Respiratory system, lung, and chest wall impedances in anesthetized dogs

We measured impedances of the respiratory system (Zrs) and lung (ZL) in anesthetized and paralyzed dogs at frequencies between 4 and 64 Hz. Zrs was measured at functional residual capacity (FRC) and with mean transpulmonary pressures (Ptp) of 12 and 30 cmH2O; ZL was measured with the chest wall open at FRC and Ptp = 12 cmH2O. From these data we derived chest wall impedances at FRC and 12 cmH2O. Effective resistances of the respiratory system, lung, and chest wall were all frequency dependent. At frequencies below 22 Hz frequency dependence of respiratory system resistance was due to the frequency dependence of the chest wall, whereas at higher frequencies it was due to frequency dependence of the lung. Analysis of the impedance data between 4 and 32 Hz provided estimates of resistance, inertance, and compliance of the lung, chest wall, and respiratory system. At FRC, 41% of the respiratory system resistance was due to the lung and 59% was associated with the chest wall. Nearly all of the respiratory inertance (98%) was due to the lung. Lung compliance was approximately twice that of the chest wall, the former accounting for approximately one-third of respiratory elasticity. As lung volume was increased, respiratory resistance and compliance decreased; inertance also decreased, although this change was not significant.