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

1 Department of Biomedical Engineering, Boston University, Boston 02215; and 2 Division of Pulmonary and Critical Care Medicine, Brigham and Women's Hospital, Boston, Massachusetts 02115 We measured lung impedance in rats in closed chest (CC), open chest (OC), and isolated lungs (IL) at four transpulmonary pressures with a optimal ventilator waveform. Data were analyzed with an homogeneous linear or an inhomogeneous 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 measurements. The hysteresivity ( ) 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 ) in IL at low transpulmonary pressure was similar to in the tissue strip. We conclude that is primarily determined by lung connective tissue, and its elevated estimates from impedance data in the CC and OC conditions are a consequence of compartment-like heterogeneity being greater in CC and OC conditions than in the IL. tissue resistance; tissue elastance; configuration; compartmental behavior