Annals of Vascular Surgery
Volume 21, Issue 6 , Pages 719-722, November 2007

Risk of Heparin-Induced Thrombocytopenia from Heparin-Bonded Vascular Prostheses

  • Leila Mureebe

      Affiliations

    • Division of Vascular Surgery, New York Presbyterian Hospital, New York, NY
    • Corresponding Author InformationCorrespondence to: Leila Mureebe, MD, Division of Vascular Surgery, New York Presbyterian Hospital, Columbia University Medical Center, 5141 Broadway, Suite 3-169, New York, NY 10034, USA
    • ,
  • Joseph A. Graham

      Affiliations

    • Division of Vascular Surgery, University of Missouri Health Care, Columbia, MO
    • ,
  • Ruth L. Bush

      Affiliations

    • Division of Vascular Surgery, Texas A & M University Health Science Center, Scott & White Memorial Hospital and Clinic, Temple, TX
    • ,
  • Donald Silver

      Affiliations

    • Division of Vascular Surgery, University of Missouri Health Care, Columbia, MO

Article Outline

Heparin can be bonded to vascular devices to improve their patency. The purpose of this study was to determine if a clinically utilized heparin-bonded Dacron® graft (HBG) places patients at risk for heparin-induced thrombocytopenia (HIT) and its complications. A commercially available HBG was divided into 1 cm long segments, which were incubated in platelet-poor plasma (PPP) for 24 hr at 37°C. Control segments of non-heparin-bonded Dacron graft were similarly treated. After incubation, aliquots of PPP were assayed for heparin content. Additional graft segments were immersed in PPP from HIT-positive patients to determine if the effluent from the HBG led to platelet activation. Heparin was discharged from the HBG (1.82 ± 0.08 units/cc) but not from the control group (0.00 units/cc, p < 0.05). Platelet aggregation occurred in 85.7% of the plasma samples with leached heparin when mixed with normal donor platelets and plasma containing heparin antiplatelet antibodies (HAAbs). None of the control grafts caused any type of aggregation. HBG may contribute to the development of HAAb that can lead to HIT in previously unaffected patients, and HIT causes a prothrombotic state which counteracts the theoretic advantages of an HBG. Patients receiving an HBG should be made aware of these possibilities.

 

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Introduction 

Infrainguinal bypass grafting is commonly performed for treatment of peripheral arterial disease. Several studies demonstrate the superiority of autologous greater saphenous vein over prosthetic graft as a conduit for infrainguinal bypass. Autogenous vein not only acts as a conduit for blood flow but also inhibits thrombus formation via generation of tissue plasminogen activator and nitric oxide.1, 2, 3 In a meta-analysis evaluating bypass graft patency rates, the pooled primary graft patency was 77.2% for bypasses to the above-knee popliteal artery using vein and 64.8% when bypass was to the below-knee position at 5 years.4 However, prior use for other procedures or the presence of either phlebitis or varicosities renders the vein either unavailable or unusable.

The need for an acceptable alternative conduit has been a major concern in vascular surgery. Several synthetic conduits have been developed. Textile grafts of polyester (Dacron®; DuPont, Wilmington, DE) and the nontextile grafts of polyurethane and expanded polytetrafluoroethylene (ePTFE) have been evaluated as vascular reconstructive conduits. Pooled primary graft patency was 57.4% for ePTFE when used as a conduit for bypass to the above-knee popliteal artery position.4 In addition to inferior patency results, these grafts are subject to material fatigue and infection. To date, no prosthetic has matched the durability results of vein for below-knee bypass. Various venous adjuncts to prosthetic conduit have been evaluated, resulting in some improvement in patency over prosthetic conduit alone.

