CLOTTING CASCADE & PLATELETS

QUESTION 1: A 66 y/o male with a history of DM, HTN and CAD is to undergo urgent surgery for an expanding 6.4 cm infrarenal abdominal aortic aneurysm.  He had a MI 2 months ago at which time he underwent percutaneous coronary angioplasty with placement of drug-eluting stents to the LAD and RCA.  Current medications include clopidogrel and aspirin, which were started after the stent placement.  With respect to the combonation of these two drugs and the intraoperative risk of bleeding it is true that:
A. both drugs exert their antiplatelet effect through the same mechanism
B. clopidogrel does not add significantly to the risk of bleeding posed by aspirin
C. if these drugs were stopped one day prior to surgery, the risk of bleeding would be minimized
D. both drugs can be antagonized
E. clopidogrel should be discontinued 7 days prior to surgery in elective cases where surgical hemostasis is needed

QUESTION 2: A 62 y/o male with history of DM, HTN and CAD is admitted tot he hospital for unstable angina.  He was recently diagnosed with heparin-induced thrombocytopenia type II and is now scheduled to undergo off-pump CABG for severe three-vessel coronary artery disease.  A suitable agent for intraoperative anticoagulation would be:
A. heparin
B. warfarin
C. argatroban
D. danaparoid
E. clopidogrel

(answers at bottom of post)

HEMOSTASIS AND BLOOD COAGULATION

When a vessel is damaged/ruptured, hemostasis is achieved by 4 mechanisms:
1. vascular constriction
2. formation of platelet plug
3. formation of blood clot from blood coagulation
4. growth of fibrous tissue into blood clot to close hole in vessel permanently (fibrin mesh)

1. VASCULAR CONSTRICTION

• Local myogenic spasm
• Platelets release a vasoconstrictor substance called thromboxane A2

2. PLATELET PLUG

• Platelets = thrombocytes; 150,000-300,000 per microliter
• Surface of cell membrane of platelets contain glycoproteins that adhere to injured areas of vessel (repulses adherence to normal endothelium) - especially exposed collagen from deep within vessel wall
• Platelet membrane contains large amt of phospholipids that activate blood-clotting process
• 1/2 life in blood of 8-12 days; eliminated by tissue macrophage system (spleen)
• Platelets that come in contact with collagen fibers in damaged vascular wall immediately change and become "sticky" - adhere to a protein called von Willebrand factor in tissue wall.
• ADP and thromboxane A2 secreted by plts act on nearby plts to activate them as well which adhere to original activated platelets --> platelet plug
• This is initially loose but just enough to stop bleeding until stronger fibrin threads/mesh forms

3. FORMATION OF BLOOD CLOT

• Activator substances from traumatized vascular wall, from plts and from blood proteins adhere to traumatized vascular wall to initiate clotting process.  Within 3-6 min entire opening is filled with clot
• After 20 min to an hr clot retracts

4. FORMATION OF FIBRIN MESH

• Blood clot invaded by fibroblasts within few hours after clot is formed and continues for 1-2 weeks

BLOOD COAGULATION:

1. Net result of activating coagulation cascade is formation of a complex of activated substances collectively called prothrombin activator
2. Prothrombin activator converts prothrombin ---> thrombin
3. Thrombin acts as an enzyme to convert fibrinogen ---> fibrin that create fibrin mesh which stabilizes platelet plug
4. Thrombin also activates fibrin-stabilizing factor that strengthens fibrin mesh

Summary: prothrombin activator --> prothrombin --> thrombin --> fibrinogen --> fibrin mesh

FORMATION OF PROTHROMBIN ACTIVATOR:
Formed in 2 ways:
1. Extrinsic Pathway (begins with trauma to vascular wall and surrounding tissues)
2. Intrinsic Pathway (begins with trauma to blood or exposure to collagen)
*Both converge into the Common Pathway

(Click to view larger image)



*Most of the clotting factors (proteins) circulate in the inactive forms.  When converted to active forms, their enzymatic actions cause the cascade reactions of the clotting process.  This active form is designated as "a".  Eg. Factor VIIIa  or Factor 8a = activated factor 8

Extrinsic Pathway:

• Release of tissue factor from damaged vessel
• Tissue factor activates Factor 7 (thromboplastin)
• Factor 7a activates Factor 10 (start of Common Pathway)
• Factor 10a complexes with Factor 5 to make the prothrombin activator

Summary: Tissue factor --> Factor 7a --> Factor 10a + 5 = prothrombin activator --> prothrombin --> thrombin --> fibrinogen --> fibrin (mesh)

Intrinsic Pathway:

• Blood trauma causes activation of Factor 12
• Factor 12a activates Factor 11
• Factor 11a activates Factor 9
• Factor 9a + Factor 8 activates Factor 10 (start of Common Pathway)
• Factor 10a complexes with Factor 5 to make prothrombin activator

Summary: Factor 12 --> 12a --> Factor 11a --> Factor 9a + 8 --> Factor 10 + 5 = prothrombin activator

