Heparin-Induced Thrombotic Thrombocytopenia Syndrome (HITTS) is a prothrombotic disorder caused by IgG mediated antibodies to complexes of platelet factor 4 (PF4) and heparin.
The detection of HITTS antibodies plus one of the following.
unexplained drop in platelet count by 30-50%
venous or arterial thrombosis
skin lesions at heparin injection site
The antibodies bind to the PF4-heparin complexes on the platelet surface inducing activation. The activated platelets increase the release and surface expression of PF4, creating a positive feedback loop in which further release of PF4 promotes further platelet activation.
Warfarin can induce a paradoxical, hypercoagulable state usually within 3 to 10 days of therapy initiation, associated with inadequate heparin overlap, and thought to be due to an imbalance between anticoagulant and procoagulant pathways. The anticoagulants protein C and protein S have a shorter half-life than other vitamin K–dependent factors (II, IX, and X), resulting in a deficiency of both proteins early in treatment. This increases the chance of thrombosis and subsequent skin necrosis.
Alternative anticoagulants during cardio-pulmonary bypass (CPB) :
Direct thrombin inhibitor (half-life 25 min)
Bivalirudin 1mg/kg IV, followed by 2.5mg/kg/hr. Use additional boluses of 0.5-1mg/kg to maintain ACT 2.5 x baseline or > 600s or APTT ~ 200s.
Cease the infusion 15 min prior to planned separation.
Use ecarin clotting times to monitor to target blood concentration 15mcg/mL.
Factor Xa inhibitor (half-life 18-24 hours)
Danaparoid 7500 units with 1500 units in prime to achieve a level of at least 1 unit/mL
during CPB. ACT does not correlate with anti-Xa activity.
Antiplatelet agent plus heparin
Tirofiban 10mcg/kg bolus with infusion 0.15mcg/kg/min followed by heparin
400 IU/kg (Restore protocol).
Minimise stagnant blood in circuit and cardiotomy suction, and continually flush cardioplegia circuit.
1 Stop heparin, defer procedure and come off bypass if practical.
2 Urgently source alternative anticoagulants.
3 Scan for clot within circuit 21 , avoid stasis, maintain circuit flow and give volume.
4 Monitor for oxygenator failure 32 .
5 Monitor lactate for evidence of organ malperfusion.
6 Do not give platelets.
7 Use sodium citrate as anticoagulant for cell salvage.
8 Send blood for platelet factor 4/polyanion antibody assay and platelet activation test.
9 Prepare for significant postbypass bleeding.
10 Avoid postoperative warfarin until platelet count recovered.