Thrombosis and Anticoagulant Therapy

Thrombosis results when the balance between maintenance of hemostasis and blood fluidity is upset, such that the tendency to clot exceeds the protective effect endogenous anticoagulant mechanisms

Virchow’s Triad (1) endothelial injury, (2) abnormal blood flow, and (3) hypercoagulability predispose to thrombus formation.

Mechanical factors: Turbulent flow

(1) disrupts laminar flow and brings platelets into contact with endothelium
(2) prevents dilution of activated clotting factors
(3) retards the inflow of clotting factor inhibitors
(4) promotes endothelial cell activation

Endothelial injury, regardless of the cause, exposes subendothelial collagen, causes platelets to adhere, exposes tissue factor, and results in a local depletion of prostacycline and plasminogen activators.

Abnormalities of blood: Platelet activation, in thrombosis, platelet destruction results in release of vasoactive and vaso-damaging substances (5-HT, Epi, TxA2, b -thromboglobulin, lysosomal enzymes, PF-4).

Coagulation abnormalities

(1) Ý of coagulation factors - V, VII, VIII, X are seen in thrombosis, pregnancy, hyperthyroidism,

(2) inappropriate activation (patients with liver disease can’t clear activated factors)

(3) deficiency or resistance to naturally occurring protease inhibitors

(4) deficient fibrinolysis

Protective mechanisms include: intact endothelium (prostacycline, NO, tPA. Heparin sulfate, Thrombomodulin), endogenous coagulation inhibitors (ATIII, APC), Dilution of activated clotting factors and disruption of platelet aggregates by blood flow, and clearance of activated coagulation factors by the liver.

Þ APC binds to V and VIII. Leiden mutation doesn’t allow APC to bind to V.

Þ C4b binding protein binds Protein S, free PS is needed for APC. Ý C4b BP in inflammatory conditions (ß free PS)

Most thrombotic events are due to stasis. Often in the venous circulation of the LEs following surgery or bed rest.

Acquired forms of unwanted clotting occur as with antiphospholipid Abs (lupus anticoagulants), in the presence of intravascular devices (prosthetic heart valves, venous catheters), in patients with certain types of malignancies, and DIC.

The genetically determined tendency to thrombosis is known as inherited thrombophilia.

Arterial thrombi consist of platelet masses and fibrin "white thrombus". In venous thrombosis, vessel wall is nl.

Venous thrombi occur where blood flow in sluggish, and consist of large amount of fibrin with numerous red cells "red thrombus". They cause obstructed blood flow and emboli.

Thromboembolic disorders

(1) Hereditary Hypercoagulable Conditions

Antithrombin III: most severe deficiency, also the most rare. The most important naturally occurring anticoagulant whose major function is to inhibit thrombin and F Xa. The rate of thrombin inhibition by ATIII is increased by heparin and forms the basis for the therapeutic action of heparin. ATIII deficiency is manifested by venous thromboses and PE, occurring in adults, frequently during pregnancy or after a surgery. Autosomal dominant inheritance pattern. Thrombotic risk in ATIII deficiency is > PC deficiency or PS deficiency (esp. in pregnancy). Thrombosis before 25 y/o in 50% of patients. ATIII levels from 20- 70% by functional assay with concordant reduction by immunoassay.

Protein C deficiency: PC is a Vit K zymogen, when activated by the thrombin-thrombomodulin complex, degrades Va and VIIa on the platelet and endothelial cell surface, and also stimulates fibrinolysis. ~50% of PC deficient family members will have their 1st thrombotic event by age 40. The risk of venous thrombosis is 8-10-fold higher than the general population. In heterozygotes, PC levels average 50%. Homozygotes have a much higher risk of thrombosis, including purpura fulminans shortly after birth and warfarin-induced skin necrosis.

Protein S deficiency: PS is a Vit K protein, serves as the cofactor for APC. PS circulates as free and bound forms. The level of free PS (the active molecule) is determined by a binding protein, C4b which increases in inflammation.

Activated Protein C resistance (APRC): Factor Va is a cofactor in the prothrombinase complex which converts prothrombin to thrombin. It is inactivated by APC which cleaves Va at Arg506.

A single point mutation was discovered in the factor V gene, resulting in a conversion of Arg506 to Gln Factor V Leiden. This mutation prevents the cleavage (and therefore inactivation) of factor Va at this site by APC.

APC-R is present in 20% of consecutive patients with DVT, and in 50% of selected patients. APC-R is thus most important cause of hereditary thrombophilia. Approximately 6% of the population have the mutation. ~25% of those with APC-R have a thrombotic event by age 50. 10-fold Ý risk of thrombosis for homozygous patients than heterozygotes.

