• ٧٨٦
Enroll For Free
← Back to All Calculators
Clinical Calculators
Alveolar-Arterial Oxygen Gradient
Clinical Calculator

Alveolar-Arterial (A-a) Oxygen Gradient

Calculates the A-a gradient, P/F ratio, a/A ratio, and respiratory index to assess the degree of pulmonary gas exchange impairment from ABG values.

Patient & ABG Values
► Advanced Options — Water Vapor Pressure & Respiratory Quotient

RQ is typically 0.8 on a mixed diet. Values range from 0.7 (fat metabolism) to 1.0 (carbohydrate metabolism). Water vapor pressure is 47 mmHg at normal body temperature (37°C).

Calculated Results
PAO2 — Alveolar O2
— mmHg
Calculated
A-a Gradient
— mmHg
Expected A-a Gradient
— mmHg
2.5 + 0.21 × Age
P/F Ratio
— mmHg
a/A Ratio
—
Respiratory Index
—

Clinical Interpretation

    Hypoxemia Severity (PaO2)
    Formulas Used

    Equations

    PAO2     = FiO2 x (Patm - PH2O) - PaCO2 / RQ
A-a      = PAO2 - PaO2
Exp A-a  = 2.5 + 0.21 x Age   (primary formula)
           Age / 4 + 4          (simplified formula)
P/F      = PaO2 / FiO2(fraction)
a/A      = PaO2 / PAO2
RI       = (PAO2 - PaO2) / PaO2   [= A-a / PaO2]
    Reference — Normal Values
    Parameter Normal Value Unit
    PAO2 (room air)~100mmHg
    PaO2 (room air)80–100mmHg
    A-a gradient (young, <30 yrs)<10mmHg
    A-a gradient (elderly)<20–25mmHg
    P/F ratio≥400mmHg
    a/A ratio>0.75—
    Respiratory Index<0.4—
    ARDS — Berlin Criteria (P/F Ratio)
    Berlin Definition of ARDS — Requires bilateral infiltrates + non-cardiogenic pulmonary edema + PEEP ≥5 cmH2O
    Mild ARDS
    P/F 200–300
    with PEEP ≥5 cmH2O
    Moderate ARDS
    P/F 100–200
    with PEEP ≥5 cmH2O
    Severe ARDS
    P/F <100
    with PEEP ≥5 cmH2O
    Medical Disclaimer: This calculator is intended for educational and clinical reference purposes only. All results must be interpreted within the complete clinical context by a qualified healthcare professional. Do not use as a sole basis for clinical decisions. Always consult current clinical guidelines and institutional protocols.

    Select a Template for your Cv