Prototype Continuous Flow Ventricular Assist Device Supported on Magnetic Bearings

Abstract This article describes a prototype continuous flow pump (CFVAD2) fully supported in magnetic bearings. The pump performance was measured in a simulated adult human circulation system. The pump delivered 6 L/min of flow at 100 mm Hg of differential pressure head operating at 2,400 rpm in water. The pump is totally supported in 4 magnetic bearings: 2 radial and 2 thrust. Magnetic bearings offer the advantages of no required lubrication and large operating clearances. The geometry and other properties of the bearings are described. Bearing parameters such as load capacity and current gains are discussed. Bearing coil currents were measured during operation in air and water. The rotor was operated in various orientations to determine the actuator current gains. These values were then used to estimate the radial and thrust forces acting on the rotor in both air and water. Much lower levels of force were found than were expected, allowing for a very significant reduction in the size of the next prototype. Hemolysis levels were measured in the prototype pump and found not to indicate damage to the blood cells.     

Characterization of a Magnetic Bearing System and Fluid Properties for a Continuous Flow Ventricular Assist Device

This article presents the performance test results of the CFVAD3 continuous flow blood pump in an artificial human circulation system. The CFVAD3 utilizes magnetic bearings that support a thin pancake impeller, the shape of which allows for a very compact pump whose total axial length is less than 5 cm with a radial length of about 10 cm. This gives a total volume of about 275 cc. The impeller itself has 4 vanes with a designed operating point of 6 L/min at 100 mm Hg of differential pressure and 2,000 rpm. The advantages of magnetic bearings, such as large clearance spaces and no mechanical wear, are elaborated upon. Furthermore, bearing model parameters such as load capacity and current gains are described. These parameters in conjunction with the operating conditions during testing are then used to estimate the fluid forces, stiffness, and damping properties while pumping. Knowledge of these parameters is desirable because of their effects on pump behavior. In addition, a better plant model will allow more robust control algorithms to be devised that can boost pump performance and reliability.     

Test Controller Design, Implementation, and Performance for a Magnetic Suspension Continuous Flow Ventricular Assist Device

A new continuous flow ventricular assist device using full magnetic suspension has been designed, constructed, and tested. The magnetic suspension centers the centrifugal pump impeller within the clearance passages in the pump, thus avoiding any form of contact. The noncontact operation is designed to give very high expected mechanical reliability, large clearances, low hemolysis, and a relatively small size compared to current pulsatile devices. A unique configuration of magnetic actuators on the inlet side and exit sides of the impeller provides full 5 axis control and suspension of the impeller. The bearing system is divided into segments which allow for 3 displacement axes and 2 angular control axes. The controller chosen for the first suspension tests consists of a decentralized set of 5 proportional integral derivative (PID) controllers. This document describes both the controller and an overview of some results pertaining to the magnetic bearing performance. The pump has been successfully operated in both water and blood under design conditions suitable for use as a ventricular assist device.     

Motor Feedback Physiological Control for a Continuous Flow Ventricular Assist Device

The response of a continuous flow magnetic bearing supported ventricular assist device, the CFVAD3 (CF3) to human physiologic pressure and flow needs is varied by adjustment of the motor speed. This paper discusses a model of the automatic feedback controller designed to develop the required pump performance. The major human circulatory, mechanical, and electrical systems were evaluated using experimental data from the CF3 and linearized models developed. An open-loop model of the human circulatory system was constructed with a human heart and a VAD included. A feedback loop was then closed to maintain a desired reference differential pressure across the system. A proportional-integral (PI) controller was developed to adjust the motor speed and maintain the system reference differential pressure when changes occur in the natural heart. The effects of natural heart pulsatility on the control system show that the reference blood differential pressure is maintained without requiring CF3 motor pulsatility.     

