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Effect of Mechanical Ventilation on Inflammatory Mediators in Patients With Acute Respiratory Distress Syndrome
A Randomized Controlled Trial
V. Marco Ranieri, MD;
Peter M. Suter, MD;
Cosimo Tortorella, MD, PhD;
Renato De Tullio, MD;
Jean Michel Dayer, MD;
Antonio Brienza, MD;
Francesco Bruno, MD;
Arthur S. Slutsky, MD
JAMA. 1999;282:54-61.
ABSTRACT
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Context Studies have shown that an inflammatory response may be elicited by mechanical ventilation used for recruitment or derecruitment of collapsed lung units or to overdistend alveolar regions, and that a lung-protective strategy may reduce this response.
Objective To test the hypothesis that mechanical ventilation induces a pulmonary and systemic cytokine response that can be minimized by limiting recruitment or derecruitment and overdistention.
Design and Setting Randomized controlled trial in the intensive care units of 2 European hospitals from November 1995 to February 1998, with a 28-day follow-up.
Patients Forty-four patients (mean [SD] age, 50 [18] years) with acute respiratory distress syndrome were enrolled, 7 of whom were withdrawn due to adverse events.
Interventions After admission, volume-pressure curves were measured and bronchoalveolar lavage and blood samples were obtained. Patients were randomized to either the control group (n=19): tidal volume to obtain normal values of arterial carbon dioxide tension (35-40 mm Hg) and positive end-expiratory pressure (PEEP) producing the greatest improvement in arterial oxygen saturation without worsening hemodynamics; or the lung-protective strategy group (n=18): tidal volume and PEEP based on the volume-pressure curve. Measurements were repeated 24 to 30 and 36 to 40 hours after randomization.
Main Outcome Measures Pulmonary and systemic concentrations of inflammatory mediators approximately 36 hours after randomization.
Results Physiological characteristics and cytokine concentrations were similar in both groups at randomization. There were significant differences (mean [SD]) between the control and lung-protective strategy groups in tidal volume (11.1 [1.3] vs 7.6 [1.1] mL/kg), end-inspiratory plateau pressures (31.0 [4.5] vs 24.6 [2.4] cm H2O), and PEEP (6.5 [1.7] vs 14.8 [2.7] cm H2O) (P<.001). Patients in the control group had an increase in bronchoalveolar lavage concentrations of interleukin (IL) 1 , IL-6, and IL-1 receptor agonist and in both bronchoalveolar lavage and plasma concentrations of tumor necrosis factor (TNF)  , IL-6, and TNF- receptors over 36 hours (P<.05 for all). Patients in the lung-protective strategy group had a reduction in bronchoalveolar lavage concentrations of polymorphonuclear cells, TNF-, IL-1, soluble TNF- receptor 55, and IL-8, and in plasma and bronchoalveolar lavage concentrations of IL-6, soluble TNF- receptor 75, and IL-1 receptor antagonist (P<.05). The concentration of the inflammatory mediators 36 hours after randomization was significantly lower in the lung-protective strategy group than in the control group (P<.05).
Conclusions Mechanical ventilation can induce a cytokine response that may be attenuated by a strategy to minimize overdistention and recruitment/derecruitment of the lung. Whether these physiological improvements are associated with improvements in clinical end points should be determined in future studies.
INTRODUCTION
Acute respiratory distress syndrome (ARDS) is a clinical syndrome characterized by severe hypoxemia, stiff lungs, and decreased respiratory system compliance. Early ARDS is characterized by acute and diffuse endothelial and epithelial injury termed diffuse alveolar damage,1 which leads to increased vascular permeability with protein-rich exudative edema. Although originally thought to be relatively homogeneous, a number of recent studies have highlighted the marked heterogeneity of the pathological process with consolidation in the dependent regions of the lung and relatively normal aeration of the nondependent regions.2-3
Despite apparent improvement in management and outcome of ARDS, the mortality rate of ARDS remains high, ranging from 35% to 65%.4-5 Mechanical ventilation delays mortality in many patients with acute respiratory failure and is used to maintain adequate systemic oxygenation and to rest the respiratory muscles. However, over the last 2 decades, it has become evident that mechanical ventilation itself can augment or cause acute lung injury. This has been demonstrated in animal studies6-9 and has been highlighted in a recent randomized trial, which found that the mortality rate from ARDS could be reduced by 40% when a lung-protective strategy was used compared with a conventional strategy of mechanical ventilation.10 The lung-protective strategy in this trial involved limiting peak airway pressures or tidal volumes and using positive end-expiratory pressure (PEEP) levels that were individualized to each patient based on respiratory system mechanics, as assessed by the volume-pressure curve.10-11 One reason for the efficacy of this approach may have been the decreased stress on the lung since Mead et al12 have predicted that in a nonuniformly inflated lung, shear forces due to expansion of collapsed alveolar regions surrounded by expanded regions could exceed 100 cm H2O even though the transpulmonary pressure may only be 30 cm H2O.
