The hypoxaemia in V/Q mismatch normally responds well to oxygen therapy by improving alveolar oxygenation in low V/Q areas. Blood is diverted to other areas and low V/Q regions are created to compensate. This occurs in areas where perfusion is limited, for example in pulmonary embolism, resulting in alveolar units with a high V/Q. High V/Q: this refers to alveolar units with poor perfusion compared with ventilation.In response, hypoxia-induced pulmonary vasoconstriction occurs in low V/Q areas, diverting blood to better-ventilated areas of the lung. This may be seen in airway disease or interstitial lung disease where ventilation is reduced. This results in hypoxaemia due to low alveolar oxygen levels. Low V/Q: this refers to alveolar units with poor ventilation compared with perfusion.During a disease process the V/Q ratio varies in the lung based on location and type of pathological process: In disease there may be an increase in the V/Q mismatch leading to hypoxaemia. As such the V/Q ratio at the apices is around 3.3 compared to 0.6 at the bases. Ventilation is also higher at the bases compared to the apices but the differential is to a far lesser degree.
Whilst standing, perfusion is better to the bases when compared with the apices due to gravitational and hydrostatic forces. In normal health there is a mismatch in V/Q that differs depending on the part of the lung. In the average adult the alveoli are ventilated by 4L of air and perfused by 5L of blood each minute, giving a V/Q ratio of 0.8 (4:5). This is the ratio of alveolar ventilation to alveolar perfusion. V/Q refers to the ventilation-to-perfusion ratio. Hypercapnia (+/- hypoxaemia) normally results from reduced alveolar ventilation or increased alveolar dead space. Whilst V/Q mismatch, shunts and diffusion limitation cause hypoxaemia they may also feature hypercapnia if the disease process is severe enough. Though below we discuss them separately they are related and often occur together. There are a number of processes that lead to hypoxaemia and hypercapnia. Hypoxaemia is most commonly caused by a V/Q mismatch, hypercapnia is often caused by alveolar hypoventilation.
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