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Ann Thorac Surg 1997;63:367-370
© 1997 The Society of Thoracic Surgeons

Original Article: Cardiovascular

Development of an Implantable Ventricular Assist System

Michael P. Macris, MD, Steven M. Parnis, O. H. Frazier, MD, John M. Fuqua, Jr, Robert K. Jarvik, MD

Cullen Cardiovascular Research Laboratories, Department of Cardiovascular Surgical Research, Texas Heart Institute, Houston, Texas, and Transicoil Inc, Valley Forge, Pennsylvania


   Abstract
Top
 Footnotes
 Abstract
Introduction
 Material and Methods
 Results
 Comment
 References
 
Background. This study describes the present state of progress in the development of the Jarvik 2000 ventricular assist system.

Methods. Designed for implantation in the human thorax, the system consists of a small (25 cm3, 90 g) intraventricular axial-flow blood pump that transmits power and data via internal electronics and a transcutaneous energy transfer system. The pump is powered by portable internal and external polymer lithium ion batteries. The only moving part, the pump rotor, contains a permanent magnet of a brushless direct-current motor that mounts an axial-flow impeller and partial magnetic thrust support, with blood-immersed radial and thrust bearings. The motor uses a redundant coil and electric lead design, which permits continued operation in case of wire breakage.

Results. Seven calves have been supported for an average of 107 days (range, 40 to 162 days) with prototypes of the Jarvik 2000 ventricular assist system. No physiologic complications have occurred. When its user is at rest, the pump produces flows of 5 to 6 L/min with a decreased arterial pulse contour. Renal and hepatic functions have remained normal throughout the duration of all studies. Mean plasma free hemoglobin levels ranged from 4.3 to 11.4 mg/dL (mean, 6.3 mg/dL) for each study. Pathologic analyses of the heart and kidneys revealed no damage related to the device.

Conclusions. These studies indicate that the Jarvik 2000 ventricular assist system is feasible in animals and holds promise for long-term support of patients.


   Introduction
Top
 Footnotes
 Abstract
 Introduction
Material and Methods
 Results
 Comment
 References
 
Implantable, pulsatile ventricular assist systems (VASs) are now used routinely as bridges to transplantation in patients with end-stage heart disease. Hundreds of patients worldwide have been supported by externally powered left ventricular assist systems [1]. Patients supported by such systems may be able to resume active lifestyles. In addition, these devices promote physical rehabilitation of the patient, which may improve the chance of survival after transplantation [2, 3]. Selected patients have even been allowed to leave the hospital to await transplantation at home, significantly reducing their hospital costs.

The Texas Heart Institute and Transicoil Inc are collaborating on a new implantable VAS, the Jarvik 2000 (Jarvik Research, Inc, New York, NY) that may be placed entirely within the thorax. The axial-flow blood pump (Fig 1Go) is positioned within the left ventricular apex. State-of-the art batteries, along with electronics for control and data transfer, will be implanted in the chest wall.



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  		Fig 1. . Jarvik 2000 axial-flow blood pump.

 
This system may be used for (1) temporary or permanent support for patients with severe circulatory insufficiency after postcardiotomy or postinfarction cardiogenic shock, (2) support for critically ill patients awaiting cardiac transplantation, and (3) long-term support for patients with end-stage heart disease. The Jarvik 2000 VAS is being developed by sequentially integrating the major subsystems and ancillary components ultimately required for preclinical evaluations.


   Material and Methods
Top
 Footnotes
 Abstract
 Introduction
 Material and Methods
Results
 Comment
 References
 
Device Description
The Jarvik 2000 VAS is an intraventricular axial-flow system consisting of a blood pump, electronics and batteries, transcutaneous energy transfer system, and external batteries and monitoring unit. These major components, along with their corresponding weights and displacement volumes, are listed in Table 1Go. The small (90 g) titanium pump has a displacement volume of 25 cm3 and can provide in excess of 11 L/min of blood flow (Fig 2Go). The pump's only moving part, the rotor, contains a permanent magnet of a brushless direct-current motor and mounts an axial-flow impeller. The rotor is supported by ceramic bearings, which are immersed in the bloodstream [4]. The impeller is powered by electromagnetic fields across the motor "air gap," through which the blood flows. All blood-contacting surfaces are titanium. In addition, the risk of thrombus formation is reduced by a shortened inflow cannula.


