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Describe and/or define sepsis/septic shock terms and symptomsS. S., a 65-year-old man who lived alone, was cutting vegetables and accidentally cut his finger. He casually washed the bleeding site, put a Band-Aid on the deep cut and continued cooking. Two days later, he removed the Band-Aid but the cut appeared open and oozing. Later that day, he replaced the Band-Aid with a fresh one. A few days later he noticed that his finger had become very red and swollen and he could see a small amount of pus at the site. He had also developed a low grade fever and malaise. The following day he went to see his physician who cleaned the cut and gave him a broad-spectrum antibiotic. However, S. S did not follow the dosing instructions and stopped taking the antibiotic when he felt better after 3 days. A few days later, he developed a high fever, chills, severe fatigue, tachycardia, and growing mental confusion. A relative visiting S. S called 911 and he was taken to the local emergency room. On admission, his temperature was 40 °C, his blood pressure was 88/60 mm Hg, his respiratory rate was 22, his pulse was 111. Blood chemistry revealed a BUN of 55 mg/dl and creatinine of 1.8 mg/dl. Blood cultures grew out Gram-negative rods that were identified as E. coli. He was diagnosed with severe septic shock as a result of inadequate treatment of a wound infection.
Laboratory values on admission:
| Table 1. Laboratory Data: |
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| HEMATOLOGY On admission | Normal values | |
| Hematocrit (%) | 26 | 41-50% |
| Hemoglobin (g/dl) | 7 | 13 - 18 gm/dL |
| White cells (per mm3) | 39,606 | 5-10,000 |
| Neutrophils (%) | 78 | 48-73 |
| Lymphocytes (%) | 8 | 18 - 48 % |
| Monocytes (%) | 3 | 0 - 9 % |
| Eosinophils (%) | 0 | 0 - 5 % |
| Basophils (%) | 0 | 0 - 2 % |
| Band forms (%) | 11 | <3% |
| Platelets (per mm3) | 16,000 | 150,000-450,000 |
| Mean Corpuscular Volume (fl) | 92 | 80 - 100 fl |
| BUN | 55 mg/dl | 7-30mg/dl |
| Serum creatinine |
1.8 mg/dl |
0.6-1.2mg/dl |
| Red Cell morphology | 1+schistocytes | none |
| Prothrombin time (sec) | 13.4 | 10-12.5 |
| Partial
Thromboplastin time (sec) |
33.2 | 20-36 |
| D - dimer (mu g/ml) | 3.0 | <0.25 |
| Fibrinogen (mg/dl) | 344 | 200-400 |
| FDP (mu g/ml) | 32 | <10 |
| Erythrocyte Sedimentation rate (mm/hr) | 60 | 0-15 millimeters per hour |
| PA CO2 (mm Hg) | 28 | 40 |
| PA O2 (mm Hg) | 68 | 80-100 |
| MICROBIOLOGY | e coli | |
| Blood culture | E. coli | e. coli |
| Urine culture | E. coli | e. coli |
| Urine output (ml/hr) | 18 | >30ml/hr |
S. S was transfused, given fluids, appropriate antibiotics, and PEEP assisted ventilation. It was considered that if he continued to deteriorate, with an increased risk of death as indicated by an acute physiology and chronic health evaluation score (in critically ill patients) APACHE II score greater than or equal to 25 or dysfunction of two or more organs, he would be given activated protein C (Drotrecogrin Alfa), recently reported to decrease mortality and to ameliorate organ dysfunction in patients with severe sepsis.
Septic shock results from a complex progression of disease that is described as severe infection and moves from a systemic inflammatory response syndrome (SIRS) to sepsis and severe sepsis to septic shock. Left untreated, septic shock can lead to death, with or without multiple organ dysfunction syndrome (MODS).