ARDS is an inflammatory disease and clinical studies have suggested increased mortality in patients who continue to manifest elevated cytokine levels during their clinical course.13-16 Recent experimental studies in various animal models have provided 3 lines of evidence suggesting that mechanical ventilation can initiate or exacerbate an inflammatory response: (1) pathologic evidence of neutrophil infiltration,17-18 (2) increased cytokine levels in lung lavage,8 and (3) increased cytokine levels in the systemic circulation.9, 19 In addition, ventilatory strategies that were designed to minimize ventilator-induced lung injury (low end-inspiratory lung volumes or high end-expiratory lung volumes) in these animal studies were associated with strikingly lower cytokine levels.8 To examine the influence of mechanical ventilation on lung and systemic cytokine levels in patients with ARDS, we compared a ventilatory strategy designed to minimize ventilator-induced lung injury (high PEEP, low end-inspiratory stretch) with a conventional ventilatory strategy.
METHODS
Patient Selection
Following approval of institutional review boards and after obtaining informed consent from the patient or next of kin, patients were recruited from the intensive care units of the university hospitals of Bari, Italy, and Geneva, Switzerland. Inclusion criteria were (1) age of 18 years or older and (2) diagnosis of ARDS based on American-European Consensus Conference criteria.20 Exclusion criteria were (1) anticipated to require mechanical ventilation less than 48 hours (determined by the attending physician), (2) more than 8 hours of mechanical ventilation prior to admission to the study, (3) cardiogenic pulmonary edema (clinically suspected, or pulmonary artery occlusion pressure >18 mm Hg), (4) history of ventricular fibrillation or tachyarrhythmia, unstable angina, or myocardial infarction within preceding month, (5) preexisting chronic obstructive pulmonary disease, (6) major chest wall abnormalities (kyphoscoliosis, open or flail chest), chest tube with persistent air leak, or abdominal distention, (7) pregnancy, (8) known intracranial abnormality, and (9) enrollment in another interventional study.
Study Protocol
Patients were sedated (0.01 mg/kg of fentanyl citrate and 5-20 mg of diazepam), paralyzed (4-8 mg of pancuronium bromide), and ventilated for 1 to 2 hours with a fixed ventilatory management protocol consisting of volume-targeted control mechanical ventilation: PEEP of 10 cm H2O, fraction of inspired oxygen (FIO2) of 100%, tidal volume of 5 to 8 mL/kg (ideal body weight), and inspiratory to expiratory ratio of 1:2. The PEEP was then removed and a volume-pressure curve was measured as described below. The volume-pressure curve of the respiratory system of patients with early ARDS often has a characteristic sigmoidal shape with a lower inflection point (Pflex) thought to approximate the pressure required to reopen collapsed lung regions and an upper inflection point (UIP) thought to correspond to the pressure at which overdistention of some lung units occurs.3 The PEEP was then restored and bronchoalveolar lavage fluid and blood samples were taken 20 to 30 minutes later. A full set of laboratory, hemodynamic, and respiratory variables was obtained. The Acute Physiology and Chronic Health Evaluation II21 and lung injury scores22 (for both scores, higher values indicate greater illness severity) and the number of organ failures as defined by Knaus et al23 were also calculated. At the end of this fixed ventilatory period, patients were randomly assigned to a control group or a lung-protective strategy group. The patients were assigned by a concealed allocation approach using opaque sealed envelopes containing the randomization schedule. Baseline blood gases were then measured 2 to 3 hours after the assigned ventilatory strategy was initiated.
