Compliance is a measurement of the distensibility of the lung
Compliance of both the chest wall and the lung tissue is known as Total Lung Compliance
Total Lung Compliance = Change in volume / Change in pressure
Static Compliance is the truest measure of the compliance of the lung tissue
It is measured when there are no gases flowing into or out of the lungs
Static Compliance = Exhaled Tidal Volume / (Plateau Pressure-PEEP)
The Plateau Pressure is obtained by instituiting a 2 second inspiratory pause at the peak of inspiration. This pause creates the condition of no gases flowing into the lungs.
This reflects the pressure due to the elastic recoil forces of the lung tissue alone . No pressure resulting from the flow of gases is measured.
Normal value for Static Compliance is 70-100 mL/cm H2O. This means that for every 1 cm H2O pressure change in the lungs , there is a change in volume of 70-100 mL of gas
As Dynamic Compliance is a measurement taken while gases are moving in the lungs , it measures both the Compliance of the lung tissue and the resistance to air flow
It is easier to obtain as it doesn’t require the inspiratory hold maneuver
Dynamic Compliance = Exhaled Tidal Volume / (Peak Inspiratory Pressure-PEEP)
The normal value for Dynamic Compliance is 50-80 mL/cm H2O
Dynamic Compliance measures are always smaller than Static Compliance because Peak Airway Pressure is always greater than Plateau Pressure
A decrease in Dynamic Compliance may indicate a decrease in lung compliance or an increase in airway resistance.