ANAESTHESIA AND ANTICOAGULANTS

1. ANAESTHESIA AND ANTICOAGULANTS Done by: Dr. Ahmad Alrefaie

2. Hemostasis • Prevention of blood loss whenever a vessel is severed or ruptured. • It is a combination of events that occur due to physical and chemical forces. • Achieved by several mechanisms: • Vascular spasm. • Formation of platelet plug. • Formation of blood clot as a result of coagulation. • Growth of fibrous tissue.

5. The ultimate step in clot formation is conversion of FIBRINOGEN , a soluble plasma protein into FIBRIN , an insoluble thread like molecule. • The conversion is catalyzed by the enzyme THROMBENat the site of injury. • Thrombin exist in the plasma in the form of an inactive precursor called PROTHROMBIN.

6. Prothrombin convert’s into thrombin by FACTOR X, a plasma clotting factor. • Factor X present in the blood in inactive form and must be converted into it’s active form by another activated factor, and so on.

7. Anticoagulants

8. Why we use anticoagulants? • Prophylaxis and treatment for deep venous thrombosis (DVT) and pulmonary embolism which are commonly associated with surgical procedures. • Mechanical heart valves. • Cardiac arrhythmias.

9. Who are patient at risk for DVT? • Major lower limb or pelvic surgery. • Trauma patient. • Malignancy ( increase the risk 7-fold). • Central neuraxial block significantly reduces the incidence of DVT after orthopaedic surgery but additional prophylaxis is necessary to reduce the rate to acceptable levels.

10. Aspirin • Also called acytelsalicylic acid. • Impair platelet function by inhibiting platelet cyclo-oxygenase (COX). • Aspirin inhibits COX irreversibly, Therefore the antiplatelet effect of aspirin persists until a new platelet population is manufactured (at least 7 days).

11. Indications • Local analgesic effect. • Antipyretic. • Anti-inflammatory. • Antiplatelet.

12. COX 1 • Continuously stimulated by the body. • Its concentration in the body remain stable. • Creates prostaglandins used for basic house keeping throughout body. • Prostaglandins stimulate normal body functions such as stomach mucous production, regulation of gastric acid and kidney water excretion.

13. COX 2 • Induced ( normally not present in cells). • Built only in special cells (A549 lung cells). • Used for signaling pain and inflammation. • Produces prostaglandins for inflammatory response. • Stimulated only as part of immune response.

14. It is safe to proceed with central and periphral nerve block in patients taking Aspirin.

15. NSAIDs • Analgesic, antipyretic and, in higher doses, anti-inflammatory drugs. • Impair platelet function by inhibiting platelet cyclo-oxygenase (COX). • NSAIDs inhibit COX reversibly. • Platelet function returns to normal within 3 days after stopping NSAIDs. • It is safe to proceed with central and periphral nerve block in patients taking NSAIDs.

16. COX 2 inhibitors • Anti-inflammatory drugs that selectively inhibit COX 2. • They do not affect platelet function. • It is safe to proceed with central and periphral nerve block in patients taking COX 2 alone. • They can potentiate the effect of warfarin by increasing the prothrombin time ( PT ).

17. Clopidogrel • A thienopyridine derivative. • It is a potent antiplatelet agent. • It inhibits ADP-induced platelet aggregation and binding between platelets and fibrinogen. • The effect is irreversible and platelet function does not return to normal until at least 7 days after stopping the drug.

18. It is used in combination with aspirin in patients with acute coronary syndrome. • It should be discontinued 7 days before surgery, central neuraxial and peripheral block. • If an antiplatelet effect must be maintained, aspirin can be substituted safely.

19. Unfractionated heparin • Indications: • Thromboprophylaxis. • Therapeutic anticoagulation. • Subcutaneous thromboprophylactic doses are seldom associated with bleeding complications.

20. Central and periphral block in thromboprophylaxis dose: the dose should be stoped 4 hours before or more than one hour after the procedures. • Catheter should be removed 2-4 hours after the last dose. • In therapeutic dose: activated partial thromboplastin time (APTT) should be normal before attempting a block or removing a catheter.

21. Patients who have been receiving unfractionated heparin for more than 4 days should have a platelet count, because the incidence of heparin-induced thrombocytopenia is about 3%.

22. LMWHs • Indications: • Thromboprophylaxis. • Therapeutic anticoagulation. • Have longer half-lives than unfractionated heparin, which allows once daily administration. • They have anti-Xa activity. • There is no monitoring test for routine use.

23. Central and periphral block in thromboprophylaxis dose: the dose should be stoped 12 hours before the block or catheter removal. • The first dose is given within 6 hours of surgery or 2 hours after the block. • In therapeutic dose: it takes about 24 hours for coagulation to return to normal. Therefore, an interval of 24 hours should elapse before attempting block.

24. Fondaparinux • Indications: for thromboprophylaxis. • It is a synthetic pentasaccharide, which has potent anti-Xa activity. • It has a longer elimination half-life than LMWH ( 17 hours in young patients and 21 hours in healthy elderly patients ). • It is administered 6 hours after surgery. • An interval of at least 24 hours should elapse before removal of neuraxial or peripheral nerve catheters.

25. Warfarin • Indications: • Thromboprophylaxis in AF. • Post prosthetic heart valve replacement. • Treatment of DVT or PE. • Central and periphral block: INR ≤ 1.5, this normally takes about 4 days after stoping warfarin. • If a LMWH or unfractionated heparin has been administered in place of warfarin, the recommended intervals discussed above should be observed before performing any block.

26. Anticoagulants perioperatively • Warfarin should be stopped 2-4 days preoperatively, and the PT time monitored daily (INR ≤ 1.5). • If INR prolonged: • Administer vitamin K. • Fresh frozen plasma. • It is often appropriate to start an alternative anticoagulant, such as LMWH or unfractionated heparin, until warfarin is re-established and the INR is back in the therapeutic range postoperatively.

27. After minor surgery: warfarin may be restarted on the first postoperative day. • After major surgery: an infusion of unfractionated heparin may be used to maintain anticoagulation ( with control by APTT ) until warfarin therapy is restarted. • Unfractionated heparin is reversed rapidly with protamine 1 mg for every 100 units of heparin.

28. Unfractionated heparin is preferable to LMWH because it may be monitored more easily and reversal titrated more accurately.

29. Summary • Aspirin and NSAIDs: No contraindication. • Clopidogrel: Stop 7 days before surgery, central and peripheral block. • Warfarin: INR ≤ 1.5. After minor surgery: start on the first postoperative day. After major surgery: an infusion of unfractionated heparin may be used to maintain anticoagulation.

30. Unfractionated heparin: • Thromboprophylaxis dose: stop 4 hours before or > than one hour after the procedures. Catheter should be removed 2-4 hours after the last dose. • Therapeutic dose: (APTT) should be normal before attempting a block or removing a catheter.

31. LMWH: • Thromboprophylaxis dose: the dose should be stoped 12 hours before the block or catheter removal. The first dose is given within 6 hours of surgery or 2 hours after the block. • Therapeutic dose: the dose should be stoped 24 hours before the block.

32. Fondaparinux: Start 6 hours after surgery. Stop 24 hours before removal of neuraxial or peripheral nerve catheters. REFERENCE: • AnaesthesiaUK • Europian Journal of Anaesthesiology2007 • Medical Physiology, Guyton • Fundamentals of Physiology • Text book of Anaesthesia, Aitkenhead

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