*Hemophilia lacks Factor 8

ANTICOAGULANTS:
The blood has natural anticoagulants:
1. the fibrin fibers that are formed during the process of clotting
2. an alpha-globulin called antithrombin III or (antithrombin-heparin cofactor)
3. heparin is also found naturally in blood but in low concentrations
4. Plasminogen, when activated by endogenous t-PA, becomes plasmin, which digests fibrin fibers and other coagulants (fibrinogen, F 5, F 8, prothrombin, F 12)
*t-PA = tissue plasminogen activator (released by injured tissues)

HEPARIN:
negatively charged conjugated polysaccharide
Mechanism: when it combines with antithrombin III, the effectiveness of antithrombin III for removing thrombin increases by 100-1000 fold.  In the presence of heparin, removal of free thrombin from the circulating blood by antithrombin III is almost instantaneous.
The heparin-antithrombin III complex also removes several other coagulation factors: 12a, 11a, 10a, and 9a. (Intrinsic pathway)

Check PTT
Antagonized by positively charged PROTAMINE

WARFARIN (Coumadin):
Mechanism: Inhibiting the enzyme that activates Vitamin K (vitamin K epoxide reductase complex 1 - VKOR c1).  By inhibiting VKOR c1, warfarin decreases available active form of Vitamin K

Vitamin K dependent factors: 2, 7, 9, 10, protein C & S
Check PT and INR

• Prothrombin Time (PT) gives an indication of the [ ] of prothrombin in the blood.  Tissue factor is mixed with a sample of blood which activates the prothrombin-to-thrombin reaction by the extrinsic pathway.  The time required for coagulation to take place is known as the prothrombin time.  Normal is 12 seconds.  However, results may vary greatly from sample to sample from the same individual.
• International Normalized Ratio (INR) was devised as a way to standardize measurements of PT.  INR is the ratio of the person's prothrombin time to a normal control sample raised to the power of the ISI (international sensitivity index).  Normal is 0.9 to 1.3.  Patients on warfarin should have an INR of 2.0 to 3.0

ASPARIN:
Mechanism: Arachidonic acid is converted by an enzyme, cyclooxegenase 1 (COX), to thromboxane A2 - which activates platelets.  ASA irreversibly blocks COX enzymes.

CLOPIDOGREL (plavix):
Mechanism: prevents platelets from releasing the 2b/3a receptor during an activated state which is responsible for platelets connecting (sticking) to one another to form platelet plugs.

(click to view larger image)

Inactivated fibrinogen also serves to connect platelets to one another via 2b/3a receptor.  Integralin serves as an antagonist to the 2b/3a receptor (sits on receptor and renders it inactive)



Clotting Factors and their Synonyms :

• Fibrinogen = Factor 1   (normal level is 100-700 mg/dl)
• Prothrombin = Factor 2
• Tissue factor = Factor 3 or tissue thromboplastin
• Calcium = Factor 4
• Factor 5 = Proaccelerin; labile factor; Ac-globulin (Ac-G)
• Factor 7 = thromboplastin; serum prothrombin conversion accelerator (SPCA)
• Factor 8 = antihemophilic factor A
• Factor 9 = antihemophilic factor B; plasma thromboplastin component (PTC)
• Factor 10 = Stuart factor
• Factor 11 = antihemophlic factor C; plasma thromboplastin antecedent (PTA)
• Factor 12 = Hageman factor
• Factor 13 = fibrin-stabilizing factor
• Prekallikrein = Fletcher factor
• high-moleculare weight kininogen = Fitzgerald factor

*von Willabran Factor is usually complexed with Factor 8

ANSWER 1: (E) Aspirin irreversibly blocks the production of thromboxane A2 in platelets, whereas clopidogrel exerts its antiplatelet effect by blocking the platelet ADP receptor.  Because of their discrete mechanisms, the effect of these drugs is additive or even synergistic, which results in a higher risk for excessive bleeding during surgical procedures.  There are no antagonists available for these medications.  Because of their long duration of action, stopping the drugs one day prior to surgery would not decrease the risk of bleeding.  Current recommendations are to stop clopidogrel 7 days prior to surgery in elective cases where surgical hemostasis is needed.

ANSWER 2: (C) Use of heparin in a patient with heparin-induced thrombocytopenia type II can lead to life-threatening thrombotic complications and is therefore not indicated.  Warfarin is an oral anticoagulant.  Danaparoid is a mixture of nonheparin glycosaminoglycans and is approved for prophylaxis of deep vein thrombosis.  It can be administered SC or IV, however, it has a long half-life of about 24 hrs.  Clopidogrel is an antiplatelet agent for the secondary prevention of stroke and the reduction of cardiac events after percutaneous coronary intervention.  Argatroban is a direct thrombin inhibitor with a relatively short half-life of 1 to 2 hours that can be continously infused during CABG surgery.

References:
McGraw-Hill Specialty Board Review
Guyton, Hall. Medical Physiology. Ch. 36: Hemostasis and Blood Coagulation