Initial Responses to Trauma
Initial responses to trauma involve several systems: hemostatic, inflammatory, endocrine and neurological. The two-hit model of post-injury MOF, described in a previous section and illustrated in Fig. 61-1, was derived from a two-way translational model in which information was exchanged between epidemiological studies, predictive models, and basic investigations using in vivo and in vitro models. As mentioned above, severe trauma patients are resuscitated into an early state of systemic inflammation or SIRS. The American College of Chest Physicians and the Society of Critical Care Medicine defined SIRS as two or more of the following criteria: (1) temperature less than 36.8°C or greater than 38.8°C, (2) heart rate more than 90/min, (3) respiratory rate more than 20 breaths/min or pco2 less than 32 mm Hg, and (4) WBC less than 4000/mL or greater than 12,000 mL or greater than 10% immature forms.
The current framework for the immunoinflammatory response to trauma is illustrated in Fig. 61-5. SIRS is the manifestation of the immunoinflammatory activation that occurs in response to ischemia–reperfusion (I/R) injury and factors released from disrupted tissue. The injured cell releases endogenous damage-associated molecular patterns (DAMPs, alarmins), which are analogous to the microbial pathogen-associated molecular patterns (PAMPs), released in sepsis.45,46,47 This analogy is not surprising as the human mitochondria derived from bacterial endosymbionts during the evolutionary process. Both PAMPS and DAMPS/alarmins activate innate immunity.
Response to trauma: hemostatic, inflammatory, endocrine and neurological systems interaction.
Liu et al48 showed that up to 70% of unique proteins measured in the plasma of blunt trauma victims were intracellular molecules that could function as DAMPS/alarmins and trigger pattern recognition receptors. Our laboratory was the first to describe the proteome description of human mesenteric lymph collected from critically ill or injured patients using a label-free semiquantitative mass spectrometry based approach.49 A total of 477 proteins were identified, including markers of hemolysis, extracellular matrix components, and general tissue damage in addition to the classical serum proteins. Hemolysis, a common event post-trauma, releases hemoglobin to the extracellular environment, where it becomes a redox-reactive DAMP molecule.50 Extracellular hemoglobin can also bind to PAMPs and triggers toll-like receptor-mediated signal transduction. High levels of extracellular hemoglobin can generate reactive oxygen species, which affects the innate immunity.
Among the proteins we identified in the mesenteric lymph, there were several markers of tissue damage including the cytoskeletal proteins actin, tubulin, cofilin-1, spectrin, gelsolin, and profilin-1.49 We also detected 16 mitochondrial proteins suggesting the presence of lysed mitochondria, presumably from cell death. There have been reports showing that circulating mitochondrial DNA and formyl peptides can mediate organ injury through PMN activation.45
We recently performed a mass spectrometric analysis of the plasma metabolome of severely injured patients, including a subgroup that underwent resuscitative thoracotomy in the Emergency Department of the Denver Health Medical Center, compared to healthy controls.51 Metabolomics describes the metabolites present in a biologic matrix and is reflective of the host’s pathologic state or response to stimuli. We observed a trauma-dependent metabolic state of hypercatabolism that provides carbon and nitrogen sources to compensate for trauma-induced energy consumption and negative nitrogen balance. This state is characterized by sugars consumption, lipolysis and fatty acid use, accumulation of ketone bodies, proteolysis and nucleoside breakdown. Unexpectedly, metabolites of bacterial origin (including tricarballylate and citramalate) were detected in the plasma of these trauma patients (see the section “Role of the Gut” later in the chapter).
Plasma from trauma patients showed accumulation of mannitol, heme, and oxidative stress markers.51 It should be noted that mannitol’s increase could reflect also therapy (as it is present in blood transfusions), although our data suggested limited transfusion effect, as we detected no difference in metabolite levels between resuscitative thoracotomy patients (who received limited transfusions) and other trauma patients, whose blood samples preceded intravenous fluid or blood products.
This metabolomic analysis confirmed the presence of an altered lipidomic profile, a hallmark of trauma-induced metabolic adaptation.51 Fatty acid mobilization (accumulation of acylcarnitines) and lipid breakdown (buildup of ketone bodies and breakdown products of fatty acids—eg, choline, glycerol, ethanolamine, glycerol phosphate, and glycerophosphocholine moieties) were observed. In addition, elevations in post-injury proinflammatory arachidonate metabolites (prostaglandin E2 and leukotriene B4) supported the proposed immunomodulatory effect of diets balancing the ratio of omega-3 and omega-6 fatty acids in reducing post-injury MOF.52 Moreover, the hyperactivation of lipid metabolism in response to trauma results in the accumulation of anionic compounds such as ketone bodies, further promoting acidosis.
