It should be pointed out that in a healthy individual the A-a Gradient is roughly 4 - 8 mm Hg, representing some slight ventilation-perfusion defects within the normal lung as well as the small right-left shunt contributed by the bronchial circulation. In general, diffusion defects, ventilation-perfusion defects, and right-left shunts result in a widened A-a Gradient whereas hypoventilation and residence at high altitudes do not. Calculation of the A-a Gradient helps distinguish basic pathogenic causes of hypoxemia.It should be pointed out that right-left shunts also result in a widened A-a Gradient as these are simply an extreme subtype of ventilation-perfusion defects. Consequently, diseases caused by ventilation-perfusion and diffusion defects result in a widening of the A-a Gradient beyond its normal value. However, this will be significantly different than the empirically-determined partial pressure of oxygen in the arterial blood. Because the value of arterial carbon dioxide largely remains normal in these diseases, the "Alveolar Gas Equation" misleadingly gives a normal value for the partial pressure of alveolar oxygen.Although ventilation-perfusion and diffusion defects can significantly alter oxygenation of blood, they typically do not affect elimination of carbon dioxide from the circulation. As noted from the Alveolar Gas Equation, the theoretical partial pressure of alveolar oxygen is derived from the partial pressure of alveolar carbon dioxide which in practice is assumed to be equivalent to the arterial carbon dioxide partial pressure. So the Gradient is equal to 1 : The Gradient 4 2 2. Gradient Change in YChange in X : Have a play (drag the points): Examples: The Gradient 3 3 1. To calculate the Gradient: Divide the change in height by the change in horizontal distance. The "Alveolar Gas Equation" (See: Alveolar Oxygen) used to calculate the theoretical partial pressure of alveolar Oxygen assumes a healthy lung with no ventilation-perfusion defects or diffusion defects. The Gradient (also called Slope) of a straight line shows how steep a straight line is.Thus: A-a Gradient = (Theoretical Alveolar Oxygen) - (Actual Arterial Oxygen).The A-a Gradient is simply determined by subtracting the actual partial pressure of arterial oxygen from the theoretical value for alveolar oxygen obtained from the "Alveolar Gas Equation" (See: Alveolar Oxygen).Calculation of this value is a useful tool in categorizing the pathophysiological source of hypoxemia. The "A-a Gradient", or "Alveolar-arterial Gradient", refers to the difference in the theoretical partial pressure of alveolar oxygen compared to the empirically determined oxygen tension within arterial blood.
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