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Ann Thorac Surg 1999;68:785-789
© 1999 The Society of Thoracic Surgeons
a Cardiovascular Devices Division, University of Ottawa Heart Institute and WorldHeart Corporation, Ottawa, Ontario, Canada
Address reprint requests to Dr Mussivand, Cardiovascular Devices Division, University of Ottawa Heart Institute, 1053 Carling Ave, Rm H560B, Ottawa, ON, Canada, K1Y 4E9
e-mail: tofym{at}heartinst.on.ca
Presented at the Fourth International Conference on Circulatory Support Devices for Severe Cardiac Failure, Houston, TX, Oct 3–5, 1997.
Abstract
Background. Ventricular assist devices (VADs) have been shown to be effective for short- or long-term circulatory support. Devices are either being adapted or newly designed for longer term or permanent support, with the goal to provide patients with improved quality of life. Since 1990, a program has been in place to develop a totally implantable, permanent VAD.
Methods. A multidisciplinary team is developing this VAD with specific goals in mind: (1) that it have an intrathoracic position, (2) that it be a totally implantable device without any percutaneous connections, and (3) that it be possible to communicate with the device from remote locations. These goals would allow for complete patient mobility and flexibility for follow-up.
Results. The electrohydraulically actuated VAD combines the blood pump, volume displacement chamber, energy converter, and internal electronic module into a single compact unit. The device called the HeartSaver VAD is powered by a transcutaneous energy transfer system and can be remotely monitored and controlled. Prototypes of different versions of the device have been tested in vitro and in vivo with satisfactory performance.
Conclusions. The prototypes of the HeartSaver VAD have functioned well under test conditions and fulfilled the outlined goals. Further development and testing of the design are being conducted before clinical availability.
There is little question that the incidence of end-stage heart failure is increasing in the world. Numerous therapeutic alternatives are being assessed as the limitations of traditional medications and cardiac transplant are recognized. As experience has been gained with the short-term use of currently available circulatory support devices, the duration of support has been extended quite successfully [1–3]. Current systems are being adapted and new technologies are being developed to allow for longer term and permanent support with circulatory support devices.
Several major issues have been identified as limitations of the current technologies. One important problem with current devices is that the location of devices in the abdomen may not be ideal as complications associated with adjacent organs and diaphragmatic defects may occur [4]. An intrathoracic location for such devices may be preferable.
Another complication associated with existing devices that require percutaneous connections is the relatively high incidence of infection, which affects 40% to 50% of recipients. Previously, it has been suggested that a totally implantable system without such percutaneous connections could have a major impact in reducing this incidence of infection [5, 6]. Finally, to increase cost effectiveness, hospital stays and visits should be minimized. To aid in this compact controllers and biotelemetry systems allowing remote control and assessment of these devices would be advantageous.
The Cardiovascular Devices Division of the University of Ottawa Heart Institute and the WorldHeart Corporation has focused on these issues with the ongoing development of the HeartSaver Ventricular Assist Device (VAD), an intrathoracically implantable VAD without percutaneous connections. This device combines total implantability with an intrathoracic position, transcutaneous power transfer, and remote communication capabilities, thus offering future recipients the potential of an enhanced quality of life.
Device description
The HeartSaver VAD has undergone several conformal changes since its inception in 1990 (Fig 1). Although it continues to be based on an electrohydraulic actuating mechanism, the change to a compact implantable unit with the blood pump, volume displacement chamber, energy converter, and internal electronic module encapsulated together has allowed the device to be shaped for an intrathoracic position.
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The implantable unit (version 5.1) has a total volume displacement of 480 mL and a weight of 680 g (Fig 3). It consists of a 70-mL blood chamber with a flexible polyurethane diaphragm within a rigid housing. The silicone-based hydraulic fluid is pumped during systole through the energy convertor that consists of a bidirectional brushless DC motor, a bladed impeller, and a bladed housing. The hydraulic fluid actuates the flexible blood chamber diaphragm that ejects the blood from the chamber. The blood chamber fills passively during diastole with the hydraulic fluid returning to the volume displacement chamber through a one-way valve. The diastolic filling may be augmented with reversal of the motor in the active filling mode. Filling and ejection of blood is monitored using Hall Effect sensors and a magnet embedded in the blood pumping diaphragm, which allows the position of the flexing diaphragm to be dynamically determined throughout the pumping cycle by the internal electronic module. Mechanical valves (Medtronic-Hall, extended side tilting disk; Medtronic Inc, Minneapolis, MN) mounted in the inflow and outflow cannulas, were used for the in vitro and in vivo testing described herein. As this device is now nearing clinical use, these mechanical valves have recently been replaced with bioprosthetic valves (Carpentier-Edwards, Baxter Healthcare Corp, Irvine, CA) to reduce anticoagulation requirements.
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Total implantability and transcutaneous energy transfer
In addition to not requiring external venting, total implantability would only be possible if power for the device was completely internal or could be delivered transcutaneously.
To provide power to the device without percutaneous lines, a transcutaneous energy transfer (TET) system was developed [8, 9]. The TET system transfers power from an external source across intact skin and tissue to the device using electromagnetic induction. This is accomplished using a pair of wire coils, one implanted subcutaneously and one located directly over the implanted coil on the skin surface. By selecting an appropriate operating frequency and coil geometry, a coupling coefficient suitable for power transfer across intact skin and tissue can be obtained.