Control Group
Mechanical ventilation was performed using control mechanical ventilation with a respiratory rate of 10 to 15/min, an inspiratory to expiratory ratio of 1:2, and a tidal volume targeted to maintain the PaCO2 between 35 and 40 mm Hg. For safety reasons, when a plateau airway pressure of 35 cm H2O was reached, the tidal volume was not increased further, irrespective of PaCO2.24 A PEEP trial on 100% FIO2 was performed using incremental (3-5 cm H2O) levels from 3 to 15 cm H2O to determine the PEEP level that produced the greatest improvement in arterial oxygen percent saturation (SaO2) without worsening hemodynamics (>10% drop in mean blood pressure). The FIO2 was then decreased until SaO2 was decreased by 1% to 2% or below 90%.
Lung-Protective Strategy Group
The following aspects of the ventilatory strategy were used as in the control group: control mechanical ventilation, respiratory rate of 10 to 15/min, inspiratory to expiratory ratio of 1:2, and FIO2 of 100%. The tidal volume and PEEP values were set to minimize stress on the lung. Tidal volume was set to obtain a value of plateau pressure (Pplat) less than pressure at the UIP regardless of PaCO2. The PEEP was set at 2 to 3 cm H2O higher than the pressure at Pflex. If UIP and Pflex could not be determined on the volume-pressure curve, a tidal volume of 5 to 8 mL/kg (ideal body weight) and a PEEPtotal level of 15 cm H2O were applied, respectively.10 If arterial pH was less than 7.15, tidal volume was increased until a Pplat of 35 cm H2O was reached or until arterial pH was more than 7.15 mm Hg. The FIO2 was then decreased until the SaO2 fell by 1% to 2% or below 90%.
After admission, protocol withdrawal could occur if 1 of the following a priori conditions occurred within the first 24 hours: (1) a 20% increase or decrease in Acute Physiology and Chronic Health Evaluation II21 and lung injury scores,22 (2) an increase or decrease in the initial number of organ failures,23 (3) need for high levels of epinephrine (>0.15 mg/kg per minute) or norepinephrine (>0.1 mg/kg per minute), (4) the patient was ready to be weaned from mechanical ventilation, as determined by the attending intensivist, and (5) high risk of death defined as hemodynamic instability, arrhythmia, or hypoxemia, not responsive to 20 to 30 minutes of standard treatment. Additional ARDS cointerventions (inhaled nitric oxide or prostacycline, administration of almitrine or steroids, prone position) were not allowed in either group during the study. Patients were cared for by attending physicians not involved in the protocol and other management decisions were made at their discretion. All measurements were repeated 24 to 30 hours and 36 to 40 hours after randomization. Sedation was maintained throughout the study.
Study Procedures and Outcome Measures
Static Inflation Volume-Pressure Curve of the Respiratory System
The volume-pressure curve was constructed by plotting the different inflation volumes against the corresponding values of Pplat as assessed by end-inspiratory occlusion at different tidal volumes.3 Each occlusion was maintained until a stable Pplat was observed. Total PEEP (applied PEEP plus auto-PEEP) was measured by performing an end-expiratory occlusion3 (Servo 300, Siemens-Elema, Stockholm, Sweden). Flow (pneumotachograph, Fleisch, Lausanne, Switzerland), pressure (Validyne pressure transducer, Northridge, Calif), and volume (digital integration of flow signal, Anadat software package, Montreal, Quebec) were measured using standard techniques described previously.3 The Pflex and UIP on the volume-pressure curve were quantified using a computer step-by-step regression analysis.3
Pulmonary and Systemic Inflammatory Mediators
