Contribution of quasi-static tissue hysteresis to the dynamic alveolar pressure-volume loop

We obtained dynamic and flow-interrupted (quasi-static) pressure-volume loops from the lungs of anesthetized paralyzed open-chest mongrel dogs by measuring tracheal flow and pressure and alveolar pressure (PA) in three different regions using alveolar capsules. We used continuous tidal ventilation to obtain dynamic PA-volume loops and used the single-breath-interrupter technique to construct quasi-static pressure-volume loops for the same tidal volume (VT). We used three different VT's (15 and 20 ml/kg and inspiratory capacity) under control conditions and a VT of 15 ml/kg after methacholine-induced bronchoconstriction. We found that quasi-static hysteresis was negligible under control conditions for VT of 15 and 20 ml/kg. Quasi-static hysteresis became more important (36 +/- 11% of the corresponding dynamic PA-volume loop) during inspiratory capacity ventilation and after induced bronchoconstriction (27 +/- 12% of the corresponding dynamic PA-volume loop). We conclude that during tidal breathing near functional residual capacity "true" static hysteresis is negligible and that purely viscoelastic processes can explain lung mechanical behavior. For higher volume ventilation and after methacholine-induced constriction, quasi-static hysteresis accounted for a more important portion of dynamic tissue hysteresis. This suggests either that a more complex model, e.g., one including plastic processes, should be invoked or that the lung exhibits longer viscoelastic time constants as peak distending stresses become greater.