Þ APC sensitivity ratio: aPPT (with APC added) / aPPT (without added APC) If lower than control you’ve demonstrated APC resistance, confirm with genetic test.

Others: Dysfibrinogenemia, Homocystinuria, and Possibly heparin cofactor II deficiency, Plasminogen deficiency, Ý PAI, Ý a -2-antiplasmin, or a -2-macroglobulin deficiency

Prevalence of hereditary disorder ~30% in pts with (1) positive family history of thrombosis (2) history of thrombosis and (3) 1st event <40 y/o.

(2) Acquired Hypercoagulable Conditions

Atherosclerosis, Vasculitis, artificial surfaces, stasis, Hyperviscosity, Malignancy (Migratory superficial thrombophlebitis a.k.a. Trousseau's syndrome), Nonbacterial thrombotic endocarditis, DIC, or chemotherapy-related, Oral contraceptives, Nephrotic syndrome, Trauma, surgery (immobilization, reduced fibrinolytic activity, DIC), Pregnancy, Heparin-induced thrombocytopenia, Paroxysmal nocturnal hemoglobinuria, TTP, and Myeloproliferative disorders.

Lupus Anticoagulant (LAC) a.k.a. Antiphospholipid Syndrome: development of anti-PL Abs associated with venous and arterial thrombosis, recurrent fetal loss, and thrombocytopenia. LAC is an Ab (IgG or IgM) that prolongs PL-dependent coagulation tests Ý PT and PTT without a history of bleeding. Dilute Russell viper venom time which reflects Xa activity is Ý but corrects with added PL. Occurs in the absence of SLE, can be drug-induced (procainamide, phenothiazines) or due to HIV. Ý anticardiolipin Ab titers. LAC is a thrombotic condition although surgical bleeding can occur if there is associated prothrombin deficiency or thrombocytopenia.

Clinical Features of the Thrombophilic Patient

Family history of thrombosis, Thrombosis <45 yrs, Idiopathic thrombosis, Thrombosis after trivial provocation, Thrombosis at unusual site (e.g. IVC, mesenteric vein, sagittal sinus, renal vein, hepatic vein (Budd-Chiari syndrome), axillary vein, retinal vein), Superficial thrombophlebitis, Warfarin-induced skin necrosis, Thrombosis with seemingly adequate anticoagulant therapy, Neonatal purpura fulminans (homozygous Protein C deficiency).

Clinical features consistent with hypercoagulable state

Labs: ATIII, PC (activity), PS (total and free reported), F V Leiden, platelet count, PT, aPTT, lupus anticoagulant, anticardiolipin Ab titers, immunologic and functional assays of fibrinogen, thrombin clotting time, blood homocysteine.

Work-up for associated disorders: SLE, PNH, nephrotic syndrome, and malignancy

Þ Avoid possible influences of acute thrombosis or anticoagulant Treatment. Repeat testing is frequently needed. Family studies are critical in confirming hereditary thrombophilia.

Conditions in which ATIII, PC, or PS levels are reduced:

ANTITHROMBIN III	PROTEIN C	PROTEIN S
Neonatal period	Neonatal period	Neonatal period
Pregnancy		Pregnancy
Liver disease	Liver disease	Liver disease
DIC	DIC	DIC
Nephrotic Syndrome	Obesity	SLE
Major surgery		Inflammation
L-asparaginase	L-asparaginase	L-asparaginase
Estrogens		Estrogens
Heparin
(Warfarin can increase)	Warfarin	Warfarin

Management of Thromboembolic Disease

Anticoagulant therapy is used to prevent propagation of the clot, thereby reducing the risk of PE and death.

Therapeutic approaches have been inhibition of blood coagulation (e.g. heparin) and thrombolytic therapy (e.g. tPA).

(1) Heparin is a naturally occurring GAG purified for clinical use from either porcine intestine or bovine lung. It acts by binding to lysine groups in ATIII via a pentasaccharide sequence, inducing a conformational change, greatly accelerating the inhibitory action of ATIII on thrombin and factors Xa. Heparin acts only in the peripheral blood and the does not affect the biosynthesis of any coagulation factor.

Poorly absorbed from the GI tract. Must be given either by continuous or intermittent IV infusion.

Heparin displays irregular pharmacokinetics; a rapid elimination phase followed by a more gradual disappearance phase. The response increases disproportionately in intensity and duration as the dose increases and varies widely among patients.

anticoagulant effect is monitored most commonly by the aPTT. (increased by 1.5-2 fold)

Clinical uses of heparin

DVT, PE, Prophylaxis of thromboembolism, CV disease (MI, angioplasty unstable angina, following fibrinolytic therapy, embolic stroke from intracardiac source, occlusion of large and medium size arteries, Intravascular devices)

Complications of heparin therapy

Hemorrhage: The risk of bleeding Ý with dose, and is Ý in patients with liver, renal or cardiac disease, and cancer.

