The "Changes in Pulmonary Compliance" AND "Disease-Related Obstruction" Page!     

Compliance is effectively a measure of elasticity of an organ.  Mathematically, we determine compliance as follows:

                                  change in volume
compliance (C) = --------------------------------
                                   change in pressure

Note also that pulmonary edema will cause a functional decrease in compliance of the lung because pulmonary edema represents a functional thickening of the lung.  As the interstitium of the lung is very small, fluid quickly makes its way into the alveolar spaces.  This movement of fluid increases the thickness of fluid lining the alveolar wall, increasing the surface tension working to collapse the alveolar sacs.  A build-up of mucus in the alveolar compartments associated with any type of respiratory infection will also increase the thickness of the fluid layer within the alveolar sacs.  All of these effects represent "DECREASED COMPLIANCE."

Emphysema results in increased compliance, a situation which might best be described as grossly accelerated aging of the lung.  As the alveolar walls rupture and break down, the lung becomes less resistant to air flow.  This is not a good situation as the area available for gas exchange is also decreased.  You may want to go out into the Internet and look up the etiology of emphysema.  Many persons are surprised to find that there are at least two genetic forms of emphysema.

A) Normal Inspiration - uptake of air is rapid, plateaus and then drops off quickly.

B) Thickening of the parenchyma of the lung (eg. pulmonary edema, chronic pneumonia with scarring, edema, fluid in lung, scarring of lung, fibrosis). The lungs are harder to fill and there is less volume available.  Because of the reduced tissue compliance, air tends to be quickly expelled from the lungs.

C) Tearing (destruction) of the parenchyma of the lung (eg. emphysema, accelerated aging of the lung).  The lungs are easier to fill but they tend not to collapse as they are less elastic.  This means it is necessary to generate greater positive pressures in the intrapleural space in order to exhale.  Patients with advanced emphysema may purse their lips upon expiration to create resistance to air flow.  This increased resistance increases pressure in the thoracic cavity, generating a greater positive pressure in the intrapleural space and assisting in expiration.  Unfortunately, in advanced emphysema, the high positive pressures necessary to expel air from the lungs may cause collapse of the respiratory bronchioles during expiration, making it even more difficult to exhale.  For this reason, emphysema can be classified both as an "exchange" disease and as a "chronic airway limitation."

D) Diseases of the conducting pathways (eg. constriction with tracheitis, tumors in air pathways, chronic obstructive pulmonary disease) or reduction in airway diameter and increased resistance to flow in asthma, anaphylactic shock, constriction of the smooth muscles of the bronchioles.  Lungs are harder/slower to fill and may be harder yet to empty as high positive pressures needed to force air out may collapse small airways!  The term Chronic Obstructive Pulmonary Disease (COPD) is still used to describe these conditions although the term Chronic Airway Limitations (CAL) is coming into vogue; the word "obstructive" is considered to be too limiting by some, whereas "limitation" could be used to refer to any situation in which airflow through the air conducting pathways is impaired.