Dynamic nonlinearity of lung tissue: effects of strain amplitude and stress level

Lung tissue presents substantial nonlinear phenomena not accounted for by linear models; however, nonlinear approaches are less available. Our aim was to characterize the behavior of total harmonic distortion, an index of nonlinearity, in lung tissue strips under sinusoidal deformation at a single frequency as a function of strain amplitude and operational stress. To that end, lung parenchymal strips from healthy rats (n = 6) were subjected to sinusoidal deformation (1 Hz) at different strain amplitudes (Δε = 4, 8, 12, 16, and 20%) and operating stresses (σ(op) = 6, 8, 10, 12, 14, and 16 hPa). Additional rats (n = 9) were intratracheally instilled with saline or bleomycin (2.5 U/kg, 3 times 1 wk apart), killed 28 days after the last instillation, and their lung tissue strips were studied at 5 and 10 hPa σ(op) and 5% Δε. In both cases, harmonic distortion (HD%) of input (strain) and output (stress) signals were determined. In healthy strips, HD% increased linearly with Δε, stress amplitude, and minimum stress by cycle variations, but showed no significant change with σ(op) levels. A prediction model could be determined as a function of operational stress and stress amplitude. Harmonic distortion was significantly increased in bleomycin-treated strips compared with controls and showed positive correlation with E behavior in both normal and diseased strips. We concluded that HD% can be useful as a single and simple parameter of lung tissue nonlinearity.