Coagulopathy in the equine critical care patient

Vet Clin Equine 20 (2004) 231–251

Coagulopathy in the equine critical care patient Barbara L. Dallap, VMD Department of Clinical Studies, University of Pennsylvania School of Veterinary Medicine, New Bolton Center, 382 West Street Road, Kennett Square, PA 19348, USA

A prevalent emerging concept in the last decade has focused on coagulopathic dysfunction as a component of a severe systemic inflammatory response as opposed to a separate primary disease process. In this light, the role of DIC as a potential contributor to multiorgan dysfunction has also been evaluated. Despite countless experimental and clinical trials in both human and veterinary medicine, it is unclear whether end-organ damage results from microvascular fibrin accumulation and resulting hypoxemia and tissue ischemia or from dramatically upregulated inflammatory responses—or both. There is some evidence that the severity of the host’s inflammatory response to the inciting event may be extremely critical in the development of coagulopathy and may play an important role in patient outcome [1]. It has been hypothesized that the amplification of the host response, from inflammatory process to sustained coagulopathy, may be a result of alterations in endothelial cell function secondary to tumor necrosis factor (TNF) and interleukin (IL-1 and IL-6) stimulation, in combination with the contribution of activated complement components [2]. One study points toward IL-1–enhanced expression of procoagulant activity in human vascular endothelial cells [3], and it has been proposed that prolongation of such a response could be responsible for the development of uncompensated coagulopathy [2]. As triggering events of abnormal vascular response and intravascular procoagulant activity, endotoxin and sepsis have been a focus of research in human and veterinary medicine and may suggest a direction in the search for successful treatment of DIC [4–8]. Absorption of endotoxin through the gastrointestinal tract is more commonly documented in the critically ill patient than full-blown sepsis [9], directing research toward control of the inflammatory responses to endotoxemia [10].

E-mail address: [email protected] 0749-0739/04/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.cveq.2003.11.002

232

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

DIC is often characterized by increased intravascular thrombin formation in the face of concurrent hemorrhagic diathesis, further complicating treatment choices made by the clinician. Treatment of the underlying disease process remains the cornerstone of therapy and almost the only point on which most clinicians agree. Therapy directed toward the goal of restoration of normal physiologic hemostatic function is essential but remains a challenge in the face of the repeated emergence of procoagulant triggering stimuli. A focus of clinical management should be earlier recognition and intervention, because recent experimental studies have demonstrated that rapid detection and treatment of subclinical coagulopathies reduce morbidity and mortality associated with the development of fulminant DIC [11,12]. As equine veterinarians struggle with critically ill patients, who are extremely endotoxin sensitive, the challenges remain early detection of coagulopathy in patients with identified at-risk diseases and identification of effective anticoagulant or component therapy to restore physiologic anticoagulant pathways. Etiology and historical perspective After approximately 50+ years of human experimental and clinical studies examining the pathogenesis, diagnosis, and treatment of DIC, this much remains clear: the process is extremely complicated with respect to its activation and propagation, and there will be no simple method of quickly identifying patients afflicted or easy discovery of effective therapies. Historically, DIC was initially recognized as a hemorrhagic diathesis resulting from intravascular consumptive coagulation, which seemed to be the phase of the disease process that was most easily recognized by the attending clinician [13]. Much more difficult to recognize, and more relevant in terms of morbidity and mortality, was the microvascular and occasional large vessel thrombi formation, which resulted in tissue hypoxia and subsequent reperfusion injury. As clinical experience with this disease process grew, it was recognized that DIC represented a portion of an entire spectrum of hemostatic dysfunction, with patients presenting at various points along a continuum [14]. As recognition of the various stages of hemostatic dysfunction (hypercoagulability, compensated or subclinical DIC, and fulminant or uncompensated DIC) became more prevalent, the debate over the true definition of DIC continued. In1994, Bick [15], a noted coagulation specialist, provided a definition of DIC containing minimally acceptable criteria: ‘‘A systemic thrombohemorrhagic disorder seen in association with well-defined clinical situations and laboratory evidence of (1) procoagulant activation, (2) fibrinolytic activation, (3) inhibitor consumption, and (4) biochemical evidence of end-organ failure.’’ As a result of the recognition of various stages of hemostatic dysfunction and the evolution of modified definitions of DIC, scoring systems describing the severity of DIC emerged. Bick’s scoring system consisted of tests from four groups of laboratory criteria representing the minimally acceptable criteria: procoagulant activation,

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

233

fibrinolytic activation, inhibitor consumption, and end-organ damage or failure [15]. The purpose of a scoring system was to establish guidelines to assess the severity of DIC, direct therapeutic intervention, and provide an objective method to evaluate efficacy of treatment [15]. The composition of a multiparametric scoring system for DIC is still debated, with individuals arguing for and against the inclusion of organ failure indicators [16]. Currently, scoring systems developed to define DIC (or sepsis) are not widely used in veterinary medicine. This may be related to the wide variation between facilities in availability of tests as well as to financial considerations inherent to practicing veterinary critical care medicine. In human medicine, DIC is commonly associated with certain disease entities as well as accepted risk factors that may predispose critically ill human patients to hemostatic dysfunction. One of the earliest recognized clinical scenarios associated with DIC was obstetric accidents, such as amniotic fluid embolism, placental abruption, retained fetus syndrome, or eclampsia [15,17]. Hemolytic crises, particularly those resulting from transfusion reaction, have been implicated as the trigger for full-blown DIC [18]. It is suspected that increased adenosine diphosphate (ADP) from lysed red cells, or the inflammatory process related to their destruction, plays a role in intravascular activation of procoagulant factors after a hemolytic event [19]. Septicemia and endotoxemia have long been associated with the development of hemostatic dysfunction. It was initially suggested that endotoxin initiated DIC only by activation of factor XII, but work emerging in the late 1990s refocused attention on tissue factor (TF), the activation and inhibition of which seems to play a key role in the host response to a variety of inciting events, including sepsis and endotoxemia [20–22]. Recent studies focus on the role of the hepatic response as the bridge between sepsis and coagulopathic dysfunction as well as on the differences in compensated and uncompensated DIC response in intraperitoneal and intravenous Escherichia coli sepsis. Progressive viremias, neoplasia, liver disease, and trauma have all been associated with hemostatic dysfunction in the human critical care patient [23–26]. In veterinary medicine, coagulopathies have been associated with a similar list of inciting causes. Neoplasia, infectious disease, and trauma are common causes of DIC in small animals [27], whereas coagulopathy secondary to gastrointestinal disorders historically has been the most prevalent cause in large animals [28].

