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	p_aCO₂ = 40	p_aO₂ = 95	pH = 7.40
ALVEOLAR GAS:
	p_ACO₂ = 36	p_AO₂ = 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-aO₂,^[1] or A–a gradient), is a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. It is used in diagnosing the source of hypoxemia.^[2]
It helps to assess the integrity of alveolar capillary unit. For example, in high altitude, the arterial oxygen PaO₂ is low but only because the alveolar oxygen (PAO₂) is also low. However, in states of ventilation perfusion mismatch, such as pulmonary embolism or right-to-left shunt, oxygen is not effectively transferred from the alveoli to the blood which results in elevated A-a gradient

Equation

The equation for calculating the A–a gradient is:

Aa~Gradient=P_{A}O_{2}-P_{a}O_{2}

^[3]

Where:

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

P_{A}O_{2}=F_{i}O_{2}(P_{{atm}}-P_{{H_{2}O}})-{\frac {P_{a}CO_{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_{i}O_{2}(P_{{atm}}-P_{{H_{2}O}})-{\frac {P_{a}CO_{2}}{0.8}}\right)-P_{a}O_{2}

On room air ( F_iO₂ = 0.21, or 21% ), at sea level ( P_atm = 760 mmHg ) 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_{a}O_{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. A conservative estimate of normal A–a gradient is less than [age in years/4] + 4. Thus, a 40-year-old should have an A–a gradient less than 14.

An abnormally increased A–a gradient suggests a defect in diffusion, V/Q (ventilation/perfusion ratio) mismatch, or right-to-left shunt.^[4]

Because A–a gradient is approximated as: (150 − 5/4(pCO₂)) – Pa_O2 at sea level and on room air (0.21x(760-47) = 149.7 mmHg for the alveolar oxygen partial pressure, after accounting for the water vapor), 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 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 high A–a gradient. This mathematical artifact makes A–a gradient more clinically useful in the setting of hyperventilation.

Mechanical ventilation

Fundamentals	Modes of mechanical ventilation Mechanical ventilation in emergencies Mechanical ventilation in neonates Nomenclature of mechanical ventilation

Modes	IMV/SIMV CMV ACV CSV PAP BPAP CPAP APRV MMV PAV ASV HFV

Related illness	ARDS Pulmonary barotrauma Pulmonary volutrauma Ventilator-associated pneumonia Oxygen toxicity Ventilator-associated lung injury

Pressure	P_EEP FiO₂ Δ_P P_IP P_S P_AW P_plat

Volumes	V_T V_E V_f

Other	C_dyn C_static P_AO₂ V_D/V_T OI A-a gradient

References

↑ Logan, Carolynn M.; Rice, M. Katherine (1987). Logan's Medical and Scientific Abbreviations. Philadelphia: J. B. Lippincott Company. p. 4. ISBN 0-397-54589-4.
↑ "iROCKET Learning Module: Intro to Arterial Blood Gases, Pt. 1". Retrieved 2008-11-14.
↑ "Alveolar-arterial Gradient". Retrieved 2008-11-14.
↑ Costanzo, Linda (2006). BRS Physiology. Hagerstown: Lippincott Williams & Wilkins. ISBN 0-7817-7311-3.

External links

A-a Oxygen Gradient online calculator

Respiratory physiology

Respiration	breath inhalation exhalation respiratory rate respirometer pulmonary surfactant compliance elastic recoil hysteresivity airway resistance bronchial hyperresponsiveness constriction dilatation mechanical ventilation

Control	pons pneumotaxic center apneustic center medulla dorsal respiratory group ventral respiratory group chemoreceptors central peripheral pulmonary stretch receptors Hering–Breuer reflex

Lung volumes	VC FRC Vt dead space CC PEF calculations respiratory minute volume FEV1/FVC ratio Lung function tests spirometry body plethysmography peak flow meter nitrogen washout

Circulation	pulmonary circulation hypoxic pulmonary vasoconstriction pulmonary shunt

Interactions	Perfusion (V) ventilation (Q) ventilation/perfusion scan zones of the lung gas exchange pulmonary gas pressures alveolar gas equation alveolar–arterial gradient hemoglobin oxygen–haemoglobin dissociation curve (Oxygen saturation 2,3-BPG Bohr effect Haldane effect) carbonic anhydrase (chloride shift) oxyhemoglobin respiratory quotient arterial blood gas diffusion capacity (DLCO)

Insufficiency	high altitude oxygen toxicity hypoxia

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Alveolar–arterial gradient

Equation

Values and meaning

See also

References

External links