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Respiratory

Alveolar–Arterial (A–a) Oxygen Gradient

Difference between alveolar and arterial oxygen tension — assesses the cause of hypoxaemia (gas-exchange defect vs hypoventilation).

Scope: Most accurate on room air. Uses respiratory quotient 0.8; the gradient widens normally with higher FiO₂.

Inputs

FieldUnitNotes
FiO₂ Fraction, 0.21 (room air) to 1.0
PaCO₂ mmHg 5–150
PaO₂ mmHg 20–700
Atmospheric pressure (optional) mmHg Default 760 (sea level)

FAQ

What is the Alveolar–Arterial (A–a) Oxygen Gradient?

Difference between alveolar and arterial oxygen tension — assesses the cause of hypoxaemia (gas-exchange defect vs hypoventilation).

What inputs does the Alveolar–Arterial (A–a) Oxygen Gradient need?

FiO₂, PaCO₂ (mmHg), PaO₂ (mmHg), Atmospheric pressure (optional) (mmHg).

Are there any limitations?

Most accurate on room air. Uses respiratory quotient 0.8; the gradient widens normally with higher FiO₂.

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