Fig. 2

The three mechanical mechanisms of ventilator-induced lung injury (VILI) include: a over-distension of tissue caused by excessive volume and pressure, b alveolar collapse and reopening with each breath secondary to surfactant deactivation, which causes a dynamic strain-induced tissue trauma, and c stress-concentrators caused by heterogeneous ventilation with open alveoli adjacent to collapsed or edema-filled alveoli. a An alveolar duct (yellow) is shown surrounded by alveoli represented by hexagons. Low volume/pressure (small arrows) do not over-distend alveolar ducts or distort surrounding alveoli. High volume/pressure (large arrows) over-distend alveolar ducts and distort surrounding alveoli that can lead to stress-failure in these tissues [40]. b Surfactant deactivation is a hallmark of ARDS and will result in alveolar collapse at end expiration and reopening during inspiration. Following loss of surfactant function at inspiration alveoli (hexagons) are fully inflated. However, unless end expiratory pressure is increased alveoli collapse at expiration (hexagons significantly reduced in size). This alveolar recruitment/derecruitment with each breath causes severe shear stress-induced tissue trauma [116, 117]. c Homogeneous ventilation is represented by uniformly open alveoli (hexagons) and the interdependence of these alveoli with shared wall results in a very stable structure [118]. Internal force lines (black arrows) are uniform across the homogeneously inflated lung tissue. [119]. Heterogeneous Ventilation, where isolated areas of alveolar collapse occur (blue arrows) disrupts the stability of alveolar interdependence such that stress is no longer evenly distributed across the tissue. Thus, heterogeneous tissue inflation causes a significant concentration of stress in the areas surrounding the collapse. Internal force lines bow in toward the collapsed alveoli and concentrate the stress, represented by the black stress lines becoming closer together, around the area of collapse. This stress-concentration would exacerbate tissue damage in the area surrounding the collapse [33]