Alveolar-Arterial Gradient

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The Arterial-alveolar gradient is a comparison of partial pressure of O2 between arterial and alveolar blood. The A-a gradient is especially useful for taking FiO2 into account when the patient is not inspiring room air. The normal value is between 5 - 25 depending on age.

A-a = ( FiO₂ × (P_B - P_H₂O)) -

PaCO₂ 0.8

- PaO₂

FiO₂ = 0.21 on room air
P_B = 760mmHg at sea level
P_H₂O = 47mmHg

Normal value = ( Age / 4 ) + 4

References

Vereerstraeten J, Schoutens A, Tombroff M, De Koster JP. Value of measurement of alveolo-arterial gradient of PCO₂ compared to pulmonary scan in diagnosis of thromboembolic pulmonary disease. Thorax. 1973; 28(3): 306-312.

Alveolar-Arterial Gradient

References

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