What is a VQ Mismatch?

A VQ Mismatch in respiratory pathophysiology is a problem with either the Ventilation (air going in and out of the lungs) or the Perfusion (Oxygen and Co2 diffusion at the alvioli and the pulmonary arteries). VQ ratios compare the amount of air reaching the alveoli to the amount of blood reaching the alveoli.

Ventilation Problems include: inadequate rate or tidal volume during respiration. For example, the person has an adequate rate of respirations (say, 18 breaths per minute), but the tidal volume is insufficient). This would be considered a V/Q mismatch relating to a problem with the V side of the equation.

Alternatively, you may have a perfusion problem. For example, excess pulmonary dead space,such as: emphasema, bronchitis, pneumonia, atelectasis, low pulmonary artery pressures, RVF, lack of haemoglobin availability (as a result of haemorrhage or Carbon Monoxide Poisoning).

Extreme VQ Ratios

The following are extreme VQ Ratios:

An area of no ventilation (VQ ratio of zero) is called a shunt;

An area of no perfusion (VQ ratio approaching infiniti) is called physiological dead space. 

Examples of a VQ Mismatch

The following are examples of disorders that result in a VQ Mismatch:

Patients who have COPD (chronic obstructive pulmonary disease) have a high ventilation rate, but a poor perfusion level at the alvioli due to damage alvioli. This results in a high V/Q ratio.

Patients who have a pulmonary embolism have an adequate ventilation rate, but the oxygen getting to the alvioli is ineffective because it is unable to perfuse the blood which has clotted in the pulmonary artery. This results in a high V/Q ratio.