Complete Alveolar Gas Equation Calculator

It estimates ideal alveolar oxygen tension (PAO₂) from inspired oxygen fraction, barometric pressure, water-vapour pressure, PaCO₂, and respiratory exchange ratio. The clinical form used on this page assumes inspired CO₂ is negligible. It is not suitable for direct use with significant CO₂ rebreathing, non-negligible inspired CO₂, an unconfirmed closed or abnormal breathing circuit, or an unknown inspired-gas composition. PAO₂ is a model estimate rather than a direct arterial or alveolar measurement.

Sources: [1, 2, 3, 4]

The complete equation uses PAO₂ = PIO₂ − PaCO₂ × [FiO₂ + (1 − FiO₂) / R]. The common clinical approximation uses PAO₂ ≈ PIO₂ − PaCO₂ / R and omits the inspired-versus-expired-volume correction. The difference is often modest on room air but grows as FiO₂ rises, so the two values are labeled separately.

Sources: [1, 2, 3]

No. PAO₂ is the equation's estimated ideal alveolar oxygen tension. PaO₂ is measured in arterial blood, and SpO₂ is an estimate of haemoglobin oxygen saturation. They describe different quantities and are not interchangeable.

Sources: [4, 5, 7]

Use arterial PaCO₂ obtained during the same stable oxygen and ventilatory conditions being modeled. The equation assumes arterial PaCO₂ approximates mean alveolar PCO₂. A venous value, end-tidal value, or old sample is not automatically interchangeable.

Sources: [1, 7]

Select room air to use 21%. For supplemental oxygen, enter a measured or device-set percentage from 1 to 100. Do not enter a decimal fraction such as 0.40. The page does not infer FiO₂ from flow alone, and variable-performance devices can make an assumed FiO₂ uncertain.

Sources: [6, 7]

Lower barometric pressure reduces humidified inspired oxygen tension and calculated PAO₂. Sea-level mode uses 760 mmHg. Custom mode requires a known local or measured pressure; this calculator does not convert altitude to pressure and does not model hyperbaric environments.

Sources: [1, 2, 6]

R is the carbon-dioxide-production to oxygen-consumption ratio used in the equation. The selectable 0.80 value is a standard clinical assumption, not a measured value for every person or metabolic state. Use a custom value only when the context justifies it.

Sources: [1, 3]

The 47 mmHg term represents saturated water-vapour pressure at 37°C. It is part of the usual body-temperature alveolar gas equation and is not a user-adjustable humidity setting on this page.

Sources: [1, 3]

No. Calculated PAO₂ alone does not diagnose shunt, ventilation–perfusion mismatch, pulmonary embolism, respiratory failure, or another condition. Interpretation requires the relevant measurements and clinical context; the separate A–a Gradient page also requires paired arterial PaO₂.

Sources: [4, 5, 7]

No. The page reproduces physiology equations and their assumptions. It does not prescribe oxygen, select a device or FiO₂, change a ventilator, or replace clinical assessment, monitoring, and an applicable care pathway.

Sources: [6, 7]