Ventilator-induced lung injury without biotrauma?

A study of macrophage mechanical properties and functional modulation based on the Young's modulus of PLGA-PEG fibers

The immune response of macrophages plays an important role in defending against viral infection, tumor deterioration and repairing of contused tissue. Macrophage functional differentiation induced by nanodrugs is the leading edge of current research, but nanodrugs have toxic side effects, and the influence of their physical properties on macrophages is not clear. Here we create an alternative way to modulate macrophage function through PLGA-PEG fibers' Young's modulus. Previously, we revealed that by controlling the Young's modulus of the fibers from kPa to MPa, all the fibers entered murine macrophage cells (RWA 264.7) in a similar manner, and based on that, we found that macrophages' mechanical properties were affected by the fibers' Young's modulus, that is, hard fibers with a Young's modulus of ∼1 MPa increased the cell average Young's modulus, but did not affect the cell shape, while soft fibers with a Young's modulus of ∼100 kPa decreased the cell average Young's modulus and modulated the cell shape to a more spherical one. On the other hand, only the soft fibers induced proinflammatory cytokine secretion, indicating an M1 macrophage functional modulation by low Young's modulus fibers. This study explored the mechanical properties of the interactions between PLGA-PEG fibers and cells, in particular, when guiding the direction of the modulation of macrophage function, which is of great significance for the applications of material biology in the biomedical field.

Prevention and Amelioration of Rodent Ventilation-Induced Lung Injury with Either Prophylactic or Therapeutic feG Administration

PURPOSE

Mechanical ventilation is a well-established therapy for patients with acute respiratory failure. However, up to 35% of mortality in acute respiratory distress syndrome may be attributed to ventilation-induced lung injury (VILI). We previously demonstrated the efficacy of the synthetic tripeptide feG for preventing and ameliorating acute pancreatitis-associated lung injury. However, as the mechanisms of induction of injury during mechanical ventilation may differ, we aimed to investigate the effect of feG in a rodent model of VILI, with or without secondary challenge, as a preventative treatment when administered before injury (prophylactic), or as a therapeutic treatment administered following initiation of injury (therapeutic).

METHODS

Lung injury was assessed following prophylactic or therapeutic intratracheal feG administration in a rodent model of ventilation-induced lung injury, with or without secondary intratracheal lipopolysaccharide challenge.

RESULTS

Prophylactic feG administration resulted in significant improvements in arterial blood oxygenation and respiratory mechanics, and decreased lung oedema, bronchoalveolar lavage protein concentration, histological tissue injury scores, blood vessel activation, bronchoalveolar lavage cell infiltration and lung myeloperoxidase activity in VILI, both with and without lipopolysaccharide. Therapeutic feG administration similarly ameliorated the severity of tissue damage and encouraged the resolution of injury. feG associated decreases in endothelial adhesion molecules may indicate a mechanism for these effects.

CONCLUSIONS

This study supports the potential for feG as a pharmacological agent in the prevention or treatment of lung injury associated with mechanical ventilation.

Characterization of the seven-day course of pulmonary response following unilateral lung acid injury in rats

BACKGROUND

Aspiration of gastric acid is an important cause of acute lung injury. The time course of the pulmonary response to such an insult beyond the initial 48 hours is incompletely characterized. The purpose of this study was to comprehensively describe the pulmonary effects of focal lung acid injury over a seven day period in both directly injured and not directly injured lung tissue.

METHODS

Male Wistar rats underwent left-endobronchial instillation with hydrochloric acid and were sacrificed at 4, 24, 48, 96 or 168 h after the insult. Healthy non-injured animals served as controls. We assessed inflammatory cell counts and cytokine levels in right and left lung lavage fluid and blood, arterial oxygen tension, alterations in lung histology, lung wet-to-dry weight ratio and differential lung perfusion.

RESULTS

Lung acid instillation induced an early strong inflammatory response in the directly affected lung, peaking at 4-24 hours, with only partial resolution after 7 days. A less severe response with complete resolution after 4 days was seen in the opposite lung. Alveolar cytokine levels, with exception of IL-6, only partially reflected the localization of lung injury and the time course of the functional and histologic alterations. Alveolar leucocyte subpopulations exhibited different time courses in the acid injured lung with persistent elevation of alveolar lymphocytes and macrophages. After acid instillation there was an early transient decrease in arterial oxygen tension and lung perfusion was preferentially distributed to the non-injured lung.

CONCLUSION

These findings provide a basis for further research in the field of lung acid injury and for studies exploring effects of mechanical ventilation on injured lungs. Incomplete recovery in the directly injured lung 7 days after acid instillation suggests that increased vulnerability and susceptibility to further noxious stimuli are still present at that time.

