The First HIT: Initial Inflammatory, Hormonal, and Immune Responses to Trauma
The model of postinjury MOF, described in a previous section, 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 criteria26: (1) temperature <36.8°C or >38.8°C; (2) heart rate >90/min; (3) respiratory rate >20 breaths/min or pco2 <32 mm Hg; and (4) WBC <4,000/mL or >12,000 mL or greater than 10% immature forms.
Figure 61-3 illustrates the current framework for the immunoinflammatory response to trauma. SIRS is the manifestation of the immunoinflammatory activation that occurs in response to ischemia–reperfusion (I/R) injury and released factors from disrupted tissue, that is, the damage-associated molecular patterns (DAMPs). In trauma, hemorrhagic shock following injury causes whole-body hypoperfusion, followed by subsequent reperfusion during resuscitation, which circulates cytokines, proinflammatory lipids, and proteins that prime polymorphonuclear neutrophils (PMNs) within 3–6 hours after injury.17 The CARS includes (a) apoptotic loss of intestinal lymphocytes and epithelial cells, (b) PMN and monocytic deactivation, (c) anergy characterized by suppressed T-cell proliferative responses, and (d) a shift from a THI to a TH2 phenotype.18 In the past, most studies attributed SIRS to hyperactivity of the innate immune system and CARS to dysfunction of the adaptive immune system, but recent evidence suggests that interactions between the innate and adaptive immune systems induce both SIRS and CARS and that the predominant mechanism for MOF is the balance between proinflammatory and counterinflammatory states.27
Immunoinflammatory response to trauma.
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 postinjury MOF.18,28 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 and primed neutrophils to reach the systemic circulation. Via the thoracic duct, these mediators reach the pulmonary circulation and affect the lungs before any other organ, which is consistent with human studies demonstrating that postinjury respiratory dysfunction is an obligate event that precedes heart, liver, and kidney failure.29,30
Role of the PMN and Other Cells
PMN kinetics are different between MOF patients and non-MOF patients. Both groups develop neutrophilia at 3 hours postinjury; however, in patients who develop MOF there is a rapid neutropenia between 6 and 12 hours postinjury suggesting end-organ sequestration.31 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-8, IL-6, and TNF-α. In non-MOF, neutrophil priming and neutrophilia are not followed by neutropenia, and resolve over the next 36 hours without end-organ damage.32
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.17,33 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 sepsis, further supporting that adherence of neutrophils to endothelium is a critical step in local tissue injury.33
Circulating monocytes and tissue macrophages also become primed after severe injury and most authorities agree that microvascular endothelium has an integral role in postinjury priming of the innate inflammatory response.32 Finally, other studies have demonstrated that the organ damage is also dependent on complement activation through the classical pathway mediated by natural IgM antibody produced by B1 lymphocytes.27
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.33 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-g, 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.33
Recently, Jastrow et al. assessed the temporal cytokine expression (every 4 hours during 24 hours postinjury) during shock resuscitation in severely injured torso trauma patients.34 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 the MOF compared with those in the non-MOF subgroup at each time interval. Adams et al. demonstrated that IL-8 can activate PMNs via two different receptors, and differential early expression of these receptors may provide an explanation for why only selected patients develop MOF.35
The cytokine pattern after trauma also differs for patients developing early (less than 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-8.36 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 postinjury. This underscores the importance of measuring inflammatory mediators very early and at short intervals after injury.27 Indeed, a recent 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 post trauma.37
Pamps, Alarmins, and Damps
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 (e.g., NF-κB). A new awareness of the close relationship between trauma- and pathogen-evoked responses recently emerged and the term “alarmin” was proposed to differentiate the endogenous molecules that signal tissue and cell damage.38 Together, alarmins and PAMPs comprise the DAMPs. Alarmins are rapidly released after nonprogrammed cell death but not by apoptotic cells. They recruit and activate receptor-expressing cells of the innate immune system, and also promote adaptive immunity responses. An example of an alarmin is the high-mobility group box 1 (HMGB1), 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 postinjury.38–40
A recent study by Zhang et al. showed that injury releases mitochondrial DAMPs (MTDs) into the circulation with functionally important immune consequences. 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 provide the key link between trauma, inflammation, and SIRS.41
TLRs are transmembranal proteins present in most body cell types, which form the major pattern recognition receptors that transduce signals in response to DAMPs.42 They were shown to participate in the recognition of endogenous alarmins released from damaged tissues after I/R injuries. Innate immune system responses are then initiated, including NF-aB activation, cell activation, and proinflammatory cytokine production.42 Inhibition of TLR2 or TLR4 seems to be beneficial in I/R injury in certain organs (hepatic, renal, cerebral, and heart) but not in gut I/R injuries. 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 TR2 and TR4 in individual organs, our understanding of how the several TLR members interact among each other in I/R injuries is still limited, which may hinder the interpretation of interventions aimed at a specific TLR.42
Heat Shock Proteins (Hsps)
HSPs are a family of molecular chaperones (e.g., 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.43 Extracellular 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 appear to be responsible for the reported in vitro cytokine effects of HSPs.43 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.
