The HeartMate II: Design and Development of a FullySealed Axial Flow Left Ventricular Assist System

*David J. Burke, *Edward Burke, *Farzad Parsaie, *Victor Poirier, †Kenneth Butler,†Douglas Thomas, †Lynn Taylor, and †Timothy Maher

*Thermo Cardiosystems Inc. (now Thoratec Corp.), Woburn, Massachusetts; and †Nimbus Inc. (now Thoratec Corp.),Rancho Cordova, California, U.S.A.

Abstract: Our group is developing the control and powertransmission components required to implement a perma-nent and fully sealed left ventricular assist system (LVAS).Starting with the percutaneously powered HeartMate IIblood pump, our development efforts are focused in thefollowing areas: a complete redesign of the transcutaneousenergy transmission system (TETS) to include a rectifica-tion network and autonomous voltage regulation withinthe secondary coil, a hermetically sealed electronics pack-age containing a miniaturized implementation of the ex-

isting redundant drive and control electronics with severalpower-input options, an implanted rechargeable lithiumion battery pack capable of providing up to 1 h of unte-thered operation, implantable electrical connectors thatallow components to be connected after placement in thebody or to be replaced if needed, and a radio telemetrysubsystem to transmit diagnostic information and to per-mit remote adjustment of selected parameters. KeyWords: Left ventricular assist system—Sealed system—Axial flow blood pump—Rotary blood pump.

Despite the widespread acceptance of several per-cutaneous and extracorporeal heart assist devices(1), only a fully sealed system can reduce the poten-tial for infection and provide the quality of life nec-essary for a permanent left ventricular assist system(LVAS). To this end, Thermo Electron, the parentcompany of Thermo Cardiosystems (TCI), devel-oped its first fully sealed LVAS in the early 1980s(2). The implanted portion comprised (Fig. 1) anactuator coupled to a pusher-plate blood pump, aflexible compliance volume, controller/battery pack,and a transcutaneous energy transmission system(TETS) coil. While innovative for its time, the vol-ume of implanted components and their complexityprevented the realization of a truly practical system.

During this period, TCI’s percutaneously poweredleft ventricular assist devices (LVADs) gained clini-cal acceptance which resulted in FDA approval of

the pneumatically driven HeartMate I in 1994 andthe electrically powered HeartMate VE in 1999. Inmany respects, these percutaneous systems offersome significant advantages over a fully sealed sys-tem, for example, the ability to place much of theactuation and control complexity outside the bodythus permitting maintenance or replacement of anyexternal component. This allows regular mainte-nance and the replacement of any external compo-nent. However, quality of life issues arising from thepresence of a percutaneous line and its potential as aconduit for infection makes an externally driven sys-tem unattractive for long-term (permanent) ven-tricular assist. Moreover, these diaphragm-typepumps require implanted valves and are subject tomechanical wear that limits them to 2 to 3 years ofoperation.

As clinical experience with these first-generationLVADs accumulates, TCI and others are developingthe next generation based on the principle of con-tinuous axial or centrifugal flow (3). These devicesaddress many of the inherent problems with the pre-vious generation of positive displacement bloodpumps. Some of the most important improvementsinclude the lack of valves, no compliance volume,small areas of blood contact, mechanical simplicity,

Received January 2001.Presented in part at the 8th Congress of the International So-

ciety for Rotary Blood Pumps, held September 6–9, 2000, inAachen, Germany.

Address correspondence and reprint requests to Dr. David J.Burke, Manager of Electrical Engineering, Thoratec Corp., 470Wildwood St., Woburn, MA 01888-2697. E-mail: david.burke@thoratec.com

Artificial Organs25(5):380–385, Blackwell Science, Inc.© 2001 International Society for Artificial Organs

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and small overall size. The HeartMate II is our sec-ond generation axial-flow blood pump that, with 1moving part, is approximately the size of a D-cellbattery. Even though this pump is one-eighth thesize of the HeartMate VE, it has the same outputcapabilities (4,5). Figure 2 shows the housing for thebrushless DC motor and the high-speed impellerwith its precision bearings. In vivo testing indicatesbearing life of over 5 years. In terms of size, simplic-ity, and reliability, the HeartMate II forms the idealplatform on which to build our next generationsealed LVAS.

