A–a Gradient Calculator (Alveolar Gas Equation) for Dogs & Cats
Alveolar oxygen (PAO₂), the alveolar–arterial oxygen gradient and the PaO₂/FiO₂ ratio from an arterial blood gas.
Results are calculated in your browser. Always double-check before administering and use your clinical judgement.
How it is calculated
PAO₂ = (PB − PH₂O) × FiO₂ − (PaCO₂ ÷ 0.8), with PH₂O = 47 mmHg [1].
A–a gradient = PAO₂ − PaO₂. The normal A–a oxygen difference in an animal breathing room air is less than 15 mmHg at sea level, although up to 25 mmHg is considered normal [1].
PaO₂/FiO₂ ratio = PaO₂ ÷ FiO₂.
Worked example
Room air at sea level, PaCO₂ 40, PaO₂ 90: PAO₂ = 99.7 → A–a = 9.7 mmHg.
Frequently asked questions
What is a normal A–a gradient in dogs?
Less than 15 mmHg in an animal breathing room air at sea level, although up to 25 mmHg is considered normal [1].
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References
For veterinary professionals. Information supports — never replaces — clinical judgement; always check the current product label and local regulations.