Using Hybrid Magnetic Bearings to Completely Suspend the Impeller of a Ventricular Assist Device

Abstract Clinically available blood pumps and those under development suffer from poor mechanical reliability and poor biocompatibility related to anatomic fit, hemolysis, and thrombosis. To alleviate these problems concurrently in a long-term device is a substantial challenge. Based on testing the performance of a prototype, and on our judgment of desired characteristics, we have configured an innovative ventricular assist device, the CF-VAD4, for long-term use. The design process and its outcome, the CFVAD4 system configuration, is described. To provide unprecedented reliability and biocompatibility, magnetic bearings completely suspend the rotating pump impeller. The CFVAD4 uses a combination of passive (permanent) and active (electric) magnetic bearings, a mixed flow impeller, and a slotless 3-phase brushless DC motor. These components are shaped, oriented, and integrated to provide a compact, implantable, pancake-shaped unit for placement in the left upper abdominal quadrant of adult humans.     

Continuous Monitoring of Cardiac Rhythms in Left Ventricular Assist Device Patients

Monitoring of cardiac rhythms is of major importance in the treatment of heart failure patients with left ventricular assist devices (LVADs) implanted. A continuous surveillance of these rhythms could improve out‐of‐hospital care in these patients. The aim of this study was to investigate cardiac rhythms using available pump data only. Datasets (n = 141) obtained in the normal ward, in the intensive care unit, and during bicycle ergometry were analyzed in 11 recipients of a continuous flow LVAD (59.1 ± 9.7 years; male 82%). Tachograms and arrhythmic patterns derived from the pump flow waveform, and a simultaneously recorded ECG were compared, as well as heart rate variability parameters such as: the average heart beat duration (RR interval), the standard deviation of the beat duration (SDNN), the root‐mean‐square of the difference of successive beat durations (RMSSD), and the number of pairs of adjacent beat duration differing by >50 ms divided by the number of all beats (pNN50). A very good agreement of cardiac rhythm parameters from the pump flow compared with ECG was found. Tachycardia, atrial fibrillation, and extrasystoles could be accurately identified from the tachograms derived from the pump flow. Also, Bland–Altman analysis comparing pump flow with ECG indicated a very small difference in average RR interval of 0.3 ± 1.0 ms, in SSDN of 0.5 ± 2.7 ms, in RMSSD of 1.0 ± 5.6 ms, and in pNN50 of 0.3 ± 1.0%. Continuous monitoring of cardiac rhythms from available pump data is possible. It has the potential to reduce the out‐of‐hospital diagnostic burden and to permit a more efficient adjustment of the level of mechanical support.     

Axial Flow Ventricular Assist Device: System Performance Considerations

Abstract: A cooperative effort between Baylor College of Medicine and NASA/Johnson Space Center is under way to develop an implantable left ventricular assist device for either pulmonary or systemic circulatory support for more than 3 months' duration. Using methodical evaluation and testing, an implantable axial pump has been systematically improved. These improvements include the addition of an inducer as a pumping element in front of the impeller and the construction of an efficient brushless direct current motor. To date, less than 10 W of power is required to generate 5 L/min flow against 100 mm Hg. An index of hemolysis of 0.021 g/100 L has been achieved. Two-day in vivo feasibility studies in calves are under way to evaluate the antithrombogenic nature of the pump. Further improvements in system efficiency, hemolytic performance, and the antithrombogenic nature of the pump are expected with the use of empirical studies, computer flow modeling, and in vivo testing in calves.     

Estimation of the Minimum Pump Speed to Prevent Regurgitation in the Continuous Flow Left Ventricular Assist Device: Left Ventricular Drainage versus Left Atrial Drainage

Abstract: Due to the fact that centrifugal and axial pumps do not require valves, there is a possibility of back flow when the pump speed is low. To estimate the minimum required pump speed to prevent this regurgitation, an in vitro simulation test was conducted. A pulsatile pump simulated the natural heart while a centrifugal pump simulated the continuous flow left ventricular assist device (LVAD). The LVAD flow was attained from the left atrial (LA) drainage or left ventricular (LV) drainage. The minimum or regurgitate flow was observed in the systolic phase with LA drainage and in the diastolic phase with LV drainage. LV drainage always provided higher flow than LA drainage at the same pump speed. These differences are due to the various total pressure heads of the LVAD. To prevent the regurgitation, the LVAD should maintain a certain pump speed which can create positive flow against the aortic systolic pressure with LA drainage and against the aortic diastolic pressure with LV drainage. These required pump speeds can be identified by the LVAD flow-pressure curve.     

Improved Flow Straighteners Reduce Thrombus in the NASA/DeBakey Axial Flow Ventricular Assist Device

Abstract: A small axial flow ventricular assist device (VAD) measuring 3 inches long and 1 inch in diameter is in development. The pump consists of a spinning inducer/ impeller, a flow straightener (FLS), and a diffuser enclosed in a cylindrical flow tube. The impeller has rod-shaped permanent magnets embedded within its 6 blades and is activated magnetically by the motor stator which is positioned outside the flow tube. At the completion of a previous study, the FLS was identified as a thrombogenic area. The aim of the present study was to evaluate the thrombogenicity of redesigned FLSs (swept-back and bulbous types), compared with standard type (STD) FLS. A total of 15 pumps (STD, n = 7; swept-back, n = 4; and bulbous, n = 4) were sequentially implanted into 4 calves paracorporeally in a short-term ex vivo test. The STD and bulbous FLSs experienced thrombus formation, but the swept-back FLS was thrombus free during a 48 h screening test.     

Thrombolysis for Suspected Intrapump Thrombosis in Patients With Continuous Flow Centrifugal Left Ventricular Assist Device

The current recommended anticoagulation regimen during continuous flow centrifugal left ventricular device support is a combination of antiplatelet therapy as well as oral anticoagulation. Despite this, pump thrombosis occurs in rare situations. We report the risk factors and nonsurgical management and outcomes of five patients implanted with continuous flow centrifugal left ventricular assist devices who displayed clinical, hemodynamic, and laboratory features of intrapump thrombosis. This information may support the use of intravenous thrombolytics for suspected pump thrombus in these newer generation devices.     

Design, Manufacturing, and Testing of a Paracorporeal Pulsatile Ventricular Assist Device: S˜o Paulo Heart Institute VAD

Abstract: This paper describes the design of a ventricular assist device (VAD), its manufacturing, and testing. The VAD presented is pulsatile, with a free-floating membrane, smooth internal surfaces, and pericardial valves. It comprehends also a pneumatic driving unit capable of operating in the "full to empty," EKG synchronized or asynchronous modes. In vitro tests were performed to assess its mechanical durability, hydrodynamic performance, and hemolysis. To optimize cannulas and implant techniques, we performed in vivo tests in 22 sheep and 8 calves. In these tests, we also evaluated hemolysis and the device's capacity to normalize hemodynamic parameters during induced cardiac failure. The VAD worked for 4,000 h without failure in a mock circulatory loop. In full to empty mode, it displayed a rate-mediated "Starling-like" performance. Optimum output was achieved with a systole duration of 40% of the cycle. The in vitro hemolysis index (IH) was 6.7 ±2.1. Hemolysis in animal experiments was clinically nonsignificant. In calves with induced cardiac failure, the VAD was able to normalize hemodynamic parameters within 120 min.     

Ex Vivo Phase 1 Evaluation of the DeBakey/NASA Axial Flow Ventricular Assist Device

Abstract: A small ventricular assist device intended for long-term implantation has been developed by a cooperative effort between the Baylor College of Medicine and the NASA/Johnson Space Center. To date, in vitro tests have been performed to address hemolysis and pump performance issues. In this Phase 1 study, we assessed the durability and atraumatic features aiming for 2 day implantation. Eight pumps were implanted in 2 calves as paracorporeal left ventricular assist devices. The pump running times ranged from 18 to 203 h (78.1 ± 23.7; mean ± SEM). All the pump implantations were terminated because of thrombus formation. Plasma-free hemoglobin levels were below 13.7 mg/dl, except for 1 case complicated by inflow cannula obstruction. The pump speed was maintained between 10,100 and 11,400 rpm. Pump outputs were from 3.6 to 5.2 L/min. The electrical power required by the system ranged between 9 and 12 W. Clinically there was no detectable organ dysfunction noted, and postmortem evaluation demonstrated no pump related adverse effects in either calf except for small kidney infarctions. Thrombus deposition was observed mainly at the hub portions and the flow straightener.     