We also detected significant increases in proteolysis as shown by the accumulation of several amino acids (alanine, aspartate, cysteine, glutamate, histidine, lysine, and phenylalanine) and cyclic dipeptide cyclo(glu-glu), which have been reported to have biologic activity including an immunomodulatory role in the stimulation of T lymphocytes.51 Glutamate and cysteine accumulation could fuel new reduced glutathione (GSH) synthesis (Fig. 61-2), thereby serving as physiologic protection from the increase in trauma-dependent oxidative stress. In addition, there was significant nucleoside breakdown for metabolic purposes as demonstrated by trauma patients’ increased levels of purine (xanthine, hypoxanthine, and inosine) and pyrimidine catabolites (uracil, 5,6-dihydrocuracil, and 3-aminobutyrate).51
Notably, we did not observe post-injury accumulation of glutamine.51 A potential explanation is that trauma enhanced consumption of this specific amino acid for direct cellular energy production or for fueling transamination reactions. However, although glutamine supplementation in critically ill patients has been a long-sought therapeutic approach, no evidence has been produced to date demonstrating the benefit of glutamine supplementation.53 While the levels of most amino acids were increased in response to trauma, tryptophan and its associated metabolites decreased in trauma samples. Finally, increased nicotinamide, a breakdown product of the purine metabolite NAD, may signal exhaustion of NAD+/NADH reservoirs potentially compromising many energy and redox-related processes dependent on these cofactors.
In trauma, hemorrhagic shock following injury produces whole-body hypoperfusion, followed by subsequent reperfusion during resuscitation, which stimulates the release of cytokines, proinflammatory lipids, chemokines, and proteins that prime polymorphonuclear neutrophils (PMNs) within 3–6 hours after injury.17 The systemic anti-inflammatory response syndrome (SARS) mediated defects in adaptive immunity include decreased antigen presentation, macrophage paralysis, depressed T-cell proliferative responses, increased lymphocyte and dendritic cell apoptosis, and shift from TH1 to TH2 lymphocyte phenotype.18,19
As discussed in the first section, recent evidence has challenged the concept of sequential pro- and anti-inflammatory responses and proposed that these processes occur simultaneously.20 In this Glue Grant study, investigators isolated blood leukocytes from a subgroup of 167 adult severe blunt trauma patients (out of 1637 adult, blunt trauma patients enrolled by seven US trauma centers), who consented to blood sampling (within 12 hours and at 1, 4, 7, 14, 21, and 28 days post-injury). The genetic expression of the leukocytes of these patients was compared to three different groups: (1) 37 healthy controls matched on age, sex, and ethnicity, (2) 133 adults with severe burn injuries, and (3) 4 healthy adults who received low-dose bacterial endotoxin. Their data suggested that severe blunt trauma produced significant changes in the expression of over 80% of the leukocyte transcriptome over the first 28 days compared to healthy subjects. The closer to the injury point, the larger the changes detected. The term “genomic storm” was coined to reflect the extent of the leukocyte transcriptome reprioritization in response to severe injury. The overexpressed genes were related to both the innate and adaptive immunity, while genes related to T-cell function and antigen presentation had decreased expression. The genomic response to blunt injury was remarkably similar to the response observed in groups 2 (burns) and 3 (endotoxemia). In addition, abnormal gene expression was long lasting, that is, still present 28 days after injury, but no evidence of a second-hit was observed. Post-injury complications were associated with greater and prolonged overexpression compared to patterns expressed by patients who had uncomplicated recoveries, although there was no major differences in which genes were invoked. Also, somewhat surprising was the minimal effect of the severity of anatomic injury (as measured by the ISS), acidosis and blood transfusions within the first 12 hours on gene expression. Based on this model, the authors proposed that rather than sequential, the SIRS-SARS response is synchronous and that it is the failure to return to homeostasis that defines the patients who will have a complicated recover.
It should be noted, however, that despite providing compelling evidence, the abovementioned investigation had a relatively small sample, limited to blunt torso trauma, and focused on circulating leukocytes.20 It is conceivable that there are different expression patterns in localized inflammation in different tissues, as well as in lymphoid organs and the reticuloendothelial system.
