fraction (0.21 = room air)
⚕️ For clinical reference only. Not a substitute for professional judgment.
A–a Gradient Respiratory
Alveolar-arterial oxygen gradient — differentiates hypoxaemia causes. Elevated A-a gradient suggests ventilation-perfusion mismatch or shunt.
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Enter ABG values
Enter FiO2, PaCO2, PaO2, and patient age.
ABG Parameters
mmHg
mmHg
years
Formula
PAO2 = (FiO2 x (760-47)) - (PaCO2 / 0.8)
A-a Gradient = PAO2 - PaO2
Normal = Age/4 + 4 (approximate upper limit)
Elevated: V/Q mismatch, shunt, diffusion impairment
Normal: hypoventilation, low FiO2
When to use
- Unexplained hypoxaemia — elevated gradient confirms intrapulmonary cause
- PE assessment — elevated A-a gradient common in PE
- Differentiating pneumonia, ARDS, shunt from hypoventilation
- Assessing response to supplemental oxygen