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Eur J Cardiothorac Surg 2004;25:958-963
© 2004 Elsevier Science NL

HeartMate® VE LVAS design enhancements and its impact on device reliability

R.D. Dowlinga*, S.J. Parkb, F.D. Paganic, A.J. Tectord, Y. Nakae, T.B. Icenoglef, V.L. Poirierg,1, O.H. Frazierh

a University of Louisville, Louisville, KY, USA
b California Pacific Medical Center, San Francisco, CA, USA
c University of Michigan Health System, Ann Arbor, MI, USA
d St Lukes Medical Center, Milwaukee, WI, USA
e New York Presbyterian Hospital, New York, NY, USA
f Sacred Heart Medical Center, Spokane, WA, USA
g Thoratec Corporation, Pleasanton, CA, USA
h St Lukes Episcopal Hospital, Houston, TX, USA

Received 22 October 2003; received in revised form 4 March 2004; accepted 5 March 2004.

* Corresponding author. Address: Jewish Hospital Heart and Lung Institute, 217 East Chestnut Street, Louisville, KY 40202, USA. Tel.: +1-502-561-2180; fax: +1-502-584-2819
e-mail: rdowling{at}ucsamd.com


   Abstract
Top
 Abstract
1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusion
 References
 
Objective: The HeartMate® VE left ventricular assist system (LVAS) has supported more than 2300 patients and has been shown to be effective for bridge to cardiac transplantation and has demonstrated improved outcomes in survival as a destination therapy. Improvements in device durability are needed as bridge to transplant times increase and as we move into the era of LVAD as destination therapy. The purpose of this study is to determine if design enhancements to the HeartMate LVAS have improved device reliability and durability. Methods: A retrospective analysis of serious mechanical failures was performed in 1865 devices (1458 VE, 407 XVE). The analysis of data included devices used to support patients from September 1998 for bridge to transplantation and destination therapy. Serious mechanical failures were defined as inflow valve dysfunction, percutaneous lead breaks, diaphragm fractures or punctures, bearing failures, outflow graft erosion and pump disconnects. Results: Median device duration for the VE was 97 days (max 1206 days), and 85 days (max 517 days) for the XVE. A total of 134 serious mechanical failures occurred and included inflow valve dysfunction (5.3% VE, 2.4% XVE) (P=0.853), percutaneous lead breaks (1.9% VE, 0% XVE) (P<0.001), diaphragm fractures (0.1% VE, 0% XVE) (P=0.134), outflow graft erosion (0.2% VE, 0% XVE) (P=0.1096), pump disconnects (0.1% VE, 0% XVE) (P=0.1336) and bearing failures (0.6% VE, 0.2% XVE) (P=0.5538). Of the XVEs 97% were free of serious mechanical failures at 6 months and 82% at 1 year compared to 92 and 73% for the VE, respectively. The 6-month difference between the devices was statistically significant (P=0.0063) and there was no statistically significant difference at 1 year (P=0.1492). Conclusions: Preliminary experience with the HeartMate XVE LVAS demonstrated a significant reduction in percutaneous lead breaks. Early trends indicate positive impact of recent design modifications on XVE performance. These design modifications may improve device durability and reliability, which is crucial as we enter the era of LVADs as an alternative to medical therapy.

Key Words: Left ventricular assist device • Assisted circulation


   1. Introduction
Top
 Abstract
 1. Introduction
2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusion
 References
 
Ventricular assist devices have become a widely accepted therapy for patients with end-stage heart failure. The majority of experience with implantable left ventricular assist devices (LVADs) has occurred in patients who are supported as a bridge to cardiac transplantation [1–3]. However, the ultimate goal of implantable LVADs is to provide a permanent solution for heart failure, as an alternative to medical therapy, thus providing some hope for improved survival for the thousands of transplant ineligible patients who are destined to die from uncontrolled heart failure.

The HeartMate vented electric (VE) left ventricular assist system (LVAS) (Thoratec Corp., Pleasanton, CA) is widely used for bridge to transplantation and recently became the first LVAD to receive Food and Drug Administration (FDA) approval for permanent use in patients with end-stage heart failure ineligible for heart transplantation. The FDA approval was primarily based on the Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure (REMATCH) trial that compared the HeartMate VE LVAS and optimal medical therapy for patients with end-stage heart failure who were ineligible for heart transplantation. The trial demonstrated that LVAD therapy provided a clear survival benefit and improved quality of life [4].

As we move into the era of LVADs as a destination therapy, improvements in device reliability and durability are needed. Device malfunctions can result in significant morbidity and mortality for patients requiring LVAD therapy [5]. The probability of HeartMate VE LVAS device failure in the REMATCH trial was 35% at 24 months, and device replacement was required in 10 of 68 (14.7%) device patients [4].

