Alveolar-Arterial Gradient - MedSchool
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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.
PaO₂
PaCO₂
 
FiO₂
Atm
 
Age
 
Calculate
 
A-a = ( FiO₂ × (PB - PH₂O)) -
PaCO₂ 0.8
- PaO₂

FiO₂ = 0.21 on room air
PB = 760mmHg at sea level
PH₂O = 47mmHg

Normal value = ( Age / 4 ) + 4
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