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

a/A ratio

The calculated a/a ratio.

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. Ratios and percentages become unstable when their denominator is small, uncertain, or defined differently between samples.

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.

• 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.

• Oxygenation Index OI

  Compute the oxygenation index as mean airway pressure x FiO2 x 100 divided by arterial oxygen tension.

• OI Variant With PEEP

  Compute an oxygenation index variant as mean airway pressure times the inspired oxygen percentage divided by arterial oxygen tension, without a scaling factor of 100.