The Effect of Positive End-Expiratory Pressure on Lung Micromechanics Assessed by Synchrotron Radiation Computed Tomography in an Animal Model of ARDS

Abstract

Modern ventilatory strategies are based on the assumption that lung terminal airspaces act as isotropic balloons that progressively accommodate gas. Phase contrast synchrotron radiation computed tomography (PCSRCT) has recently challenged this concept, showing that in healthy lungs, deflation mechanisms are based on the sequential de-recruitment of airspaces. Using PCSRCT scans in an animal model of acute respiratory distress syndrome (ARDS), this study examined whether the numerosity (ASnum) and dimension (ASdim) of lung airspaces change during a deflation maneuver at decreasing levels of positive end-expiratory pressure (PEEP) at 12, 9, 6, 3, and 0 cmH2O. Deflation was associated with significant reduction of ASdim both in the whole lung section (passing from from 13.1 ± 2.0 at PEEP 12 to 7.6 ± 4.2 voxels at PEEP 0) and in single concentric regions of interest (ROIs). However, the regression between applied PEEP and ASnum was significant in the whole slice (ranging from 188 ± 52 at PEEP 12 to 146.4 ± 96.7 at PEEP 0) but not in the single ROIs. This mechanism of deflation in which reduction of ASdim is predominant, differs from the one observed in healthy conditions, suggesting that the peculiar alveolar micromechanics of ARDS might play a role in the deflation process.

Gravitational distribution of regional opening and closing pressures, hysteresis and atelectrauma in ARDS evaluated by electrical impedance tomography

“…Opening and closing pressures were also higher in the dependent lung, underlying the role of gravity in the distribution of lung edema and transpulmonary pressure [ 22 ]. We disclosed values of opening pressure beyond 30 cmH 2 O, confirming that recruitment is a continuous phenomenon [ 23 ] during tidal breath insufflation [ 3 , 5 ].…”

“…Acute respiratory distress syndrome (ARDS) is characterized by the development of bilateral acute lung inflammation and edema as a consequence of direct or indirect injury [ 1 ]. Although inflammation is diffuse, lung edema is not homogeneously distributed and its impact on the mechanical properties of each lung region depends on several factors, such as gravity [ 2 ] or distance from the pleural surface [ 3 ]. The alveolar distending pressure, i.e., the transpulmonary pressure, presents, therefore, an uneven distribution in the lungs.…”

Prevention and treatment of acute lung injury with time-controlled adaptive ventilation: physiologically informed modification of airway pressure release ventilation

“…Synchrotron phase-contrast imaging can measure R/D at acinar length scales over short time frames and has demonstrated that lung collapse in the microenvironment differs between normal and acutely injured lungs [53][54][55][56]. Scaramuzzo et al first measured tissue collapse in the microenvironment of the normal lung with graded reductions in PEEP.…”

“…They assessed the numerosity (ASnum) and dimension (ASdim) of airspaces during lung deflation and found that the primary mechanism by which the lung loses volume was reduced ASnum secondary to alveolar and small airway derecruitment [53]. In a subsequent paper, Scaramuzzo showed in an ARDS model that the mechanism of lung deflation was reduced ASdim, which differs from the mechanism of normal lung deflation (ASnum) [54]. Broche et al showed that "compliant collapse", which is described "as a structural collapse of the airway wall along a certain length" is the primary mechanism of airway closure in the acutely injured lung [56].…”

Acute Respiratory Distress Syndrome (ARDS): Pathophysiological Insights and Lung Imaging

“…A different imaging method has been studied by Scaramuzzo et al [12]. Their paper is oriented to the definition of lung behavior during decremental PEEP application in a rabbit model of ARDS.…”