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Ann Thorac Surg 1999;68:790-794
© 1999 The Society of Thoracic Surgeons
a Nimbus Inc, Rancho Cordova, California, USA
b Tracor Analysis and Applied Research, Austin, Texas, USA
c McGowan Center for Artificial Organ Development, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
Address reprint requests to Mr Butler, Nimbus Inc, 2945 Kilgore Rd, Rancho Cordova, CA 95670
e-mail: kcbutler{at}pacbell.net
Presented at the Fourth International Conference on Circulatory Support Devices for Severe Cardiac Failure, Houston, TX, Oct 3–5, 1997.
Abstract
Background. Nimbus Inc, and the University of Pittsburgh’s McGowan Center for Artificial Organ Development have been collaborators on rotary blood pump technology initiatives since 1992. Currently, a major focus is an innovative ventricular assist system (IVAS) that features an implantable, electrically powered axial flow blood pump. In addition to the blood pump, a major development item is the electronic controller and the control algorithm for modulating pump speed in response to varying physical demand.
Methods. Methods used in developing the IVAS include computational fluid dynamic modeling of the pump’s interior flow field, flow visualization of the flow field using laser-based imaging, computer simulation of blood pump-physiological interactions, vibroaccoustic monitoring, and an extensive in vivo test program.
Results. Results to date, which are presented below, include successful in vivo tests of blood pumps with blood-immersed bearings, and feasibility demonstration of vibroacoustic monitoring in this application.
Conclusions. This unique blend of industrial experience and technologies with the University-based Research and Development Center has greatly enhanced the progress made on this IVAS project.
As part of a National Institutes of Health-sponsored program to develop next-generation mechanical circulatory support systems, the research team of Nimbus Inc and the University of Pittsburgh (UOP) is developing an innovative ventricular assist system (IVAS) that features an implantable, electrically powered axial flow blood pump. This IVAS has the potential to offer safe and cost-effective rehabilitation to patients in end-stage cardiac failure. Compared with existing clinical devices currently being used for this purpose, the axial-flow IVAS has significant advantages with respect to implanted component simplicity, size, and weight. In addition, the cost of our axial-flow IVAS as a clinical product will be significantly lower than today’s pulsatile-based systems.
IVAS description
The device Nimbus/UOP is developing embodies a small integrated pump-motor assembly that connects into the circulatory system through an apical cannula whose inlet draws blood flow from the left ventricular apex, and an outflow cannula that is anastomosed to the ascending aorta. Figure 1 displays the blood pump/motor unit. Figure 2 depicts one concept to anatomical placement of the blood pump and cannulae. Here, the pump is placed below the left costal margin and under the left rectus abdominal muscle. The inflow cannula exits the left ventricular apex and crosses the diaphragm in the costophrenic angle, entering the subrectus pocket under the left rib margin to attach to the pump. The outflow cannula is tunneled back under the sternum to the ascending aorta.
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The latter configuration requires implanted components in addition to the pump/cannulae. These include a miniature internal controller, an internal battery, and the secondary coil of an energy transmission circuit. This coil lies under the skin, adjacent to an external coil. Electrical power applied to the external coil is inductively coupled to the implanted coil. Hence, power is transmitted to the implanted controller transcutaneously, thus avoiding the need for direct wire access and a skin penetration site. Table 1 lists the components that comprise the fully implantable IVAS version. A patient would also carry or wear an external battery power source to operate the IVAS.
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Torque required to drive the pump rotor is developed by an integral electric motor. The pump’s 12-mm-diameter duct is actually a thin-walled tube that passes through the bore of the motor’s coil windings. The motor’s rotor is a permanent magnet located in the hub of the pump’s rotor. The location of the pump rotor in the tube places the motor magnet in the proper position with respect to the coils, which is at the centerline of the winding’s axis, and centered longitudinally with respect to the coil’s length. Excitation current sequentially commutated to the coils creates a spinning magnetic field, thereby imparting torque and angular velocity to the motor magnet (ie, pump rotor).
The pump rotor spins on two bearings located at either end. Both bearings react against radial and axial thrust loads. Each bearing’s stationary element is located in the hub of the respective inlet and outlet stators; the bearings themselves are similar to standard ball-cup jewel bearings. The outer boundary of the bearing’s adjacent static and moving surfaces are washed directly by the main blood flow. To date, in our testing, the preferred bearing materials embody combinations of state-of-the-art ceramic technology.