Pathophysiology Coagulation The primary goal of the hemostatic system is to maintain adequate blood flow to tissues while preventing a hemorrhagic crisis. Efficient clot formation and retraction are essential for successful hemostatic function. Equally important is the corresponding fibrinolysis that follows clot formation,

234

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

preventing excessive intravascular or extravascular fibrin deposition and restoring appropriate end-organ perfusion. Normal hemostatic function can be viewed as a highly regulated balance-controlling mechanism of clot formation and lysis, with significant contributions from inflammatory cells and cytokines. It is important to realize that the inhibitors of the pathways of thrombin formation are critical to the progression of clinical coagulopathy and have become a focus of research and prospective clinical trials. Therefore, a significant portion of the discussion here is focused on these systems as well as on the more recently clarified interactions between the coagulation pathways. For almost 200 years, the activation of prothrombin to thrombin and subsequent conversion of fibrinogen to fibrin have been investigated as the primary events in the activation of clot formation. The Roman numeral system that sometimes haunts clinicians was established essentially in the order of factor discovery and was put into use by Paul Orwen, who described parahemophilia [29]. Initial work focused on the ‘‘intrinsic’’ ability of blood to coagulate, which led to the discovery of factor XII and its associated cascade resulting in intravascular clot formation [30]. This work was tied into earlier studies of hemophilia A and B and other factor deficiencies associated with clinically recognized bleeding disorders. The assimilation of such studies resulted in the acceptance of the familiar intrinsic and common coagulation pathways as well as the conceptualization of coagulation as an explosive ‘‘cascade’’ that dramatically amplifies itself at each step. Because of the prevalence of clinically recognized bleeding disorders of the time, much work was focused on the intrinsic and common pathways. Factor XIII was identified as the factor that stabilized the fibrin clot by means of active cross-linking, and deficiency was clinically correlated to bleeding from surgical wounds after an operation [31]. The extrinsic pathway received little attention until the critical discovery that TF-activated factor VII complex could activate factors IX and X [32]. TF is a small glycoprotein transmembrane cell receptor originally classified in the cytokine family [33]. It is present in monocytes and vascular endothelial cells. Therefore, TF exposed during extravascular tissue trauma and, perhaps more importantly, activated by inflammatory cells and cytokines results in activation of both pathways of coagulation. The TF pathway moved into a prominent position in coagulation research. Studies in recent years have focused on control of the tissue factor pathway inhibitor (TFPI) as a means of restoring physiologic hemostasis [34–37]. TFPI can bind activated factor X and inhibit it. Gando et al [34] demonstrated that TF and neutrophil elastase (an enzyme released from activated neutrophils known to affect anticoagulofibrinolytic factors) concentrations were higher in nonsurvivors than in survivors in patients with sepsis-associated systemic inflammatory response syndrome (SIRS) and multi-organ dysfunction syndrome (MODS). In human endotoxemia, it has been demonstrated that TFPI-attenuated endotoxin induced activation of coagulation [21].

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

235

It is well accepted that phospholipids are required for normal coagulation and are provided by platelets, damaged endothelial cells, and alterations in inflammatory cells. Coordination between these phospholipids and activated vitamin K–dependent serine proteases (II, VII, IX, and X) in combination with their cofactors (V and VIII) results in the coagulation explosion that follows any significant vascular damage or inflammatory event. Numerous feedback activation loops have been examined more closely in recent years. Activated factor X enhances factor IX activation in combination with the TF–factor VIIa complex and also increases production of activated cofactors V and VIII [38]. Factor IXa is 50 times more efficient at activating factor X than the TF–factor VIIa complex [38]. In situations where the TFPI is functioning appropriately, factor X can only be activated by factor IXa, thereby decreasing endotoxin or tissue trauma activation of coagulation. The conceptual approach relating to feedback activation and TF activation triggered a renewed interest in the role of inflammatory cytokines and the development of DIC. Inflammation and disseminated intravascular coagulation The activation of the hypercoagulable state can be triggered by endothelial damage or TF activation, and depending on the cross-reactivity of activated serine proteases, inhibition of the fibrinolytic system may occur simultaneously. It appears that the proinflammatory cytokine cascade related to trauma- and sepsis-mediated organ failure is inextricably intertwined with DIC. Emerging from the Margaux Conference on Critical Illness in November 2000 [1] is the concept that DIC is no longer thought to be an isolated epiphenomenon but, instead, one of the components of an imbalanced and massively activated inflammatory response. Cytokines implicated in the activation of TF expression on vascular endothelium and inflammatory cells are TNF, IL-1a, IL-1b, IL-6, and IL-8 [39]. Endotoxin increases the expression of TF on monocytes and endothelial cells in vitro and is primarily mediated by the action of IL-1b [40]. In human and nonhuman primate studies, IL-1, IL-6, and TNF induced activation of coagulation in vivo. Specific inhibitors of TNF did not result in a difference in measured coagulation parameters but did seem to be protective against mortality [41]. The role of cytokines in sustained hemostatic dysfunction requires further investigation, and variation between species may exist. Fig. 1 depicts a brief representation of proposed inflammatory mediators that may be involved in the development of DIC in critically ill equine patients. Inhibitors of coagulation Antithrombin (AT) is a plasma serine protease inhibitor that acts primarily on thrombin, factor X, and the TF–factor VIIa complex, thereby controlling procoagulant activity at the most critical steps. In higher levels, it has

236

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

Fig. 1. Coagulation and inflammation.

inhibitory effects on factors IXa, XIa, and XIIa as well as on plasmin. Physiologically, it is thought to be the most important inhibitor of coagulation, responsible for up to 70% of the plasma anticoagulant ability [28]. Heparin sulfate serves as the reusable AT cofactor, creating a conformational change in the AT molecule that results in a dramatic upregulation of the binding of AT to associated serine proteases. Horses are reported to have larger concentrations of AT than human beings or other species [42,43]. The signif-

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

237

icance of increased AT in the horse is unclear at this point, but a relation to inefficient effect on thrombin or increased procoagulant factors has been hypothesized [28]. Other serine protease inhibitors include a2-macrobulin, which inhibits thrombin and kallikrein, and a2-antiplasmin. a2-Antiplasmin is the primary inhibitor of plasmin but also can have a small antagonistic effect on factors Xa and XIIa as well as on thrombin. The protein C–protein S–thrombomodulin system has received a great deal of attention as a possible site for therapeutic intervention in cases of clinical coagulopathy. Thrombin binds to thrombomodulin expressed on cellular surfaces of endothelial cells and activates protein C, which inactivates factors Va and VIIIa, resulting in a significant anticoagulant effect [39]. Protein S serves as a cofactor for protein C. It is possible that this process is impaired in sepsis via consumption or downregulation of thrombomodulin on endothelial cell surfaces. The use of recombinant human protein C (rhAPC) in the clinical setting has proceeded through stage II clinical trials, in which investigators believed it was well-tolerated and resulted in a dosedependent reduction in D-dimer and IL-6 levels [44]. Fibrinolysis and its inhibitors Plasmin is the principal enzyme involved in the degradation of fibrin clots. It is the end product of the fibrinolytic system, which is typically activated simultaneously with activation of coagulation, and can be thought as the body’s built-in anticoagulation system. Plasmin is formed as its inactive precursor (plasminogen) by the liver, kidneys, and eosinophils [28], and it circulates freely in plasma as plasminogen. Plasminogen activation can be initiated by a series of reactions initiated by the intrinsic pathway and plasma kallikrein or by the activity of tissue or urokinase type activators. Tissue plasminogen activator (tPA) has a high affinity for fibrin and is produced primarily by endothelial cells. tPA is present in high levels in the fibrin clot, and stasis of blood upstream causes an increase in tPA release from vascular endothelium [28]. In studies where chimpanzees were challenged with lowdose endotoxin, increased levels of tPA were observed before activation of the coagulation system [41]. Additionally, monoclonal antibodies inhibiting the coagulation response in human beings and nonhuman primates challenged with endotoxin did not inhibit the fibrinolytic response [41,45]. In cases of sepsis or severe endotoxemia, it must be considered that fibrinolysis might be activated by mechanisms other than those related to clot formation [39]. Coagulation testing and identifying equine patients at risk Clinical manifestations of disseminated intravascular coagulation Fulminant DIC or hemostatic dysfunction can occur in the equine critical care patient because of severe endotoxemia, neoplasia, sepsis, acute trauma or hemorrhage, or possibly through hypoxia-ischemia syndrome in neonates.

