In a spontaneously breathing individual, the total compliance is about 0.1 L/cm H20. The value for compliance varies considerably, depending on a person's posture, position, and active breathing. It can range from 0.05 L/cm H20 to 0.17 L/cm H2O (50 to 170 mL/cm H20).
For intubated and mechanically ventilated patients with normal lungs and a normal chest wall, compliance varies from 40 to 50 mL/cm H20 in males and 35 to 45 mL/cm H20 in females to as high as 100 mL/cm H20 in either gender.
Monitoring of changes in compliance is a valuable means of assessing changes in the patient's condition during mechanical ventilatory support. Because compliance usually is measured under conditions of no gas flow, it is referred to as a static compliance or static effective compliance.
Diseases that reduce the compliance of the lungs or chest wall increase the pressure required to inflate the lungs. Acute respiratory distress syndrome (ARDS) is an example of such a disease. Conversely, emphysema is an example of a pulmonary problem that leads to increased compliance. With emphysema, less pressure is required to inflate the lungs.
For patients on mechanical ventilatory support, static compliance is calculated as:
C lung =(Exhaled tidal volume)/(Plateau pressure - EEP)
EEP is the end-expiratory pressure, which is also known as baseline pressure; that is the baseline from which the patient breathes.