Amplitude-modulated Ventilation: Results
The dynamic relaxation properties of surfactant samples collected from control and injured animals were compared by measuring film surface tension as a function of time following a step change in bubble volume of 50 percent. Surface tension profiles as a function of time for both control and ALI samples conformed to a single time constant model, were similar in magnitude (control Vj = 0.14 ± 0.04, n = 8; ALI v, = 0.15 ± 0.01, n = 8; p = NS by Student’s t test), and were approximately an order of magnitude smaller than the fastest time constant describing lung compliance changes in the intact lung.
Lung tissue viscoelastic behavior exclusive of surface film effects was characterized in terms of linear stress relaxation functions relating tissue stress to strain using an approach similar to that described above. Relaxation functions for both control and ALI parenchymal strips were similar and demonstrated two time constant behavior (control v, = 0.23 ± 0.08, v, = 1.70 ± 0.23, n = 3; LPS v, = 0.27 ±0.11, v2= 1.41 ±0.17, n = 3; p = NS by Student’s t test).
Histopathologic findings confirmed the presence of neutrophil infiltration, hyaline membrane formation, and alveolar septal thickening in ALI samples which was not present in control samples. Representative photomicrographs are shown in Figure 3. buy allegra
Applying Surface Film and Parenchijmal Tissue Strip Results to a Model of Alveolar Stability to Better Understand AMV. Tissue strip relaxation responses and surface film responses to step changes in surface area were incorporated into a microstructural model of alveolar mechanics and stability to assess whether the dynamics observed in vitro could explain the compliance responses to a deep inflation in vivo. This model, developed by Stamenovic, postulates that two types of forces, shear forces and normal forces, contribute to alveolar instability and collapse. It further considers the lung as an isotropic continuum. This assumption allows the constituitive relationship between transpulmonary pressure and lung volume to be expressed in terms of two independent measurable tissue properties: the shear modulus |i (the ratio of the shear stress applied to the tissue divided by the resulting shear angle) which is a quantitative expression for the lung’s ability to resist shear deformation, and the bulk modulus к (lung elastance normalized to one over lung volume), which expresses the lung’s ability to resist normal radial deformation. These moduli are not constants but are functions of tissue and surface film properties and the transpulmonary distending pressures of the lung. If a given region of parenchyma is subject to either external destabilizing shear or inward destabilizing traction, then the criteria for maintenance of stability.
Figure 3. Photomicrographs of LPS treated and control lungs. LPS treated lungs demonstrate neutrophil infiltration, hyaline membrane formation, and septal thickening.
Category: Respiratory Symptoms
Tags: gas exchanging, lung compliance, lung inflation, lung tissue, parenchyma, transpulmonary pressure