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- Materials and Methods
Background: There is considerable variation in the coagulation profile of dogs with disseminated intravascular coagulation (DIC), making it difficult to assess overall hemostatic function.
Objectives: To characterize the overall hemostatic state in dogs with DIC, by use of tissue factor-activated thromboelastography (TF-TEG), and to determine whether there is an association between hemostasis and outcome.
Animals: 50 dogs with DIC.
Methods: Dogs admitted to the intensive care units, with an underlying disease known to predispose to DIC, were prospectively assessed with TF-TEG. Citrated blood samples were collected daily during hospitalization and an extended coagulation panel and TF-TEG were performed. Diagnosis of DIC was based on expert opinion.
Results: Hemostatic dysfunction was observed on the TF-TEG profile in 33/50 of the dogs, of which 22/50 were hypercoagulable and 11/50 were hypocoagulable based on the TF-TEG G value alone. There were significant differences in k, α, and MA values (P < .0001) among hypo-, normo-, and hypercoagulable dogs. There was a significant difference in case fatality rate between hypo- (64%) and hypercoagulable (32%) dogs (relative risk = 2.38; P= .04). Dogs that died had significantly lower antithrombin activity (P= .03) and higher d-dimer concentration (P= .03) than survivors.
Conclusions: The most common overall hemostatic abnormality in dogs diagnosed with DIC was hypercoagulability, and there was significant difference in survival between hyper- and hypocoagulable dogs. The results suggest TF-TEG is valuable in the assessment of hemostatic function in dogs diagnosed with DIC.
Disseminated intravascular coagulation (DIC) is a complicated and dynamic hemostatic disorder characterized by variable imbalances of the components of the hemostatic system. DIC always occurs secondary to an underlying disease that, among others, causes an uncontrolled systemic inflammatory response.1,2 In the initial stage of DIC, the patient is thought to be hypercoagulable because of circulating inflammatory mediators, which cause activation of hemostasis through increased exposure of tissue factor (TF) as well as inhibitor consumption.2 Consumption of coagulation factors and increased fibrinolytic activity, if not compensated, can lead to a hypocoagulable state with overt clinical symptoms of bleeding.3 Owing to the progressive nature of DIC, the clinical signs vary considerably and range from no signs of DIC (nonovert DIC), accompanied by no or mild changes in traditional hemostasis parameters (activated partial thromboplastin time [aPTT], prothrombin time [PT], d-dimer, fibrinogen, and platelet count), to signs of organ failure, associated with microvascular thrombosis in vital organs, finally culminating in overt bleeding symptoms (overt DIC).1,3 The diverse clinical characteristics of DIC make initial diagnosis and optimization of treatment very challenging.
The traditional theoretical and diagnostic approach to the hemostatic system divides it into primary hemostasis (vascular tone and the platelet plug), secondary hemostasis (coagulation), fibrinolysis (breakdown of the clot), and the presence of endogenous anticoagulants, which limit clot formation to the site of injury. Accordingly, assessment of a dog's hemostatic capabilities in DIC is traditionally performed with tests of primary hemostasis, including platelet count and platelet function tests, and secondary hemostasis through plasma-based assays designed to further localize defects such as the aPTT (intrinsic pathway) and PT (extrinsic pathway). The fibrinolytic system is traditionally evaluated with measurements of degradation products, including fibrinogen degradation products (FDPs) and d-dimer. Endogenous antithrombotic ability has been evaluated through measurement of antithrombin activity (AT) and concentrations of protein C (PC) and protein S (PS).4 Specialized individual coagulation factor tests can be performed to further localize the defect. All of these tests of the secondary and fibrinolytic systems are performed on citrated plasma samples and target very specific elements in the hemostatic system and thus potentially discount important cellular factors. Although this approach makes it possible to diagnose DIC effectively and systematically, it can be difficult from a clinical perspective to piece together a picture of a dog's overall hemostatic capability and to predict or monitor the effect of treatment with anticoagulant or procoagulant medication with this traditional approach, especially if the dog is suspected of being hypercoagulable.
