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J Thorac Cardiovasc Surg 2002;124:1063-1067
© 2002 The American Association for Thoracic Surgery

Editorials

Adverse events in the use of HeartMate vented electric and Novacor left ventricular assist devices: Comparing apples and oranges

From the Department of Surgery, Division of Cardiothoracic Surgery, and the Department of Medicine, Division of Cardiology, Washington University School of Medicine, St Louis, Mo.

Received for publication Jan 9, 2002. Accepted for publication Feb 3, 2002. Address for reprints: Michael K. Pasque, MD, Division of Cardiothoracic Surgery, Suite 3108 Queeny Tower, Barnes-Jewish Hospital, One Barnes-Jewish Hospital Plaza, St Louis, MO 63110 (E-mail: pasquem{at}msnotes.wustl.edu).

	Introduction

Top Introduction Thromboembolism Anticoagulation Device reliability Conclusion References

 
There are two Food and Drug Administration-approved, implantable, electrical left ventricular assist devices (LVADs) currently available to cardiothoracic surgeons for use in patients with end-stage heart failure. These are the HeartMate vented electric (VE) and Novacor devices, marketed by Thoratec Corporation (Pleasanton, Calif) and WorldHeart Corporation (Ottawa, Ontario, Canada), respectively. Until recently, objective data concerning device-use end points for these two LVADs was limited. Device choice, therefore, has been influenced primarily by single center reports, anecdotal communications, or marketing rhetoric. For the first time, however, we now have important objective information characterizing anticipated adverse event rates based on clinical outcomes in large multicenter trials and objective reports of in vitro durability testing. Clinicians who accept the responsibility of using this complex therapy in patients with end-stage heart failure must be expected to objectively analyze this available information.

First things first. We are the surgical and medical directors of the heart failure service at Washington University Medical School at Barnes-Jewish Hospital in St Louis, Missouri. In this capacity, we have served as the institutional principal investigators of the Novacor bridge-to-transplant and INTrEPID trials at Barnes-Jewish Hospital. We have no financial interest in, and do not serve on the medical advisory boards of, either of the companies that market these two devices. We currently have both devices available at our institution for the treatment of end-stage heart failure.

	Thromboembolism

Top Introduction Thromboembolism Anticoagulation Device reliability Conclusion References

 
Both of the currently available LVADs require that nonphysiologic surfaces be placed in contact with the recipient blood pool, therefore rendering a finite incidence of thromboembolism as a recognized complication of their use. Much has been made of the novel textured "biologic" blood-contacting surface of the HeartMate VE device, which has allowed use of this device without routine systemic anticoagulation, other than platelet-inhibiting drugs. Much has also been made of the history of thromboembolism from the smooth "nonbiological" surface of the Novacor LVAD and its propensity to thromboembolism despite mandated anticoagulation with heparin or warfarin.

Information concerning thromboembolism is available for these devices from their respective "user's manuals" which, in both cases, contain summaries of their Food and Drug Administration pre-market approval submission data derived from large multicenter bridge-to-transplant trials. ^1,2 Obviously, the definitions used in reporting adverse events are of critical importance in any attempt at comparison. These events and their accompanying definitions have been gathered from user's manuals, investigator brochures, and journal publications and are summarized in Table 1. Unfortunately, because of the widely variable, often ill-defined, and usually overlapping character of the definitions used in these reports, direct comparison is nearly impossible. For example, the "neurological dysfunction" rate reported in the current HeartMate VE user's manual is 27%, ¹ whereas an "embolism (central nervous system)" rate of the Novacor device is reported in their user's manual as 26.9%. ² Are these two rates comparable? Probably not (see Table 1). Further, two other pertinent event categories are reported in the Novacor user's manual. A "neurologic deficit" rate is reported as 41%, ² whereas a separate non-neurological thromboembolic event rate of 14.7% is also reported from the same patient data. ² Unfortunately, a total thromboembolic rate cannot be derived by simply adding these rates since some patients had events that fit into all three categories and therefore would be counted three times. The HeartMate VE user's manual does not report a separate non-neurological thromboembolic rate although it does report a global thromboembolic rate of 12%. ¹

