Physiology, Hemostasis

Issues of Concern

Hyper-coagulation. The hemostatic cascade is meant to control hemorrhage and be a protective mechanism. At times, this process is triggered inadvertently while the blood is within the lumen of the blood vessel and without any bleeding.[1] This situation leads to a pathologic phenomenon of thrombosis, which can have catastrophic complications by obstructing blood flow leading to ischemia and even infarction of the tissues supplied by the occluded blood vessels. In this way, a physiologic process becomes a pathologic process leading to morbidity and/or mortality. Some of the examples include Antiphospholipid antibody syndrome, Factor 5 Leiden mutation, Protein C deficiency, protein S deficiency, Prothrombin gene mutation, etc.

Hypo-coagulation. When there is any defect in the functionality of any component of this hemostatic cascade, it can lead to ineffective hemostasis and inability to control hemorrhage; this can lead to severe blood loss, hemorrhage and also complications that can hence ensue due to the inhibited blood supply to vital organs. Some of the examples include Von Willebrand disease, hemophilia, disseminated intravascular coagulation, deficiency of the clotting factors, platelet disorders, collagen vascular disorders, etc.

Iatrogenic Coagulopathy. Medicine is currently in the era of widespread use of antiplatelet agents like aspirin, clopidogrel, ticagrelor and anticoagulants like warfarin, heparin, low molecular weight heparin, rivaroxaban, apixaban, dabigatran, fondaparinux amongst others for various commonly encountered clinical conditions like cardiac stenting/ percutaneous coronary intervention, atrial fibrillation, deep venous thrombosis, pulmonary embolism, and many more. The way these medications affect the functionality of the various components of clotting cascade can help patients with their clinical conditions. However, it can lead to bleeding/thrombosis in cases of inappropriate dosage, non-compliance, medication interactions, and result in significant morbidity and mortality. 

Cellular Level

There are various cellular components in the process of coagulation. Most notably are those processes associated with the endothelium, platelets, and hepatocytes.

Endothelium. Clotting factors III and VIII originate from the endothelial cells while the clotting factor IV comes from the plasma.[2][3] Factor III, IV, and VIII all undergo K dependent gamma-carboxylation of their glutamic acid residues, which allows for binding with calcium and other ions while in the coagulation pathway.[4]

Platelets. These are non-nucleated disc-like cells created from megakaryocytes that arise from the bone marrow. They are about 2 to 3 microns in size. Some of their unique structural elements include plasma membrane, open canalicular system, spectrin and actin cytoskeleton, microtubules, mitochondria, lysosomes, granules, and peroxisomes.[5] These cells release proteins involved in clotting and platelet aggregation.

Hepatocytes. The liver produces the majority of the proteins that function as clotting factors and as anticoagulants.

Development

Embryology. The development of the coagulation system begins in the fetus. The various clotting factors and the coagulation proteins initially get expressed in the endothelial cells during early gestation. They usually are undetectable in the plasma until after the first trimester. There is a transient gap in the development and maturation of the hemostatic proteins from the early second trimester until term due to some unclear mechanisms. Due to the similar structure and functionality of the hemostatic coagulative proteins in the fetus and the similarity in the platelet expression, there is a rarity in the occurrence of any thrombotic/hemorrhagic complications in the healthy fetus unless there is any form of uteroplacental insufficiency due to any maternal or fetal factors.[6]

Organ Systems Involved

The physiology of hemostasis involves the:

• Vasculature
• Liver
• Bone marrow

All of these systems help with the production of the clotting factors, vitamins, and cells for appropriate functionality of hemostasis.

Function

Hemodynamic Stability. Under normal circumstances, there exists a fine balance between the procoagulant and anticoagulant pathway. This mechanism ensures control of hemorrhage as needed and cessation of pro-coagulant pathway activation beyond the injury site/or without any bleeding. When this equilibrium becomes compromised under any condition, this may lead to thrombotic/bleeding complications.[4] The hemostatic system also helps in wound healing.

Cardiovascular System. PGA1 and PGA2 cause peripheral arteriolar dilation. Prostacyclin produces vasodilation, and thromboxane A2 causes vasoconstriction. Prostacyclin inhibits platelet aggregation and produces vasodilation whereas thromboxane A2 and endoperoxides promote platelet aggregation and cause vasoconstriction. The balance between the prostacyclin and thrombox­ane A2 determines the degree of platelet plug forma­tion. Thus, prostaglandins greatly influence temporary hemostasis. 

Mechanism

Vaso Constriction. Within about 30 minutes of damage/trauma to the blood vessels, vascular spasm ensues, which leads to vasoconstriction. At the site of the disrupted endothelial lining, the extracellular matrix (ECM)/ collagen becomes exposed to the blood components.[7]

Platelet Adhesion. This ECM releases cytokines and inflammatory markers that lead to adhesion of the platelets and their aggregation at that site which leads to the formation of a platelet plug and sealing of the defect. The platelet adhesion is a complex process mediated by interactions between various receptors and proteins including tyrosine kinase receptors, glycoprotein receptors, other G-protein receptors as well as the von Willebrand Factor (vWF). The von Willebrand Factor functions via binding to the Gp 1b-9 within the platelets.[7]

Platelet Activation. The platelets that have adhered undergo very specific changes. They release their cytoplasmic granules that include ADP, thromboxane A2, serotonin, and multiple other activation factors. They also undergo a transformation of their shape into a pseudopodal shape which in-turn leads to release reactions of various chemokines. P2Y1 receptors help in the conformational changes in platelets.[7]

Platelet Aggregation. With the mechanisms mentioned above, various platelets are activated, adhered to each other and the damaged endothelial surface leading to the formation of a primary platelet plug.

