Hypoxemia: abnormally low levels of oxygen in the blood. Hypoxemia may be mild to severe and leads to shortness of breath. Low blood oxygen can result from a variety of conditions that affect either the ability of the heart to pump blood to the lungs to receive oxygen or the function of the lungs themselves. Breathing at high altitudes can also cause hypoxemia. The level of oxygen in the blood can be measured by a test performed on a sample of arterial blood (referred to as an arterial blood gas measurement) or by an instrument known as a pulse oximeter , which is clipped onto the finger.