A-a gradient

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The Alveolar-arterial gradient (A-a gradient), is used to compare the causes of hypoxemia.

A-a gradient Equation

A - a gradient = PA_O2 - Pa_O2

Where:
PA_O2 = alveolar PO₂ (calculated from the alveolar gas equation)
Pa_O2 = arterial PO₂ (measured in arterial blood)

The Alveolar Gas equation:

PA_O2 = PI_O2 - PA_CO2/R

Where:
PI_O2 = FI_O2 x (760 mmHg - 47mmHg) = inspired O₂
       - FI_O2 = 21% at sea level atmospheric pressure = 0.21
       - 760 mmHg = atmospheric pressure at sea level
       - 47 mmHg = H₂O pressure
PA_CO2 = 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

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¹Costanzo, Linda (2006). Brs Physiology. Hagerstwon: Lippincott Williams & Wilkins. ISBN 0781773113. 

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Articles on A-a gradient	Most recent articles on A-a gradient • Most cited articles on A-a gradient • Review articles on A-a gradient • Articles on A-a gradient in N Eng J Med, Lancet, BMJ
Media (Slides, Video, Images, MP3) on A-a gradient	Powerpoint slides on A-a gradient • Images of A-a gradient • Photos of A-a gradient • Podcasts & MP3s on A-a gradient • Videos on A-a gradient
Evidence Based Medicine Regarding A-a gradient	Cochrane Collaboration on A-a gradient • Bandolier on A-a gradient • TRIP on A-a gradient
Cost Effectiveness of A-a gradient	Cost Effectiveness of A-a gradient
Clinical Trials Involving A-a gradient	Ongoing Trials on A-a gradient at Clinical Trials.gov • Trial results on A-a gradient • Clinical Trials on A-a gradient at Google
Guidelines / Policies / Government Resources (FDA/CDC) Regarding A-a gradient	US National Guidelines Clearinghouse on A-a gradient • NICE Guidance on A-a gradient • NHS PRODIGY Guidance • FDA on A-a gradient • CDC on A-a gradient
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Pharmacology Resources on A-a gradient	Dosing of A-a gradient • Drug interactions with A-a gradient • Side effects of A-a gradient • Allergic reactions to A-a gradient • Overdose information on A-a gradient • Carcinogenicity information on A-a gradient • A-a gradient in pregnancy • Pharmacokinetics of A-a gradient •
Genetics, Pharmacogenomics, and Proteinomics of A-a gradient	Genetics of A-a gradient • Pharmacogenomics of A-a gradient • Proteomics of A-a gradient
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Commentary on A-a gradient	Blogs on A-a gradient
Patient Resources on A-a gradient	Patient resources on A-a gradient • Discussion groups on A-a gradient • Patient Handouts on A-a gradient • Directions to Hospitals Treating A-a gradient • Risk calculators and risk factors for A-a gradient
Healthcare Provider Resources on A-a gradient	Symptoms of A-a gradient • Causes & Risk Factors for A-a gradient • Diagnostic studies for A-a gradient • Treatment of A-a gradient
Continuing Medical Education (CME) Programs on A-a gradient	CME Programs on A-a gradient
International Resources on A-a gradient	A-a gradient en Espanol • A-a gradient en Francais
Business Resources on A-a gradient	A-a gradient in the Marketplace • Patents on A-a gradient
Informatics Resources on A-a gradient	List of terms related to A-a gradient

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Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .