Clinical Lab ( ABG )

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

A-a Gradient EquationA - a gradient = PAO2 - PaO2 Where:

PAO2 = alveolar PO2 (calculated from the alveolar gas equation)

PaO2 = arterial PO2 (measured in arterial blood)

The Alveolar Gas EquationPAO2 = 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 MeaningThe 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.