Ventilator-induced lung injury: historical perspectives and clinical implications
© de Prost et al; licensee Springer. 2011
Received: 25 June 2011
Accepted: 23 July 2011
Published: 23 July 2011
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© de Prost et al; licensee Springer. 2011
Received: 25 June 2011
Accepted: 23 July 2011
Published: 23 July 2011
Mechanical ventilation can produce lung physiological and morphological alterations termed ventilator-induced lung injury (VILI). Early experimental studies demonstrated that the main determinant of VILI is lung end-inspiratory volume. The clinical relevance of these experimental findings received resounding confirmation with the results of the acute respiratory distress syndrome (ARDS) Network study, which showed a 22% reduction in mortality in patients with the acute respiratory distress syndrome through a simple reduction in tidal volume. In contrast, the clinical relevance of low lung volume injury remains debated and the application of high positive end-expiratory pressure levels can contribute to lung overdistension and thus be deleterious. The significance of inflammatory alterations observed during VILI is debated and has not translated into clinical application. This review examines seminal experimental studies that led to our current understanding of VILI and contributed to the current recommendations in the respiratory support of ARDS patients.
The prognosis of the acute respiratory distress syndrome (ARDS) has improved dramatically within the past decades, with in-hospital mortality rates ranging from 90% in the seventies  to approximately 30% in a recent study . Reduction of the tidal volume delivered to mechanically ventilated patients, and thus of the stress applied to their lungs, unambiguously contributed to improving outcomes, as demonstrated by the ARDSnet study, which showed a 22% higher survival in patients who received lower (6 mL/kg) than in those who received larger (12 mL/kg) tidal volumes . Interestingly, almost one decade before the ARDSnet study was published, the concept of "permissive hypercapnia"  had already led to the use of lower tidal volumes by clinicians and well-conducted observational studies had evidenced significant decrease in the mortality of patients suffering from ARDS . Indeed, compelling physiological evidence had been drawn from experimental studies that had described the deleterious effects of mechanical ventilation using high peak inspiratory pressures on lungs, regrouped under the term ventilator-induced lung injury (VILI) [6–8]. In addition to this "volutrauma," so-called "low-volume" injury associated with the repeated recruitment and derecruitment of distal lung units has been incriminated in the development of VILI and forms the rationale for the use of positive end-expiratory pressure (PEEP) [9–11]. We reviewed seminal experimental studies that led to our current understanding of VILI and contributed to the current recommendations in the respiratory support of ARDS patients.
Only 3 years after the first description of ARDS was made , Mead et al. developed the conceptual basis for VILI from the analysis of the mechanical properties of the lungs using a theoretical model of lung elasticity . They suggested that the forces acting on lung parenchyma can be actually much greater than those applied to the airway, and theorized that the pressure tending to expand an atelectatic region at a transpulmonary pressure of 30 cm H2O surrounded by fully expanded lung would be approximately 140 cm H2O . In a visionary statement, the authors concluded that "mechanical ventilation, by applying high transpulmonary pressures to heterogeneously expanded lungs, could contribute to the development of lung hemorrhage and hyaline membranes." In 1974, Webb and Tierney demonstrated for the first time that mechanical ventilation could generate lung lesions in intact animals . Rats ventilated with peak inspiratory pressures of 30 or 45 cm H2O developed pulmonary edema within 60 and 20 min, respectively. Interestingly enough, when a 10-cm H2O PEEP was applied and the level of end-inspiratory pressure kept constant, the amount of lung edema was lessened . Although the authors suggested that low tidal ventilation should be used, they recently mentioned that "this article seemed to interest few clinicians or investigators for a decade or more, perhaps because a similar degree of injury in patients was not apparent" and acknowledged that "in retrospect, it seems almost irresponsible that we didn't publicize our concerns that such ventilator patterns might be harmful to humans" . Since this seminal publication, our knowledge of VILI has considerably increased .
The increase in transmural microvascular pressure, even if modest, contributes to the severity of pulmonary edema, which may be fulminating during VILI, because any increase in the driving force will have a dramatic effect on edema formation in the face of an altered microvascular permeability .