The design of the HeartMate VE LVAS dates back to 1975 and clinical trials were initiated in 1991 after more than 16 years of development and more than 95 pre-clinical implants [6,7]. Since that time, the HeartMate VE LVAS has supported more than 2300 patients. This extensive clinical experience has led to numerous design changes and product improvements over the years [8]. Recently, the HeartMate VE LVAS underwent a number of design improvements and was renamed the HeartMate XVE LVAS. The modifications were designed to improve pump reliability and durability, ease of use, patient comfort and to decrease device-related complications and nuisance alarms. The purpose of this study is to determine if design enhancements have improved device reliability and durability.


   2. Materials and methods
Top
 Abstract
 1. Introduction
 2. Materials and methods
3. Results
 4. Discussion
 5. Conclusion
 References
 
2.1. Device
The HeartMate XVE LVAS (Fig. 1) is electrically powered and consists of an implanted blood pump, external system controller, and external power supply components. The system utilizes a pusher-plate blood pump that is capable of providing a stroke volume of 83 ml and generating pulsatile blood flow up to 10 l/min. The pump consists of rigid titanium housing divided in half by a flexible diaphragm. One half functions as the blood chamber, while the opposite half contains a motor that provides the force to move the diaphragm and eject blood from the pump. The blood-contacting surfaces of the pump chamber and its conduits are textured to promote the formation of a biologic neointima lining that markedly reduces the thrombogenicity of the device. As a result of this surface and the use of tissue valves, anticoagulation is not required for most patients. A percutaneous lead containing a vent and electric wires exits the motor chamber and then the skin. The vent serves to transfer air in and out of the motor chamber and the electrical wires are connected to the microprocessor-based system controller. The system controller powers the pump and continuously monitors and reports on system function. Power is provided to the system by either a pair of wearable, rechargeable batteries or by connection to a power-based unit.



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  		Fig. 1. HeartMate XVE system with cannulation.

 
2.2. Clinical impact of malfunctions
The serious device-related malfunctions discussed involve many implications to patient outcomes. Serious device malfunctions, in some cases, have led to patient death; however, many malfunctions can be managed with device repair or replacement. Patients have managed to survive malfunctions that have occurred in both the inpatient and home-discharge settings.

2.3. Design modifications
A series of improvements were made to the current HeartMate XVE LVAS (Fig. 2) compared to the previous HeartMate VE LVAS model. To date, the most significant enhancements incorporated into the HeartMate XVE system to improve reliability and durability was made to the percutaneous lead, LVAS pump, outflow conduit, and system controller (Table 1).



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  		Fig. 2. HeartMate XVE design modifications.

 

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  		Table 1. Summary of major modifications

 
One of the major XVE modifications was a redesigned percutaneous lead. Seven base materials were changed in the percutaneous lead and included a high flex cadium–copper alloy, high-strength silicone, hardenable stainless steel, fibers, and polycarbonate-based polyurethane and two independent coil springs. Percutaneous lead length was increased 7.62 cm to the vent filter and 15.2 cm from the vent filter to controller. Percutaneous lead diameter was decreased by 1.4 mm at the exit site. These changes were made to improve lead flexibility, strength, kink and fatigue resistance, improve patient comfort and decrease the potential for infectious complications. Another change includes design and material modifications to the vent adapter to reduce the likelihood of breakage during long-term use.

The HeartMate XVE also includes some of the modifications made to a later version of the HeartMate VE, most notably, the addition of an outflow graft bend relief and locking screw rings. The bend relief is a polyester graft that is positioned over the outflow graft to prevent graft kinking and abrasion of the outflow graft. Outflow graft kinking and abrasion is believed to cause graft erosion and blood loss as well as high pump chamber pressure, which could contribute to increase the stress on the inflow valve. Another significant improvement was the locking screw rings, a ratchet-type locking mechanism that eliminates the use of conventional sutures to secure the grafts and valves, as well as conduits to the inflow and outflow body of the pump. The screw rings were designed to reduce the possibility of blood loss due to accidental dislodgement from loosening of screw rings with sutures.

High diaphragm stresses, caused by diaphragm buckling, can result in diaphragm fracture. This has been addressed by two means. First, by repositioning the diaphragm support structure axially 0.150 in. to bring the diaphragm flange closer to the piston at the end of each stroke. In this position, buckling should be eliminated which reduces the likelihood of diaphragm fractures. A second modification is the incorporation of modified software for the automatic mode of operation. The software will in essence decrease the amount of time that the pump chamber pressure is elevated and significantly reduce the pump chamber pressure, which should decrease the amount of stress on the diaphragm, motor bearings and inflow valve.

