Coagulation

1

How can you identify a patient at risk for perioperative bleeding?

Preoperative evaluation for bleeding risk includes a focused history, physical examination, a review of all medications and dietary supplements, appropriate laboratory testing and a consideration of the bleeding risk inherent to the scheduled surgical procedure. Questions should be asked about prior bleeding in nonsurgical settings (e.g., tendency to form large hematomas after minor trauma, severe bleeding while brushing teeth) and significant bleeding with prior surgical procedures not normally associated with significant bleeding (e.g., dental extractions). Prior surgery without the need for transfusion suggests the absence of a clinically significant inherited coagulation disorder. However, it does not rule out the subsequent interim development of an acquired coagulation disorder (liver or renal disease, hematological malignancy, etc.) in a patient who reports “recent easy bleeding/bruising.” Preoperative coagulation studies may confirm a clinical suspicion that a patient has a bleeding disorder, but no evidence supports the value of routine preoperative coagulation studies in asymptomatic patients. However, those with a personal history of bleeding will likely bleed again. With regard to von Willebrand disease (vWD), it was demonstrated that a standardized preoperative/antepartum “bleeding questionnaire” was equivalent to laboratory testing for predicting bleeding after dental extractions, and superior to laboratory testing for the prediction of surgical bleeding.

2

Do dietary supplements and herbal remedies cause bleeding?

Many medications unrelated to the coagulation system, herbal remedies, over-the-counter dietary supplements, foods, fruits, vegetables, spices, and vitamins have been demonstrated to have varying degrees of antiplatelet, antithrombotic and/or anticoagulant activity, but most (with a few notable exceptions) have not been unequivocally demonstrated to confer clinically significant bleeding risk in-and-of themselves when taken as directed and/or consumed in normal quantities. Notable exceptions may include certain herbal or dietary supplements (e.g., ginko, ginseng, garlic, omega-3-fatty acids, coenzyme Q ₁₀ , D vitamins) and some anesthesiologists advise patients to discontinue the use of such supplements for at least 2 weeks preoperatively to decrease the risk of perioperative bleeding and/or complications of planned neuraxial procedures.

3

What is the difference between hemostasis and coagulation?

Hemostasis is the overall process by which bleeding is stopped. Coagulation is the formation of a fibrin clot at the site of blood vessel injury. In nonpathological states, a balance must exist between bleeding and clotting if blood is to remain liquid, and tissue perfusion distal to the site of a vessel injury is to continue, once vessel injuries have been repaired. Thus the overall process of hemostasis must include checks and balances to mitigate the effects of excessive coagulation and dissolution of clots. The hemostatic mechanism therefore includes the following: vasoconstriction of the injured vessel, coagulation at the site of vessel injury, and fibrinolysis.

4

Describe the process of coagulation

Coagulation (confusingly enough) is subdivided into primary hemostasis and secondary hemostasis.

5

What is primary hemostasis?

Primary hemostasis refers to the formation of a preliminary platelet plug at the site of vessel injury. Exposure of the subendothelial collagen results in the adherence of platelets to the site of injury and their activation. Platelet activation results in degranulation, shape change, aggregation, and exposure of the fibrinogen receptor (glycoprotein IIb/IIIa). A preliminary platelet plug forms as many platelets all bind to the same strands of fibrinogen.

6

What is secondary hemostasis?

Secondary hemostasis refers to the ultimate fibrin crosslinking and reinforcement of the platelet plug developed during primary hemostasis. The additional fibrin needed locally to stabilize the platelet plug and create a true fibrin clot comes from the extrinsic and intrinsic coagulation pathways.

