One-year experience with the platelet glycoprotein IIb/IIIa antagonist tirofiban and heparin during cardiopulmonary bypass in patients with heparin-induced thrombocytopenia type II

Effects of tirofiban on haemostatic activation in vitro

BACKGROUND

Thrombin plays a critical role in normal haemostasis and pathological thrombosis. Heparin has long been a mainstay choice of antithrombotic regimen in cardiac patients, but persistent thrombin generation seems to occur during heparin therapy. Because platelets are integral to primary haemostasis and clot formation, we evaluated the use of tirofiban (Aggrastat),a platelet inhibitor, as a therapy to improve heparin sensitivity and delay thrombin formation.

METHODS

Blood samples were obtained from healthy subjects (n=8) and cardiac surgical patients (n=34). Thrombin formation was measured in platelet-rich plasma with a Thrombogram-Ascent fluorescent plate reader system. Platelet inhibition by tirofiban was evaluated with Plateletworks, and the interaction of tirofiban and heparin (>1.5 U ml(-1)) on clot formation was evaluated with Sonoclot Analyzer or kaolin activated clotting times (ACTs).

RESULTS

Addition of tirofiban (70-280 ng ml(-1)) progressively delayed onset of thrombin generation triggered by adenosine diphosphate (ADP). Plateletworks showed platelet inhibition with tirofiban (>35 ng ml(-1)), whereas heparin per se failed to produce platelet inhibition at 7 U ml(-1). Heparin (1.5 U ml(-1)) slowed the onset and rate of fibrin formation on Sonoclot analyses, and this was further slowed after addition of tirofiban (70 ng ml(-1)) to heparin-containing blood samples. Significant increases in ACT at all heparin concentrations were observed with the addition of tirofiban (70 ng ml(-1)). The addition of antithrombin (0.2 units/ml) to heparinized blood samples further prolonged ACTs, but the difference was not statistically significant when compared with heparin alone.

CONCLUSION

Tirofiban delays platelet activation-mediated thrombin generation and prolongs ACT in heparinized blood.

Heparin-induced thrombocytopenia and cardiac surgery

Unfractionated heparin given during cardiopulmonary bypass is remarkably immunogenic, as 25% to 50% of postcardiac surgery patients develop heparin-dependent antibodies during the next 5 to 10 days. Sometimes, these antibodies strongly activate platelets and coagulation, thereby causing the prothrombotic disorder, heparin-induced thrombocytopenia. The risk of heparin-induced thrombocytopenia is 1% to 3% if unfractionated heparin is continued beyond the first postoperative week. When cardiac surgery is urgently needed for a patient with acute or subacute heparin-induced thrombocytopenia, options include an alternative anticoagulant (bivalirudin, lepirudin, or danaparoid) or combining unfractionated heparin with a platelet antagonist (epoprostenol or tirofiban). As heparin-induced thrombocytopenia antibodies are transient, unfractionated heparin alone is appropriate in a patient with previous heparin-induced thrombocytopenia whose antibodies have disappeared.

Heparin-induced thrombocytopenia and cardiac surgery

Unfractionated heparin given during cardiopulmonary bypass is remarkably immunogenic, as 25% to 50% of postcardiac surgery patients develop heparin-dependent antibodies during the next 5 to 10 days. Sometimes, these antibodies strongly activate platelets and coagulation, thereby causing the prothrombotic disorder, heparin-induced thrombocytopenia. The risk of heparin-induced thrombocytopenia is 1% to 3% if unfractionated heparin is continued throughout the postoperative period. When cardiac surgery is urgently needed for a patient with acute or subacute heparin-induced thrombocytopenia, options include an alternative anticoagulant (bivalirudin, lepirudin, or danaparoid) or combining unfractionated heparin with a platelet antagonist (epoprostenol or tirofiban). As heparin-induced thrombocytopenia antibodies are transient, unfractionated heparin alone is appropriate in a patient with previous heparin-induced thrombocytopenia whose antibodies have disappeared.

