MRCP revision: Alveolar-arterial gradient (A-a gradient)

Saturday 7 July 2012

Alveolar-arterial gradient (A-a gradient)

$Aa~Gradient=P_AO_2-P_aO_2$

$P_AO_2=F_iO_2(P_{atm}-P_{H_2O})-\frac{P_aCO_2}{0.8}$

$Aa~Gradient=\left(F_iO_2(P_{atm}-P_{H_2O})-\frac{P_aCO_2}{0.8}\right)-P_aO_2$

Calculated A-a gradient = 150mmHg - PaCO2/0.8 - PaO2
Expected A-a gradient = Age/4 + 4

Normal A-a gradient suggest hypoventilation (hypoxia proportional to low ventilatory effort)
Increased A-a gradient suggest defect in diffusion, V/Q mismatch or right-left shunt

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Saturday 7 July 2012

Alveolar-arterial gradient (A-a gradient)

2 comments: