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Ann Thorac Surg 1999;68:775-779
© 1999 The Society of Thoracic Surgeons

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Innovative Circulatory Support Systems

Development of a totally implantable biventricular bypass centrifugal blood pump system

^a Department of Surgery, Baylor College of Medicine, Houston, Texas, USA

Address reprint requests to Dr Nosé, Department of Surgery, Baylor College of Medicine, One Baylor Plaza, Houston, TX 77030;
e-mail: ynose{at}bcm.tmc.edu

Presented at the Fourth International Conference on Circulatory Support Devices for Severe Cardiac Failure, Houston, TX, Oct 3–5, 1997.

Abstract

Background. During the past 2 years, the development of a totally implantable biventricular bypass rotary blood pump system has been made.

Methods. An extracorporeal gyro centrifugal pump, the CIE3, was miniaturized and developed into the PI601, a totally implantable plastic pump. Two-day anatomic and hemodynamic feasibility studies demonstrated that these two pump systems were easily implantable inside a calf’s abdominal wall, directly under the diaphragm. The priming volume of the pump was 20 mL, with sufficient cardiac outputs at approximately 2,000 rpm and requiring less than 10 W of power. Two-week antithrombogenic screening tests also revealed these pump systems to be quite antithrombogenic. In addition, 1-month system reliability studies demonstrated fail-safe reliable performances.

Results and Conclusions. Encouraged by these preliminary studies, the PI601 model was converted to the permanently implantable titanium gyro pump PI702 model. The long-term implantations were initiated approximately 3 months ago, and two such long-term LVAD studies are currently underway with no sign of difficulty (October 10, 1997). They were followed 283 days and 72 days, respectively. Both terminated due to functional inflow obstruction. There were no blood clots or emboli at autopsy.

Recently, the beneficial effects of the use of an implantable left ventricular assist device not only for bridge to transplantation but also as an aid to myocardial recovery are being clearly demonstrated [1]. However, approximately 25% of these patients develop right heart failure syndrome, including multiorgan failure and sepsis [2]. Thus, many researchers recognize the need for a totally implantable biventricular support system. This is further confirmed by the excellent clinical results of the CardioWest Total Artificial Heart [3]. However, it is becoming quite convincing that we should not remove dilating cardiomyopathic hearts expecting their recovery in 6 months to 1 year [4]. It is also being demonstrated that atherosclerotic vessels would begin to become normalized after monthly lipofiltration or lipoadsorption procedures for at least 2 years [5, 6]. Thus, it is becoming quite urgent to develop a totally implantable biventricular assist device that would be operational for longer than 2 years. Unfortunately, the currently available implantable pulsatile ventricular assist devices are quite large, and it is impossible to implant two of them intracorporeally [7, 8]. Miniaturization of these devices is mandatory. However, not only theoretically, but also practically, reducing the size of a pulsatile blood pump further is very difficult [9, 10].

Under the circumstances, the introduction of a rotary blood pump is the only way to develop substantially smaller blood pumps [9]. An axial flow blood pump would be the smallest due to the high rotational speed of its impeller, which would be in the range of 10,000 to 12,000 rpm [10]. Unfortunately, high rotational speeds of the impeller impose more wear on the bearing system of the pump. Thus, their theoretical life expectancy is less than 2 years. To develop a blood pump that will last longer than 2 years, it is mandatory that the rotational speed of a rotary blood pump’s impeller be reduced to less than 3,000 rpm, although the size of the pump needs to be somewhat larger [11].

Two identical centrifugal blood pumps of such performance characteristics could be implanted in the abdominal wall as a left and a right ventricular bypass device. Their inflows should be inserted directly into the ventricles (Fig 1). Each pump should independently control pump output with its own independent control and actuation system. The pump should accommodate more than 8 L/min cardiac output, yet be small enough to implant two of the devices in an individual weighing less than 45 kg.

View larger version (144K): [in this window] [in a new window]   Fig 1. Two centrifugal blood pumps are implanted in the abdominal wall, directly under the diaphragm. The pump shown on the left is the right ventricular assist device (RVAD), and the pump shown on the right side is the left ventricular assist device (LVAD). (TETS = transcutaneous energy transmission system.)

1. Two-day tests of biventricular bypass implantation. Aims of these 2-day studies are to demonstrate the anatomic compatibility, acceptable hemodynamics, and minimum blood trauma.
   
2. Two-week tests of a single pump implantation. The aim of these studies is to demonstrate antithrombogenic features of the pump. Any pump system that does not produce any thromboembolic phenomena during this 2-week test period will most likely remain blood clot-free for an extended time [12].
   
3. One-month test for system reliability. By extending the above mentioned 2-week test for an additional 2 weeks, it is possible to prove the system reliability or serve as in vivo burn in test.
   
4. Chronic animal tests. If the above mentioned studies are satisfactorily completed, the implantable pump actuator system should be converted to a durable system. The plastic components of the pump should be converted to Ti-alloy metallic components. To maintain durable electronic performances, hermetic sealing, antiflex and moisture resistance cable assembly, and an anti-high temperature electronics package should be provided.
   

All animals subjected to this program have received humane care in compliance with the "Guide for the Care and Use of Laboratory Animals" published in the National Institutes of Health (NIH publication 85-23, revised 1985).

