Histologic features of thrombosis events with a centrifugal left ventricular assist device.

BACKGROUND: Histology of thrombosis events in left ventricular assist devices (LVADs) may point to differences between the etiology of either ingested or de novo thrombus formation within LVADs. Materials ingested by the pump would have features suggestive of lifting and folding, whereas thrombi formed de novo would have uniform, parallel layers. This study tested this hypothesis in a cohort of explanted HeartWare Ventricular assist devices (HVADs) (Medtronic, Miami Lakes, Florida). METHODS: Histology of thrombi from 59 explanted HVAD pumps were classified as presumed ingested, presumed de novo, or undeterminable on the basis of pre-defined criteria. The apparent size and location of the thrombotic materials were noted. RESULTS: Histologically, all thrombotic materials were either presumed to be ingested (73%; 95 of 130 total histology cassettes examined) or of undeterminable origin (27%; 35 of 130 histology cassettes). Undetermined origin commonly was due to a lack of sufficient material for analysis. The larger materials (>800 mm3) tended to be in the inflow region. The most common finding was smaller thrombotic materials (<150 mm3) within the pump (64%; 38 of 59 HVADs); when these smaller materials were ingested by the pump, they were most often found within the smaller flow pathways within the pump. CONCLUSIONS: Our study suggests that the thrombi within HVAD pumps are commonly ingested materials rather than de novo thrombus formation within the pump. Further research to understand the source of this ingested material and the consideration to mitigate this complication should be considered.

Accuracy of the HVAD Pump Flow Estimation Algorithm.

Controller algorithms are an important feature for assessment of ventricular assist device performance. Flow estimation is one algorithm implemented in the HeartWare continuous-flow ventricular assist device pump system. This parameter estimates flow passing through the pump and is calculated using speed, current, and hematocrit. In vitro and in vivo studies were conducted to assess the algorithm accuracy. During in vitro testing, three pumps were tested in four water-glycerol solutions at 37°C with viscosities equivalent to hematocrits of 20, 30, 40, and 50%. By using a linear regression model, a correlation coefficient of >0.94 was observed between measured and estimated flow for all conditions. In vivo studies (n = 9) were conducted in an ovine model where a reference flow probe was placed on the outflow graft and speed was adjusted from 1,800 to 4,000 revolutions per minute. During in vivo experiments, estimated pump flow (mean, minimum, and maximum) was compared with measured pump flow. The best-fit linear regression equation for the data is y = 0.96x + 0.54, r = 0.92. In addition, waveform fidelity was high (r > 0.96) in normal (i.e., nonsuction) cases where flow pulsatility was >2 L/min. The flow estimation algorithm demonstrated strong agreement with measured flow, both when analyzing average waveform magnitude and fidelity.

Design Concepts and Preclinical Results of a Miniaturized HeartWare Platform: The MVAD System.

OBJECTIVE: Ventricular assist device (VAD) miniaturization is one design trend that may result in less-invasive implantation techniques and more versatility with patient selection. The MVAD System is a miniature, continuous-flow device implanted in the ventricle. The pump is capable of delivering between 0 and 7 L/min of flow at a mean arterial pressure of 75 mm Hg. The impeller was optimized from its original design to improve hydraulic performance, minimize shear regions, and enhance the impeller's radial stiffness. These studies evaluated the MVAD System with modified impeller in the preclinical setting. METHODS: This modified pump design was tested through chronic studies (n = 6) in a healthy ovine model where 4 animals were implanted for a duration of 30 ± 5 days and 2 animals were implanted for a duration of 90 ± 5 days. The pump was placed in the left ventricular apex with the outflow graft anastomosed to the descending aorta. Postoperatively, no anticoagulant or antiplatelet therapies were administered throughout the study duration. RESULTS: All 6 animals reached their elective date of kill, demonstrating no evidence of organ compromise or device-related complications. Average pump parameters did not deviate significantly, and average rotational speed, pump flow, and power consumption were 14095 ± 139 RPM, 4.1 ± 0.4 L/min, and 4.3 ± 0.1 W, respectively. Examination of pump components postexplant demonstrated no mechanical wear or thrombus formation. CONCLUSIONS: Hemocompatibility and biocompatibility of the modified MVAD System were demonstrated through pump parameters, blood chemistry panels, and histopathology analysis.

Six-year in-vitro reliability results of the HeartWare HVAD pump.

As a result of stagnant heart transplantation rates, ventricular assist devices (VADs) have become a widely accepted therapy for the treatment of advanced-stage heart failure. Long-term reliability of VADs will become increasingly vital as the population of destination therapy patients expands. In this study, eight HVAD pumps (n = 8) completed a 6-year reliability test in the HeartWare Life Cycle Testing System, an in-vitro mock circulatory loop that simulated physiologic pressures and flows. Cumulative runtime for the pumps was 2,408 ± 60 days. During this time, no device failures of any type occurred. These results strongly support the durability of the pump design.

