Journal of Applied Physiology, Vol 67, Issue 3 1013-1019, Copyright © 1989 by American Physiological Society
Flow and volume dependence of expiratory resistance in anesthetized cats
M. Skaburskis, F. Shardonofsky and J. Milic-Emili
Meakins-Christie Laboratories, McGill University, Montreal, Quebec, Canada.
In five anesthetized paralyzed cats, mechanically ventilated with tidal
volumes of 36-48 ml, the isovolume pressure-flow relationships of the lung
and respiratory system were studied. The expiratory pressure was altered
between 3 and -12 cmH2O for single tidal expirations. Isovolume
pressure-flow plots for three lung volumes showed that the resistive
pressure-flow relationships were curvilinear in all cases, fitting Rohrer's
equation: P = K1V + K2V2, where P is the resistive pressure loss, K1 and K2
are Rohrer's coefficients, and V is flow. Values of K1 and K2 declined with
lung inflation, consistent with the volume dependence of pulmonary (RL) and
respiratory system resistances (Rrs). During lung deflation against
atmospheric pressure, RL and Rrs tended to remain constant through most of
expiration, resulting in a nearly linear volume-flow relationship. In the
presence of a fixed respiratory system elastance, the shape of the
volume-flow profile depended on the balance between the volume and the flow
dependence of RL and Rrs. However, the flow dependence of RL and Rrs
indicates that their measured values will be affected by all factors that
modify expiratory flow, e.g., respiratory system elastance, equipment
resistance, and the presence of respiratory muscle activity.