As described in the Epidemiology section, there has been a notable decrease in the incidence of late MOF. Furthermore, late MOF rarely results in death, but leads to prolonged ICU stays and need for mechanical ventilation.33 This observation has led to the description of a new phenotype, namely, the persistent inflammation, immunosuppression, and catabolism syndrome (PICS).19 These patients have manageable organ dysfunction, suffer recurrent inflammatory insults/nosocomial infections, have a persistent acute phase response with neutrophilia and lymphopenia, experience progressive loss of lean body mass (despite good nutritional supplementation), poor wound healing, and develop decubitus ulcers. They are discharged to acute long-term care facilities, where they die an indolent death or experience sepsis recidivism requiring ICU readmission. The elderly with baseline comorbidities and sarcopenia as especially prone. This refractory clinical phenotype translates to long-term impairment of cognitive and functional status from which recovery is uncertain.19,54 Clinically, PICS was initially defined by the following criteria: ICU stay greater than or equal to 14 days, persistent inflammation (C-reactive protein concentration >150 μg/dL and retinol binding protein concentrations <10 μg/dL), immunosuppression (crudely defined by a total lymphocyte count <800/mm3), and a catabolic state (serum albumin <3.0 mg/dL, creatinine height index <80%, and weight loss >10% or BMI <18 kg/m2 during the current hospitalization). Studies are underway to better define the phenotype, its true significance and novel interventions to prevent it or its progression. Multiple mechanisms are likely involved and multimodality interventions will be required. As the population ages, this MOF phenotype is likely to be next challenging horizon in surgical critical care.
As we are learning from the study of trauma-induced coagulopathy (see Chapter 13), the acute traumatic insult may result in a variety of post-injury clinical phenotypes depending on several factors such as severity and duration of shock, the specific organs and tissues injured, mechanism, extent and location of tissue disruption, time from injury to resuscitation, resuscitation protocols, etc.55,56,57 Therefore, it is entirely possible that all these theories apply to explain the development of diverse MOF phenotypes.
The gut is the last organ to have its circulation restored after ischemia, and is thought to play a pivotal role in the pathogenesis of post-injury MOF.18,47 Initially, the dominant hypothesis linking the gut to MOF was related to bacterial translocation: intestinal mucosa increased permeability allowed gut bacteria and/or endotoxin to be translocated to the circulation leading to sepsis and MOF. However, inconsistent results regarding the role of bacteria and endotoxin in the genesis of MOF led to experiments demonstrating that the mesenteric lymph acted as a bridge between the gut and the systemic circulation, allowing gut-derived inflammatory mediators to reach the systemic circulation.58,59,60 Via the thoracic duct, these mediators reach the pulmonary circulation and affect the lungs before other organs. This is consistent with human studies demonstrating that post-injury respiratory dysfunction almost always precedes heart, liver, and kidney failure.61
Our recent description of the post-injury plasma metabolome highlighted the presence of citramalate (methylmalonate) and tricarballylate, likely of bacterial origin.51 These findings suggest that bacterial metabolites, and not bacteria themselves as previously thought, may translocate during reperfusion of ischemic splanchnic beds to manifest systemic pathology. Alternatively, bacterial metabolites such as citramalate may elaborate systemically during trauma-induced hemolysis.62 The precise source of these nonmammalian metabolites and their role in post-injury pathophysiology deserve further investigation, which could include MS-metabolomic analysis for bacterial metabolites in postshock mesenteric lymph and blood product cell lysates.
Role of the PMN and Macrophages
PMN kinetics is different between MOF patients and non-MOF patients. Both groups develop neutrophilia at 3 hours post-injury; however, in patients who develop MOF there is a rapid neutropenia between 6 and 12 hours post-injury suggesting end-organ sequestration.63 PMNs marginated to end organs cause direct local cytotoxic cellular effects via degranulation, and the release of nitric oxide and reactive oxygen species. They also have remote systemic proinflammatory cytokine effects, releasing proinflammatory mediators including IL-6, IL-8, and TNF-α. In non-MOF patients, neutrophil priming and neutrophilia are not followed by neutropenia, and neutrophil counts normalize over the ensuing 36 hours without end-organ damage.64
Following trauma there is an immediate increase in adhesion molecules, including L-selectin and CD18, which allow PMNs to slow and roll along the endothelium and marginate out of circulation.65 Antibodies directed against the CD11b/CD18 components of the adhesion receptor complex between leukocytes and endothelium significantly attenuate lung injury and prevent the neutropenia associated with tissue sequestration during experimental sepsis; further supporting that adherence of neutrophils to endothelium is a critical step in local tissue injury.65