Exhaled nitric oxide and carbon monoxide in mechanically ventilated brain-injured patients

The inflammatory influence and biological markers of prolonged mechanical-ventilation in uninjured human lungs remains controversial. We investigated exhaled nitric oxide (NO) and carbon monoxide (CO) in mechanically-ventilated, brain-injured patients in the absence of lung injury or sepsis at two different levels of positive end-expiratory pressure (PEEP). Exhaled NO and CO were assessed in 27 patients, without lung injury or sepsis, who were ventilated with 8 ml kg(-1) tidal volumes under zero end-expiratory pressure (ZEEP group, n  =  12) or 8 cm H2O PEEP (PEEP group, n  =  15). Exhaled NO and CO was analysed on days 1, 3 and 5 of mechanical ventilation and correlated with previously reported markers of inflammation and gas exchange. Exhaled NO was higher on day 3 and 5 in both patient groups compared to day 1: (PEEP group: 5.8 (4.4-9.7) versus 11.7 (6.9-13.9) versus 10.7 (5.6-16.6) ppb (p  <  0.05); ZEEP group: 5.3 (3.8-8.8) versus 9.8 (5.3-12.4) versus 9.6 (6.2-13.5) ppb NO peak levels for days 1, 3 and 5, respectively, p  <  0.05). Exhaled CO remained stable on day 3 but significantly decreased by day 5 in the ZEEP group only (6.3 (4.3-9.0) versus 8.1 (5.8-12.1) ppm CO peak levels for day 5 versus 1, p  <  0.05). The change scores for peak exhaled CO over day 1 and 5 showed significant correlations with arterial blood pH and plasma TNF levels (r s  =  0.49, p  =  0.02 and r s  =  -0.51 p  =  0.02, respectively). Exhaled NO correlated with blood pH in the ZEEP group and with plasma levels of IL-6 in the PEEP group. We observed differential changes in exhaled NO and CO in mechanically-ventilated patients even in the absence of manifest lung injury or sepsis. These may suggest subtle pulmonary inflammation and support application of real time breath analysis for molecular monitoring in critically ill patients.

Poloxamer 188 facilitates the repair of alveolus resident cells in ventilator-injured lungs

RATIONALE

Wounded alveolus resident cells are identified in human and experimental acute respiratory distress syndrome models. Poloxamer 188 (P188) is an amphiphilic macromolecule shown to have plasma membrane-sealing properties in various cell types.

OBJECTIVES

To investigate whether P188 (1) protects alveolus resident cells from necrosis and (2) is associated with reduced ventilator-induced lung injury in live rats, isolated perfused rat lungs, and scratch and stretch-wounded alveolar epithelial cells.

METHODS

Seventy-four live rats and 18 isolated perfused rat lungs were ventilated with injurious or protective strategies while infused with P188 or control solution. Alveolar epithelial cell monolayers were subjected to scratch or stretch wounding in the presence or absence of P188.

MEASUREMENTS AND MAIN RESULTS

P188 was associated with fewer mortally wounded alveolar cells in live rats and isolated perfused lungs. In vitro, P188 reduced the number of injured and necrotic cells, suggesting that P188 promotes cell repair and renders plasma membranes more resilient to deforming stress. The enhanced cell survival was accompanied by improvement in conventional measures of lung injury (peak airway pressure, wet-to-dry weight ratio) only in the ex vivo-perfused lung preparation and not in the live animal model.

CONCLUSIONS

P188 facilitates plasma membrane repair in alveolus resident cells, but has no salutary effects on lung mechanics or vascular barrier properties in live animals. This discordance may have pathophysiological significance for the interdependence of different injury mechanisms and therapeutic implications regarding the benefits of prolonging the life of stress-activated cells.

Persistence after birth of systemic inflammation associated with umbilical cord inflammation

Intrauterine inflammation is followed by elevated concentrations of inflammation-related proteins in the newborn's blood. Many of these proteins have short half-lives. The persistence of this postnatal inflammation has not previously been investigated. In a sample of 834 infants born before the 28th week of gestation, 12% (103) had grade 1 or 2, and 17% (142) had grade 3, 4, or 5 umbilical cord inflammation. Concentrations of nine proteins previously shown to be associated with umbilical cord inflammation at birth were measured on the first postnatal day and at two weekly intervals after birth. We evaluated the hypothesis that children who had umbilical cord inflammation were no more likely than others to have elevated concentrations of inflammation-related proteins in postnatal blood. The concentrations of seven of the nine proteins [C-reactive protein (CRP), myeloperoxidase (MPO), IL1β, IL8, TNFα, intercellular adhesion molecule-1 (ICAM3), and matrix metalloproteinase (MMP9)] showed a tendency to be elevated on day 7 among infants with funisitis. Adjusting for gestational age, growth restriction, and three postnatal exposures (ventilation on day 7, presumed and definite early bacteremia, and Bell stage III necrotizing endocolitis) did not diminish the elevated odds ratios of concentrations in the top quartile (for gestational age and day the specimen was obtained) of MPO, IL1β, TNFα, IL8, ICAM3, and MMP9. The persistence of a relationship between umbilical cord inflammation and elevated blood concentrations of inflammation-related proteins on postnatal day 7 suggests the existence of phenomena that contribute to a reinforcement loop and thereby sustained systemic inflammation.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