238

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

Clinical signs of fulminant DIC in the horse often include oral petechiae, abnormal bleeding from venipuncture sites, or an increased tendency of large peripheral vessels to thrombosis. An early report describing DIC in six horses [46] reported abnormal hemorrhage and venous thrombosis as the most common signs. Clinical signs in the horse most likely represent fulminant DIC. Probably more relevant, if one ascribes to the concept that identification of subclinical DIC followed by subsequent aggressive intervention results in better outcome [11], is the early microvascular thrombi formation in the hypercoagulable stage, which may ultimately affect end-organ perfusion. Future research should focus on earlier identification of at-risk equine patients and evaluation of aggressive early intervention. Studies describing the development of DIC secondary to a primary gastrointestinal disorder are most common in the literature, and changes in the peritoneal and peripheral coagulation profiles are described [47]. Early work by Pablo et al [48] described the association between prolonged (6 hours) jejunal strangulation and the development of DIC in an experimental pony model. Interestingly, ponies with significantly abnormal coagulation parameters and subsequent death did not show specific signs of fulminant DIC. A 1992 study reported the development of DIC in 23 horses with colic, 22 of which required surgical intervention, with a higher incidence in horses with lesions that resulted in devitalized small intestine [49]. Excessive hemorrhage was observed in 19 of these patients either at surgery or in the immediate postoperative period. Although this represented a small portion of all horses presented for colic over the study period (3.5%), the low survival in the group (34%) emphasizes the need for careful coagulation monitoring of surgical colic cases. An evaluation of eight hemostatic parameters (platelet count, plasma fibrinogen, plasma AT III, partial thromboplastin time [PTT], prothrombin time [PT], thrombin clotting time, soluble fibrin monomer, and fibrin[ogen] degradation products [FDPs]) in 24 horses presented for colic revealed at least one abnormal parameter in all horses [50]. The average number of abnormal coagulation parameters (five versus two) was much higher in nonsurvivors than in survivors, supporting the association of higher mortality with increasing hemostatic dysfunction. A larger study [51] analyzing hemostasis in 233 horses with colic revealed that nonsurvivors had evidence of hypercoagulability most evident in the 24- and 48-hour periods after admission. Fibrinolytic activity was increased in all patients presented for colic regardless of intestinal ischemia or outcome, suggesting a possible acute phase response secondary to gastrointestinal insult. Fibrinolytic activity in horses with acute gastrointestinal disease was evaluated more closely in a study measuring tPA and plasminogen activator inhibitor type-1 (PAI-1) for 5 days after admission [52]. The authors concluded that fibrinolysis is initially inhibited in response to acute gastrointestinal disease, as demonstrated by increased PAI-1 and low tPA, and that persistently increased PAI-1 levels were associated with poor prognosis for survival. Recent studies indicate that coagulation profiles may

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

239

be abnormal in horses with colitis [53] or surgical treatment of large colon volvulus [54] before the observation of clinical signs of DIC. In horses with colitis, horses were eight times more likely to die or be euthanized if they had subclinical DIC, defined as three of six coagulation test results being abnormal. Seventy percent of horses with surgical treatment for large colon volvulus had abnormal results in three of six coagulation tests, and horses with abnormal results in four of six tests were significantly more likely to be euthanized [54]. The attending surgeon responsible for case management reported clinical signs of possible coagulopathy in only 2 of 27 horses receiving surgical treatment for large colon volvulus, and no coagulation testing was ordered in any horse by the primary clinician. This provides further evidence for the importance of early and accurate identification of horses with coagulation dysfunction. Coagulation dysfunction or DIC has also been reported in neonatal foals with septicemia [55] as well as in a horse with disseminated neoplasia [46]. A horse presented for anorexia and chronic weight loss was diagnosed with disseminated cholangiosarcoma after first demonstrating overt clinical signs of coagulopathy and having a prolonged PT, activated partial thromboplastin time (APTT), and increased levels of FDPs. Foals that were presumed septic (sepsis score identified) had decreased fibrinogen, plasminogen activator, a2-antiplasmin, and AT [55]. Foals with detectable plasma endotoxin had significantly prolonged PT, APTT, and decreased plasminogen and AT. Coagulation parameters in neonatal foals and people can be quite different from those in adult horses [56] and people [57], and careful evaluation of coagulation tests in the neonatal population as well as individual clinical laboratory normal values are indicated when trying to evaluate coagulation dysfunction in a neonate. Although studies like these are much less prevalent in the literature, they help to identify additional equine critical care patients at risk for development of a coagulopathy. For human and veterinary patients alike, the problem continues to be prompt and accurate identification of hemostatic dysfunction early enough to intervene successfully. It seems that by the time many of the standard coagulation test results, such as APTT, PT, platelet count, AT, and fibrinogen, are significantly abnormal, the therapeutic window of opportunity has closed. An additional problem in early identification as well as in the overall applicability of many clinical research efforts is the lack of a readily agreed on definition of DIC. Criteria for DIC vary considerably in prospective clinical trials and among clinicians. Many clinicians consider a patient to have DIC if three of five coagulation test results are abnormal and require that the patient have clinical signs consistent with a coagulopathy. Unfortunately, coagulopathies with different inciting causes may have a variety of different test results abnormal in the initial stages, which makes point-of-care testing difficult and misleading, particularly in a veterinary emergency clinic setting. This often results in running entire coagulation profiles on at-risk patients, which can be costly and not always readily available. Further, if the aim is to

