732
SCIENCE & PRACTICE
Septic shock: pathogenesis
Septic shock is a clinical syndrome that has become increasingly important in the last 40 years. The condition is most common in hospitals, particularly in patients with other underlying diseases. Although patients with diseases such as plague or typhoid fever caused by "classical" gram-negative pathogens may present with the clinical picture of septic shock, it is only with the increasing incidence since the 1950s of disease caused by gramnegative bacilli of the normal host flora that the sepsis/septic shock syndromes have been defined. We believe that the definitions of the sepsis/septic shock syndromes proposed recently by Bone (table 1)1 should be provisionally accepted the standard definitions. Septic shock has traditionally been recognised as a consequence of gram-negative bacteraemia, but it may also be caused by gram-positive organisms, fungi, and, probably, viruses and parasites. Table II summarises the organisms isolated and mortality in three recent studies of sepsis syndrome/septic shock. The incidence of gram-negative organisms varied between 30% and 80%, and that of gram-positive organisms between 6% and 24%. In one prospective study of the aetiology of the sepsis syndrome,2 the aetiologic agent was not determined in more than half of the patients. Worth noting is the fact that the severity of septic shock as reflected by mortality did not depend on the type of organisms responsible for the syndrome. Study of the pathophysiology of septic shock concentrated initially on the interactions of lipopolysaccharide (LPS) from the cell wall of gramnegative bacteria with various humoral pathways, but attention now focuses on the central role of macrophages and of the cytokines released upon stimulation by most if not all of the recognised agents of septic shock. Thus, this review as
TABLE I-DEFINITIONS OF SEPSIS SYNDROME AND OF SEPTIC SHOCK
will address the known humoral pathways that are activated during septic shock, and discuss the present state of knowledge on the role of cytokines, in particular tumour necrosis factor (TNF), in the pathogenesis of septic shock. We will also review the mechanisms by which LPS interacts with macrophages, with the purpose of evaluating the potential for anti-LPS and anticytokine agents in the therapy of septic shock.
Bacterial cell wall components and septic shock The exotoxins produced by some bacteria (such as exotoxin A produced by Pseudomonas aeruginosa, or the toxic shock syndrome toxin produced by some Staphylococcus aureus) can initiate septic shock, but it is the bacteria themselves, in particular their cell wall components, that are primarily responsible for the development of septic shock. These components are potent activators of numerous humoral pathways, and they also activate macrophages and other cell types involved in the inflammatory processes. The prime initiator of gram-negative bacterial septic shock is endotoxin, an LPS component of the bacterial outer membrane. Circulating endotoxin in the blood appears to be a predictor of poor outcome in some clinical settings such as meningococcaemia,sbut the levels of endotoxin required to trigger the cascade of events in septic shock may vary greatly. Indeed, it has been observed that bacterial products such as cell wall components, including endotoxin itself and staphylococcal and streptococcal toxins, may greatly increase host sensitivity to endotoxin, thus rendering toxic otherwise harmless levels of endotoxin.6 For these reasons measurement of endotoxin has not yet become standard clinical practice. The outermost part of the endotoxin molecule consists of a series of oligosaccharides (fig 1) that are structurally and antigenically diverse and are responsible for the 0 serotype of gram-negative bacteria. Internal to the 0 side chains are the core oligosaccharides, and these are structurally rather similar in common gram-negative bacteria. To the core oligosaccharide is bound a lipid part, lipid A. The structure of lipid A is highly conserved, and it is lipid A that is responsible for most of the toxicity of endotoxin. Some natural lipid As and synthetic lipid A analogues with different sugar and acyl residues are less or not at all endotoxically active in vitro and in vivo. This observation has led to development of lipid A analogues that can block ADDRESSES. Division of Infectious Diseases, Department of Medicine, Centre Hospitalier Universitaire Vaudois, 1011 Lausanne, Switzerland (Prof M. P. Glauser, MD, G. Zanetti, MD, J.-D. Baumgartner, MD); and Infectious Diseases Unit, Departments of Bacteriology and Medicine, Hammersmith Hospital and Royal Postgraduate Medical School, London, UK (J. Cohen, FRCP). Correspondence to Prof M. P. Glauser.