Chronic Survival of Calves Implanted with the DeBakey Ventricular Assist Device

The DeBakey ventricular assist device (VAD) is a miniaturized, electromagnetically driven axial flow pump capable of generating in excess of 10 L/min output. The VAD was evaluated in 19 calves during experiments designed to test iterative modifications in the system and to determine the safety of the DeBakey VAD for intermediate to long-term implant. Five of the animals died or were euthanized during the perioperative period (i.e., Days 1-5) due to complications associated with bleeding (n = 3), sudden cardiac arrest (n = 1), or pump occlusion due to a muscle remnant associated with coring (n = 1). The remaining 14 animals survived from 7-145 days. Ten of the 14 animals survived 30 or more days, and 2 animals survived 93 and 145 days before elective euthanasia. Pump function was evaluated in the 14 calves that survived beyond the perioperative period. Pump output at implantation averaged 3 L/min while output at 100 days (n = 2) averaged 4.22 L/min. The electrical current did not change across time during the study, indicating normal operation of the bearings. Pumps consumed less than 10.5 W of power for all support durations. Hemolysis did not occur; the average daily plasma free hemoglobin varied from 2.0 to 8.0 mg/dl. Evaluation of serum biochemical data showed that implantation of the DeBakey VAD in calves with normal hearts did not impair end organ function; BUN, creatinine, and total bilirubin varied minimally within the normal range. The white blood cell count of implanted animals remained within the normal range throughout the study.     

Left Ventricle Afterload Impedance Control by an Axial Flow Ventricular Assist Device: A Potential Tool for Ventricular Recovery

Ventricular assist devices (VADs) are increasingly used for supporting blood circulation in heart failure patients. To protect or even to restore the myocardial function, a defined loading of the ventricle for training would be important. Therefore, a VAD control strategy was developed that provides an explicitly definable loading condition for the failing ventricle. A mathematical model of the cardiovascular system with an axial flow VAD was used to test the control strategy in the presence of a failing left ventricle, slight physical activity, and a recovering scenario. Furthermore, the proposed control strategy was compared to a conventional constant speed mode during hemodynamic changes (reduced venous return and arterial vasoconstriction). The physiological benefit of the control strategy was manifested by a large increase in the ventricular Frank–Starling reserve and by restoration of normal hemodynamics (5.1 L/min cardiac output at a left atrial pressure of 10 mm Hg vs. 4.2 L/min at 21 mm Hg in the unassisted case). The control strategy automatically reduced the pump speed in response to reduced venous return and kept the pump flow independent of the vasoconstriction condition. Most importantly, the ventricular load was kept stable within 1%, compared to a change of 75% for the constant speed. As a key feature, the proposed control strategy provides a defined and adjustable load to the failing ventricle by an automatic regulation of the VAD speed and allows a controlled training of the myocardium. This, in turn, may represent a potential additional tool to increase the number of patients showing recovery.     