Circulating monocytes and tissue macrophages also become primed after severe injury and most authorities agree that microvascular endothelium has an integral role in post-injury priming of the innate inflammatory response.64 Finally, other studies have demonstrated that the organ damage is dependent on complement activation through the classical pathway mediated by natural IgM antibody produced by B1 lymphocytes.66
Our group demonstrated the predictive role of thrombocytopenia, especially when persistent, as a predictor of post-injury multiple organ failure.67,68 In the mid-1990s, Gawaz et al measured the platelet surface expression of fibrinogen receptor on GPIIb-IIIa, of von Willebrand factor receptor GPIb, and of granule glycoproteins (thrombospondin, GMP-140, GP53) using flow cytometry and platelet-specific monoclonal antibodies. These authors observed irreversible degranulation of granule glycoproteins in MOF patients, which correlated positively with the severity of organ dysfunction.69 Platelet-neutrophil interaction has been shown to be important in different models of acute lung injury including acid-aspiration, sepsis and transfusion injury.70 In fact, blocking this interaction completely reversed acute lung injury in animal models.71 The Matthay group showed in a mouse model that transfusion-related acute lung injury (TRALI) depended on participation by both platelets and neutrophils, and that depletion of platelets or antiplatelet therapy with aspirin prevented TRALI. Similarly, in a rat model of trauma/hemorrhagic shock, we observed that animals pretreated with a platelet P2Y12 receptor antagonist were protected from post-injury acute lung injury.72 Furthermore, isoflurane, an halogenated ether known to interfere with platelet-granulocyte aggregation, attenuated acute lung injury (ALI) through an anti-platelet mechanism, partially through inhibition of the platelet ADP pathway.73 We translated these observations to the clinical sphere, and documented that pre-injury antiplatelet therapy was associated with a decreased risk of lung dysfunction and multiple organ failure in high-risk blunt trauma patients who received blood transfusions.74 Recently, Boyle et al confirmed that therapy with aspirin (a potent, long-lasting suppressor of platelet aggregation, via inhibition of cyclo-oxygenase enzymes that prevent thromboxane A2 production), given either before or during hospital stay, was associated with a reduction in ICU mortality among patients with established adult-respiratory distress syndrome (ARDS).75 The results of the multicenter trial LIPS-A (Lung Injury Prevention with Aspirin, clinicaltrials.gov identifier NCT01504867) should provide interesting evidence on the therapeutic use of this antiplatelet agent.
The hemodynamic, metabolic, and immune responses are mainly regulated by endogenous mediators referred to as cytokines, produced by diverse cell types at the site of injury and by systemic immune cells.65 Cytokines bind to specific cellular receptors resulting in activation of intracellular signaling pathways that regulate gene transcription and influence immune cell activity, differentiation, proliferation, and survival. They also regulate the production and activity of other cytokines, which may either augment or attenuate the inflammatory response. There is also significant overlap in bioactivity among different cytokines.33 Cytokines can be classified into proinflammatory (TNF-α, MIP, GM-CSF, IFN-γ, IL-1, IL-2, IL-6, IL-8, IL-17, etc.) and anti-inflammatory cytokines (IL-4, IL-10, IL-13), which downregulate synthesis of the proinflammatory cytokines.65
Jastrow et al76 assessed the temporal cytokine expression (every 4 hours during 24 hours post-injury) during shock resuscitation in severely injured torso trauma patients. Median concentrations of IL-1 receptor antagonist (IL-1Ra), IL-8, eotaxin, granulocyte colony-stimulating factor (GCSF), granulocyte-macrophage colony-stimulating factor (CSF), inducible protein 10 (IP-10), monocyte chemotactic protein-1, and macrophage inflammatory protein-1 (MIP-1) were significantly greater in MOF patients compared with those in the non-MOF subgroup at each time interval. Adams et al77 demonstrated that IL-8 can activate PMNs via two different receptors, and differential early expression of these receptors may provide an explanation for why some patients are at higher to develop MOF.
The cytokine pattern after trauma also differs for patients developing early (<3 days) versus late (>3 days) MOF. Analysis of cytokine serum biomarkers revealed that whereas early onset MOF was associated with an initial peak of IL-6 and IL-8 followed by a comparatively rapid return to baseline values, late MOF was characterized by a significant secondary increase of the proinflammatory cytokines IL-6 and IL-8.78 Moreover, plasma levels of soluble tumor necrosis factor-alpha receptors (sTNF-R p55, sTNF-R p75) progressively increased during the 10-day observation period, and higher values were associated with lethal outcome.