The recent discovery of the cell-based, TF/factor VII-dependent model of hemostasis has increased our understanding of the complex biochemistry of physiologic hemostasis and has forced a re-evaluation of the traditional view of the intrinsic and extrinsic pathways of coagulation.5,6 Concurrently, the important role of activated platelets in amplification of thrombin generation in the cell-based hemostasis model has been recognized.7–9 In inflammation, cytokine-induced platelet activation and TF expression on cytokine-activated mononuclear cells lead to the systemic activation of coagulation.10–13 Although citrated plasma contains many of the factors involved in coagulation, whole blood contains both the soluble factors and intravascular cells active in physiologic and pathologic hemostasis, incorporating TF and phospholipid-bearing cells such as platelets and leukocytes. Although thromboelastography (TEG) is not a new method, its potential use in assessing hemostatic disorders has resurfaced after the assay was automated and new activators were introduced, allowing for rapid and global assessment of hemostatic function in whole blood in dogs.14 More specifically, TEG evaluates all the steps in hemostasis, including initiation, amplification, and propagation as well as fibrinolysis, including the interaction of platelets and leukocytes with the proteins of the coagulation cascade. Thus, TEG combines evaluation of the traditional plasma components of coagulation with the cellular components.15 Recently, a TF-activated TEG assay on citrated whole blood has been validated in dogs, and the assay has been shown to have a low analytical variation compared with many of the traditional plasma-based coagulation assays in normal healthy dogs.14,16 TEG has been used to evaluate hypercoagulability in dogs with parvoviral infection and to evaluate platelet dysfunction in dogs with hypothermia.17,18 TEG has also been cited in a few abstracts, but the total amount of published material on dogs is sparse.a,b,c,d TEG is increasingly used to monitor hemostatic function in humans after cardiac and hepatic surgery and to optimize blood-product selection and usage, but the role of TEG also includes platelet-mapping assays as well as diagnosis and treatment of both hypo- and hypercoagulable states.19–23
With DIC, the major challenge for the clinician is to make the diagnosis in the early, hypercoagulable, and nonovert stage of the disease. There is general consensus that early intervention in humans with DIC increases the patient's chances of survival. Although TEG cannot eliminate the need for plasma-based coagulation analyses in the initial diagnosis of DIC in dogs, it can be used to detect hypo-, normo-, and hypercoagulability, potentially allowing for more distinct stratification of dogs with DIC before treatment. Therefore, the objective of the present study was to characterize the overall hemostatic state in dogs presenting with DIC irrespective of clinical signs of bleeding, using the newly validated TEG assay, with the overall aims of (1) differentiating among hypo-, normo-, and hypercoagulable subgroups of DIC with TF-TEG and (2) examining whether there is an association between subgroup of DIC and outcome.