The indirect thrombin inhibitor heparin has been used as an adjunct to reduce the thrombogenicity of prosthetic grafts.5, 6, 7, 8, 9 Several problems with these grafts have become apparent. They are subject to excess bleeding from the graft10 and leaching of the heparin over time.11 Despite these hurdles, several studies report favorable outcomes utilizing heparin-bonded vascular grafts (HBGs).12, 13

Heparin is immunogenic in some patients, leading to the generation of heparin-associated antiplatelet antibodies (HAAbs). These antibodies play an important role in the development of heparin-induced thrombocytopenia (HIT). The immunologically mediated form of HIT is characterized by the development of thrombotic and sometimes hemorrhagic complications in the setting of heparin administration. Heparin released from intravascular catheters or grafts has been shown to cause HIT. The incidence of HIT due to implantation of heparin-bonded catheters is 0.5-1%.14 HBGs have been shown to induce a prothrombotic state in the presence of HAAbs.11, 15

This study was undertaken to determine whether heparin leached from a commercial Dacron HBG. We also sought to determine whether heparin released from the graft caused platelet aggregation in the presence of HAAbs.

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Materials and Methods 

Materials 

An 8 mm diameter Dacron HBG was utilized for these studies (InterGard™ heparin knitted polyester graft; InterVascular, Montvale, NJ). The control graft was of the same diameter without heparin bonding (InterGard knitted polyester graft).

General Methods 

All data points were repeated in triplicate. All experiments were blinded, and blinding was not broken until data were analyzed. All statistics were analyzed by Student's t-test.

The HBG was sectioned into 1 cm long sections. Sections of control graft were similarly treated. These sections of graft were incubated in 5 cc of platelet-poor plasma (PPP) at varying temperatures and durations. Treatment conditions were 24 hr at 37°C. Aliquots of the PPP after incubation were sent to an independent laboratory for heparin assay to determine the concentration of heparin eluted from the graft into the PPP (American Red Cross, Columbia, MO).

Aggregometry 

Our method for the detection of HAAbs has been described previously.16 Briefly, platelet-rich plasma (PRP) from healthy donors and PPP from both healthy donors and patients known to have HIT were prepared by means of differential centrifugation of whole blood collected in a 3.8% citrate solution. The PRP and the PPP (after incubation with either HBG or control graft) were combined and incubated for 3 min at 37°C. This mixture was further incubated for an additional 15 min in a platelet aggregometer, with 25 μL of one of the following three sources of heparin (final concentration, 1 anti-Xa U/mL): porcine intestinal mucosa (Solopak Laboratories, Elk Grove, IL), bovine lung heparin (Upjohn, Kalamazoo, MI), and enoxaparin (Lovenox; Rhone-Poulenc Rorer Pharmaceuticals, Collegeville, PA). Platelet aggregation was measured with a dual-channel aggregometer (Chronolog, Havertown, PA). Aggregation was defined as a greater than 20% change in optical density.

If aggregation did not occur, 25 μL of adenosine diphosphate (10−4 mol/L solution) was added to ensure the proper reactivity of the donor PRP. The specificity of this test was increased with the use of a two-point platelet aggregation assay. All samples underwent testing with high-concentration unfractionated heparin (final concentration 100 anti-Xa U/mL). If platelet aggregation occurred with high-concentration heparin therapy, the reactions were considered nonspecific.

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Results 

Heparin leached off of all the HBG samples (100%). Heparin was discharged from the HBG in a concentration of 1.82 ± 0.08 units/cc. The control group exhibited no heparin in the effluent PPP (0.00 units/cc, p < 0.05) (Fig. 1).

  • View full-size image.
  • Fig. 1 

    Mean (±1 standard deviation) concentration of heparin in effluent after incubation in equal amounts of graft in PPP. There is a constant amount of heparin released from the HBG and none from the control graft. ∗p < 0.05.

