Physiology, Alveolar to Arterial Oxygen Gradient

The A-a gradient, or the alveolar-arterial gradient, measures the difference between the oxygen concentration in the alveoli and arterial system. The A-a gradient has important clinical utility as it can help narrow the differential diagnosis for hypoxemia. The A-a gradient calculation is as follows: A-a Gradient = PAO2 – PaO2. With PAO2 representing alveolar oxygen pressure and PaO2 representing arterial oxygen pressure. The arterial oxygen pressure (PaO2) can be directly assessed with an arterial blood gas test (ABG) or estimated with a venous blood gas test (VBG). The alveolar oxygen pressure (PAO2) is not easily measured directly; instead, it is estimated using the alveolar gas equation: PAO2 = (Patm – PH2O) FiO2 – PaCO2/RQ (Please see the article on the alveolar gas equation for more information.) In a perfect system, no A-a gradient would exist: oxygen would diffuse and equalize across the capillary membrane, and the pressures in the arterial system and alveoli would be equal (resulting in an A-a gradient of zero). However, there is a physiologic V/Q mismatch in the lungs due to heterogeneity in apical vs. basilar perfusion and ventilation. This mismatch is, in part, responsible for the slight difference in oxygen tension between the alveoli and arterial blood. So there exists a physiologic A-a gradient that changes based on a patient's age. The expected A-a gradient can be estimated with the following equation: A-a gradient = (Age + 10) / 4 The value calculated for a patient's A-a gradient can assess if their hypoxia is due to the dysfunction of the alveolar-capillary unit, for which it will elevate, or due to another reason, in which the A-a gradient will be at or lower than the calculated value using the above equation.