240

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

treat DIC aggressively in the subclinical stage, perhaps the focus should be on evaluation of the degree of hemostatic dysfunction, thus making debate over the definition of DIC less critical. Standard clinical testing for coagulation dysfunction In the clinical veterinary setting, routine coagulation testing most commonly consists of platelet count, PT, APTT, AT, fibrinogen, and FDPs. Additional hemostatic parameters, such as tPA, PAI-1, thrombin-antithrombin (TAT), plasminogen concentration, thrombin time, and soluble fibrin monomer, have been explored in the critically ill equine patient [47,48,50–55], but significant changes in the hemostatic evaluation of clinical cases have not yet occurred as a result. A seemingly practical approach to coagulation testing would be to determine the most predictive tests with respect to outcome in the equine patient, develop a multiparametric model predicting outcome, and run only those tests that are most useful. The problem with this approach is the fact that coagulopathy is typically a secondary syndrome initiated by anything from sepsis to a devitalized large colon. We now realize that the host inflammatory response is critical in the development of DIC, SIRS, and MODS, and one would presume that the cytokine response to bacteremia versus massive acute release of endotoxin, which probably occurs in horses with severe large colon volvulus, might be quite different. Because of this inherent variability, it is difficult to say which tests will be most useful. Further research might result in the determination of the most prognostic coagulation tests for specific equine at-risk populations. Platelet count (platelets/lL) Although not specific for the development of coagulopathy, thrombocytopenia is a common feature of DIC. Horses typically have lower platelet counts than other species, and the development of coagulopathy is generally associated with thrombocyte counts less than 100,000 platelets/lL [28]. Thrombocytopenia is a more consistent feature of acute coagulopathy as opposed to a more prolonged compensated course of hemostatic dysfunction. Decreased platelet count might be a result of consumption or immunemediated destruction as a component of a SIRS response. It is also possible that platelet function may be abnormal during the development of coagulopathy, perhaps in part because of circulating plasmin-mediated degradation products of fibrin or fibrinogen adhering to platelet surfaces. Platelet function testing, if available, could be useful in identifying patients with subclinical DIC. In most cases of fulminant DIC, platelet function is almost always abnormal and function tests do not significantly add to confirming the diagnosis [15]. Thrombocytopenia was observed in ponies associated with strangulating obstruction of the jejunum in an experimental model [48]. A study evaluating the association between equine colitis and the development of subclinical DIC

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

241

reported that 50% of the horses evaluated had platelet counts less than 100,000 platelets/lL [53]. In horses with surgical treatment for large colon volvulus, there was a significant association between the development of thrombocytopenia and mortality [54]. Prothrombin time (seconds) PT evaluates the efficiency of fibrinogen conversion to fibrin through the extrinsic and common pathways. PT can be determined on a chemistry analyzer or on point-of-care PT/PTT analyzers. Most clinicians consider a 20% increase in PT compared with controls abnormal. PT can be prolonged in the face of hypofibrinogenemia (\100 mg/dL in human beings) [58] or with degradation or consumption of factor VII, V, or X. Decreased plasma prothrombin concentration to less than 30% of normal results in prolonged PT [15]. In human patients diagnosed with DIC, only approximately 50% have prolonged PT, and some believe that measurement of PT is not a sensitive measure of coagulopathy [14]. Factors like hypercoagulability or early degradation products may alter the test results. PT measurement may be more useful in the equine patient. Continued reliance on PT and PTT in veterinary medicine could be a result of financial constraints and lack of availability of additional coagulation tests. Prolonged PT was observed in 25% of horses presented with colic and seemed to be a good predictor of outcome [50]. In a study evaluating horses that developed DIC secondary to acute gastrointestinal crisis, 58% had significantly prolonged PT [49]. An association between strangulating gastrointestinal lesions and prolonged PT was observed in a larger (n = 233) study evaluating hemostatic parameters in horses with colic [51]. Similarly, in horses with large colon volvulus, individuals with prolonged PT were much more likely to be euthanized, perhaps reflective of the degree of vascular compromise to the large colon [54]. PT in normal foals can be significantly longer at birth than in older foals [56]. Foals with presumed sepsis demonstrated prolonged PT compared with their nonseptic matched controls [55]. The increased PT at birth in a normal foal requires clinicians to use caution when interpreting coagulation testing in a neonatal population. Activated partial thromboplastin time (seconds) APTT evaluates the efficiency of the intrinsic and common pathways of fibrin formation and can be similarly measured using a chemistry analyzer or point-of-care testing. A time increase 20% longer than that of control indicates a prolonged APTT. In human beings, APTT becomes prolonged when factor VIII, IX, or XI is decreased to 20% of its normal plasma concentration [58]. Plasmin degradation of factors V, VIII:C, IX, and XI can result in prolonged APTT. Only 50% to 60% of human patients with DIC have a prolonged APTT [15], however, and a normal APTT does not rule out

242

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

hemostatic dysfunction. The degree of plasmin degradation of coagulation factors could depend on the degree of activation of the fibrinolytic system in response to the triggering event. Similar to PT, hypercoagulability or circulating FDPs potentially affect APTT, making some skeptical of its usefulness in human patients with a suspected coagulopathy [14]. Prolonged APTT has been reported in the literature in equine patients with coagulopathy. Prolonged APTT was the most frequently observed coagulation abnormality in two studies evaluating hemostatic indices in horses with acute abdominal disease [49,50]. Both studies found that the test was relatively sensitive but not specific or strongly correlated with outcome. Similar to PT, prolonged APTT was observed in horses with strangulating intestinal lesions in a study evaluating 233 horses presented for acute abdominal discomfort [51]. In horses with a primary complaint of acute colitis, prolonged APTT was commonly observed in horses developing subclinical DIC [53], but association with outcome was not evaluated. APTT is prolonged in foals at birth compared with foals at 1 month of age, similar to PT [56]. The differences are quantitatively small, making it incumbent that the clinician carefully evaluate APTT in this population. Sepsis can be a differential diagnosis in neonates with prolonged APTT and should be considered in individuals suspected of having a coagulopathy [55]. Antithrombin (% activity) AT is thought to be the most potent inhibitor of clot formation. It is a singlechain glycoprotein belonging to the serpin family and is produced in the liver parenchymal cells [59]. AT has a strong affinity for thrombin and binds irreversibly in a 1:1 arrangement. AT also has inhibitory effects on factors Xa, XIa, and XIIa as well as on plasmin and kallikrein. Heparins dramatically enhance the efficacy of AT, presumably through a change in stearic configuration after binding to lysine sites [60], but the exact mechanism is not fully elucidated. In addition to inhibiting many of the enzymes of coagulation, AT is thought to have anti-inflammatory properties by binding to endothelial cell heparin-like substances and causing the release of prostacyclin [61]. High morbidity and mortality are associated with AT activity levels of 60% or less in human patients [14], and many believe that AT is a useful test in diagnosing and monitoring the patient with suspected coagulopathy [14,15,59,60]. AT loss can be associated with inherited disorders in people, consumption, endotoxin inhibition, liver dysfunction, or protein loss. Although decreased AT activity in patients is often thought to be caused by consumption, a recent study contradicts this theory, correlating decreased AT only to albumin loss (presumably renal) and reduced liver function [62]. When patients were categorized according to levels of albumin and choline esterase (liver function), no differences in AT were observed in DIC versus non-DIC groups. Additionally, there was no correlation between TAT levels and AT.

