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.

FiO2

Inspired oxygen fraction between 0 and 1.

Atmospheric pressure

Atmospheric pressure; must be greater than zero.

PaCO2

Arterial carbon dioxide pressure; must be greater than zero.

PH2O

Water vapour pressure; defaults to 47 mmHg.

Respiratory quotient

Respiratory quotient between 0.7 and 1; defaults to 0.8.

The implementation follows this calculation rule: 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.^[1]

Alveolar PO2

Ideal alveolar partial pressure of oxygen.

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

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

• A-a Gradient Sea Level

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

• a/A Ratio

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