Arterial PO2 is decreased in hypoxia due to
COPD presents with Hypoxic Hypoxia. Hypoxic hypoxia is the most common form of hypoxia seen clinically. 
The diseases that cause it can be roughly divided into those in which the gas exchange apparatus fails, those such as congenital heart disease in which large amounts of blood are shunted from the venous to the arterial side of the circulation, and those in which the respiratory pump fails. 
Lung failure occurs when conditions such as pulmonary fibrosis produce alveolar-capillary block, or there is ventilation-perfusion imbalance. 
Pump failure can be due to fatigue of the respiratory muscles in conditions in which the work of breathing is increased or to a variety of mechanical defects such as pneumothorax or bronchial obstruction that limit ventilation. 
It can also be caused by abnormalities of the neural mechanisms that control ventilation, such as depression of the respiratory neurons in the medulla by morphine and other drugs.
Cyanide poisoning
CO poisoning
COPD
Shock