Haematology and Immunology - Haemostasis & Clotting

Question 1

Describe and draw the clotting cascade

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Answer 1

Extrinsic pathway (measured by PT)
Acts as trigger in response to tissue damage & release of tissue factor. Factor 7 is converted to 7a, which catalyses conversion of Factor 10 to 10a.

10a, 5, and Ca2+ convert prothrombin to thrombin.

Enough factor 7a, 10a and thrombin will trigger the intrinsic pathway, amplifying the cascade

Intrinsic pathway (measured by APTT)
12 converted to 12a, which converts 11 to 11a, converting 9 to 9a. 8 & 9a convert 10 to 10a

Thrombin converts 13 to 13a, and fibronigen to fibrin. These join to form a stable clot.

Question 2

Describe and draw the fibrinolytic pathway

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Answer 2

Kallikrein, 11a, 12a, urokinase and tissue plasminogen activator facilitate the conversion of plasminogen to plasminogen. This conversion is also blocked by TXA.

Plasminogen activator inhibitor inhibits the action of urokinase and tissue plasminogen activator.

Plasmin breaks down fibrin into its degradation products (Eg D-Dimers), but is inhibited by thrombin-activated fibrinolysis inhibitor, and by α2-Antiplasmin.

Question 3

How does Heparin work?

Answer 3

Endogenous molecule, glycosaminoglycan

Strongly electronegative & acidic, so binds to proteins & amines

Reversibly binds to lysine residue to potentiate antithrombin III (α2 globulin, which inactivates thrombin & plasmin), increasing its affinity by 1000x.

Antithrombin III also inactivates serine proteases, affecting the following factors.
9, 10, 11, 12 (10 at the lowest dose) - all part of the intrinsic clotting pathway, and therefore activity measured by APTT.

At very high doses, also inhibits fibrin mediated platelet activation.

Side effects:
Bleeding, hypersensitivity, hypotension
Alopecia & osteoporosis in long-term treatment
Inhibition of aldosterone (measure K+ levels if given >7/7)

Heparin induced thrombocytopaenia
HIT 1 (non-immune mediated, temporary, mild - 10% of patients within 72 hours)
HIT 2 (immune mediated - 3% of cases 5-10 days after treatment)

Question 4

Compare Heparin to Low Molecular Weight Heparin

Answer 4

Size (Daltons): 3,000-30,000 vs 4000-6,500
Mechanism: Binds antithrombin III & other factors vs antithrombin III and factor 10 only
Route: IV & Subcut vs Subcut
Pharmacokinetics: 50% protein bound, rapid onset, half-life of 90 minutes, 40% bioavailability & rapid breakdown vs 10% protein bound, half-life 3-5 hours, not rapidly degraded
Monitoring: APTT vs Anti-Xa activity
Reversal: Stop infusion, can use protamine vs protamine only partially effective
Uses: Thromboprophylaxis (Bypass, ECMO, cell salvage, dialysis), neither cross the placenta
Neuraxial Blockade: Stop & restart 4 hours either side vs 12 hours if pLMWH, 24 hours if tLMWH
Side Effects: LMWH far less likely to cause HIT

Question 5

How can Heparin be reversed?

Answer 5

Protamine sulphate - positively charged basic protein, binds and inactivates heparin

1mg inactivates 100 Units
Max dose 50mg in 10 mins

Side effects:
Anaphylaxis (fish allergy)
Myocardial depression
Bradycardia
Hypotension (histamine release)
Pulmonary hypertension (thromboxane release)

Question 6

Discuss Dabigatran

Answer 6

Mechanism of action: Direct thrombin inhibitor - Dabigatran exilate is a prodrug, metabolised by esterases to dabigatran
Pharmacokinetics
-Absorption: 6.5% bioavailability
-Distribution: 35% protein bound, peak effect 2 hours
-Metabolism: Hepatic
-Elimination: 80% Renal, 20% Faecal, Half life 15 hours
Side effects: Bleeding, anaemia, nausea
Advantages: Oral, rapid onset, no monitoring required
Disadvantages: Expensive in itself, and Idarucizumab (reversal) is expensive

Question 7

What are the parameters of thromboelastography?

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Answer 7

R Time FFP
Time from stimulation until beginning of clot formation (determined by clotting factors)
K Time Cryoprecipitate
Time from initial clot formation, until width deviation of 20mm on trace - normal 3-5 mins, determined by fibrinogen
Alpha Angle FFP
Rate of clot formation - normal 50-70degrees, determined by fibrinogen
Maximum Amplitude Platelets
Maximum extent of clot formation - normal 50-70mm, determined by platelets
LY30 TXA
Lysis at 30 minutes, should be less than 8%

Question 8

What are the causes of DIC (Disseminated Intravascular Coagulopathy?

Answer 8

Infection
Bleeding
Trauma
Shock
Adenocarcinoma
Transfusion