Diagnostic Assessment
A CBC and differential
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Tests for serum electrolyte levels and renal and hepatic function are recommended
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Coagulation status, as calculated by prothrombin time (PT), activated partial thromboplastin time (aPTT or PTT), fibrinogen, FDP and D-dimer can reflect the potential for disseminated intravascular clotting (DIC).
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Arterial blood gas (ABG) analysis measures the amount of oxygen, carbon dioxide, and acidity.
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Elevated serum lactate levels indicate the presence of hypoperfusion, with higher figures equating to greater degrees of shock and mortality rates.
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Urinalysis and culturing can be used to rule out the presence of UTIs.
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Gram staining may not only document bacterial infection, but also influence the type of initial antibiotic therapy chosen.
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Blood cultures
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Etiology Of Sepsis
Sepsis occurs when pathogenic microorganisms cross host barriers and overwhelm defenses. Gram-negative bacteria - such as Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Neisseria meningitides - are key pathogens. Other causes include Gram-positive organisms such as Staphylococcus aureus, coagulase-negative Staphylococcus, Streptococcus pneumoniae, Streptococcus pyogenes, and Enterococcus. Rickettsiae, viruses, fungi and polymicrobial sepsis have also been noted as causative agents. In neonates, sepsis is most likely caused by group B Streptococci, E. coli bearing a pathogenic K1 capsule, Klebsiella sp. and Enterobacter sp.
According to the American College of Chest Physicians (ACCP) and the Society of Critical Care Medicine, several physiological effects occur as a result of the systemic inflammatory response syndrome (SIRS). Two or more of the following criteria must be present for a SIRS diagnosis: Body temperature can increase above 38C or drop below 36C; individuals might experience tachycardia (over 90 bpm) or tachypnea (more than 20 breaths per minute or PaCO2 less than 32 mm Hg); and white blood cell counts may be greater than 12,000/mm3, less than 4000/mm3, or over 10 percent band cells. It is important to note that sepsis/septic shock can occur without the identification of bacteria in the blood.
As the disease progresses to severe sepsis, patients generally experience both earlier symptoms, along with organ dysfunction, hypoperfusion, or hypotension. Confusion or altered mental states, elevated plasma lactate levels, and oliguria (decreased urine output of less than 30 ml) are just some examples of the perfusion abnormalities observed.
| Table 1: Definitions of Sepsis-Related Terms (adapted from the Society for Critical Care Medicine) |
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| Sepsis | A systemic inflammatory response to infection |
| Severe sepsis | Sepsis with one or more dysfunctional organs or systems (e.g., cardiovascular dysfunction as indicated by hypotension and shock that is resistant to fluid resuscitation, respiratory dysfunction as indicated by hypoxemia, renal dysfunction as indicated by anuria or oliguria) |
| Systemic inflammatory response syndrome (SIRS) | A syndrome in which inflammatory mediator release, particularly cytokines such as TNF, secondary to an infectious and/or traumatic insult, causes the patient to present with at least two of the following conditions:§ alterations in body temperature (>38°C or <36°C)§ heart rate >90 beats/min§ alterations in respiratory function (rate >20 breaths/min or PaCO2 <32 mmHg) § alterations in WBC count (>12,000/mm3 or <4,000/mm3 or >10% immature forms) |
| Compensatory anti-inflammatory response syndrome (CARS) | A syndrome in which anti-inflammatory mediator release overcompensates for the systemic inflammatory response to an infectious and/or traumatic insult leading to a state of immune suppression, increased susceptibility of the critically ill patient to infection, and impaired recovery |
| Septic shock | Severe sepsis with hypotension that is resistant to fluid resuscitation and requires pharmacological intervention (vasopressors and/or inotropic agents) |
| Multiple organ dysfunction syndrome (MODS) | A syndrome in which the hypotension and hypoperfusion, secondary to the pathophysiological alterations in severe sepsis, result in dysfunction in more than one organ or system |
| Table 2. Clinical Definition of Sepsis/Septic Shock |
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TYPE |
CHARACTERISTICS |
| Moderate sepsis | Body Temperature >38°C or <36°C§ Heart rate >90 beats/min§ Respiratory rate 20 breaths/min or partial pressure of arterial CO2 <32 mm Hg§ White count >12,000/mm3, or >10% immature band forms§ Evidence of infection |
| Severe sepsis | Sepsis-associated lactic acidosis, oliguria, or acute alteration of mental status |
| Septic shock | Sepsis-induced hypotension (i.e., systolic blood pressure <90 mm Hg) despite adequate fluid resuscitation. Patients treated with vasopressors or inotropic medication may not be hypotensive at the time of measurement. |
From: Schrier and Wang, NEJM 2004, 35:159-169.