One possible solution to the lack of uniformity in thromboembolic event definitions is for individual centers to analyze their event rate data by using the definitions used in reporting the other device's event rates. For example, the thromboembolic event rate of our single-center experience with the Novacor device can be examined with the use of the HeartMate VE user's manual definitions of thromboembolic events. In 30 Novacor implants we have seen a "neurological dysfunction" (using the HeartMate VE user's manual definition) rate of 20% (compared with 27% reported by the HeartMate VE multicenter study) and a "thromboembolism" rate of 10% (compared with 12% reported by the HeartMate VE multicenter study). Once again, our data represent a single-center experience, not a multicenter trial. In addition, the majority of our Novacor implants were performed after the introduction of the new conduits (2/98) that reportedly have significantly reduced the thromboembolic event rate after implantation of the Novacor device. ⁴

The recently reported REMATCH trial ⁵ (Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure) provided additional insights regarding the thromboembolic risk of the HeartMate VE device. Once again, definitions remain problematic. In this trial, 24% of implant patients had "serious neurological events," including "stroke, transient ischemic attacks, and toxic or metabolic encephalopathy." Fully 10% of patients in the device implant group had events that fit study criteria for an "ischemic stroke." "Serious" adverse events were defined as those adverse events that "caused death or permanent disability, were life-threatening, or required or prolonged hospitalization." This trial was performed in an older group of nontransplant candidates whose risk of "serious neurological events" would be expected to be higher than in the bridge-to-transplant patient population.

	Anticoagulation

Top Introduction Thromboembolism Anticoagulation Device reliability Conclusion References

 
Anticoagulation with heparin, followed by warfarin and antiplatelet medications, is routinely recommended after Novacor assist device implantation. According to the user's manual accompanying the HeartMate VE device, antiplatelet medications are recommended, but systemic anticoagulation with heparin or warfarin is not routinely required. This recommendation deserves further scrutiny in light of the thromboembolic potential suggested by the above-described reports, especially when related to other cardiovascular conditions for which more aggressive anticoagulation is considered the standard of care. For instance, the incidence of stroke in a population of patients with chronic atrial fibrillation is only 5% per year and the incidence of stroke or valve thrombosis in patients with a St Jude Medical bileaflet mechanical valve (St Jude Medical, Inc, St Paul, Minn) in the aortic position who are not anticoagulated with warfarin is only 2% to 6% per year. ^6-13 Yet the unequivocal recommendation, and certainly the standard of care in these familiar clinical settings, is routine anticoagulation with heparin or warfarin. The HeartMate VE device has a frank thromboembolic rate of at least 12% over an average implant duration of only 112 days. The "neurological dysfunction" rate (including transient ischemic attacks) of 27% suggests that the thromboembolic potential may be considerably higher. Anticoagulation, however, is still not routinely recommended.

The recommendation to not anticoagulate may have been reasonable at the time of this novel device's introduction. Unfortunately, the claims do not appear to be borne out by the subsequent trial data. It is undoubtedly true that some of the embolic events reported with HeartMate VE LVAD use were due to device infection, as they are with all LVADs. These embolic events may or may not have been prevented by anticoagulation. Nonetheless, it is hard to believe that the risk of anticoagulation, which is routinely recommended and used in association with almost all other ventricular assist devices, outweighs an embolic risk that ranges from 12% to 27%. Just because the HeartMate VE device may (or may not) have a lower incidence of thromboembolic events than competing devices does not stand as a rationale for avoidance of anticoagulation in a patient population with such a high risk of thromboembolism. In light of the available data, avoidance of systemic anticoagulation as the "standard of care" in these patients warrants reconsideration.

	Device reliability

Top Introduction Thromboembolism Anticoagulation Device reliability Conclusion References

 
Information on medical device durability is critical in the clinical decision-making process regarding the implantation of all medical devices, let alone those that perform the vital role of circulatory support. We have come to expect near perfection in implantable medical devices because of the reliability records established by many of the medical device companies. Certainly, the incredible record of the pacemaker and implantable defibrillator industry, representing a technology of nearly the same age as the implantable LVAD, represents the benchmark. With over 30 years of development, LVAD failure rates, when used in the relatively short average time frame of the bridge-to-transplant application, could reasonably be expected to be 1% or less. With the impending reality of the use of these devices as permanent therapy (in patients with end-stage heart failure who are not transplant candidates), reliability issues are of critical importance. Obviously, when clinicians are faced with decisions regarding possible permanent device implantation in nontransplant candidates, they must have some reasonable expectation of the potential length of support that is being offered to their patients by each available device. In this regard, two sources of reliability information are available for both devices: in vitro durability testing data and actual in vivo human use durability information.

