Alveolar-arterial gradient

Pathophysiology sample values

BMP/ELECTROLYTES:
Na+=140	Cl−=100	BUN=20	/
			Glu=150
K+=4	CO2=22	PCr=1.0	\
ARTERIAL BLOOD GAS:
HCO3-=24	paCO2=40	paO2=95	pH=7.40
ALVEOLAR GAS:
	pACO2=36	pAO2=105	A-a g=10
OTHER:
Ca=9.5	Mg2+=2.0	PO4=1
CK=55	BE=−0.36	AG=16
SERUM OSMOLARITY/RENAL:
PMO = 300	PCO=295	POG=5	BUN:Cr=20
URINALYSIS:
UNa+=80	UCl−=100	UAG=5	FENa=0.95
UK+=25	USG=1.01	UCr=60	UO=800
PROTEIN/GI/LIVER FUNCTION TESTS:
LDH=100	TP=7.6	AST=25	TBIL=0.7
ALP=71	Alb=4.0	ALT=40	BC=0.5
		AST/ALT=0.6	BU=0.2
AF alb=3.0	SAAG=1.0		SOG=60
CSF:
CSF alb=30	CSF glu=60	CSF/S alb=7.5	CSF/S glu=0.4

The Alveolar-arterial gradient (A-a gradient), is a measure of the difference between the alveolar concentration of oxygen and the arterial concentration of oxygen. It is used in diagnosing the source of hypoxemia.

Equation

The equation for calculating the A-a gradient is:

$Aa~Gradient=P_AO_2-P_aO_2$

Where:

• P_AO₂ = alveolar PO₂ (calculated from the alveolar gas equation)

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

• P_aO₂ = arterial PO₂ (measured in arterial blood)

In its expanded form, the A-a gradient can be calculated by:

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

On room air ( F_iO₂=0.21, or 21% ), at sea level ( P_atm=760mmHg ) assuming 100% humidity in the alveoli, a simplified version of the equation is:

$Aa~Gradient=\left(150-\frac{5}{4}(P_{CO_2})\right)-P_aO_2$

Values and meaning

The A-a gradient is useful in determining the source of hypoxemia. The measurement helps isolate the location of the problem as either intrapulmonary (within the lungs) or extrapulmonary (somewhere else in the body).

A normal A-a gradient for a young adult non-smoker breathing air, is between 5-10 mmHg. Normally, the A-a gradient increases with age. For every decade a person has lived, their A-a gradient is expected to increase by 1 mmHg.

An abnormally increased A-a gradient suggests a defect in diffusion, V/Q (ventilation/perfusion ratio) mismatch, or right-to-left shunt.

Because A-a gradient is approximated as: (150 - 5/4(P_CO2)) - Pa_O2, the direct mathematical cause of a large value is that the blood has a low P_O2, a low P_CO2, or both. CO2 is very easily exchanged in the lungs and low P_CO2 directly correlates with high minute ventilation; therefore a low arterial P_CO2 indicates that extra respiratory effort is being used to oxygenate the blood. A low Pa_O2 indicates that at the patient's current minute ventilation (whether high or normal) is not enough to allow adequate oxygen diffusion into the blood. Therefore the A-a gradient essentially demonstrates a high respiratory effort (low arterial P_CO2) relative to the achieved level of oxygenation (arterial P_O2). A high A-a gradient could indicate a patient breathing hard to achieve normal oxygenation, a patient breathing normally and attaining low oxygenation, or a patient breathing hard and still failing to achieve normal oxygenation.

If lack of oxygenation is proportional to low respiratory effort, then the A-a gradient is not increased; a healthy person who hypoventilates would have hypoxia, but a normal A-a gradient. At an extreme, high CO2 levels from hypoventilation can mask an existing Aa gradient. This mathematical artifact makes A-a gradient more clinically useful in the setting of hyperventilation.

See also

• Pulmonary gas pressures