Keywords
coagulation - inflammation - sepsis - tissue factor - antithrombin - protein C - fibrinolysis
Sepsis is a clinical syndrome that is caused by an infection, often associated with
bacteremia and characterized by the presence of systemic signs and symptoms of inflammation.[1] When sepsis leads to organ failure, the term severe sepsis is used. The incidence of sepsis is estimated to be approximately 2.5 per 1,000 in
the Western world and shows a rapid 8.7% annual increase over the past 20 years.[2] Total in-hospital mortality of sepsis is around 20%, whereas severe sepsis is associated
with mortality rates of 40 to 50%.[3] Treatment of sepsis is focused on adequate antibiotic therapy, source control, and
appropriate supportive care and organ function replacement, if required.
Virtually all patients with sepsis have coagulation abnormalities. These abnormalities
range from subtle activation of coagulation that can only be detected by sensitive
markers for coagulation factor activation to somewhat stronger coagulation activation
that may be detectable by a small decrease in platelet count and subclinical prolongation
of global clotting times to fulminant disseminated intravascular coagulation (DIC),
characterized by simultaneous widespread microvascular thrombosis and profuse bleeding
from various sites.[4]
[5] Septic patients with severe forms of DIC may present with manifest thromboembolic
disease or clinically less apparent microvascular fibrin deposition that predominantly
presents as multiple organ dysfunction.[5]
[6] Alternatively, severe bleeding may be the leading symptom,[7] but quite often a patient with DIC has simultaneous thrombosis and bleeding. Bleeding
is caused by consumption and subsequent exhaustion of coagulation proteins and platelets,
because of the ongoing activation of the coagulation system.[8] In its most severe form this combination may present as the Waterhouse-Friderichsen
syndrome, commonly seen during fulminant meningococcal septicemia, although many other
microorganisms may cause this clinical state.[9]
Incidence of Coagulation Abnormalities in Sepsis
Incidence of Coagulation Abnormalities in Sepsis
Clinically relevant coagulation abnormalities may occur in 50 to 70% of patients with
sepsis, whereas approximately 35% of patients will meet the criteria for DIC (see
further).[1]
[10] In general, the incidence of thrombocytopenia (platelet count <150 × 109/L) in critically ill medical patients is 35 to 50%.[11]
[12] Typically, the platelet count decreases during the first 4 days on the intensive
care unit.[13] Sepsis is a clear risk factor for thrombocytopenia in critically ill patients and
the severity of sepsis correlates with the decrease in platelet count.[14] Main factors that contribute to thrombocytopenia in patients with sepsis are impaired
platelet production, increased consumption or destruction, or sequestration in the
spleen or at the endothelial level. Impaired production of platelets in the bone marrow
may seem contradictory to the high levels of platelet production-stimulating proinflammatory
cytokines, such as tumor necrosis factor (TNF)-α and interleukin (IL)-6, and high
concentration of circulating thrombopoietin in patients with sepsis, which theoretically
should stimulate megakaryopoiesis in the bone marrow.[15] However, in a substantial number of patients with sepsis marked hemophagocytosis
may occur, consisting of active phagocytosis of megakaryocytes and other hematopoietic
cells by monocytes and macrophages, hypothetically because of stimulation with high
levels of macrophage colony stimulating factor (M-CSF) in sepsis.[16] Platelet consumption probably also plays an important role in patients with sepsis,
because of ongoing generation of thrombin. Platelet activation, consumption, and destruction
may also occur at the endothelial site as a result of the extensive endothelial cell-platelet
interaction in sepsis, which may vary between different vascular beds in various organs.[17] A prolonged global coagulation time (such as the prothrombin time [PT] or the activated
partial thromboplastin time [aPTT]) occurs in 14 to 28% of patients.[18] Other coagulation test abnormalities include high fibrin split products (in 99%
of patients with sepsis)[19]
[20] and low levels of coagulation inhibitors, such as antithrombin and protein C (90%
of sepsis patients).[20]
[21]
Pathogenetic Pathways in the Coagulopathy of Sepsis
Pathogenetic Pathways in the Coagulopathy of Sepsis
In recent years the mechanisms involved in the pathological derangement of coagulation
in patients with sepsis have become increasingly clear. Apparently, various mechanisms
at different sites in the hemostatic balance act simultaneously toward a procoagulant
state. It has become clear that the most important mediators that orchestrate this
imbalance of the coagulation system during sepsis are cytokines.[22] Increasing evidence points to extensive cross-talk between these two systems, whereby
inflammation leads not only to activation of coagulation, but coagulation also considerably
affects inflammatory activity.[23] Interestingly, systemic activation of coagulation and inflammation in sepsis can
have some organ-specific manifestations that are relevant for the specific organ dysfunction
as a consequence of severe sepsis.[24]
The principal initiator of thrombin generation in sepsis is tissue factor. The evidence
that points to a pivotal role of the tissue factor/factor VIIa system in the initiation
of thrombin generation comes from studies of human endotoxemia or cytokinemia, which
did not show any change in markers for activation of the contact system.[25] Furthermore, abrogation of the tissue factor/factor VII(a) pathway by monoclonal
antibodies specifically directed against tissue factor or factor VIIa activity resulted
in a complete inhibition of thrombin generation in endotoxin-challenged chimpanzees
and prevented the occurrence of DIC and mortality in baboons, that were infused with
Escherichia coli.[26]
[27] However, other than in severe meningococcemia,[28] it has proved difficult to demonstrate ex vivo tissue factor expression on monocytes
of septic patients or experimental animals systemically exposed to microorganisms.