The physical basis of ventilator-induced lung injury

Although mechanical ventilation (MV) is a life-saving intervention for patients with acute respiratory distress syndrome (ARDS), it can aggravate or cause lung injury, known as ventilator-induced lung injury (VILI). The biophysical characteristics of heterogeneously injured ARDS lungs increase the parenchymal stress associated with breathing, which is further aggravated by MV. Cells, in particular those lining the capillaries, airways and alveoli, transform this strain into chemical signals (mechanotransduction). The interaction of reparative and injurious mechanotransductive pathways leads to VILI. Several attempts have been made to identify clinical surrogate measures of lung stress/strain (e.g., density changes in chest computed tomography, lower and upper inflection points of the pressure-volume curve, plateau pressure and inflammatory cytokine levels) that could be used to titrate MV. However, uncertainty about the topographical distribution of stress relative to that of the susceptibility of the cells and tissues to injury makes the existence of a single 'global' stress/strain injury threshold doubtful.

Protective mechanical ventilation does not exacerbate lung function impairment or lung inflammation following influenza A infection

The degree to which mechanical ventilation induces ventilator-associated lung injury is dependent on the initial acute lung injury (ALI). Viral-induced ALI is poorly studied, and this study aimed to determine whether ALI induced by a clinically relevant infection is exacerbated by protective mechanical ventilation. Adult female BALB/c mice were inoculated with 104.5 plaque-forming units of influenza A/Mem/1/71 in 50 μl of medium or medium alone. This study used a protective ventilation strategy, whereby mice were anesthetized, tracheostomized, and mechanically ventilated for 2 h. Lung mechanics were measured periodically throughout the ventilation period using a modification of the forced oscillation technique to obtain measures of airway resistance and coefficients of tissue damping and tissue elastance. Thoracic gas volume was measured and used to obtain specific airway resistance, tissue damping, and tissue elastance. At the end of the ventilation period, a bronchoalveolar lavage sample was collected to measure inflammatory cells, macrophage inflammatory protein-2, IL-6, TNF-α, and protein leak. Influenza infection caused significant increases in inflammatory cells, protein leak, and deterioration in lung mechanics that were not exacerbated by mechanical ventilation, in contrast to previous studies using bacterial and mouse-specific viral infection. This study highlighted the importance of type and severity of lung injury in determining outcome following mechanical ventilation.

Extracellular matrix and mechanical ventilation in healthy lungs: back to baro/volotrauma?

PURPOSE OF REVIEW

The extracellular matrix plays an important role in the biomechanical behaviour of the lung parenchyma. The matrix is composed of a three-dimensional fibre mesh filled with different macromolecules, including proteoglycans which have important functions in many lung pathophysiological processes, as they regulate tissue hydration, macromolecular structure and function, response to inflammatory agents, and tissue repair and remodelling. The aim of this review is to describe the role of mechanical ventilation on pulmonary extracellular matrix structure and function.

RECENT FINDINGS

Recent experimental and clinical data suggest that in healthy lungs, mechanical ventilation with tidal volume ranging between 7 and 12 ml/kg in the absence of positive end-expiratory pressure may lead to endothelial, extracellular matrix and peripheral airways damage without major inflammatory response. Several mechanisms may explain damage to the lung structure induced by mechanical ventilation: regional overdistension, 'low lung volume' associated with tidal airway closure, and inactivation of surfactant.

SUMMARY

Tidal volume reduction to 6 ml/kg may be useful during mechanical ventilation of healthy lungs. The study of the extracellular matrix may be useful to better understand the pathophysiology of ventilator-induced lung injury in healthy and diseased lungs.

Cell mechanics of alveolar epithelial cells (AECs) and macrophages (AMs)

Cell mechanics provides an integrated view of many biological phenomena which are intimately related to cell structure and function. Because breathing constitutes a sustained motion synonymous with life, pulmonary cells are normally designed to support permanent cyclic stretch without breaking, while receiving mechanical cues from their environment. The authors study the mechanical responses of alveolar cells, namely epithelial cells and macrophages, exposed to well-controlled mechanical stress in order to understand pulmonary cell response and function. They discuss the principle, advantages and limits of a cytoskeleton-specific micromanipulation technique, magnetic bead twisting cytometry, potentially applicable in vivo. They also compare the pertinence of various models (e.g., rheological; power law) used to extract cell mechanical properties and discuss cell stress/strain hardening properties and cell dynamic response in relation to the structural tensegrity model. Overall, alveolar cells provide a pertinent model to study the biological processes governing cellular response to controlled stress or strain.