• The partial pressure of arterial oxygen typically ranges between 90 - 100 mm Hg. When the value of arterial oxygen tension falls below 80 mm Hg, this is considered a state of "Hypoxemia". A large variety of disease processes can result in hypoxemia and these can be categorized according to several basic pathophysiological mechanisms as described in the sections below. The categories described below can be distinguished clinically using a number of simple tests which include calculation of the A-a Gradient and the response of arterial oxygen to oxygen therapy. The primary clinical concern in a hypoxemic patient is the development of body-wide hypoxia. The reader should be careful to distinguish the concept of "Hypoxemia" from that of "Hypoxia". Although hypoxemia is a possible cause of hypoxia, it is certainly not the only etiology that can yield hypoxia.