Respiratory

A-a Gradient

Calculates the alveolar-arterial oxygen gradient to determine causes of hypoxia.

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Parameters

A-a Gradient Result

Formula

PAO₂ = [FiO₂ × (Patm - PH₂O)] - (PaCO₂ / 0.8)
A-a Gradient = PAO₂ - PaO₂
Expected A-a Gradient = (Age / 4) + 4
*Assumes Patm=760 mmHg and PH₂O=47 mmHg.

Clinical Pearls

An elevated A-a gradient indicates an intrinsic lung issue (like PE, pneumonia, ARDS, or pulmonary edema). A normal A-a gradient in a hypoxic patient points to an extrinsic cause (opiate overdose causing hypoventilation, or high altitude).

Limitations

Calculation relies on assumptions of atmospheric pressure and respiratory quotient which can vary. The gradient also artificially widens when a patient is placed on supplemental oxygen (high FiO2).