Alveolar–Arterial Oxygen Gradient (A–aDO₂)
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Fixed assumptions: barometric pressure 760 mmHg (sea level), water vapor 47 mmHg at 37 °C and respiratory exchange ratio 0.8. This model may be inaccurate with altitude, pressure changes, high oxygen concentration or unknown delivered FiO₂. Oxygen flow rate is not converted into FiO₂.
A-aDO₂ mmHg
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Estimated alveolar PO₂ (PAO₂) — mmHg
Method: simplified alveolar gas equation (PB 760, PH₂O 47, R 0.8)
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Formula, assumptions and sources
Formula
Estimated PAO₂ = FiO₂ × (760 − 47) − PaCO₂/0.8; A–aDO₂ = PAO₂ − PaO₂ (mmHg). FiO₂ can be entered as an explicit fraction or percentage. Changing this unit clears the previous FiO₂ value.
Hypothetical example
Sea level, FiO₂ 0.21, PaO₂ 90 mmHg, PaCO₂ 40 mmHg: PAO₂ 99.73 mmHg, A–aDO₂ 9.73 mmHg.
Scope
This simplified model does not adjust barometric pressure, respiratory quotient or inspired gas flow. Negative gradients are displayed without clamping and require reassessment of assumptions, FiO₂, samples and timing. It does not diagnose conditions, provide age-specific norms or recommend oxygen therapy.
Review
Sources checked 2026-11-10.