The primary goal of this study was to test the hypothesis that a panel of biomarkers that reflects the complex pathophysiology of acute lung injury can be used to diagnose ALI/ARDS among a group of critically ill patients with severe traumatic injuries. From an initial panel of 21 biomarkers of inflammation, lung epithelial and endothelial cell injury, fibrosis and dysregulated coagulation and fibrinolysis, a combination of 7 biomarkers were used to develop a diagnostic model. The final model includes markers of lung epithelial injury (RAGE), collagen deposition (PCPIII), cardiac dysfunction (BNP), endothelial activation and injury (Ang2) and inflammation (IL-10, TNF-α and IL-8)). This diagnostic model had excellent performance for differentiating ALI/ARDS cases from controls with an area under the ROC curve of 0.86. A simplified model using the three top performing biomarkers was also tested (RAGE, PCPIII, BNP); this model also had excellent discriminatory power with an area under the ROC curve of 0.83.
The gold standard for diagnosis of ALI/ARDS in the current study was the American European Consensus definitions 8
. Despite widespread use of these definitions for enrollment into clinical trials, ALI and ARDS remain underdiagnosed and undertreated 9,10
. Furthermore, these consensus definitions may not be ideal in trauma patients. In one study of patients with trauma induced ARDS 32
defined by standard consensus definitions, there was little uniformity among the patients who met consensus criteria when comparing severity of illness scores, incidence of multi-organ failure and incidence of sepsis. The authors concluded that application of the consensus criteria for the diagnosis of ALI/ARDS in trauma patients captures a widely disparate group with poor specificity for identifying patients at risk for prolonged respiratory failure and associated complications 32
. In addition, at least three studies have found that a diagnosis of ALI/ARDS is not an independent predictor of mortality in patients with severe trauma 33–35
, in contrast to other etiologies of ALI/ARDS where the diagnosis is associated with increased mortality 1,36
. In the current nested case control study, patients with a clinical diagnosis of ALI/ARDS had significantly higher mortality and longer duration of mechanical ventilation and ICU stay compared to patients with clear chest radiographs or hydrostatic pulmonary edema. When the biomarker panel was used to classify patients as high or low probability of ALI/ARDS, substantial differences in important clinical outcomes including duration of mechanical ventilation and duration of ICU stay were seen.
In addition to its diagnostic performance, the multimarker panel provides novel insight into the pathogenesis of trauma-associated ALI/ARDS. Although several of the biomarkers that were studied have been previously studied in early trauma-associated ALI/ARDS 1
, the majority of the biomarkers most highly associated with ALI/ARDS in our study have not been previously studied in a large trauma population including CC16, RAGE, PCPIII, Ang2 and BNP. The diagnostic value of RAGE is of particular interest, providing biochemical confirmation of the critical role of alveolar epithelial injury in trauma-associated ALI/ARDS, a concept that has been suggested in several prior studies 16,37–42
and was first emphasized in pathologic specimens by Bachofen and Weibel 38
. Although RAGE is expressed in several organs, RAGE is most highly expressed in the lung where it is an alveolar type-1 epithelial cell associated protein localized primarily to the basal membrane of the type 1 cells. RAGE is a multi-ligand binding receptor that binds advanced glycation end products, amyloid beta-peptide, s100 proteins and high-mobility group box-1. RAGE-ligand interaction results in intracellular signaling and NF-kB activation 15
. Two previous human studies found RAGE to be elevated in patients with acute lung injury compared to either normal controls 15
or to patients with less severe acute lung injury 39
. Since RAGE is a marker of alveolar epithelial injury, its release into the plasma implies damage to the lung epithelium as one important mechanism of early injury in trauma-induced ALI/ARDS. Injury to the alveolar epithelium is associated with impaired alveolar fluid clearance in both experimental and clinical studies 42,43
and in experimental models, hemorrhagic shock-induced lung injury causes impaired alveolar fluid clearance 44–46
. Thus, alveolar edema in trauma-induced ALI/ARDS may be accounted for in part by epithelial injury.
