HITTS. Heparin-Induced Thrombotic Thrombocytopenia Syndrome.

 Heparin-Induced Thrombotic Thrombocytopenia Syndrome (HITTS)

is a prothrombotic disorder caused by IgG mediated antibodies

to complexes of platelet factor 4 (PF4) and heparin.

Diagnosis

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
• anaphylactoid reactions

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 duringCPB.

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.

Management during cardio-pulmonary bypass (CPB) :

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.

Source:Kristine Wardle | Paul Sadleir

read also: Perioperative bleeding risk factors