A-a Gradient & P/F Suite
A-a Gradient & P/F Suite - a deterministic tool in Sophie Well's Clinical Math & Conversions group.
Open the A-a Gradient & P/F Suite → Runs in your browser. No signup, no tracking.
What this is
Run the two bedside oxygenation indices side by side. The A-a gradient uses the alveolar gas equation - PAO2 = FiO2*(Patm - PH2O) - PaCO2/RQ - to estimate the alveolar-to-arterial oxygen difference, with age-adjusted upper limits of normal. The P/F ratio (PaO2/FiO2) is the ARDS Berlin Definition's severity axis - mild 200-300, moderate 100-200, severe under 100 - and applies whether the patient is on room air or a known FiO2. Sophie Well computes both from a single ABG plus FiO2 input.
When to use it
Use this when a patient is hypoxemic and you want to distinguish a V/Q-mismatch / shunt picture (elevated A-a gradient) from pure hypoventilation (normal A-a gradient with high CO2), or when you are documenting ARDS severity for a multidisciplinary chart. Both numbers are bedside-fast and require no specialized equipment beyond an ABG; together they steer the workup toward parenchymal versus pump versus ventilatory failure faster than either does alone.
References
West JB. Respiratory Physiology. ARDS Berlin Definition: JAMA 2012;307(23):2526.
Worked example: A-a gradient ~9.7 mmHg (expected ~14 by age); P/F ratio 429 (Normal).
Sophie Well is a reference and educational tool. Not medical, legal, or financial advice. Does not replace clinician judgment, professional billing review, or legal counsel.