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

243

Decreased AT levels have been reported in horses with acute gastrointestinal disease. In an initial study evaluating hemostatic parameters in horses with colic, AT was decreased in 29% of the horses studied and was of the greatest prognostic value with respect to outcome [50]. In horses with large colon torsion, significantly decreased AT that failed to improve over time was associated with nonsurvival [63]. In a larger group of horses presented with a variety of gastrointestinal problems, low plasma AT was associated with nonsurvival [51]. Evaluation of intraperitoneal and intravascular coagulation parameters resulted in the conclusion that increased AT in the peritoneal cavity was associated with strangulating gastrointestinal lesions and that differences existed between plasma and peritoneal coagulation parameters in horses with gastrointestinal tract disease [52]. Dramatic increases in peritoneal concentrations of AT could possibly explain corresponding low plasma AT and would correlate with decreased survival. In horses with colitis, hypoproteinemia was associated with decreased AT activity 48 hours after admission [53], correlating with the study refuting consumption as the main cause of depleted AT activity in patients with DIC [62]. Measuring AT activity seems useful in predicting outcome in certain equine patient populations and would be even more useful in monitoring therapy should the use of replacement AT come to fruition in veterinary medicine. Fibrinogen (mg/dL) Hypofibrinogenemia is sometimes a feature of human patients with a coagulopathy but is not considered to be particularly sensitive. Decreased fibrinogen concentration is usually indicative of severe acute DIC, and other tests should certainly be used to attempt to evaluate the activity of the fibrinolytic system. In the horse, hypofibrinogenemia is not common in cases with fulminant DIC, possibly because of the liver’s ability to generate large amounts of the acute phase protein in response to significant inflammatory insult [28]. It is even possible that decreased fibrinogen could be a result of decreased production by the liver, reflective of altered hepatic function. Failure of an anticipated fibrinogen response may be relevant in horses with acute severe gastrointestinal disorders, such as colitis or large colon volvulus, because fibrinogen concentrations were significantly lower in nonsurvivors compared with survivors in recent studies evaluating coagulation parameters in these groups [53,54]. Clinicians should perhaps be suspicious of hemostatic dysfunction in a horse with a fibrinogen concentration of 180 to 200 mg/dL 48 hours after surgical treatment for large colon volvulus. Fibrin(ogen) degradation products (lg/mL) FDPs are produced during plasmin degradation of fibrinogen or crosslinked fibrin. Fragments X, Y, D, and E could comprise the increased FDP level; therefore, FDP levels are not specific for the degradation of cross-linked

244

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

fibrin. The presence of increased FDPs can indicate increased fibrinolytic activity, plasmin presence, or overwhelming of the monophagocytic system of the liver to clear blood of FDPs appropriately. There are a variety of methods for measuring FDPs, but the most common methods use monoclonal antibodies against fragments D and E, available in a slide latex-agglutination technique. FDPs can be increased as a result of DIC or, perhaps more commonly in people, can be associated with thrombotic or thromboembolic events. FDPs are increased in a large percentage of human patients with DIC [15], but abnormalities in FDPs have not been proven to be particularly predictive of outcome. FDP levels in horses with acute gastrointestinal disease are significantly increased, but FDP increases have been observed in survivors and nonsurvivors alike, making the test a poor prognostic indicator [50,51,53]. Additional clinical testing for coagulation dysfunction TAT and D-dimer levels have shown promise in further characterization of DIC and prediction of outcome in certain human patient populations. TAT and D-dimer levels have been evaluated in equine and canine patients with DIC [54,64–67]. D-dimer D-dimer analysis uses a monoclonal antibody that detects a neoantigen resulting from cross-linking of c-chains, thereby specifically identifying fibrin degradation products as opposed to FDPs. Semiquantitative and quantitative latex agglutination kits for D-dimer measurement are now commercially available. Historically, there was excellent correlation with increased D-dimer concentration and the development of DIC in people, and many believed it to be an extremely useful test in identifying patients with coagulopathy, particularly if used in conjunction with AT and fibrinopeptide A concentration, a diagnostic test for thrombin acting on fibrinogen [15]. D-dimer analysis has been used as a prognostic indicator for horses with colic [66] and in the evaluation of dogs with DIC [67]. The D-dimer concentration (and FDPs) increases relatively quickly in DIC, deep vein thrombosis, or severe inflammation. Therefore, these tests are quite sensitive but not extremely specific, and a negative test result is much more helpful than a positive test result in predicting outcome. In prospective clinical trials in our clinic, it seemed that the usefulness of D-dimer concentrations was somewhat dependent on the presenting complaint when used to predict outcome. If the primary disease was severe or involved vascular compromise of the intestine (ie, large colon volvulus), all patients had increased levels and the concentration was not helpful in predicting survival [54]. Consideration of the primary disease should be part of the thought process

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

245

when evaluating increased D-dimer concentration before making a diagnosis of coagulopathy. Because there are currently many tests available for D-dimer analysis, one must be cautious when evaluating and comparing study and test results. Thrombin-antithrombin complexes (lg/L) TAT levels are useful in evaluating the human patient with DIC. The test gives an estimation of the consumption of AT via complex formation with thrombin and also reflects the degree of activation of thrombin. An increased TAT concentration is evidence of procoagulant activity and serine protease inhibitor consumption. Because the TAT complex is stable and has a relatively short half-life, it is reflective of recent coagulation activity [68]. TAT complexes have been useful in characterizing a relation between degree of hemostatic dysfunction and SIRS in human patients [69]. The enzymelinked immunosorbent assay (ELISA) TAT test developed for human use (Enzygnost; Dade-Behring, Wilmington, DE) has been evaluated in the horse and was found useful in identifying hypercoagulability in horses with colic [64,65]. Nonsurvivors were found to have the greatest increase in TAT concentration. A significantly increased TAT concentration was strongly associated with the development of coagulopathy and poor outcome in horses with surgical treatment of large colon volvulus [54]. Unfortunately, at this time, the only test that is validated in the horse is a multiwell ELISA technique that is not convenient or cost-effective in the clinical setting. Viscoelastic methods of coagulation evaluation The revitalization of viscoelastic methods of coagulation evaluation may emerge as useful tools for coagulation and platelet function analysis. Thromboelastography (TEG) and the Sonoclot analyzer (Sienco, Wheat Ridge, CO) were used originally to monitor hypercoagulability in a variety of patients undergoing organ transplant or cardiovascular bypass procedures. TEG works by measuring the shear forces of clot formation generated on a pin in a small cup containing a blood sample during oscillation. The Sonoclot analyzer has a sensitive sensor vibrating in a cuvette containing a blood sample, which records clot mechanical properties during clot formation. Citrated blood is commonly used with both analyzers. TEG was evaluated in the cat and dog in the late 1970s and was recently used to identify hypercoagulable states in dogs suffering from parvoviral enteritis [70]. The Sonoclot analyzer has recently been used for evaluation of patients undergoing liver transplantation, cardiac bypass surgery, orthopedic surgery, or systemic anticoagulation for a variety of reasons. It has also been used to evaluate hypercoagulability in uremic patients and women with highrisk pregnancies. The analyzer has also proved useful to evaluate the effects of different high-volume fluid resuscitation protocols on coagulation and to assess rheologic properties of red blood cells undergoing different methods