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- Materials and Methods
This study demonstrates that with TF-activated TEG, it is possible to distinguish between different stages of DIC in dogs and that the most common overall hemostatic abnormality in dogs with DIC is hypercoagulability. There was a 2.4 × higher RR of 28-day mortality in dogs that were hypocoagulable on TEG compared with those that were hypercoagulable, demonstrating the potential use of the assay as a prognostic indicator. The low AT level and high d-dimer in 28-day nonsurvivors compared with 28-day survivors suggest severe thromboembolic disease in this group of dogs; however, these markers were not able to demonstrate prognostic significance. This finding is in accordance with the findings of several previous studies.27–30
Laboratory testing in the management of any clinical condition is relevant only if it can be used to indicate and guide the appropriate institution of therapeutic measures or monitor the status of a disease state. Although useful in its indication of worsening in the primary condition or raising awareness of otherwise unsuspected disease, there has been no noticeable breakthrough as to how recognition of hemostatic dysfunction in dogs with DIC could affect outcome. As such, meaningful progress in DIC testing in dogs over the years has largely faltered, because there has been no universally proven diagnostic tests or therapy for DIC. A plausible explanation for the lacking progress in diagnosis and treatment of DIC is that the laboratory diagnosis of DIC in dogs is not standardized and therefore the hemostatic function tests used are not consistent. In veterinary medicine, DIC is often diagnosed based on three or more abnormal hemostatic parameters such as aPTT, PT, fibrinogen, d-dimer, platelet count, and red blood cell morphology, along with predisposing disease, which is a sensitive but unspecific approach. Currently, there is neither consensus nor an obvious golden standard for the diagnosis of DIC in dogs. In an attempt to increase both sensitivity and specificity of diagnosis of DIC, in this study diagnosis was based on expert evaluation of an extended coagulation panel. A similar approach has recently been used in a human study evaluating applicability of a scoring scheme developed by the ISTH for the diagnosis of DIC.24
This study also demonstrates that with TF-activated TEG, it is possible to differentiate among subgroups of DIC in dogs. Accompanied by the finding that case fatality rate was significantly lower in the hypercoagulable group, it supports the assumption that early or aggressive intervention in dogs with DIC might be associated with outcome. This is in accordance with interpretation from humans, where it is believed that aggressive intervention in the early hypercoagulable stage of DIC, through supportive or antithrombotic therapy, while the underlying disease is treated, may minimize thromboembolic complications and delay or even prevent progression to overt signs, thus increasing the individual's chances of survival.3,31–33 Assessment of hypercoagulability and thrombosis in dogs is very difficult with routinely used coagulation assays such as d-dimer, which has mainly negative predictive value for thromboembolism.34,35 Thus, there has been a need for improved assay methods that enable easy and near patient assessment of the overall hemostatic state. With the ability to detect hypercoagulability in DIC, TEG provides the clinician with the unique ability to identify dogs that are in the proinflammatory and hypercoagulable state of DIC and offers the novel possibility of clearly differentiating this group of dogs from those in a consumptive but still nonovert stage of DIC, and as such, TEG may potentially be useful for individualization of treatment. Further studies are needed to address whether hypercoagulable dogs with DIC will benefit from therapeutic intervention by anticoagulant therapy, and whether TEG can be used to guide and individualize such treatment.
Hypocoagulability in this study was observed in only 22% of the dogs, which is much lower than what has been suggested in previous studies, where up to 80% of dogs had variable prolongation of one or more of the routine coagulation tests or thrombocytopenia, believed to be indicative of a hypocoagulant state.29,36,37 The discrepancy between those observations and the results of the TEG analysis can perhaps be explained by the fact that routine coagulation tests are plasma based, whereas the TEG assay is a whole-blood-based assay. Thus, TEG includes both cellular and plasma components important for initiation, amplification, propagation, and lysis of the forming blood clot. The findings indicate that an assay including cellular as well as plasma components, such as TEG, gives a more reliable evaluation of the overall hemostatic state than plasma-based assays alone, with the added benefit that TEG enables overall assessment of hemostasis when the results of plasma assays are ambiguous.
All of the TEG parameters examined in this study are, theoretically, influenced by abnormal hemostasis in both directions and should therefore be affected in both hypo- and hypercoagulable states. Accordingly, the results indicate that even though the dogs, overall hemostatic state was categorized based solely on TEG G value, there were significant differences in k, α, and MA values among hypo-, normo-, and hypercoagulable dogs. Thus as expected, hypocoagulable dogs had prolonged k, lower α and MA compared with normocoagulable dogs, whereas hypercoagulable dogs had shortened k, higher α and MA compared with normocoagulable dogs, indicating that several components of the hemostatic system were affected in these dogs. The results further indicate that it is most likely not necessary to include more TEG parameters when dividing dogs into hypo-, normo-, and hypercoagulable groups.
