Approach to the A-a Gradient A-a gradient = PAO2 - PaO2 A-a gradient = [FiO2 x (Patm - 47) - (PaCO2/0.8)] - PaO2 Simplified A-a (assumes Room Air + Sea Level) [150- (PaCO2/0.8)] - PaO2 Normal Gradient = (Patient's Age + 10) / 4 Compare A-a gradient to normal age adjusted gradient A-a Gradient ≤ Age-Adjusted Gradient • Alveolar Hypoventilation: CNS of Respiratory Center, Obesity Hypoventilation Syndrome, Muscular Weakness, Loss of Chest Wall Elasticity (flail chest) • Low PiO2: Extreme Elevation, Low Fio2 A-a Gradient > Age-Adjusted Gradient • Right to Left Shunt - Anatomic Shunt: Intracardiac, Pulmonary AVMs, Hepatopulmonary Syndrome - Physiologic Shunt: Atelectasis, ARDS, Pneumonia and more • V/Q Mismatch: Pulmonary Edema, Atelectasis, Pulmonary Embolism, Obstructive Lung Disease, Interstitial Disease and more • Diffusion Defect: Pulmonary Fibrosis, Interstitial Lung Disease Unremarkable Labs @UnremarkableLab #AA #Gradient #alveolar #arterial #pulmonary #Hypoxia #hypoxemia #differential #diagnosis
