Amplitude-modulated Ventilation: Methods
We studied this hypothesis in an animal model of ALI. Male Hartley guinea pigs, weight 550 to 600 g, were treated with inhaled solubilized endotoxin (time = 0) followed by continuous intravenous infusion (time = 12 to 18 h) while dynamic lung compliance was recorded during mechanical ventilation. Intermittent deep lung inflations to peak pressures 1.5 times that noted during tidal ventilation (7 ml/kg) were administered, and lung compliance measured as a function of time thereafter. Results in ALI animals were compared to those measured in control saline solution-treated animals. Following completion of measurements in the intact animals, lungs were lavaged, and crude surfactant pellets prepared from the cell-free superna-tants of the fluid. Animals were killed, their lungs degassed and exsanguinated, and subpleural parenchymal strips prepared for organ bath studies of tissue relaxation behavior in the absence of surface film effects. The dynamic behavior of the surfactant pellets and tissue strips was characterized by surfactometry and by length tension measurements in a perfused organ bath system, respectively. These dynamic measurements of surface film and parenchymal tissue properties were then used to predict the compliance response of lung tissue to a deep inflation by using a microstructural model of alveolar stability which assumes that changes in lung compliance following a deep inflation reflect alveolar stabilization and recruitment. Degree of damage was assessed histopathologically by light microscopic examination of formalin-fixed samples stained with hematoxylin and eosin. canada health and care mall
Compliance Measurements in Animals, Surface Film Step Responses, and Tissue Strip Relaxation Profiles
Changes in dynamic lung compliance following a deep lung inflation for both control (n = 3) and ALI (n = 6) animals were fitted to an exponential relaxation decay model of the form: Cdyn(t) = L a, exp (v. t) where Cdyn is dynamic lung compliance, and a. and are model coefficients selected to minimize the error between model predictions and experimental results. The data are summarized in Figure 2 and demonstrate that following a deep inflation, compliance changes in control lungs as a function of time were characterized by a single short time constant (v, = 1.25 s), while compliance changes in damaged lungs were significantly larger (p = 0.03 Student’s t test) and more lasting, consistent with a two time constant model (v, = 1.4, v2 = 10.1 s).
Figure 2. Lung compliance as a function of time in control and ALI lungs following a deep lung inflation. Responses have been fit to a two time constant exponential relaxation model. Damaged lungs demonstrated a larger and more lasting dynamic compliance response to a deep inflation than control lungs.
Category: Respiratory Symptoms
Tags: gas exchanging, lung compliance, lung inflation, lung tissue, parenchyma, transpulmonary pressure