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.

Exhaled tidal volume

A finite measured or assumed value, expressed in mL.

Plateau pressure

A finite measured or assumed value, expressed in cmH2O.

Total PEEP

A finite measured or assumed value, expressed in cmH2O.

The implementation follows this calculation rule: Compute total respiratory system compliance as exhaled tidal volume divided by the driving pressure between plateau pressure and total PEEP.^[1]

Total compliance

The calculated total compliance in mL/cmH2O.

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.

• Static Compliance

  Compute static respiratory compliance as tidal volume divided by plateau pressure minus PEEP.

• Dynamic Compliance

  Compute dynamic respiratory compliance as tidal volume divided by peak inspiratory pressure minus PEEP.

• Total Airway Resistance

  Compute total airway resistance as the difference between peak inspiratory pressure and pause plateau pressure divided by inspiratory flow.