It has been shown, however, that low-dose endotoxemia in healthy subjects results
in an 125-fold increase in tissue factor mRNA levels in blood monocytes.[29] Another source of tissue factor may be its localization on polymorphonuclear cells,[30] although it is unlikely that these cells actually synthesize tissue factor in substantial
quantities.[31] Based on the observation of transfer of tissue factor from leukocytes to activated
platelets on a collagen surface in an ex vivo perfusion system, it is hypothesized
that this “blood borne” tissue factor is transferred between cells through microparticles
derived from activated mononuclear cells.[32]
Platelets play a pivotal role in the pathogenesis of coagulation abnormalities in
sepsis. Platelets can be activated directly, for example by proinflammatory mediators,
such as platelet activating factor.[33] Once thrombin is formed, this will activate additional platelets. Activation of
platelets may also accelerate fibrin formation by another mechanism. The expression
of P-selectin on the platelet membrane not only mediates the adherence of platelets
to leukocytes and endothelial cells but also enhances the expression of tissue factor
on monocytes.[34] The molecular mechanism of this effect relies on nuclear factor kappa-B (NFκB) activation,
induced by binding of activated platelets to neutrophils and mononuclear cells. P-selectin
can be relatively easily shed from the surface of the platelet membrane and soluble
P-selectin levels have been shown to be increased during systemic inflammation.[34]
In general, activation of coagulation is regulated by three major anticoagulant pathways:
antithrombin, the protein C system, and tissue factor pathway inhibitor (TFPI). During
sepsis-induced activation of coagulation, the function of all three pathways can be
impaired. Experimental models indicate that at the time of maximal activation of coagulation
in sepsis, the fibrinolytic system is largely shutoff. The acute fibrinolytic response
to inflammation is the release of plasminogen activators, in particular tissue-type
plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA), from
storage sites in vascular endothelial cells. However, this increase in plasminogen
activation and subsequent plasmin generation is counteracted by a delayed but sustained
increase in plasminogen activator inhibitor type 1 (PAI-1).[35] Of interest, studies have shown that a functional mutation in the PAI-1 gene, the 4G/5G polymorphism, not only influenced the plasma levels of PAI-1, but
it was also linked to clinical outcome of meningococcal septicemia. Patients with
the 4G/4G genotype had significantly higher PAI-1 concentrations in plasma and an
increased risk of death.[36] Further investigations demonstrated that the PAI-1 polymorphism did not influence
the risk of contracting meningitis as such, but probably increased the likelihood
of developing septic shock from meningococcal infection.[37]
Inflammation and the Coagulopathy of Sepsis
Inflammation and the Coagulopathy of Sepsis
Similar to almost all systemic inflammatory responses to infection, the derangement
of coagulation and fibrinolysis in sepsis is mediated by several cytokines. Most proinflammatory
cytokines have been shown to activate coagulation in vitro. In patients with sepsis,
high levels of cytokines are detectable in the circulation and experimental bacteremia
or endotoxemia results in the transient enhancement of serum levels of these cytokines.[25] Consecutively, tumor necrosis factor becomes first detectable, followed by an increase
in circulating levels of interleukin-6 (IL-6) and interleukin 1 (IL-1). Several experimental
and clinical studies have focused on the roles of these cytokines in the pathogenesis
of DIC.
