Alveolar-arterial oxygen gradient
Calculates PAO2 with the alveolar gas equation and the gradient against the measured PaO2.
When to use
Hypoxaemia where the alveolar and extrapulmonary contributions need separating, with a blood gas and a known FiO2.
Pearls and pitfalls
- Barometric pressure enters the equation: about 760 mmHg at sea level, and it falls with altitude.
- The respiratory exchange ratio is assumed to be 0.8, the value used in the alveolar gas equation.
- The expected gradient rises with age, roughly a quarter of the age plus four.
- The respiratory index is the gradient divided by PaO2 and tracks the severity of gas exchange.
Fill in the fields to see the result.
Formula
PAO2 = (barometric pressure − 47) × FiO2 − PaCO2 ÷ 0.8; gradient = PAO2 − PaO2; expected for age = age ÷ 4 + 4; respiratory index = gradient ÷ PaO2.
Evidence
Alveolar gas equation applied to blood gas values, with the age expected gradient and the respiratory index described in a cohort of patients with COVID-19.
References
- Alveolar Arterial Gradient and Respiratory Index in Predicting the Outcome of COVID-19 Patients; a Retrospective Cross-Sectional Study. Arch Acad Emerg Med. 2022;10(1):e27. doi:10.22037/aaem.v10i1.1543
- Central hypoventilation with hypercapnia: the silent face of ventilatory control. J Bras Pneumol. 2026;52(1):e20260320. doi:10.36416/1806-3756/e20260320
Frequently asked questions
What is Alveolar-arterial oxygen gradient?
Calculates PAO2 with the alveolar gas equation and the gradient against the measured PaO2. Hypoxaemia where the alveolar and extrapulmonary contributions need separating, with a blood gas and a known FiO2.
How is the result calculated?
PAO2 = (barometric pressure − 47) × FiO2 − PaCO2 ÷ 0.8; gradient = PAO2 − PaO2; expected for age = age ÷ 4 + 4; respiratory index = gradient ÷ PaO2.
How should the result be interpreted?
Alveolar gas equation applied to blood gas values, with the age expected gradient and the respiratory index described in a cohort of patients with COVID-19.
What are the sources?
Alveolar Arterial Gradient and Respiratory Index in Predicting the Outcome of COVID-19 Patients; a Retrospective Cross-Sectional Study. Arch Acad Emerg Med. 2022;10(1):e27. Central hypoventilation with hypercapnia: the silent face of ventilatory control. J Bras Pneumol. 2026;52(1):e20260320.