Ventilation perfusion relationships

Disturbances in the relationship of ventilation and perfusion may result in hypoxemia, a deficiency of oxygen in the blood. The four major causes of hypoxemia are hypoventilation, alveolar-capillary diffusion impairment, shunt, and ventilation perfusion mismatch.1 In this article factors which determine ventilation and gas exchange will be identified and abnormalities leading to hypoxemia will be examined. The alveolar 02 tension (PAO2) is determined by the balance between the addition of 02 to the alveoli from ventilation and the removal of 02 from the alveoli by pulmonary capillary blood. If alveolar ventilation is low, PAO 2 will decrease and PACO2 will increase, resulting in hypoxemia and hypercarbia. This is known as hypoventilation. Causes of hypoventilation include anesthetics, narcotics, muscle relaxants, trauma to the chest wall, spinal cord injuries, and central nervous system depression. Arterial 02 tension (PaO 2) can be maintained with increases in inspired oxygen while arterial CO 2 tension (PaCO 2) will always increase during hypoventilation. The second cause of hypoxemia is impaired diffusion. With normal breathing, gas is transported through the airways to the alveoli by convective or bulk gas flow. At the alveolar capillary membrane the mechanism of gas transport is diffusion. When the alveolar-capillary membrane is thickened by disease, the difference between 02 in the alveoli and 02 in the end capillary blood may increase because of impaired diffusion. CO 2 diffuses 20 times more rapidly than oxygen, and elevation of CO2 is not usually seen with impaired diffusion. The third cause of hypoxemia is right to left shunt in which blood enters the arterial system