Hypoxemia with increased (A-a) DO2 Hypoxemia with increased (A-a) DO2 is caused by Low ventilation / perfusion (V / Q) ratio (a type of V / Q mismatch) Right-to-left shunting Severely impaired diffusing capacity Low V / Q ratio is one of the more common reasons for hypoxemia and contributes to the hypoxemia occurring in COPD and asthma.