The histopathology of acute lung injury (ALI) is characterized by alveolar septal thickening, hyaline membrane formation, and neutrophil infiltration. The physiologic consequence of these abnormalities is hypoxemia due primarily to alveolar instability and collapse with an increase in the number of gas exchanging units with zero ventilation perfusion ratios (ie, an increase in shunt fraction). Treatment strategies for patients with ALI are supportive and focus on maintaining oxygen delivery to vital organs, guarding against complications, and providing adequate nutrition. A major component of this supportive care plan involves the application of positive pressure ventilation (generally with positive end-expiratory pressure [PEEP]) to open collapsed alveoli and decrease shunt, thereby improving blood oxygenation, a critical determinant of oxygen delivery.
The use of positive pressure ventilation to improve gas exchange can have both positive and negative effects, however, due to the complex microstructure of the damaged lung. Specifically, there is evidence to suggest that in the setting of ALI, several heterogeneous populations of alveoli exist as follow: (1) relatively normal alveoli with intact microstructure and normal compliance; (2) nonatelectatic, structurally damaged alveoli with abnormally low compliance; (3) atelectatic, moderately damaged alveoli whose patency can be restored through the application of PEEP at “physiologically safe” levels; and (4) atelectatic, severely damaged, or totally destroyed alveoli whose functional integrity cannot be restored through the application of PEEP at physiologically safe levels. While current ventilator therapy attempts to improve gas exchange by applying positive pressure to open atelectatic, potentially functional alveoli, the elevated pressures are transmitted to other regions of the lung parenchyma as well, and may cause local damage over time. mycanadianpharmacy
We have been investigating a nonconventional alternative mode of ventilation which utilizes intermittent, rather than continuous, application of elevated distending pressures to help stabilize damaged alveoli in ALI. This approach, which we have designated as amplitude modulated ventilation (AMV), is based on the hypothesis that the increase in shunt fraction that occurs during ALI is due largely to alveolar collapse, and the time constants for alveolar closure following a deep inflation are long compared to the respiratory cycle. Therefore, the stabilizing effect of a single high pressure breath may last for a period encompassing multiple subsequent breaths. If this hypothesis were correct, then high pressure breaths imposed every fourth or fifth breath (Fig 1) would be as effective in maintaining alveolar patency and improving gas exchange as continuous high pressures, and yet be less likely to cause tissue rupture (macroscopic barotrauma) or propagation of lung injury (microscopic barotrauma) to regions with previously functional gas exchanging units.
Figure 1. Airway pressure and tidal volume profiles during one form of amplitude modulated ventilation implemented using volume cycled ventilation with amplitude modulation in the form of intermittent single large breaths.
Category: Respiratory Symptoms
Tags: gas exchanging, lung compliance, lung inflation, lung tissue, parenchyma, transpulmonary pressure