733
TABLE II-TYPE OF MICROORGANISM ISOLATED AND MORTALITY IN THREE STUDIES OF SEPSIS SYNDROME/SEPTIC SHOCK
the toxic effects of endotoxin
or
act
as
endotoxin
antagonists.7 Activation of humoral
pathways
The alternative complement pathway can be activated experimentally by LPS and gram-positive cell-wall components. The classical pathway is mainly activated by complexes of cell-wall components and antibodies. The anaphylatoxins C3a and C5a that result from activation of these pathways are reponsible for a series of inflammatory events that have been implicated in the pathophysiology of septic shock. These events include vasodilation and increased vascular permeability, which may be partly responsible for haemodynamic changes, platelet aggregation, and aggregation and activation of granulocytes, processes that have been implicated in the pathogenesis of the adult respiratory distress syndrome.8 An important effect of the stimulation of complement is activation of neutrophils (fig 2). Activated neutrophils become adherent to each other and to vascular endothelium. The subsequent release of arachidonic acid derivatives, cytotoxic products of molecular oxygen and lysosomal enzymes, produces additional local vasoactive effects on the microvasculature and endothelial cell cytotoxicity, resulting in capillary leakage. Increased concentrations of activated complement have been associated with fatal outcome in septic shock of gram-positive and gram-negative origin.9
Fig 1-Diagrammatic representation
It is well known that the derivatives of arachidonic acid metabolism that cause vasodilation, platelet aggregation, and neutrophil activation may contribute to the pathogenesis of septic shock. Such derivatives are found in increased concentrations after experimental endotoxin challenge and in humans with septic shock The role of inhibitors/antagonists of the pathways of arachidonic acid metabolism in the prevention and treatment of septic shock is under investigation. Activated neutrophils, a key element in the inflammatory response, probably play an important part in the pathogenesis of septic shock in that they contribute to vascular and tissue injuries. Activated leucocytes adhere to each other, to endothelial cells, and to tissues through interactions of receptors (on endothelial cells) and ligands (on inflammatory cells) that are mediated by specific adhesion molecules (fig 2). The adhesion process is essential for most functions of leucocytes, such as chemotaxis, phagocytosis, and cytotoxicity," and blocking of the adhesion process by monoclonal antibodies prevents tissue injury and improves survival in animal models of septic shock. Factor XII (Hageman factor) of the coagulation cascade has long been known to have a central role in the pathogenesis of septic shock. It is activated by peptidoglycan residues and teichoic acid from the cell wall of gram-positive
organisms (S
aureus,
of the structure of endotoxin
streptococci, pneumococci)
(LPS).
as
734
Fig 2-Proposed pathway for the interactions of humoral factors and cytokines in the pathogenesis of septic shock. as by LPS and lipid A from gram-negative Activated factor XII triggers both the intrinsic bacilli.12,13 coagulation pathway, through activation of factor XI, and endothelial cells and macrophages to produce tissue factor, which in turn activates the extrinsic coagulation pathway (fig 2). The activation of these pathways may lead to the consumption of coagulation factors and to disseminated intravascular coagulation (DIC). Tissue factor produced upon stimulation of macrophages and endothelial cells by LPS has been shown to have a major role in inducing DIC, since anti-tissue factor prevented LPS-induced DIC in rabbits.14 TNF is also an activator of the extrinsic pathway of coagulation, and therefore may contribute to the perturbations of coagulation in septic shock. 15 In addition to being induced by activation of complement and the arachidonic acid cascade, hypotension in septic shock may also result from LPS-activated factor XII that converts prekallikrein into kallikrein. Kallikrein in turn cleaves high-molecular-weight kininogen to release bradykinin, a potent hypotensive agent. 16 Hypotension also results from release of another potent vasodilator, endothelium-derived relaxing factor, recently identified as nitric oxideP Generation of nitric oxide occurs in macrophages and in cultured endothelial cells. While it appears that LPS-induced nitric oxide release by macrophages takes several hours, endothelial cells react within minutes, a phenomenon that might contribute to the rapid fall in blood pressure associated with endotoxic shock. 18 Endogenous opiod peptides may play a part in septic shock, because opioid peptide secretion can be induced by endotoxin, and the administration of an opioid antagonist, naloxone, reverses endotoxin-induced hypotension under some experimental conditions. However, the importance of endorphins in the pathophysiology of shock is still incompletely understood.19
efficiently
The
cytokine network
Monocytic cells probably have a pivotal role in mediation of the biological effects of LPS (fig 2). First, they can remove
and detoxify LPS from the blood, thus having a beneficial effect. Second, LPS-stimulated monocytes produce cytokines such as TNF and interleukin 1 (IL-1). Several binding sites for LPS on the cell surface of macrophage have been described .20-24 LPS can also interact with the monocytic cell membrane after binding to plasma molecules. An acute-phase protein called LPS-binding protein (LBP) has been shown to bind to the lipid A moiety of LPS.25 LPS-LBP complexes are a ligand for the CD14 receptors on monocytes and macrophages.24 LPS when complexed with LBP can stimulate production of TNF by macrophages at concentrations far below those required for stimulation by LPS alone.24,25 This suggests that recognition of the presence of LPS is important for an efficient response to infection with gram-negative bacteria. Thus, a principal function of LBP may be to enhance the ability of the host to detect LPS early in infection. The intravascular activation of inflammatory systems involved in septic shock is mainly the consequence of a dysregulation in the production of various cytokines. As well as being produced by macrophages, several cytokines are also produced by lymphocytes, endothelial cells, and other cells triggered by microbial products. The systemic release of large amounts of various cytokines is associated with fatal outcome in human septic shock .26-28 One of these cytokines, TNF, is now regarded as a central mediator of the pathophysiological changes associated with LPS release and, probably, also with shock due to microorganisms that do not contain LPS. In animal models, anti-TNF antibodies given prophylactically before bolus intravenous injections of LPS or gram-negative bacteria, or given therapeutically, have been effective in preventing mortality. However, although these promising results have prompted development of strategies to counteract overproduction or release of TNF and hence prevent or treat septic shock, several important pathophysiological considerations should be kept in mind. First, TNF and other cytokines are released into the bloodstream during the first hour after bolus injection of LPS or live bacteria and disappear thereafter from the
