Generalized bronchial lesions, which would cause expiratory disturbance even in spontaneous respiration, are not found in emphysematous lungs. Obstructive expiratory disturbance as one of the most characteristic clinical symptoms of pulmonary emphysema can therefore, appear only in forced expiration, as a result of, compression of large bronchi by extraneous force originating from the thoracic wall. The prevalence of forced expiration in emphysema patients is attributed to deteriorated efficiency of spontaneous expiration, and expiratory dynamics are developed with the aid of anatomical findings of autopsied lungs. The strain and stress of the pulmonary elastic system is expressed as
F(x)=
A (
eax-1) where
x is the strain,
F(x) the stress at x, and A and
a are constants, of which the latter is determined by the physical property of the elastic system. The differential equation of, spontaneous expiration is d
t/d
P=-3
RV0/
P2. (
eax-1) (1+
x)
3/ (
az+
a-2)
eax+2 and
P=
Eeax-1/ (1+
x) 2where
P is intrapulmonary pressure,
t, time,
R, total airway resistance,
V0, total lung volume in relaxation, and E, the density of the pulmonary elastic system. In emphysematous lungs
V0 is increased to 2 to 3 times the normal, while
E is lowered to half the normal value. Computer analysis of the expiratory differential equation with the parameters obtained from the measurement on autopsied lungs reveals that expiratory efficiency is extremely lowered in emphysematous lungs. The main causative agent of the disturbance is the enlarged
V0, whereas the reduction in
E plays a rather subordinate role.
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