In vitro durability testing has been performed in a relatively similar fashion for both devices by their respective manufacturers. A set number of each of these two devices were run in vitro to either failure or a fixed termination period. The data from these studies were then subjected to reliability modeling to 90% confidence intervals by their respective manufacturers. The results are documented in Table 2. The 2-month and 1-year reliability data for the HeartMate VE device was taken from the HeartMate SNAP-VE user's manual. ¹ To our knowledge, the 2- and 3-year reliability data for the HeartMate VE device have not been published. The Novacor durability information at 2-month, 1-year, 2-year, and 3-year time periods was taken from their published test information. ^2,14

Clinical use reliability information is also available from several published sources. Although the device failure rate was reported as only 1% in the HeartMate VE bridge-to-transplant trial, 9% of patients had to hand pump or hook their device to the pneumatic console because of component failure. ^1,3 In other words, nearly 1 of every 10 patients had serious device component failure at an average implant duration of 112 days (<4 months). Admirably, 72% of the patients who used the unique back-up components of the HeartMate VE device survived to transplantation. Failure modes were apparently multiple and usually not predictable.

In addition, the REMATCH trial supplies more clinical use information on the durability of the HeartMate VE device over a longer implant duration. Of considerable importance, there were no system failures that caused death or resulted in device replacement at 12 months of follow-up. ⁵ Probability of device failure, however, at 24 months was 35%. The device was replaced in 10 patients. Failure of the LVAD resulted in 7 deaths in a cohort of only 22 patients who were "at risk" beyond 12 months of support duration. Once again, multiple failure modes were demonstrated, including inflow valve failure, outflow graft erosion, rupture of the lining, motor failure, and wear on the bearings. ⁵

Information regarding the reliability of the Novacor device during clinical use is available from the US and European bridge-to-transplant experience. Unfortunately, well-documented clinical longevity information, as offered by the HeartMate VE REMATCH Trial, is not available because of the early status of the Novacor INTrEPID trial. In the US and European bridge use, however, it is noteworthy that 7 Novacor patients were supported beyond 3 years with 2 of these patients supported beyond 4 years. In more than 1300 implants, representing 361 patient-years of support, there has been only one documented case of pump failure resulting in device stoppage (from an encapsulation leak that fouled the electronics on the first postoperative day) that required immediate device replacement. To date, wear of the pump main bearing represents the only long-term device failure mode. Bearing wear is apparently detectable by device parameter interrogation and tracking and has allowed elective device replacement on 6 occasions during long-term support.

	Conclusion

Top Introduction Thromboembolism Anticoagulation Device reliability Conclusion References

 
When compared with other available devices, the HeartMate VE LVAD appears to have an admirable record with regard to thromboembolic event rates, even without systemic anticoagulation in the majority of the reported patients. Whether routine anticoagulation in all patients would have resulted in a further reduction of thromboembolic rates is unknown. The Novacor thromboembolic rates appear to be higher than those reported with the HeartMate VE device, although the often unknown, overlapping, and widely disparate definitions of thromboembolic events make direct comparison difficult. Despite the apparent advantage in thromboembolic event rates demonstrated by the HeartMate VE bridge-to-transplant trial, the actual rate of clinically significant thromboembolic events may not have differed widely from the Novacor bridge experience. This is suggested by the fact that an apparently higher percentage of the Novacor bridge patients remained transplant candidates and actually survived to transplantation (78%) than in the HeartMate VE bridge trial (70%).

The HeartMate VE continues to have an advantage over the Novacor device in that anticoagulation with heparin and warfarin is not routinely recommended. The threat of stroke, transient ischemic attack, and non-neurological peripheral embolism is not, however, eliminated by the "biologic" surface of the HeartMate VE device. Quite to the contrary, the neurological dysfunction and thromboembolic rates documented in the bridge-to-transplant and REMATCH trials are considerable, especially considering that a significant number of these patients received warfarin at some point during their support. The combined thromboembolic rate of the HeartMate VE device, which may be several fold higher than that reported in unanticoagulated patients who have chronic atrial fibrillation or who have a St Jude Medical mechanical valve in the aortic position, would indicate that the routine recommendation for isolated antiplatelet therapy, without heparin or warfarin, may deserve reexamination.

