Calc Function

    • Calcs that help predict probability of a diseaseDiagnosis
    • Subcategory of 'Diagnosis' designed to be very sensitiveRule Out
    • Disease is diagnosed: prognosticate to guide treatmentPrognosis
    • Numerical inputs and outputsFormula
    • Med treatment and moreTreatment
    • Suggested protocolsAlgorithm

    Disease

    Select...

    Specialty

    Select...

    Chief Complaint

    Select...

    Organ System

    Select...

    Patent Pending

    A-a O₂ Gradient

    Assesses for degree of shunting and V/Q mismatch.
    Favorite
    When to Use
    Why Use
    • Patients with unexplained hypoxia.
    • Patients with hypoxia exceeding the degree of their clinical illness.

    The A-a Gradient can help determine the cause of hypoxia; it pinpoints the location of the hypoxia as intra- or extra-pulmonary.

    mm Hg
    mm Hg
    %
    mm Hg
    years

    Result:

    Please fill out required fields.

    Next Steps
    Evidence
    Creator Insights

    Advice

    After calculating the A-a Gradient and determining if it is elevated or normal, one can rule out several causes of hypoxia.

    Formula

    A-a O2 Gradient = [ (FiO2) × (Atmospheric Pressure - H2O Pressure) - (PaCO2/0.8) ] – PaO2 from ABG

    Normal Gradient Estimate = (Age/4) + 4

    Facts & Figures

    The 5 Causes of Hypoxemia

    Causes of HypoxemiaA-a O2 Gradient Shift
    1V/Q Mismatch (ex: PNA, CHF, ARDS, atelectasis, etc)Elevation
    2Shunt (ex: PFO, ASD, PE, pulmonary AVMs)Elevation
    3Alveolar Hypoventilation (ex: interstitial lung dz, environmental lung dz, PCP PNA)Elevation
    4Hypoventilation (ex: COPD, CNS d/o, neuromuscular dz, etc)Depression
    5Low FiO2 (ex: high altitude)Depression
    Dr. H. Fred Helmholz

    About the Creator

    H. Fred Helmholz, MD, (d. 2012) spent most of his career at the Mayo Graduate School of Medicine. He was active in most aspects of respiratory care which included research and clinical practice in high-altitude medicine, oxygen therapy, pulmonary function testing, and mechanical ventilation.

    To view Dr. H. Fred Helmholz's publications, visit PubMed