In the respiratory system, ventilation/perfusion (V/Q) mismatch refers to the pathological discrepancy between ventilation (V) and perfusion (Q) resulting in an abnormal ventilation/perfusion (V/Q) ratio.
A mismatch can contribute to hypoxemia and often signifies the presence or worsening of an underlying pulmonary condition.
Gas exchange thus becomes highly inefficient leading to hypoxemia, as measured by arterial oxygenation.
In conditions with right to left shunts, there are also ventilation perfusion defects with high A-a gradients.
Patient with parenchymal lung diseases will have an increased A-a gradient with moderate response to oxygen therapy.