Early feasibility testing and engineering development of the transapical approach for the HeartWare MVAD ventricular assist system.

Implantation of ventricular assist devices (VADs) for the treatment of end-stage heart failure (HF) falls decidedly short of clinical demand, which exceeds 100,000 HF patients per year. Ventricular assist device implantation often requires major surgical intervention with associated risk of adverse events and long recovery periods. To address these limitations, HeartWare, Inc. has developed a platform of miniature ventricular devices with progressively reduced surgical invasiveness and innovative patient peripherals. One surgical implant concept is a transapical version of the miniaturized left ventricular assist device (MVAD). The HeartWare MVAD Pump is a small, continuous-flow, full-support device that has a displacement volume of 22 ml. A new cannula configuration has been developed for transapical implantation, where the outflow cannula is positioned across the aortic valve. The two primary objectives for this feasibility study were to evaluate anatomic fit and surgical approach and efficacy of the transapical MVAD configuration. Anatomic fit and surgical approach were demonstrated using human cadavers (n = 4). Efficacy was demonstrated in acute (n = 2) and chronic (n = 1) bovine model experiments and assessed by improvements in hemodynamics, biocompatibility, flow dynamics, and histopathology. Potential advantages of the MVAD Pump include flow support in the same direction as the native ventricle, elimination of cardiopulmonary bypass, and minimally invasive implantation.

HeartWare controller logs a diagnostic tool and clinical management aid for the HVAD pump.

Continuous-flow ventricular assist devices (VADs) are a viable therapy for the treatment of end-stage heart failure, offering support for bridge-to-transplantation and destination therapy. As support duration for VADs continues to rise, patient management and device maintenance will play an increasingly crucial role. The HeartWare Ventricular Assist System has currently been implanted in >4,000 patients worldwide. The HeartWare controller stores approximately 30 days of VAD data including pump rotational speed, power consumption, and estimated VAD flow. Routine assessment of controller log files can serve as a pump performance tool and clinical management aid, assisting the clinician to make accurate and timely diagnoses. Here, we discuss the controller's data collection system as well as present the process for evaluation and reporting of controller log files to clinicians.

In vivo evaluation of the HeartWare MVAD Pump.

OBJECTIVE: The current design trend for left ventricular assist devices (LVADs) is miniaturization, which aims to increase the treatable patient population and enable new treatment indications by reducing surgical trauma and the complications associated with device implantation. The MVAD Pump (HeartWare Inc, Framingham, MA) is a small, axial VAD that uses magnetic and hydrodynamic impeller technology and incorporates wide helical flow channels to minimize shear stress. In this study, we implanted the MVAD Pump in an ovine model to evaluate device hemocomaptiblity, biocompatibility, performance, and safety. METHODS: The MVAD Pump was implanted in an ovine model (n = 9) for 90 days. The pump was implanted through a thoracotomy and secured to the LV apex with a gimbaled sewing ring, which allowed for intraoperative adjustment of the insertion depth and angle of the inflow cannula. Serum analytes and coagulation parameters were analyzed at specific intervals throughout the study period. Pump flow, speed, and power were recorded daily to monitor device performance. Sheep were electively euthanized at 90 days for pathologic and histologic analysis. RESULTS: In this study, results demonstrated the safety, reliability, hemocompatability, and biocompatibility of the MVAD Pump. Nine animals were implanted for 90 ± 5 days. No complications occurred during surgical implantation. Seven of the 9 animals survived until elective sacrifice. Each sheep that survived to the scheduled explant appeared physically normal, with no signs of cardiovascular or other organ compromise. The 2 sheep that were euthanized early showed no evidence of device-related issues. CONCLUSIONS: The MVAD Pump was successfully implanted through a thoracotomy and demonstrated excellent hemodynamic support with no device malfunctions throughout the study period.

The effects of continuous and intermittent reduced speed modes on renal and intestinal perfusion in an ovine model.

The effects of the continuous-flow output on renal and intestinal microcirculation have not been extensively studied. To address this, the Heartware HVAD pump loaded with continuous and intermittent reduced speed (IRS) modes was implanted in four sheep and then operated at low and high speeds to mimic partial and complete unloading of the left ventricle. Then microsphere and positron emission tomography/computed tomography (PET/CT) studies were used to assess renal and intestinal tissue perfusion at various pump speeds and flow modes as compared with baseline (pump off). Arterial and venous oxygen (T02) and carbon dioxide (TCO2) contents were measured to assess changes in intestinal metabolism. Renal and intestinal regional blood flows did not produce any significant changes compared with baseline values in either continuous or IRS modes and speeds. The venous TO2 and TCO2 significantly increased in continuous and IRS modes and speeds compared with baseline. Our data suggested that renal and intestinal tissue perfusions were not adversely affected by continuous and IRS modes either in partial or complete unloading. Intestinal venous hyperoxia and increased TCO2 may be the evidence of intestinal arteriovenous shunting along with increased intestinal tissue metabolism. Longer-term studies are warranted in chronic heart failure models.