration results in de-oxygenated blood going to the heart from the lungs via the pulmonary veins.
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.
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. [11]: 229
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.