735
circulation.29 A similar pattern has been found in children with fulminant meningococcaemia.26 If similar patterns were to be found during most cases of septic shock in humans, anti-TNF antibodies would be less likely to be effective when administered late in the course of shock. However, bolus intravenous challenge models and the fulminant course of meningococcal septicaemia might not reflect most clinical situations in which release of LPS or bacteria from an infectious focus could be more sustained and subacute. Indeed, when serum concentrations of TNF and other cytokines were measured prospectively in 70 patients with septic shock, it was found that TNF and IL-11 could be detected up to 10 days after the onset of shock in non-survivors.28 These results indicate that concentrations of TNF and other cytokines are sustained in patients presenting with gram-negative septic shock. Moreover, models of severe subacute gram-negative infection have shown a pattern of TNF release different from that after bolus inoculation, and anti-TNF antibodies failed to prevent death in these models. 30,31 Thus, the release of TNF in most clinical cases of septic shock is probably different from that in fulminant gram-negative infections, and new therapies should be devised accordingly. Second, cytokines other than TNF are involved in inducing a shock-like state in animals. For example, TNF and IL-1 act synergistically in inducing shock in mice and rabbits, and y-interferon is a central mediator of the Shwartzman reaction in mice. In one clinical study, outcome was not only related to the concentration of TNF, but also to that of IL-1 and y-interferon.27 Direct proof of the central role of IL-1 in septic shock comes from animal experiments in which specific blocking of the binding of IL-1 to its cell-surface receptor, by means of monoclonal antibodies or IL-1 receptor antagonist, prevented the detrimental effects of LPS or Escherichia coli inoculation. 32,33 Thus, several cytokines are probably involved in the pathogenesis of septic shock, so that blocking TNF alone might not be sufficient to reverse the condition, and cocktails might be necessary. Third, when considering the use of anti-TNF therapies, the role of TNF in defence against infection must be taken into account. TNF and the other cytokines participate in defence against infection and are mediators that increase natural resistance by, for example, up-regulation of the cytolytic activity of lymphocytes and of the complement receptors and oxidative burst of polymorphonuclear leucocytes, activation of macrophages, and stimulation of proliferation of B, T, and progenitor cells. Thus, blocking cytokines in patients with septic shock with the purpose of counteracting "harmful" cytokine concentrations, may well interfere with the "physiological" cytokine concentrations that are needed for proper control of infection. This may result in a worsening of the very infections that were responsible for the development of septic shock.
experimental
Conclusions Several approaches to the treatment and prevention of septic shock are now being considered that aim to suppress and/or inhibit one or other of several pathways and cytokines, but it should be remembered that the syndrome most probably results from complex interactions between all these pathways and cytokines. Thus, more precise delineation is required of the roles of each mechanism contributing to the pathogenesis of septic shock. This will help to identify the subsets of patients that might benefit
from administration of anticytokine antibodies, and the need for other cytokine inhibitors or anti-inflammatory agents.
REFERENCES
Sepsis, the sepsis syndrome, multi-organ failure: a plea for comparable definitions. Ann Intern Med 1991; 114: 332-33. 2. Bone RC, Fisher CJ, Clemmer TP, Slotman GJ, Metz GA, Balk RA. A controlled clinical trial of high-dose methylprednisolone in the treatment of severe sepsis and septic shock. N Engl J Med 1987; 317: 1. Bone RC.
653-58.
Ispahani P, Pearson NJ, Greenwood D. An analysis of community and hospital-acquired bacteremia in a large teaching hospital in the United Kingdom. QJ Med 1987; 241: 427-40. 4. Calandra T, Glauser MP, Schellekens J, Verhoef J, the Swiss-Dutch J5 Immunoglobulin Study Group. Treatment of gram-negative septic shock with human IgG antibody to Escherichia coli J5: a prospective, double-blind, randomized study. J Infect Dis 1988; 158: 312-19. 5. Brandtzaeg P, Kierulf P, Gaustad P, et al. Plasma endotoxin is a predictor of multiple organ failure and death in systemic meningococcal disease. J Infect Dis 1989; 159: 195-204. 6. Galanos C, Freudenberg MA, Matsuura M. Mechanisms of the lethal action of endotoxin and endotoxin hypersensitivity. In: Friedman H, Klein TW, Nokano M, Nowotny A, eds. Endotoxin, advances in experimental medicine and biology. New York: Plenum Press, 1990: 3.
603-619. 7. Stütz P, Liehl E.
Lipid A analogs aimed at preventing the detrimental effects of endotoxin. In: Glauser MP, Young LS, eds. Infectious disease clinics of North America. Philadelphia: W. B. Saunders (in
press). Jacobs HS. The role of activated complement and granulocytes in shock states and myocardial infarction. J Lab Clin Med 1981; 98: 645-54. 9. Hack CE, Nuijens JH, Felt-Bersma RJF, et al. Elevated plasma levels of the anaphylatoxins C3a and C4a are associated with a fatal-outcome in sepsis. Am J Med 1989; 86: 20-26. 10. Reines HD, Halushka PV, Cook JA, Wise WC, Rambo W. Plasma thromboxane concentrations are raised in patients dying with septic shock. Lancet 1982; ii: 174-75. 11. Springer TA. Adhesion receptors of the immune system. Nature 1990; 8.