The second top performing biomarker in the diagnostic model was PCP-III, a marker of collagen synthesis. PCPIII is cleaved from the precursor procollagen molecule by extracellular proteases during collagen synthesis and reflects fibrogenesis 47
. Elevated levels have been described in fibrotic lung diseases 48,49
as well as hepatic cirrhosis 50
and cardiac fibrosis 51
. Levels of PCPIII were five-fold higher in the pulmonary edema fluid of patients with ALI/ARDS compared to patients with hydrostatic pulmonary edema 47
; among patients with ALI/ARDS, higher levels were associated with increased mortality, a finding that was replicated in a study of bronchoalveolar lavage fluid from patients with ARDS 52
. Higher levels in plasma of patients with ALI/ARDS have also been reported in a few small studies 53,54
. In one study of 57 patients with severe trauma, elevated PCPIII levels were associated with higher mortality, longer duration of mechanical ventilation and poorer oxygenation 55
. The finding of elevated plasma PCPIII levels in trauma patients with ALI/ARDS supports the hypothesis that lung fibrosis is triggered early in the course of ALI/ARDS. However, it should be noted that increased PCPIII levels in trauma patients may also reflect wound healing, fibrosis in other organs and decreased clearance due to hepatic dysfunction 55
BNP was also a top performing biomarker. BNP is the main clinical marker used to differentiate hydrostatic pulmonary edema from increased permeability pulmonary edema,23
and for this reason it was included in the original panel of biomarkers for testing. BNP is secreted by the cardiac ventricles in response to increased stretch, as occurs in the setting of increased circulating volume or decompensated systolic or diastolic heart failure. We found that BNP levels were significantly lower in the patients with ALI/ARDS compared to those without ALI/ARDS, suggesting a higher prevalence of volume overload and/or cardiac dysfunction in the control group in this study. However, as a single biomarker, BNP performed very poorly for the discrimination of ALI/ARDS with an area under the curve of 0.64. The remaining four biomarkers in the model reflect other aspects of the pathophysiology of ALI/ARDS endothelial injury and activation and inflammation.
This study has some limitations. The diagnostic model was designed to differentiate patients with ALI/ARDS from those without evidence of lung injury (clear chest radiograph) and those with hydrostatic (cardiogenic) pulmonary edema. Differentiation of patients with ALI/ARDS from patients with hydrostatic pulmonary edema can be particularly challenging and may require invasive hemodynamic monitoring 23
. However, we were only able to identify 11 patients with hydrostatic pulmonary edema in our cohort; it is difficult to predict whether the model will perform as well in a larger group of patients where hydrostatic edema is more prevalent. Also, it is possible that some of the patients with ALI/ARDS had a combination of both increased permeability and elevated hydrostatic pressure as the cause of their pulmonary edema, as has been documented in a recent large multicenter clinical trial 56
. We also did not attempt to distinguish between ALI/ARDS due to pulmonary contusion and ALI/ARDS due to other causes such as aspiration of gastric contents, or severe hemorrhagic shock with multiple blood product transfusions. Pulmonary contusion is histologically similar to ALI/ARDS from other causes 57
and the underlying cause of ALI/ARDS may be difficult to ascertain with confidence even in the relatively homogeneous severe trauma population. Another limitation is the retrospective nature of this study. Although most of the clinical data was collected prospectively as part of the parent study, none of the investigators were involved in clinical care of the patients in this study, and all determinations of clinical diagnosis of ALI/ARDS were made by chart review and review of the chest radiographs. A prospective analysis of this diagnostic biomarker panel in unselected trauma patients will be needed for validation. Finally, the model was developed in patients who developed ALI/ARDS due to severe trauma. Although trauma is an important cause of ALI/ARDS, other clinical disorders including aspiration and infectious causes are also important. Thus, the value of this biomarker panel for diagnosis and pathogenesis of ALI/ARDS in patients with other risk factors for ALI requires further study.