Prevention of Blood Clotting in the Normal Vascular

Prevention of Blood Clotting in the Normal Vascular System-Intravascular Anticoagulants • 1. Surface Factors: most important factors for preventing clotting in normal vascular system: • (1) the smoothness of the endothelial cell surface- prevents contact activation of the intrinsic clotting system; • (2) a layer of glycocalyx on the endothelium (glycocalyx is a mucopolysaccharide adsorbed to the surfaces of the endothelial cells), which repels clotting factors and platelets, • (3) a protein bound with the endothelial membrane, thrombomodulin, which binds thrombin. And the thrombomodulin-thrombin complex also activates a plasma protein, protein C, that acts as an anticoagulant by inactivating activated Factors V and VIII (both activated by thrombin)

Thus • When the endothelial wall is damaged-loss of smoothness and glycocalyx-thrombomodulin • activates both Factor XII and the platelets, thus setting off the intrinsic pathway of clotting • If Factor XII and platelets come in contact with the subendothelial collagen, the activation is even more powerful

2. Antithrombins : Fibrin & Antithrombin III • Among the most important anticoagulants in the blood are those that remove thrombin • The most powerful are: • (1) the fibrin fibers that themselves are formed during the process of clotting and • (2) an alpha-globulin called antithrombin III or antithrombin-heparin cofactor- ie usually activated by heparin

So • During clotting, about 85 -90 % of the thrombin formed is adsorbed to the fibrin fibers; - Helps prevent spread of thrombin into the remaining blood and prevents excessive spread of the clot • Thrombin that does not adsorb to the fibrin fibers soon combines with antithrombin III • Blocks convertion of fibrinogen and also inactivates the thrombin itself during the next 12 to 20 minutes.

3. Heparin. • another powerful anticoagulant, but normal blood conc in low, so has significant anticoagulant effects only under special physiologic conditions • produced by many different cells, but especially the basophilic mast cells located in the pericapillary connective tissue throughout the body • The basophils also release small quantities of heparin • Mast cells abundant in tissue surrounding the capillaries of the lungs and to a lesser extent capillaries of the liver. • Easy to understand cos large quantities are needed in these areas because the capillaries of lungs & liver receive many embolic clots formed in slowly flowing venous blood

Uses • Alone, little/no anticoagulant properties, but combined with antithrombin III, effectiveness of antithrombin III by more than a hundredfold • Thus in the presence of excess heparin, removal of free thrombin from the circulating blood by antithrombin III is almost instantaneous • used widely as a pharmacological agent in medical practice to prevent intravascular clotting. • The complex of heparin and antithrombin III also removes activated Factors XII, X, and IX.

Conditions That Cause Excessive Bleeding • can result from deficiency of any one of the many blood-clotting factors. • Three types of that have been studied to the greatest extent are discussed here: • (1)Liver disease & vitamin K deficiency, • (2) Deficiency of Factor VIII-v. WF complex • (3) thrombocytopenia (platelet deficiency).

Liver disease &Vitamin K Deficiency • Almost all the blood-clotting factors are formed by the liver. • Vit K needed for: prothrombin, Factors VII, IX, X & protein C. • Deficiency & subsequent insufficiency of these coagulation factors can lead to serious bleeding tendencies. • Vitamin K continually synthesized in the intestinal tract by bacteria, so that vitamin K deficiency seldom occurs in the normal person as a result of vitamin K absence from the diet (except in neonates before they establish their intestinal bacterial flora). • However, in GI disease, deficiency can occurs as a result of poor absorption of fats from the gastrointestinal tract • Therefore, diseases of the liver such as hepatitis, cirrhosis, etc can sometimes depress the clotting system

Vit K deficiency • Vitamin K is fat-soluble absorbed along with the fats. • Another cause of vitamin K deficiency is failure of the liver to secrete bile into the GIT (bile ducts obstruction / liver disease) • Because of this, vitamin K is injected into all surgical patients with liver disease or with obstructed bile ducts before performing the surgical procedure. • If given to a deficient patient 4 to 8 hours before the operation and the liver parenchymal cells are at least one-half normal in function, sufficient clotting factors will be produced to prevent excessive bleeding during the operation.

Deficiency of Factor VIII-v. WF complex • Complex made up of 2 active components, a large component with a molecular weight and a smaller component with a molecular weight of about 230, 000. • The smaller component is Factor VIII, activates factor X in the intrinsic pathway, and deficiency causes classic hemophilia. • Another bleeding disease with somewhat different characteristics called von Willebrand's disease results from loss of the large component.

