Injurious ventilatory strategies increase cytokines and c-fos m-RNA expression in an isolated rat lung model

Prolonged alveolocapillary barrier damage after acute cardiogenic pulmonary edema

OBJECTIVES

To determine whether acute cardiogenic pulmonary edema is associated with damage to the alveolocapillary barrier, as evidenced by increased leakage of surfactant specific proteins into the circulation, to document the duration of alveolocapillary barrier damage in this setting, and to explore the role of pulmonary parenchymal inflammation by determining if circulating tumor necrosis factor-alpha is increased after acute cardiogenic pulmonary edema.

DESIGN

Prospective, observational study.

SETTING

Critical care, cardiac intensive care, and cardiology wards of a tertiary-care university teaching hospital.

PATIENTS

A total of 28 patients presenting with acute cardiogenic pulmonary edema and 13 age-matched normal volunteers.

INTERVENTIONS

Circulating surfactant protein-A and -B and tumor necrosis factor-alpha were measured on days 0 (presentation), 1, 3, 7, and 14. Clinical markers of pulmonary edema were documented at the same times.

MEASUREMENTS AND MAIN RESULTS

Surfactant protein-A and -B were elevated on day 0 compared with controls (367 +/- 17 ng/mL vs. 303 +/- 17 and 3821 +/- 266 ng/mL vs. 2747 +/- 157 [mean +/- sem], p <.05), and although clinical, hemodynamic and radiographic variables improved rapidly (p <.001), surfactant protein-A and -B rose further until day 3 (437 +/- 22, p <.001, 4642 +/- 353, p <.01). Tumor necrosis factor-alpha was elevated at presentation (p <.05), doubled by day 1 (6.98 +/- 1.36 pg/mL, p <.05), remained elevated on day 3 (5.72 +/- 0.96 pg/mL, p <.05), and peak levels were related to chest radiograph extravascular lung water score (r(p) = 0.64, p =.003).

CONCLUSIONS

Although the initial increase in plasma surfactant protein-A and -B may represent hydrostatic stress failure of the alveolocapillary barrier, the prolonged elevation, when hemodynamic abnormalities have resolved, and the delayed elevation of tumor necrosis factor-alpha are consistent with pulmonary parenchymal inflammation, which may further damage the alveolocapillary barrier. This prolonged physiologic defect at the alveolocapillary barrier after acute cardiogenic pulmonary edema may partly account for the vulnerability of these patients to recurrent pulmonary fluid accumulation.

Lung-protective ventilation strategies in acute lung injury

OBJECTIVES

To review the challenges of providing mechanical ventilatory support for respiratory failure while avoiding ventilator-associated lung injury in patients with acute lung injury. To review the results of several randomized clinical trials of lung-protective ventilation strategies using conventional mechanical ventilators.

DATA SOURCES

Published reports of clinical trials comparing clinical outcomes of patients with acute lung injury, randomized to mechanical ventilation with either a lung-protective or a control, conventional, standard, or traditional approach.

DATA EXTRACTION AND SYNTHESIS

Lung-protective mechanical ventilation strategies are designed to prevent injury from overdistention by using lower tidal volumes and lower inspiratory pressures (volume- and pressure-limited ventilation) or injury from ventilation with atelectasis and alveolar flooding at end-expiration (open-lung ventilation). In one trial, clinical outcomes were better in the study group that received combined volume- and pressure-limited and open-lung strategies compared with the study group that received a conventional approach. Of four trials focusing on volume- and pressure-limited ventilation alone, three did not demonstrate improvements in clinical outcomes, whereas one demonstrated a substantial reduction in mortality and an increase in ventilator-free days. The different results in these four trials may be attributable to differences in tidal volumes between the study groups, chance variation, or differences in the management of respiratory acidosis.

CONCLUSIONS

Evidence supports the use of a volume- and pressure-limited approach to mechanical ventilation in patients with acute lung injury. It is not yet clear whether the open-lung approach will further reduce mortality in patients receiving volume- and pressure-limited ventilation support.

Physiologic rationale for ventilator setting in acute lung injury/acute respiratory distress syndrome patients

OBJECTIVES

To review the physiologic approach to setting mechanical ventilation in acute lung injury/acute respiratory distress syndrome.

DATA SOURCES

MEDLINE search from 1979 to the present.

DATA SELECTION

Personal selection of some articles we believe relevant for understanding acute lung injury/acute respiratory distress syndrome physiopathology and its physiologic management.

DATA SUMMARY

Knowing the underlying pathology is key to estimating the potential for recruitment. The potential for recruitment is rather low when the consolidation of pulmonary units exceeds collapse, as in diffuse pneumonia. In contrast, when pulmonary unit collapse exceeds consolidation, as in acute lung injury/acute respiratory distress syndrome from extrapulmonary origin, the potential for recruitment may be high. To exploit the potential for recruitment, a transpulmonary pressure greater than the opening pressure must be applied to the lung. To do so, chest wall elastance must be measured or estimated. To avoid collapse after recruitment, a positive end-expiratory pressure greater than the compressive forces operating on the lung and an alveolar ventilation sufficient to prevent absorption atelectasis must be provided. Indeed, avoidance of stretch (low airway plateau pressure) and prevention of cyclic collapse and reopening (adequate positive end-expiratory pressure and alveolar ventilation) are the physiologic cornerstones of mechanical ventilation in acute lung injury/acute respiratory distress syndrome. When considering all the randomized clinical trials reported so far, it is tempting to speculate that transpulmonary pressure and stresses, rather than tidal volume per se, are the key factors that may have an impact on mortality.

CONCLUSIONS

The majority of physiologic, experimental, and clinical trial data converge on one simple concept: treat the lung gently.

Mechanical stretch induces MMP-2 release and activation in lung endothelium: role of EMMPRIN

High-volume mechanical ventilation leads to ventilator-induced lung injury. This type of lung injury is accompanied by an increased release and activation of matrix metalloproteinases (MMPs). To investigate the mechanism leading to the increased MMP release, we systematically studied the effect of mechanical stretch on human microvascular endothelial cells isolated from the lung. We exposed cells grown on collagen 1 BioFlex plates to sinusoidal cyclic stretch at 0.5 Hz using the Flexercell system with 17-18% elongation of cells. After 4 days of cell stretching, conditioned media and cell lysate were collected and analyzed by gelatin, casein, and reverse zymograms as well as Western blotting. RT-PCR of mRNA extracted from stretched cells was performed. Our results show that 1) cyclic stretch led to increased release and activation of MMP-2 and MMP-1; 2) the activation of MMP-2 was accompanied by an increase in membrane type-1 MMP (MT1-MMP) and inhibited by a hydroxamic acid-derived inhibitor of MMPs (Prinomastat, AG3340); and 3) the MMP-2 release and activation were preceded by an increase in production of extracellular MMP inducer (EMMPRIN). These results suggest that cyclic mechanical stretch leads to MMP-2 activation through an MT1-MMP mechanism. EMMPRIN may play an important role in the release and activation of MMPs during lung injury.

Effects of sustained inflation and postinflation positive end-expiratory pressure in acute respiratory distress syndrome: focusing on pulmonary and extrapulmonary forms

OBJECTIVE

To investigate whether the response to sustained inflation and postinflation positive end-expiratory pressure varies between acute respiratory distress syndrome with pulmonary (ARDS(exp)) and extrapulmonary origin (ARDS(exp)).