References:
Russell, J.A. Management of Sepsis. 2006. NEJM. 355:1699-1713.
Bone, R. C., C. L. Sprung, and W. J. Sibbald. 1992. Definitions for sepsis and organ failure. Crit Care Med
20:724-726.Members of the American College of Chest Physicians/Society of Critical Care Medicine Consensus
Conference Committee.1992. Definitions for sepsis and organ failure and guidelines for the use of
innovative therapies in sepsis. Crit Care Med 20: 864–874Levy, M. M., M. P. Fink, J. C. Marshall, E. Abraham, D. Angus, D. Cook, J. Cohen, S. M. Opal, J. L. Vincent, and G. Ramsay. 2003. 2001 SCCM/ESICM/ACCP/ATS/SIS International Sepsis Definitions Conference. Intensive Care Med 29:530-538.
Hotchkiss, R. S., and I. E. Karl. 2003. The pathophysiology and treatment of sepsis. N Engl J Med
348:138-150.Janeaway JA, Travers P, Walport M and M.J. Shlomchik (eds). Immunobiology, 6th ed. Garland Science, NY 2005.
Internet Links - Microbiology:
QUESTIONS
and ANSWERS:
QUESTIONS: The innate immune response to bacteria is initially driven by the interaction of bacteria and/or their products with innate immune cells. Severe sepsis/septic shock can result from an early, overactive innate immune response following bacterial infection resulting in overproduction of inflammatory mediators that lead to a cascade of events, potentially ending in death, often within only a few days.
1. What cells are the major components of the innate immune response and what is their primary function?
Answer 1. Monocytes/macrophages and neutrophils function primarily to rid the body of infectious agents.
2. Describe how endotoxin (LPS) from Gram-negative bacteria activates cells of the innate immune system?
Answer 2. LPS binds to factors in serum, such as LPS-binding protein (LBP); this complex then binds to CD14, expressed strongly on the surface of Monocytes/macrophages and weakly on neutrophils; the binding of LPS to CD14 causes movement of molecules on the cell surface and the formation of a receptor complex that includes other surface molecules such as TLR4 bound to a soluble protein MD-2. This complex interaction results in transduction of a signal via TLR4. What products are produced by this activation? Proinflammatory cytokines, including TNF, IL-1, IL-6, chemokines, lipid mediators, tissue factor, etc.
2a.) What products are produced by this activation?
Answer 2a). Proinflammatory cytokines, including TNF, IL-1, IL-6, chemokines, lipid mediators, tissue factor, etc.
2b.) Which of these products contribute to the cascade of events leading to further deterioration of the patient?
Answer 2b). The release of TNF stimulates the activation of other cells, including endothelial cells. The release of chemokines, especially IL-8, results in the recruitment (chemotaxis) of neutrophils to the site of infection. The release of tissue factor leads to disseminated intravascular clotting (DIC). The release of lipid mediators such as platelet activating factor (PAF) leads to platelet activation and chemotaxis.
3. What is chemotaxis?
Answer 3. The movement of cells along a chemical gradient, from low concentration to high concentration.
3a.) How do neutrophils get from the blood to sites of infection? What molecules are involved in transmigration (diapedesis) across the endothelium?