Although inflammatory mediators’ levels vary greatly according to injury-related factors as well as the patient’s individual characteristics, most studies agree that the changes start very early post-injury.79 This underscores the importance of measuring inflammatory mediators very early and at short intervals after injury.66 Indeed, a 2009 German study including 58 multiple injured patients (ISS >16) found that IL-6, IL-8, and IL-10 differentiate patients with MOF (n = 43) and those without MOF (n = 15) within 90 minutes of injury.79
The 1994 “danger theory” of the inflammatory response following trauma or infection proposed that the role of the immunological system was more than differentiating self from nonself but to protect the body from danger.46,80,81,82,83,84 In the “danger model,” immunological responses are triggered by specific types of cell death. If a healthy, undamaged cell dies an apoptotic death, it is scavenged without triggering an immune response. Conversely, a noncontrolled and abnormal cell damage via trauma or infection, causing lysis or apoptosis, releases intracellular contents and signals “danger,” triggering both innate and adaptive responses.81,83,85 As Matzinger eloquently put it “the Danger model holds that the immune system is governed from within, responding to endogenous signals that originate from stressed or injured cells.”85
Pathogen-associated molecular patterns (PAMPs) are exogenous microbial molecules that alert the organism to pathogens and are recognized by cells of the innate and acquired immunity system, primarily through Toll-like receptors (TLRs), and activate several signaling pathways (eg, NF-κB).80 The new awareness of the close relationship between trauma- and pathogen-evoked responses led to proposal of the term “alarmin” to differentiate the endogenous molecules derived from cell damage from the pathogen-derived molecules.80 Together, alarmins and PAMPs comprise the DAMPs. Major DAMPs are HMGB1 (high mobility group box protein-1), heat-shock proteins, uric acid, and DNA. The HMGB1 is a nuclear protein which binds to nucleosomes and promotes DNA bending. HMGB1 has been associated with SIRS and end-organ damage in animals, and shown to be elevated in trauma patients more than 30-fold above healthy controls as early as 1 hour post-injury.80,86,87
A recent study by Zhang et al showed that injury releases mitochondrial DAMPs (MTDs) into the circulation with functionally important immune consequences.45 The authors suggest that since mitochondria are evolutionary endosymbionts derived from bacteria, the released MTDs have conserved similarities to bacterial PAMPs. Thus, these MTDs signal through innate immune pathways identical to those activated in sepsis to create a sepsis-like state. This mechanism may be the key link between trauma, inflammation, and SIRS.
TLRs are transmembranal proteins present in most body cell types, which form the major pattern recognition receptors that transduce signals in response to DAMPs.88 They were shown to participate in the recognition of endogenous alarmins released from damaged tissues after ischemia/reperfusion injuries. Innate immune system responses are then initiated, including NF-kappa-B activation, cell activation, and proinflammatory cytokine production. Inhibition of TLR2 or TLR4 seems to be protective from ischemia/reperfusion injury in certain organs (hepatic, renal, cerebral, and heart) but not in the gut. It is conceivable that because the gut mucosa is continuously exposed to local bacterial endotoxins, local TLRs are uniquely regulated to prevent persistent inflammatory activity. In spite of the distinct roles played by TLR2 and TLR4 in individual organs, our understanding of how the multiple TLR members interact among each other in ischemia/reperfusion injuries is still limited, which may hinder the interpretation of interventions aimed at a specific TLR.
HSPs are a family of DAMP molecular chaperones (eg, Hsp70 and Hsp90) necessary for the folding of newly synthesized proteins in the cell and also for the protection of proteins during exposure to stressful situations such as heat shock, which causes proteins folded previously to unfold.89 Extracellular heat shock proteins (HSPs) can interact with several receptors (including TLRs), and have been implicated in inducing secretion of proinflammatory cytokines. However, highly purified HSPs do not show any cytokine effects suggesting recombinant HSP products may be contaminated with PAMPs that may be responsible for the reported in vitro cytokine effects of HSPs. Thus, the reported HSP’s role in antigen presentation and cross-presentation and in vitro cytokine functions may be attributable to molecules bound to or chaperoned by HSPs.