246

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

of storage. The Sonoclot analyzer produces a tracing (‘‘signature’’) that represents the rate of clot formation, platelet activity, and strength of the clot. Preliminary studies in our hospital using the Sonoclot analyzer to evaluate coagulation in neonatal foals with presumed sepsis revealed a significant prolongation in activated clot time in nonsurviving patients (B.L. Dallap, unpublished data, 2002). We have also found the information regarding platelet function to be useful clinically in evaluating critically ill foals with suspected coagulopathies. Hopefully, further work evaluating viscoelastic methods of coagulation analysis will provide with us a simple test that correlates well with coagulation function in the DIC patient. Earlier identification and intervention in patients with various degrees of coagulation dysfunction will hopefully improve morbidity and mortality. Viscoelastic coagulation testing should also be more useful in determining the efficacy of anticoagulant or component therapy chosen for treatment because it is consistently more sensitive than standard coagulation profiles after anticoagulation therapy in human patients. Treatment Treatment for DIC is difficult and somewhat controversial. It is well accepted that treatment of the primary disease is paramount in achieving successful reversal of coagulopathy and restoring normal physiologic hemostasis. Appropriate fluid therapy and adequate perfusion are essential to ensure appropriate cellular metabolism at the tissue level in patients prone to microvasculature thrombi. DIC develops in the horse most commonly as a result of endotoxemia, and therapies directed at decreasing the effect of endotoxin could potentially increase the probability of survival. Flunixin meglumine (0.25 mg/kg intravenously three times daily) has been shown to decrease prostaglandin release in response to endotoxin [71] and could possibly decrease the associated inflammatory response. Polymixin B binds endotoxin in a dose-dependent fashion and may decrease the effects of endotoxemia in horses at risk for development of DIC. Plasma and platelet substitutes (component therapy), anticoagulants, coagulation inhibitor concentrates, and various anti-inflammatories have been used in the treatment of DIC. Close evaluation of any abnormal hemostatic parameters, in conjunction with any overt clinical signs, should be used to direct therapy toward the portion of the coagulation system that is most affected. Fresh-frozen plasma, depending on quality, storage, and handling, may provide coagulation factors and variable amounts of AT. Specially prepared cryoprecipitates used in human medicine can provide factor VII, von Willebrand Factor (vWF), fibronectin, fibrinogen, and factor XIII. Platelet-rich plasma can be obtained relatively inexpensively in an equine referral hospital by using platelet collection kits and two-speed centrifugation techniques. Platelet concentration is not as great as in plateletrich concentrates produced and used in human medicine, but administration

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

247

of platelet-rich plasma is still superior to whole blood transfusion for the treatment of profound thrombocytopenia (\50,000 platelets/lL). The primary anticoagulant used in the horse is heparin. Controlled prospective studies demonstrating its efficacy in the treatment of DIC in the horse have not been done. At low doses, heparin increases the activity of AT, inactivates factor Xa, and inhibits the conversion of prothrombin to thrombin. At higher doses, heparin inactivates thrombin, preventing conversion of fibrinogen to fibrin. Heparin facilitates the AT-mediated inhibition of factors IX, X, XI, and XII. At high levels, depending on individual response, it may inactivate factor XIII, resulting in unstable clots. The dose commonly cited in the literature is 15 to 80 U/kg as a bolus, with continuous rate infusion dosing ranging from 5 to 25 U/kg/h. Potential adverse effects of unfractionated heparin include thrombocytopenia, and erythrocyte agglutination. Additionally, there is wide variation of efficacy in individuals, making patient monitoring critical. Low-molecular-weight heparin (LMWH) is used in people with DIC and various stages of coagulopathy. The advantages include higher bioavailability, longer half-life, and more reliable dose-dependent anticoagulant effects. Adverse effects, such as thrombocytopenia and anemia, are observed much less commonly in human patients treated with LMWH as opposed to unfractionated heparin. The pharmacokinetics of two LMWHs have been evaluated in the horse [72]. The doses were determined to be 50 to 100 U/kg subcutaneously for dalteparin and 40 to 80 U/kg subcutaneously for enoxaparin, with no significant adverse effects seen after a single dose. Aspirin has been used to decrease platelet aggregation in horses with thrombotic tendencies. Aspirin inhibits cyclooxygenase, decreasing thromboxane A2 production, which results in decreased platelet aggregation. Because platelets are unable to produce more cyclooxygenase, they are irreversibly affected. Doses of 15 to 100 mg/kg orally two or three times daily or 30 mg/kg orally once daily or every other day have been used in the horse. Side effects can include exacerbation of gastric ulceration, gastrointestinal bleeding, and increased platelet dysfunction. Flunixin meglumine and pentoxifylline may be useful in controlling the inflammatory response associated with a triggering or recurring event. As mentioned earlier, flunixin meglumine may be helpful if the trigger is endotoxin related. Pentoxifylline is a methylxanthine derivative that decreases blood viscosity and increases red blood cell deformability. It has also been reported to inhibit TNFa and possibly IL-1 and IL-6. Although publications in the human literature report success with anticoagulant therapies like rhAPC, AT, and factor VII administration, prospective clinical studies in the horse have not been done, and financial constraints may make treatment with such therapeutics unlikely. LMWH is also quite expensive at this time, and clinical trials are needed to demonstrate therapeutic efficacy. The best chance for success in treatment of a coagulopathy in the equine critical care patient is early identification, removal or

248

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

treatment of the inciting cause, and therapy directed at restoration of normal physiologic hemostasis.