346: 425-34. 12. Kalter ES, van Dijk WC, Timmerman A, Verhoef J, Bouma BN. Activation of purified human plasma prekallikrein triggered by cell wall fractions of Escherichia coli and Staphylococcus aureus. J Infect Dis 1983; 148: 682-91. 13. Spika JS, Peterson PK, Wilkinson BJ, et al. Role of peptidoglycan from Staphylococcus aureus in leukopenia, thrombocytopenia, and complement activation associated with bacteremia. J Infect Dis 1982; 146: 227-34. 14. Warr TA, Mohan Rao LV, Rapaport SI. Disseminated intravascular coagulation in rabbits induced by administration of endotoxin or tissue factor: effect of anti-tissue factor antibodies and measurement of plasma extrinsic pathway inhibitor activity. Blood 1990; 75: 1481-89. 15. van der Poll T, Büler HR, ten Cate H. Activation of coagulation after administration of tumour necrosis factor to normal subjects. N Engl J Med 1990; 322: 1622-27. 16. Colman RW. The role of plasma proteases in septic shock. N Engl J Med 1989; 320: 1207-09. 17. Palmer RMJ, Ferrige AG, Moncada S. Nitric oxide release accounts for the biological activity of endothelium-derived relaxing factor. Nature
1987; 327: 524-526. 18. Vane JR, Angaard EE, Botting RM. Regulatory functions of the vascular endothelium. N Engl J Med 1990; 323: 27-36. 19. Hackshaw KV, Parker GA, Roberts JW. Naloxone in septic shock. Crit Care Med 1990; 18: 47-51. 20. Hampton RY, Golenbock DT, Raetz CRH. Lipid A binding sites in membranes of macrophage tumor cells. J Biol Chem 1988; 263: 14802-07. 21. Lei MG, Morrison DC. Specific endotoxic lipopolysaccharide-binding proteins on murine splenocytes. I. Detection of lipopolysaccharide binding sites on splenocytes and splenocyte subpopulation. J Immunol 1988; 141: 996-1005. 22. Wright SD, Detmers PA, Aida Y, et al. CD18-deficient cells respond to lipopolysaccharide in vitro. J Immunol 1990; 144: 2566-71. 23. Hara-Kuge S, Amano F, Nishijima M, Akamatsu Y. Isolation of a lipopolysaccharide (LPS)-resistant mutant, with defective LPS binding, of cultured macrophage-like cells. J Biol Chem 1990; 265: 6606-10. 24. Wright SD, Ramos RA, Tobias PS, Ulevitch RJ, Mathison JC. CD 14, a receptor for complexes of lipopolysaccharide (LPS) and LPS binding protein. Science 1990; 249: 1431-33.
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25. Schumann
RR, Leong SR, Flaggs GW, et al. Structure and function of lipopolysaccharide binding protein. Science 1990; 249: 1429-31. 26. Waage A, Halstensen A, Espevik T. Association between tumor necrosis factor in serum and fatal outcome in patients with meningococcal disease. Lancet
1987; i: 355-57. 27. Girardin E, Grau G, Dayer J, Roux-Lombard P, J5 Study Group, Lambert PH. Tumour necrosis factor and interleukin-1 in serum of children with severe infectious purpura. N Engl J Med 1988; 319: 397-400. 28. Calandra T, Baumgartner JD, Grau GE, et al. Prognosis values of tumor necrosis factor/cachectin, interleukin-1, alpha-interferon and gammainterferon in the serum of patients with septic shock. J Infect Dis 1990; 161: 982-87. 29. Michie HR, Manogue KR, Spriggs DR, et al. Detection of circulating tumor necrosis factor after endotoxin administration. N Engl J Med 1988; 318: 1481-86.
30. Bagby GJ, Plessala KJ, Wilson LA. Thompson KJ, Nelson S. Divergent efficacy of antibody to tumor necrosis factor-&agr; in intravascular and peritonitis models of sepsis. J Infect Dis 1990; 163: 83-88.
B, Falk W, Männel DN, Krammer PH. Requirement of endogenous tumor necrosis factor/cachectin for recovery from experimental peritonitis. J Immunol 1990; 145: 3762-66. 32. McIntyre KW, Stepan GJ, Kolinsky KD, et al. Inhibition of interleukin 1 (IL-1) binding and bioactivity in vitro and modulation of acute inflammation in vivo by IL-1 receptor antagonist and anti-IL-1 receptor monoclonal antibody. J Exp Med 1991; 173: 931-39. 33. Wakabayashi G, Gelfand JA, Burke JF, Thompson RC, Dinarello CA. A specific receptor antagonist for interleukin 1 prevents Escherichia coli-induced shock in rabbits. FASEB J 1991; 31. Echtenacher
5: 338-43.
