Ventilation-Perfusion Mismatch
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 causesdiversion of bloodto 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 T1RFmay 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. Thisimpairs 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 issmooth muscle contraction which causes an increased resistance to alveolar airflow. In COPD, inflammatory changes inducestructural 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 circulationlimits blood flow to alveoli. As a result, blood isredirected 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.
