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



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