Because TNF is the first cytokine to appear in the circulation after infusion of bacteria
or endotoxin and exerts potent procoagulant effects in vitro, it was initially thought
that activation of coagulation was mediated by TNF. However, in studies using various
strategies to block TNF activity, it became clear that the endotoxin-induced increase
in TNF could be completely abolished whereas activation of coagulation was unchanged,
although the effects on anticoagulant pathways and fibrinolysis seemed to be driven
by TNF.[25] Also, in baboons infused with a lethal dose of E. coli, treatment with an anti-TNF antibody had little or no effect on fibrinogen consumption.[38] Moreover, clinical studies in septic patients with an anti-TNF monoclonal antibody
did not show a beneficial effect of this treatment.[39] In subsequent studies the role of IL-6 was investigated. It could be shown that
infusion of a monoclonal anti–IL-6 antibody resulted in the complete abrogation of
endotoxin-induced activation of coagulation in chimpanzees.[40] In addition, studies in cancer patients receiving recombinant IL-6 indicated that
indeed thrombin is generated following the injection of this cytokine.[41] Thus, these data suggest that IL-6 rather than TNF is relevant as a mediator for
the induction of the procoagulant response in DIC. Though IL-1 is a potent agonist
of tissue factor expression in vitro, its role has not been clarified in vivo. Administration
of a IL-1 receptor antagonist partly blocked the procoagulant response in a sepsis
model in baboons and treatment of patients with an IL-1 receptor inhibitor-reduced
thrombin generation.[42] However, most procoagulant changes after an endotoxin challenge occur well before
IL-1 becomes detectable in the circulation, leaving a potential direct role of IL-1
in coagulation activation in sepsis an unresolved issue.
Coagulation proteases and protease inhibitors not only interact with coagulation protein
zymogens, but also with specific cell receptors to induce signaling pathways. In particular,
protease interactions that affect inflammatory processes may be important in sepsis.
The most important mechanisms by which coagulation proteases influence inflammation
is by binding to so-called protease-activated receptors (PARs), of which four types
(PAR 1–4) have been identified—all belonging to the family of transmembrane domain,
G-protein coupled receptors.[43] A peculiar feature of PARs (in contrast to most other receptors of the superfamily)
is that they serve as their own ligand. Proteolytic cleavage by an activated coagulation
factor leads to exposure of a neo-amino terminus that activates the same receptor
(and possibly adjacent receptors), initiating transmembrane signaling. PARs 1, 3,
and 4 are thrombin receptors whereas PAR 2 cannot bind thrombin but can be activated
by the tissue factor-factor VIIa complex, factor Xa, and trypsin. PAR 1 can also serve
as receptor of the tissue factor-factor VIIa complex and factor Xa.
There is also considerable cross-talk between physiological anticoagulant pathways
and inflammatory mediators. Antithrombin can act as a mediator of inflammation, for
example, by direct binding to neutrophils and other leucocytes and thereby attenuating
cytokine and chemokine receptor expression.[44] In addition, there is mounting evidence that the protein C system also has an important
function in modulating inflammation.[45] Indeed, activated protein C has been found to inhibit endotoxin-induced production
of TNF-α, IL-1β, IL-6, and IL-8 by cultured monocytes/macrophages.[46] Further, activated protein C abrogates endotoxin-induced cytokine release and leukocyte
activation in rats in vivo.[47] Blocking the protein C pathway by a monoclonal antibody in septic baboons exacerbates
the inflammatory response, as evidenced by increased levels of proinflammatory cytokines
and more leukocyte infiltration and tissue destruction at histological analysis.[48] Mice with a one-allele targeted disruption of the protein C gene (resulting in heterozygous
protein C deficiency) not only have a more severe coagulation response to endotoxin
but also demonstrate significant differences in inflammatory responses, as shown by
higher levels of circulating proinflammatory cytokines.[49]
Diagnostic Approach to the Coagulopathy in Sepsis
Diagnostic Approach to the Coagulopathy in Sepsis
It is important to realize that apart from DIC, there are several other reasons for
coagulation abnormalities in patients with sepsis ([Table 1]). Although thrombocytopenia is common in patients with severe sepsis, this may also
be caused by other (sometimes simultaneously occurring) diseases, such as immune thrombocytopenia,
medication-induced bone marrow depression, heparin-induced thrombocytopenia, or thrombotic
microangiopathies.[50] It is very important to properly diagnose these causes of thrombocytopenia, as they
may require distinctive treatment strategies.[17] Laboratory tests can be helpful in differentiating the coagulopathy in sepsis from
various other hemostatic disorders, such as vitamin K deficiency or liver failure.