Blind bronchoalveolar lavage was performed using a telescoping catheter (Ballard, Draper, Utah) with 2 aliquots of 40 to 50 mL of sterile isotonic saline. Lavage with a third aliquot was performed if there was less than 30 to 40 mL of recovered fluid from the first 100 mL. When a diffuse infiltrate was seen on a chest x-ray, bronchoalveolar lavage was performed in the right lower or middle lobe. When an area of localized pulmonary infiltration was present, bronchoalveolar lavage was blindly performed in the lower lobe of the opposite lung.25 The first aliquot was discarded13-15 and the remaining bronchoalveolar lavage fluid was rapidly filtered through sterile gauze and then spun at 4°C at 400g for 15 minutes. A microscopic cell count was performed on the cell pellet using standard techniques.25 The supernatant was centrifuged at 80,000g for 30 minutes at 4°C to remove the surfactant-rich fraction and then concentrated 10-fold on a 5000 molecular weight cut-off filter (Amicon, Beverly, Mass) under nitrogen. The concentrated supernatant was then frozen at -70°C. Blood samples (20 mL) obtained from a central venous line were placed in a specimen tube containing heparin, centrifuged at 1500g for 10 minutes, and then the plasma was aspirated and stored at -70°C. All cytokine determinations on the bronchoalveolar lavage fluid and plasma were carried out in duplicate in Geneva, Switzerland (with the technician blinded to ventilation strategy) using a solid-phase enzyme-linked immunosorbent assay method based on the quantitative immunometric sandwich enzyme immunoassay technique.25 Reagents for the various cytokines were obtained from several sources (tumor necrosis factor [TNF] , soluble TNF- receptors [TNF-sR55 and TNF-sR75], interleukin [IL] 6, and IL-8 from Medgenix, Fleures, Belgium); IL-1 and IL-1 receptor antagonist [IL-1Ra] from Immunotech, Marseille, France).
Ventilator-Free Days and Mortality
In a post hoc fashion, we calculated the number of ventilator-free days (days without mechanical ventilation after extubation) during the 28 days immediately after study entry.26 Results were scored from 0 (worst outcome) to 28. The number of patients who were alive at 28 days was also recorded.
Statistics
The values for the cytokine concentrations were not normally distributed so we performed log10 transformations to normalize the data to permit the application of parametric statistics. To evaluate differences over time of cytokine values within each group, repeated measures analysis of variance by Bonferroni method was used. To evaluate differences between the 2 groups, the Fisher exact test for categorical variables, the t test with unequal variance for continuous variables, and the Mann-Whitney rank sum test for ordinal variables were used. All tests of significance were 2-tailed, and P<.05 was accepted as significant.
RESULTS
From November 1995 to February 1998, 44 patients were enrolled (34 in Bari and 10 in Geneva). Seven protocol patients withdrew because of an increase in the initial number of organ failures (1 in each group), need for high doses of norepinephrine (1 in each group), high risk of death (1 in the lung-protective strategy group), and initiation of weaning from mechanical ventilation (1 in each group). Baseline characteristics, underlying condition responsible for ARDS, and respiratory mechanics on entry are presented in Table 1. Most patients had sepsis and multiple trauma as the underlying condition responsible for ARDS. The Pflex was measurable in all patients, while the UIP was not measurable in 10 patients (4 in the control group and 6 in the lung-protective strategy group). There was no significant difference in Pflex or UIP between groups (Table 1).
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Table 1. Patient Characteristics at Baseline*
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Values of ventilator settings and of arterial blood gases 2 to 3 hours after randomization are presented in Table 2. In the control group, PEEP levels were lower than Pflex in every patient and values of Pplat were higher than UIP in all but 4 patients. By design, PEEP levels were greater than Pflex and Pplat levels were lower than UIP in every patient in the lung-protective strategy group. The PaO2 did not differ between groups, although FIO2 was slightly higher in the control group. A significant increase in PaCO2 (permissive hypercapnia) was observed in the lung-protective strategy group.
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Table 2. Patients 2 to 3 Hours After Randomization by Group*
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With the exception of bronchoalveolar lavage concentrations of TNF-sR75 (lower in the control group, P<.05; Table 3) and plasma values of IL-6 (lower in the control group, P<.01), values of inflammatory mediators prior to randomization did not differ between groups (Figure 1 and Figure 2, Table 3).
