PARTITIONING OF AIRWAY AND LUNG-TISSUE PROPERTIES - COMPARISON OF IN-SITU AND OPEN-CHEST CONDITIONS

The purpose of this study was to investigate under physiological breat hing conditions whether airway and lung tissue properties are differen t in situ and in open-chest conditions. We measured lung input impedan ce in dogs from 0.2 to 8 Hz with an optimal ventilator waveform at fou r tidal volumes (VT; from 75 to 450 ml) in intact animals using an eso phageal balloon as well as after opening the chest. The lung impedance from both conditions was partitioned into airway and tissue compartme nts as characterized by airway resistance and inertance (law) and tiss ue damping (G) and elastance (H) parameters respectively. All paramete rs except law depended to some extent on VT. The in situ tissue G and H slightly decreased with VT while in the open-chest condition; G decr eased and H increased slightly with VT. We found small but significant differences between the mechanical properties of the airway and lung tissues in situ and in open-chest conditions. Over the total populatio n, the G, airway resistance, and law parameters were 13% (not signific ant), 35% (P < 0.001), and 31% (P < 0.001) smaller, respectively, in s itu than in the open-chest condition. However, the H was 15% larger in situ (P < 0.001). Although we cannot completely rule out certain arti facts, these differences most likely reflect real alterations in the l ung due to the different configurations and possible differences in th e distribution of pleural pressures in the two conditions. The G being smaller and E being larger in situ resulted in hysteresivity (G/H) 36 % smaller in situ (P < 0.001). Consequently, energy dissipation in the lung tissues relative to energy storage as characterized by hysteresi vity is somewhat increased in the open-chest configuration. This may b e anticipated because the mechanical efficiency of the lungs is expect ed to be better in its natural state than in any artificially maintain ed condition that was the end result of an invasive intervention.