The complement system is considered a major component of the innate immunity response with several functions including: recognize and eliminate microorganisms, clear immune complexes and apoptotic cells, and mediate inflammation.82 However, research has shown complement to also enhance the adaptive response.90 The complement system connects the innate and adaptive immune responses and also links the immune system with the coagulation system.91
Complement system activation occurs immediately after trauma leading to production of biologically active peptides.47,82 Proinflammatory complement activation products include C3a, C3b, and C5a (chemotaxis of leukocytes; degranulation of phagocytic cells, mast cells, and basophils; smooth muscle contraction and increased vascular permeability), and leads directly to the generation of the terminal C5b–C9 complex (the complement membrane-attack complex) that leads to lysis of the target cells at the end stage of the complement activation cascade. Furthermore, complement activation results in the production of oxygen free radicals, arachidonic acid metabolites, and cytokines. However, excessive intravascular C5a may lead to neutrophil function “paralysis,” rendering them incapable to respond to C5a or other chemo-attractants, a paradox that has been equated to a “Pyrrhic victory of the innate immunity.”92 Several studies suggest that complement activation, especially serum C3 and C3a levels as well as C5a, reflects severity and treatment of injury and organ failure.93,94,95,96
Complement regulatory proteins (CD55, CD46, CD55, CD59), the C5a receptor (CD88) inhibitors of complement, such as C4b-binding protein (C4BP) and factor I, modulate the complement cascade and protect against complement-mediated tissue destruction. Several studies in polytrauma patients indicate that these regulatory factors are significantly altered post-injury suggesting “a trauma-induced complementopathy.”96,97,98
Oxidative stress occurs when the level of toxic reactive oxygen intermediates (ROIs) overcomes endogenous antioxidant defenses as a result from either oxidant production excess or antioxidant defense depletion.99 ROIs are normally generated by mitochondrial oxidation, metabolism of arachidonic acid, activation of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in phagocytes, and activation of xanthine oxidase (XO) and play important roles in cellular homeostasis, mitosis, differentiation, and signaling.47 Excess ROIs, however, cause direct oxidative injury to cellular proteins and nucleic acids, and disrupt cell membranes by inducing lipid peroxidation.88,99
Ischemia/reperfusion (I/R) injury leads to significant disturbances in the production of ROIs.47,88 During ischemia, hypoxemia leads to a shift from aerobic to anaerobic metabolism, with consumption of and decreased production of adenosine triphosphate (ATP). As ATP decreases, changes in cell membrane permeability result in intracellular Na+ increase causing cellular swelling and cell membrane damage. ATP reduction also increases cytosolic Ca2+ levels leading to phospholipase and protease activation resulting in cell damage. Increased ATP hydrolysis is followed by rising levels of AMP and purine metabolites. As reperfusion increases O2 availability, oxidation of purines produces urate and superoxide radicals, which can produce the toxic hydroxyl radicals. In addition, superoxide radicals may be generated by a plasma membrane–associated NADPH oxidase system, which can be activated by macrophages, neutrophils, and other immunologic cells.100 ROI secreted from PMNs after I/R injury induces cytokines, chemokines (IL-8), HSP, and adhesion molecules (P-selectin, ICAM-1) leading to cell and tissue damage.47
Under normal conditions, NO production greatly exceeds O2– production in the endothelial cell (EC).101 However, with reperfusion, the balance between NO and O2– shifts in favor of O2–. The relatively low level of NO by constitutive endothelial nitric oxide synthase (NOS-I) reacts with the now abundant O2– to generate peroxynitrites, leaving little NO available to reduce arteriolar tone, prevent platelet aggregation, and minimize PMN adhesion to EC.102 In addition, NO seems to upregulate the production of proinflammatory cytokines.101 Thus, altering the redox state of the cell may contribute to the ongoing inflammatory cytokine production and progression to MOF. ROIs also play a role as second messengers in the intracellular signaling pathways of inflammatory cells, in particular, activation of NF-κB and activator protein 1 (AP-1), which can be activated by both oxidants and antioxidants depending on the cell type and on intracellular conditions.99
Endogenous antioxidant defenses, including enzymatic (superoxide dismutase, catalase, glutathione peroxidase) and nonenzymatic (vitamins E and C, provitamin A, glutathione, bilirubin, urate) groups that combat oxidative stress, were the focus of interventions to modulate the inflammatory response to critical illness. However, the results of the large randomized clinical trials REDOX100,103,104 and METAPLUS,105 both demonstrating harm in systemic administration of antioxidants, have discouraged research in this direction.53 For both glutamine and antioxidants, the greatest potential for harm was observed in patients with MOF that included renal dysfunction.104 It is possible, that antioxidants can only be effective at specific sites and/or specific cells as opposed to a systemic alteration in the redox balance.101
Abdominal Compartment Syndrome
A hallmark of the post-injury inflammatory state is generalized capillary leak and associated tissue edema. There has been change resurgent interest in this phenomenon as intra-abdominal hypertension has accompanied the recent widespread application of damage control procedures (see Chapter 38).106 Increased intra-abdominal pressure (IAP) is associated with a host of physiologic derangements that include high ventilator pressures, decreased cardiac output, and impaired renal function, a constellation of signs that are named abdominal compartment syndrome (ACS). Usually associated with abdominal injuries (primary ACS), these effects can also be observed following extra-abdominal injury or following large-volume resuscitation for nontraumatic or nonsurgical conditions (secondary ACS).107
The incidence of ACS varies greatly according to the resuscitation strategy but is primarily dependent on the severity of injury and the degree of shock. Mesenteric I/R increases microvascular permeability, causing bowel wall edema that can directly cause decreased bowel motility, decreased barrier function, and further capillary leak. Splanchnic hypoperfusion during shock is compounded by even mild increases in IAP (which can reduce the abdominal perfusion pressure) and low plasma oncotic pressure from crystalloid resuscitation (which worsens edema). As with any compartment syndrome, the increased pressure within the compartment rises above postcapillary pressure causing a functional venous and lymphatic obstruction. Under these conditions, the intestinal epithelium microvilli secrete fluid into the gut lumen. Transudation of free fluid into the peritoneal space also contributes to further IAP increases. The combination of bowel wall edema, intraluminal fluid secretion, and fluid accumulation in the peritoneal cavity further increases IAP and gut ischemia, which affects the inflammatory response as described above. While ACS physiologic effects usually reverse on decompression, the immunomodulatory effects may persist and trigger MOF.108
ACS incidence has lowered to as 0–2% in high-performing trauma centers, a substantial decrease in this highly lethal condition compared to 15% reported by earlier studies.106 Although life-threatening post-injury ACS is preventable, most patients with major shock and trauma develop some transient increase in intra-abdominal pressure. If this pre-ACS state is combined with renal impairment or severe lung injury, MOF is unknown.106 As an isolated entity, sub-ACS IAH is probably negligible, but in combination with other distant organ dysfunctions, this syndrome could be detrimental.