DESIGN

Prospective clinical study.

SETTING

Multidisciplinary intensive care unit in a university hospital.

PATIENTS

A total of 11 patients with ARDS and 13 patients with ARDS.

INTERVENTIONS

A 7 ml/kg tidal volume, 12-15 breaths/min respiratory rate, and an inspiratory/expiratory ratio of 1:2 was used during baseline ventilation. Positive end-expiratory pressure levels were set according to the decision of the primary physician. Sustained inflation was performed by 45 cm H2O continuous positive airway pressure for 30 secs. Postinflation positive end-expiratory pressure was titrated decrementally, starting from a level of 20 cm H2O to keep the peripheral oxygen saturation between 92% and 95%. Fio2 was decreased, and baseline tidal volume, respiratory rate, inspiratory/expiratory ratio were maintained unchanged throughout the study period.

MEASUREMENTS AND MAIN RESULTS

Blood gas, airway pressure, and hemodynamic measurements were performed at the following time points: at baseline and at 15 mins, 1 hr, 4 hrs, and 6 hrs after sustained inflation. After sustained inflation, the Pao2/Fio2 ratio improved in all of the patients both in ARDS(p) and ARDS(exp). However, the Pao2/Fio2 ratio increased to >200 in four ARDS(p) patients (36%) and in seven ARDS(p) patients (54%). In two of those ARDS patients, the Pao2/Fio2 ratio was found to be <200, whereas none of the ARDS(p) patients revealed Pao2/Fio2 ratios of <200 at the 6-hr measurement. Postinflation positive end-expiratory pressure levels were set at 16.7 +/- 2.3 cm H O in ARDS(p) and 15.6 +/- 2.5 cm H2O in ARDS. The change in Pao /Fio ratios was found statistically significant in patients with ARDS(p) (p =.0001) and with ARDS(p) (p =.008). Respiratory system compliance increased in ARDS patients (p =.02), whereas the change in ARDS was not statistically significant.

CONCLUSIONS

Sustained inflation followed by high levels of postinflation positive end-expiratory pressure provided an increase in respiratory system compliance in ARDS; however, arterial oxygenation improved in both ARDS forms.

Advances in the Understanding of Clinical Manifestations and Therapy of Severe Sepsis: An Update for Critical Care Nurses

Severe sepsis is a major public health concern and a burden on the healthcare system. Despite improvements in efforts to control the source of infection and increased recognition by healthcare providers of patients with the disease, the mortality rate remains unacceptably high, from 30% to 50%. The systemic inflammatory response syndrome criteria are used as diagnostic indicators of sepsis when they occur in patients with known or suspected infection. The outcome of a patient with severe sepsis is often related to the occurrence of sepsis-induced multiple organ dysfunction syndrome. Multiple organ dysfunction syndrome appears to result from a cascade of organism-related factors, inflammatory mediators, endothelial injury, disturbed hemostasis, and microcirculatory abnormalities. In patients with severe sepsis, derangements of inflammation and coagulation are tightly linked. Although numerous clinical trials focused on interventions in one or the other of the inflammatory and coagulation systems failed to show reduced mortality due to sepsis, a member of a new class of drugs called “cogins” was effective. In its active form, protein C has anti-inflammatory, antithrombotic, and profibrinolytic properties that can reduce organ injury associated with severe sepsis. A recombinant form of activated protein C, drotrecogin alfa (activated), significantly reduces 28-day mortality due to all causes in patients with severe sepsis and has an acceptable safety profile. This review provides an overview of severe sepsis, highlighting recent advances in treatment of the disease and the role of critical care nurses.

High oxygen concentrations predispose mouse lungs to the deleterious effects of high stretch ventilation

Mechanical ventilation is a necessary intervention for patients with acute lung injury. However, mechanical ventilation can propagate acute lung injury and increase systemic inflammation. The exposure to >21% oxygen is often associated with mechanical ventilation yet has not been examined within the context of lung stretch. We hypothesized that mice exposed to >90% oxygen will be more susceptible to the deleterious effects of high stretch mechanical ventilation. C57B1/6 mice were randomized into 48-h exposure of 21 or >90% oxygen; mice were then killed, and isolated lungs were randomized into a nonstretch or an ex vivo, high-stretch mechanical ventilation group. Lungs were assessed for compliance and lavaged for surfactant analysis, and cytokine measurements or lungs were homogenized for surfactant-associated protein analysis. Mice exposed to >90% oxygen + stretch had significantly lower compliance, altered pulmonary surfactant, and increased inflammatory cytokines compared with all other groups. Our conclusion is that 48 h of >90% oxygen and high-stretch mechanical ventilation deleteriously affect lung function to a greater degree than stretch alone.

Mechanical ventilation-induced pneumoprotein CC-16 vascular transfer in rats: effect of KGF pretreatment

After air-blood barrier injury, "pneumoproteins" specific to lung epithelial distal airspaces reaching the bloodstream are putative markers of lung hyperpermeability. The contribution of mechanical ventilation (MV) to this leakage is unknown. To explore this issue, 16-kDa Clara cell protein (CC-16) concentration was quantified in bronchoalveolar lavages (BALFs) and/or sera of rats first exposed either to ambient air or to 48 h of hyperoxia-induced acute lung injury and then ventilated for 2 h according to one of the following strategies: 1) spontaneous ventilation (SV), 2) very-low-volume high PEEP (VLVHP, where PEEP is positive end-expiratory pressure), 3) low-volume zero PEEP, 4) moderate-volume low PEEP, and 5) high-volume zero PEEP (HVZP). Results show that total proteins in BALFs increased with time and MV, with little impact from hyperoxia preexposure. CC-16 content decreased in BALFs but increased in the bloodstream during MV, suggesting intravascular leakage. Lung overdistension may result either from high-volume (HVZP) or high-PEEP (VLVHP) MV, and it was the most potent inducer of CC-16 leakage (P < 0.05 vs. SV). In the VLVHP group, pretreatment with keratinocyte growth factor was efficient in reducing blood CC-16 transfer.

Heat shock proteins and ventilator-induced lung injury

In this review, we discuss the heat shock response, a specific example of gene expression that has been studied over the past 25 years, and its relevance to acute lung injury and other critical conditions. The heat shock response has been observed in virtually all organisms and involves the rapid induction of a set of highly conserved genes that encode heat shock proteins (HSPs). The HSP70 family represents the most prominent eukaryotic group of HSPs. It has been suggested that members of the HSP70 family act in the protection of cellular damage by binding to denatured or abnormal proteins after heat shock, thereby preventing protein aggregation. The capacity of HSPs to subserve cytoprotection has produced considerable interest from the perspective of elucidating the pathophysiology of organ damage and dysfunction. Several studies support the hypothesis that HSPs are cytoprotective In addition, recent investigations have demonstrated that HSP70 is released into the systemic circulation and is involved in the activation of innate immunity.

