ANAESTHESIA AND ANTICOAGULANTS Done by: Dr. Ahmad Alrefaie

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.

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 THROMBEN at the site of injury. Thrombin exist in the plasma in the form of an inactive precursor called PROTHROMBIN.

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.

Anticoagulants

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.

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.

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).

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

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.

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.

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

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.

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 ).

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.

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.

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

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.

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%.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

Fondaparinux: Start 6 hours after surgery 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

THANK YOU