A-a Gradient (Alveolar-Arterial O₂)

A-a Gradient (Alveolar-Arterial O₂) | Online Free Tool | Elysia Tools

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

1Enter PaO₂ and PaCO₂ values from an arterial blood gas.
2Select the inspired oxygen fraction, or FiO₂, used when the sample was collected.
3Enter the patient’s age; optionally adjust atmospheric pressure for altitude instead of using the default sea-level value of 760 mmHg.
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.

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

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

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Elevated A-a gradient on room air

2. Normal A-a gradient with elevated PaCO₂

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