The following picture shows the Ventilation Perfusion Ratio. Please click the picture to be enlarged
Normal Ratio (A)
In the healthy lung, a given amount of blood passes an alveolus and is matched with an equal amount of gas (A). The ratio is 1:1 (ventilation matches perfusion).
Low Ventilation–Perfusion Ratio: Shunts (B)
Low ventilation–perfusion states may be called shuntproducing disorders. When perfusion exceeds ventilation, a shunt exists (B). Blood bypasses the alveoli without gas exchange occurring. This is seen with obstruction of the distal airways, such as with pneumonia, atelectasis, tumor, or a mucus plug.
High Ventilation–Perfusion Ratio: Dead Space (C)
When ventilation exceeds perfusion, dead space results (C). The alveoli do not have an adequate blood supply for gas exchange to occur. This is characteristic of a variety of disorders, including pulmonary emboli, pulmonary infarction, and cardiogenic shock.
Silent Unit (D)
In the absence of both ventilation and perfusion or with limited ventilation and perfusion, a condition known as a silent unit occurs (D). This is seen with pneumothorax and severe acute respiratory distress syndrome.
