3 CE CREDITS CE Article 2 Hemostasis At a Glance ❯❯ Cody Alcott, DVM ❯❯ Charles Brockus, DVM, ❯❯ Brett Sponseller, DVM, Response to initial David M. Wong, DVM, PhD, DACVIM, DACVP PhD, DACVIM MS, DACVIM Charles River Labs Iowa State University vascular injury Sparks, Nevada Page 78 Iowa State University Platelet plug formation Abstract: Hemostasis, the coagulation cascade, and clot formation present a daunting list of and primary hemostasis Page 79 factors, pathways, and interactions to equine clinicians. A basic knowledge of hemostasis is necessary to evaluate various disease processes in horses. Initial injury to the vascular en- Endothelial cell reactions Page 80 dothelium results in local vasoconstriction and formation of a platelet plug. This initial response reduces blood loss from the damaged vessel but may be inadequate at maintaining hemostasis Secondary hemostasis alone. Therefore, the intrinsic, extrinsic, and common coagulation pathways interact with one Page 80 another to form thrombin and ultimately stabilize the platelet plug. Thrombin accomplishes this The coagulation cascade: by converting soluble fibrinogen to insoluble fibrin, resulting in clot stabilization. Alternatively, the extrinsic pathway fibrinolysis promotes resolution of clot formation. Because of multiple positive and negative Page 80 feedback interactions as well as multiple circulating mediators and inhibitors, coagulation and The coagulation cascade: fibrinolysis are finely controlled systems in healthy patients. However, perturbations of the the intrinsic pathway coagulation cascade in disease states can result in severe complications or death. Page 81 Common pathway lterations in coagulation and Response to Initial Vascular Injury Page 81 fibrinolysis are relatively uncom- The main function of the coagulation sys- Clot formation Amon in horses, but when they tem is to control hemorrhage from the Page 82 occur, especially in critically ill patients, vascular endothelium when it has been T h r o m b i n general knowledge of the associated damaged by trauma or inflammation. Page 82 pathways is vital to equine clinicians to Conversely, anticoagulants and fibrinolytic Clot prevention properly diagnose, monitor, and treat mechanisms maintain patency and ade- Page 82 primary or secondary disease processes quate blood flow. The regulation of these involving these systems (TABLE 1 lists mechanisms is a tightly knit system of local Fibrinolysis coagulation factor synonyms). Clinicians and systemic feedback control pathways Page 83 must also be able to interpret and under- that maintains a delicate balance between Diagnostic testing stand the available diagnostic tests used pro- and anticoagulant processes. of coagulation to evaluate these systems. This article Hemostasis is defined as the arrest of Page 83 reviews coagulation, fibrinolysis, and bleeding by vasoconstriction and coagu- Adult versus neonatal diagnostic testing to evaluate these sys- lation and is an essential part of normal differences in diagnostic tems in horses. functioning of the cardiovascular system. tests Page 86 FIGURE 1 Primary and secondary hemostasis. (A) Initial vessel injury results in local vasocon- striction via reflex myogenic spasm and release of endothelin and thromboxane. (B) Formation of the platelet plug results from platelet adhesion, activation, recruitment, and aggregation. (vWF = von Willebrand’s factor) 78 Compendium Equine: Continuing Education for Veterinarians® | March 2009 | CompendiumEquine.com FREE Hemostasis CE TABLE 1 Coagulation Factor Synonyms and initiator of platelet adherence and forma- tion of the platelet plug, known as primary hemostasis. Likewise, damaged endothelium FACTOR SYNONYM activates the extrinsic and intrinsic coagula- I Fibrinogen tion enzyme cascades via tissue factor and collagen fiber exposure. The direct result of II Prothrombin activating either or both coagulation cascades III Tissue factor, thromboplastin is conversion of prothrombin to thrombin; this subsequently results in the conversion of IV Calcium soluble fibrinogen to insoluble fibrin and sta- V Proaccelerin, labile factor bilization of the clot (secondary hemostasis). Platelet adherence and activation combined VI — with activation of the coagulation cascades result in a blood clot or thrombus formation to VII Proconvertin, stable factor stop hemorrhage. VIII Antihemophilic factor Platelet Plug Formation IX Christmas factor and Primary Hemostasis X Stuart-Prower factor Essential to progression of the clotting pro- cess is the development of a platelet plug via XI Plasma thromboplastin antecedent platelet adhesion, activation, recruitment, and aggregation (FIGURE 1). Platelets are circulating XII Hageman factor c y topla sm ic fr ag ment s of megak ar yoc y