Material and methods

Pump–actuator system
The gyro C1E3 centrifugal blood pump was used as the start-up device for this system development [13]. This extracorporeal blood pump had already demonstrated its antitraumatic and antithrombogenic features after 1 month of paracorporeal implantations [14]. To meet the design criteria for the development of a totally implantable biventricular assist system, further miniaturization of the C1E3 pump was attempted while maintaining a completely sealless blood-contacting chamber design. The implantable plastic gyro pump model PI601 was thus developed for the preliminary studies. A C1E3 blood pump has a double-pivoted bearing-supported impeller, with an eccentric inlet port and secondary vanes at the bottom of the impeller. The impeller is activated by the magnetic coupling principle. The possible stagnant regions inside the blood pump are around the top and bottom bearings. However, these areas are well washed by the secondary blood flows introduced by the eccentric inlet port for the top-bearing region and the secondary vanes at the bottom of the impeller for the bottom-bearing region. These secondary blood flows induce additional blood trauma but prevent possible blood clot formation at these regions (Figs 2 and 3 ). After reviewing various types of actuators, the Vienna thin motor actuator was chosen for this permanent gyro pump. The special size-matched actuator–pump subsystem was developed together with the Vienna group (Fig 4). The in vitro feasibility studies were conducted satisfactorily with polycarbonate fabricated pumps. The bearing systems were the same as those of the C1E3 pump, composed of the alumina ceramic male bearings and the ultra–high molecular weight polyethylene female bearings.

View larger version (139K): [in this window] [in a new window]   Fig 2. A picture of the C1E3 centrifugal pump. At the top of the pump housing, the top female bearing housing is seen. The eccentric inlet port of the pump is located on the right side of the bearing housing.

View larger version (100K): [in this window] [in a new window]   Fig 3. The impeller of the C1E3 pump. The white ceramic male bearing shaft is shown at the center. The impeller spins supported by the top and bottom male bearings in the housing of the pump.

View larger version (94K): [in this window] [in a new window]   Fig 4. The PI601 pump–actuator package is shown on the right. The hermetically sealed actuator subsystem is also shown at the left.

View larger version (119K): [in this window] [in a new window]   Fig 5. The PI601 right ventricular assist device (at the top) and the PI601 left ventricular assist device (at the bottom).

Results

Fabrication of titanium pump (PI702)
Encouraged by these preliminary studies, conversion of the plastic pump to a titanium pump was made (Fig 6). A titanium alloy of 6% aluminum and 4% vanadium was chosen for the pump material. The reason for this choice was to use the same material as the material already used successfully in the TCI-Heartmate [15]. Although their blood-contacting surface was texturized [16], blood compatibility was clinically well demonstrated. However, before fabricating the pump with this alloy, simple platelet adhesion tests [17] were performed with a smooth surface material of 0.2-µm surface roughness to prove its feasibility as a pump material. Figure 7 shows the results of these studies. The smooth surface titanium alloy demonstrated the minimum platelet adhesion among all materials subjected to these studies. Because the pivot-bearing systems used for the C1E3 pump already demonstrated their longer than 2 years life expectancy [18], it was decided to keep the design and material combination of the pivot-bearing systems the same as that of the C1E3 pump. All the other parts were converted from plastic to titanium. There was no change in dimensions between the PI601 and the PI702 pump. The same actuator controller was used for the PI702. Their hydrodynamic performances [19] were studied (Table 3 ) and demonstrated that performances of this titanium PI702 model pump was almost equivalent to those of the PI601 pump. In vitro hemolysis studies of the PI702 comparing other previous models of the centrifugal pumps were conducted (Table 4). The Normalized Index of Hemolysis [20] value of the PI702 was in the range of 0.004 g/100 L. Although it was slightly higher than those of the other models, this value is still in the range of one-tenth of those of pulsatile blood pumps. The key features of the PI702 system are summarized in Table 5

View larger version (113K): [in this window] [in a new window]   Fig 6. The titanium permanently implantable centrifugal gyro pump model PI702.

View larger version (18K): [in this window] [in a new window]   Fig 7. Platelet adhesion studies. The smooth surface Ti6Al4V shows the lowest level of platelet adhesion among all test specimens.

Animal 1 survived for 283 days and animal 2 survived for 72 days. The reason of termination for both experiments was the functional inflow occlusion due to overgrowth of experimental animals. No blood clots or emboli were revealed inside the pumps or in the downstream organs.

In conclusion:

1. Biventricular bypass implantations of two gyro centrifugal pump systems (PI601 model) demonstrated their anatomic and hemodynamic feasibility in the abdominal wall.
   
2. The PI601 plastic pumps demonstrated thrombus free in in vivo screening studies.
   
3. The PI601 pump–actuator combination demonstrated reliable 1 month in vivo screening test performances not only as a left ventricular assist device but also as a right ventricular assist device.
   
4. Successful conversion from the plastic pump (PI601) to the metallic pump was achieved. The permanently implantable titanium centrifugal gyro pump (PI702 model) was produced.
   
5. Long-term in vivo studies of the PI702 pumps were successfully initiated.
   

Acknowledgments

This project is funded by NEDO, the Ministry of Industry and Commerce, government of Japan, and by Kyocera Corporation, Japan. The authors acknowledge other collaborating investigators on this project and contributors to the paper. They include John C. Baldwin, MD, Goro Ohtsuka, MD, Yukio Ohashi, MD, Yoshiyuki Takami, MD, Shingo Yamane, PhD, Akinori Sueoka, PhD, Juergen Mueller, and Julia Glueck.

References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): George V. Letsou Ernst Wolner Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Nosé, Y. Articles by Schima, H. Search for Related Content PubMed PubMed Citation Articles by Nosé, Y. Articles by Schima, H.