Additional sutures to the valve commissure were added to strengthen the attachment to the graft material. Bench testing has shown that the combination of additional stitches behind the commissure and one across the commissure has doubled the life expectancy of the valves in high-speed cyclic testing compared to the old design (personal communication, V. Poirier, Thoratec Corp.).

The designed benefits of the HeartMate XVE LVAS improvements primarily reduce the risk of device-related adverse events and improve device durability. For patients, improvement in device durability, a reduction in device-related adverse events, would also improve patient's Quality of Life [4].

The serious device malfunctions have been addressed as a result of clinical manifestation of the various failure modes associated with the different LVAD components. In each of the failure types, the manufacturer was able to simulate in the in vitro setting, similar failure modes experienced in the clinical setting. Table 2 describes the type of in vitro analysis conducted to confirm the failure type.


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  		Table 2. Reproduction of failure modes

 
2.4. Methods
A retrospective analysis of serious mechanical failures was performed in 1865 HeartMate devices used from September 1998 to June 2003. Data was obtained from Thoratec's device tracking and complaint handling databases. The analysis of data included 1458 HeartMate VE LVAS and 407 HeartMate XVE LVAS used to support patients for bridge to transplantation and destination therapy indications. Serious mechanical failures were defined as inflow valve dysfunction, percutaneous lead breaks, outflow graft erosions, pump disconnects, diaphragm fractures or punctures and bearing failures. Six month and 1 year freedoms from serious malfunction were analyzed using the product-limit method of Kaplan and Meier. A limitation of the analysis may include the under-reporting of implantation and product complaint data to Thoratec's device registry (mandatory in USA, but voluntary outside USA).


   3. Results
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
4. Discussion
 5. Conclusion
 References
 
There were a total of 774 cumulative years of support for all devices (VE, 641 years; XVE, 133 years). Median duration of support for the 1458 VE devices was 97 days with a maximum duration of 1206 days compared to median support duration of 85 days for the XVE and a maximum of 517 days.

A total of 134 serious mechanical failures was reported within the first year of LVAS support (Table 3). Inflow valve dysfunction was the most common mechanical failure and occurred in 78/1457 (5.3%) VE devices and 10/407 (2.4%) XVE devices (P=0.853). There were 28 percutaneous lead breaks, 4 cases of outflow graft erosion, 2 pump disconnects and 2 diaphragm fractures in the VE device. None of these complications occurred in the XVE group. Bearing failures occurred in nine VE pumps at a mean duration of 294 days (range 210–363 days) and one XVE pump at 249 days.


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  		Table 3. Serious mechanical failures 1-year post-implant

 
Median duration of LVAS support to device failure included: inflow valve dysfunction, 211 days (range 0–805 days); percutaneous lead breaks, 289 days (range 13–616 days); diaphragm factures 457.5 days (range 157–643 days); outflow graft erosion 233 days (range 8–569 days); pump disconnect 98 days (range 33–163 days); and bearing failures 430 days (range 210–792 days). Overall freedom from serious mechanical failure for the XVE was 97% at 6 months and 82% at 1 year compared to 92 and 73% for the VE, respectively (Fig. 3) . The 6-month difference between the devices was statistically significant (P=0.0063), however, there was no statistically significant difference at 1 year (P=0.1492).



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  		Fig. 3. Freedom from serious mechanical failures at 6 months and 1 year.

 
Device improvements were staged over time resulting in a number of VE pumps with some of the modifications found in the XVE and a number of XVE pumps without all of the most recent device enhancements. Therefore, further analysis was performed comparing serious mechanical failures before and after specific modifications (Table 3).

There were no diaphragm fractures in 411 pumps implanted after repositioning the diaphragm support structure axially compared to a 0.4% incidence prior to the modification. One case of outflow graft erosion was reported after introducing the bend relief and in this particular case there was kinking beyond the bend relief due to an outflow graft that was not trimmed adequately. There were no reported cases of pump disconnects after introducing the locking screw rings and no pump lead breakage associated with the new percutaneous lead. The incidence of inflow valve incompetence is 5% after introduction of the outflow graft bend relief. However, the influence of the revised software and additional valve sutures on the incidence of inflow valve incompetence has not been analyzed (Table 4).


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  		Table 4. Impact of improvements on device performance

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
5. Conclusion
 References
 
Comparisons of serious mechanical failures before and after design improvements have been encouraging in this study and in large single center experiences [5]. Early results suggest a trend toward improvement in freedom from serious device malfunction at 1 year. However, because XVE modifications are fairly recent, more experience past 1 year is needed to fully understand the impact on long-term device performance.