Acute renal failure leads to dysregulation of lung salt and water channels

BACKGROUND

Renal ischemia/reperfusion (I/R) injury and the acute respiratory distress syndrome (ARDS) frequently coexist in the intensive care setting, and this combination is associated with a high mortality. Recent experimental data demonstrate that renal I/R injury leads to an increase in pulmonary vascular permeability, similar to that observed in ARDS. However, the effects of renal I/R injury on alveolar fluid clearance-of potential importance in the setting of increased permeability-are unknown. We investigated the effects of renal I/R injury on pulmonary epithelial sodium channel (ENaC), Na,K-ATPase and aquaporin expression as a first step in addressing this question.

METHODS

Sprague Dawley rats were subjected to four protocols: (1) surgery for bilateral I/R injury, (2) sham surgery, (3) surgery for unilateral I/R injury, or (4) bilateral nephrectomy. Lung tissue was examined for Na channel, Na,K-ATPase, aquaporin-1, and aquaporin-5 expression. Northern and Western blots were performed.

RESULTS

Renal I/R injury and bilateral nephrectomy both led to marked down-regulation of pulmonary ENaC, Na,K-ATPase and aquaporin-5 but not aquaporin-1 compared to sham surgery. These changes were not influenced by the animals' volume status. In contrast, unilateral I/R with an intact contralateral kidney did not lead to down-regulation of channel down-regulation.

CONCLUSIONS

Ischemic acute renal failure leads to down regulation of pulmonary ENaC, Na,K-ATPase and aquaporin-5, but not aquaporin-1. Since bilateral nephrectomy but not single kidney I/R injury also leads to lung changes, these changes are likely mediated by systemic effects of acute renal failure (ARF), such as "uremic toxins," rather than reperfusion products. These changes may modulate lung dysfunction, susceptibility to lung injury, or both.

Inflammation and bronchopulmonary dysplasia

Pulmonary inflammation is a key feature in the pathogenesis of bronchopulmonary dysplasia (BPD). This inflammatory process, induced by multiple risk factors, is characterized by the presence of inflammatory cells, cytokines and an arsenal of additional humoral mediators in the airways and pulmonary tissue of preterm infants with the condition. Several mediators have a direct detrimental effect on pulmonary structures by affecting cell integrity and inducing apoptosis. An imbalance between pro-inflammatory and anti-inflammatory factors can generally be considered to be a hallmark of lung injury. Intrauterine exposure to pro-inflammatory cytokines or antenatal infection may prime the fetal lung such that minimally injurious postnatal events provoke an excessive pulmonary inflammatory response that most certainly affects normal alveolization and pulmonary vascular development in preterm infants with BPD.

Response of alveolar cells to mechanical stress

The purpose of this review is to highlight areas in alveolar cell biology in which our understanding of the effects of mechanical stress have been advanced in the last year, focusing on intracellular signal transduction pathways, the surfactant system, and cell injury and repair. Mechano-transduction pathways are only now beginning to be elucidated in alveolar cells. The importance of the mitogen-activated protein kinase, G protein, and growth factor systems is emphasized. The research conducted in the last year has also stressed the importance of alveolar cell cross-talk, with surfactant exocytosis being facilitated through parathyroid hormone-related peptide and leptin and calcium in interstitial fibroblasts and endothelial cells, respectively. Finally, the importance of deformation-induced plasma membrane breaks is emphasized. Alveolar cells were found to exocytose intracellular lipid vesicles to the plasma membrane-not only to prevent cell breaks but also to reseal cell breaks. This dynamic process was a stronger determinant of cell breaks than the prestress properties of the cytoskeleton. All of these exciting findings provide further potential treatment targets for ventilator-induced lung injury.

Mechanisms of surface-tension-induced epithelial cell damage in a model of pulmonary airway reopening

Airway collapse and reopening due to mechanical ventilation exerts mechanical stress on airway walls and injures surfactant-compromised lungs. The reopening of a collapsed airway was modeled experimentally and computationally by the progression of a semi-infinite bubble in a narrow fluid-occluded channel. The extent of injury caused by bubble progression to pulmonary epithelial cells lining the channel was evaluated. Counterintuitively, cell damage increased with decreasing opening velocity. The presence of pulmonary surfactant, Infasurf, completely abated the injury. These results support the hypotheses that mechanical stresses associated with airway reopening injure pulmonary epithelial cells and that pulmonary surfactant protects the epithelium from this injury. Computational simulations identified the magnitudes of components of the stress cycle associated with airway reopening (shear stress, pressure, shear stress gradient, or pressure gradient) that may be injurious to the epithelial cells. By comparing these magnitudes to the observed damage, we conclude that the steep pressure gradient near the bubble front was the most likely cause of the observed cellular damage.

Ventilator-associated lung injury

Mechanical ventilation is indispensable in support of patients with respiratory failure who are critically ill. However, use of this technique has adverse effects, including increased risk of pneumonia, impaired cardiac performance, and difficulties associated with sedation and paralysis. Moreover, application of pressure to the lung, whether positive or negative, can cause damage known as ventilator-associated lung injury (VALI). Despite difficulties in distinguishing the effects of mechanical ventilation from those of the underlying disorder, VALI greatly assists patients with the most severe form of lung injury, acute respiratory distress syndrome (ARDS). Moreover, modification of mechanical ventilation so that VALI is kept to a minimum improves survival of patients with ARDS. Here, we outline the effects of mechanical ventilation on injured lungs and explore the underlying mechanisms.

High in comparison with low tidal volume ventilation aggravates oxidative stress-induced lung injury

Ventilator settings influence the development and outcome of acute lung injury. This study investigates the influence of low versus high tidal volume (V(t)) on oxidative stress-induced lung injury. Isolated rabbit lungs were subjected to one of three ventilation patterns (V(t)-positive end-expiratory pressure, PEEP): LVZP (6 ml/kg-0 cm H(2)O), HVZP (12 ml/kg-0 cm H(2)O), LV5P (6 ml/kg-5 cm H(2)O). These ventilation patterns allowed a comparison between low and high V(t) without dependence on peak inspiratory pressure (PIP). Infusion of hypochlorite (1000 nmol/min) or buffer (control) was started at t=0 min. Pulmonary artery pressure (PAP), PIP and weight were continuously recorded. Capillary filtration coefficient [K(f,c) (10(-4) ml s(-1) cm H(2)O(-1) g(-1))] was gravimetrically determined (-15/30/60/90/120 min).PIP averaged 5.8+/-0.6/13.9+/-0.6/13.9+/-0.4 cm H(2)O in the LVZP, HVZP and LV5P groups. PIP, K(f,c) or PAP did not change in control groups, indicating that none of the ventilation patterns caused lung injury by themselves. Hypochlorite-induced increase in K(f,c) but not hypochlorite-induced increase in PAP, was significantly attenuated in the LVZP-/LV5P- versus the HVZP-group (K(f,c,max.) 1.0+/-0.23/1.4+/-0.40 versus 3.2+/-1.0*). Experiments with hypochlorite were terminated due to excessive edema (>50 g) at 97+/-2.2/94.5+/-4.5 min in the LVZP-/LV5P-group versus 82+/-3.8* min in the HVZP-group (*: P<0.05). Low V(t) attenuated oxidative stress-induced increase in vascular permeability independently from PIP and PEEP.

