Normal PaO2/FiO2 ratio is 300 to 500 mmHg. PaO2/FiO2 ratio : It is a measure of Oxygen transfer across the alveolar capillary membrane. The expected A-a gradient can be estimated with the following equation: A-a Gradient PAO2 - PaO2 where PAO2 is predicted alveolar oxygen pressure (based on Alveolar Oxygen Equation above) where PaO2 is measured arterial oxygen pressure A-a Gradient FIO2 x (760 - 47) - ( PaCO2 /0.8) - PaO2 where FIO2 on room air 0. A-a gradient varies with age and can be estimated from the following equation, assuming the patient is breathing room air. So there exists a physiologic A-a gradient that changes based on a patient's age.

To calculate the Gradient: Divide the change in height by the change in horizontal distance. This mismatch is, in part, responsible for the slight difference in oxygen tension between the alveoli and arterial blood. The Gradient (also called Slope) of a straight line shows how steep a straight line is. However, there is a physiologic V/Q mismatch in the lungs due to heterogeneity in apical vs. The A-a gradient is calculated as the alveolar partial pressure of oxygen (PAO) minus the arterial partial pressure of oxygen (PaO): A-a gradient PAO - PaO The alveolar value is calculated, based on the patients inspired FiO and the PCO from their blood gas result, while the arterial value is the PaO from the patients blood gas result. 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). (Please see the article on the alveolar gas equation for more information.) The alveolar oxygen pressure (PAO2) is not easily measured directly instead, it is estimated using the alveolar gas equation: 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). With PAO2 representing alveolar oxygen pressure and PaO2 representing arterial oxygen pressure. The A-a gradient calculation is as follows: The A-a gradient has important clinical utility as it can help narrow the differential diagnosis for hypoxemia.

The A-a gradient, or the alveolar-arterial gradient, measures the difference between the oxygen concentration in the alveoli and arterial system.

Aa gradient