The TET system provides two major functions: (1) provides operating power from an external power source (battery, wall socket, etc.) to the implanted device; and (2) provides power to recharge the implanted battery, which is used as a backup power supply. The implanted battery also provides the patient with the ability to bathe, shower, swim, and partake in other activities unencumbered by any external components.
Biotelemetry
To monitor and control the device without the need for percutaneous connections, a biotelemetry system was developed [9–11]. This system uses infrared data communications to transfer control and monitoring information across intact skin and tissue. Electronic modules consisting of multiple infrared receivers and transmitter components are mounted in both of the energy transfer coils, establishing a continuous bidirectional communications path between the implanted device and the outside of the body. The infrared method was selected based primarily on its immunity to electrical noise, which ensures a high level of accuracy for this life-critical application.
Remote capabilities
A remote biotelemetry capability has been added to the HeartSaver VAD System, which allows the device to be controlled and monitored using public communications infrastructure (telephone lines, ATM, satellite, Internet, etc) by a healthcare provider at a remote site. This feature is expected to offer patients improved convenience and quality of life by reducing the number of visits to the hospital or clinic for routine device or patient check-ups. Basically the patient can connect the wearable external controller to a telephone line or cellular phone at his home or other location, and a healthcare provider at the hospital or clinic can have access to control and monitoring functions of the implanted device. This remote communications capability should also help to alleviate the fears that some patients have exhibited related to leaving the hospital with a complex implanted medical device. In addition, as the patient will no longer be required to return as frequently to the hospital for routine device or patient check-ups, the cost of follow-on patient care could also be reduced.
Quality of life
The HeartSaver VAD System is expected to deliver enhanced quality of life to the device recipients in both the short and long term. The intrathoracic position should reduce incision length and abdominal manipulation and thereby reduce postoperative pain and minimize associated complications. Total implantability will offer improved patient mobility and hopefully early patient discharge. Finally, the remote biotelemetry capability will allow for ease of follow-up with patients living away from hospital-based resources.
Results
In vitro testing
Testing of the individual components and the entire HeartSaver VAD System is ongoing. Various versions of the device have been tested on modified Donovan mock loops for up to 6.4 years (Fig 4). One of the early systems has now run for more than 6.4 years, which is equivalent to approximately 200 million cycles (Fig 5). Although the results from this testing may not be directly relevant to the final device configuration, they have provided valuable insight into the potential durability of the developed system, subsystems, and components.
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In vivo testing
The entire HeartSaver VAD System (version 4) including the TET and Biotelemetry subsystems was assessed in vivo during a series of 11 bovine experiments [12]. During these experiments, the animals were supported for periods up to 5 days to assess system design, implantability, and overall function.
To allow further development of the TET and Biotelemetry systems independent of the implantable unit, version 5 was tested with a percutaneous power and information cable. Testing focused on performance of inflow and outflow cannulas, device positioning in the calf model in preparation for chronic testing of the final design, and team preparation. A total of 13 experiments were performed in animals with a mean weight of 102 ± 4 kg. Duration of implantation ranged from 13.5 hours to 30 days. Modifications resulting from these tests included: (1) abandoning cardiopulmonary bypass support during cannula implantation to reduce pulmonary sequelae of cardiopulmonary bypass, (2) use of a stainless-steel wire reinforced cannula to prevent kinking and use of a Dacron coating to prevent abrasion of adjacent tissue, and, (3) changes in the control software and an improved monitoring system for blood chamber filling and ejection (Fig 6).
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The reason for termination of the various studies was as follows: 6 electively terminated due to achieving various goals, 3 due to insufficient inflow to the device, 3 due to pulmonary dysfunction, 3 due to bleeding/tamponade, 2 due to respiratory insufficiency, 2 due to faulty electrical connections, 2 due to housing defects resulting in hydraulic fluid loss, 1 due to thromboembolic complications (related to the anticoagulation protocol), 1 due to ventricular fibrillation during implant, 1 due to an air embolism during device implant, and 1 due a power failure in the facility. As can be seen by these results, most of the study terminations were due to the medical/surgical complications experienced during the development of a suitable implantation protocol in the animal model. Each of these issues have now been specifically addressed and satisfactorily resolved.
Comment
The HeartSaver VAD development has achieved the major design goals, resulting in a totally implantable, intrathoracic VAD. The system has undergone in vitro and in vivo evaluation, establishing the feasibility of an intrathoracically implanted VAD without percutaneous connections. Further modifications to the design of the device will be based on the results from ongoing testing. Once the design has been finalized, formal in vitro reliability testing and chronic in vivo assessment will be carried out before human clinical trials.
Acknowledgments
This work is supported in part by the University of Ottawa Heart Institute, Industry Canada and WorldHeart Corporation.
References
This article has been cited by other articles:
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  P. J. Hendry, T. V. Mussivand, R. G. Masters, M. E. Bourke, G. M. Guiraudon, K. S. Holmes, K. D. Day, and W. J. Keon The HeartSaver left ventricular assist device: an update Ann. Thorac. Surg., March 1, 2001; 71 (2007): S166 - S170. [Abstract] [Full Text] [PDF]
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 A K Mahmood, J M Courtney, S Westaby, M Akdis, and H Reul Critical review of current left ventricular assist devices Perfusion, September 1, 2000; 15(5): 399 - 420. [PDF]
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