The application of PEEP results in less severe lung lesions when end-inspiratory volume is kept constant. This might be related to a reduction of tidal volume and the stabilization of terminal units. Webb and Tierney showed that at 45 cm H2O teleinspiratory pressure, edema was less severe when a 10 cm H2O PEEP was applied and attributed this effect to the preservation of surfactant activity . It was later confirmed that for a same end-inspiratory pressure, rats ventilated with zero end-expiratory pressure exhibited larger amounts of lung edema, as determined by extravascular lung water measurement, than those ventilated with PEEP. However, in the presence of PEEP edema remained confined to the interstitium, whereas there was alveolar flooding in its absence . The application of PEEP during VILI development was associated with a preservation of the integrity of the alveolar epithelium and the only ultrastructural abnormalities observed were endothelial blebbing and interstitial edema . This beneficial effect of PEEP might be related to the reduction of cyclic recruitment-derecruitment of lung units, which causes the abrasion of the epithelial airspace lining by interfacial forces [10, 11]. These phenomena of repeated opening and closing of distal lung units have been theorized to provide an explanation why large cyclic changes in lung volume promote the development of edema. Indeed, for an identical increase in mean airway pressure, ventilation of hydrochloric acid-injured dog lungs with a large tidal volume and a low PEEP resulted in more severe edema than did ventilation with a small tidal volume and a high PEEP . The effect of the amplitude of tidal volume on alveolar epithelium protein permeability, end-inspiratory pressures being kept constant by manipulating PEEP level, was further confirmed in rats using noninvasive scintigraphic techniques . The alveolar albumin permeability-surface area product, measured from the clearance of an intra-tracheally instilled 99 mTc-labeled albumin solution, dramatically increased, and in a dose-dependent manner, when VT was increased from 8 to 24 and 29 mL/kg . Finally, the decrease in cardiac output secondary to the increase in intrathoracic pressures has been demonstrated to account for a part of the PEEP-induced reduction in pulmonary edema . All in all, the beneficial effects of PEEP during high-volume ventilation (i.e., reductions in both the amount of edema and the severity of cell damage) involve a combination of hemodynamic alterations, shear stress reduction, and surfactant modifications.
Because of the heterogeneity of lung volume reduction, ventilation will be redistributed toward the more compliant zones, which may favor their overinflation. In the absence of an accurate tool for measuring ventilatable lung volume, analysis of the pressure-volume (PV) curve may help understand how pre-existing lung injury interacts with ventilator-induced injury. The decrease in compliance associated with lung edema is related to the reduction in the ventilatable lung volume, the so-called "baby lung" observed during ARDS [39, 40]. In addition, the upper inflection point (UIP) of the PV curve represents the lung volume at which lung compliance begins to diminish and thus is a surrogate of the beginning of overinflation [41, 42]. Both respiratory system compliance and the position of the UIP may allow for a better understanding of the consequences of high-volume ventilation. Indeed, the amount of pulmonary edema produced by high-volume ventilation in animals given ANTU  was inversely proportional to the respiratory system compliance measured during the first breaths, i.e., before any damage due to ventilation had occurred. In other words, the lower the lung compliance after ANTU infusion, the higher the amount of edema during VILI. Similarly, animals with an UIP occurring at lower pressures (indicating an earlier onset of overdistension) developed more edema than those with an UIP occurring at higher pressures. This suggests that reduced lung distensibility predisposes to the noxious effects of high volume ventilation.
This concept was further strengthened during experimental reduction of the ventilatable lung volume by instillation of a viscous liquid in distal airways of rats. As with ANTU, the higher the compliance and volume of the UIP after instillation of the liquid but before the onset of high-volume ventilation, the lower was the amount of edema observed after high peak pressure ventilation, suggesting that the UIP is a marker of the amount of ventilatable lung volume, and a predictor of the development of edema during mechanical ventilation .
The lower inflection point (LIP) of the PV curve corresponds to the volume and pressure at which there is the greatest increase in the compliance of the respiratory system. This point may reflect the reexpansion of atelectatic parenchyma and has been considered to indicate the minimal pressure required to recruit collapsed alveoli, as setting the PEEP level according to this point has been shown to improve oxygenation of ARDS patients [44, 45]. Sykes' group showed that setting PEEP above the LIP in rabbit models of surfactant depletion (after repeated alveolar lavage) improved oxygenation and lessened lung damage compared with lower PEEP levels [9, 46]. This lessening of pathological alterations was observed even with ventilator settings that achieved identical mean airway pressures in the low and high PEEP groups . These observations were confirmed in isolated surfactant-depleted nonperfused lungs . In contrast, those findings could not be replicated by Sykes' group in a rabbit model of hydrochloric acid instillation , suggesting that this strategy for setting PEEP based on the LIP of the pressure-volume curve is only beneficial in conditions associated with major alveolar instabilities, such as encountered during surfactant depletion. Moreover, Lichtwarck-Aschoff et al. showed that when PEEP was set at the LIP in surfactant-depleted piglets, there was a decrease in compliance during tidal volume insufflation, which indicated overinflation . The authors concluded that the PEEP level that allows compliance to remain constant during the full tidal volume insufflation cannot be routinely derived from analysis of the pressure-volume curve.
