The constant phase description of lung input impedance is Zin = Raw + (G + H )/ assuming the inertance of airway gas is negligible and Raw = airway resistance, G = tissue damping, H = tissue elasticity and = hysteresivity = G/H. There is concern that non-uniform air-flow conditions in the airways at low lung volume produce changes in Zin which are incorrectly interpreted as alterations in the lungs tissue properties. The tissue properties of the constant phase model can be written in terms of quasi-static PV loop parameters described by Hildebrandt (Bul. Math. Biophy., 31, 1968, JAP,35, 1973) as: k = AH/dP dV (see fig. 1) , = 1/((/4k)2 -1)1/2 , = 2/ tan-1(1/ ), Edyn =dP/dV(1/(1+ 2)1/2), H = Edyn(-1), G = (4AH/dV2) (-1), where is the quasi-static radial frequency at which the loops were recorded. The equivalent constant phase parameters calculated from quasi-static P-V loop analysis at low frequencies are not complicated by airway airflow inequalities. An analysis of small P-V loops recorded at various points during a complete inflation-deflation cycle of excised rat lungs shows that the of lung tissue decreases then increases as a function of end expiratory pressure during lung deflation and does not have to be explained by unequal airflow patterns in the airways.
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