static respiratory system compliance during mechanical ventilation =
= (minute tidal volume) / ((plateau inspiratory pressure) - (PEEP))
dynamic respiratory system compliance during mechanical ventilation =
= (minute tidal volume) / ((peak inspiratory pressure) - (PEEP))
where:
• pressures are in cm H20
• minute tidal volume is in mLs
• plateau pressure is measured by inserting a 0.3- 0.5 second pause at the end of the inspiratory phase, with the exhalation port closed to maintain the lungs in inspiration
• compliance is liters per cm water and is the opposite of "stiffness"
• PEEP is positive end-expiratory pressure
• If the inspiratory pressure is high, the volume needs to adjusted for the volume of gas which expands the ventilator tubing but which does not involve the patient.
Interpretation:
• normal static respiratory system compliance during mechanical ventilation is 70-100 mL per cm H2O
• normal dynamic respiratory system compliance during mechanical ventilation is 60-100 mL per cm H2O
Comparison of peak and plateau inspiratory pressures:
(1) If airway obstruction is present, plateau and peak pressures differ; if airway obstruction is minimal, plateau and peak pressures are similar.
(2) If (peak inspiratory pressure) - (plateau inspiratory pressure) > 10 cm H20, suspect increased airway resistance.