Septic shock: treatment
The initial approach to management of the patient in septic shock is institution of corrective measures that are designed, firstly, to confirm and characterise the condition and to correct rapidly any potentially reversible factors, and, secondly, to begin specific therapy of the underlying cause. A detailed discussion of these measures is beyond the scope of this article, but several general principles emerge. Firstly, it is essential to ensure adequate oxygenation and to establish suitable means to monitor the haemodynamic status, which ideally will include measurement of right atrial and pulmonary capillary wedge pressure as well as intra-arterial pressure. Modem management practice in shock emphasises that the administration of fluid, inotropes, and vasopressors must be tailored to the needs of each patient and will usually need frequent modification as the condition evolves. Metabolic abnormalities such as hypoxaemia or severe acidosis need to be identified and remedied. Sometimes these abnormalities are not immediately apparent; for example, hypocalcaemia is a common complication of rhabdomyolysis and may be the cause of refractory hypotension. Finally, every effort must be made to identify the source of sepsis in order to drain abscesses and choose the most appropriate empirical antimicrobial therapy. Although it will seldom be possible (or perhaps even desirable) to use an antibiotic directed against just one bacterial species, outcome is improved by choosing a regimen that proves to be active against the infecting
organism.2 Despite the improvement in outcome of patients with shock and multi-organ failure made by applying the above principles, mortality in patients with severe, established shock remains 50% to 75%. In the USA, septic shock is estimated to cause 100 000 deaths annually.3 This high mortality has stimulated considerable interest in improving outcome by applying insights into the basic mechanisms of the disease. The possibility of manipulating the host factors that seem to mediate tissue damage has received particular attention. Here we review clinical experience with these
why many doctors felt strongly that high-dose steroids given early in shock were beneficial.4 Although there were experimental data that tended to confirm this impression, clinical fmdings were confused. Most early trials were flawed in design, but in the last 10 years three major studies have overcome many of the early difficulties. Sprung et als compared the effects of methylprednisolone (30 mg/kg), dexamethasone (6 mg/kg, one or two doses), and no steroids in patients with severe established shock. Steroids delayed death but did not reduce overall mortality; in addition, patients given dexamethasone had an increased incidence of bacterial superinfections. Subsequently, two further studies have evaluated the effect of early intervention with high-dose methylprednisolone in septic shock.6,7Both studies were placebo-controlled, and both were careful to control for the many variables that can influence outcome. Importantly, both trials insisted that patients only be enrolled if they could be treated within 2 h of shock being recognised. Results of the two trials were strikingly similar: neither found any evidence of benefit from steroids, and steroid recipients had significantly more secondary bacterial infections. Taken together, these studies provide no support for the routine use of high-dose steroids in septic shock. The experience of Hoffman et al8 was different. They reported a double-blind, placebo-controlled trial of dexamethasone in severe typhoid fever. Patients who were in shock or who had an abnormal level of consciousness received dexamethasone 3 mg/kg, followed by eight doses of 1 mg/kg over the next 48 h. There were 2 deaths in the 20 patients given dexamethasone compared with 10 deaths in the 38 placebo recipients (p=0003). The favourable outcome in this small study contrasts with the opposite result reported in the two large trials noted above.6,7However, it is of interest that there appears to be a less clearcut association between cytokine levels and outcome in typhoid fever compared with other types of gram-negative sepsis9 (Richens, Exley, and Cohen, unpublished observations). to understand
types of treatments. ADDRESSES:
High-dose steroids Corticosteroids are anti-inflammatory and antipyretic. They induce a feeling of well-being in the patient which can give a subjective sense of improvement. Furthermore, they have profound effects on many of the mediator systems implicated in the pathogenesis of shock, and it is not difficult
Infectious Diseases Unit, Departments of Bacteriology and Medicine, Hammersmith Hospital and Royal Postgraduate Medical School, London, UK (J. Cohen, FRCP); and Division of Infectious Diseases, Department of Medicine, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland (Prof M. P Glauser, MD) Correspondence to Dr J Cohen, Infectious Diseases Unit, Department of Bacteriology, Royal Postgraduate Medical School, Du Cane Road, London W12 ONN, UK
SCIENCE & PRACTICE
Septic shock: pathogenesis
Septic shock is a clinical syndrome that has become increasingly important in the last 40 years. The condition is most common in hospitals, particularly in patients with other underlying diseases. Although patients with diseases such as plague or typhoid fever caused by "classical" gram-negative pathogens may present with the clinical picture of septic shock, it is only with the increasing incidence since the 1950s of disease caused by gramnegative bacilli of the normal host flora that the sepsis/septic shock syndromes have been defined. We believe that the definitions of the sepsis/septic shock syndromes proposed recently by Bone (table 1)1 should be provisionally accepted the standard definitions. Septic shock has traditionally been recognised as a consequence of gram-negative bacteraemia, but it may also be caused by gram-positive organisms, fungi, and, probably, viruses and parasites. Table II summarises the organisms isolated and mortality in three recent studies of sepsis syndrome/septic shock. The incidence of gram-negative organisms varied between 30% and 80%, and that of gram-positive organisms between 6% and 24%. In one prospective study of the aetiology of the sepsis syndrome,2 the aetiologic agent was not determined in more than half of the patients. Worth noting is the fact that the severity of septic shock as reflected by mortality did not depend on the type of organisms responsible for the syndrome. Study of the pathophysiology of septic shock concentrated initially on the interactions of lipopolysaccharide (LPS) from the cell wall of gramnegative bacteria with various humoral pathways, but attention now focuses on the central role of macrophages and of the cytokines released upon stimulation by most if not all of the recognised agents of septic shock. Thus, this review as
TABLE I-DEFINITIONS OF SEPSIS SYNDROME AND OF SEPTIC SHOCK
will address the known humoral pathways that are activated during septic shock, and discuss the present state of knowledge on the role of cytokines, in particular tumour necrosis factor (TNF), in the pathogenesis of septic shock. We will also review the mechanisms by which LPS interacts with macrophages, with the purpose of evaluating the potential for anti-LPS and anticytokine agents in the therapy of septic shock.
