Ensuring adequate matching of ventilation and perfusion of the lungs 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.
The ventilation rate (V) refers to the volume of gas inhaled and exhaled from the lungs in a given time period, usually a minute. This is calculated by multiplying the tidal volume (volume of air inhaled and exhaled in a single breath) by the respiratory rate. In an average man, the ventilation rate is roughly 6L/min.
The perfusion (Q) of the lungs refers to the total volume of blood reaching the pulmonary capillaries in a given time period.
The ideal V/Q ratio would be 1 for maximally efficient pulmonary function. However, the ratio varies depending on the part of the lung concerned. For example, when standing up straight, the ratio is roughly 3.3 in the apex of the lung, and only 0.63 in the base. This means that ventilation exceeds perfusion towards the apex, and that perfusion exceeds ventilation towards the base.
The different ratios for different areas are due to the relation of the area to the heart. Areas of lung below the heart have increased perfusion relative to ventilation due to gravity, reducing the V/Q ratio. As such the overall value in the average human lung is closer to 0.8.
Gravity triggers these changes in ventilation and perfusion through two different mechanisms:
- 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
As perfusion increases with gravity, the apical and middle zones of the lung see the greatest relative increase in their perfusion rate with an increased cardiac output, such as during exercise.
If alveolar ventilation and alveolar blood flow are not matched, this will be reflected in the V/Q ratio. When there is inadequate ventilation the V/Q reduces, and gas exchange within the affected alveoli is impaired. As a result, the capillary partial pressure of oxygen (pO2) falls and the partial pressure of carbon dioxide (pCO2) rises.
In response to this, hypoxic vasoconstriction causes diversion of blood to better ventilated parts of the lung. However, in most physiological states the haemoglobin in these well ventilated alveolar capillaries will already be saturated. This means that red cells will be unable to bind additional oxygen to increase the pO2. As a result, the pO2 level of the blood remains low, which acts as a stimulus to cause 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. Here we will consider the more common causes. Reduced ventilation primarilly affects oxygen levels, as carbon dioxide is more soluble and continues to diffuse despite the impairment. Thus, the initial effect of reduced ventilation is type 1 respiratory failure (T1RF), with reduced pO2 and a normal/low pCO2.
All causes of T1RF may progress to type 2 respiratory failure with low pO2 and elevated pCO2 if they are sufficiently severe.
In pneumonia the alveoli are filled with exudate. This impairs the delivery of air to the alveoli and lengthens the diffusion pathway for the respiratory gases. This results in reduced ventilation and can cause hypoxia, and therefore T1RF.
Asthma and chronic obstructive pulmonary disease (COPD) can also result in a reduced ventilation. In asthma there is smooth muscle contraction which causes an increased resistance to alveolar airflow. In COPD, inflammatory changes induce structural airway damage. This leads to impaired gas exchange, which can worsen in an acute exacerbation.
The effect of reduced ventilation is hypoxia. However, as the rest of the lung can still remove CO2, hypercapnia does not occur. In cases of severely limited ventilation, hypercapnia may develop.
Clinical Relevance – Reduced Perfusion of the Lungs
A pulmonary embolism can result in reduced perfusion of the lungs. Obstruction of some regions of pulmonary circulation limits blood flow to alveoli. As a result, blood is 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.