Pharmacotherapy of heparin- induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a life-and-limb threatening condition that is associated with the development of antibodies that activate platelets and the coagulation system in the presence of unfractionated heparin or low molecular weight heparin. The binding of antibody to heparin-PF-4 complexes can activate platelets, leading to an acute, often catastrophic, thrombotic diathesis. The most common laboratory finding is the development of thrombocytopenia 5 or more days after beginning heparin treatment, which occur in up to 1 - 5% of patients exposed to heparin, depending on type of heparin and indication for anticoagulation. The onset of thrombocytopenia can be immediate or delayed for several weeks after the exposure to heparin. Approximately 50 - 60% of patients who develop HIT manifest acute venous or arterial thrombosis and a significant percentage of these patients die or develop vascular gangrene of a limb that requires amputation. Given the severe sequelae associated with HIT, recognition and immediate medical management is essential. Treatment of a patient with HIT is complex, as there are several different anticoagulants now available which have been shown to be useful. Optimal management depends on each patient's individual clinical manifestations, as well as the need for ongoing anticoagulation therapy. No single agent or treatment approach can be considered to be 'standard practice' as very few clinical trials have been completed, compare different treatment options. The use of warfarin alone in a patient with HIT, must be avoided in order to avoid the possibility of further activating coagulation, which may hasten the development of venous limb gangrene. There are several different tests available that detect HIT antibodies and each has different sensitivity and specificity for HIT. In this review we discuss the epidemiology and natural history of HIT, risk factors associated with the development of HIT and the clinical and laboratory tests that aid in the diagnosis and treatment. Special emphasis is given to addressing the management of HIT in special populations, particularly patients with renal or liver disease, acute coronary syndromes, pregnancy, paediatrics and patients who require cardiopulmonary bypass surgery.

Patients with a history of type II heparin-induced thrombocytopenia with thrombosis requiring cardiac surgery with cardiopulmonary bypass: a prospective observational case series

Heparin-induced thrombocytopenia with thrombosis (HITT) type II is a life-threatening complication of heparin therapy that most often occurs after 5-10 days of exposure to heparin. Anticoagulation is a significant concern for patients with HITT type II being prepared for cardiac surgery requiring cardiopulmonary bypass (CPB). We report a case series of 12 patients with a history HITT type II who underwent CPB and cardiac surgery. Six patients did not express the antibody that mediates HITT type II immediately before surgery. Heparin was used as the anticoagulant for the duration of CPB only, and all these patients did well without thrombotic complications. Six patients expressed the antibody that mediates HITT type II immediately before surgery. Hirudin was used as the anticoagulant for CPB in these patients. The ecarin clotting time was used to guide hirudin therapy during CPB. The patients receiving hirudin did well, but they had a large amount of bleeding, required transfusions of multiple allogeneic blood products, and had a frequent rate of reexploration of the mediastinum after CPB.

Current agents for the treatment of patients with heparin-induced thrombocytopenia

Several counterintuitive treatment paradoxes complicate the management of immune heparin-induced thrombocytopenia (HIT). For example, simple discontinuation of heparin often fails to prevent subsequent HIT-associated thrombosis. Thus, current treatment guidelines recommend substituting heparin with a rapidly acting alternative anticoagulant (eg, danaparoid, lepirudin, or argatroban) even when HIT is suspected on the basis of thrombocytopenia alone ("isolated HIT"). Another paradox-coumarin (warfarin) anticoagulation-can lead to venous limb gangrene in a patient with HIT-associated deep-vein thrombosis. Thus, warfarin is not recommended during acute thrombocytopenia secondary to HIT. However, warfarin can be given as overlapping therapy with an alternative anticoagulant, provided that (1) initiation of warfarin is delayed until substantial platelet count recovery has occurred (to at least above 100 x 10(9)/L); (2) low initial doses of warfarin are used; (3) at least 5 days of overlapping therapy are given; and (4) the alternative agent is maintained until the platelet count has normalized. It has recently been recognized that HIT antibodies are transient and usually do not recur upon subsequent re-exposure to heparin. This leads to a further paradox-patients with previous HIT can be considered for a brief re-exposure to heparin under exceptional circumstances; for example, heart surgery requiring cardiopulmonary bypass, if HIT antibodies are no longer detectable using sensitive assays. For patients with acute or recent HIT who require urgent heart surgery, other approaches include use of alternative anticoagulants (eg, lepirudin or danaparoid) for cardiopulmonary bypass or antiplatelet agents (eg, tirofiban or epoprostenol) to permit intraoperative use of heparin.

New Treatment Options for Heparin-Induced Thrombocytopenia

Heparin, in various forms, is used in a variety of conditions including prophylaxis and treatment of thromboembolic disorders. Although the most common adverse effect related to the use of heparin is bleeding, heparin-induced thrombocytopenia (HIT) can also occur. Two types of HIT exist, HIT type I and type II. HIT type I is a mild and self-limiting disease in which the patient’s platelet count may decrease slightly but will recover with continued treatment. This is in contrast to HIT type II, which may lead to potentially devastating complications. Patients with HIT type II are at increased risk for thromboembolic complications that are mediated through autoimmune reactions and therefore require anticoagulation with danaparoid or one of the direct thrombin inhibitors, including lepirudin, argatroban, or bivalirudin. Limited data are available on the use of these agents in special populations including patients who are pregnant or those undergoing procedures such as percutaneous coronary intervention, cardiopulmonary bypass and dialysis, and pregnancy. Future therapies may include the use of unfractionated heparin in combination with glycoprotein IIb/IIIa inhibitors during cardiopulmonary bypass.