Bacterial cell wall components and septic shock The exotoxins produced by some bacteria (such as exotoxin A produced by Pseudomonas aeruginosa, or the toxic shock syndrome toxin produced by some Staphylococcus aureus) can initiate septic shock, but it is the bacteria themselves, in particular their cell wall components, that are primarily responsible for the development of septic shock. These components are potent activators of numerous humoral pathways, and they also activate macrophages and other cell types involved in the inflammatory processes. The prime initiator of gram-negative bacterial septic shock is endotoxin, an LPS component of the bacterial outer membrane. Circulating endotoxin in the blood appears to be a predictor of poor outcome in some clinical settings such as meningococcaemia,sbut the levels of endotoxin required to trigger the cascade of events in septic shock may vary greatly. Indeed, it has been observed that bacterial products such as cell wall components, including endotoxin itself and staphylococcal and streptococcal toxins, may greatly increase host sensitivity to endotoxin, thus rendering toxic otherwise harmless levels of endotoxin.6 For these reasons measurement of endotoxin has not yet become standard clinical practice. The outermost part of the endotoxin molecule consists of a series of oligosaccharides (fig 1) that are structurally and antigenically diverse and are responsible for the 0 serotype of gram-negative bacteria. Internal to the 0 side chains are the core oligosaccharides, and these are structurally rather similar in common gram-negative bacteria. To the core oligosaccharide is bound a lipid part, lipid A. The structure of lipid A is highly conserved, and it is lipid A that is responsible for most of the toxicity of endotoxin. Some natural lipid As and synthetic lipid A analogues with different sugar and acyl residues are less or not at all endotoxically active in vitro and in vivo. This observation has led to development of lipid A analogues that can block ADDRESSES. Division of Infectious Diseases, Department of Medicine, Centre Hospitalier Universitaire Vaudois, 1011 Lausanne, Switzerland (Prof M. P. Glauser, MD, G. Zanetti, MD, J.-D. Baumgartner, MD); and Infectious Diseases Unit, Departments of Bacteriology and Medicine, Hammersmith Hospital and Royal Postgraduate Medical School, London, UK (J. Cohen, FRCP). Correspondence to Prof M. P. Glauser.
733
TABLE II-TYPE OF MICROORGANISM ISOLATED AND MORTALITY IN THREE STUDIES OF SEPSIS SYNDROME/SEPTIC SHOCK
the toxic effects of endotoxin
or
act
as
endotoxin
antagonists.7 Activation of humoral
pathways
The alternative complement pathway can be activated experimentally by LPS and gram-positive cell-wall components. The classical pathway is mainly activated by complexes of cell-wall components and antibodies. The anaphylatoxins C3a and C5a that result from activation of these pathways are reponsible for a series of inflammatory events that have been implicated in the pathophysiology of septic shock. These events include vasodilation and increased vascular permeability, which may be partly responsible for haemodynamic changes, platelet aggregation, and aggregation and activation of granulocytes, processes that have been implicated in the pathogenesis of the adult respiratory distress syndrome.8 An important effect of the stimulation of complement is activation of neutrophils (fig 2). Activated neutrophils become adherent to each other and to vascular endothelium. The subsequent release of arachidonic acid derivatives, cytotoxic products of molecular oxygen and lysosomal enzymes, produces additional local vasoactive effects on the microvasculature and endothelial cell cytotoxicity, resulting in capillary leakage. Increased concentrations of activated complement have been associated with fatal outcome in septic shock of gram-positive and gram-negative origin.9
Fig 1-Diagrammatic representation
It is well known that the derivatives of arachidonic acid metabolism that cause vasodilation, platelet aggregation, and neutrophil activation may contribute to the pathogenesis of septic shock. Such derivatives are found in increased concentrations after experimental endotoxin challenge and in humans with septic shock The role of inhibitors/antagonists of the pathways of arachidonic acid metabolism in the prevention and treatment of septic shock is under investigation. Activated neutrophils, a key element in the inflammatory response, probably play an important part in the pathogenesis of septic shock in that they contribute to vascular and tissue injuries. Activated leucocytes adhere to each other, to endothelial cells, and to tissues through interactions of receptors (on endothelial cells) and ligands (on inflammatory cells) that are mediated by specific adhesion molecules (fig 2). The adhesion process is essential for most functions of leucocytes, such as chemotaxis, phagocytosis, and cytotoxicity," and blocking of the adhesion process by monoclonal antibodies prevents tissue injury and improves survival in animal models of septic shock. Factor XII (Hageman factor) of the coagulation cascade has long been known to have a central role in the pathogenesis of septic shock. It is activated by peptidoglycan residues and teichoic acid from the cell wall of gram-positive
organisms (S
aureus,
of the structure of endotoxin
streptococci, pneumococci)
(LPS).