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Figure 1. Levels of Inflammatory Mediators in Bronchoalveolar Lavage Fluid
Individual trends of polymorphonuclear (PMN) cells, interleukin (IL) 1, in the bronchoalveolar lavage fluid of the 2 groups of patients. Time 1 indicates 24 to 30 hours after study entry and time 2, 36 to 40 hours after study entry. Horizontal bars indicate mean values. P values are for repeated measures analysis of variance vs entry.
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Figure 2. Levels of Inflammatory Mediators in Bronchoalveolar Lavage Fluid and Plasma
Individual trends of tumor necrosis factor (TNF-), interleukin (IL) 8 and IL-6 in plasma and bronchoalveolar lavage fluid in the 2 groups of patients. Time 1 indicates 24 to 30 hours after study entry and time 2, 36 to 40 hours after study entry. Numbers of patients differ from enrolled group size because measurements were not available for all subjects. Horizontal bars indicate mean values. P values are for repeated measures analysis of variance for time 2 vs entry.
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There was a trend for polymorphonuclear cell levels in bronchoalveolar lavage to increase over time in the control group (Figure 1). In the control group, bronchoalveolar lavage fluid concentration of IL-1 (P<.001), TNF- (P<.05), and IL-6 (P<.01) increased over time, as did plasma levels of TNF- (P<.01) and IL-6 (P<.001) (Figure 1 and Figure 2). The concentrations of other inflammatory mediators did not change significantly. A significant (P<.01) increase over time of both TNF- receptors in the bronchoalveolar lavage fluid and plasma concentrations and in the bronchoalveolar lavage IL-1Ra (P<.01) was observed in the control group (Table 3).
A significant reduction over time in bronchoalveolar lavage concentrations of polymorphonuclear cells (P<.001), IL-1 (P<.05), TNF- (P<.001), IL-8 (P<.001), and IL-6 (P<.005), and in plasma concentrations of IL-6 (P<.002) was observed in the lung-protective strategy group (Figure 1 and Figure 2). Plasma concentrations of TNF- and IL-8 did not change significantly over time in the lung-protective strategy group (Figure 2). A significant reduction over time of bronchoalveolar lavage concentrations of TNF-sR55 (P<.001) and bronchoalveolar lavage and plasma concentrations of TNF-sR75 (P<.001) and IL-1Ra (P<.05 and P<.001, respectively) were observed in the lung-protective strategy group (Table 3).
With the exception of IL-1Ra in the bronchoalveolar lavage, values of inflammatory mediators 36 to 40 hours after randomization were significantly (P<.05 to P<.001) lower in the lung-protective strategy group.
The post hoc analysis showed that the mean (SD) number of ventilator-free days26 in the lung-protective strategy group was higher (P<.01) than in the control group (12 [11] vs 4 [8] days, respectively). Mortality rates at 28 days from admission were 38% and 58% in the lung-protective strategy and control groups, respectively (P=.19).
COMMENT
The major finding of this study is that mechanical ventilation—the therapeutic modality that is invariably used in the treatment of ARDS—can itself lead to an increase in cytokine levels in the lung, as well as in the systemic circulation. These results may partially explain the development of multiple organ failure in many patients with ARDS,27 the high mortality rate of this syndrome (35%-65%),4-5 and perhaps the decrease in mortality observed in a recent study that used a lung-protective strategy.10
Over the past decade, numerous studies have suggested that mechanical ventilation can cause or exacerbate acute lung injury. This is particularly true in patients with ARDS because of the widespread, heterogeneous distribution of consolidated/atelectatic regions, which produces a small lung volume available for ventilation.2 Using computed tomography, Gattinoni et al2 showed that the lungs of patients with ARDS are highly asymmetrical along the vertical axis with a small nondependent lung region continuously open to ventilation, and a dependent consolidated, atelectatic region. In between, there is a region that can be recruited or derecruited depending on the particular ventilatory strategy used.2 In such patients, mechanical ventilation could lead to injury due to overdistention as more of the tidal volume is distributed to the small, relatively normal alveolar regions2, 7, 28 and/or repeated recruitment or derecruitment of alveolar units that may be exacerbated with ventilation at low PEEP levels.2, 29-30
Recent in vitro studies using human alveolar macrophage cultures31 and ex vivo preparations of rat and mice lungs8, 19 "stretched" with injurious ventilatory strategies have demonstrated an up-regulation of the cytokine response, which may also lead to a systemic cytokine response in vivo.9, 19 The present study is the first in humans to demonstrate that mechanical ventilation per se may be an important factor in determining the pulmonary and systemic cytokine levels in patients with ARDS. Of particular importance, the lack of improvement in the cytokine response in the control group occurred using tidal volumes and PEEP levels commonly used in the treatment of patients with ARDS. Furthermore, our data indicate that bronchoalveolar lavage concentrations of neutrophils and various cytokines decreased significantly over time with a ventilatory strategy aimed at minimizing injury by maintaining the lung recruited throughout the respiratory cycle, and by avoiding overdistention.