Currently, state-of-the-art bioengineering methods are being applied to evaluate the pump’s basic hydraulic design. These involve laser-based flow visualization studies, and modeling of the pump’s flow field using computational fluid dynamics (CFD). The overall purpose of this effort is to improve the flow efficiency of the pump and optimize the interior flow field relative to minimizing stagnant or recirculation areas, and to examine off-design extremes. These efforts have been discussed in detail [1].
Nimbus/UOP’s latest axial flow IVAS pump version has an overall length of approximately 7.0 cm and an outer diameter of 4.0 cm. Most of its structural parts are machined from 6A14V titanium. Other features of a fully implantable system are listed in Table 1.
Pump-left ventricular dynamics
A key characteristic of rotary pump hydraulic performance is contained in the pump’s pressure-flow (H-Q) relationships. Figure 3 illustrates steady flow H-Q curves measured over a range of speeds for the Nimbus/UOP axial-flow IVAS pump circulating bovine blood. As shown, this family of curves defines the relationship of flow generated by the pump versus pressure difference across the pump. Each curve is for a particular rotating speed.
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In vivo studies
As of this writing, a total of 19 IVAS implants have been performed in Jersey calves at the University of Pittsburgh. Of the 19 implants, seven utilized our initial blood pump configuration (model 1), and 12 used our current configuration (model 2). All animals received humane care in compliance with the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health (NIH publication 85-23, revised 1985).
In vivo studies have focused on demonstrating that bearings do not wear excessively, and thrombus does not form at the bearing sites. For each bearing material combination under consideration in the model 1 and model 2 versions, and after baseline inspection and surface characterization, we conduct a 30-day in vivo basic biocompatibility screening to assess hemolysis, signs of emboli, and any depositions on the pump or bearing surfaces. A particular pump prototype is deemed to have successfully completed a 30-day in vivo experiment if the bearings are thrombus free, no emboli or infarcts are detected in the kidneys, and acceptably low hemolysis is documented. When this occurs, the pump is cleaned, resterilized, and implanted for a 6-month duration to investigate longer-term biocompatibility and bearing wear effects. Thus far, two IVAS prototypes have successfully completed both a 30-day and a 6-month in vivo trial. At explant after performing flawlessly for 6 months, the pumps were free of all deposits. There was only a single small infarct in the left kidney in one of the animals, whereas the right kidney was free of any infarcts. Additional details regarding one of the 6-month implants have been described [2].
A third series of in vivo trials are also underway for the purpose of developing algorithms to regulate pump speed (rpm) in a closed-loop fashion, which is required for a fully implantable version. These experiments are being conducted for 30 days, and require that the animal be highly instrumented in order to monitor cardiac and systemic pressures and flow rates as a function of the physiologic state (eg, sleeping, awake, exercising, etc) of the animal. Additional details of these assessments can be found in reference [1].
Overall, the results of our in vivo implants have been quite positive. We have identified bearing combinations that perform satisfactorily, operating directly in the main blood flow, and with acceptably low wear over 6 months. The IVAS appears to be well tolerated by the animal, which eats and drinks ad libitum, nor is hemolysis a problem. Blood chemistries remain within normal ranges throughout the various implant periods, and there is no detectable physical sign of organ dysfunction. Finally, no consistent mechanical problems have been identified. The blood pumps have functioned normally as per the H-Q curves of Figure 3, at flow rates that are adequate for LV support of adult patients in cardiac failure.
Vibroacoustic fault monitoring
When a mechanical system such as the Nimbus/UOP IVAS operates, its internal vibrations are coupled to the pump housing, which in turn drives the surrounding medium creating a time-varying pressure field that propagates through the medium. It is possible to sense the vibrations on the housing with accelerometers or the (acoustic) pressure field transthoracically with a contact microphone. When the nature of the vibrations changes, for example, due to bearing wear, these changes can be detected in the accelerometer and acoustic signals, thus providing a potential means of early detection of pump faults that portend future failure. Currently monitored quantities (voltage, current) appear to provide warning only a very short time (minutes, perhaps a few hours) before failure occurs, whereas in other applications involving rotating machinery, vibroacoustic warnings occur as early as 90 days in advance of failure [3]. Vibroaccoustic fault monitoring has also been applied to the in vivo assessment of the physical integrity of the Björk-Shiley Convexo-Concave heart valve [4]. Recognition of the potential for patient benefit is thus the motive for undertaking the use of vibroacoustic fault monitoring to monitor the Nimbus/UOP IVAS on the bench and under actual use conditions in vivo.