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  		Table 1. . Components of the Jarvik 2000 Ventricular Assist Device
 


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  		Fig 2. . In vitro flow/pressure curves with the Jarvik 2000 blood pump. The pump was tested in a 3.3 centistoke glycerol/water mixture under steady-state flow conditions. The pump achieved speeds as high as 13,000 rpm, which can generate more than 11 L/min. (AoP = aortic pressure; LVP = left ventricular pressure.)

 
Animal Model
Various prototypes of the Jarvik 2000 axial-flow blood pump have been implanted in 7 calves weighing 68 to 93 kg. All animals in these studies received humane care in compliance with the "Principles of Laboratory Animal Care" (National Society of Medical Research) and the "Guide for Care and Use of Laboratory Animals" (National Institutes of Health publication 85-23, revised 1985).

Device Implantation
The surgical technique for implantation of the Jarvik 2000 does not require cardiopulmonary bypass and has been previously described [5]. Briefly, under general anesthesia and after line placement, a left lateral thoracotomy is performed at the fifth intercostal space, and the fifth rib is removed. A 16-mm low-porosity woven Dacron graft is preclotted with autologous, nonheparinized whole blood. After heparin (1 mg/kg) is intravenously administered, the graft is anastomosed to the descending thoracic aorta in an end-to-side fashion using a partial occluding vascular clamp. Next, a Dacron, tapered, silicone-collared sewing ring is attached to the left ventricular apex using either interrupted, pledgeted braided 2-0 polyester sutures or 3-0 polypropylene continuous monofilament sutures. A small cruciate incision is then made in the apex, and a conical obturator is inserted into the left ventricular cavity. The obturator is used to apply back pressure against the apical endocardium, and a cylindric knife is used to core the ventricular apex. The obturator and knife are removed as a unit, and the ventricular core is manually plicated to prevent bleeding. The pump is then inserted into the cored ventricle and is secured by tightly drawing a pursestring suture around the silicone collar (Fig 3Go). Air is removed from the graft, and pumping is initiated.



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  		Fig 3. . Jarvik 2000 axial-flow blood pump in place within the left ventricular apex.

 
In Vivo Studies
Since 1993, the Jarvik 2000 axial-flow pump has been implanted in 7 calves at our institution [5, 6]. To prevent encapsulation and reduce susceptibility to infection, the blood pump is placed within the left ventricular apex. All pumps were implanted in the left ventricular–descending thoracic aorta position. In these studies, electronics and batteries were not implanted. Instead, each pump was powered and controlled by a percutaneous drive line. In 2 calves, pump flow was measured by implanted ultrasonic flow probes (Transonics Inc, Ithaca, NY) around the outflow graft. In vivo physiologic and pump parameters were evaluated as previously described [5, 6]. System performance was monitored using outputs from external control and power units. To assess pump performance, treadmill studies were performed at various pump speeds, both at rest and during exercise. Hematologic and biochemical data were obtained throughout the studies. Animals were orally anticoagulated with daily doses of warfarin (2.5 to 25 mg), dipyridamole (400 mg), and aspirin (1 g).


   Results
Top
 Footnotes
 Abstract
 Introduction
 Material and Methods
 Results
Comment
 References
 
Seven calves were supported by the Jarvik 2000 VAS for an average of 107 days (70 to 162 days) (Table 2Go). There were no operative deaths, and all 7 calves had uncomplicated postoperative recoveries. Renal function, as measured by serial serum creatinine and blood urea nitrogen levels, remained normal throughout all studies [6] (see Table 2Go). Plasma free hemoglobin levels ranged from 4.3 to 11.4 mg/dL (average, 6.3 mg/dL; normal range, 1 to 4 mg/dL) for each study and revealed minimal device-related hemolysis.


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  		Table 2. . Laboratory Dataa
 
During the course of all experiments, each device was maintained at a continuous speed of 10,000 revolutions per minute (rpm). Average flow rate in all studies ranged from 5 to 6 L/min. Figure 4Go demonstrates the speed (in rpm) and power (in watts) needed to maintain normal pump operation in calf 4 (pump flow ~6 L/min). Arterial pulse pressure remained at around 20 mm Hg and correlated well with changes in pump current. Daily recorded minimum and maximum current levels (amperes) for calf 4 are presented in Figure 5Go. Changes in current corresponded with changes in arterial pulse pressure (ie, a decrease in the arterial pulse pressure would show a decrease in {delta} current, the maximum - minimum current), demonstrating that the pump can function safely in an asynchronous manner that maintains pulsatile systemic arterial wave forms. This change represents a mean difference of 0.04 ± 0.02 (standard deviation) amperes. The change occurs because of the differential pressure load on the pump; thus, the torque load on the motor varies with the pulse pressure caused by natural ventricular contraction. In other words, the pulsatile flow produced by the pump is the result of continuous pump flow and native cardiac output. If, for example, the native heart were fibrillating, pump flow would be nonpulsatile. Mean current was stable throughout the duration of the study.



