Aa Gradient Calculator

Compute the Alveolar Arterial Oxygen Gradient

The alveolar arterial (A-a) oxygen gradient compares alveolar oxygen pressure with arterial oxygen pressure from a blood gas sample. An elevated gradient suggests impaired gas exchange from ventilation perfusion mismatch, shunt, or diffusion limitation. Enter age, fraction of inspired oxygen (FiO₂), PaCO₂, PaO₂, atmospheric pressure, and water vapor pressure to get instant results with step by step math.

A-a Gradient Formulas

$$P_AO_2 = F_iO_2 \times (P_{atm} - P_{H_2O}) - \dfrac{PaCO_2}{0.8}$$

$$A-a\ gradient = P_AO_2 - PaO_2$$

$$Expected\ gradient = \dfrac{age}{4} + 4$$

FiO₂ is entered as a percent and converted to a fraction. The 0.8 factor reflects respiratory quotient assumptions in the alveolar gas equation.

Clinical Context

Young healthy adults often have a small gradient. Expected values rise with age even when lungs are normal. Compare measured and expected gradients together rather than relying on a single cutoff. For related respiratory tools, try the BMI Calculator when assessing obesity related hypoventilation risk.