SYSTEM CONFIGURATION

As shown in Fig. 3, the HeartMate II sealed LVASis designed around a modular concept. The carefulallocation of component functionality, combinedwith the use of connectors that facilitate the replace-ment of any component, including the blood pump,

makes this approach possible. Replacement issuesaside, a modular approach lets the system expand tomeet the changing needs of the patient. A conven-tional bridge-to-transplant percutaneous implantcould be easily upgraded to a permanent system ifthe transplant eligibility of the patient changed.Similarly, in a pediatric application, a percutaneoussystem could serve until the child was large enoughto accommodate the additional implanted compo-nents. In a normal postoperative situation, the sys-tem may be powered percutaneously (Fig. 4) until

FIG. 1. The photograph is of the fully sealed HeartMate I typeLVAS that was developed and tested in the early 1980s.

FIG. 2. The HeartMate II axial flow LVAD showing the flow statorwith the pump impeller (top) and the pump housing with integralmotor (bottom) is shown.

FIG. 3. The diagram presents the HeartMate II sealed systemcomponents: (from top ) the external TETS coil, the implantedTETS coil, the external batteries in holsters, the HeartMate IILVAD, the control module, the internal battery module, and theTETS power oscillator.

FIG. 4. A temporary external power option for the HeartMate IIsealed system is depicted.

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the site of the implanted TETS coil heals. Oncehealed, the system can be powered via the TETS,and the battery pack may be installed or replacedwithout disrupting normal LVAD operation.

THE ENERGY TRANSMISSION SYSTEM

The TETS is not only the primary energy sourcefor the implanted components but also the enablingtechnology that makes a sealed LVAD possible. Ourpresent device represents the next generation of theoriginal TETS designed at Thermo Electron in theearly1980s (6).

The TETS incorporates an external power oscilla-tor that converts DC power to high-frequency ACpower. An external primary coil together with animplanted secondary coil form a highly efficient air-core transformer that transmits this AC powerthrough the skin. A rectifier circuit is connected tothe output of the implanted secondary coil to pro-duce the regulated DC power that powers the bloodpump and charges the internal batteries.

The secondary coil is 7.3 cm in diameter and 1.9cm tall. It is given a size and shape that creates ashallow mound when it is implanted subcutaneously.The mound helps to locate the flat external primarycoil, whose inner diameter is chosen to approximatethe perimeter of the mound. A simple elastic ban-dage is all that is necessary to hold and maintainproper alignment of the lightweight and flexible pri-mary coil.

One of the significant changes made to the im-planted portion of the new TETS design places thepower conditioning circuitry, shown in the fore-ground of Fig. 5, inside the secondary coil. Movingthis circuitry into the otherwise unused space in thecoil module reduces the size and heat load of theimplanted motor control module. More importantly,by performing the AC-to-DC conversion within thecoil, TETS’ efficiency and reliability are improvedbecause DC power then can be delivered to the con-trol module using conventional wire and connectorsrather than resorting to the use of large and inflex-ible Litz wire. The power conditioning circuitry itselfwas redesigned to increase reliability and efficiency.Under normal coupling, the TETS is capable of de-livering over 20 W of power at 14 V and will con-tinue to operate efficiently with a primary coil mis-alignment of up to 50% in either axis.

The belt-mounted power oscillator obtains itspower from either a main supply or a pair of batter-ies worn by the patient. The oscillator package,shown in Fig. 6, contains audio and visual indicatorsto signal hazard conditions as well as a batterycharge level display.

CONTROL MODULE

The implanted control module (Fig. 7) forms boththe functional and the physical core of the implantedsystem. Housed in a hermetically sealed titaniumcase, it accepts power from the TETS and batterymodule while providing the motor drive signals tothe blood pump.

