A more gradual positive end-expiratory pressure increase reduces lung damage and improves cardiac function in experimental acute respiratory distress syndrome

Does 'sub-threshold' ventilatory stress promote healing after lung injury?

Excessive tidal stretching may initiate damage or retard healing after lung injury. However, it is seldom considered whether intracycle power and ventilatory forces of lesser magnitude than those required to cross an injury threshold might stimulate or accelerate beneficial adaptive responses. Acute lung injury is a dynamic process that may exhibit phase-dependent reparative responses to mechanical stress broadly similar to physical training, body trauma or sepsis. We propose that lower stress may not always be better through all phases of ARDS; moderately high tidal airway pressures that stay below the threshold of global injury may have potential to speed healing of the injured lung.

Effects of mechanical ventilation on the interstitial extracellular matrix in healthy lungs and lungs affected by acute respiratory distress syndrome: a narrative review

BACKGROUND

Mechanical ventilation, a lifesaving intervention in critical care, can lead to damage in the extracellular matrix (ECM), triggering inflammation and ventilator-induced lung injury (VILI), particularly in conditions such as acute respiratory distress syndrome (ARDS). This review discusses the detailed structure of the ECM in healthy and ARDS-affected lungs under mechanical ventilation, aiming to bridge the gap between experimental insights and clinical practice by offering a thorough understanding of lung ECM organization and the dynamics of its alteration during mechanical ventilation.

MAIN TEXT

Focusing on the clinical implications, we explore the potential of precise interventions targeting the ECM and cellular signaling pathways to mitigate lung damage, reduce inflammation, and ultimately improve outcomes for critically ill patients. By analyzing a range of experimental studies and clinical papers, particular attention is paid to the roles of matrix metalloproteinases (MMPs), integrins, and other molecules in ECM damage and VILI. This synthesis not only sheds light on the structural changes induced by mechanical stress but also underscores the importance of cellular responses such as inflammation, fibrosis, and excessive activation of MMPs.

CONCLUSIONS

This review emphasizes the significance of mechanical cues transduced by integrins and their impact on cellular behavior during ventilation, offering insights into the complex interactions between mechanical ventilation, ECM damage, and cellular signaling. By understanding these mechanisms, healthcare professionals in critical care can anticipate the consequences of mechanical ventilation and use targeted strategies to prevent or minimize ECM damage, ultimately leading to better patient management and outcomes in critical care settings.

Biological impact of restrictive and liberal fluid strategies at low and high PEEP levels on lung and distal organs in experimental acute respiratory distress syndrome

Background: Fluid regimens in acute respiratory distress syndrome (ARDS) are conflicting. The amount of fluid and positive end-expiratory pressure (PEEP) level may interact leading to ventilator-induced lung injury (VILI). We therefore evaluated restrictive and liberal fluid strategies associated with low and high PEEP levels with regard to lung and kidney damage, as well as cardiorespiratory function in endotoxin-induced ARDS.

Methods: Thirty male Wistar rats received an intratracheal instillation of Escherichia coli lipopolysaccharide. After 24 h, the animals were anesthetized, protectively ventilated (V_T = 6 ml/kg), and randomized to restrictive (5 ml/kg/h) or liberal (40 ml/kg/h) fluid strategies (Ringer lactate). Both groups were then ventilated with PEEP = 3 cmH₂O (PEEP3) and PEEP = 9 cmH₂O (PEEP9) for 1 h (n = 6/group). Echocardiography, arterial blood gases, and lung mechanics were evaluated throughout the experiments. Histologic analyses were done on the lungs, and molecular biology was assessed in lungs and kidneys using six non-ventilated animals with no fluid therapy.

Results: In lungs, the liberal group showed increased transpulmonary plateau pressure compared with the restrictive group (liberal, 23.5 ± 2.9 cmH₂O; restrictive, 18.8 ± 2.3 cmH₂O, p = 0.046) under PEEP = 9 cmH₂O. Gene expression associated with inflammation (interleukin [IL]-6) was higher in the liberal-PEEP9 group than the liberal-PEEP3 group (p = 0.006) and restrictive-PEEP9 (p = 0.012), Regardless of the fluid strategy, lung mechanical power and the heterogeneity index were higher, whereas birefringence for claudin-4 and zonula-ocludens-1 gene expression were lower in the PEEP9 groups. Perivascular edema was higher in liberal groups, regardless of PEEP levels. Markers related to damage to epithelial cells [club cell secreted protein (CC16)] and the extracellular matrix (syndecan) were higher in the liberal-PEEP9 group than the liberal-PEEP3 group (p = 0.010 and p = 0.024, respectively). In kidneys, the expression of IL-6 and neutrophil gelatinase-associated lipocalin was higher in PEEP9 groups, regardless of the fluid strategy. For the liberal strategy, PEEP = 9 cmH₂O compared with PEEP = 3 cmH₂O reduced the right ventricle systolic volume (37%) and inferior vena cava collapsibility index (45%).

Conclusion: The combination of a liberal fluid strategy and high PEEP led to more lung damage. The application of high PEEP, regardless of the fluid strategy, may also be deleterious to kidneys.