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





    Chief Complaint


    Organ System


    Patent Pending

    A-a O₂ Gradient

    Assesses for degree of shunting and V/Q mismatch.
    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


    Please fill out required fields.

    Next Steps
    Creator Insights


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


    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 Hypoxemia A-a O2 Gradient Shift
    1 V/Q Mismatch (ex: PNA, CHF, PE, ARDS, atelectasis, etc) Elevation
    2 Shunt (ex: PFO, ASD, pulmonary AVMs) Elevation
    3 Alveolar Hypoventilation (ex: interstitial lung dz, environmental lung dz, PCP PNA) Elevation
    4 Hypoventilation (ex: COPD, CNS d/o, neuromuscular dz, etc) Depression
    5 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

    About the Creator
    Dr. H. Fred Helmholz