The effect of heparin can be reversed rapidly with PROTAMINE.

Thrombocytopenia occurs in 5% of patients after 7-10 days.

In most patients it is due to platelet agglutination and reversible, even if heparin is continued.

In some patients, heparin-dependent Abs develop leading to widespread arterial thrombosis which is frequently fatal.

Heparin resistance can develop in the setting of PE, DIC, and inflammatory conditions. Excess PF4 (a heparin-neutralizing protein), thrombocytosis and low levels of ATIII are factors. Obese patients will frequently require Ý doses.

Osteoporosis with long-term use

(2) Low molecular weight heparins (LMWHs), made from standard heparin, inhibit F Xa, but not thrombin. Greater bio-availability than unfractionated heparin because of less binding to plasma proteins and endothelial cells. No loading dose needed. Longer T_1/2 and more predictable anticoagulant effect allow QD or BID administration without monitoring. They are equally effective as heparin in prevention of thrombosis and in treating venous thrombosis, possibly fewer bleeding complications. The cost of LMWH is higher than standard heparin, overall costs of treatment are about the same because the necessity of closely monitoring standard heparin treatment. Can be administered as outpatient and save some money in patients without contraindications.

(3) Warfarin (Coumadin) interferes with the synthesis of Vit K factors (II, VII, IX, X). Inhibits Vit K1 reductase and Vit K1 epoxide reductase. Results in ß regeneration of active Vit K, necessary for the g -carboxylation of glutamic acid residues of the Vit K proteins (including PC and PS). Because these glutamic acid residues require carboxylation to bind calcium, all of the affected factors are rendered inactive by the lack of the active hydroquinine form of Vit K.

Pharmacology: T_1/2 of 36-42 hours, full effect is not realized until all the Vit K factors are at lowest levels (3-5 days). Dose adjustments may also have to be made in patients on other medications due to the large number of drug interactions with warfarin. Other drugs can potentiate or interfere with its action by altering drug absorption, protein binding, receptor site affinity and metabolism or excretion. Consider drug interactions when starting a patient on warfarin and titrate the dose carefully.

Levels are monitored with INR and PT.

Clinical uses: venous thromboembolism, prevention of embolism in patients with valvular heart disease, prosthetic heart valves or A fib, prevention of acute MI in patients with peripheral arterial disease, prevention of stroke and PE after MI.

Complications: Bleeding, usually in the form of bruising, hematuria and GI bleeding, but CNS bleeds occur and can be fatal.

The risk of bleeding Ý with use of aspirin or comorbid conditions such as renal or hepatic disease.

Þ Bleeding while in the therapeutic INR range may be from renal or GI bleeding.

Management of bleeding from Warfarin: withhold drug and monitor (takes 3-4 days), administer Vit K (IV, oral, SQ), FFP (for immediate reversal), or Prothrombin complex concentrate.

Warfarin crosses the placenta, CANNOT USE IN PREGNANCY

(4) Hirudin and hirulog: from medicinal leeches. Direct thrombin inhibitors, and have an advantage over heparin in that they inhibit fibrin-bound thrombin and they do not depend on the presence of ATIII.

(5) Thrombolytic agents The most commonly used agents- tPA, urokinase and streptokinase.

All cause degradation of fibrin within an existing thrombus by and promoting plasmin activity (through activation of plasminogen). They also degrade circulating fibrinogen leading to a systemic lytic state.
Streptokinase has limited usefulness because it is derived from a bacterial source and can induce allergic reactions.

Clinical uses (1) in coronary artery disease, (2) arterial occlusive disease, (3) acute ischemic stroke, and (4) acute PE.

Complications hemorrhage, in ~30% of patients, after recent invasive procedures Absolute contraindications to thrombolytic therapy include: hemorrhagic stroke, CNS neoplasm, recent cranial surgery or trauma, uncontrolled HTN, major surgery, and recent GI or GU hemorrhage.

Management of Patients with Thrombophilia

High Risk – lifelong anticoagulation

Þ 1 or more episodes with hereditary or acquired thrombophilia

Þ 2 or more thrombotic episodes at different sites plus special clinical features

Moderate Risk – prophylaxis during high risk times

Þ asymptomatic carriers of ATIII, PS or PC deficiency

Þ asymptomatic acquired hypercoagulable states

Pregnant – Heparin used with frequent surveillance. NO WARFARIN. LMWH administered at home with post-partum Warfarin for 4-6 wks.