Recent Progress on the Vibrating Flow Pump as a Totally Implantable Ventricular Assist Device

This study describes the present state of progress in the development of the vibrating flow pump (VFP) ventricular assist system. We have proceeded with development aiming at a totally implantable ventricular assist system with smaller size and lighter weight appropriate for Asians like the Japanese by increasing the drive frequency. An actuator is important for the development of the miniature sized and lightweight artificial heart. We applied a linear motor for the mechanical part at first. The step motor was applied after that. This form may be best if we want the lightweight small sized motor for an actuator. The cross slider form is applied at present. It succeeded in the miniaturization compared with the linear motor. In the VFP-type ventricular assist system, the blood contact parts are a central vibration tube with inflow and outflow chambers. We designed round diaphragms to prevent thrombus formation. In addition, we developed an energy transmission system for total implantation. The VFP creates a high frequency oscillated blood flow. It has a unique flow pattern. Brain blood flow increased although the total flow of the circulation did not change in the frequency of 25 to 30 Hz. The quantitative evaluation of the autonomic nerve function during the left heart assistance with an oscillated blood flow was carried out by spectral analysis. Some influences on an autonomic nerve were observed by the VFP left heart assistance. We will continue development research with the aim of clinical application.     

Development of an Axial Flow Ventricular Assist Device: In Vitro and In Vivo Evaluation

Abstract: A collaborative effort between Baylor College of Medicine and NASA/Johnson Space Center is underway to develop an axial flow ventricular assist device (VAD). We evaluated inducer/impeller component designs in a series of in vitro hemolysis tests. As a result of computational fluid dynamic analysis, a flow inducer was added to the front of the pump impeller. According to the surface pressure distribution, the flow inducer blades were connected to the impeller long blades. This modification eliminated high negative pressure areas at the leading edge of the impeller. Comparative studies were performed between inducer blade sections that flowed smoothly into the impeller blades (continuous blades) and those that formed discrete separate pumping sections (discontinuous blades). The inducer/impeller with continuous blades showed significantly (p < 0.003) lower hemolysis with a normalized index of hemolysis (NIH) of 0.018 ±< 0.007 g/100 L (n = 3), compared with the discontinuous model, which demonstrated an NIH of 0.050 ± 0.007 g/100 L (n = 3). The continuous blade model was evaluated in vivo for 2 days with no problems. One of the pumps evaluated ran for 5 days in vivo although thrombus formation was recognized on the flow straightener and the inducer/impeller. As a result of this study, the pump material was changed from polyether polyurethane to polycarbonate. The fabrication method was also changed to a computer numerically controlled (CNC) milling process with a final vapor polish. These changes resulted in an NIH of 0.0029 ± 0.0009 g/100 L (n = 4). which is a significant (p < .0001) value 6 times less than that of the previous model. This model was used for in vivo studies and achieved 9 days of operation with a sufficient flow between 3.6 and 4.7 L/min against 80 to 100 mm Hg mean arterial pressure. Plasma free hemoglobin levels remained at 2–3 mg/dl with a hematocrit of 20%.     

Design Method of a Foldable Ventricular Assist Device for Minimally Invasive Implantation

To date, ventricular assist devices (VADs) have become accepted as a therapeutic solution for end‐stage heart failure patients when a donor heart is not available. Newer generation VADs allow for a significant reduction in size and an improvement in reliability. However, the invasive implantation still limits this technology to critically ill patients. Recently, expandable/deployable devices have been investigated as a potential solution for minimally invasive insertion. Such a device can be inserted percutaneously via peripheral vessels in a collapsed form and operated in an expanded form at the desired location. A common structure of such foldable pumps comprises a memory alloy skeleton covered by flexible polyurethane material. The material properties allow elastic deformation to achieve the folded position and withstand the hydrodynamic forces during operation; however, determining the optimal geometry for such a structure is a complex challenge. The numerical finite element method (FEM) is widely used and provides accurate structural analysis, but computation time is considerably high during the initial design stage where various geometries need to be examined. This article details a simplified two‐dimensional analytical method to estimate the mechanical stress and deformation of memory alloy skeletons. The method was applied in design examples including two popular types of blade skeletons of a foldable VAD. Furthermore, three force distributions were simulated to evaluate the strength of the structures under different loading conditions experienced during pump operation. The results were verified with FEM simulations. The proposed two‐dimensional method gives a close stress and deformation estimation compared with three‐dimensional FEM simulations. The results confirm the feasibility of such a simplified analytical approach to reveal priorities for structural optimization before time‐consuming FEM simulations, providing an effective tool in the initial structural design stage of foldable minimally invasive VADs.     