Hemophilia • 85% are hemophilia A or classic hemophilia caused by an abnormality/deficiency of Factor VIII • In 15% by deficiency of Factor IX- Hemophilia B/Christmas disease (named after 1 st patient) • Both inherited as X-linked recessive disease- so rare in females • If one X chromosomes is deficient- hemophilia carrier, transmitting dx to some male offsprings & the carrier state to females • Bleeding usually does not occur except after trauma, but degree of trauma required to cause severe/prolonged bleeding maybe so mild & hardly noticeable. Eg, bleeding for days after tooth extraction • Effective therapy is injection of purified Factor VIII. • Cost is high & availability limited cos it can be gathered only from human blood and only in extremely small quantities

Von Willibrand’s disease • Though synthesized differently from VIII, both come together in plasma (endothelial cells/platelets) • Factor serves mainly in platelet adhesion to subendothelial collagen but also carries factor VIII • Autosomal dominant disorder, though rare recessive variants now identified • Spontaneous bleeding from mucous membrane, excessive bleeding from joints, menorrhagia and prolonged bleeding time with normal platelet count • Bleeding into joints uncommon like in hemophilia

Thrombocytopenia • small punctate hemorrhages occur throughout all the body tissues • The skin of such a person displays many small, purplish blotches, giving the disease the name thrombocytopenic purpura. • Levels as low as 10, 000/l are frequently lethal. • Even without making specific platelet counts in the blood, suspect thrombocytopenia if blood fails to retract • Most known as idiopathic thrombocytopenia • Treated by giving fresh whole blood transfusions and splenectomy is often helpful

Disseminated Intravascular Coagulationconsumption coagulopathy • Activation of clotting mechanism in widespread areas of the circulation • Often results from presence of large amounts of traumatized/ dying tissue in the body - releases great quantities of tissue factor • Clots are small but numerous, and plug a large share of the small peripheral blood vessels- diminishes O 2 & nutrients delivery a situation that leads to or exacerbates circulatory shock. • Patient begins to bleed cos many factors are removed by widespread clotting , so few procoagulants remain for normal hemostasis • Seen in widespread septicemia, circulating bacteria or bacterial toxins -especially endotoxins-activate the clotting mechanisms • Plugging of small peripheral vessels • It is partly for this reason that septicemic shock is lethal in more than 85% patients.

Thrombus or an embolus • A clot inside a blood vessel = thrombus • inappropriate activation of the hemostatic process in an uninjured or slightly injured vessel. • in a large blood vessel will decrease blood flow through that vessel (termed a mural thrombus). • In small vessel, blood flow maybe cut-off (an occlusive thrombus) resulting in death of tissue • Can dislodge, becomes free-floating = an embolus. • People with hypercoagulable states have increased risk for blood clots developing in arteries & veins • From venous system or right side of the heart generally flow into lungs to cause pulmonary arterial embolism • Arterial clots can increase risk for stroke, heart attack

Causes • (1) Any roughened endothelial surface of a vessel-as may be caused by arteriosclerosis, infection, or trauma • (2) when blood flows very slowly through blood vessels, - small quantities of thrombin and other procoagulants are always being formed

Risk factors • Different risk factors or events egs: • Immobility (including prolonged inactivity, long trips by plane or car) • Pregnancy • Obesity • Smoking, Oral contraceptives, certain cancers, Trauma, certain surgeries, Age (increased risk for people over age 60)

Some current treatments • Anticoagulants – medicine that prevents clots from forming • Clot busters – medicine that dissolves blood clots • Catheter-directed thrombolysis – a procedure in which a long tube, called a catheter, is surgically inserted and directed toward the blood clot where it delivers clot-dissolving medication • Thrombectomy – surgical removal of a clot

Use of t-PA in Treating Intravascular Clots • Genetically engineered t-PA (tissue plasminogen activator) is available. • If delivered directly to a thrombosed area through a catheter, effective in activating plasminogen to plasmin, which dissolves some intravascular clots. • Eg, if used within the first hour after thrombotic occlusion of a coronary artery, the heart is often spared serious damage

Anticoagulants for Clinical Use • The most useful clinically are heparin and the coumarins • Heparin as an Intravenous • Extracted from several different animal tissues and prepared in almost pure form • Injection of relatively small quantities, causes clotting time to increase instantaneously, from about 6 to 30 min or more • immediately preventing or slowing further development of a thromboembolic condition. • The action lasts about 1. 5 to 4 hours and is destroyed by an enzyme in the blood known as heparinase

Coumarins e. g. warfarin • blocks the action of vitamin K by competing with Vit K for reactive sites in the enzymatic processes formation of prothrombin and the other clotting factors • When given, plasma levels of prothrombin and Factors VII, IX, and X (all liver formed) • The coagulation process is not blocked immediately but must await the natural consumption of the prothrombin and the other affected coagulation factors already present in the plasma. • Normal coagulation usually returns 1 to 3 days after discontinuing coumarin therapy.

