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Ann Thorac Surg 2001;71:S166-S170
© 2001 The Society of Thoracic Surgeons
a Division of Cardiac Surgery, University of Ottawa Heart Institute, Ottawa, Ontario, Canada
b Division of Cardiac Anesthesia, University of Ottawa Heart Institute, Ottawa, Ontario, Canada
c Division of Cardiovascular Devices, University of Ottawa Heart Institute, Ottawa, Ontario, Canada
Address reprint requests to Dr Hendry, Division of Cardiac Surgery, Ottawa Heart Institute, Room H207, 40 Ruskin St, Ottawa, ON K1Y 4W7, Canada
Presented at the Fifth International Conference on Circulatory Support Devices for Severe Cardiac Failure, New York, NY, Sept 15–17, 2000.
Abstract
Background. Ventricular assist devices have been shown to be effective as bridges to transplantation and recovery for patients with end-stage heart failure. Current technology has been limited because of the need for percutaneous connections with controllers. The HeartSaver ventricular assist device (VAD) (World Heart Corporation, Ottawa, Ontario, Canada) was developed with the intention of having a completely implantable, portable VAD system. The system consists of an electrohydraulic blood pump, internal and external battery power, and a transcutaneous energy transfer and telemetry unit that allows for power transmission through the skin. Control of the device may be achieved locally or remotely through a variety of communication systems.
Methods. The device has been modified with the Series II preclinical version being available for in vitro (mock loop) and in vivo (bovine model) testing.
Results. Seventeen Series II devices have been functional on mock loops or other testing trials for an accumulated 900 days of operation. There have been eight acute experiments using a bovine model to test various components as they have become available from manufacturing. Mean pump output was 10.4 ± 1.1 L/min in full-fill/full-eject mode. Changes in the last 24 months include (1) cannula redesign for better port alignment and integration of tissue valves; (2) battery redesign to convert to new lithium-ion cells; (3) optimized infrared information and electromagnetic inductance energy transmission through various skin thicknesses and pigmentation; and (4) improved reliability of internal and external controller hardware and software.
Conclusions. Modifications have been required to optimize the HeartSaver VAD’s performance. The final HeartSaver VAD design will be produced in the near future to allow for formal in vitro and in vivo testing before clinical implantation.
As the prevalence of congestive heart failure increases, numerous medical and surgical strategies for treatment are being investigated. Since the early 1960s, there have been great developments in the area of mechanical circulatory support. The early devices (which have only recently achieved US Food and Drug Administration approval) have shown that patients can be supported successfully for long periods of time while waiting for either cardiac transplantation or recovery of the natural heart. The current thrust of technology development is to provide systems that improve on their predecessors by being totally implantable, having a transcutaneous power supply and telemetry while providing normal physiologic hemodynamics for the patient.
These were the goals set out for the development of the HeartSaver ventricular assist device (VAD). After the first prototype was produced in 1990, there have been several iterations of the device. Since last reported [1], further refinements have occurred resulting in a preclinical version that will be submitted to formal testing for regulatory approval.
Device description
The HeartSaver VAD (World Heart Corporation, Ottawa, Ontario, Canada) consists of three implantable components including the HeartSaver VAD blood pump, an internal battery, and a transcutaneous energy and information transfer (TEIT) coil. External components include the external controller, the external battery, and the external energy and information transfer coil.
The implantable VAD blood pump (Fig 1) combines the blood sac, volume displacement chamber, electrohydraulic axial flow pump, and internal control electronics into a single unit capable of being implanted in the thoracic cavity. The geometric design and configuration of the implantable unit was based on a series of cadaver fit trials [2] and fluid dynamics studies [3]. The intrathoracic implant site was chosen for potential advantages over intraabdominal implantation, namely, (1) the device can be implanted via median sternotomy, eliminating the need for more extensive dissection in the abdomen; (2) the diaphragm remains relatively intact since it does not need to be perforated for cannulation of the natural heart; (3) the cannulation can be extremely short and direct (Fig 2); and (4) the volume displacement chamber lies adjacent to the lung, allowing the pressure of the system to equalize with atmospheric pressures (a necessity for implantable pulsatile devices), and when integrated into the VAD unit eliminates the need for percutaneous venting of the device.