Cytokines are a diverse group of inflammatory mediators (low-molecular-weight proteins) that are produced by numerous cell types, which initiate and orchestrate the host's response to different stresses such as bacteremia, shock, and thermal injury.32 Cytokines act on many target cells and can affect all organs to elicit physiological and biochemical responses to critical illness.33 Cytokines interact with highly specific cell-surface receptors, causing a series of intracellular signaling events that typically result in de novo protein synthesis and the elaboration of other cytokines within the target cell. If these processes are not regulated, they may result in excessive amplification of the inflammatory cascade and the overproduction of proinflammatory mediators with the consequent uncontrolled activation of the immune system.32-33 This can lead to a number of clinical sequelae, including disseminated intravascular coagulation, renal insufficiency, acute pancreatitis, multiple system dysfunction syndrome (MODS), and ARDS.1, 34 Indeed, there is evidence that in patients with MODS, the highest levels of cytokines in the blood are found downstream from the most affected organ.35 The relevance of cytokines in ARDS is evident from human studies showing an increase in bronchoalveolar lavage IL-8 concentrations that precede clinical evidence of disease,36 and the observation that bronchoalveolar lavage concentrations of TNF-, IL-1, IL-6, and IL-8 were higher at presentation in patients who died of ARDS, compared with those who survived.13-15,25 Concentrations of bronchoalveolar lavage cytokines remained elevated over time in nonsurvivors, suggesting that persistent elevation of polymorphonuclear cells and inflammatory cytokines may delay or preclude resolution of the pulmonary14-15 and systemic13, 16 inflammatory processes. Recent data suggest that modulation of the inflammatory response by administration of methylprednisolone in patients with unresolving ARDS was associated with improvement in lung injury and MODS scores and reduced mortality.37
In patients developing ARDS, only a small percentage die of hypoxia and/or hypercarbia.27 Rather, lung injury appears to predispose patients to the development of a systemic inflammatory response that culminates in MODS and death.4, 27 The classic explanation for the development of MODS in ARDS is the development of superinfection.27 However, a number of recent studies have suggested another mechanism. In ARDS, the alveolar epithelial-endothelial barrier is disrupted,1 and cytokines produced in the lungs may enter the systemic circulation.38 This represents a potential mechanism leading to the subsequent development of MODS.35, 38 It has been suggested that mechanical ventilation may play a major role in this regard38 by causing an increase in production of pulmonary cytokines as well as by increasing alveolar capillary permeability that would increase the transfer of intrapulmonary cytokines from the alveolar or airway compartment to the systemic circulation. In addition, both high-peak pressures as well as the absence of PEEP have been shown to increase bacterial translocation from the lung into the bloodstream in animal models of intratracheal instillation of bacteria,39 providing another mechanism by which mechanical ventilation can produce systemic manifestations. Our data suggest that, at least in some patients, the persistence of elevated lung lavage and systemic cytokine concentrations over 36 hours may be due in part to the ventilatory strategy used to treat the underlying respiratory failure, although it is possible that the somewhat higher FIO2 levels in the control group may play a role in this regard.40