The mechanical failure rate of the HeartMate VE device is of considerable concern. To our knowledge, this failure rate far exceeds that associated with any implantable mechanical device that is currently used to treat any other medical condition in the United States. The fact that the failure modes are multiple and unpredictable suggest that attempts to retro-fix the problems may be problematic. This is of specific concern when the clinical goal is permanent implantation for long-term circulatory support in patients with end-stage heart failure.

	References

Top Introduction Thromboembolism Anticoagulation Device reliability Conclusion References

 

1. HeartMate Sutures Not APplied Vented Electric (SNAP-VE) Left Ventricular Assist System (LVAS): directions for use. Thoratec Corporation, Pleasanton (CA). Document 29123, revision B; April 25, 2001. p. 1-66.
   
2. Physician's Manual Novacor LVAS. Baxter Healthcare Corporation, Novacor Division, Technical Support Department, Deerfield (IL). Document GCP 9105-1097, revision B; 1998. p. 1-1-16-2.
   
3. Frazier OH, Rose EA, Oz MC, Dembitsky W, McCarthy P, Radovancevic B, et al. Multicenter clinical evaluation of the HeartMate vented electric left ventricular assist system in patients awaiting heart transplantation. J Thorac Cardiovasc Surg. 2001;122:1186-95.[Abstract/Free Full Text]
   
4. Portner PM, Jansen PGM, Oyer PE, Wheeldon DR, Ramasamy N. Improved outcomes with an implantable left ventricular assist system: a multicenter study. Ann Thorac Surg. 2001;71:205-9.[Abstract/Free Full Text]
   
5. Rose EA, Gelijns AC, Moskowitz AJ, Heitjan DF, Stevenson LW, Dembitsky W, et al. Long-term use of a left ventricular assist device for end-stage heart failure. N Engl J Med. 2001;345:1435-43.[Abstract/Free Full Text]
   
6. Ad N, Cox JL. Stroke prevention as an indication for the maze procedure in the treatment of atrial fibrillation. Semin Thorac Cardiovasc Surg. 2000;12:56-62.[Medline]
   
7. Wolf PA, Abbott RD, Kannel WB. Atrial fibrillation as an independent risk factor for stroke: the Framingham study. Stroke. 1991;22:983-8.[Abstract/Free Full Text]
   
8. Hart RG, Sherman DG, Easton JD, Cairns JA. Prevention of stroke in patients with nonvalvular atrial fibrillation. Neurology. 1998;51:674-8 1.[Abstract/Free Full Text]
   
9. The Boston Area Anticoagulation Trial for Atrial Fibrillation Investigators. The effect of low-dose warfarin on the risk of stroke in patients with nonrheumatic atrial fibrillation. N Engl J Med. 1990;323:1505-11.[Abstract]
   
10. Pass HI, Sade RM, Crawford FA, Hohn AR. Cardiac valve prostheses in children without anticoagulation. J Thorac Cardiovasc Surg. 1984;87:832-5.[Abstract]
    
11. Ribeiro PA, Al Zaibag M, Idris M, Al Kasab S, Davies G, Mashat E, et al. Antiplatelet drugs and the incidence of thromboembolic complications of the St Jude Medical aortic prosthesis in patients with rheumatic heart disease. J Thorac Cardiovasc Surg. 1986;91:92-8.[Abstract]
    
12. Hartz RS, LoCicero J 3rd, Kucich V, DeBoer A, O'Mara S, Meyers SN, et al. Comparative study of warfarin versus antiplatelet therapy in patients with a St Jude Medical valve in the aortic position. J Thorac Cardiovasc Surg. 1986;92:684-90.[Abstract]
    
13. Baudet EM, Oca CC, Roques XF, Laborde MN, Hafez AS, Collot MA, et al. A 5-year experience with the St Jude Medical cardiac valve prosthesis: early and late results of 737 valve replacements in 671 patients. J Thorac Cardiovasc Surg. 1985;90:137-44.[Abstract]
    
14. Lee J, Miller PJ, Chen H, Conley MG, Carpenter JL, Wihera JC, et al. Reliability model from the in vitro durability tests of a left ventricular assist system. ASAIO J. 1999;45:595-601.[Medline]
    

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