what is vq mismatch in the lungs

Ventilation Perfusion mismatch or “V/Q defects” are defects in total lung ventilation perfusion ratio. it is a condition in which one or more areas of the lung receive oxygen but no blood flow, or they receive blood flow but no oxygen due to some diseases and disorders.
The V/Q ratio of a healthy lung is approximately equal to 0.8, as normal lungs are not perfectly matched., which means the rate of alveolar ventilation to the rate of pulmonary blood flow is roughly equal.
The ventilation perfusion ratio can be measured by measuring the A-a gradient i.e. the alveolar-arterial gradient.
Let us consider some scenarios where there is a defect in ventilation and/ or perfusion of the lungs.
In condition such as pulmonary embolism, the pulmonary blood flow is affected, thus the ventilation of the lung is adequate, however there is a perfusion defect with defect in blood flow. Gas exchange thus becomes highly inefficient leading to hypoxemia as measured by arterial oxygenation. A ventilation perfusion scan or lung scintigraphy shows some areas of lungs being ventilated but not adequately perfused. This also leads to a high A-a gradient which is not responsive to oxygen.
In conditions with right to left shunts, there is again a ventilation perfusion defect with high A-a gradient. However, the A-a gradient is responsive to oxygen therapy. In cases of right to left shunts more of deoxygenated blood mixes with oxygenated blood from the lungs and thus to a small extent the condition might neutralize the high A-a gradient with pure oxygen therapy. Patient with parenchymal lung diseases will have an increased A-a gradient with moderate response to oxygen therapy.
A patient with hypoventilation will have complete response to 100% oxygen therapy.