A-a gradient: Difference between revisions
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==Overview== | ==Overview== | ||
The '''Alveolar-arterial gradient''' (A-a gradient), is used to compare the causes of [[hypoxemia]]. | The '''Alveolar-arterial gradient''' (A-a gradient), is used to compare the causes of [[hypoxemia]]. | ||
== A-a | ===A-a Gradient Equation=== | ||
'''A - a gradient = PA<sub>O2</sub> - Pa<sub>O2</sub>''' | '''A - a gradient = PA<sub>O2</sub> - Pa<sub>O2</sub>''' | ||
Where: | Where: | ||
<br>'''PA<sub>O2</sub>''' = alveolar PO<sub>2</sub> (calculated from the alveolar gas equation) | <br>'''PA<sub>O2</sub>''' = alveolar PO<sub>2</sub> (calculated from the alveolar gas equation) | ||
<br>'''Pa<sub>O2</sub>''' = arterial PO<sub>2</sub> (measured in arterial blood) | <br>'''Pa<sub>O2</sub>''' = arterial PO<sub>2</sub> (measured in arterial blood) | ||
== The Alveolar Gas | ===The Alveolar Gas Equation=== | ||
'''PA<sub>O2</sub> = PI<sub>O2</sub> - PA<sub>CO2</sub>/R''' | '''PA<sub>O2</sub> = PI<sub>O2</sub> - PA<sub>CO2</sub>/R''' | ||
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<br>'''R''' = respiratory exchange ratio or respiratory quotient = 0.8 under normal conditions | <br>'''R''' = respiratory exchange ratio or respiratory quotient = 0.8 under normal conditions | ||
== Values and Meaning | ===Values and Meaning=== | ||
The normal A-a gradient is < 10 mmHg, but can range from 5-20 mmHg in a normal individual. An increased A-a gradient suggests a diffusion defect, V/Q (ventilation/perfusion) defect, or right-to-left shunt.<sup>1</sup> | The normal A-a gradient is < 10 mmHg, but can range from 5-20 mmHg in a normal individual. An increased A-a gradient suggests a diffusion defect, V/Q (ventilation/perfusion) defect, or right-to-left shunt.<sup>1</sup> | ||
---- | ---- | ||
<sup>1</sup>{{cite book | last = Costanzo | first = Linda | title = Brs Physiology | publisher = Lippincott Williams & Wilkins | location = Hagerstwon | year = 2006 | isbn = 0781773113 }} | <sup>1</sup>{{cite book | last = Costanzo | first = Linda | title = Brs Physiology | publisher = Lippincott Williams & Wilkins | location = Hagerstwon | year = 2006 | isbn = 0781773113 }} | ||
[[Category:Pulmonology]] | [[Category:Pulmonology]] | ||
Revision as of 17:51, 26 September 2011
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Overview
The Alveolar-arterial gradient (A-a gradient), is used to compare the causes of hypoxemia.
A-a Gradient Equation
A - a gradient = PAO2 - PaO2
Where:
PAO2 = alveolar PO2 (calculated from the alveolar gas equation)
PaO2 = arterial PO2 (measured in arterial blood)
The Alveolar Gas Equation
PAO2 = PIO2 - PACO2/R
Where:
PIO2 = FIO2 x (760 mmHg - 47mmHg) = inspired O2
- FIO2 = 21% at sea level atmospheric pressure = 0.21
- 760 mmHg = atmospheric pressure at sea level
- 47 mmHg = H2O pressure
PACO2 = arterial PCO2 measured in plasma
R = respiratory exchange ratio or respiratory quotient = 0.8 under normal conditions
Values and Meaning
The normal A-a gradient is < 10 mmHg, but can range from 5-20 mmHg in a normal individual. An increased A-a gradient suggests a diffusion defect, V/Q (ventilation/perfusion) defect, or right-to-left shunt.1
1Costanzo, Linda (2006). Brs Physiology. Hagerstwon: Lippincott Williams & Wilkins. ISBN 0781773113.