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The VAD has four principle modes of operation: (1) single beat (used for removing air after implantation); (2) fixed beat rate; (3) full-fill/full-eject, in which full stroke of the electrohydraulic mechanism occurs; and (4) default mode (used in emergencies). The default mode has three safety levels that can be initiated in the case of an emergency or electronic failure. First, there is a completely redundant, full function software mode that will engage upon a primary software failure. Second, there is a default software mode of a fixed 60 beats per minute operation upon failure of the secondary software system. Finally, there is a hardware default mode of a fixed 60 beats per minute operation upon failure of the tertiary software system.
Transcutaneous energy transfer and telemetry
To eliminate the need for percutaneous connections to monitor, control, and power the implantable VAD unit, a TEIT system was developed. This system allows energy and information to be transferred through intact skin and tissue [4–6] by the two coils shown in Figure 3. One coil may be implanted subcutaneously in either the deltopectoral region or upper abdomen and connected by a cable to the implantable VAD unit. The other coil rests on the skin directly over the implanted coil and is connected by a cable to the external controller (Fig 4).
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hours of operating time, which should allow the patient complete mobility away from external components providing total freedom of movement and the ability to partake in a wide variety of activities such as bathing, showering, swimming, and so on. The implanted VAD unit is monitored and controlled by the information transfer system part of the TEIT. This system uses transcutaneous infrared data communications between the implanted VAD unit and the wearable external controller through the coil system. The external controller provides device status displays and warning alarms to the patient.
During implantation of the device and patient recovery in the hospital, and during remote device monitoring after patient discharge, the clinician will use the clinical user interface (Fig 5). This system provides all the monitoring and control functions for the device, including real-time monitoring of device output. To provide ease of use, the clinical user interface is equipped with an radiofrequency modem to allow wireless communication with the device through the external controller. This feature may be useful during device implantation as it eliminates the need to drag cables into the sterile field. Furthermore, during the in-hospital recovery phase it will allow the patient to move around freely, while maintaining continuous monitoring of device function.
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Anatomical fit
Models of the most recent design of the implantable components were placed in fresh human cadavers to assess fit and implantability. Measurements and photographs were taken to determine feasibility of an implant procedure.
In vitro experiments
After device fabrication, all units were inspected and subjected to a burn-in period to ensure proper performance. Units were either placed on the mock loop for long-term reliability testing or tested before each in vivo experiment under the following conditions: an inflow pressure between 2 and 25 mm Hg and a mean afterload pressure between 60 and 160 mm Hg. Two operating modes were tested including a fixed rate between 40 and 160 BPM, and full-fill/full-eject mode. The function of the devices was tested in open air and while submerged in saline at 37°C. For in vivo trials, after acceptable performance the units were packaged and sterilized for implantation. For each implant, two complete system were prepared and delivered to the operating room.
In vivo experiments
A bovine model has been used to test the devices. Series II systems were used in acute experiments to test components, develop the implant procedure, and assess hemodynamics of the device working in situ.
Results
Anatomical fit
Device size and geometry is critical for successful intrathoracic implantation. To assess the optimal size and geometry for intrathoracic device placement, fit studies in cadavers and intraoperative human fit trials were conducted using previous version models [1]. Following changes in device conformation, new models were used in follow-up fit studies that suggested that there was acceptable fit in patients as small as 64 kg. However, to ensure anatomical fit in a wider range of patients, efforts have been ongoing to further reduce the volume and weight of the implantable VAD unit, resulting in a current device volume of approximately 530 mL.
Surgical implantation procedure
Based on fit studies in cadavers, the operative approach will be suggested as follows (Fig 7): after median sternotomy, the pericardium will be opened on the right side to allow for a large portion of the sac to be used to encircle the outflow cannula at the end of the procedure. The left pleural will be opened completely to allow the surgeon to assess the left chest. If the left thorax is large enough to accept the device without any further manipulation, cannulation will proceed. If the device does not fit easily in the thorax, then better positioning may be accomplished as follows: the diaphragm can be separated from the rib cage to create a subphrenic/supraperitoneal space in which the inferior aspect of the VAD can be placed. The superior part of the VAD containing the volume displacement chamber will be left in the pleural cavity in contact with the anterior surface of the inferior lobe of the left lung. The left ventricular apex will be cannulated with the patient on cardiopulmonary bypass. The inflow and outflow cannulas will be attached to the VAD after it is placed in the left chest. The TEIT and internal battery cables will be brought out through the inferior margin of the device pocket and connected to the TEIT and battery located in separate pockets on either side of the midline above the umbilicus. Finally, the outflow cannula will be connected to a suction tube to allow for complete removal of air from the cannulas and VAD. The cannula is cut to an appropriate length and anastomosed to aorta. The pericardium is sutured around the outflow graft before standard chest closure.