7

Describe platelet activation

Platelet adherence to exposed subendothelial collagen is via their glycoprotein receptor Gp1b mediated by von Willebrand factor (vWF). Substances like collagen, thrombin, and epinephrine activate phospholipases A and C in the platelet plasma membrane, resulting in the formation of thromboxane A ₂ (TXA ₂ ) and the degranulation of platelet alpha- and dense granules. Platelet granules contain a variety of procoagulant factors, including: serotonin, adenosine diphosphate (ADP), TXA ₂ , vWF, factor V, fibrinogen, and fibronectin, all of which assist in the process by activating platelets, promoting aggregation of platelets, recruiting more platelets to the plug and initiating secondary hemostasis (discussed later). ADP released locally during degranulation initiates shape change (that exposes electronegatively charged phospholipids on the platelet surface that will activate secondary hemostasis through the “contact activation pathway”, as discussed later), as well as a decrease in cyclic adenosine monophosphate (cAMP). Falling cAMP levels in conjunction with other secondary messengers alters the membrane glycoproteins IIb and IIIa to form the activated fibrinogen receptor (GPIIbIIIa). A platelet plug thus forms by many activated platelets binding to fibrinogen through the GPIIbIIIa receptor. Platelet plug formation is referred to as primary hemostasis .

8

What are the extrinsic and intrinsic coagulation pathways?

The classic depiction of the extrinsic and intrinsic pathways ( Fig. 10.1 ) as two completely separate processes is no longer accepted because of multiple points of interaction between the two (factors from each can activate factors in the other), although it remains useful conceptually for the interpretation of in vitro tests of coagulation and hemostasis. Both the extrinsic and the intrinsic pathways lead to activation of factor X (which will then cleave prothrombin to thrombin, which will then cleave fibrinogen to fibrin; these latter steps are known as the common pathway ). Modern terminology for the extrinsic pathway is “the tissue factor pathway,” and that for the intrinsic pathway is “the contact activation pathway.” The intrinsic pathway is also sometimes known as the amplification pathway .

9

Describe the tissue factor (extrinsic) pathway. What are the laboratory tests for this pathway?

The tissue factor (TF) pathway (see Fig. 10.1 ) is triggered by the exposure of TF at the site of blood vessel damage, which combines with Factor VII to form the activated TF-VIIa complex, which activates Factor X, creating prothrombinase (Xa + Va cofactor). The “extrinsic” or tissue factor pathway thus consists of the activated TF-VIIa complex and the Xa/Va complex (prothrombinase) that cleaves prothrombin to thrombin (thus beginning the “final common pathway” of coagulation resulting in fibrin formation). The prothrombin time (PT) and international normalized ratio (INR) are measurements of clotting via the tissue factor (extrinsic) pathway. The PT uses thromboplastin (mixture of TF + calcium + phospholipid) to activate coagulation in the ex vivo laboratory assay. A normal PT is 10 to 12 seconds. The INR was devised to standardize the results of the PT because different laboratory tests use different formulations of thromboplastin to perform the test. With each batch of thromboplastin, each manufacturer provides an “international sensitivity index rating” (ISI) to which the patient’s PT is compared, resulting in the INR. An INR of 0.8 to 1.2 is considered normal. Therapeutic anticoagulation with warfarin generally requires an INR 2 to 3, but an INR of greater than 3 may be desirable for certain anticoagulation indications.

10

Describe the contact activation (intrinsic) pathway. What are the laboratory tests for this pathway?

In vivo, secondary hemostasis can be initiated via the contact activation (intrinsic) pathway in more than one way. When there is vessel injury, the contact activation pathway gets activated by factors from the TF (extrinsic) pathway, as well as by the electronegatively charged phospholipids on the platelet surface and by exposed collagen. Once activated, a series of reactions take place on the activated platelet surface to generate a local burst of thrombin. This contact activation pathway can also be activated by contact with other electronegatively charged surfaces/molecules, such as the electronegatively charged phospholipids in amniotic fluid, or on foreign surfaces, like glass or plastic (e.g., in laboratory tests) or by cardiopulmonary bypass and extracorporeal membrane oxygenation (ECMO) circuits.

