Hb [Ref: Ganong 23/e p617; http://en.wikipedia.org/wiki/Hypoxemiaj Hypoxemia is decreased paial pressure of oxygen in blood. It is the oxygen that is dissolved in the blood not attached with hemoglobin; thus it does not depend on Hemoglobin. Hypoxemia is one of the types of Hypoxia kla Hypoxic Hypoxia. The Hypoxia dependent on hemoglobin is anemic Hypoxia. Hypoxia is defined as 0, deficiency at the tissue level. Hypoxia has been divided into four types. (1) Hypoxic hypoxia, in which the P02 of the aerial blood is reduced; (2) Anemic hypoxia, in which the aerial P02 is normal but the amount of hemoglobin available to carry 02 is reduced; (3) Stagnant or Ischemic hypoxia, in which the blood .flow to a tissue is so low that adequate 02 is not delivered to it despite a normal P02 and hemoglobin concentration; and (4) Histotoxic hypoxia, in which the amount of 02 delivered to a tissue is adequate but, because of the action of a toxic agent, the tissue cells cannot make use of the 02 supplied to them. Hypoxic Hypoxia (Hypoxemia) Hypoxic Hypoxia is the most common form of hypoxia seen clinically. Causes: 1.Low inspired paial pressure of oxygen (low Pi02) 2. Alveolar hypoventilation 3.Impairment of diffusion across blood-gas membrane 4.Ventilation-perfusion inequality 5. Shunt In contrast, primary hemoglobin deficiency, e.g. anemia is not a cause, since it doesn't decrease the paial pressure of oxygen in blood. Low inspired oxygen paial pressure (low PiO2) If the paial pressure of oxygen in the inspired gas is low, then a reduced amount of oxygen is delivered to the alveoli of the lung. The reduced oxygen paial pressure can be a result of reduced fractional oxygen content (low Fi02) or simply a result of low barometric pressure, as can occur at high altitudes. This reduced Pi02 can result in hypoxemia even if the lungs are normal. Alveolar hypoventilation If the alverolar ventilation is low, there may be insufficient oxygen delivered to the alveoli each minute. (e.g., airway obstruction, depression of the brain's respiratory center, or muscular weakness). Impaired diffusion Impaired diffusion across the blood-gas membrane in the lung can cause hypoxemia, e.g. pneumonia, pulmonary fibrosis Shunt Shunting of blood from the right side to the left side of the circulation (right-to-left shunt) is a powerful cause of hypoxemia. The shunt may be intracardiac (tong. cyanotic hea disease) or may be intrapulmonary.(Note that Left-toright shunting does not cause hypoxemia.) Ventilation-perfusion inequality Ventilation-perfusion inequality (or ventilation-perfusion mismatch) is a common cause of hypoxemia in people with lung disease. It is the areas of the lung with ventilation/perfusion ratios that are less than one (but not zero) that cause hypoxemia by this mechanism. A ventilation/perfusion ratio of zero is considered a shunt. (Not sure how PCO\ is linked with hypoxemia or PaO2 ; possibl! hypoxemia causes lryperventilation which leads todecreased alveolar and aeial PCO2)