The 28-day death rate in the TEG hypocoagulable group of dogs was twice that in the hypercoagulable group, indicating that there is a large potential for improvement of therapy in this group of dogs. In both humans and dogs, therapy for DIC is not evidence based and therefore often empirically directed at correcting the imbalance in the hemostatic system, eg, through transfusion of packed red blood cells, fresh frozen plasma (FFP), and heparinization, while treating the underlying disease aggressively. The response to treatment with FFP and heparin is unpredictable and, until now, no laboratory tests have been available to accurately predict or monitor treatment response in these patients. Many drugs modulating hemostasis are available, but limited evidence-based information is available regarding their use in veterinary medicine. TEG may be able to help predict and monitor the response to some of those therapies.
Thirty-three percent of the dogs had normal overall hemostatic capability when evaluated with TF-activated TEG. The death rate in this group of dogs was higher than in the hypercoagulable but lower than in the hypocoagulable dogs, indicating that these dogs might be in a transitory phase between hyper- and hypocoagulable states. Although the dogs included in this study were evaluated with TEG during several days, the average hospitalization was only 3 days, which is unfortunately not long enough to detect significant trends in the TEG profile. Additional longitudinal studies are therefore needed to further characterize this group of dogs with DIC in order to establish whether these dogs are in a transitory phase between hyper- and hypocoagulable states and whether serial measurements will be of benefit to predict outcome.
Another limitation of the study is that the validation of the TF-activated TEG in normal healthy dogs does not account for potential effects of increases in endogenous circulating TF that may influence the results in sick dogs. The relationship between TEG results and endogenous TF activity was not examined in this study, but might be an important area for future research.
Overall, the diversity of the hemostatic changes observed in this group of dogs with DIC emphasizes the complexity of the syndrome and highlights the need for a more differentiated approach to diagnosis and treatment. TEG as a test of global hemostasis may potentially provide us with an option for more individually tailored treatment plans for patients with DIC in the near future, which could have a positive effect on the ability to treat these dogs with DIC. Thus, future studies should be aimed at providing evidence as to what specific therapy is of benefit to the hypocoagulable and hypercoagulable dogs with DIC and whether the TEG assay will be of value in monitoring patients receiving such therapy.
In conclusion, the TF-activated TEG assay confirmed that overall hemostatic dysfunction is common in dogs with DIC, but most important documented, for the 1st time, that the most frequent abnormality is hypercoagulability and that the case fatality rate was significantly lower in the hypercoagulable than in the hypocoagulable dogs with DIC. The higher mortality rate in the hypocoagulable group is related to many factors, including cause, response to treatment, type of treatment, overall health of the animal, etc., and additional studies are necessary before definitive conclusions can be drawn about mortality rates comparing hyper- to hypocoagulable states. However, the findings suggest that dogs with DIC could benefit from assessment of their overall hemostatic state before supportive therapy guided against DIC and that TF-activated TEG is of value in this regard.
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aTuman KJ, McCarthy RJ, Patel RB, Ivankovich AD. Quantification of aprotinin reversal of severe fibrinolysis in dogs using thromboelastography. Anesth Analg 1993;76:S439 (abstract)
bMousa S. Synergistic interactions between GPIIb/IIIa antagonists and low molecular weight heparin in inhibiting platelet-fibrin clot dynamics in human blood and in canine model using thromboelastography. Blood 2002;100:3986 (abstract)
cTuman K, Naylor B, Spiess B, et al. Effects of hematocrit on thromboelastography and sonoclot analysis. Anesthesiology 1989;71:A414 (abstract)
dSpiess B, McCarthy R, Ivankovich A. Primary fibrinolysis or D.I.C. differentiated by different viscoelastic tests. Anesthesiology 1989;71:A415 (abstract)
eVacuette, Greiner Bio-One International AG, Kremsmunster, Austria
fTEG 5000 Haemostasis Analyzer, Haemoscope Corporation, Niles, IL
gInnovin, Dade Behring, Marburg, Germany
hACL9000, Instrumentation Laboratory, Warrington, UK
iAdvia 120, Bayer A/S, Lyngby, Denmark
jNycoCard READER II, Medinor A/S, Denmark
kGraphPad Prism v4.01, GraphPad Software, San Diego, CA