A low-profile discoid shaped housing with a 10 cmdiameter provides a degree of anatomical compat-

FIG. 6. The belt-worn TETS power oscillator for the HeartMate IIsealed system is shown.

FIG. 5. The photograph is of the implanted TETS coil showingthe circular telemetry antenna and the power conditioning cir-cuitry that resides within the body of the coil.

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ibility that gives the surgeon a range of placementoptions. This is further enhanced by the use of elec-trical connectors on the controller that allow cablesfrom the other implanted components to be routedindividually with the least amount of surgical traumabefore the final connections are made. Additionally,connectors allow different component configura-tions and make module replacement possible.

Inside the module is a single circuit board contain-ing the motor drive, control, and telemetry electron-ics. The key feature of the control module is a pair offully redundant motor control and drive systems.These systems function in a primary/backup configu-ration in which a failure of the primary system au-tomatically enables the backup system. During eachprogram loop, the primary controller calculates akey which enables it to strobe a watchdog circuit.Any disruption of normal program execution willproduce an invalid key that cannot strobe the watch-dog circuit, thereby allowing it to time out and trans-fer operation to the backup controller. Because bothcontrol systems are identical, the LVAS will operatein the backup mode without any loss of functionality.In addition to the redundant hardware, an extensivesuite of diagnostic monitoring routines executes con-tinuously to provide status information to the exter-nal monitoring system.

BATTERY MODULE

To improve the quality of life, our sealed systemincludes an implanted battery module that enablesour patients to be completely unplugged for a lim-

ited period of time each day. Along with bathing oreven swimming, an implanted battery facilitates awhole range of normal activities that are more easilyaccomplished with the external TETS coil removed.The battery module shown in Fig. 8 occupies 92 ccand weighs approximately 150 g. It is a self-contained power subsystem housed in a hermeticallysealed titanium case that includes both the batterycells and the supporting electronics.

Lithium ion is the only battery technology with anenergy density sufficient to provide a useful amountof operating time in a reasonably sized implantablemodule. With this in mind, our battery module con-tains 4 hermetically sealed 4.1 V lithium ion pris-matic cells (inset, Fig. 8), each rated at 550 mA-h(Model R11098, Wilson Greatbatch, Clarence, NY,U.S.A.). Assuming 1 complete discharge cycle eachday, we estimate that our battery module will havean 18 to 24 month service life before requiring re-placement. During that period, the maximum oper-ating time will drop from 1 h to approximately 20min. On-board monitoring circuitry allows us tomeasure the actual battery charge status and makevery accurate predictions of remaining capacity. Themodule also contains the battery charging electron-ics, overvoltage suppression, undervoltage protec-tion, and excess temperature protection devices. Byplacing all of the required intelligence within the bat-tery module, no changes to the controller hardwareor software are required to accommodate periodicimprovements in battery technology.

IMPLANTED CONNECTORS

We have developed a practical and reliable im-plantable connector that is robust enough to with-stand mishandling while at the same time is easilyconnected in a wet, slippery environment. The basicconnector design may be used for either panelmount or inline mating applications. It comprises a

FIG. 7. The control module for the HeartMate II sealed systemshows the 3 implantable connectors.

FIG. 8. An implanted battery module with 1 of the 4 lithium ionprismatic cells that are contained within it is shown.

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titanium body, off-the-shelf pin and socket inserts,and a double O-ring environmental seal. Figure 9shows an open view of the inline connector configu-ration in which each half is 4.8 cm long with a 1.25 cmbarrel diameter. The panel mounted configurationuses the same mating surfaces as the male section ofthe inline connector but without the strain relief orgrip ring. A complimentary set of tabs and detentsform a self-locking screw ring that allows the con-nector to be mated without tools yet prevents retrac-tion due to movement or vibration. Each connectormay be keyed in 1 of 3 separate positions to preventconnection to the wrong component.