Recommendations from a consensus conference on mechanical ventilation24 suggested that in patients with ARDS, inflation pressure should ideally be maintained at less than 35 cm H2O by decreasing tidal volume to as low as 5 mL/kg. The consensus conference also suggested that PEEP is useful in supporting oxygenation and may prevent lung damage. To verify the clinical impact of these recommendations, several randomized clinical trials have been conducted.10, 41-42 Stewart et al41 found no difference in mortality in their comparison of low (6-7 mL/kg) and high (10-12 mL/kg) tidal volume. In that study,41 PEEP was set as the minimal value that achieved acceptable arterial oxygen saturation (89%-93%) with nontoxic FIO2 values ( 0.5). Similar results were obtained by Brochard et al42 in a study in which they compared a strategy aimed at limiting Pplat to 25 cm H2O vs a more conventional ventilatory approach using a mean tidal volume of 10.3 mL/kg; both arms used a similar level of PEEP. In another study, Amato et al10 tested the hypothesis that the limitation of end-inspiratory volume and the use of PEEP levels that would prevent end-expiratory collapse would decrease mortality in patients with ARDS. Ventilation settings in the protective ventilation group were defined according to the inspiratory volume-pressure curves and resulted in a tidal volume and PEEP levels of 4 to 6 mL/kg and 15 to 20 cm H2O compared with 10 to 12 mL/kg and 6 to 8 cm H2O in the control group, respectively. This strategy was associated with significantly improved survival.10 Given the correlation between the persistence of the pulmonary and systemic inflammatory response and outcome in patients with ARDS,13-15 our data may help explain these findings.10
In our study, the post hoc analysis of ventilator-free days26 showed a significant reduction in the lung-protective strategy group, and mortality at 28 days after randomization tended to be lower in the lung-protective strategy group. These results were somewhat surprising to us since the protocol was designed to be a 36-hour study to assess cytokine levels. However, on termination of the study, there was a strong tendency for patients to be maintained on the ventilatory strategy that they had been assigned to in the experimental protocol until they were ready to be weaned. We should emphasize, however, that the study design did not include a strict weaning protocol for both groups and was not designed to assess these clinical end points; hence, these comparisons should be viewed as post hoc analyses, which must clearly be examined in appropriate randomized clinical trials.
In conclusion, we found that conventional mechanical ventilation of patients with ARDS is associated with a local and systemic cytokine response that is sustained over 36 hours; furthermore, this response may be attenuated by a ventilatory strategy designed to minimize ventilator-induced lung injury. Additional studies, assessing optimum ventilatory strategies, as well as other novel approaches specifically targeting the inflammatory response, are required to evaluate their effect on physiological as well as clinical end points in patients with ARDS requiring mechanical ventilation.
AUTHOR INFORMATION
Members of the Lung-Protective Strategy Ventilatory Strategy Group Steering Committee: In addition to the authors, members were Tommaso Fiore, MD, Salvatore Antonaci, MD, Filomena Puntillo, MD, Luciana Mascia, MD, PhD (Policlinico Hospital, Bari), and Salvatore Grasso, MD (Di Venere Hospital, Bari).
Funding/Support: This study was supported by grant 96-00934-CT04 from the Consiglio Nazionale delle Ricerche (Italy), grant 32-33902-92 from the Swiss National Research Foundation (Switzerland), and grant 8558 from the Medical Research Council (Canada).
Acknowledgment: The authors thank G. Assennato, MD, and M. Carbonara, MD, for their help in the statistical analysis and Eliane Agrebi and Anne McClair-Turnbull for secretarial assistance.
Corresponding Author and Reprints: V. Marco Ranieri, MD, University of Toronto, Mount Sinai Hospital, Room 656A, 600 University Ave, Toronto, Ontario, Canada M5G 1X5 (e-mail: marco.ranieri{at}utoronto.ca).
Author Affiliations: Istituto di Anestesiologia e Rianimazione (Drs Ranieri, Brienza, and Bruno), Servizio di Pneumologia (Dr De Tullio), Dipartimento di Medicina Interna (Dr Tortorella), Università di Bari, Ospedale Policlinico, Bari, Italy; Divisions of Surgical Intensive Care (Dr Suter), Immunology, and Allergy (Dr Dayer), Universitè de Genèva, Hôpital Cantonal Universitaire, Geneva, Switzerland; and Department of Medicine, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, University of Toronto, Toronto, Ontario (Dr Slutsky). Dr Ranieri is now at Mount Sinai Hospital, University of Toronto.
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