Alveolar Gas Equation (PAO₂ Calculator)

Alveolar Gas Equation (PAO₂ Calculator) | 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
Output type: json
Sample coverage: 4
API ready: Yes

Overview

The Alveolar Gas Equation Calculator estimates alveolar oxygen pressure (PAO₂) from PaCO₂, FiO₂, atmospheric pressure, and respiratory quotient using PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ. It uses sea-level pressure and an RQ of 0.8 by default and provides a directional comparison with expected PAO₂ values.

When to use

Estimate PAO₂ from arterial PaCO₂ and inspired oxygen values.
Adjust the calculation for altitude by entering a different atmospheric pressure.
Check how calculated PAO₂ compares with the expected value on room air at sea level.

How it works

1Enter PaCO₂ in mmHg and FiO₂ as a fraction, such as 0.21 for room air.
2Optionally replace the default atmospheric pressure of 760 mmHg and RQ of 0.8.
3The calculator applies PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ.
4The JSON result reports PAO₂, inputs, calculation method, and a directional category.

Use cases

Review arterial blood gas inputs by estimating the expected alveolar oxygen pressure.
Compare room-air PAO₂ at sea level with PAO₂ at reduced atmospheric pressure at altitude.
Explore how FiO₂, PaCO₂, atmospheric pressure, and RQ affect the calculated PAO₂.

Examples

1. Sea-level room-air PAO₂ estimate

Clinician reviewing an arterial blood gas

Background

The patient has a PaCO₂ of 40 mmHg while breathing room air at sea level.

Problem

Estimate the expected alveolar oxygen pressure and compare it with the textbook value near 100 mmHg.

How to use

Enter PaCO₂ as 40 and FiO₂ as 0.21. Leave atmospheric pressure and RQ at their defaults.

{"paco2":40,"fio2":0.21,"decimalPlaces":1}

Outcome

The calculator returns PAO₂ of approximately 99.7 mmHg and categorizes it as normal PAO₂ for sea-level room air.

2. PAO₂ estimate at Denver altitude

Respiratory physiology learner

Background

A person at higher altitude has a PaCO₂ of 35 mmHg while breathing room air. Atmospheric pressure is entered as 630 mmHg.

Problem

Show how reduced atmospheric pressure changes the estimated alveolar oxygen pressure.

How to use

Enter PaCO₂ as 35, FiO₂ as 0.21, and atmospheric pressure as 630 mmHg. Use the default RQ of 0.8.

FAQ

What does this calculator measure?

It estimates alveolar oxygen partial pressure, or PAO₂, in mmHg.

What FiO₂ should I enter for room air?

Enter 0.21 for room air.

What are the default atmospheric pressure and RQ?

The defaults are 760 mmHg for sea level and 0.8 for a mixed diet.

Can I calculate PAO₂ at altitude?

Yes. Enter the local atmospheric pressure in mmHg instead of using the sea-level default.

Does this replace a complete A-a gradient interpretation?

No. This tool focuses on PAO₂ and does not provide a full A-a gradient interpretation. It is not medical advice.

{"paco2":35,"fio2":0.21,"patm":630,"decimalPlaces":1}

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Continue with connected tools and hubs

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. Sea-level room-air PAO₂ estimate

2. PAO₂ estimate at Denver altitude

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