Complement system activation occurs immediately after trauma leading to production of biologically active peptides.28 Proinflammatory peptides include C3a, C3b, C4b (chemotaxis of leukocytes; degranulation of phagocytic cells, mast cells, and basophils; smooth muscle contraction and increased vascular permeability), and C5b-9 or 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. Several studies suggest that complement activation, especially serum C3 and C3a levels, reflects severity and treatment of injury.28
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.44 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.28 Excess ROI, however, causes direct oxidative injury to cellular proteins and nucleic acids, and cell membrane destruction by inducing lipid peroxidation.28,42,44
I/R injury leads to significant disturbances in the production of ROIs.28,42 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 alters cytosolic Ca2+ levels leading to phospholipase and protease activation and 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 then 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.45 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.28
Under normal conditions, NO production greatly exceeds O2− production in the endothelial cell (EC). 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.46 In addition, NO seems to upregulate the production of proinflammatory cytokines.28 Thus, altering the redox state of the cell may contribute to the ongoing inflammatory cytokine production and progression to MOF.42 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.44
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, have been the focus of interventions to modulate the inflammatory response to critical illness, as detailed later.
Traditionally, the injury stress response was viewed as a neuroendocrine reflex mediated via counterregulatory hormones (cortisol, glucagon, and epinephrine) that altered substrate metabolism while the body is in a state of repair. Adrenaline is released and suppresses insulin secretion but stimulates secretion of growth hormone and renin, proteolysis and glycogenolysis, enhancing hepatic-mediated gluconeogenesis.33 Glucagon is released by pancreatic islet cells that increase hepatic glucose production from substrate that arises from tissue catabolism. The liver produces acute phase reactants such as opsonins (CRP), protease inhibitors, hemostatic agents (fibrinogen), and transporters (transferrin).33
The hypothalamic–pituitary–adrenal (HPA) axis is activated during stress, stimulating the release of adrenal corticotropic hormone (ACTH) from the pituitary gland, which induces the release of cortisol from the adrenal cortex.47 The HPA axis and the immune response are intrinsically linked in a negative feedback loop in which activated immune cells produce specific cytokines that activate the HPA axis increasing cortisol release that in turn suppresses the immune response and further cytokine release.47 Cytokines also act directly on the adrenal cortex and on glucocorticoid receptors (GR), present in most cells. In addition, different types of stress (sepsis, trauma, elective surgery) seem to be associated with the release of distinct mediators that may inhibit or stimulate cortisol production by acting on the HPA axis, adrenal cortex, and/or GR.
The HPA response is pivotal for survival as adrenal insufficiency (AI) increases the mortality of critically ill or injured patients.47 In the Section “Interventions,” we will return to this important topic in the treatment and prevention of MOF.
Less clear is the role of sexual hormones. Choudhry et al. in Alabama have studied extensively the influence of gender in the response to trauma and hemorrhage (TH) in animal models.48 They suggested that immune and cardiac dysfunctions after TH are depressed in adult males and ovariectomized/aged females, while both are maintained in castrated males and in proestrus females. The female reproductive cycle seems to be an important variable in the regulation of lung injury after TH.49 One of their most recent studies showed that enhanced hepatic heme oxygenase (HO-1) in proestrus- and estradiol-pretreated ovariectomized females modulates inflammatory responses and protects liver following TH.49
In contrast with the animal studies, clinical investigations have shown controversial results regarding a protective effect of female gender. In a 2006 study by the Alabama group, female polytrauma victims younger than 50 years with an ISS >25 suffered significantly less MOF and sepsis and had lower plasma cytokines compared with age-matched males.50 This study however included a relatively small number of women (n = 37), of whom over half were older than 50 years of age; thus, a type II error in the postmenopausal women was possible. A more recent study by the “Inflammation and the Host Response to Injury Investigators” attempted to characterize the gender dimorphism after injury with specific reference to the reproductive age of the women (young, <48 years of age, vs. old, >52 years of age) in a cohort of 1,036 severely injured trauma patients.51 The independent protective effect of female gender on MOF rates remained significant in both premenopausal and postmenopausal women when compared with similarly aged men suggesting that factors other than sex hormones were responsible for the gender-based differences after injury.
