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Ann Thorac Surg 2000;70:533-537
© 2000 The Society of Thoracic Surgeons

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Original articles: cardiovascular

Alterations in coagulation after implantation of a pulsatile Novacor LVAD and the axial flow micromed DeBakey LVAD

^a Department of Anesthesia, Deutsches Herzzentrum Berlin, Berlin, Germany
^b Department of Cardiothoracic and Vascular Surgery, Deutsches Herzzentrum Berlin, Berlin, Germany

Address reprint requests to Dr Koster, Deutsches Herzzentrum Berlin, Augustenburgerplatz 1, D-13353 Berlin, Germany
e-mail: koster{at}dhzb.de

	Abstract

Top Abstract Introduction Patients and methods Results Comment References

 
Background. The MicroMed DeBakey left ventricular assist device (LVAD) is a chamber and valveless axial flow blood pump. We investigated parameters of the coagulation system in patients after implantation of the axial flow LVAD and patients following implantation of a pulsatile Novacor LVAD.

Methods. Six consecutive patients of both groups were investigated over a period of 6 weeks after implantation. ß-Thromboglobulin, platelet factor 4, factor XIIa, thrombin/antithrombin complexes, plasmin/₂-antiplasmin complexes, and D-Dimer levels were measured.

Results. With the exception of the plasmin/₂-antiplasmin levels in the Novacor group, all parameters were elevated in both groups. The levels of ß-thromboglobulin, platelet factor 4, factor XIIa, and plasmin/₂-antiplasmin were significantly increased in the axial flow LVAD group.

Conclusions. The axial flow LVAD strongly influences the systems of contact activation and fibrinolysis. The elevation of platelet proteins appears to follow platelet damage. Although no thromboembolic events were observed in both groups, elevation of thrombin/antithrombin complexes provides convincing evidence of an increased activation of the coagulation system and the concomitant risk for the development of thromboembolism.

	Introduction

Top Abstract Introduction Patients and methods Results Comment References

 
Implantation of a left ventricular assist device (LVAD) may serve as a bridge to transplantation or for the recovery of the failing heart [1]. Most of the LVADs, such as the Novacor (Baxter, Oakland, CA) or the HeartMate system (Thermo Cardiosystems Inc, Woburn, MA) are based on the principle of pulsatile blood flow [2–5]. This pulsatile blood flow is achieved by magnetic or pneumatic compression of the blood in the chamber of the device. In contrast, the recently introduced MicroMed DeBakey LVAD (MicroMed Technology, The Woodlands, TX) works by the production of an axial blood flow generated by a turbine with approximately 7,500 to 12,500 r/min.

This flow is generated by a rotating impeller, which sucks the blood from the left ventricular apex and pumps it into the ascending thoracic aorta. The pulsatile flow of the patient’s heart and the axial flow of the LVAD are in parallel, which leads to a reduction of the arterial pressure amplitude. This amplitude depends on the difference between the pressure of the device and the native left ventricle (contingent on its residual function). Similar to the Novacor or HeartMate system, an external portable control battery pack allows the patients mobility. Compared with the other systems, the MicroMed DeBakey LVAD is smaller in size and lighter. This contributes to easier implantation when compared with the other systems. The device has been developed by the space agency NASA in collaboration with Baylor College of Medicine in Houston, TX.

However, the implantation of all mechanical assist devices is followed by changes in the coagulation system [6–8]. In patients following implantation of a VAD, an increased bleeding tendency in the early postoperative period and thrombus formation with consequent embolism in the later postoperative period have been described [6, 7].

The aim of the present investigation was to asses the parameters of the coagulation system, including the procoagulant coagulation cascade, the contact activation system, the fibrinolytic system, and platelet function in patients after implantation of the DeBakey LVAD, and to compare this with observations in patients following implantation of the Novacor system.

	Patients and methods

Top Abstract Introduction Patients and methods Results Comment References

 
Patients
Twelve patients, three women and nine men, were enrolled in the investigation. The age ranged from 32 to 63 years, with a mean of 47 ± 7.7 SD. In 10 patients, the implantation of the device was necessary because of dilated cardiomyopathy and, in 2, because of ischemic cardiomyopathy. All patients received an isolated left ventricular assist device. Six patients received an axial flow MicroMed DeBakey LVAD and the other 6 patients received a pulsatile Novacor LVAD.

