Kimitaka Tasai

Development of a Pivot Bearing Supported Sealless Centrifugal Pump for Ventricular Assist

Since 1991, in our laboratory, a pivot bearing-supported, sealless, centrifugal pump has been developed as an implantable ventricular assist device (VAD). For this application, the configuration of the total pump system should be relatively small. The C1E3 pump developed for this purpose was anatomically compatible with the small-sized patient population. To evaluate an-tithrombogenicity, ex vivo 2-week screening studies were conducted instead of studies involving an intracorpore-ally implanted VADs using calves. Five paracorporeal LVAD studies were performed using calves for longer than 2 weeks. The activated clotting time (ACT) was maintained at approximately 250 s using heparin. All of the devices demonstrated trouble-free performances over 2 weeks. Among these 5 studies, 3 implantations were subjected to 1-month system validation studies. There were no device-induced thrombus formations inside the pump housing, and plasma-free hemoglobin levels in calves were within the normal range throughout the experiment (35, 34, and 31 days). There were no incidents of system malfunction. Subsequently, the mass production model was fabricated and yielded a normalized index of hemolysis of 0.0014, which was comparable to that of clinically available pumps. The wear life of the impeller bearings was estimated at longer than 8 years. In the next series of in vivo studies, an implantable model of the C1E3 pump will be fabricated for longer term implantation. The pump-actuator will be implanted inside the body; thus the design calls for substituting plastic for metallic parts.

The Baylor total artificial heart. Flow visualization studies.

To analyze the flow patterns of the left blood chamber of the Baylor total artificial heart (TAH) and to evaluate influences of the inflow valve angle to the flow patterns, flow visualization studies were performed. The inflow valve angle of the left housing was changed by 20 degrees orthogonal to the inflow tube, and comparison studies of the modified and unmodified models were made. For evaluating sectional flow patterns, a laser light was used, the clear transparent housing was scanned segmentally, and flow patterns were recorded on high contrast film for measuring flow velocities. A signal was used that synchronized the timing of the camera shutter to the pusher-plate movement signal. With the modified 20 degree inflow valve direction, there were better closing characteristics of the inflow valve leaflets. At the same time, we could successfully reduce the vortex formation at the inflow port, which may cause thrombus formation. We also have improved the washout during the diastolic phase in not only the bottom area, but in the entire pumping chamber. This flow visualization setup is simple and inexpensive. It is useful not only for validation of global flow patterns, but also for validation of local flow velocities of various blood pumps.

A PIVOT BEARING-SUPPORTED CENTRIFUGAL PUMP FOR A LONG-TERM ASSIST HEART

A pivot bearing-supported centrifugal blood pump has been developed. It is a compact, cost effective, and anti-thrombogenic pump with anatomical compatibility. A preliminary evaluation of five paracorporeal left ventricular assist studies were performed on pre-conditioned bovine (70-100 kg), without cardiopulmonary bypass and aortic cross-clamping. The inflow cannula was inserted into the left ventricle (LV) through the apex and the outflow cannula affixed with a Dacron vascular graft was anastomosed to the descending aorta. All pumps demonstrated trouble free performance over a two-week screening period. Among these five studies, three implantations were subjected for one month system validation studies. All the devices were trouble free for longer than 1 month. (35, 34, and 31 days). After achieving one month studies, all experiments were terminated. There was no evidence of device induced thrombus formation inside the pump. The plasma free hemoglobin levels were within normal ranges throughout all experiments. As a consequence of these studies, a mass production model C1E3 of this pump was fabricated as a short-term assist pump. This pump has a Normalized Index of Hemolysis of 0.0007 mg/100L and the estimated wear life of the impeller bearings is longer than 8 years. The C1E3 will meet the clinical requirements as a cardiopulmonary bypass pump. For the next step, a miniaturized pivot bearing centrifugal blood pump PI-601 has been developed for use as a permanently implantable device after design optimization. The evolution from C1E3 to the PI-601 converts this pivot bearing centrifugal pump as a totally implantable centrifugal pump. A pivot bearing centrifugal pump will become an ideal assist pump for the patients with failing heart.

Development of an Implantable Centrifugal Ventricular Assist Device (CVAD)

The centrifugal ventricular assist device (CVAD) was developed for long-term circulatory support, and is capable of either intracorporeal implantation or paracorporeal placement. The pump was designed based on our antithrombogenic concepts: (1) sealless pump casing, (2) elimination of stationary parts, and (3) blood flow acceleration under the impeller. To meet conditions (1) and (2), a pivot bearing system was adopted to support the impeller. The inlet port was placed slightly off-center and inclined 60° towards the same direction as the outlet port. This port configuration not only yielded a space where an inlet cup bearing could be directly embedded but also allowed for a significant reduction of the pump height, hence, resulting in easier placement inside the body cavity. Two small secondary vanes were installed in the bottom of the impeller to satisfy condition (3). Five paracorporeal left ventricular (LV) AD studies, using calves, were performed to evaluate the antithrombogenic design of the pump. The first two cases were subjected to 2-week tests. With the activated clotting time (ACT) kept at 250 s with heparin, the initial two cases had trouble-free performances over the 2 weeks. Following these successful results, another three cases were subjected to 1-month validation studies, in which there was no device-induced thrombus formation inside the pump housing. These results confirm that the CVAD, the C1E3, meets the requirements for a 1-month paracorporeal LVAD.

