A-a gradient

Overview
The Alveolar-arterial gradient (A-a gradient), is used to compare the causes of hypoxemia.

A-a Gradient Equation
A - a gradient = PAO2 - PaO2 Where: PAO2 = alveolar PO2 (calculated from the alveolar gas equation) PaO2 = arterial PO2 (measured in arterial blood)

The Alveolar Gas Equation
PAO2 = PIO2 - PACO2/R

Where: PIO2 = FIO2 x (760 mmHg - 47mmHg) = inspired O2        - FIO2 = 21% at sea level atmospheric pressure = 0.21 - 760 mmHg = atmospheric pressure at sea level - 47 mmHg = H2O pressure PACO2 = arterial PCO2 measured in plasma R = respiratory exchange ratio or respiratory quotient = 0.8 under normal conditions

Values and Meaning
The normal A-a gradient is < 10 mmHg, but can range from 5-20 mmHg in a normal individual. An increased A-a gradient suggests a diffusion defect, V/Q (ventilation/perfusion) defect, or right-to-left shunt.