TELEMETRY

Although other methods are used (7), a radiobased telemetry system offers noncontact communi-cation at a distance, high reliability, and ease ofimplementation. Our telemetry system is based onlow-power radio frequency (RF) transceiver circuitslocated internally in the control module and exter-nally in the TETS power oscillator as well as in thepatient monitor. Internally, a miniature coaxial cablefrom the control module carries the RF signal to theantenna. Figure 6 shows how the antenna is fabri-cated as a tuned loop on a printed circuit board thatis embedded in the center of the TETS secondarycoil. Each transceiver is a commercial hybrid circuit,available in a range of frequencies that satisfies theregulatory requirements of our international mar-kets. The telemetry system is designed for short-range data transmission with a maximum outputpower of less than 1 milliwatt. To maintain integrity,data are transmitted digitally in encoded packetsthat include an identification header and terminatewith a cyclical redundancy check code.

While not required for active control of theLVAD, telemetry provides a vital link between theimplanted system and the external monitor regard-less of whether the patient is powered externally orusing the internal battery. The internal data packetcontains information regarding hemodynamic per-formance, internal diagnostics, power consumption,internal battery status, and alarms. During normaloperation, the belt-worn TETS power oscillator re-quests a new data packet from the internal controllerevery 5 s. Each received packet is examined by theoscillator, which extracts the relevant alarm and sta-tus information for local display. Similarly, theHeartMate Patient Monitor, when placed 1 to 2 mfrom the patient, can process the same data packetand present a detailed report of system status. Inaddition to status reporting, the monitor is used tochange the LVAD operating parameters and updateseveral calibration constants. The Patient Monitoralso contains the appropriate interfaces to allowwired or wireless remote patient monitoring.

CONCLUSIONS

The HeartMate II sealed system is the product ofover 20 years of development and clinical experienceat Nimbus and TCI. It incorporates the reliability,safety, and usability required for a long-term LVAS.Not only does the HeartMate II have the capabilityfor permanent assist, but it also opens entirely newareas of clinical application that include children andsmall adults.

Above all, this system will provide our patientswith a life that is simply not possible with the first-generation assist systems of today. The absence ofskin penetrations removes the fear of infection whilefacilitating a range of normal activities, such as bath-ing, which permits the quality of life with an implantto approach that of life without an implant. Further,our modular system design allows a range of con-figuration options for specific circumstances and,should problems occur, simplifies replacement of im-planted components, which in turn speeds the pa-tient’s recovery.

REFERENCES

1. Frazier OH. Mechanical cardiac assistance: Historical per-spectives. Semin Thorac Cardiovasc Surg 2000;12:207–20.

2. Szycher M, Clay W, Gernes D, Sherman C. Thermedics’ ap-proach to ventricular support systems. Journal of BiomedicalApplications 1986;1:39–105.

FIG. 9. An open view of the inline configuration of the implant-able electrical connector is shown.

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3. Goldstein DJ, Oz MC, eds. Cardiac Assist Devices. Armonk,NY: Futura Publishing Company, 2000.

4. McCarthy PM, Sabik JF. Implantable circulatory support de-vices as a bridge to heart transplantation. Semin Thorac Car-diovasc Surg 1994;6:174–80.

5. Butler KC, Dow JJ, Litwak P, Kormos RL, Borovetz HS.Development of the Nimbus/University of Pittsburgh innova-tive ventricular assist system. Ann Thorac Surg 1999;68:790–4.

6. Sherman C, Daly B, Dasse K, Clay W, Szycher M. Research

and development: systems for transmitting energy throughintact skin. Waltham, MA: Thermo Electron Corp. FirstTechnical Report XC NO1-HV-0-2903-3. Devices and Tech-nology Branch, National Heart, Lung and Blood Institute,1983.

7. Mussivand T, Hum A, Diguer M, Holmes KS, Vecchio G,Masters RG, Hendry PJ, Keon WJ. A transcutaneous energyand information transfer system for implanted medical de-vices. ASAIO J 1995;41:M253–8.

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