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A-a Gradient (Alveolar-Arterial O₂)

· Last updated：2026-08-10

This tool is for informational purposes only and does not constitute medical, financial, or legal advice; consult a qualified professional for advice.

Calculate the alveolar-arterial O₂ gradient to distinguish hypoxemia causes. PAO₂ = FiO₂×(P_atm−47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂. Normal ≈ (Age+10)/4. Elevated → V/Q mismatch, shunt, diffusion impairment. Derived from StatPearls, LITFL, Cornell PICU, and Martin 1999. Not medical advice.

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Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category
Health
Input types
number, select
Output type
json
Sample coverage
4
API ready
Yes

Overview

The A-a Gradient calculator estimates the alveolar-arterial oxygen gradient from PaO₂, PaCO₂, FiO₂, age, and atmospheric pressure. It calculates PAO₂ with the alveolar gas equation, then compares the result with an age-adjusted expected normal value to help assess hypoxemia patterns. It is not medical advice.

When to use

  • Assess whether hypoxemia is associated with a normal or elevated A-a gradient.
  • Review arterial blood gas results using a selected FiO₂ and atmospheric pressure.
  • Support clinical reasoning about hypoventilation versus conditions associated with V/Q mismatch, shunt, or diffusion impairment.

How it works

  1. 1Enter PaO₂ and PaCO₂ values from an arterial blood gas.
  2. 2Select the inspired oxygen fraction, or FiO₂, used when the sample was collected.
  3. 3Enter the patient’s age; optionally adjust atmospheric pressure for altitude instead of using the default sea-level value of 760 mmHg.
  4. 4The calculator computes PAO₂, subtracts PaO₂ to find the A-a gradient, and reports the expected normal value, ratio, and category.

Use cases

  • Interpret arterial blood gas results in respiratory and critical-care education.
  • Compare oxygenation calculations at sea level and at altitude.
  • Estimate whether a low PaO₂ occurs with an approximately normal or significantly elevated A-a gradient.

Examples

1. Elevated A-a gradient on room air

Clinician or medical learner

Background

A 60-year-old patient has an arterial blood gas showing PaO₂ of 55 mmHg and PaCO₂ of 32 mmHg while breathing room air.

Problem

Determine whether the A-a gradient is within the expected age-adjusted range.

How to use

Enter PaO₂ 55, PaCO₂ 32, select FiO₂ 0.21, enter age 60, and leave atmospheric pressure at its default value.

{"pao2":55,"paco2":32,"fio2":"0.21","age":60,"decimalPlaces":1}

Outcome

The calculator estimates PAO₂ at 109.7 mmHg and an A-a gradient of 54.7 mmHg, compared with an expected normal value of 17.5 mmHg. It categorizes the result as a significantly elevated A-a gradient.

2. Normal A-a gradient with elevated PaCO₂

Medical student or respiratory educator

Background

A 40-year-old patient has PaO₂ of 60 mmHg and PaCO₂ of 70 mmHg on room air.

Problem

Assess whether the low PaO₂ is accompanied by a widened A-a gradient.

How to use

Enter PaO₂ 60, PaCO₂ 70, select FiO₂ 0.21, enter age 40, and use the default atmospheric pressure.

FAQ

What does the A-a gradient measure?

It is the difference between estimated alveolar oxygen pressure, PAO₂, and arterial oxygen pressure, PaO₂.

What formula does the calculator use?

PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / 0.8, and A-a = PAO₂ − PaO₂.

What does a normal A-a gradient suggest?

A normal value can be consistent with hypoventilation or other causes of low oxygen that do not substantially widen the alveolar-arterial difference.

What does an elevated A-a gradient suggest?

An elevated value may occur with V/Q mismatch, shunt, or diffusion impairment, but it must be interpreted with the clinical context.

Why can atmospheric pressure be changed?

Atmospheric pressure is lower at altitude, so adjusting it can make the PAO₂ estimate more representative of the measurement conditions.

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{"pao2":60,"paco2":70,"fio2":"0.21","age":40,"decimalPlaces":1}

Outcome

The estimated PAO₂ is 62.2 mmHg and the A-a gradient is 2.2 mmHg, below the expected normal value of 12.5 mmHg. The calculator categorizes it as a normal A-a gradient.