Innovative practices of ventilatory support with pediatric patients

OBJECTIVES

The recognition that alveolar overdistension rather than peak inspiratory airway pressure is the primary determinant of lung injury has shifted our understanding of the pathogenesis of ventilator-induced side effects. In this review, contemporary ventilatory methods, supportive treatments, and future developments relevant to pediatric critical care are reviewed.

DATA SYNTHESIS

A strategy combining recruitment maneuvers, low-tidal volume, and higher positive end-expiratory pressure (PEEP) decreases barotrauma and volutrauma. Given that appropriate tidal volumes are critical in determining adequate alveolar ventilation and avoiding lung injury, volume-control ventilation with high PEEP levels has been proposed as the preferable protective ventilatory mode. Pressure-related volume control ventilation and high-frequency oscillatory ventilation (HFOV) have taken on an important role as protective lung strategies. Further data are required in the treatment of children, confirming the preliminary results in specific lung pathologies. Spontaneous breathing supported artificially during inspiration (pressure support ventilation) is widely used to maintain or reactivate spontaneous breathing and to avoid hemodynamic variation. Volume support ventilation reduces the need for manual adaptation to maintain stable tidal and minute volume and can be useful in weaning. Prone positioning and permissive hypercapnia have taken on an important role in the treatment of patients undergoing artificial ventilation. Surfactant and nitric oxide have been proposed in specific lung pathologies to facilitate ventilation and gas exchange and to reduce inspired oxygen concentration. Investigation of lung ventilation using a liquid instead of gas has opened new vistas on several lung pathologies with high mortality rates.

RESULTS

The conviction emerges that the best ventilatory treatment may be obtained by applying a combination of types of ventilation and supportive treatments as outlined above. Early treatment is important for the overall positive final result. Lung recruitment maneuvers followed by maintaining an open lung favor rapid resolution of pathology and reduce side effects.

CONCLUSIONS

The methods proposed require confirmation through large controlled clinical trials that can assess the efficacy reported in pilot studies and case reports and define the optimal method(s) to treat individual pathologies in the various pediatric age groups.

One-lung ventilation with high tidal volumes and zero positive end-expiratory pressure is injurious in the isolated rabbit lung model

We tested the hypothesis that one-lung ventilation (OLV) with high tidal volumes (VT) and zero positive end-expiratory pressure (PEEP) may lead to ventilator-induced lung injury. In an isolated, perfused rabbit lung model, VT and PEEP were set to avoid lung collapse and overdistension in both lungs, resulting in a straight pressure-time (P-vs-t) curve during constant flow. Animals were randomized to (a) nonprotective OLV (left lung) (n = 6), with VT values as high as before randomization and zero PEEP; (b) protective OLV (left lung) (n = 6), with 50% reduction of VT and maintenance of PEEP as before randomization; and (c) control group (n = 6), with ventilation of two lungs as before randomization. The nonprotective OLV was associated with significantly smaller degrees of collapse and overdistension in the ventilated lung (P < 0.001). Peak inspiratory pressure values were higher in the nonprotective OLV group (P < 0.001) and increased progressively throughout the observation period (P < 0.01). The mean pulmonary artery pressure and lung weight gain values, as well as the concentration of thromboxane B(2), were comparatively higher in the nonprotective OLV group (P < 0.05). A ventilatory strategy with VT values as high as those used in the clinical setting and zero PEEP leads to ventilator-induced lung injury in this model of OLV, but this can be minimized with VT and PEEP values set to avoid lung overdistension and collapse.

IMPLICATIONS

One-lung ventilation with high tidal volumes and zero positive end-expiratory pressure (PEEP) is injurious in the isolated rabbit lung model. A ventilatory strategy with tidal volumes and PEEP set to avoid lung overdistension and collapse minimizes lung injury during one-lung ventilation in this model.

Science review: mechanisms of ventilator-induced injury

Acute respiratory distress syndrome (ARDS) and acute lung injury are among the most frequent reasons for intensive care unit admission, accounting for approximately one-third of admissions. Mortality from ARDS has been estimated as high as 70% in some studies. Until recently, however, no targeted therapy had been found to improve patient outcome, including mortality. With the completion of the National Institutes of Health-sponsored Acute Respiratory Distress Syndrome Network low tidal volume study, clinicians now have convincing evidence that ventilation with tidal volumes lower than those conventionally used in this patient population reduces the relative risk of mortality by 21%. These data confirm the long-held suspicion that the role of mechanical ventilation for acute hypoxemic respiratory failure is more than supportive, in that mechanical ventilation can also actively contribute to lung injury. The mechanisms of the protective effects of low tidal volume ventilation in conjunction with positive end expiratory pressure are incompletely understood and are the focus of ongoing studies. The objective of the present article is to review the potential cellular mechanisms of lung injury attributable to mechanical ventilation in patients with ARDS and acute lung injury.

Measurement of regional lung function in rats using hyperpolarized 3helium dynamic MRI

Dynamic regional lung function was investigated in rats using a radial acquisition cine (RA-CINE) pulse sequence together with hyperpolarized (HP) (3)He gas delivered by a constant flow ventilator. Based on regional differences in the behavior of inspired air, the lung was conceptually divided into two regions (the major airways and the peripheral airspace) for purposes of functional analysis. To measure regional function in the major airways, a large RF flip angle (24 degrees) was applied to reduce (3)He magnetization in the peripheral airspace, and signal intensity (SI) was normalized with the projected airway diameter to estimate local airflow. Higher normalized signal intensity was observed in the left branch airway as compared to the right branch airway. To determine regional function in the peripheral airspace, a small RF flip angle (6 degrees) was used. Incremental increases of peripheral SI in successive lung images were consistent with the increase in lung volume. A new "skipping" scanning strategy using dummy frames allows a trade-off between the number of frames acquired for dynamic information, the RF flip angle, and the penetration depth of (3)He magnetization into the lung. This work provides a novel approach to simultaneously assess dynamic regional function and morphology.

Effects of respiratory rate, plateau pressure, and positive end-expiratory pressure on PaO2 oscillations after saline lavage

One of the proposed mechanisms of ventilator-associated lung injury is cyclic recruitment of atelectasis. Collapse of dependent lung regions with every breath should lead to large oscillations in PaO2 as shunt varies throughout the respiratory cycle. We placed a fluorescence-quenching PO2 probe in the brachiocephalic artery of six anesthetized rabbits after saline lavage. Using pressure-controlled ventilation with oxygen, ventilator settings were varied in random order over three levels of positive end-expiratory pressure (PEEP), respiratory rate (RR), and plateau pressure minus PEEP (Delta). Dependence of the amplitude of PaO2 oscillations on PEEP, RR, and Delta was modeled by multiple linear regression. Before lavage, arterial PO2 oscillations varied from 3 to 22 mm Hg. After lavage, arterial PO2 oscillations varied from 5 to 439 mm Hg. Response surfaces showed markedly nonlinear dependence of amplitude on PEEP, RR, and Delta. The large PaO2 oscillations observed provide evidence for cyclic recruitment in this model of lung injury. The important effect of RR on the magnitude of PaO2 oscillations suggests that the static behavior of atelectasis cannot be accurately extrapolated to predict dynamic behavior at realistic breathing frequencies.