Platelet aggregation occurred in 85.7% of samples from the HBG and none of the control graft samples (p < 0.05). The aggregation studies demonstrated variable reaction strengths, which may reflect the variability seen among donors and the reaction of the antibodies and a variety of epitopes.17, 18

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Discussion 

An ideal alternative vascular conduit continues to elude the vascular community. The ready availability of prosthetic makes it an attractive option, if modifications improve patency rates to be comparable to venous conduit. Unmodified prosthetics, both polyester and ePTFE, have poor patency rates when applied to the arterial tree below the knee. One chemical modification that has the potential to reduce the thrombogenicity of these grafts is the addition of heparin. Heparin, an indirect inhibitor of thrombin, is an effective systemic anticoagulant and has properties other than anticoagulation.19, 20, 21, 22, 23, 24

A 2001 study by McCollum et al.12 demonstrated superior primary patency of this HBG compared to PTFE. At 1 year, the primary patency of HBG was 70% compared to 56% for the uncoated grafts. A follow-up study in 2004 included 179 patients who underwent bypasses to the above-knee popliteal artery and 30 patients who underwent bypasses to the below-knee popliteal artery; although the patency rates of the HBG were better at 3 years, at 5 years there was no statistical difference between the HBG and PTFE for either target.13 Lambert and colleagues25 reviewed their experience using this HBG and found a primary patency of 69% at 4 years in patients with above-knee bypasses. Their data also showed a primary patency of 58% over the same period for the below-knee bypasses in which a vein cuff was used. Their data suggest no advantage for the use of the HBG. There were eight major amputations in their series, for a 17% amputation rate. According to the manufacturer's data, use of this graft is associated with an amputation rate of 2.7%, a thrombosis rate of 27.7%, and a primary patency rate of 70.6%. However, data showed amputation, thrombosis, and primary patency rates of 0.4%, 17.1%, and 91.6%, respectively, for the InterGard knitted Dacron graft without heparin coating.

The potential issue of HIT or a hypercoagulable state was not addressed in the above studies. HIT and the thrombotic complications associated with the syndrome are not dose-related, and the presence of the heparin on the graft could potentially become the initiating event.14, 15 Our study shows that heparin released into patient serum can lead to aggregation in susceptible plasma. There is a possibility that even bound heparin in the bound phase may potentially lead to development of HAAbs and HIT. Furthermore, the method by which heparin is bonded to the graft may affect the development of HAAbs, which we did not evaluate. The graft in this study utilizes heparin of porcine mucosal origin coupled to the polyester graft surface using molecules of tridodecyl methyl ammonium chloride (TDMAC). Heparin and TDMAC form an insoluble complex, and the heparin bonds to the graft surface by the action of hydrophilic forces of the TDMAC and the polyester. Collagen (bovine origin) coating is used on the surface of the graft to decrease the transgraft bleeding and to slow the release of heparin from the graft.

Once the diagnosis of HIT is made, the estimated morbidity of HIT is between 5% and 20%. Morbidity is most commonly associated with thrombotic events.26 Surgeons should realize that the incidence of HIT is higher in patients with vascular disease than in the general population.15 Treatment of HIT requires elimination of all sources of heparin. In the case of an HBG, this would involve removal of the vascular graft if heparin was still being eluted or was still bonded to the graft in any concentration. Once the heparin source has been removed, a hypercoagulable state still exists in many patients and may require treatment with an alternate anticoagulant such as a direct thrombin inhibitor.

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Conclusion 

The potential for HIT is apparent with the leaching of the heparin from the graft studied. Furthermore, the luminal surface of the graft exposes heparin to circulating plasma and may lead to the formation of HAAbs even if not liberated into the plasma. Our current experience is only with a single graft. We have not evaluated any other HBGs, including those manufactured with ePTFE. Although HIT from graft-bound or leached heparin has not yet been reported clinically, caution should be exercised in placing these grafts in patients with any history of HAAbs or HIT. All patients should be counseled on the possibility of HIT.

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References 

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PII: S0890-5096(07)00288-9

doi:10.1016/j.avsg.2007.07.016

Annals of Vascular Surgery
Volume 21, Issue 6 , Pages 719-722, November 2007

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