Prevention of Blood Coagulation Outside the Body • Although blood removed from the body and held in a glass test tube normally clots in about 6 minutes, blood collected in siliconized containers often does not clot for 1 hour or more. • The reason for this delay is that preparing the surfaces of the containers with silicone prevents contact activation of platelets and Factor XII, the two principal factors that initiate the intrinsic clotting mechanism. • Conversely, untreated glass containers allow contact activation of the platelets and Factor XII, with rapid development of clots. • Heparin can be used for preventing coagulation of blood outside the body as well as in the body. especially used in surgical procedures in which the blood must be passed through a heart-lung machine or artificial kidney machine and then back into the person.

Substances that decrease Ca+ concentration • oxalate compound: causes precipitation of calcium oxalate from the plasma and thereby decreases the ionic calcium level • negatively charged citrate ion: deionizes the blood calcium to prevent coagulation. Common ones are in the form of sodium, ammonium, or potassium citrate

Citrates used more • because oxalate is toxic to the body • After injection, the citrate ion is removed from the blood within a few minutes by the liver and is polymerized into glucose or metabolized directly for energy. • But if the liver is damaged or if large quantities of citrated blood or plasma are given too rapidly the citrate ion may not be removed quickly enough, and the citrate can, under these conditions, greatly depress the level of calcium ion

Blood Coagulation Tests 1. Bleeding Time • When a sharp-pointed knife is used to pierce the tip of the finger or lobe of the ear, bleeding ordinarily lasts for 1 to 6 minutes • Time depends largely on the depth of the wound and the degree of hyperemia in the finger or ear lobe at the time of the test. • Lack of any one of several of the clotting factors can prolong the bleeding time, but it is especially prolonged by lack of platelets

2. Clotting Time • Many methods, one most widely used is to collect blood in a chemically clean glass test tube and then to tip the tube back and forth about every 30 sec until the blood clots • By this method, normal clotting time is 6 to 10 mins. • Procedures using multiple test tubes have also been devised for determining clotting time more accurately • But, clotting time varies widely, depending on method used, so it is no longer used in many clinics • Instead, measurements of the clotting factors themselves are made, using sophisticated chemical procedures.

Prothrombin Time and others • Prothrombin time gives an indication of the concentration of prothrombin in the blood • Blood is immediately oxalated so that none of the prothrombin change into thrombin • Then, excess of calcium ion and tissue factor is quickly mixed with the oxalated blood • The excess calcium nullifies the effect of the oxalate, and the tissue factor activates the prothrombin-to-thrombin reaction by means of the extrinsic clotting pathway • The time required for coagulation is determined mainly by prothrombin concentration is known as the prothrombin time. • The normal time is about 12 seconds • Similar tests have been devised to determine the quantities of other blood clotting factors

Hypercoagulability in pregnancy • The propensity of pregnant women to develop thrombosis • Physiologically adaptive mechanism to prevent post partum bleeding • However, when combined with an additional underlying hypercoagulable states, the risk of thrombosis or embolism may become substantial

Pregnancy & clotting • Changes plasma levels of many clotting factors • fibrinogen & thrombin levels increase, but the other major anticoagulants, protein C and antithrombin III, remain constant. • Fibrinolysis impaired by an increase plasminogen activator inhibitor-2 (PAI-2) from the placenta. • Venous stasis may occur at the end of 1 st trimester due to enhanced compliance of vessel walls by a hormonal effect • Also can cause hypercoagulability by other factors e. g. post partum prolonged bed rest in case of delivery by forceps or C/S

During menstrual cycle • Lining of uterus thickens to get ready for pregnancy. Then, during your period, your body sheds the uterus lining along with blood • The amount lost is usually 4 &12 teaspoons/cycle • The body typically has anticoagulants to keep menstrual blood liquid, but when your period is heavy and blood rapidly expelled, there's not enough time for anticoagulants to work- enables clots to form. • Temporary thick, heavy flow isn't cause for concern.