HYPOXIA

Hypoxia is defined as a deficiency of oxygen at the tissue level. There are many potential causes of hypoxia, but they can be classified into four general categories:

• Hypoxic hypoxia also termed hypoxemia, in which the arterial pO2 is reduced.
• Anemic hypoxia or carbon monoxide hypoxia, in which the arterial pO2 is normal but the total oxygen content of the blood is reduced because of inadequate numbers of erythrocytes, deficient or abnormal hemoglobin, or competition for the Hb molecule by carbon monoxide.
• Ischemic hypoxia (also called hypo perfusion hypoxia), in which blood flow to the tissues is too low.
• Histotoxic hypoxia, in which the quantity of oxygen reaching the tissues is normal but the cell is unable to utilize the oxygen because a toxic agent- cyanide for e.g., has interfered with the cell's metabolic machinery. Hypoxic hypoxia is a common cause of hypoxia.

The primary causes of hypoxic hypoxia in disease are as follows:

• Hypoventilation may be caused by : 1) A defect anywhere along the respiratory control pathway, from the medulla through the respiratory muscles.2) Severe thoracic age abnormalities. 3) Major obstruction of the upper airway.
• Diffusion impairment results from thickening of the alveolar membranes or a decrease in their surface area. Some of the diseases that produce hypoxia also produce carbon dioxide retention and an increased arterial pCO2 (hypercapnia).
• A shunt is an anatomical abnormality of the cardiovascular system that causes mixed venous blood to bypass ventilated alveoli. Arterial pCO2, generally does not increase because the effect of the shunt on arterial pCO2 is counter-balanced by the increased ventilation reflexively stimulated, by the hypoxemia.
• Ventilation-perfusion inequality  is by far the most common cause of hypoxemia. It occurs in chronic obstructive lung diseases. Arterial pCO2 may be normal or increased, depending upon how much ventilation is reflexively stimulated.

The administration of pure oxygen may cause such patients to stop breathing; consequently, such individuals are typically treated with a mixture of air and oxygen rather than 100% O2.