Tidal volume increases do not affect alveolar mechanics in normal lung but cause alveolar overdistension and exacerbate alveolar instability after surfactant deactivation

OBJECTIVE

We utilized microscopy to measure the impact of increasing tidal volume on individual alveolar mechanics (i.e., the dynamic change in alveolar size during tidal ventilation) in the living porcine lung.

DESIGN

In three anesthetized, mechanically ventilated pigs, we observed normal alveoli (n = 27) and alveoli after surfactant deactivation by Tween 20 lavage (n = 26) at three different tidal volumes (6, 12, and 15 mL/kg). Alveolar area was measured at peak inspiration (I) and at end expiration (E) by image analysis and I minus E was calculated as an index of alveolar stability (I-Edelta).

MEASUREMENTS AND MAIN RESULTS

In normal alveoli, increasing tidal volume did not change alveolar area at I (6 mL/kg = 9726 +/- 848 microm; 15 mL/kg = 9,637 +/- 884 microm ), E (6 mL/kg = 9747 +/- 800 microm; 15 mL/kg = 9742 +/- 853 microm ), or I-Edelta (6 mL/kg = -21 +/- 240 microm; 15 mL/kg = -105 +/- 229 microm ). In contrast, with surfactant deactivation, increasing tidal volume significantly increased alveolar area at I (6 mL/kg = 11,413 +/- 1032 microm; 15 mL/kg = 13,917 +/- 1214 microm ), at E (6 mL/kg = 10,462 +/- 906 microm; 15 mL/kg = 12,000 +/- 1066 microm ), and I-Edelta (6 mL/kg = 825 +/- 276 microm; 15 mL/kg = 1917 +/- 363 microm ). Moreover, alveolar instability (increased I-Edelta) was significantly increased at all tidal volumes with altered surface tension when compared with normal alveoli.

CONCLUSIONS

We conclude that high tidal volume ventilation does not alter alveolar mechanics in the normal lung; however, in the surfactant-deactivated lung, it causes alveolar overdistension and exacerbates alveolar instability.

Critical role for CXCR2 and CXCR2 ligands during the pathogenesis of ventilator-induced lung injury

Mortality related to adult respiratory distress syndrome (ARDS) ranges from 35% to 65%. Lung-protective ventilator strategies can reduce mortality during ARDS. The protective strategies limit tidal volumes and peak pressures while maximizing positive end-expiratory pressure. The efficacy of this approach is due to a reduction of shear-stress of the lung and release of inflammatory mediators. Ventilator-induced lung injury (VILI) is characterized by inflammation. The specific mechanism(s) that recruit leukocytes during VILI have not been elucidated. Because the murine CXC chemokines KC/CXCL1 and MIP-2/CXCL2/3, via CXCR2, are potent neutrophil chemoattractants, we investigated their role in a murine model of VILI. We compared two ventilator strategies in C57BL/6 mice: high peak pressure and high stretch (high peak pressure/stretch) versus low peak pressure/stretch for 6 hours. Lung injury and neutrophil sequestration from the high-peak pressure/stretch group were greater than those from the low-peak pressure/stretch group. In addition, lung expression of KC/CXCL1 and MIP-2/CXCL2/3 paralleled lung injury and neutrophil sequestration. Moreover, in vivo inhibition of CXCR2/CXC chemokine ligand interactions led to a marked reduction in neutrophil sequestration and lung injury. These findings were confirmed using CXCR2(-/-) mice. Together these experiments support the notion that increased expression of KC/CXCL1 and MIP-2/CXCL2/3 and their interaction with CXCR2 are important in the pathogeneses of VILI.

Effect of ventilator-induced lung injury on the development of reperfusion injury in a rat lung transplant model

OBJECTIVE

Although mechanical ventilation can potentially worsen preexisting lung injury, its importance in the setting of lung transplantation has not been explored. This study was undertaken to examine the effect of 2 ventilatory strategies on the development of ischemia-reperfusion injury after lung transplantation.

METHODS

In a rat lung transplant model animals were randomized into 2 groups defined by the ventilatory strategy during the early reperfusion period. In conventional mechanical ventilation the transplanted lung was ventilated with a tidal volume equal to 50% of the inspiratory capacity of the left lung and a low positive end-expiratory pressure. In minimal mechanical stress ventilation the transplanted lung was ventilated with a tidal volume equal to 20% of the inspiratory capacity of the left lung, and positive end-expiratory pressure was adjusted according to the shape of the pressure-time curve to minimize pulmonary stress.

RESULTS

After 3 hours of reperfusion, oxygenation from the transplanted lung was significantly higher with minimal mechanical stress ventilation than with conventional ventilation. In addition, elastance, cytokine levels, and morphologic signs of injury were significantly lower in the group with minimal mechanical stress ventilation.

CONCLUSIONS

This study demonstrates that the mode of mechanical ventilation used in the early phase of reperfusion of the transplanted lung can influence ischemia-reperfusion injury, and a protective ventilatory strategy on the basis of minimizing pulmonary mechanical stress can lead to improved lung function after lung transplantation.

Accuracy of displayed values of tidal volume in the pediatric intensive care unit

OBJECTIVES

To assess the accuracy of the expired tidal volumes (VT(E)) displayed by one of the most frequently used ventilators that measures exhaled volume at the expiratory valve.

DESIGN

Prospective study.

SETTING

The intensive care units of a pediatric tertiary referral center in London, UK.

PATIENTS

A total of 56 intubated children aged between 3 wks and 16.6 yrs who were clinically stable and ventilated with a Servo 300 ventilator.

INTERVENTIONS

The CO2SMO Plus respiratory monitor, which measures flow at the airway opening, was validated using calibrated syringes and appropriate tracheal tubes and connections. Simultaneous in vivo recordings of VT(E) from the Servo 300 and CO2SMO Plus were compared before (displayed Servo VT(E)) and after (effective Servo VT(E)) compensating for ventilator circuit compliance.

MEASUREMENTS AND MAIN RESULTS

The in vitro accuracy of the CO2SMO Plus was within +/-5% over a wide range of volumes and measurement conditions. The displayed Servo 300 VT(E) overestimated the true VT(E) by between 2% and 91%. The magnitude of error varied within and between children, according to pressure change (peak inspiratory pressure minus positive end-expiratory pressure), VT(E), and circuit size. Mean (sd) error was 32% (20%) in 40 children with displayed Servo VT(E) of <160 mL and 18% (6%) in 16 subjects with displayed Servo VT(E) of >/=160 mL. After correcting for gas compression, effective VT(E) from the Servo 300 underestimated the true VT(E) by up to 64% in the smallest infants but continued to overestimate by as much as 29% in older children.

CONCLUSIONS

The accuracy of tidal volume values is crucially dependent on the site of measurement. Unless measured at the airway opening, displayed values are an inconsistent and misleading indicator of the true volumes delivered.

Mechanisms of ventilator-induced lung injury in premature infants

Mechanical ventilation in premature infants may injure the lungs or exacerbate the pre-existing condition that led to the need for mechanical ventilation. Ventilator-induced lung injury (VILI) may be associated with alveolar structural damage, pulmonary oedema, inflammation, and fibrosis. This injury is not uniform and is associated with surfactant dysfunction. Recovery from VILI includes clearance of pulmonary oedema and alveolar structural repair. Mechanisms of VILI include high airway pressure (barotrauma), large gas volumes (volutrauma), alveolar collapse and re-expansion (atelectotrauma), and increased inflammation (biotrauma). Injury to the lung may lead to other organ dysfunction. The premature lung is more susceptible to VILI, and lung injury may exacerbate the disturbance of lung development that occurs after birth. Therapies targeting specific processes in lung injury, and which complement the protective ventilator management strategies to avoid atelectotrauma and lung overdistension are an area of active research.