as
734
Fig 2-Proposed pathway for the interactions of humoral factors and cytokines in the pathogenesis of septic shock. as by LPS and lipid A from gram-negative Activated factor XII triggers both the intrinsic bacilli.12,13 coagulation pathway, through activation of factor XI, and endothelial cells and macrophages to produce tissue factor, which in turn activates the extrinsic coagulation pathway (fig 2). The activation of these pathways may lead to the consumption of coagulation factors and to disseminated intravascular coagulation (DIC). Tissue factor produced upon stimulation of macrophages and endothelial cells by LPS has been shown to have a major role in inducing DIC, since anti-tissue factor prevented LPS-induced DIC in rabbits.14 TNF is also an activator of the extrinsic pathway of coagulation, and therefore may contribute to the perturbations of coagulation in septic shock. 15 In addition to being induced by activation of complement and the arachidonic acid cascade, hypotension in septic shock may also result from LPS-activated factor XII that converts prekallikrein into kallikrein. Kallikrein in turn cleaves high-molecular-weight kininogen to release bradykinin, a potent hypotensive agent. 16 Hypotension also results from release of another potent vasodilator, endothelium-derived relaxing factor, recently identified as nitric oxideP Generation of nitric oxide occurs in macrophages and in cultured endothelial cells. While it appears that LPS-induced nitric oxide release by macrophages takes several hours, endothelial cells react within minutes, a phenomenon that might contribute to the rapid fall in blood pressure associated with endotoxic shock. 18 Endogenous opiod peptides may play a part in septic shock, because opioid peptide secretion can be induced by endotoxin, and the administration of an opioid antagonist, naloxone, reverses endotoxin-induced hypotension under some experimental conditions. However, the importance of endorphins in the pathophysiology of shock is still incompletely understood.19
efficiently
The
cytokine network
Monocytic cells probably have a pivotal role in mediation of the biological effects of LPS (fig 2). First, they can remove
and detoxify LPS from the blood, thus having a beneficial effect. Second, LPS-stimulated monocytes produce cytokines such as TNF and interleukin 1 (IL-1). Several binding sites for LPS on the cell surface of macrophage have been described .20-24 LPS can also interact with the monocytic cell membrane after binding to plasma molecules. An acute-phase protein called LPS-binding protein (LBP) has been shown to bind to the lipid A moiety of LPS.25 LPS-LBP complexes are a ligand for the CD14 receptors on monocytes and macrophages.24 LPS when complexed with LBP can stimulate production of TNF by macrophages at concentrations far below those required for stimulation by LPS alone.24,25 This suggests that recognition of the presence of LPS is important for an efficient response to infection with gram-negative bacteria. Thus, a principal function of LBP may be to enhance the ability of the host to detect LPS early in infection. The intravascular activation of inflammatory systems involved in septic shock is mainly the consequence of a dysregulation in the production of various cytokines. As well as being produced by macrophages, several cytokines are also produced by lymphocytes, endothelial cells, and other cells triggered by microbial products. The systemic release of large amounts of various cytokines is associated with fatal outcome in human septic shock .26-28 One of these cytokines, TNF, is now regarded as a central mediator of the pathophysiological changes associated with LPS release and, probably, also with shock due to microorganisms that do not contain LPS. In animal models, anti-TNF antibodies given prophylactically before bolus intravenous injections of LPS or gram-negative bacteria, or given therapeutically, have been effective in preventing mortality. However, although these promising results have prompted development of strategies to counteract overproduction or release of TNF and hence prevent or treat septic shock, several important pathophysiological considerations should be kept in mind. First, TNF and other cytokines are released into the bloodstream during the first hour after bolus injection of LPS or live bacteria and disappear thereafter from the
735
circulation.29 A similar pattern has been found in children with fulminant meningococcaemia.26 If similar patterns were to be found during most cases of septic shock in humans, anti-TNF antibodies would be less likely to be effective when administered late in the course of shock. However, bolus intravenous challenge models and the fulminant course of meningococcal septicaemia might not reflect most clinical situations in which release of LPS or bacteria from an infectious focus could be more sustained and subacute. Indeed, when serum concentrations of TNF and other cytokines were measured prospectively in 70 patients with septic shock, it was found that TNF and IL-11 could be detected up to 10 days after the onset of shock in non-survivors.28 These results indicate that concentrations of TNF and other cytokines are sustained in patients presenting with gram-negative septic shock. Moreover, models of severe subacute gram-negative infection have shown a pattern of TNF release different from that after bolus inoculation, and anti-TNF antibodies failed to prevent death in these models. 30,31 Thus, the release of TNF in most clinical cases of septic shock is probably different from that in fulminant gram-negative infections, and new therapies should be devised accordingly. Second, cytokines other than TNF are involved in inducing a shock-like state in animals. For example, TNF and IL-1 act synergistically in inducing shock in mice and rabbits, and y-interferon is a central mediator of the Shwartzman reaction in mice. In one clinical study, outcome was not only related to the concentration of TNF, but also to that of IL-1 and y-interferon.27 Direct proof of the central role of IL-1 in septic shock comes from animal experiments in which specific blocking of the binding of IL-1 to its cell-surface receptor, by means of monoclonal antibodies or IL-1 receptor antagonist, prevented the detrimental effects of LPS or Escherichia coli inoculation. 32,33 Thus, several cytokines are probably involved in the pathogenesis of septic shock, so that blocking TNF alone might not be sufficient to reverse the condition, and cocktails might be necessary. Third, when considering the use of anti-TNF therapies, the role of TNF in defence against infection must be taken into account. TNF and the other cytokines participate in defence against infection and are mediators that increase natural resistance by, for example, up-regulation of the cytolytic activity of lymphocytes and of the complement receptors and oxidative burst of polymorphonuclear leucocytes, activation of macrophages, and stimulation of proliferation of B, T, and progenitor cells. Thus, blocking cytokines in patients with septic shock with the purpose of counteracting "harmful" cytokine concentrations, may well interfere with the "physiological" cytokine concentrations that are needed for proper control of infection. This may result in a worsening of the very infections that were responsible for the development of septic shock.