Because such conditions, however, may also occur simultaneously with for example DIC,
this differentiation is not always simple.[51]
[52]
Table 1
Causes of coagulation abnormalities in critically ill patients
|
Thrombocytopenia
|
|
Sepsis
|
|
DIC
|
|
Massive blood loss
|
|
Thrombotic microangiopathy
|
|
Heparin-induced thrombocytopenia
|
|
Immune thrombocytopenia
|
|
Drug-induced thrombocytopenia
|
|
Abnormal global coagulation times[a]
|
|
Coagulation factor deficiency
|
|
• Synthesis: liver failure
|
|
• Loss: massive bleeding
|
|
• Consumption: DIC
|
|
Vitamin K deficiency
|
|
Use of vitamin K antagonists
|
|
Use of unfractionated heparin
|
|
inhibiting antibody and/or antiphospholipid antibody
|
Abbreviation: DIC, disseminated intravascular coagulation.
a Prothrombin time and/or activated partial thromboplastin time.
According to the current understanding of sepsis-associated coagulation abnormalities,
the determination of soluble fibrin in plasma appears to be crucial.[53]
[54] In general, the sensitivity of these assays for severe coagulation activation or
DIC is relatively higher than the specificity. Indeed, initial clinical studies indicate
that if the concentration of soluble fibrin has increased above a defined threshold,
a diagnosis of DIC can be made.[19]
[55] Most of the clinical studies show a sensitivity of 90 to 100% for the diagnosis
of DIC but a rather low specificity.[56] Fibrin degradation products (FDPs) may be detected by specific ELISA tests or by
latex agglutination assays, allowing rapid and bedside determination in emergency
cases.[57] None of the available assays for FDPs discriminates between degradation products
of cross-linked fibrin and fibrinogen degradation, which may cause spuriously high
results.[58] The specificity of high levels of FDPs is therefore limited, and many other conditions,
such as trauma, recent surgery, inflammation, or venous thromboembolism, are associated
with elevated FDPs. Recently developed tests are specifically aimed at the detection
of neoantigens on degraded cross-linked fibrin. One of such tests detects an epitope
related to plasmin-degraded cross-linked γ-chain, resulting in fragment D-dimer. These tests better differentiate degradation of cross-linked fibrin from fibrinogen
or fibrinogen degradation products.[59]
Consumption of coagulation factors leads to low levels of coagulation factors in patients
with sepsis. In addition, impaired synthesis, for example, due to impaired liver function
or a vitamin K deficiency, and loss of coagulation proteins due to massive bleeding,
may play a role as well.[60] Although the accuracy of the measurement of one-stage clotting assays in DIC has
been contested (due to the presence of activated coagulation factors in plasma), the
level of coagulation factors appears to correlate well with the severity of DIC.[60] Measurement of fibrinogen has been widely advocated as a useful tool for the diagnosis
of DIC but in fact is not very helpful to diagnose DIC in most cases.[8]
[61] Fibrinogen acts as an acute-phase reactant, and despite ongoing consumption, plasma
levels can remain well within the normal range for a long time. In a consecutive series
of patients the sensitivity of a low fibrinogen level for the diagnosis of DIC was
only 28% and hypofibrinogenemia was detected in very severe cases of DIC only. Sequential
measurements of fibrinogen might be more useful and provide diagnostic clues.
Thromboelastography (TEG) is a method that has been developed decades ago and provides
an overall picture of ex vivo coagulation. Modern techniques, such as rotational thromboelastography
(ROTEG), enable bedside performance of this test and have again become popular recently
in acute care settings.[62] The theoretical advantage of TEG over conventional coagulation assays is that it
provides an idea of platelet function as well as fibrinolytic activity. Hyper- and
hypocoagulability as demonstrated with TEG was shown to correlate with clinically
relevant morbidity and mortality in several studies,[63]
[64] although its superiority over conventional tests has not unequivocally been established.[65] Also, TEG seems to be overly sensitive to some interventions in the coagulation
system, such as administration of fibrinogen, of which the therapeutic benefit remains
to be established. There are no systematic studies on the diagnostic accuracy of TEG
for the diagnosis of DIC; however, the test may be useful for assessing the global
status of the coagulation system in critically ill patients.