New model of ventilator-associated pneumonia in immunocompetent rabbits

OBJECTIVE

Despite the high rate of therapeutic failures in ventilator-associated pneumonia, up to now there has been no animal model specifically designed for antimicrobial evaluation. A rabbit model of ventilator-associated pneumonia is described for the first time in this study. DESIGN Prospective, randomized experimental study.

SETTING

An animal research laboratory.

SUBJECTS

Male New Zealand healthy rabbits (n = 44).

INTERVENTIONS

After oral intubation and an hour of mechanical ventilation, animals in the ventilator-associated pneumonia group (n = 22) were infected intrabronchially with a calibrated inoculum of. The nonventilated pneumonia group (n = 22) was composed of animals that received the same inoculum in the absence of mechanical ventilation. Rabbits from both groups were randomly killed 3, 6, 12, 24, or 48 hrs after inoculation. Pneumonia evaluation was based on histologic (macroscopic and microscopic score) and bacteriologic (bacterial count) findings.

MAIN RESULTS

Infected animals undergoing mechanical ventilation rapidly developed a progressive bilateral and multifocal pneumonia. Lung bacterial mean (sd) concentration was 6.48 (0.71) log10 colony-forming units (cfu) per gram of tissue at the 48th hour, whereas bacteremia occurred in most cases. In the nonventilated pneumonia group, pneumonia was less severe in terms of bacterial count (3.18 [1.86] log10 cfu/g; p <.05), and spleen cultures remained negative. In addition, microscopic examination revealed noninfectious lung injury in the ventilator-associated pneumonia group, especially hyaline membrane filling alveolar spaces. Of note, these features were never observed in the nonventilated pneumonia group.

CONCLUSIONS

An animal model of ventilator-associated pneumonia was obtained in immunocompetent rabbits. Histopathologic and bacteriologic features were similar to those found in humans. Obviously, pneumonia was more severe when animals underwent mechanical ventilation, especially in terms of systemic spread. Noninfectious lung injury corresponding to ventilation-induced lung injury may explain the difference. This model emphasizes the strong impact of both mechanical ventilation and infection on lung because they seem to act synergistically when causing alveolar damage. Moreover, it seems well suited to testing antimicrobial effectiveness.

Postnatal lung inflammation increased by ventilation of preterm lambs exposed antenatally to Escherichia coli endotoxin

Chorioamnionitis resulting in exposure of the fetal lung to inflammation is frequent before preterm delivery. The initiation of mechanical ventilation in the preterm recruits granulocytes to the lungs and increases proinflammatory cytokine expression in the lungs. We hypothesized that when the prematurely born newborn with chorioamnionitis was ventilated, inflammation would increase. Therefore, we asked whether inflammatory exposure to the fetal lung caused by intra-amniotic endotoxin (10 mg, Escherichia coli 055:beta 5) given at 100 d gestation would alter the inflammatory responses to the mechanical ventilation in surfactant-treated preterm lambs delivered at 130 d gestation. Cells in alveolar washes, proinflammatory cytokine expression, and surfactant protein mRNA expression were not different for saline and endotoxin exposed lambs that were not ventilated. The endotoxin- and saline-exposed control animals had similar lung function for 6 h of ventilation. Bronchoalveolar lavage fluid from the ventilated and antenatal endotoxin-exposed animals contained 5.7 times more monocytes, 12 times more lymphocytes, and a nonsignificant increase in neutrophils. Cells from the bronchoalveolar lavage fluid expressed 3-fold more IL-6 and IL-8 mRNA than did cells from the saline exposed comparison animals. An antenatal exposure of the fetal lung to endotoxin enhanced the subsequent inflammatory response of the ventilated preterm lung.

Cellular biomechanics in the lung

Mechanical forces affect both the function and phenotype of cells in the lung. In this symposium, recent studies were presented that examined several aspects of biomechanics in lung cells and their relationship to disease. Wound healing and recovery from injury in the airways involve epithelial cell spreading and migration on a substrate that undergoes cyclic mechanical deformation; enhanced green fluorescent protein-actin was used in a stable cell line to examine cytoskeletal changes in airway epithelial cells during wound healing. Eosinophils migrate into the airways during asthmatic attacks and can also be exposed to cyclic mechanical deformation; cyclic mechanical stretch caused a decrease in leukotriene C(4) synthesis that may be dependent on mechanotransduction mechanisms involving the production of reactive oxygen species. Recent studies have suggested that proinflammatory cytokines are increased in ventilator-induced lung injury and may be elevated by overdistention of the lung tissue; microarray analysis of human lung epithelial cells demonstrated that cyclic mechanical stretch alone profoundly affects gene expression. Finally, airway hyperresponsiveness is a basic feature of asthma, but the relationship between airway hyperresponsiveness and changes in airway smooth muscle (ASM) function remain unclear. New analysis of the behavior of the ASM cytoskeleton (CSK) suggests, however, that the CSK may behave as a glassy material and that glassy behavior may account for the extensive ASM plasticity and remodeling that contribute to airway hyperresponsiveness. Together, the presentations at this symposium demonstrated the remarkable and varied roles that mechanical forces may play in both normal lung physiology as well as pathophysiology.

Conventional mechanical ventilation of healthy lungs induced pro-inflammatory cytokine gene transcription

We investigated the potential inflammatory reaction induced by mechanical ventilation (MV) using 10 ml/kg tidal volume and no positive end-expiratory pressure (PEEP) in control (C, n = 8), spontaneously breathing (SB, n = 12) and mechanically ventilated (MV, n = 12) rabbits with normal lungs. After 6 h (MV and SB groups) or immediately (C group), lungs were removed for measurement of wet-to-dry (W/D) weight ratio and for bronchoalveolar lavage (BAL). Pulmonary mechanics were also studied. MV animals developed a modest but significant (P < 0.01) impairment of arterial blood oxygenation and had higher W/D lung weight ratio than C ones. In MV group, BAL macrophage count was greater (P < 0.05) than in SB one. MV induced an upregulation of MCP-1, TNF-alpha, and IL-1beta gene transcription (mRNAs), without significant elevation of the corresponding protein cytokines in the BAL supernatant, except for MCP-1 (P < 0.05). These data suggest that MV, even using moderate tidal volume, elicits a pro-inflammatory stimulus to the lungs.

