Alveolar–arterial O₂ gradient
Distinguishes hypoxemia due to hypoventilation or low inspired pressure (normal gradient) from V/Q mismatch, shunt or diffusion impairment (increased gradient).
Calculate with transparency
Check the population, units and version. The result shows the formula or classification; it does not make an automatic treatment decision.
Preparing the form…
- Check the method
- Enter the data
- Check the result
Method · Limits of application
Alveolar equation RQ 0.8/vapour 47 mmHg; Mellemgaard 1966 gradient 2.5+0.21 age; age/4+4 approximation
The calculation uses a water-vapor pressure of 47 mmHg at 37 °C and a fixed respiratory quotient of 0.8; this is a steady-state assumption and can vary with diet. Enter FiO₂ as a percentage, pressures in mmHg and an appropriate barometric pressure. The approximate age-based reference must not be automatically extrapolated to high FiO₂ or altitude. The gradient helps assess oxygenation but does not, on its own, determine the cause of hypoxemia. The coefficients of the local age-based reference still require checking against the complete cited study.
Conditions of use
Check the population, units, inclusion and exclusion criteria, and version in the original source. A result alone does not establish a diagnosis, discharge decision or prescription.
Documented parameters
- FiO₂ · %
- PaO₂ · mmHg
- PaCO₂ · mmHg
- Age · years
- Local barometric pressure (default 760) · mmHg
4/4 reference cases checked. Numerical tests are not clinical validation.
Relationship with cancer research
General context — relationship to be defined
A resource for general clinical contexts. This edition does not assign a specific oncological relationship; selecting it for a study requires a justified question, population and purpose.