References [1] Marshall J. Inflammation, coagulopathy, and the pathogenesis of multiple organ dysfunction syndrome. Crit Care Med 2001;29(7 Suppl):S99–104. [2] Taylor F. The inflammatory–coagulant axis in the host response to gram-negative sepsis: regulatory roles of proteins and inhibitors of tissue factor. New Horiz 1994;2(4):555–65. [3] Bevilacqua M, Pober J, Majeau J, Cotran R, Gimbrone M Jr. Interleukin 1 (IL-1) induces biosynthesis and cell expression of procoagulant activity in human vascular endothelial cells. J Exp Med 1984;160(2):618–23. [4] Pu Q, Wiel E, Corseaux D, Bordet R, Azrin M, et al. Beneficial effect of glycoprotein IIb/ IIIa inhibitor (AZ-1) on endothelium in Escherichia coli endotoxin-induced shock. Crit Care Med 2001;29(6):1181–8. [5] Taylor F, Wada H, Kinasewitz G. Description of compensated and uncompensated disseminated intravascular coagulation (DIC) responses (non-overt and overt DIC) in baboon models of intravenous and intraperitoneal Escherichia coli sepsis and in the human model of endotoxemia: toward a better definition of DIC. Crit Care Med 2000;28(9 Suppl): S12–9. [6] Tsuchiya R, Kyotani K, Scott M, Nishizono K, Ashida Y, et al. Role of platelet activating factor in development of thrombocytopenia and neutropenia in dogs with endotoxemia. Am J Vet Res 1999;60(2):216–21. [7] Dhainaut JF, Marin N, Mignon A, Vinsonneau C. Hepatic response to sepsis: interaction between coagulation and inflammatory processes. Crit Care Med 2001;29(7 Suppl):S42–7. [8] Sheu J, Hung W, Kan Y, Lee Y, Yen M. Mechanisms involved in the antiplatelet activity of Escherichia coli lipopolysaccharide in human platelets. Br J Haemotol 1998;103(1): 29–38. [9] Bates D, Parsonnet J, Ketchum P, Miller E, Novitsky T, et al. Limulus amebocyte lysate assay for detection of endotoxin in patients with sepsis syndrome. Clin Infect Dis 1998; 27(3):582–91. [10] Marshall J, Creery D. Preclinical models of sepsis. Sepsis 1998;2:187–97. [11] Wada H, Wakita Y, Nakase T, Shimura M, Hiyoyama K, et al. Outcome of disseminated intravascular coagulation in relation to the score when treatment was begun. Thromb Haemost 1995;74(3):848–52. [12] Wada H, Sakuragawa N, Mori Y, Takagi N, Nakasaki T, et al. Hemostatic molecular markers before the onset of disseminated intravascular coagulation. Am J Hematol 1999; 60(4):273–8. [13] Rodriquez-Erdman F. Bleeding due to increased intravascular blood coagulation: hemorrhagic syndromes caused by consumption of blood-clotting factors (consumption coagulopathies) [abstract]. N Engl J Med 1965;273:1370. [14] Bick R. Disseminated intravascular coagulation and related syndromes: a clinical review. Semin Thromb Hemost 1988;14(4):299–338. [15] Bick R. Disseminated intravascular coagulation. Objective criteria for diagnosis and management. Med Clin North Am 1994;78(3):511–43. [16] Cartlet J, Taylor F, Levi M, Artigas A, ten Cate H, et al. Clinical expert round table discussion (session 4) at the Margaux Conference on Critical Illness: sepsis: inflammation disorder, coagulation disorder, or both? A challenge for clinicians. Crit Care Med 2001; 29(7 Suppl):S107–8. [17] Steiner P, Lushbaugh C. Maternal pulmonary embolism by amniotic fluid as a cause of shock and unexplained deaths in obstetrics. JAMA 1941;117:1245–50.

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

249

[18] Plewa M, King R, Fenn-Buderer N, Gretzinger K, Renvart D, et al. Hematologic safety of intraosseous blood transfusion in a swine model of pediatric hemorrhagic hypovolemia. Acad Emerg Med 1995;2(9):799–809. [19] Bilgrami S, Cable R, Posciotto P, Rowland F, Greenberg B. Fatal DIC and pulmonary thrombosis following a blood transfusion in a patient with severe autoimmune hemolytic anemia and HIV infection. Transfusion 1994;34(3):248–52. [20] Camerer E, Kolsto A, Prydz H. Cell biology of tissue factor, the principal initiator of blood coagulation. Thromb Haemost 1996;81(1):1–41. [21] Mann K. Biochemistry and physiology of blood coagulation. Thromb Haemost 1999; 82(2):165–74. [22] Osterud B, Bjorklid E. The tissue factor pathway in disseminated intravascular coagulation. Semin Thromb Hemost 2001;27(6):605–15. [23] Bell W. The fibrinolytic system in neoplasia. Semin Thromb Hemost 1996;22(6):459–78. [24] Prentice C. Acquired coagulation disorders. Clin Haematol 1985;14(2):413–21. [25] Chiaretti A, Piastra M, Pulitanos S, Pietrini D, DeRosa G, et al. Prognostic factors and outcome of children with severe head injury: an eight year experience. Childs Nerv Syst 2002;18(3–4):129–36. [26] Gando S. DIC in trauma patients. Semin Thromb Hemost 2001;322(4):222–8. [27] Slappendel R. Disseminated intravascular coagulation. Vet Clin North Am Small Anim Pract 1988;18(1):169–84. [28] Morris D. Recognition and management of disseminated intravascular coagulation in horses. Vet Clin North Am Equine Pract 1988;4(1):115–43. [29] Wright I. The nomenclature of blood clotting factors. Thromb Diath Haemost 1962;7: 381–8. [30] Ratnoff O, Colpy J. A familial hemorrhagic trait associated with a deficiency of a clotpromoting fraction of plasma. J Clin Invest 1955;34:602–13. [31] Duckert F, Jung D, Schmerling D. A hitherto undescribed congenital haemorrhagic diathesis probably due to fibrin stabilizing factor deficiency. Throm Diath Haemorrh 1961; 5:179–81. [32] Osterud B, Rapaport S. Activation of factor IX by the reaction product of tissue factor and factor VII: additional pathway for initiating blood coagulation. Proc Natl Acad Sci USA 1977;74(12):5260–4. [33] Ruf W, Mueller B. Tissue factor signaling. Thromb Haemost 1999;82(2):175–82. [34] Gando S, Kameue T, Morimoto Y, Matsunda N, Hayakawa M, et al. Tissue factor production not balanced by tissue factor pathway inhibition in sepsis promotes poor prognosis. Crit Care Med 2002;30(8):1729–34. [35] Levi M, de Jonge E, van der Pol T, ten Cate H. Advances in the understanding of the pathogenetic pathways of disseminated intravascular coagulation result in more insight in the clinical picture and better management strategies. Semin Thromb Hemost 2001;27(6): 569–75. [36] Osterud B, Bjorklid E. The tissue factor pathway in disseminated coagulation. Semin Thromb Hemost 2001;27(6):605–16. [37] Taylor F. Response of anticoagulant pathways in disseminated intravascular coagulation. Semin Thromb Hemost 2001;27(6):619–32. [38] de Jonge E, Dekkers P, Creasey A, Hack C, Paulson S, et al. Tissue factor pathway inhibitor dose dependently inhibits coagulation activation without influencing the fibrinolytic and cytokine response during human endotoxemia. Blood 2000;95(4): 1124–9. [39] van der Poll T, de Jonge E, Levi M. Regulatory role of cytokines in disseminated intravascular coagulation. Semin Thromb Hemost 2001;27(6):639–51. [40] Wharram B, Fitting K, Kunkel S, Remick D, Merritt S, et al. Tissue factor expression in endothelial cell/monocyte cocultures stimulated by lipopolysaccharide and/or aggregated IgG. Mechanisms of cell:cell communication. J Immunol 1991;146(5):1437–45.