Interactions of lung stretch, hyperoxia, and MIP-2 production in ventilator-induced lung injury

The use of positive pressure mechanical ventilation can cause ventilator-induced lung injury (VILI). We hypothesized that hyperoxia in combination with large tidal volumes (VT) would accentuate noncardiogenic edema and neutrophil infiltration in VILI and be dependent on stretch-induced macrophage inflammatory protein-2 (MIP-2) production. In rats ventilated with VT 20 ml/kg, there was pulmonary edema formation that was significantly increased by hyperoxia. Total lung neutrophil infiltration and MIP-2 in bronchoalveolar lavage (BAL) fluid were significantly elevated, in animals exposed to high VT both on room air (RA) and with hyperoxia. Hyperoxia markedly augmented the migration of neutrophils into the alveoli. Anti-MIP-2 antibody blocked migration of neutrophils into the alveoli in RA by 51% and with hyperoxia by 65%. We concluded that neutrophil migration into the alveoli was dependent on stretch-induced MIP-2 production. Hyperoxia significantly increased edema formation and neutrophil migration into the alveoli with VT 20 ml/kg, although BAL MIP-2 levels were nearly identical to VT 20 ml/kg with RA, suggesting that other mechanisms may be involved in hyperoxia-augmented neutrophil alveolar content in VILI.

Repeated derecruitments accentuate lung injury during mechanical ventilation

OBJECTIVE

This study was performed to test the hypothesis that derecruitment itself might accentuate lung injury during mechanical ventilation.

SETTING

Randomized, controlled trial.

SETTING

Experimental laboratory.

SUBJECTS

New Zealand White rabbits (2.8-3.5 kg).

INTERVENTION

Twenty-four rabbits were ventilated in pressure-controlled mode with constant tidal volume (10 mL/kg). After lung injury was induced by repeated saline lavage (PaO2 <100 torr, 13.3 kPa), a pressure-volume curve was drawn to calculate the lower inflection point (Pflex), and randomization was done. The control group (n = 8) received ventilation with positive end-expiratory pressure (PEEP) fixed at Pflex for 3 hrs. The nonderecruitment group (n = 8) was ventilated at PEEP of 2 mm Hg (2.7 cm H2O) for the initial hour and then PEEP of Pflex for the remaining 2 hrs. The derecruitment group (n = 8) was ventilated for 3 hrs with six 30-min cycles consisting of 10 mins at PEEP of 2 mm Hg (2.7 cm H2O) and 20 mins at PEEP of Pflex to induce repeated derecruitments.

MEASUREMENTS AND MAIN RESULTS

Variables of gas exchange, mechanics, and hemodynamics were measured, and histologic evaluation was done. In the control group, Pao2 remained >500 torr (66.7 kPa) for 3 hrs. In the nonderecruitment group, PaO2 was 40 +/- 16 (mean +/- SD) torr (5.3 +/- 2.1 kPa) at 1 hr but increased to >500 torr (66.7 kPa) for the remaining 2 hrs after increase in PEEP to Pflex. In the derecruitment group, there was progressive decline in Pao2 with each derecruitment to 220 +/- 130 torr (29.3 +/- 17.3 kPa) at 3 hrs (p <.05 compared with other groups). Histologically there was more hyaline membrane formation in the derecruitment group compared with control (p <.05) and significantly higher mean bronchiolar injury score in the derecruitment group (1.92 +/- 0.78) than both control (0.50 +/- 0.50) and nonderecruitment (0.78 +/- 0.42) groups (p <.05).

CONCLUSION

Repeated derecruitments can accentuate lung injury during mechanical ventilation.

Hypercapnic acidosis is protective in an in vivo model of ventilator-induced lung injury

To investigate whether hypercapnic acidosis protects against ventilator-induced lung injury (VILI) in vivo, we subjected 12 anesthetized, paralyzed rabbits to high tidal volume ventilation (25 cc/kg) at 32 breaths per minute and zero positive end-expiratory pressure for 4 hours. Each rabbit was randomized to receive either an FI(CO(2)) to achieve eucapnia (Pa(CO(2)) approximately 40 mm Hg; n = 6) or hypercapnic acidosis (Pa(CO(2)) 80-100 mm Hg; n = 6). Injury was assessed by measuring differences between the two groups' respiratory mechanics, gas exchange, wet:dry weight, bronchoalveolar lavage fluid protein concentration and cell count, and injury score. The eucapnic group showed significantly higher plateau pressures (27.0 +/- 2.5 versus 20.9 +/- 3.0; p = 0.016), change in Pa(O(2)) (165.2 +/- 19.4 versus 77.3 +/- 87.9 mm Hg; p = 0.02), wet:dry weight (9.7 +/- 2.3 versus 6.6 +/- 1.8; p = 0.04), bronchoalveolar lavage protein concentration (1,350 +/- 228 versus 656 +/- 511 micro g/ml; p = 0.03), cell count (6.86 x 10(5) +/- 0.18 x 10(5) versus 2.84 x 10(5) +/- 0.28 x 10(5) nucleated cells/ml; p = 0.021), and injury score (7.0 +/- 3.3 versus 0.7 +/- 0.9; p < 0.0001). We conclude that hypercapnic acidosis is protective against VILI in this model.

Injurious ventilation induces widespread pulmonary epithelial expression of tumor necrosis factor-alpha and interleukin-6 messenger RNA

OBJECTIVE

We examined the hypothesis that injurious strategies of mechanical ventilation alter the expression and distribution within the lung of tumor necrosis factor-alpha and interleukin-6 that are both duration and ventilation strategy dependent.

SUBJECTS

Male Sprague Dawley rats.

INTERVENTIONS

Lungs from rats were preserved immediately after death or were randomized to ex vivo ventilation with either a) noninjurious ventilation; b) high end-inspiratory lung volume with positive end-expiratory pressure (PEEP); c) high end-inspiratory lung volume without PEEP; or d) intermediate lung distension without PEEP, for periods ranging from 30 mins to 3 hrs.

MEASUREMENT AND MAIN RESULTS

Changes in cytokines were assessed by in situ hybridization, immunocytochemistry, simultaneous in situ hybridization and immunocytochemistry, Northern analysis, and enzyme-linked immunosorbent assay. Whereas minimal expression of tumor necrosis factor-alpha and interleukin-6 mRNA was found in lungs subjected to noninjurious ventilation, the three injurious strategies resulted in a diffuse increase in expression of tumor necrosis factor-alpha and interleukin-6. The principal cells involved were the bronchial, bronchiolar, and alveolar epithelium. The changes in tumor necrosis factor-alpha mRNA and protein expression were dependent on both duration of ventilation and the ventilation strategy used.

CONCLUSIONS

The vast pulmonary epithelium is a major contributor to ventilation-induced changes in cytokine production and may play an important role in the pathogenesis of lung injury and systemic sequelae in ventilated subjects.

Intratidal compliance-volume curve as an alternative basis to adjust positive end-expiratory pressure: a study in isolated perfused rabbit lungs

OBJECTIVE

Repeated collapse and reopening of alveoli have been shown to aggravate lung injury, which could be prevented by positive end-expiratory pressure (PEEP). Yet, how to adjust optimum PEEP is a matter of debate. We suggest a new strategy to adjust PEEP, which is based on the analysis of the intratidal compliance-volume curve. This approach was compared with a strategy based on the static pressure-volume curve. Furthermore, two other ventilator settings were investigated. One served as a negative control likely to provoke atelectasis, and the other was expected to induce overdistension.

DESIGN

Prospective, randomized block design.

SETTING

Laboratory.

SUBJECTS

Isolated, perfused, and ventilated rabbit lungs.