The goals of transfusion in the injured patient are to maintain adequate oxygen-carrying capacity. However, there is substantial, solid evidence that blood transfusions are a risk factor for the development of MOF independent of shock or injury severity.37,67,109,110,111 Indeed, blood transfusions fit many of the criteria for causation, that is, strength of the association, temporal relationship (risk factor precedes outcome), dose–response relationship, consistency, and reproducibility. Knowledge of this association has resulted in more judicious use of blood products during resuscitation (see Chapter 12).37 Despite this reduction in the use of red blood cells, early blood transfusion remains one of the most powerful independent risk factor for post-injury MOF.33 Reductions in blood transfusion in the resuscitation period correlate with improved outcome and less MOF.37 Blood products are immunoactive, contain proinflammatory cytokines and lipids, and have an early immunosuppressive effect predisposing the patient to SARS, infection, and late MOF.111
The age of transfused blood is also important, with progressive daily increases in proinflammatory cytokines in stored blood products. Transfusing packed red blood cells (PRBCs) stored more than 3 weeks in the first 6 hours post-injury is associated with a higher rate of MOF, while PRBC units with shorter storage times are associated with decreased MOF prevalence and less morbidity and mortality. Leukodepletion does not remove the potential for blood to act as a second hit, as PRBCs contain proinflammatory mediators. Initially, this was believed to be related to the generation of the PAF, IL-6, and IL-8 during storage. Later studies found that biologically active lipids (lysophosphatidylcholines) accumulate in stored blood and are capable of PMN priming. “Passenger leukocytes” present in stored blood have been implicated as pivotal components by the finding that prestorage leukoreduction decreases the PMN priming and transfusion-mediated lung injury in animal models. Proposed mechanisms include induction of T-cell anergy in the recipient, decreased natural killer cell function, altered ratio of T-helper to T-suppressor cells, and soluble proinflammatory cytokines produced by leukocytes during storage. Together, these effects can suppress the recipient immune system and promote a worse proinflammatory state leading to increased infection and complications.
Proteomic analysis of PRBC and leukoreduced-PRBC, after depletion of the 14 most abundant proteins, detected PRBC-related proteins including: structural proteins, protein 14-3-3, band 3, the α- and β-hemoglobin chains, and enzymes involved in essential metabolic function such as glycolysis and redox cycling to protect against oxidative damage.112 Furthermore, storage of unmodified PRBC supernatant resulted in increased concentrations of both leukocyte specific proteins (moesin, MPO, MMP-8, MMP-9) and platelet specific proteins (integrin alpha 2b, platelet glycoprotein V) which are removed by prestorage leukoreduction. Several structural proteins also increased including actin, α-actinin-1, vinculin, and talin-1. The latter three are important scaffolding proteins responsible for stable presentation of adhesion molecules. In addition, several glycolytic enzymes also increased including α-enolase, involved in plasmin activation and extracellular matrix degradation.113,114 Conversely, mannose-binding lectin 2, the complement protein C4b, protein C, and prothrombin significantly diminished in PRBC supernatant, which could inhibit innate immunity. Leukoreduced PRBC showed accumulation of an active phospholipase, which could explain the production of arachidonic acid, and the 5-LO metabolites 5-, 12-, and 15-HETE via cleavage of lipids from RBC membranes, which have the capacity to cause transfusion-related acute lung injury in vivo.115
Other blood-derived products (platelets, plasma, and coagulation factors) are also immunoactive and could act as second hits (see Chapter 13). Transfusion of fresh frozen plasma (FFP) has been shown to be associated with MOF among severely injured, massively transfused patients, although no details about when plasma was given or about plasma:PRBC ratios were available in that study.109 Proteomic analysis of fresh frozen plasma by our group suggested that plasma of female donors had higher levels of factor V and antiproteases as well as complement factors H and C4b compared to male donor plasma.116 These preliminary data were derived from a small sample but suggest that, during resuscitation, female donor plasma may more efficiently augment factor V levels, increase circulating antiprotease concentrations, thus potentially decreasing indiscriminate alternative pathway activation of complement.