These discrepancies are not trivial because they underlie the concept of "protective ventilation" during acute lung injury. Indeed, whereas numerous experimental studies have demonstrated that lung overinflation--regional or global--leads to VILI [6–8, 36, 49], the genesis of lesions at low lung volume is much more debated . Such lesions could result from repetitive opening and collapse of distal airways/alveoli, a mechanism termed "atelectrauma" . However, as explained earlier, this phenomenon might be limited to certain particular settings (e.g., surfactant depletion) and might not be relevant to edematous lungs. For instance, Hubmayr's group challenged this concept using both elegant experimental settings and insightful mathematical models [50, 52]. They concluded that distal airways do not close and open during ventilation when PEEP is set below the LIP but, instead, demonstrated that the LIP reflects the movement of liquid or foam in the airways: when a liquid column is present in the airways, it opposes a marked resistance to the airflow; after a certain pressure threshold the liquid is propelled into the alveoli where it can distribute in a much larger volume than in the airways. As a result, there is an abrupt gain in volume at constant (or even decreased) pressure that translates into a prominent knee on the PV curve. In such circumstances, no epithelial lesion is generated and the LIP may be considered as an artifact. There is no doubt that a certain level of PEEP is beneficial during VILI [6, 9, 29, 46], but there is no firm demonstration that this level must necessarily be "high" rather than "low" and may be deduced from the presence of a LIP on the PV curve. Interestingly, this controversy about the respective contribution of overall lung distension and of cyclic recruitment-derecruitment has its exact counterpart for the management of ARDS: it is beyond doubt that tidal volume reduction saves lives , whereas the improvement of prognosis with higher PEEP is highly disputable [2, 53, 54]. This clinical controversy will be addressed elsewhere in this article.
Reducing the stress applied to the lungs by lowering tidal volume improved the survival of ARDS patients . However, the mortality rate remains high, between 30 and 60% depending on the study [2, 69]. Thus, a considerable amount of studies aiming at developing new ventilator strategies or pharmacological treatments of VILI have been published.
Despite promising experimental results that suggest that they could suppress air-liquid interfaces and allow for reopening of collapsed or liquid-filled areas, surfactant administration  and partial liquid ventilation with perfluorocarbons  have been abandoned since the negative results of clinical trials. Synthetic surfactant administration failed to improve oxygenation  and to improve lung mechanics  in ARDS patients. This could be related to the type of surfactant tested as another study using a natural surfactant in a pediatric population with acute lung injury was associated with increased survival . Partial liquid ventilation with perfluorocarbons at both "high" (20 mL/kg) and "low" (10 mL/kg) doses did not improve outcome of ARDS patients . Such negative results might have been anticipated from the results of experimental studies that had previously demonstrated that ventilator-induced pulmonary edema was aggravated in animals given such high doses of perfluorocarbons because they favored gas trapping in the distal lung .
Two randomized, controlled trials showed no effect of prone positioning on outcome of ARDS patients [77, 78]. However, Mancebo et al. demonstrated that prone positioning was associated with a trend toward higher survival when administered early during the course of the disease and for as much as 20 h per day . A recent meta-analysis found that prone positioning was associated with improved mortality in the most hypoxemic patients (i.e., having a PaO2/FiO2 ratio < 100 mmHg) . These results are in keeping with experimental studies that evidenced that prone ventilation lessened the histological injury associated with high peak pressure ventilation in a dog model of oleic-acid lung injury . These protective effects likely stem from a more homogenous distribution of ventilation associated with prone ventilation .