Derivation of Indices of Left Ventricular Contractility in the Setting of Continuous‐Flow Left Ventricular Assist Device Support

It is important to accurately monitor residual cardiac function in patients under long‐term continuous‐flow left ventricular assist device (cfLVAD) support. Two new measures of left ventricular (LV) chamber contractility in the cfLVAD‐unloaded ventricle include I_Q, a regression coefficient between maximum flow acceleration and flow pulsatility at different pump speeds; and K, a logarithmic relationship between volumes moved in systole and diastole. We sought to optimize these indices. We also propose RI_Q, a ratio between maximum flow acceleration and flow pulsatility at baseline pump speed, as an alternative to I_Q. Eleven patients (mean age 49 ± 11 years) were studied. The K index was derived at baseline pump speed by defining systolic and diastolic onset as time points at which maximum and minimum volumes move through the pump. I_Q across the full range of pump speeds was markedly different between patients. It was unreliable in three patients with underlying atrial fibrillation (coefficient of determination R² range: 0.38–0.74) and also when calculated without pump speed manipulation (R² range: 0.01–0.74). The K index was within physiological ranges, but poorly correlated to both I_Q (P = 0.42) and RI_Q (P = 0.92). In four patients there was excellent correspondence between RI_Q and I_Q, while four other patients showed a poor relationship between these indices. As RI_Q does not require pump speed changes, it may be a more clinically appropriate measure. Further studies are required to determine the validity of these indices.     

In Vivo Biocompatibility Evaluation of a New Resilient,Hard‐Carbon,Thin‐Film Coating for Ventricular Assist Devices

The purpose of this study was to evaluate in vivo the biocompatibility of BioMedFlex (BMF), a new resilient, hard‐carbon, thin‐film coating, as a blood journal bearing material in Cleveland Heart's (Charlotte, NC, USA) continuous‐flow right and left ventricular assist devices (RVADs and LVADs). BMF was applied to RVAD rotating assemblies or both rotating and stator assemblies in three chronic bovine studies. In one case, an LVAD with a BMF‐coated stator was also implanted. Cases 1 and 3 were electively terminated at 18 and 29 days, respectively, with average measured pump flows of 4.9 L/min (RVAD) in Case 1 and 5.7 L/min (RVAD) plus 5.7 L/min (LVAD) in Case 3. Case 2 was terminated prematurely after 9 days because of sepsis. The sepsis, combined with running the pump at minimum speed (2000 rpm), presented a worst‐case biocompatibility challenge. Postexplant evaluation of the blood‐contacting journal bearing surfaces showed no biologic deposition in any of the four pumps. Thrombus inside the RVAD inlet cannula in Case 3 is believed to be the origin of a nonadherent thrombus wrapped around one of the primary impeller blades. In conclusion, we demonstrated that BMF coatings can provide good biocompatibility in the journal bearing for ventricular assist devices.     

Totally Implantable Ventricular Assist System Using a Vibrating Flow Pump

Abstract: A totally implantable ventricular assist system (VAS), including a drive system and a percutaneous electric energy transmission system, was developed and evaluated in acute animal experiments using adult goats. This newly designed VAS mainly consists of a vibrating tube, coils, magnets, and a jelly-fish valve as the outlet valve. For energy transmission, a new implantable transmitter with a plain weave structure was proposed as a noncon-tacting transform by using the spinal amorphous magnetic fibers. The fluid mechanical and hemodynamic properties and the efficiency of the energy transmission system were evaluated in acute animal experiments using healthy adult goats. This vibrating electromagnetic artificial heart (AH) could generate more than 10 L/min as output volume, with 10 Hz vibration using 20 volts as supplied voltage. The total efficiency of the percutaneous energy transmission system was 76%, and temperature increases were within the acceptable range, suggesting the usefulness of our newly developed implantable VAS.