Proteomic analyses of platelet supernatants of healthy donors suggested a storage and sex-dependent impairment of blood coagulation mediators, proinflammatory complement components and cytokines, energy, and redox metabolic enzymes. Specifically, the analysis of the platelet supernatants suggested storage-dependent platelet activation, which was more pronounced among female donors.117,118
Osler recognized in 1904 that “except on few occasions, the patient appears to die from the body’s response to infection rather than from the infection.”119 The association of infection and MOF has been widely explored. In the late 1970s, intra-abdominal abscess (IAA) was the inciting event in half of the cases.120 As a result of the appropriate use of presumptive antibiotics for abdominal trauma and prompt diagnosis of hollow viscus injury, the incidence of post-injury IAA decreased and its progression toward MOF followed. The epidemiology of post-injury infections changed and nosocomial pneumonia became the principal infection associated with MOF (see Chapter 3).
While the anti-inflammatory response may be protective because it limits unnecessary inflammation potentially auto-destructive, it is associated with relative immunosuppression predisposing the host to infections.121 Data from both clinical and animal studies suggest diminished production of proinflammatory type 1 and increased production of inhibitory type 2 cytokines by T cells. Monocytes/macrophages and suppressor T lymphocytes have been implicated as key modulators that downregulate immune functions.
Another area of research interest has been the potential role of persistent hypercatabolism in the development of infections.65 Although energy expenditure can increase dramatically following injury, the associated hypercatabolism is a critical metabolic alteration. If not supported by exogenous nutrients, the consequent obligatory protein turnover quickly erodes somatic protein stores and then the critical visceral mass. The resulting acute protein malnutrition causes well-documented adverse immunologic consequences and is a recognized cofactor for the development of post-injury infection (see Chapter 3).
The second operation can be considered a controlled traumatic event where surgical trauma takes the place of the initial injury. Additional stresses of secondary operations include higher intraoperative fluid administration, hypothermia, hypotension, tissue hypoxia, and intraoperative blood loss.65
The timing of the second operation has been studied over the last decade mostly as related to operative fracture fixation. In 1989, Bone et al published a prospective randomized study showing that, compared to early stabilization of femoral fractures, delayed stabilization resulted in higher incidence of adult respiratory-distress syndrome, fat embolism, and pneumonia, as well as longer ICU stay.122 While early definitive fracture fixation decreases post-injury morbidity and improves recovery, it is not without consequences when performed within the priming window. A 2003 randomized controlled trial by Pape et al demonstrated that early external fixation followed by delayed conversion to intramedullary instrumentation was associated with a decreased inflammatory response to the operative fixation.123 The same group compared the inflammatory response of injured patients with femoral shaft fracture who were divided into two groups according to their initial treatment: (1) damage control orthopedics (DC) group if the femoral fracture was initially stabilized with an external fixator and (2) intramedullary nailing (IMN) group if they underwent primary IMN.124 Despite more severe injuries in the DC group, patients had a less severe and shorter postoperative SIRS and did not suffer significantly more pronounced organ failure than the IMN group. DC patients undergoing conversion while their SIRS score was raised suffered the most pronounced subsequent inflammatory response and organ failure. According to these data, DC treatment was associated with a lesser SIRS than early total care for femur fractures.
Our group recently examined outcomes associated with early total care with IMN (ETC group) versus damage control external fixation (DC group) for 462 multiple injured patients with femoral shaft fractures.125 Although minimal differences were noted between DC and ETC groups regarding systemic complications, DC was a safer initial approach, significantly decreasing the initial operative exposure and blood loss. We also explored the application of the DC concept to spine fractures and observed similar beneficial effects regarding pulmonary complications, infections, mechanical ventilation time, and length of ICU stay.126 Collectively, these findings suggest that a secondary operation can act as an additional inflammatory insult and amplify the post-injury inflammatory response and precipitate MOF.