Hysteresivity of the lung and tissue strip in the normal rat: effects of heterogeneities

We measured lung impedance in rats in closed chest (CC), open chest (OC), a nd isolated lungs (IL) at four transpulmonary pressures with a optimal vent ilator waveform. Data were analyzed with an homogeneous linear or an inhomo geneous linear model. Both models include tissue damping and elastance and airway inertance. The homogeneous linear model includes airway resistance ( Raw), whereas the inhomogeneous linear model has a continuous distribution of Raw characterized by the mean Raw and the standard deviation of Raw (SDR ). Lung mechanics were compared with tissue strip mechanics at frequencies and operating stresses comparable to those during lung impedance measuremen ts. The hysteresivity (eta) was calculated as tissue damping/elastance. We found that 1) airway and tissue parameters were different in the IL than in the CC and OC conditions; 2) SDR was lowest in the IL; and 3) eta in IL at low transpulmonary pressure was similar to eta in the tissue strip. We con clude that eta is primarily determined by lung connective tissue, and its e levated estimates from impedance data in the CC and OC conditions are a con sequence of compartment-like heterogeneity being greater in CC and OC condi tions than in the IL.