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  		Fig 4. . Power and speed remained constant during a 5-month study in calf 4.

 


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  		Fig 5. . Minimum and maximum current levels in calf 4. The change in amperage occurs as the torque load in the motor varies with the pulse pressure caused by ventricular contraction.

 
All studies were terminated electively. Causes for termination included a broken electrical wire (n = 3), thrombus at the rotor/stator junction (n = 1), impeller blade/pump housing friction (n = 2), and infection (n = 1). Calf 4 experienced two separate incidents of electrical drive line breakage; in each case, however, the drive line was repaired without incident. Problems with the blade tip rubbing against the housing were found upon postmortem analysis of two failed pumps. Because of thrombus at the rotor/stator junction, pump components in this area were redesigned and the problem was minimized [5]. Bearing wear was essentially undetectable in any pump. Postmortem examination has shown no evidence of either thromboemboli in the kidneys or damage to intracardiac structures.


   Comment
Top
 Footnotes
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
References
 
As demonstrated in several experimental studies [5–7], the Jarvik 2000 VAS may be placed safely within the heart for a mean period of 107 days and up to a current maximum of 162 days without causing damage to intracardiac structures, can maintain pulsatile wave forms in systemic circulation, and does not lead to hemolysis. Unlike the larger implantable VASs, this miniature implantable system does not need valves; thus it contains fewer moving parts and should be less expensive to manufacture. Further, because it is small, it holds promise for long-term support of patients of all sizes, including women and small children [7]. In addition, the Jarvik 2000 may be implanted easily and does not require a laparotomy.

In the future, the internal battery and electronics systems of the Jarvik 2000 will be modified to include rechargeable polymer lithium-ion batteries and a miniaturized hybrid control system hermetically sealed in a titanium case. The control system will be programmed to meet the needs of the individual patient. A simple feedback system will alter cardiac output as required. Telemetry will allow communication between the implanted electronics and a very small personal computer for data output and changes in device parameters. Flexible polymer lithium-ion batteries will be worn externally, allowing batteries to be configured as required. A battery better suited for ease of use and comfort of the user is being designed. Studies of prototype systems are providing data that will aid in modifying the blood pump and designing a physiologic control system.


   Footnotes
Top
 Footnotes
Abstract
 Introduction
 Material and Methods
 Results
 Comment
 References
 
Presented at the Poster Session of the Thirty-second Annual Meeting of The Society of Thoracic Surgeons, Orlando, FL, Jan 29–31, 1996.

Address reprint requests to Dr Macris, Cullen Cardiovascular Research Laboratories, Texas Heart Institute (Mail Code 1-268), 1101 Bates St, Houston, TX 77030.


   References
Top
 Footnotes
 Abstract
 Introduction
 Material and Methods
 Results
 Comment
 References
 

  1. McCarthy PM. HeartMate implantable left ventricular assist device: bridge to transplantation and future applications. Ann Thorac Surg 1995;59:S46–51.
  2. Cloy MJ, Myers TJ, Stader LA, Macris MP, Frazier OH. Hospital charges for conventional therapy versus left ventricular assist system therapy in heart transplant patients. ASAIO J 1995;41:M535–9.[Medline]
  3. Pristas JM, Winowich S, Nastala CJ, et al. Protocol for releasing Novacor left ventricular assist system patients out-of-hospital. ASAIO J 1995;41:539–43.
  4. Jarvik RK. System considerations favoring rotary artificial hearts with blood-immersed bearings. Artif Organs 1995;19:565–70.[Medline]
  5. Macris MP, Myers TJ, Jarvik R, et al. In vivo evaluation of an intraventricular electric axial flow pump for left ventricular assistance. ASAIO J 1994;40:M719–22.[Medline]
  6. Parnis SM, Macris MP, Jarvik R, et al. Five month survival in a calf supported with an intraventricular axial flow blood pump. ASAIO J 1995;41:M333–6.[Medline]
  7. Kaplon RJ, Oz MC, Kwiatkowski PA, et al. Miniature axial flow pump for ventricular assistance in children and small adults. J Thorac Cardiovasc Surg 1996;111:13–8.[Abstract/Free Full Text]



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