For the diagnosis of the most extreme form of coagulation activation in critically
ill patients, DIC, a simple scoring system has been developed.[66] The score can be calculated based on routinely available laboratory tests, that
is, platelet count, prothrombin time, a fibrin-related marker (usually D-dimer), and fibrinogen. Tentatively, a score of 5 or more is compatible with DIC,
whereas a score of less than 5 may be indicative but is not affirmative for nonovert DIC. By using receiver-operating characteristics curves,
an optimal cutoff for a quantitative D-dimer assay was determined, thereby optimizing sensitivity and the negative predictive
value of the system.[56] Prospective studies show that the sensitivity of the DIC score is 93%, and the specificity
is 98%.[67]
[68] The severity of DIC according to this scoring system is related to the mortality
in patients with sepsis.[69] Linking prognostic determinants from critical care measurement scores such as Acute
Physiology and Chronic Health Evaluation (APACHE-II) to DIC scores is an important
means to assess prognosis in critically ill patients. Similar scoring systems have
been developed in Japan.[70]
Supportive Treatment of Coagulation Abnormalities in Sepsis
Supportive Treatment of Coagulation Abnormalities in Sepsis
The keystone of the treatment of hemostatic abnormalities in patients with sepsis
is the specific treatment of the sepsis by appropriate antibiotics and control of
the infectious source. However, in many cases additional supportive treatment, aimed
at circulatory and respiratory support and replacement of organ function, is required.
Coagulation abnormalities may proceed, even after proper treatment has been initiated.
In those cases, supportive measures to manage the coagulation disorder may be considered,
and they may positively affect morbidity and mortality. The increase in the insight
into the various mechanisms that play a role in the coagulation abnormalities associated
with sepsis has indeed been accommodating in the development of such supportive management
strategies.
Low levels of platelets and coagulation factors may increase the risk of bleeding.
However, plasma or platelet substitution therapy should not be instituted on the basis
of laboratory results alone; it is indicated only in patients with active bleeding
and in those requiring an invasive procedure or otherwise at risk for bleeding complications.[71] The presumed efficacy of treatment with plasma, fibrinogen, cryoprecipitate, or
platelets is not based on randomized controlled trials but appears to be rational
therapy in bleeding patients or in patients at risk for bleeding with a significant
depletion of these hemostatic factors.[72] It may be necessary to use large volumes of plasma to correct the coagulation defect.
Coagulation factor concentrates, such as prothrombin complex concentrate, may overcome
this obstacle, but these compounds may lack essential factors, such as factor V. Moreover,
in older literature caution is advocated with the use of prothrombin complex concentrates
in DIC, as it may worsen the coagulopathy due to small traces of activated factors
in the concentrate. It is, however, not clear whether this is still relevant for the
concentrates that are currently in use. Specific deficiencies in coagulation factors,
such as fibrinogen, may be corrected by administration of purified coagulation factor
concentrates.
Experimental studies have shown that heparin can at least partly inhibit the activation
of coagulation in sepsis.[73] Uncontrolled case series in patients with sepsis and DIC have claimed to be successful.
However, a beneficial effect of heparin on clinically important outcome events in
patients with DIC has never been demonstrated in controlled clinical trials.[74] Also, the safety of heparin treatment is debatable in DIC patients who are prone
to bleeding. Therapeutic doses of heparin are indicated in patients with clinically
overt thromboembolism or extensive fibrin deposition, like purpura fulminans or acral
ischemia. Patients with sepsis may benefit from prophylaxis to prevent venous thromboembolism,
which may not be achieved with standard low-dose subcutaneous heparin.[75]
In view of the deficient state of physiological anticoagulant pathways in patients
with sepsis, restoration of these inhibitors may be a rational approach.[76] Because activated protein C and antithrombin are the most important physiologic
inhibitors of coagulation and based on successful preclinical results, the use of
concentrates of these factors in patients with sepsis has been studied relatively
intensively. Most of the randomized controlled trials concern patients with sepsis,
septic shock, or both. All trials showing some beneficial effect in terms of improvement
of laboratory parameters, shortening of the duration of DIC, or even improvement in
organ function, however, failed to reduce mortality.