Methods
We developed a spectral analysis of the accelerometer and acoustic signals from unfaulted and deliberately faulted IVAS prototypes. Recordings were first obtained in vitro, and the spectrum analysis showed the classical signatures that are observed in other application areas. Example spectra are shown in Figure 4. The changes in the deliberately faulted IVAS spectrum relative to that of the unfaulted prototype were a 10- to 100-fold increase in the tonal and continuum components of the power density spectrum.
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Results
Subjective comparison of the spectra 1 day postimplant and 90 days postimplant strongly suggests that no significant bearing wear had taken place. The spectra of this pump 1 day and 90 days postimplant are shown in Figure 5. After compensation for different rpms and normalization of the continuum, the spectra appear as shown in Figure 6.
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Comment
This project is advancing quickly towards a final IVAS design to be utilized in animal trials and in vitro reliability tests. Our work conducted to date suggests that several conclusions are appropriate. (1) The nominal pressure-flow operating range of our current IVAS adequately covers hemodynamic requirements of a clinical left ventricular assist device. (2) Hemolysis performance of the blood pump is satisfactory, as determined both by in vitro and in vivo test results. (3) Nonpurged bearings located in the blood pump’s main flow stream operate satisfactorily in blood for 6 months. (4) Bearing wear is minimal in pumps inspected after 6 months of operating time either in vivo or in vitro. (5) Acoustic signals can be obtained transthoracically reliably and without great difficulty. (6) In the in vivo environment, the acoustic signature appears to be statistically stationary and repeatable. This is very important, because it is desirable for changes to occur only as a result of pump wear. 7) These results constitute strong anecdotal evidence of the feasibility of vibroacoustic fault monitoring as a means of detecting faults that precede failures in this application.
Acknowledgments
This work is being supported by Contract N01-HV-58155, issued by The National Heart, Lung, and Blood Institute, National Institutes of Health.
Footnotes
Kenneth C. Butler is an employee of Nimbus Inc. Jerry J. Dow is an employee of Tracor Analysis and Applied Research.
References
This article has been cited by other articles:
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  J. Haft, W. Armstrong, D. B. Dyke, K. D. Aaronson, T. M. Koelling, D. J. Farrar, and F. D. Pagani Hemodynamic and Exercise Performance With Pulsatile and Continuous-Flow Left Ventricular Assist Devices Circulation, September 11, 2007; 116(11_suppl): I-8 - I-15. [Abstract] [Full Text] [PDF]
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 T. A. Snyder, M. J. Watach, K. N. Litwak, and W. R. Wagner Platelet activation, aggregation, and life span in calves implanted with axial flow ventricular assist devices Ann. Thorac. Surg., June 1, 2002; 73(6): 1933 - 1938. [Abstract] [Full Text] [PDF]
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B. W. Duncan Mechanical circulatory support for infants and children with cardiac disease Ann. Thorac. Surg., May 1, 2002; 73(5): 1670 - 1677. [Abstract] [Full Text] [PDF]
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Y. Ochiai, L. A.R. Golding, A. L. Massiello, A. L. Medvedev, R. L. Gerhart, J.-F. Chen, M. Takagaki, and K. Fukamachi In vivo hemodynamic performance of the Cleveland Clinic CorAide blood pump in calves Ann. Thorac. Surg., September 1, 2001; 72(3): 747 - 752. [Abstract] [Full Text] [PDF]
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G. M. Wieselthaler, H. Schima, A. M. Lassnigg, M. Dworschak, R. Pacher, M. Grimm, and E. Wolner Lessons learned from the first clinical implants of the DeBakey ventricular assist device axial pump: a single center report Ann. Thorac. Surg., March 1, 2001; 71 (2007): S139 - S143. [Abstract] [Full Text] [PDF]
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 H. M Reul and M. Akdis Blood pumps for circulatory support Perfusion, July 1, 2000; 15(4): 295 - 311. [PDF]
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