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.
The ventilation rate (V) refers to the volume of gas inhaled and exhaled from the lungs in a given time period, usually a minute. It can be 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 is to say that ventilation exceeds perfusion towards the apex, and that perfusion exceeds ventilation towards the base.
The different ratios for different areas are due to where each area lies in relation to the heart, with areas of lung below the heart having 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, it is the apical and middle zones of the lung which see the greatest relative increase in their perfusion rate with an increased cardiac output, such as during exercise.
If there is a mismatch between the alveolar ventilation and the alveolar blood flow, this will be seen in the V/Q ratio. If the V/Q ratio reduces due to inadequate ventilation, gas exchange within the affected alveoli will be impaired. As a result, the capillary partial pressure of oxygen (pO2) falls and the partial pressure of carbon dioxide (pCO2) rises.
To manage this, hypoxic vasoconstriction causes blood to be diverted 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, but for the scope of this article we will consider the more common. Primarily reduced ventilation 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, impairing the delivery of air to the alveoli and lengthening the diffusion pathway for the respiratory gases.
Asthma and chronic obstructive pulmonary disease (COPD) may also result in a reduction in ventilation. In asthma this is caused by smooth muscle contraction, increasing resistance to airflow to the alveoli. In COPD, structural airway damage caused by inflammatory changes lead 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, unless ventilation is severely limited.
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.