A-a gradient

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Overview

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

A-a Gradient Equation

A - a gradient = PA_O2 - Pa_O2 Where:
PA_O2 = alveolar PO₂ (calculated from the alveolar gas equation)
Pa_O2 = arterial PO₂ (measured in arterial blood)

The Alveolar Gas Equation

PA_O2 = PI_O2 - PA_CO2/R

Where:
PI_O2 = FI_O2 x (760 mmHg - 47mmHg) = inspired O₂
       - FI_O2 = 21% at sea level atmospheric pressure = 0.21
       - 760 mmHg = atmospheric pressure at sea level
       - 47 mmHg = H₂O pressure
PA_CO2 = 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.¹

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¹Costanzo, Linda (2006). Brs Physiology. Hagerstwon: Lippincott Williams & Wilkins. ISBN 0781773113.

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