Devices
The MicroMed DeBakey LVAD achieves axial blood flow by a valveless rotary blood pump with inflow from the left ventricular apex and an outflow graft into the ascending aorta. The pulsatile Novacor LVAD has a polyurethane pumping chamber and an inflow graft from the left ventricular apex and an outflow graft to the ascending aorta. Both grafts are made of Dacron (C.R. Bard, Haverhill, PA) and contain biological heart valves.

Anticoagulation
In both groups, the same protocol of anticoagulation was applied. Before implantation of the LVAD, anticoagulation was performed with unfractionated heparin (Liquemin, Roche, Germany) to an activated thromboplastin time (aptt) of 40 to 60 seconds. Anticoagulation during surgery with cardiopulmonary bypass was accomplished with unfractionated heparin according to the Hepcon HMS regimen (Medtronic, Parker, CO). In all patients, during cardiopulmonary bypass, the kallikrein inhibitor aprotinin was given according to the high-dose regimen with a bolus of 2 x 10⁶ kIU for the patient, 2 x 10⁶ kIU for the priming solution, and a continuous infusion of 500,000 kIU/h. After termination of cardiopulmonary bypass, unfractionated heparin activity was totally reversed by protamine. The intravenous systemic heparinization was restarted 6 to 12 hours after surgery to achieve a target activated clotting time between 160 and 180 seconds.

Antiplatelet therapy
Antiplatelet agents were simultaneously administered according to the in vitro-induced platelet aggregation test using the method of Born. After removal of all drainage tubes, the anticoagulation was switched to coumarines and aspirin, and dipyridamole depending on the results of the platelet aggregation test.

The test were performed with adenosine diphosphate 20 µmol/L, epinephrine 100 µl/L, collagen 190 µg/mL, and arachidonic acid 500 µg/mL. The target value for adenosine diphosphate and epinephrine-induced platelet aggregation was 30% to 50% and less than 40% for arachnidonic acid. A decrease of the aggregation to a value of 70% to 90% was regarded as sufficient for collagen-induced platelet aggregation.

Antiplatelet therapy was initiated with aspirin beginning with 50 mg/d up to 200 mg/d as a maximum dosage. If patients did not respond to this therapy, dipyridamole (400 to 1,000 mg/d) was given.

Laboratory investigation
Blood samples were collected on the first and third postoperative day and, thereafter, twice a week for a period of 2 months. Platelet factor 4 (PF4) and ß-thromboglobulin (ß-TG) were measured in citrate, theophylline, adenosine, dipyramidole (CTAD) plasma. Platelet factor 4 was measured by the use of the ASSERACHROM PF4 ELISA (Boehringer, Mannheim, Germany; range of reference 0 to 0.5 IU/mL). The ß-TG tests were performed by the use of the ASSERACHROM ß-TG ELISA (Boehringer; range of reference 10 to 40 IU/mL).

Factor XIIa (FXIIa) D-Dimers, plasmin-₂-antiplasmin complexes (PAP), and thrombin-antithrombin complexes (TAT) were measured in citrated blood samples. The FXIIa testing was performed with the FXIIa ELISA (PROGEN, Heidelberg, Germany; range of reference 18 to 25 µg/mL) and the D-Dimers were evaluated by the use of the AUTO Dimertest (Organon Teknika, Eppelheim, Germany; range of reference between 130 and 200 µg/L). The measurement of TAT complexes was performed with the ENZYGNOST TAT micro ELISA (Dade Behring, Marburg, Germany; range of reference 1.0 to 4.0 µg/L) and the PAP complexes were measured by the ENZYGNOST PAP ELISA (Dade Behring; range of reference 120 to 700 µg/L).

Statistical analysis
Statistical analysis was performed by the use of the Pearson’s correlation coefficient and the Student’s t tests. Differences between groups with a p value less than 0.01 were regarded to be significant.

	Results

Top Abstract Introduction Patients and methods Results Comment References

 
Twelve patients were enrolled in the investigation. There was no significant difference in the preimplantation data between the 2 groups. Moreover, there were no significant differences in the postoperative course of the patients between the groups with regard to transfusion requirements, postoperative blood losses, duration of mechanical ventilation, parameters indicating infection (leukocyte count, C-Reactive Protein), or stay in the intensive care unit after implantation of the device.

In all patients, anticoagulation followed the same protocol and there was no significant difference in the values of the plasmatic coagulation assays, platelet count, and platelet aggregation assays between the groups. In no patient was clinical evidence of thromboembolism obvious during the period of investigation.

