Use this respiratory calculation when pressures, gas fractions, flows, and volumes use the stated units and were measured under compatible conditions. It can reproduce a defined mechanics or gas-exchange quantity.

Inspired oxygen fraction

A finite measured or assumed value from 0 through 1.

Atmospheric pressure

A finite measured or assumed value, expressed in mmHg.

Arterial CO2

A finite measured or assumed value, expressed in mmHg.

Arterial O2

A finite measured or assumed value, expressed in mmHg.

The implementation follows this calculation rule: Compute the alveolar-arterial oxygen gradient as fiO2 x (patm - 47) - paco2 / 0.8 - pao2.

A-a gradient

The calculated a-a gradient in mmHg.

These simplified equations do not replace pulmonary-function testing or clinical assessment. Barometric pressure, temperature, water-vapour pressure, respiratory quotient, leaks, ventilator timing, spontaneous effort, and measurement site can change the result. Preserve the input values and units so the result can be audited and recalculated.

Interpret the result with arterial or end-tidal gases, ventilator waveforms, oxygen saturation, and serial measurements as appropriate. Directly measured mechanics or gas exchange is preferable when the simplifying assumptions do not hold.

• A-a Gradient Sea Level

  Compute the sea level alveolar-arterial oxygen gradient as fiO2 x (760 - 47) - paco2 / 0.8 - pao2.

• Alveolar Gas Equation

  Compute the ideal alveolar oxygen pressure PAO2 = FiO2 x (Patm - PH2O) - PaCO2 / R, with water vapour pressure defaulting to 47 mmHg at body temperature and respiratory quotient R defaulting to 0.8.

• a/A Ratio

  Compute the arterial-to-alveolar oxygen ratio as PaO2 divided by the alveolar oxygen tension from the simplified alveolar gas equation.