It is caused by intrapulmonary shunting of blood with resulting in ventilation-perfusion (V / Q) mismatch due to airspace filling or collapse (eg, cardiogenic or non-cardiogenic pulmonary edema, pneumonia, pulmonary hemorrhage) or possibly airway disease (eg, sometimes asthma, COPD); or by intracardiac shunting of blood from the right - to the left-sided circulation.