Numerous cell signaling pathways are involved in the pathophysiology of VILI. As such, hundreds of studies aiming at testing pharmacologic interventions during VILI have been published: 1) studies designed to modulate microvascular permeability using blockers of stretch-activated cation channels , beta-adrenergic agonists , inhibitors of phosphotyrosine kinase , or reducing myosin light chain phosphorylation with adrenomedullin ; 2) studies testing the modulation of the imbalance between pro- and anti-inflammatory mediators in the lung. For instance, the administration of anti-TNF-α antibody [87–89] and the inhibition of MIP-2 activity [90, 91] reduced neutrophilic infiltration and lung injury; and 3) studies modulating hormonal and metabolic pathways: inhibition of the renin-angiotensin system [92, 93] and pretreatment with atorvastatin or simvastatin [94, 95] decreased alveolar-capillary barrier permeability and lung inflammation in experimental models of VILI. However, none of those pharmacological interventions has proven beneficial for the prevention and treatment of VILI in patients. Although it is probably illusory to believe that a single pharmacological intervention might be beneficial in patients, the description of those pathways illustrates the complexity of the cellular mechanisms involved in VILI.
As discussed earlier in this article, the prevention of ventilator-associated lung injury during treatment of ALI may theoretically stem from two approaches: 1) easing the strain  applied to diseased lungs through the reduction of tidal volume and therefore of end-inspiratory lung volume (which may be evaluated indirectly by inspiratory plateau pressure); and 2) reducing the so-called atelectrauma via an adequate use of PEEP. Whereas the first concept, which is undisputed on physiological grounds, received a resounding illustration with the demonstration of an improved prognosis by a simple reduction of tidal volume in ARDS patients , things are far less clear regarding the second. Indeed, as explained above, the concept of repetitive opening and closure or distal airspaces and the putative low lung volume lesions related to insufficient PEEP levels remains debated. This uncertainty formed the basis of three high-quality, independent, randomized, controlled studies [2, 53, 54], which all failed to demonstrate improved survival with a high PEEP strategy. Although a meta-analysis of these trials showed a reduction of mortality using higher PEEP levels in the most severe population , this does not constitute a definite proof but rather a hypothesis that would require another well-conducted, randomized trial to be confirmed. Moreover, the marked heterogeneity of ventilator protocols among these three studies (with resulting considerable differences in plateau pressures between one study  and the others [2, 53]) casts some doubt on the physiological rationale for this meta-analysis. Interestingly, a recent study using positron emission tomography imaging in ARDS patients to quantify lung metabolism, a surrogate of lung inflammation, showed that lung regions undergoing cyclic recruitment-derecruitment did not exhibit higher metabolism than those continuously collapsed throughout the respiratory cycle, thus questioning the concept of low lung volume injury .
Few experimental concepts have led to dramatic changes in clinical practices as the concept of VILI did. The understanding that end-inspiratory distension is the main determinant of VILI led to the reduction of tidal volume and improved the survival of ARDS patients. Yet, whether tidal volume should be set at 6 mL/kg in all patients remains unsettled , and there is currently no bedside tool that allows an accurate assessment of the aerated lung volume and thus no way to tailor tidal volume routinely. Rather than sticking to arbitrarily fixed approaches, clinicians should tailor mechanical ventilation according to patient's individual characteristics, including close monitoring of plateau pressure and maybe PEEP titration guided by esophageal pressure monitoring . In addition, opinion leaders should strive to convince clinicians not to use excessive tidal volume, which is still unfortunately the case . Indeed, two epidemiological studies suggested an association between ventilator settings (i.e., use of a tidal volume > 6 mL/kg or 700 mL  and plateau pressure > 30 cm H2O ) and the development of ARDS in mechanically ventilated patients who did not meet ARDS criteria upon hospital admission. Similarly, a relationship between the use of large tidal volumes and postoperative respiratory failure has been established after mechanical ventilation in patients undergoing a pneumonectomy, suggesting that ventilation does not need to be protracted to be deleterious . Even though the tidal volume to be used in patients with so-called normal lungs has not been defined yet, these studies suggest that protective ventilator settings could prevent the development or ARDS, particularly when a systemic insult is associated.
The story of VILI and its clinical correlates may be viewed as a success of applied physiology [105, 106]. Indeed, based on sound physiology, physicians had started to reduce tidal volume long before this strategy was proved by randomized, controlled trials [5, 107]. The considerable improvement in survival of ARDS patients with a simple and low-cost physiologic approach is at striking contrast with the failure of many randomized trials of expensive drugs to improve the prognosis of critically ill patients [105, 106]. Nevertheless, many questions persist and VILI will likely not soon be out of the spotlight.
acute respiratory distress syndrome
lower inflection point
positive end-expiratory pressure
upper inflection point
ventilator-induced lung injury.
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