A-a gradient

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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. [1]

References

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



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