Ventilation – Perfusion (V/Q) mismatch
V/Q mismatch is the presence of a degree of shunt and a degree of dead space in the same lung. It is a component of most causes of respiratory failure and is the commonest cause of hypoxaemia.
Because of the complicated structure of the lungs, it is impossible to describe this condition in anatomical terms. A patient with this condition is likely to have areas in the lungs that are better perfused than ventilated and areas that are better ventilated than perfused. This occurs in normal lungs to some extent. The difference in V/Q mismatch is that the extent to which this occurs is significantly increased.
Because of the flat upper portion of the Oxyhaemoglobin dissociation curve (fig 4), blood leaving the relatively healthy alveoli will have an oxygen saturation of about 97%. Blood leaving alveoli that do not have optimum V/Q ratios will have a much lower oxygen saturations . The admixture of all the blood leaving the alveoli results low oxygen saturations and hypoxaemia.
Figure 4. Oxygen haemoglobin dissociation curve. SaO2 = arterial haemoglobin saturations. PaO2 = partial pressure of oxygen.
In general, this cause of respiratory failure responds to oxygen therapy, although the response varies depending on the precise nature and size of the V/Q mismatch.