Extrinsic Pathway. The tissue factor binds to factor VII and activates it. The activated factor VII (factor VIIa) further activates factor X and factor IX via proteolysis. Activated factor IX (factor IXa) binds with its cofactor – activated factor VIII (factor VIIIa), which leads to the activation of factor X (factor Xa). Factor Xa binds to activated factor V (factor Va) and calcium and generates a prothrombinase complex that cleaves the prothrombin into thrombin.[4]

Intrinsic Pathway. With thrombin production, there occurs conversion of factor XI to activated factor XI (factor XIa). Factor XIa with activated factor VII and tissue factor converts factor IX to activated factor IX (factor IXa). The activated factor IX combines with activated factor VIII (factor VIIIa) and activates factor X. Activated factor X (factor Xa) binds with activated factor V (factor Va) and converts prothrombin to thrombin. Thrombin acts as a cofactor and catalysis and enhances the bioactivity of many of the aforementioned proteolytic pathways.[4]

Fibrin Clot Formation. The final steps in the coagulation cascade involve the conversion of fibrinogen to fibrin monomers which polymerizes and forms fibrin polymer mesh and result in a cross-linked fibrin clot.  This reaction is catalyzed by activated factor XIII (factor XIIIa) that stimulates the lysine and the glutamic acid side chains causing cross-linking of the fibrin molecules and formation of a stabilized clot.

Clot Resolution (Tertiary Hemostasis). Activated platelets contract their internal actin and myosin fibrils in their cytoskeleton, which leads to shrinkage of the clot volume. Plasminogen then activates to plasmin, which promotes lysis of the fibrin clot; this restores the flow of blood in the damaged/obstructed blood vessels.[4]

Related Testing

Indications. The assessment of platelet function as well as its dysfunction has become vital in the current era in multiple clinical scenarios; several examples are:

• For patients with clotting or bleeding disorders

• For patients after cardiac stenting or stroke to monitor the activity of the antiplatelet agents

• For perioperative evaluation.

Platelet Specific. Various tests have undergone development for platelet testing; they include[8][9]:

• Bleeding time (BT)

• Light transmission platelet aggregation

• Impedance platelet aggregation

• Global thrombosis test

• PFA-100/200

• VerifyNow system

• Thromboelastography (TEG)

• Flow cytometric analysis of platelet function

Coagulation Cascade Specific. There has been the development of various tests that evaluate specific events in the coagulation cascade.

• They help in the determination of where the deficiency exists in the intrinsic, extrinsic, or the final common pathways as well as identification of qualitative or quantitative defects of the specific clotting factors.

• Prothrombin time, developed in 1935, assesses the extrinsic and common coagulation cascade function.

• Activated partial thromboplastin time assesses the intrinsic and the common pathways of coagulation. 

• Thrombin time evaluates the formation of fibrin in the final common pathway of coagulation.

• The reptilase time and the various fibrinogen assays assess the fibrin formation step.

• Mixing studies, factor activity assays, and factor inhibitor assays are special tests for further evaluation of the presence of inhibitors or antibodies as well as deficiency of factors. 

Pathophysiology

General Principle. The Virchow’s triad of hypercoagulability, vascular stasis, and vascular trauma, described in 1856, remains a true predictor of thrombosis.

• Hypercoagulability

• Stasis

• Trauma

Etiologies. The physiology of coagulation undergoes alteration due to various factors, including:

• Genetics

• Medications

• Procoagulant

• Anticoagulation defects of the coagulation cascade

• Quantitative defects of the integral components of the coagulation

• Qualitative defects of the integral components of coagulation.

Clinical Presentations. With the altering of hemostatic physiology, various clinical outcomes including:

• Pulmonary embolism

• Deep vein thrombosis

• Stroke

• Myocardial infarction

Coagulopathies. Few of the disorders of coagulation include:

• Anti-thrombin 3 deficiency,

• Protein C deficiency,

• Hyperhomocysteinemia,

• Anti-phospholipid antibody syndrome

Risk Factors. Some acquired factors influencing the coagulation include[10]:

• Pregnancy

• Trauma

• Malignancy-related hypercoagulable state

• Hormone replacement therapy

• Inflammation

• Infection

• Heparin-induced thrombocytopenia

Clinical Significance

As discussed above, there are various hypercoagulable and hypercoagulable conditions resulting from defects in the coagulation pathways. The full extent is beyond the scope of this topic. Here are several examples:

• Cardiovascular. There has been increased incidence of bleeding while on antiplatelet agents and anticoagulant agents for recent myocardial infarction, stroke, cardiac stents, peripheral vascular stenting, atrial fibrillation, pulmonary embolism, deep venous thrombosis as well as many other conditions; this has led to the development and use of reversal agents.

• Renal. Pathological conditions like end-stage renal disease can lead to uremic platelet dysfunction which can be corrected with dialysis and renal replacement therapy.

• Immunological. Replenishing the deficient clotting factors, removing the antibodies against the clotting factors, use of medications to enhance or ameliorate functionality of the clotting cascade- these newer developments have led to significant advances in the field of medicine and provided treatment options for various challenging to manage clinical scenarios. Transfusion of blood products such as packed red blood cells, platelets, and clotting factors aid further in management. Prothrombin complex concentrate and other formulations are available to replace the deficient clotting factors.

• Pharmacological. Prudent use of the antiplatelet agents such as aspirin, clopidogrel, prasugrel, ticagrelor as well as the anticoagulant agents such as unfractionated heparin, low molecular weight heparin, fondaparinux, warfarin, rivaroxaban, apixaban, dabigatran, argatroban, lepirudin, as well as use of vitamin K, transfusion of blood products and specific modalities like hemodialysis, plasmapheresis and others are recommended as indicated for the management of various hemostatic disorders and can enhance patient care and improve clinical endpoints significantly.