A-a Gradient
A-a Gradient - a deterministic tool in Sophie Well's Clinical Math & Conversions group.
Open the A-a Gradient → Runs in your browser. No signup, no tracking.
What this is
Compute the alveolar-arterial oxygen gradient using the standard alveolar gas equation: PAO2 = FiO2 * (760 - 47) - (PaCO2 / 0.8), then A-a = PAO2 - PaO2. Sophie Well returns the gradient in mmHg along with the age-adjusted upper limit (roughly age/4 + 4) so a normal-vs-elevated read is one glance away.
When to use it
Use this when a patient is hypoxemic and you need to differentiate the five physiologic causes: a normal A-a gradient points to hypoventilation or low inspired oxygen; a widened gradient points to V/Q mismatch, shunt, or diffusion limitation. The standard formula assumes sea-level barometric pressure and water-vapor partial pressure; at altitude or on supplemental O2, interpret the absolute value with care.
References
West JB. Respiratory Physiology: The Essentials. PAO2 = (FiO2 * (760-47)) - (PaCO2 / 0.8).
Worked example: A-a gradient ~9.7 mmHg
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.