Ensuring that the ventilation and perfusion of the lungs are adequately matched is vital for ensuring continuous delivery of oxygen and removal of carbon dioxide from the body. In this article, we will discuss ventilation- perfusion matching, how mismatch may occur and how this may be corrected.
- In an average man, the ventilation rate (V) is roughly 5L/min
- The perfusion of the lungs (Q) is roughly 5L/min
As such, the ideal V/Q ratio would be 1, however, the ratio varies depending on the part of the lung. For example, the ratio is roughly 3.3 in the apex of the lung, and only 0.63 in the base. As such the overall value in the average human is closer to 0.8.
This variation occurs due to the position of the heart in relation to the lungs. As you move from the apex to the base of the lungs both ventilation and perfusion rise, however perfusion increases at a greater rate, resulting in a lower V/Q ratio. This allows for increased perfusion of the apical and middle zone of the lungs during exercise.
It is also worth mentioning that gravity plays a significant role on the V/Q ratio:
- Pleural pressure is increased at the base of the lungs, resulting in more compliant alveoli and increased ventilation
- Hydrostatic pressure is decreased at the apex of the lung, resulting in decreased flow and decreased perfusion
If there is a mismatch between the alveolar ventilation and the alveolar blood flow, this will be seen in the V/Q ratio. In order to maintain optimal function of the lungs, the V/Q ratio should be 1. If the V/Q ratio drops to <1, i.e. the perfusion is higher than the ventilation, the pO2 falls and the pCO2 in the alveolus will rise. Hypoxic vasoconstriction can occur, diverting blood to better ventilated parts of the lung. However, these alveoli are unlikely to be able to take up much more O2 than they already are. As a result, the pO2 remains low, which acts as a stimulus, causing hyperventilation, resulting in either normal or low CO2 levels.
A mismatch in ventilation and perfusion can arise due to either reduced ventilation of part of the lung or reduced perfusion.
Clinical Relevance – Reduced Ventilation of the Lungs
Reduced ventilation can occur for a number of reasons. This includes pneumonia, whereby the alveoli are filled with exudate, limiting the ability to maintain ventilation. Asthma and COPD may also result in a reduction in perfusion, as well as respiratory distress syndrome of the newborn, whereby reduced surfactant production results in multiple collapsed alveoli, limited its ventilation capability.
The effect of reduced ventilation is hypoxia. However, as the rest of the lung can still remove CO2, hypercapnia does not occur. Thus, the net effect of this is type 1 respiratory failure.
Clinical Relevance – Reduced Perfusion of the Lungs
Pulmonary embolism can result in reduced perfusion of the lungs. Areas of the pulmonary circulation are obstructed, limiting blood flow to alveoli. As a result, blood has to be redirected to other areas of the lung. As the other areas receive an increased blood supply, the V/Q ratio will be <1. In this case, hypoxia still occurs because a vast majority of the lung is still working with a V/Q of <1.