The results for each patient are expressed as the mean value for all 12 measurements of a single parameter over the period of 6 weeks.

In the DeBakey group, the ß-TG value ranged from 218.3 IU/mL to 285.4 IU/mL with a mean of 258.2 IU/mL and a SD of ±138.7 (Fig 1). In the Novacor group, the ß-TG value ranged from 96.0 IU/mL to 224 IU/mL with a mean of 137.8 and a SD of ±77.9 IU/mL (Fig 1). The p value between the groups was 0.002.

View larger version (15K): [in this window] [in a new window]   Fig. 1. Platelet factor 4 (PF4) (A) and ß-thromboglobulin (B). Each patient is represented by a symbol. The line represents the median of the group. For both parameters, the p value was p < 0.001 between the groups.

In the DeBakey group, the factor XIIa ranged from 1.14 to 6.28 ng/mL with a mean of 3.76 ng/mL and a SD of ±2.26 ng/mL (Fig 2). In the Novacor group, the factor XIIa ranged from 1.81 ng/mL to 5.37 ng/mL with a mean of 2.84 ng/mL and a SD of ±1.51 ng/mL (Fig 2). The p value between the groups was 0.006.

View larger version (15K): [in this window] [in a new window]   Fig. 2. Plasmin/anti-plasmin (PAP) complexes (A) and factor XIIa (B). Each patient is represented by a symbol. The line represents the median of the group. For PAP, the p value was p < 0.001 between the groups and, for factor XIIa, the p value was 0.006.

View larger version (16K): [in this window] [in a new window]   Fig. 3. D-Dimers (A) and thrombin-antithrombin (TAT) complexes (B). Each patient is represented by a symbol. The line represents the median of the group. For D-Dimers, the p value was 0.66 between the groups and, for TAT, the p value was 0.95.

The D-Dimer levels in the DeBakey group ranged from 439.0 µg/mL to 1,552 µg/mL with a mean of 11,270 µg/mL and a SD of ±769.7 µg/mL (Fig 3). The level in the Novacor group ranged from 1,026.1 µg/mL to 1,934 µg/mL with a mean of 1,320.7 µg/mL and a SD of ±544 µg/mL (Fig 3). The p value between the groups was 0.66.

	Comment

Top Abstract Introduction Patients and methods Results Comment References

 
Changes in the coagulation system in 6 patients after implantation of the axial flow DeBakey LVAD and in 6 patients after implantation of the pulsatile Novacor LVAD were investigated during the first 6 postoperative weeks. The parameters included platelet factor 4 (PF4) and ß-thromboglobulin (ß-TG), which are markers for both platelet activation and platelet degradation. Moreover, the fibrinolytic system was observed by monitoring plasmin/₂-antiplasmin (PAP) complexes and D-Dimers. The influences on the plasmatic coagulation system were monitored by the thrombin generation via measurement of the thrombin/antithrombin (TAT) complexes. Factor XIIa was monitored to investigate the influences on the contact activation system. The mean values for each group during the entire course of the investigation were elevated for all measured parameters except the PAP complexes in the Novacor group. When compared with the Novacor group, there was a significant increase in the levels of ß-TG, PF4, Factor XIIa, and the PAP complex in the DeBakey LVAD group. No significant differences between the groups was observed for the D-Dimer and TAT levels.

Pulsatile LVADs have been successfully implanted over a period of several years as a bridge to transplantation. However, despite the development of biocompatible surfaces, thromboembolism still remains a problem. After the implantation of the Novacor system, a rather high percentage of thromboembolic complications, despite elaborate anticoagulation protocols, have been observed [9]. However, it remains unclear whether thromboembolic complications result from thrombus formation on the artificial surfaces, in particular in the compression chamber of the system itself, or to distinct flow patterns of the device at the site of the valves or to thrombus formation in the native ventricle [10].

Because totally implanted pulsatile devices need a compliance volume shifting chamber, these devices are large and heavy, which restricts the mobility of the patient and limits the option for permanent implantation [11]. Therefore, the development of a nonpulsatile permanent rotary blood pump was suggested [11]. However, certain features were necessary prerequisites for such a system, namely, small size, atraumatic blood flow, low thrombogenicity, a durable simple design, and a low energy requirement with easy controllability [11]. The MicroMed DeBakey LVAD was the first such pump to become available in November 1998. Because it does not need a compliance chamber, the rotary DeBakey LVAD is small in size and light. Moreover, certain characteristics of the device’s design are thought to contribute to a reduction of thrombogenicity, for example, because there is no compliance chamber, no residual volume remains in the device, the rapid rotation of the impeller and the fast blood flow may additionally inhibit thrombus formation in the device. Because of the axial flow system, it does not need a valved conduit, which should contribute to a reduction of thrombus formation in the device and blood trauma.