experimental
Conclusions Several approaches to the treatment and prevention of septic shock are now being considered that aim to suppress and/or inhibit one or other of several pathways and cytokines, but it should be remembered that the syndrome most probably results from complex interactions between all these pathways and cytokines. Thus, more precise delineation is required of the roles of each mechanism contributing to the pathogenesis of septic shock. This will help to identify the subsets of patients that might benefit
from administration of anticytokine antibodies, and the need for other cytokine inhibitors or anti-inflammatory agents.
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Septic shock: treatment
The initial approach to management of the patient in septic shock is institution of corrective measures that are designed, firstly, to confirm and characterise the condition and to correct rapidly any potentially reversible factors, and, secondly, to begin specific therapy of the underlying cause. A detailed discussion of these measures is beyond the scope of this article, but several general principles emerge. Firstly, it is essential to ensure adequate oxygenation and to establish suitable means to monitor the haemodynamic status, which ideally will include measurement of right atrial and pulmonary capillary wedge pressure as well as intra-arterial pressure. Modem management practice in shock emphasises that the administration of fluid, inotropes, and vasopressors must be tailored to the needs of each patient and will usually need frequent modification as the condition evolves. Metabolic abnormalities such as hypoxaemia or severe acidosis need to be identified and remedied. Sometimes these abnormalities are not immediately apparent; for example, hypocalcaemia is a common complication of rhabdomyolysis and may be the cause of refractory hypotension. Finally, every effort must be made to identify the source of sepsis in order to drain abscesses and choose the most appropriate empirical antimicrobial therapy. Although it will seldom be possible (or perhaps even desirable) to use an antibiotic directed against just one bacterial species, outcome is improved by choosing a regimen that proves to be active against the infecting
organism.2 Despite the improvement in outcome of patients with shock and multi-organ failure made by applying the above principles, mortality in patients with severe, established shock remains 50% to 75%. In the USA, septic shock is estimated to cause 100 000 deaths annually.3 This high mortality has stimulated considerable interest in improving outcome by applying insights into the basic mechanisms of the disease. The possibility of manipulating the host factors that seem to mediate tissue damage has received particular attention. Here we review clinical experience with these
why many doctors felt strongly that high-dose steroids given early in shock were beneficial.4 Although there were experimental data that tended to confirm this impression, clinical fmdings were confused. Most early trials were flawed in design, but in the last 10 years three major studies have overcome many of the early difficulties. Sprung et als compared the effects of methylprednisolone (30 mg/kg), dexamethasone (6 mg/kg, one or two doses), and no steroids in patients with severe established shock. Steroids delayed death but did not reduce overall mortality; in addition, patients given dexamethasone had an increased incidence of bacterial superinfections. Subsequently, two further studies have evaluated the effect of early intervention with high-dose methylprednisolone in septic shock.6,7Both studies were placebo-controlled, and both were careful to control for the many variables that can influence outcome. Importantly, both trials insisted that patients only be enrolled if they could be treated within 2 h of shock being recognised. Results of the two trials were strikingly similar: neither found any evidence of benefit from steroids, and steroid recipients had significantly more secondary bacterial infections. Taken together, these studies provide no support for the routine use of high-dose steroids in septic shock. The experience of Hoffman et al8 was different. They reported a double-blind, placebo-controlled trial of dexamethasone in severe typhoid fever. Patients who were in shock or who had an abnormal level of consciousness received dexamethasone 3 mg/kg, followed by eight doses of 1 mg/kg over the next 48 h. There were 2 deaths in the 20 patients given dexamethasone compared with 10 deaths in the 38 placebo recipients (p=0003). The favourable outcome in this small study contrasts with the opposite result reported in the two large trials noted above.6,7However, it is of interest that there appears to be a less clearcut association between cytokine levels and outcome in typhoid fever compared with other types of gram-negative sepsis9 (Richens, Exley, and Cohen, unpublished observations). to understand
types of treatments. ADDRESSES:
High-dose steroids Corticosteroids are anti-inflammatory and antipyretic. They induce a feeling of well-being in the patient which can give a subjective sense of improvement. Furthermore, they have profound effects on many of the mediator systems implicated in the pathogenesis of shock, and it is not difficult
Infectious Diseases Unit, Departments of Bacteriology and Medicine, Hammersmith Hospital and Royal Postgraduate Medical School, London, UK (J. Cohen, FRCP); and Division of Infectious Diseases, Department of Medicine, Centre Hospitalier Universitaire Vaudois, Lausanne, Switzerland (Prof M. P Glauser, MD) Correspondence to Dr J Cohen, Infectious Diseases Unit, Department of Bacteriology, Royal Postgraduate Medical School, Du Cane Road, London W12 ONN, UK
