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.

Initial pH

Initial pH between 6 and 8.5.

Final pH

Final pH between 6 and 8.5.

Bohr coefficient

Signed Bohr coefficient d(log P50)/d(pH); defaults to -0.48.

The implementation follows this calculation rule: Compute the Bohr effect shift of hemoglobin oxygen affinity as shiftLogP50 = phiBohrCoefficient x (pH2 - pH1), where phi is the signed Bohr coefficient, negative by convention and defaulting to -0.48 for human whole blood.

P50 shift

Shift of log10(P50) in log units.

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.

• 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.

• Oxygenation Index OI

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