Changes in platelet morphology have also been described with the use of the Novacor system [12] and reports have appeared that attribute the use of rotary or centrifugal blood pumps as the cause of platelet damage [13]. These findings are supported by our results in that the levels of PF4 and ß-TG, which are released from the platelet-alpha granula, were significantly elevated when compared with the values for the Novacor device (Fig 1). Because the platelet activation was comparable in both groups and the patients were treated with antiplatelet agents according to the results of aggregometry, these elevations of PF4 and ß-TG appear to be predominantly caused by cell damage and not by cell activation.

Interestingly, in the DeBakey group, both FXIIa and PAP levels were significantly increased when compared with the Novacor group (Fig 2). These findings are of significance as PF4 inhibits FXII [14], and platelets are known to inhibit fibrinolysis [15]. Therefore, the activation of both systems may be higher as these results indicate. Because FXIIa plays a pivotal role in the initiation of fibrinolysis [16] in both the contact activation on foreign surfaces and the endothelium, the elevated FXIIa levels may be responsible for the initiation of fibrinolysis. However, there was no statistically significant difference in the other measured marker of the fibrinolytic system, the D-Dimers, although in both groups this parameter was elevated. Limitations in the assay, in particular in the higher range of the D-Dimer test, may be responsible for these findings.

However, the clinical relevance of an increased fibrinolytic activity in patients on a VAD is unclear. Although fibrinolysis in the early postoperative period may contribute to adverse effects, such as bleeding [6], moderate hyperfibrinolysis in the later periods may contribute to a reduction of thromboembolic complications.

There was no significant difference in the values for the TAT complexes in both groups. However, in both groups nearly fourfold elevated levels of TAT, as well as increased D-Dimers, indicate an excessive generation of thrombin (and fibrin). Therefore, although during the period of the investigation no thromboembolic events were observed, these elevated parameters provide evidence of a strong activation of the plasmatic coagulation cascade and a possible concomitant increase of thrombogenic risk in both groups.

We conclude that the implantation of the MicroMed DeBakey high-velocity axial flow LVAD, when compared with the established pulsatile Novacor LVAD, is associated with a significant elevation of platelet-granula proteins. This elevation seems to be caused by platelet damage by the device. Furthermore, systems of contact activation and fibrinolysis are significantly accelerated in these patients when compared with the Novacor group. The elevation of TAT complexes and D-Dimers strongly suggests an increased risk of thromboembolic events in both groups.

These findings of an increased thrombin generation and fibrinolysis have also been reported in patients with the HeartMate system, which is thought to have improved biocompatibility of the inner artificial surface because of the generation of a pseudo-endothelium layer [17, 18]. However, reduced rates of thromboembolic events using this system have been reported [17]. Therefore, it can be speculated that the balance between the systems of thrombin generation and fibrinolysis is of pivotal importance for the reduction of thromboembolic complications [17]. Although as described previously, the MicroMed DeBakey high-velocity axial flow LVAD has characteristics that are thought to reduce thrombogenicity in the device, such as lack of a compression chamber, valves, and the fast impeller rotation, its potentially increased thrombogenicity appears to be from systemic effects, such as platelet trauma and activation of plasmatic coagulation systems.

However, the balance between the coagulation systems, as a part of the inflammatory system, is not only influenced by the device itself but also by the clinical course of the patient. It may be that, during prolonged periods of implantation and ongoing recovery, the balance between the systems may be shifted toward coagulation as well as fibrinolysis. Therefore, further investigations with larger patient groups over longer periods of implantation are necessary to address this subject.

	Acknowledgments

 
I thank Miss Annette Gaussmann for the professional preparation of the figures and Miss Tonie Derwent for her editorial work on the text. Moreover, I thank Miss Cornelia Harke for the accurate performance of the laboratory work. I am also indebted to Prof Fritz Mertzlufft, Homburg Saar, for the helpful discussions during the preparation of the manuscript.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Matthias Loebe George P. Noon Roland Hetzer Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Koster, A. Articles by Hetzer, R. Search for Related Content PubMed PubMed Citation Articles by Koster, A. Articles by Hetzer, R.