• Coagulation during bypass
    • Classic teaching of the intrinsic and extrinsic pathways is not accurate of in vivo coagulation but useful for understanding lab tests
    • The PT measures extrinsic pathway
      The APTT measures the intrinsic pathway
    • Coagulation is initiated when TF is exposed at the site of injury and binds to factor VII, the resulting TF-VII complex activates IX & X. Activated X (Xa) then binds V. This TF-Xa/Va cleaves prothrombin to thrombin. Thrombin cleaves fibrinogen to fibrin and activates platelets, IX, V & VIII (amplification). Platelets also amplify thrombin. Platelets become trapped in the fibrin clot via their GPIIb/IIIa receptor.
    • Heparin is used during CPB to prevent blood clotting in the circuit. Despite this, some coagulation occurs. It facilitates the action of ATIII thereby inhibiting thrombin
    • Anticoagulation for CPB
      • UFH is a -ve charged glycosaminoglycan with MW of 3000-40000 Da. It binds to ATIII and potentates it's inhibition of Xa. The dose used is 300-400U/kg plus additional to ensure ACT of > 480s. The circuit is primed with 5000U.
    • Heparin resistance
      • Suspect if pt needs >600U/kg to achieve ACT of >480s
      • Recombinant ATIII is used (or FFP)
    • Allergy to heparin, protamine or HIT may mean need to consider alternatives
      • Danaparoid
      • Lepirudin (thrombin inhibitor)
      • Argatroban
    • Reversal is with protamine 1mg per 100U of heparin used. It neutralises heparin but can actually cause bleeding due to inhibition of platelet function
    • Monitoring anticoagulation
      • ACT
        • 1ml of blood is placed in a tube with a magnetic rod and activator (kaolin) and warmed to 37 degrees. Clotting is detected by resistance to movement of the magnetic rod in a magnetic field. The normal ACT is 100-140s and increases in a non-linear fashion with increasing heparin concentration
        • Affected by hypothermia and haemodilution so ceases to correlate well with heparin concentration when on bypass
      • Heparin concentration (near patient tests)
        • Protamine titration assay
        • High dose thrombin time
        • Heparin management test
      • TEG
        Link:Thromboelastometry
      • Lab tests
        • Guide FFP, platelets and cryo use but limited use intra operatively
    • CEACCP
      Link:ceaccp.oxfordjournals.org/content/7/6/195.full.pdf