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years. Several more recent prototype versions have functioned for more than 2 years failure free. Seventeen Series II VADs are accumulating in vitro run-time operation as follows: four units are being used in a pilot in vitro study with 320 days of accumulated operation and four are to enter into pilot in vivo testing. Nine units are being used for a variety of engineering experiments with a total of more than 420 days of operation.
In vivo experiments
To date more than 50 experiments have been performed to assess the components as they have been modified [7, 8]. Over the last 12 months there have been an additional eight completed, acute experiments. Five other experiments were not completed because of problems encountered with bleeding around or through the new titanium cannulas or irreversible ventricular fibrillation. To date, the mean VAD output has been 10.4 ± 1.1 L/min while operating in full-fill/full-eject mode. The main problems encountered in the acute experiments were with the new cannulas. Initially, there were problems with the mating of the cannula to the device, but these have been easily rectified. An ongoing problem remains the difficulty of adapting a device designed for the thoracic cavity of a human to the much different anterioposterior dimension of the calf. These difficulties will be overcome with further manipulation of the cannulas and adjustment of the cannulation site in the left ventricle. The VAD pump itself has performed well under all conditions and has been shown to be effective even with the ventricle fibrillating. Further data regarding in vivo trials will be available when this series of experiments is completed.
Comment
Over the last 24 months, Series II HeartSaver VADs have been developed and assessed. The current version has a displacement of 530 mL. Many of the improvements in Series II have focused on improved safety and reliability features of the device. With the recent acquisition of Novacor from Edwards Life Science, additional development work has been initiated to incorporate their material for the flexible components of the VAD system including the blood sac, pumping diaphragm between the blood sac and hydraulic pumping chamber, and the volume displacement chamber. Their material, segmented polyurethane solution, has been used extensively in the Novacor device in the clinical setting and has US Food and Drug Administration approval. This clinical experience will be a great asset to the development of the HeartSaver VAD system.
The electronics continue to undergo extensive refinements. These include changes to the internal battery, which will now incorporate medical grade prismatic lithium ion cells instead of NiCad batteries. Each battery cell will be monitored independently with thermal and current fuses to prevent overload. The internal controller’s electronic reliability has been increased using input protection circuitry. Additional work to improve algorithms for the motor controller has provided smoother and more efficient operation of the hydraulic axial flow DC motor within the VAD. The TEIT has been improved to allow infrared communication across skin up to 6 cm in thickness and of varying pigmentation. Finally, power transfer efficiency is being optimized that will result in reduced heat generated by the coils of the TEIT.
The development of the HeartSaver VAD is progressing well. The use of in vitro and in vivo studies to test all of the components has been critical for the identification of areas needing further improvement. Following these changes, the device will be available for formal testing before submission for regulatory approval to allow clinical implantation of the device.
Acknowledgments
We thank the staff of the Cardiovascular Devices Division and WorldHeart Corporation, whose ongoing efforts have been instrumental to the continuing development of the HeartSaver VAD.
Footnotes
Drs Hendry, Masters, Bourke, and Keon are consultants to WorldHeart Corporation; Dr Guiraudon, Mr Day, and Mr Holmes are employees of WorldHeart Corporation; Dr Mussivand is Chairman and Chief Scientific Officer of WorldHeart Corporation.
References
This article has been cited by other articles:
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  M. Haddad, R. G. Masters, P. J. Hendry, A. Kawai, J. P. Veinot, G. Lavallee, and T. V. Mussivand Intercontinental LVAS Patient Transport Ann. Thorac. Surg., November 1, 2004; 78(5): 1818 - 1820. [Abstract] [Full Text] [PDF]
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 E. L. Kukuy, M. C. Oz, and Y. Naka Long-Term Mechanical Circulatory Support Card. Surg. Adult, January 1, 2003; 2(2003): 1491 - 1506. [Full Text]
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