te pseu- XIII Fibrin-stabilizing factor, transglutaminase dopods. They contain actin–myosin molecules and thrombosthenin for platelet contraction, an endoplasmic reticulum and Golgi apparatus Hemostasis is usually initiated by vascular for enzyme production and calcium storage, trauma but can be induced by inflammation and a variety of enzymatic systems and secre- or sepsis.1 Smooth muscle contraction marks tory granules.2 Glycoproteins on the platelet the beginning of the hemostatic process surface and within platelet granules help medi- and begins instantaneously after endothe- ate adherence to the vascular endothelium and lial damage (FIGURE 1). Myogenic spasm of serve as a means of communication and acti- smooth muscle is followed by endothelial and vation among platelets. After vascular injury, platelet release of autocoid vasoconstrictive adherence is initiated largely by the binding substances—endothelin and thromboxane, of platelets to exposed subendothelial matrix respectively.2,3 Further vasoconstriction caused via von Willebrand’s factor (vWF).4 Fibrinogen by sensory nerve impulses results in a neuro- binding to platelet glycoprotein complexes genic contractile reflex of smooth muscle.2 enhances the activation of additional plate- Damaged endothelium serves as a mediator lets.5 Platelet aggregation ensues with further (C) Secondary hemostasis involves activation of the intrinsic and extrinsic path- ways, culminating in the activation of prothrombin to thrombin via the common pathway of coagulation. (D) Clot stabilization occurs via fibrin fiber formation within the clot and subsequent clot stabilization by fibrin stabiliz- ing factor (XIII). CompendiumEquine.com | March 2009 | Compendium Equine: Continuing Education for Veterinarians® 79 FREE CE Hemostasis platelet activation, forming the platelet plug. lial damage lead to exposure of subendothelial Prothrombin attaches to platelet surface recep- matrix, which is largely composed of collagen. tors as well, accumulating within the newly The combined endothelial expression of vWF formed platelet plug. Platelet activation results and tissue factor, in addition to subendothelial in platelet swelling and the development of collagen exposure, activates the coagulation membrane surface pseudopods containing cascade and supports platelet adhesion and alpha granules and dense bodies. Alpha gran- activation.5 vWF mediates the adhesion of plate- ules contain substances essential to hemostasis, lets to exposed collagen fibrils while stabilizing including adhesive proteins for vWF, fibrinogen, factor VIII; the latter plays a role in secondary plasminogen, fibronectin, and thrombospondin; hemostasis. Tissue factor initiates secondary plasma proteins IgG and albumin; cellular mito- hemostasis via the extrinsic pathway. Collagen gens of platelet-derived growth factor; coagula- exposure simultaneously causes a change in tion factors V and VIII; and protease inhibitors configuration of the proteolytic enzyme factor 6 α2-macroglobulin and α2-antiplasmin. Dense XII, which signals the activation of the intrinsic bodies activate and recruit locally circulating pathway. In summary, the endothelium plays platelets via release of ADP, ATP, serotonin, a key role in initiating, amplifying, and modu- ionized calcium, histamine, epinephrine, and lating the coagulation process in response to pyrophosphate.7 Fibronectin and thrombo- many conditions, including trauma, inflamma- spondin help reinforce and stabilize aggre- tion, sepsis, and endotoxemia.9,10 gated platelets. Platelet-derived growth factor increases smooth muscle growth while con- Secondary Hemostasis trolling tissue repair. These factors facilitate Secondary hemostasis begins with the activa- and regulate the continual adherence, activa- tion of the extrinsic and intrinsic pathways tion, and aggregation of platelets until a loose immediately after vessel damage. These path- but reversible plug forms. Further platelet plug ways converge to form prothrombin activator, stabilization is attained through the activation which catalyzes the formation of thrombin of coagulation pathways and the generation of from prothrombin. Thrombin converts soluble CriticalPo nt thrombin. fibrinogen to insoluble fibrin fibers that inter- The hemostatic pro- act with platelets, blood cells, and plasma for Endothelial Cell Reactions final formation of a stable clot. cess begins imme- Endothelial cells are responsible for initiating diately following coagulation in response to local vascular injury The Coagulation Cascade: vascular endothelial or systemic inflammation.8 Intact endothelium the Extrinsic Pathway damage. maintains normal function and limits activation of Activation of the extrinsic pathway occurs via coagulation by maintaining the