What is the A-a gradient?

The alveolar-arterial (A-a) oxygen gradient is the difference between the calculated alveolar partial pressure of oxygen (PAO2) and the measured arterial PaO2. A normal gradient means oxygen freely crosses from alveoli to blood; an elevated gradient indicates a barrier — such as a V/Q mismatch, shunt, or diffusion impairment.

What is a normal A-a gradient?

In a young healthy adult breathing room air at sea level, the A-a gradient is roughly 5–15 mmHg. It increases physiologically with age: a commonly used estimate is (Age / 4) + 4 mmHg. Values above 25–30 mmHg in most adults suggest significant pulmonary pathology.

What causes an elevated A-a gradient?

Elevated gradients are caused by V/Q mismatch (most common — pneumonia, PE, COPD exacerbation), intrapulmonary shunt (ARDS, atelectasis, hepatopulmonary syndrome), and diffusion impairment (interstitial lung disease, pulmonary edema). Hypoventilation alone (e.g., opioid overdose) produces hypoxemia with a NORMAL A-a gradient.

Why does FiO2 matter?

The alveolar gas equation uses FiO2 to compute PAO2. Room air is 0.21 (21%). Supplemental oxygen raises FiO2, which raises PAO2. The A-a gradient can widen on high FiO2 in shunt physiology, making FiO2 an important variable when comparing results over time.

What respiratory quotient (RQ) does this tool use?

The tool defaults to RQ = 0.8, which is the standard assumed value for a mixed diet. This means the alveolar equation used is: PAO2 = (FiO2 × (Patm − 47)) − (PaCO2 / 0.8). Some references use 1.0 for simplicity, but 0.8 is clinically conventional.

Is this tool a substitute for clinical judgment?

No. The A-a gradient is one piece of data. Always integrate it with the full clinical picture, ABG interpretation, imaging, and history. This tool is for educational and reference use only.

A-a Oxygen Gradient Calculator

Name: A-a Oxygen Gradient Calculator
Availability: InStock
Author: Nham Vu

Enter patient vitals and arterial blood gas values to calculate the A-a oxygen gradient and interpret the likely cause of hypoxemia.

Patient & ABG Values

FiO₂ — Fraction of Inspired O₂

Enter a value between 0.21 (room air) and 1.00 (100% O₂).

Atmospheric Pressure (P_atm) — mmHg

Default 760 mmHg at sea level. Adjust for altitude or hypobaric conditions.

PaCO₂ — Arterial CO₂ (mmHg)

Normal range: 35–45 mmHg. From the arterial blood gas report.

PaO₂ — Arterial O₂ (mmHg)

Normal range on room air: 80–100 mmHg. From the arterial blood gas report.

Patient Age (years) — for normal range reference

Enter values on the left and click Calculate to see the A-a gradient.

Summary

Enter patient vitals and arterial blood gas values to calculate the A-a oxygen gradient and interpret the likely cause of hypoxemia.

How it works

Enter the fraction of inspired oxygen (FiO2) — 0.21 for room air, higher for supplemental oxygen.
Enter the atmospheric pressure (default 760 mmHg at sea level) or select a common altitude preset.
Enter the arterial PaCO2 from the arterial blood gas (ABG) report.
Enter the arterial PaO2 from the ABG report.
The tool computes the alveolar PO2 (PAO2) using the alveolar gas equation, then subtracts PaO2 to yield the A-a gradient.
Results include an age-adjusted normal range and an interpretation of the most likely cause of hypoxemia.

Use cases

Distinguishing intrinsic lung disease from hypoventilation as the cause of low PaO2.
Evaluating pulmonary embolism, pneumonia, ARDS, or pulmonary edema at the bedside.
Monitoring a patient on supplemental oxygen to ensure oxygenation is improving.
Teaching residents and medical students the alveolar gas equation and its clinical relevance.
Rapid triage in the emergency department or ICU for patients with acute dyspnea.
Assessing patients at altitude where baseline atmospheric pressure differs.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu