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Current section: Calculators

Alveolar-Arterial (A-a) Gradient Calculator

Parameters

RequiredAllowed: ≥ 0 mmHg

RequiredAllowed: 21–100 %

RequiredAllowed: ≥ 0 mmHg

RequiredAllowed: ≥ 0 years

RequiredAllowed: ≥ 0 mmHg

RequiredAllowed: ≥ 0.1

Clinical context

Common causes of an elevated A-a gradient: • V/Q mismatch (e.g., pneumonia, asthma/COPD exacerbation, pulmonary embolism) • Shunt physiology (e.g., pneumonia, pulmonary edema, ARDS) • Diffusion limitation (e.g., interstitial lung disease, pulmonary fibrosis) Formulas used: 1) A-a gradient = PAO2 − PaO2, where PAO2 = FiO2 × (Patm − PH2O) − PaCO2/RQ 2) Expected A-a gradient ≈ (Age + 10) / 4 Notes: • RQ (respiratory quotient) is usually about 0.8 and varies with metabolic state. • PH2O (water vapor pressure) is 47 mmHg at 37°C.