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Overview
• 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. Calculation of this value is a useful tool in categorizing the pathophysiological source of hypoxemia.
Calculation
• 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).
• Thus: A-a Gradient = (Theoretical Alveolar Oxygen) - (Actual Arterial Oxygen)
Significance
• 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. 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. Although ventilation-perfusion and diffusion defects can significantly alter oxygenation of blood, they typically do not affect elimination of carbon dioxide from the circulation.
• 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. However, this will be significantly different than the empirically-determined partial pressure of oxygen in the arterial blood. Consequently, diseases caused by ventilation-perfusion and diffusion defects result in a widening of the A-a Gradient beyond its normal value. 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.
Role in Diagnosis
• Calculation of the A-a Gradient helps distinguish basic pathogenic causes of hypoxemia. 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. 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.