<Li> An area with perfusion but no ventilation (and thus a V / Q of zero) is termed "shunt". </Li> <Li> An area with ventilation but no perfusion (and thus a V / Q undefined though approaching infinity) is termed dead space . </Li> <P> 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 . </P> <Ul> <Li> A lower V / Q ratio (with respect to the expected value for a particular lung area in a defined position) impairs pulmonary gas exchange and is a cause of low arterial partial pressure of oxygen (paO). Excretion of carbon dioxide is also impaired, but a rise in the arterial partial pressure of carbon dioxide (paCO) is very uncommon because this leads to respiratory stimulation and the resultant increase in alveolar ventilation returns paCO to within the normal range . These abnormal phenomena are usually seen in chronic bronchitis, asthma, hepatopulmonary syndrome, and acute pulmonary edema . </Li> <Li> A high V / Q ratio decreases PACO and increases PAO . Because of the increased dead space ventilation, the PaO2 is reduced and thus also the peripheral oxygen saturation is lower than normal, leading to tachypnea and dyspnea . This finding is typically associated with pulmonary embolism (where blood circulation is impaired by an embolus). Ventilation is wasted, as it fails to oxygenate any blood . A high V / Q can also be observed in emphysema as a maladaptive ventilatory overwork of the undamaged lung parenchyma . Because of the loss of alveolar surface area, there is proportionally more ventilation per available perfusion area . As a contrast, this loss of surface area leads to decreased arterial pO due to impaired gas exchange (see Fick's laws of diffusion). </Li> </Ul>

Gas exchange and ventilation-perfusion relationships in the lung