The contact pathway is often initiated by contact with collagen (electronegatively charged surface) with three serum proteins: high-molecular-weight kininogen (HMWK), prekallikrein (PK), and factor XII. Although the details are beyond the scope of this text, the result is activation of factor XII, which in turn, causes activation of factor XI, IX, and X, respectively (see Fig. 10.1 ). However, neither factor XII nor its cofactors (PK and HMWK) are absolutely necessary for clinical hemostasis (because the pathway can be otherwise activated from the TF “extrinsic” pathway), and mild deficiencies of these cofactors do not result in bleeding problems. The partial thromboplastin time (PTT) is a common measure of clotting via the contact activation (intrinsic) pathway. The test is named such because partial thromboplastin is used as the activator (which eliminates the platelet variability had platelet phospholipid been used as the activator). A normal PTT is in the range of 60 to 70 seconds. The activated partial thromboplastin time (aPTT) is a more sensitive version of the PTT and is commonly used to monitor heparin therapy. A normal range for the aPTT is 30 to 40 seconds.

11

Does primary hemostasis happen before secondary hemostasis?

Although the terms primary and secondary hemostasis suggest that one happens after the other, clot initiation, amplification, and propagation all occur concurrently once the process is initiated because of the crossover of multiple factors from and between the various pathways to coagulation.

12

This has gotten confusing. What are the initiation, activation, propagation, and stabilization phases of coagulation? Describe the big picture of how this all happens

The modern understanding is that the initiation stage of coagulation takes place on TF bearing cells (cells, such as monocytes, that can bind TF and present it to a ligand), which come into play when endothelial injury occurs and TF is exposed. TF is a transmembrane glycoprotein expressed on cells outside the bloodstream and is sometimes referred to as a cell surface receptor for the serine protease Factor VIIa. The initiation phase is characterized by presentation of TF to its ligand, factor VII. The activation phase takes place when the TF–VIIa complex activates factors X and IX. Activated factor X (Xa) then binds cofactor V. This TF–Xa/Va complex cleaves prothrombin to thrombin. However, the relatively small amount of thrombin produced thus far by the classic cascades is not sufficient to produce a fibrin clot. A number of other reactions is triggered during all of this, with platelets playing a central role. As previously discussed, platelets are activated during primary hemostasis via receptors for substances, such as collagen and thrombin. The platelets, upon activation, degranulate, releasing procoagulant factors, and change their shape, exposing negatively charged membrane phospholipids. Factors IXa, Xa, and XIa also have negatively charged sites that attach to platelet phospholipid with calcium ions acting as a sandwich-like buffer. Amplification of thrombin production is mediated by enzyme reactions located on the platelet surface. The combination of enzyme, cofactor, calcium, and phospholipid surface increase the speed of these reactions many 1000-fold. This is the propagation phase, in which an explosive increase in thrombin production cleaves large amounts of fibrinogen into fibrin. Finally, the stabilization phase occurs when activated factor XIII (XIIIa) crosslinks fibrin to reinforce the platelet plug to stabilize the clot.

13

What is the function of Vitamin K in the coagulation pathways?

Vitamin K facilitates the carboxylation of factors II, VII, IX, X, protein C, and protein S by the enzyme gamma-glutamyl carboxylase.

14

How does Warfarin work?

Warfarin (and related coumarins) inhibit “production” of the “vitamin K-dependent” factors II, VII, IX, X, protein C and protein S, by blocking vitamin K epoxide reductase.

15

Why is the initiation of warfarin therapy associated with hypercoagulability?

Protein C is an inhibitory counterregulatory factor with a short half-life (~ 8 hours) that degrades factors Va and VIIIa, thus limiting the formation of factor Xa (which cleaves prothrombin to thrombin). The initiation of warfarin therapy inhibits the synthesis of anticoagulant protein C, predisposing to hypercoagulability because of the shorter half-life of protein C compared with other procoagulant clotting factors. Warfarin also inhibits the production of anticoagulant protein S but this protein has a much longer half-life (~ 30 hours).