INTERVENTIONS

Tidal volumes of 8 mL/kg of body weight were used throughout. After stabilization, the lungs were randomized to one of four protocols (lasting 120 mins; n = 6 per group). Group 1 was ventilated at zero end-expiratory pressure. In group 2, PEEP was set above the lower inflection point of the static pressure-volume curve. In group 3, adjustment of PEEP was based on the intratidal compliance-volume curve, as determined by the slice method. In group 4, increasing PEEP levels ensured a plateau airway pressure of 20-25 cm H2O likely to provoke overdistension.

MEASUREMENTS AND MAIN RESULTS

The ventilation/perfusion (VA/Q) distribution was analyzed by the multiple inert gas elimination technique. Alveolar derecruitment was indicated by shunt and low VA/Q areas as observed in group 1. In groups 2 and 3, VA/Q data initially indicated full recruitment. In contrast to group 3, shunt increased in group 2 near completion of the experiments. Group 4 showed complete recruitment, but the VA/Q distribution included high VA/Q areas.

CONCLUSIONS

The intratidal compliance-volume curve represents a rational basis for adjusting PEEP in the isolated lung model. Because this strategy does not require invasive measures and facilitates continuous assessment of ventilator settings, it may be of clinical interest.

Role of MAP kinase activation in interleukin-8 production by human BEAS-2B bronchial epithelial cells submitted to cyclic stretch

Overstretching the airways during positive pressure mechanical ventilation or attacks of acute severe asthma is associated with important biologic responses. Interleukin (IL)-8-dependent neutrophil recruitment seems to play a critical role in the process of mechanical stress-induced airway inflammation. Herein, we show that human bronchial epithelial BEAS-2B cells submitted to cyclic stretch in vitro produce IL-8, at both the mRNA and protein levels. This cellular stress "turns on" activator protein (AP)-1 and cyclic AMP (cAMP)-responding elements. The mitogen-activated protein (MAP) kinases (MAPK) p44/42, SAPK/JNK, and p38 were all rapidly activated (phosphorylated) after the initiation of the cyclic strain (5-10 min). The blockade of p38 with the pharmacologic inhibitor SB203580 abrogated IL-8 production by cell stretching, and an inhibitor of the p44/42 pathway, PD98059, partially inhibited the IL-8 response. A nonspecific tyrosine kinase inhibitor, genistein, also blocked the stretch-induced IL-8 production. This suggests that MAPK, and p38 in particular, are proximal and key intracellular signaling molecules mediating cell activation in response to cyclic stretch, a mechanical strain similar to that applied to lung epithelial cells during mechanical ventilation. Pharmacologic inhibition of the p38 pathway holds promise as a new therapeutic avenue in ventilated patients.

Different strategies to keep the lung open: a study in isolated perfused rabbit lungs

OBJECTIVE

Atelectatic alveoli can be recruited or kept open either by sustained inflation maneuvers or by positive end-expiratory pressure (PEEP). Little is known about potential interactions between both approaches. Especially, it is not known whether the recruiting effect of sustained inflation maneuvers is maintained in combination with a low PEEP, as suggested recently. In an attempt to answer this question, we combined sustained inflation maneuvers with either high or low PEEP. Both approaches were compared with a strategy likely to result in alveolar atelectasis and with another ensuring adequate alveolar recruitment by adjustment of PEEP alone.

DESIGN

Randomized block design.

SETTING

Laboratory.

SUBJECTS

Isolated perfused rabbit lungs (n = 28).

INTERVENTIONS

The lungs were ventilated with a tidal volume of 8 mL/kg. After stabilization, the lungs were randomized to one of four ventilatory strategies, which then were followed for 120 mins: a) PEEP 1 cm H2O (PEEP1, negative control); b) PEEP 1 cm H2O and 30 sec-sustained inflations (20 cm H2O) every 30 mins (SI-1); c) PEEP 3 cm H2O combined with sustained inflations (SI-3); and d) PEEP repeatedly adjusted following a previously established strategy ensuring full alveolar recruitment (DYN, positive control).

MEASUREMENTS AND MAIN RESULTS

Distribution of ventilation and perfusion (Va/Q distribution) was analyzed by the multiple inert gas elimination technique. Volume-dependent compliance within the tidal volume was determined by using the slice method. Shunt and Va/Q mismatch significantly differed between SI-1 and SI-3, indicating full alveolar recruitment only in the latter. Data of SI-1 did not differ substantially from those of PEEP1, and data obtained in SI-3 were similar to those of DYN.

CONCLUSIONS

First, enduring alveolar recruitment by sustained inflation maneuvers is only possible when the alveoli are stabilized thereafter by sufficient PEEP. Second, a ventilation strategy that uses repeated sustained inflations on a comparably high PEEP may not be superior to adequate adjustment of PEEP alone.

Dopamine increases lung liquid clearance during mechanical ventilation

Short-term mechanical ventilation with high tidal volume (HVT) causes mild to moderate lung injury and impairs active Na+ transport and lung liquid clearance in rats. Dopamine (DA) enhances active Na+ transport in normal rat lungs by increasing Na+-K+-ATPase activity in the alveolar epithelium. We examined whether DA would increase alveolar fluid reabsorption in rats ventilated with HVT for 40 min compared with those ventilated with low tidal volume (LVT) and with nonventilated rats. Similar to previous reports, HVT ventilation decreased alveolar fluid reabsorption by ~50% (P < 0.001). DA increased alveolar fluid reabsorption in nonventilated control rats (by ~60%), LVT ventilated rats (by approximately 55%), and HVT ventilated rats (by ~200%). In parallel studies, DA increased Na+-K+-ATPase activity in cultured rat alveolar epithelial type II cells (ATII). Depolymerization of cellular microtubules by colchicine inhibited the effect of DA on HVT ventilated rats as well as on Na+-K+-ATPase activity in ATII cells. Neither DA nor colchicine affected the short-term Na+-K+-ATPase alpha1- and beta1-subunit mRNA steady-state levels or total alpha1- and beta1-subunit protein abundance in ATII cells. Thus we reason that DA improved alveolar fluid reabsorption in rats ventilated with HVT by upregulating the Na+-K+-ATPase function in alveolar epithelial cells.

Cyclic stretch upregulates interleukin-8 and transforming growth factor-beta1 production through a protein kinase C-dependent pathway in alveolar epithelial cells

OBJECTIVE

Positive-pressure mechanical ventilation can injure the lung, causing oedema and alveolar inflammation, which is termed 'ventilator-induced lung injury' (VILI). We postulated that cyclic stretch upregulates the release of cytokines, which may cause lung damage, and explored which cytokines were released after cyclic stretch in type II alveolar epithelial cells (A549).

METHODOLOGY

To test this hypothesis, A549 cells were cultured on a silicoelastic membrane and interleukin (IL)-1beta, IL-8, granulocyte-macrophage colony stimulating factor, activin, transforming growth factor (TGF)-beta1, insulin-like growth factor-2 and tumour necrosis factor-alpha mRNA and protein were assessed after stimulation of the cells by cyclic stretch.

RESULTS

Cyclic stretch induced activation of protein kinase C and resulted in the release of IL-8 and TGF-beta1 from A549 cells.

CONCLUSIONS

The release of IL-8 and TGF-beta1 from alveolar epithelial cells may be a contributing factor in alveolitis associated with VILI.