Journal of Applied Physiology, Vol 59, Issue 3 732-737, Copyright © 1985 by American Physiological Society
Volume-time profile during relaxed expiration in the normal dog
J. H. Bates, M. Decramer, D. Chartrand, W. A. Zin, A. Boddener and J. Milic-Emili
Airway opening pressure, esophageal pressure, and flow were obtained during
relaxed expirations in two normal anesthetized paralyzed dogs. The
signal-to-noise ratio in the flow signals was greatly increased by
averaging 10 different signals obtained with the same lung inflation
volume. Numerical integration of an averaged flow signal then yielded the
time course of the volume of the respiratory system above functional
residual capacity (the elastic equilibrium volume). Comparison of volume
signals obtained with different inflation volumes suggests that the
resistance of the respiratory system increases with flow. The flow-volume
and semilog volume curves show that expiration is induced by two apparently
separate mechanisms: one causes emptying of most of the expired volume over
a time interval of much less than 1 s, whereas the other contributes a
relatively small amount to the expired volume over a significantly longer
time (greater than or equal to 1 s). We postulate the first mechanism to be
due to that of the respiratory system behaving like a single unit, with an
elastance that is slightly volume dependent, emptying through a single
airway which has a resistance that increases with flow. From the nature of
airway opening pressure and esophageal pressure measured after occlusion in
midexpiration, we conclude that the second mechanism is due to the
viscoelastic properties (i.e., creep) of the respiratory system. The
properties are manifest mainly in the chest wall.