Answer 3a). The regulated migration of PMN from the blood to sites of infection is a complicated multi-step process that includes traversing the blood vessel endothelium to enter surrounding tissues at or near the site of infection. Migration is initiated by the release of cytokine and chemotactic factors by resident cells (tissue macrophages) in response to stimuli by components of microorganisms. These cytokines and chemokines in turn cause changes in the endothelium and in the neutrophils that enhance their ability to migrate. A chemokine gradient formed at the source of induction results in neutrophil migration from the blood to inflamed tissue, a process known as extravasation or diapedesis. The recognition of chemokines by neutrophils occurs via receptors that are integral membrane proteins containing seven membrane-spanning helices that belong to the family of G protein coupled receptors (GPCR).
Neutrophils are recruited to the site of infection in response to a chemokine gradient whereby the concentration of chemokine at the initial site of infection is higher than the systemic concentration. In the initial steps, neutrophils exhibit selectin mediated tethering (slow rolling) along the vessel wall. This rolling phase is promoted by vasodilation and by PMN expression of selectins and selectin ligands, including L-selectin, PSGL-1 (P-selectin ligand), ESL-1 (E-selectin ligand) and endothelial cell expression of E-selectin and P-selectin. Subsequently, when a slow rolling cell encounters a chemokine (such as IL-8) that binds to surface receptors (such as CXCR2), an intracellular signaling cascade is triggered that induces the functional upregulation of integrins. The chemokine-stimulated PMNs then firmly adhere to the endothelial layer and change in shape through the interaction of integrins on the neutrophils with adhesion molecules on the endothelial cell. Integrin adhesiveness is enhanced by chemokines produced by leukocytes (e.g., IL-8) as well as by endothelial and tissue cells. Chemokines accumulate on the endothelial cell layer through glycosaminoglycan binding and activate PMN through specific recognition of their corresponding GPCR. Firm adhesion of PMN is characterized by PMN expression of integrins, LFA-1 (CD11a/CD18), Mac-1 (CD11b/CD18) and endothelial cell expression of ICAM-1, 2 and VCAM-1. The last step is transmigration of the PMN through adjacent endothelial cells in a process called diapedesis. PMN migrate through the endothelial cell layer into the underlying tissue to the chemokine source.
Activation of neutrophils by chemokines provokes the secretion of proteases that can degrade the subendothelial extracellular matrix and facilitate their migration. Transmigration is characterized by PMN expression of integrins LFA-1, Mac-1, VLA-4 and endothelial cell expression of their ligands PECAM-1 (CD31), ICAM-1 (ligand for both LFA-1 and Mac-1); LFA-1, ICAM-2 and –3. These steps are summarized in Fig. 1 and Table 1.
Figure 1
Table 1: Adhesion molecules that play a role in emigration of neutrophils through the endothelium to site of infection.
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EFFECT ON EMIGRATION |
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Selectins |
GlyCAM-1, MAdCAM-1, CD34 (sialyl Lewis-X type structures) [*activ EC] |
L-selectin (CD62L) [†PMN] |
Selectins |
Rolling (light adhesion) |
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P-selectin (CD62P) [activ EC] |
PSGL-1 (CD162) [PMN] |
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E-selectin (CD62E) [activ EC] |
CD44 [PMN] |
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+IgSF |
ICAM-1 [activ EC] |
Mac-1 (CD11b/CD18, CR3 or aM/b2) [PMN] |
b2-Integrins |
Stopping (firm adhesion) and transmigration across EC |
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LFA-1 (CD11a/CD18 or aL/b2) [PMN] |
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ICAM-2 [activ EC] |
LFA-1 (also called CD11a/CD18 or aL/b2) [PMN] |
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VCAM-1 [activ EC] |
VLA-4 (also known as a4/b1 or CD49d/CD29) [PMN] |
b1-Integrins |
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IgSF |
PECAM-1 (CD31) (cell-cell junctions) [activ EC] |
PECAM-1 (CD31) Homotypic interaction [PMN] |
IgSF |
Transmigration across EC |
*Activ EC: present on activated endothelial cells
†PMN: present on activated neutrophils
+IgSF: immunoglobulin superfamily
4. Complement plays an important role in innate immunity. What are three major functions of complement in innate immunity and describe them.