250

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

[41] van der Poll T, Levi M, van Deventer S, ten Cate H, Haagmans B, et al. Differential effects of anti-tumor necrosis factor monoclonal antibodies on systemic inflammatory responses in experimental endotoxemia in chimpanzees. Blood 1994;83(2):446–51. [42] Bernard W, Morris D, Divers T, Ramberg C. Plasma antithrombin III values in clinically normal horses: effect of sex and/or breed. Am J Vet Res 1987;48(5):866–8. [43] Green R. Clinical implications of antithrombin III deficiency in animal diseases. Compend Contin Educ Pract Vet 1984;6(6):537–45. [44] Bernard G, Wesley E, Wright T, Fraiz J, Stasek J, et al. Safety and dose relationship of recombinant human activated protein C for coagulopathy in severe sepsis. Crit Care Med 2001;29(11):2051–7. [45] van der Poll T, Coyle S, Levi M, Jansen P, Denterer M, et al. Effect of a recombinant dimeric tumor necrosis factor receptor on inflammatory responses to intravenous endotoxin in normal humans. Blood 1997;89(10):3727–34. [46] Morris D, Beech J. Disseminated intravascular coagulation in six horses. J Am Vet Med Assoc 1983;10(2):1067–72. [47] Collatos C, Barton M, Prasse K, Moore J. Intravascular and peritoneal coagulation and fibrinolysis in horses with acute gastrointestinal tract diseases. J Am Vet Med Assoc 1995; 207(4):465–70. [48] Pablo L, Purohit R, Teer P, Newton J, Hammond L. Disseminated intravascular strangulation obstruction in ponies. Am J Vet Res 1983;44(1):2115–22. [49] Welch R, Watkins J, Taylor T, Cohen N, Carter G. Disseminated intravascular coagulation associated with colic in 23 horses (1984–1989). J Vet Intern Med 1992;6(1):29–35. [50] Johnstone I, Crane S. Haemostatic abnormalities in horses with colic—their prognostic value. Equine Vet J 1986;18(4):271–4. [51] Prasse K, Topper M, Moore J, Welles E. Analysis of hemostasis in horses with colic. J Am Vet Med Assoc 1993;5(1):685–93. [52] Collatos C, Barton M, Moore J. Fibrinolytic activity in plasma from horses with gastrointestinal diseases: changes associated with diagnosis, surgery and outcome. J Vet Intern Med 1995;9(1):18–23. [53] Dolente B, Wilkins P, Boston R. Clinicopathologic evidence of disseminated intravascular coagulation in horses with acute colitis. J Am Vet Med Assoc 2002;220(7):1034–8. [54] Dallap B, Dolente B, Boston R. Coagulation profiles in 27 horses with large colon volvulus. In: Proceedings of the 11th Annual Veterinary Meeting of the American College of Veterinary Surgeons. Chicago; 2001. p. 4. [55] Barton M, Morris D, Norton N, Prasse K. Hemostatic and fibrinolytic indices in neonatal foals with presumed septicemia. J Vet Intern Med 1998;12(1):26–35. [56] Barton M, Morris D, Crowe N, Collatos C, Prasse K. Hemostatic indices in healthy foals from birth to one month of age. J Vet Diagn Invest 1995;7:380–5. [57] Chuansumrit A, Hotrakitya S, Hathirat P, Isarangkura P. Disseminated intravascular coagulation in children: diagnosis, management and outcome. Southeast Asian J Trop Med 1993;24(1):229–33. [58] Colmon RW. Hemostasis and thrombosis: basic principles and clinical practices. Philadelphia: JB Lippincott; 1982. p. 693–700. [59] Mammen E. Antithrombin: its physiological importance and role in DIC. Semin Thromb Hemost 1998;24(1):19–25. [60] Pecon J, Blackburn M. Pyridoxylation of essential lysines in the heparin-binding site of antithrombin III. J Biol Chem 1984;259(2):935–8. [61] Yamauchi T, Umeda F, Inoguchi T, Nawata H. Antithrombin III stimulates prostacyclin production by cultured aortic endothelial cells. Biochem Biophys Res Commun 1989; 163(3):1404–11. [62] Asakura H, Ontachi Y, Mizutni T, Kato M, Ito T, et al. Decreased plasma activity of antithrombin or protein C is not due to consumption coagulopathy in septic patients with disseminated intravascular coagulation. Eur J Haemtol 2001;67(3):170–5.

B.L. Dallap / Vet Clin Equine 20 (2004) 231–251

251

[63] Holland M, Kelly A, Snyder J, Steffey E, Willits N, et al. Antithrombin III activity in horses with large colon torsion. Am J Vet Res 1986;47(4):897–900. [64] Topper M, Prasse K, Morris M, Duncan A, Crow N. Enzyme-linked immunosorbent assay for thrombin-antithrombin III complexes in horses. Am J Vet Res 1996;57(4): 427–31. [65] Topper M, Prasse K. Use of enzyme-linked immunosorbent assay to measure thrombinantithrombin III complexes in horses with colic. Am J Vet Res 1996;57(4):456–62. [66] Sandholm M, Vidovic A, Puotunen-Reinert A, Sankari S, Nyholm K, et al. D-dimer improves the prognostic value of combined clinical and laboratory data in equine gastrointestinal colic. Acta Vet Scand 1995;36(2):255–72. [67] Stokol T, Brooks M, Erb H, Mauldin G. D-dimer concentrations in healthy dogs and dogs with disseminated intravascular coagulation. Am J Vet Res 2000;61(4):393–8. [68] Pizzo S. Serpin receptor 1: a hepatic receptor that mediates the clearance of antithrombin III-proteinase complexes. Am J Med 1989;87(3 Suppl):B10S–4S. [69] Garcia-Fernandez N, Montes R, Purroy A, et al. Hemostatic disturbances in patients with systemic inflammatory response syndrome (SIRS) and associated acute renal failure (ARF). Thromb Res 2000;100:19–25. [70] Otto C, Reiser T, Brooks M, Russell M. Evidence of hypercoagulability in dogs with parvoviral enteritis. J Am Vet Med Assoc 2000;217(10):1500–4. [71] Durando M, MacKay R, Linda S, Skelley L. Effects of polymixin B and Salmonella typhimurium antiserum on horses given endotoxin intravenously. Am J Vet Res 1994; 55(7):921–7. [72] Schwarzwald C, Fiege K, Wunderli-Allenspach H, Braun B. Comparison of pharmacokinetic variables for two low molecular weight heparins after subcutaneous administration of a single dose to horses. Am J Vet Res 2002;63(6):868–73.