Hypoxemia was originally defined as a deficiency of oxygen in arterial blood, and standard manuals take this to mean an abnormally low partial pressure of oxygen, content of oxygen or percent saturation of hemoglobin with oxygen, either found singly or in combination. One simple rule is that hypoxemia becomes very serious when the decreased partial pressure of oxygen in blood is less than 60 mm Hg, because that point is the beginning of the steep portion of the hemoglobin dissociation curve, where a small decrease in the partial pressure of oxygen results in a large decrease in the oxygen content of the blood.