Device improvements are ongoing and the current potential of the XVE LVAS has not been realized. Later this year, the XVE will again be modified by the addition of a new reinforced inflow valve assembly that has demonstrated encouraging in vitro result (Fig. 4) . High pressure in the pump chamber would tend to displace the current valve toward the ventricle and cause distortion of the graft. In addition, sutures that attach the vascular graft to the metal valve cage would break due to the pressure and allow the valve to oscillate in the cage. This oscillation could produce abrasion as well as distortion, leading to valve incompetence. Any extreme bending of the valve assembly would also lead to valve distortion. Distortion of the tissue valve is also possible with the current inflow valve conduit if any twisting action is used to insert the inflow tube into the left ventricle.



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  		Fig. 4. New inflow valve conduit.

 
The new valve conduit is designed to avoid all these complications. In addition, a joint was added with a locking screw ring to permit disconnection at the apex of the LV to allow for valve exchanges or pump exchanges without the need to remove the inlet tube. This permits simple valve exchanges or pump exchanges in situ. The new design permits articulation on both sides of the valve, thereby reducing the distortion by 50%. Bench testing has demonstrated a 6-fold increase in life expectancy for the new valve assembly compared to the current inflow valve (personal communication, V. Poirier, Thoratec Corp.).

All mechanical systems are subject to wear. The HeartMate XVE has four bearings in the system, two motor bearings and two driver bearings. There was one reported bearing failure in the current XVE LVAS version and testing and evaluation is underway to improve bearing durability.


   5. Conclusion
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusion
References
 
HeartMate LVAS device modifications were made based on extensive clinical experience. The causes of serious device failures, such as percutaneous lead breaks, diaphragm fractures, outflow graft erosion and pump disconnect, have been identified, analyzed and resulted in significant design modifications. Further design modifications should continue to improve device durability, which is crucial as we enter the era of LVADs as an alternative to medical therapy.


   Footnotes
 
Presented at the joint 17th Annual Meeting of the European Association for Cardio-thoracic Surgery and the 11th Annual Meeting of the European Society of Thoracic Surgeons, Vienna, Austria, October 12–15, 2003.

1 V.L. Poirier is an employee of Thoratec Corporation. Back


   References
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Conclusion
 References
 

  1. Frazier O.H., Rose E.A., McCarthy P., Burton N.A., Tector A., Levin H., Kayne H.L., Poirier V.L., Dasse K.A. Improved mortality and rehabilitation of transplant candidates treated with a long-term implantable left ventricular assist system. Ann Surg 1995;222(3):327-338.[Medline]
  2. Poirier V.L. Worldwide experience with the TCI HeartMate system: issues and future perspective. Thorac Cardiovasc Surg 1999;47(Suppl 2):316-320.
  3. El-Banayosy A., Korfer R., Arusoglu L., Kizner L., Morshuis M., Milting H., Tenderich G., Fey O., Minami K. Device and patient management in bridge-tro-transplant settings. Ann Thorac Surg 2001;71(Suppl 3):S98-S102.[Abstract/Free Full Text]
  4. Rose E.A., Gelijns A.C., Moskowitz A.J., Heitjan D.F., Stevenson L.W., Dembitsky W., Long J.W., Ascheim D.D., Tierney A.R., Levitan R.G., Watson J.T., Meier P., Ronan N.S., Shapiro P.A., Lazar R.M., Miller L.W., Gupta L., Frazier O.H., Desvigne-Nickens P., Oz M.C., Poirier V.L., Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure (REMATCH) Study Group Long-term mechanical left ventricular assistance for end-stage heart failure. N Engl J Med 2001;345(20):1435-1443.[Abstract/Free Full Text]
  5. Pagani F.D., Patel H.J., Wright S., Dyke B.D., Aaronson K.D. Significant reduction in major LVAD device failures: comparison of the heartmate® VE and XVE LVAS. J Heart Lung Transplant 2003;22(Suppl 1):S204.
  6. Frazier O.H. First use of an untethered vented electric left ventricular assist device for long-term support. Circulation 1994;89:2908-2914.[Abstract/Free Full Text]
  7. Poirier V.L. The quest for a solution we must continue. We must push forward. Am Soc Artif Intern Organs J 1993;39:856-863.
  8. Farrar D.J., Bataille O., Cotter C., Kwan K., Aulenbach C., Poirier V. Design improvements of the heartmate VE LVAS and their effectiveness in reducing adverse events. J Heart Lung Transplant 2003;22(Suppl 1):S83.



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