Gradient

Calculator for the alveolar–arterial oxygen gradient \(A\!-\!a = PAO_2 - PaO_2\), using the alveolar gas equation to estimate \(PAO_2\).

Gas Exchange Inputs

Default teaching value is often 0.8.

A–a Gradient Output

Alveolar gas equation:
Gradient formula:
Estimated PAO2:
Compute A–a:
Current A–a gradient
0.0
mmHg
Interpretation:

A–a Gradient Visualizer

A–a Gradient Scale
Higher gradients reflect greater gas-exchange abnormality
Lower gradient Higher gradient
0102550100

Clinical Significance

The A–a gradient estimates the difference between available alveolar oxygen and measured arterial oxygen. It is commonly used to assess whether hypoxemia is likely due primarily to hypoventilation or to impaired oxygen transfer.

A lower A–a gradient suggests that alveolar oxygen and arterial oxygen remain relatively close, while a larger gradient suggests impaired diffusion, ventilation-perfusion mismatch, shunt, or other gas-exchange abnormalities. The gradient is especially useful when interpreted alongside PaCO₂, FiO₂, and the broader clinical scenario.

As a broad educational framework, smaller gradients are relatively reassuring, while progressively larger gradients reflect greater oxygen transfer abnormality. Exact interpretation should be tied to age, FiO₂ level, altitude, and disease state.

The calculated value depends on the assumptions used in the alveolar gas equation, including barometric pressure, water vapor pressure, and respiratory quotient. Normal expected gradient also increases with age, so the value should not be interpreted in isolation.

References (APA 7th Edition)

  1. West, J. B., & Luks, A. M. (2021). West’s respiratory physiology: The essentials (11th ed.). Wolters Kluwer.
  2. Lumb, A. B., & Thomas, C. R. (2020). Nunn’s applied respiratory physiology (9th ed.). Elsevier.
  3. Marino, P. L. (2014). The ICU book (4th ed.). Wolters Kluwer.
  4. Kacmarek, R. M., Stoller, J. K., & Heuer, A. J. (2022). Egan’s fundamentals of respiratory care (12th ed.). Elsevier.