Abdominal Compartment Syndrome
A hallmark of the postinjury inflammatory state is generalized capillary leak and associated tissue edema. Historically, peripheral edema was considered to be a minimally significant consequence of fluid resuscitation. This view has changed with the resurgent interest in intra-abdominal hypertension that has accompanied the recent widespread application of damage control procedures (DCP). Increased intra-abdominal pressures (IAP) are accompanied by a host of physiologic derangements that include high ventilator pressures, decreased cardiac output, and impaired renal function, a constellation of signs that are named 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).52
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).52 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.52
Both primary and secondary ACS can be predicted early. The independent predictors of primary ACS are the indicators of the damage control physiology (transfer to the operating room without further imaging, temperature <34°C, hemoglobin <8 g/dL, base deficit >8 mmol/L), whereas the secondary ACS predictors are markers of uncontrolled resuscitation (>7.5 L of crystalloids before ICU admission, no indication for lifesaving surgical intervention, relatively low urine output [≤150 mL/h] on ICU admission [considering the massive resuscitation]). Identification of these independent predictors led to earlier hemorrhage control in orthopedic trauma, abandoning crystalloid-based supranormal resuscitation goals, introduction of hemostatic resuscitation, and the early application of ICU resuscitation protocols.52 The current treatment of full-blown postinjury ACS is surgical decompression. Abdominal decompression may be performed in the ICU, especially in secondary ACS cases, whereas in primary ACS repeated hemorrhage control is usually required, preferably undertaken in the OR. After decompression, open abdomen management starts with the application of temporary abdominal closure followed by timely restoration of the abdominal wall.52
The goals of transfusion in the injured patient are to maintain adequate oxygen-carrying capacity. However, there is abundant, solid evidence that blood transfusions are a risk factor for the development of MOF independent of shock or injury severity.53,54 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. Early blood transfusion is indeed the most powerful independent risk factor for postinjury MOF.53 Reductions in blood transfusion in the resuscitation period correlate with improved outcome and less MOF.22 Blood products are immunoactive, contain proinflammatory cytokines and lipids, and have an early immunosuppressive effect predisposing the patient to CARS, infection, and late MOF.54
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 >3 weeks in the first 6 hours postinjury are 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.54 Leukodepletion does not remove the potential for blood to act as a second hit, as PRBCs contain proinflammatory cytokines, including IL-8 and IL-6. The mechanism was believed to be related to the generation of the proinflammatory agents PAF, IL-6, and IL-8 during storage. Later studies found that additional biologically active cytokines and lipid mediators (lysophosphatidylcholines) accumulate in stored blood and are capable of PMN priming.54 “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.54 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.54 Together, these effects can suppress the recipient immune system and promote a worse proinflammatory state leading to increased infection and complications.
Other blood-derived products (platelets, plasma, and coagulation factors) are also immunoactive and could act as second hits.17 Indeed, transfusion of fresh frozen plasma (FFP) is associated with MOF, especially among patients who received more than 6 U of PRBCs in the first 12 hours postinjury.55
Osler was the first to recognize in 1904 that “except on few occasions, the patient appears to die from the body’s response to infection rather than from the infection.”56 The association of infection and MOF was always strong. In the late 1970s, intra-abdominal abscess (IAA) was the inciting event in half of the cases.11 As a result of the appropriate use of presumptive antibiotics in patients sustaining abdominal trauma and prompt diagnosis of hollow viscus injury, the incidence of postinjury IAA decreased and its progression toward MOF was hindered. The epidemiology of postinjury infections changed and nosocomial pneumonia became the principal infection associated with MOF.
While CARS, the anti-inflammatory response, may be protective because it limits unnecessary (potentially autodestructive) inflammation, it is associated with relative immunosuppression predisposing the host to infections.57 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 to be key modulators that downregulate immune functions.57
Another area of research interest has been the potential role of persistent hypercatabolism in the development of infections.33 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 postinjury infection.
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 consumption, hypothermia, hypotension, tissue hypoxia, and intraoperative blood loss.33
The timing of the second operation has been variously studied over the last decade mostly as related to operative fracture fixation. While early definitive fracture fixation decreases postinjury 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.58 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 (DCO) group if the femoral fracture was initially stabilized with an external fixator and (2) intramedullary nailing (IMN) group if they underwent primary IMN.59 Despite more severe injuries in the DCO group, patients had a smaller, shorter postoperative SIRS and did not suffer significantly more pronounced organ failure than the IMN group. DCO patients undergoing conversion while their SIRS score was raised suffered the most pronounced subsequent inflammatory response and organ failure. According to these data, DCO 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 (DCO group) for 462 multiple injured patients with femoral shaft fractures.60 Although minimal differences were noted between DCO and ETC groups regarding systemic complications, DCO was a safer initial approach, significantly decreasing the initial operative exposure and blood loss. Collectively, these findings strongly suggest that a secondary operation can act as an additional inflammatory insult and amplify the postinjury inflammatory response and precipitate MOF.