Biomaterial Considerations for Cardiac Prostheses

One of the most important prerequisites in developing a clinically applicable cardiac prosthesis is making it blood compatible. Two factors must be considered to prevent blood clotting inside a device: design to eliminate blood-stagnant areas within the device, and use of blood-compatible materials for the device’s blood-contacting surfaces.

The Baylor-ABI Electromechanical Total Artificial Heart: Accelerated Endurance Testing

To test the durability of each part or assembled component of the Baylor-ABI total artificial heart (TAH), the authors performed an endurance test under severe conditions. The TAH was immersed in a saline bath at 42 degrees C, which is 4-5 degrees C higher than normal body temperature. This is an accelerated endurance test because of the elevated temperatures. In this accelerated endurance test loop, the 42 degrees C heated saline was circulated not only in the pump but also outside the pump. During pumping, temperatures of the motor and outside surface of the centerpiece were continuously measured. This testing showed that during almost 4 months of pumping no electromechanical troubles were observed. Both inside (motor) and outside temperatures were stable and the differences in both temperatures were only 3-4 degrees C, demonstrating that heat generation is not a problem. The voltage and current required in this system remained constant, indicating stable and reliable performance. Based on these results, this pump is expected to run continuously over a long duration in a normal physiologic environment. This accelerated endurance test system is very suitable for estimating the influence of heat generation by the actuator of blood pumps. It is also quite useful in validating the durability of various cardiac prosthesis.

Phase 1 Ex Vivo Studies of the Baylor/NASA Axial Flow Ventricular Assist Device

The Baylor/NASA ventricular assist device (VAD) is a small, electrically driven, valveless axial flow pump that is implantable inside the chest cavity. It is intended to assist a diseased heart. In the phase 1 study of this pump development program, the 2-day pump is intended to produce an assist device for cardiopulmonary bypass (CPB) application. The main focus of this phase of the program was to develop a pump which produced minimum blood trauma. Antithrombogenic features are planned to be incorporated into the phase 2 pump. In this phase 1 study, eight pumps were implanted paracorporeally in two calves as LVADs to assess hemolysis, pump performance, efficiency, and stability, the goal for this study being a 2-day implantation. The pump running times ranged from 18 to 203 (78.1 ± 23.7; mean ± SE) h. Plasma free hemoglobin levels were below 13.7 mg/di, except for one case complicated by inflow cannula obstruction due to pannus formation. Pump speed was maintained between 10100 and 11400rpm. Pump output ranged from 3.6 to 5.11/min. The electrical power required by the system ranged from 10.5 to 12.8W. No detectable organ dysfunction was noted and postmortem evaluations demonstrated no pump-related adverse effects in any of the calves. Thrombus deposition was observed mainly at the hub area and flow straightener. For the next series of experiments (phase 2), the thrombogenic regions in these subacute experiments should be eliminated.

The Baylor Electromechanical Total Artificial Heart

A totally implantable electromechanical total artificial heart (TAH) system has been developed in our institute. This pump is very small (outer diameter, 97 mm; central thickness, 83 mm; and weight, 620 g), demonstrating a good anatomical fit in the pericardial space of 26 heart transplant recipients. The actuation mechanism is simple, and all the components are commercially available with proven longterm durability, thus allowing easier fabrication. The pump can be easily and simply controlled by reliable Hall effect sensors with left master alternate (LMA) mode. Four newly fabricated TAHs demonstrated quite similar pump performances. This TAH has a reproducible high performance with good quality assurance. In vitro performance mapping demonstrated that the pump can provide a maximum flow of 91/ min, with a high sensitivity to preload and a low sensitivity to afterload. During 4 months of accelerated endurance testing in 42°C saline, no electromechanical troubles were observed and power requirement remained constant, indicating a stable and reliable performance. After modification of the inflow valve angle, excellent flow paterns inside the blood chamber were demonstrated in this study, in which laser light and a high-speed camera were used. In vivo feasibility tests were performed successfully in eight calves for up to 1 week, demonstrating the readiness to move forward to longterm in vivo studies. This small, simple, reliable, and durable mechanically driven totally implantable TAH system is suitable for a permanent heart replacement.