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Compliance During Mechanical Ventilation

Description

Patients on mechanical ventilators who are paralyzed or otherwise unable to make respiratory movements can be assessed for the compliance of their respiratory system. High inspiratory pressures are need to ventilate either patients with airway obstruction or with stiff lungs. Airway obstruction can often be distinguished from stiff lungs by considering the peak and plateau inspiratory pressures, since the plateau pressure tends to reflect lung stiffness.


 

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.

 


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