MECHANICAL IMPEDANCES OF LUNGS AND CHEST-WALL IN THE CAT

In nine anesthetized and paralyzed cats, the mechanical impedances of the total respiratory system (Zrs) and the lungs (ZL) were measured with small-volume pseudorandom forced oscillations between 0.2 and 20 Hz. ZL was measured after thoracotomy, and chest wall impedance (Zw) was calculated as Zw = Zrs - ZL. All impedances were determined by using input airflow [input impedance (Z(i))] and output flow measured with a body box [transfer impedance (Z(t))]. The differences between Z(i) and Z(t) were small for Zrs and negligible for ZL. At 0.2 Hz, the real and imaginary parts Of ZL amounted to 33 +/- 4 and 35 +/- 3% (SD), respectively, Of Zrs. Up to 8 Hz, all impedances were consistent with a model containing a frequency-independent resistance and inertance and a constant-phase tissue part (G - jH)/omega(alpha), where G and H are coefficients for damping and elastance, respectively, omega is angular frequency, and a determines the frequency dependence of the real and imaginary parts. G/H was higher for Zw than for ZL (0.29 +/- 0.05 vs. 0.22 +/- 0.04, P < 0.01). In four cats, the amplitude dependence of impedances was studied: between oscillation volumes of 0.8 and 3 ml, GL, HL, Gw, and Hw decreased on average by 3, 9, 26, and 29%, respectively, whereas the change in G/H was small for both ZL (7%) and Zw (-4%). The values of H were two to three times higher than the quasistatic elastances estimated with greater volume changes (>20 ml).