Effects of acute lung injury on dynamic tissue properties

The dynamic properties of lung tissue after acute injury (ALI) were characterized by studying parenchymal strips prepared from guinea pigs endobronchially treated with hydrochloric acid. Forty-eight-hour samples (10 x 4 x 4 mm) from control (C), acid-damaged (D), and uninvolved (U) lungs of treated animals had elastance and resistance measured in a perfused organ bath system during uniaxial length oscillation at a mean force of 750 mg over a 1.5 (0.1-3 Hz)-fold decade frequency range and 5-fold (1-5%) strain amplitude range. Results were interpreted using a quasi-linear viscoelastic model with tissue stress expressed as an exponential function of strain. Among C strips, elastance increased linearly with the logarithm of frequency, whereas resistance decreased hyperbolically with frequency; both were strain amplitude independent. In contrast, elastance among D strips was uniformly greater (P < 0.0001) and was inversely strain amplitude dependent (P < 0.01); resistance was markedly elevated (P < 0.0001) but decreased hyperbolically with frequency similar to C strips. Resistance and elastance changes among U strips were intermediate between those for D and C strips. Histological grading of cellular infiltration, alveolar septal thickening, and hemorrhage correlated with functional changes. The frequency and amplitude dependencies and magnitude changes for resistance and elastance were similar to those previously observed in intact dog lungs after ALI. These findings suggest that ALI can affect the rheological properties of lung parenchyma independently of surfactant and that changes in lung mechanics after ALI may, in part, be determined at the parenchymal level.