Answer 4. Complement is a complex system of plasma proteins that functions in innate immunity to (1) act as chemotactic agent for neutrophils, (2) act as an opsonin to promote phagocytosis and (3) act directly in complement-mediated killing of pathogens by the assembly of the membrane attack complex (MAC), a complex that generates a pore in the bacterial membrane, resulting in bacterial killing.
The complement fragments C3a and C5a act as chemotactic factors for neutrophils.
| Opsonin/Ligand | Receptor | Cell type | Function |
| C3b, C4b, iC3b |
CR1 (CD35) |
Monocytes/macrophages, neutrophils |
Stimulates phagocytosis |
| iC3b | CR3 (CD11b/CD18) |
Monocytes/macrophages, neutrophils |
Stimulates phagocytosis |
| iC3b | CR4 (CD11c/CD18) |
Monocytes/macrophages, neutrophils |
Stimulates phagocytosis |
Briefly, for a MAC attack to occur, C5 convertase breaks down C5 into C5a and C5b. C5a diffuses away and has anaphylotoxin activity including vasodilation and promotes chemotaxis of neutrophils; C5b binds C6 and C7 to form a complex. The addition of C7 changes the conformation of the proteins so that they are hydrophobic and are able to insert into the outer membrane of the bacterium. C8 then binds to this complex and also inserts into the membrane. The complex of C5b678 then binds up to 10 to 16 C9 molecules to form a MAC. The MAC looks like a tube with a hollow center (a very short straw) or pore that allows ions, water and other small molecules to freely pass through it, resulting in an inability of the cell to maintain osmolality causing the bacterium to quickly die.
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5. What is phagocytosis? Describe in detail the events leading to killing of bacteria by
phagocytosis.
Answer
5. PMN and monocytes/macrophages are phagocytes that function in the killing of bacteria. Phagocytosis is greatly enhanced by opsonization of the bacteria with (1) complement components (C3b, C4b, iC3b) or (2) IgG. The binding of complement components to the surface of the bacterium promotes the binding of the bacterium to neutrophils via complement receptors including CR1 (CD35), CR3 (CD11b/CD18) and CR4 (CD11c/CD18). Bacteria opsonized with IgG bind to PMN via Fc receptors including FcγR1 (CD64) and FcγRIII (CD16)]. The bound pathogen is first surrounded by the phagocyte membrane and then internalized in a membrane-bounded vesicle known as a phagosome or endocytic vacuole. The phagosome becomes acidified (pH 6.0), killing some pathogens. The phagosome fuses to membrane-bound granules, called lysosomes, that contain enzymes, proteins, and peptides that can result in bacterial killing; the contents of the lysosome are released into the phagolysosome and the pathogen is destroyed.
Upon phagocytosis, neutrophils also produce a variety of other toxic products that help kill the bacterium, including nitric oxide (NO), the superoxide anion (O2–), and hydrogen peroxide (H2O2), products that are directly toxic to bacteria. A high-output form of nitric oxide synthase, iNOS2, produces nitric oxide.
Superoxide is generated by a multicomponent, membrane-associated NADPH oxidase in a process known as the respiratory burst. Neutrophils are short-lived cells, dying soon after they have accomplished a round of phagocytosis. Dead and dying neutrophils are a major component of the pus that forms in some infections; bacteria that give rise to such infections are thus known as pyogenic or pus-forming bacteria. Macrophages, in contrast, are long-lived and continue to generate new lysosomes; they will function in removal of the dead or dying
neutrophils.
6. What did S. S. do wrong?
Answer 6. He stopped taking his antibiotic too early.
