Evaluation of in vitro hemolysis and platelet activation of a newly developed maglev LVAD and two clinically used LVADs with human blood

In vitro hemolysis testing remains one of the most important performance measures to judge the hemocompatibility of a left ventricular assist device (LVAD). Clinically relevant operating conditions and appropriate testing blood are essential to infer in vitro data for potential clinical use. This in vitro study was carried out to evaluate and compare the hemolytic performance of a newly developed magnetically levitated (maglev) LVAD (CH‐VAD) with two clinically used LVADs (HVAD and HeartMate II (HMII)) using fresh human blood. A small volume (~300 mL) in vitro circulating flow loop was constructed with a LVAD generated flow of 4.5 L/min at the nominal or reported clinical operating speed for each LVAD. The blood was circulated in the loop for 4 hours with samples drawn at baseline and hourly. Plasma‐free hemoglobin (PFH) concentrations in the hourly blood samples were determined with spectrophotometry. Normalized index of hemolysis (NIH) was calculated to compare the hemolytic performance of the CH‐VAD and the two reference LVADs. Platelet activation was measured with flow cytometry. The experimental test for each device was repeated at least 7 times. The data from this study showed that all the three LVADs generated very low hemolysis (NIH <0.01 g/100 L). The CH‐VAD was found to have a significantly lower NIH value (0.00135 ± 0.00032 g/100 L) compared to the HVAD (0.00525 ± 0.00183 g/100 L) and the HMII (0.00583 ± 0.00182 g/100 L). No statistically significant difference in device‐generated hemolysis was found between the HVAD and the HMII. The level of platelet activation induced by the CH‐VAD is significantly lower than those by the HVAD and the HMII. The data suggest that the shear‐induced hemolysis and platelet activation of the CH‐VAD are acceptable relative to the two LVADs currently in clinical use.     

Investigation of the Axial Gap Clearance in a Hydrodynamic‐Passive Magnetically Levitated Rotary Blood Pump Using X‐Ray Radiography

The HeartWare HVAD is a radial rotary blood pump with a combination of passive magnetic and hydrodynamic bearings to levitate the impeller. The axial gap size between impeller and housing in this bearing and its sensitivity to speed, flow, and pressure difference is difficult to assess. Shear stresses are exceptionally high in this tiny gap making it important for blood damage and related adverse events. Therefore, the aim of this study was to measure the axial gap clearance in the HVAD at different operating conditions employing radiography. To quantify the gap size in the HVAD, the pump was positioned 30 mm in front of the X‐ray source employing a microfocus X‐ray tube with an acceleration voltage up to 300 kV. Beams were detected on a flat panel detector (Perkin Elmer XRD 1611‐CP3). The pump was connected to a tubing circuit with a throttle to adjust flow (0, 5, 10 L/min) and a water glycerol mixture to set the desired viscosity (1, 4, 8 mPas). Rotational speed was varied between 1800 and 3600 rpm. In this study, for clinically relevant conditions at 5 L/min and 2700 rpm, the axial gap was 22 µm. The gap size increased with rotational speeds dependent on the viscosity (2.8, 6.9, and 9.4 µm/1000 rpm for 1, 4, and 8 mPas, respectively), but was independent from the volume flow and the pressure head at constant speeds. In summary, using X‐ray radiographic imaging small gaps in a rotary blood pump during operation can be measured in a nondestructive contact‐free way. The axial hydrodynamic bearing gap in the HVAD pump was determined to be in the range of about three times the diameter of a red blood cell. Its dependence on operating volume flow and generated pressure head across the pump is not pronounced.     

Early Aspirin Nonresponders Identification by Routine Use of Aggregometry Test in Patients With Left Ventricle Assist Devices Reduces the Risk of Pump Thrombosis

Left ventricular assist device (LVAD) management is very challenging since many adverse events can occur in ongoing patients. Inadequate anticoagulation treatment can lead to life-threatening situations like ischemic stroke or pump thrombosis. The main intention of our study was to investigate if early identification of aspirin nonresponders by using aggregometry can improve anticoagulation management, reducing the risk of pump thrombosis.MethodsFrom December 2010 to May 2018, 24 patients were implanted with a HeartMate II (HMII), 6 received a HeartWare HVAD system--full support VAD (HVAD), and 22 received a HeartMate III (HMIII). All patients were maintained with a target INR of 2.0 to 3.0. When the aggregometry test revealed a normal platelet function, 100 mg of aspirin were initiated. Only aspirin nonresponders were early identified by repeating the aggregometry after 7 days of aspirin administration. In acetylsalicylic acid nonresponder patients, 75 mg of clopidogrel was used, and the patients were tested again. Ticlopidine (250 mg) was used when clopidogrel was unsuccessful.ResultsFour patients required modification in antiplatelet therapy. Three patients (5%), 2 HVAD and 1 HMII, suffered from pump thrombosis. One patient died as a consequence of a large intracranial hemorrhagic event following thrombolytic treatment. One patient required a pump exchange; in 1 patient, thrombolytic infusion was conducted successfully.ConclusionReported rates of pump thrombosis at 12 months for patients implanted with commonly used LVADs were 6% to 12% for axial-flow pumps and 8% with centrifugal-flow devices. In our series, the reported 5% overall incidence of pump thrombosis encourages the routine use of an aggregometry test for early identification of aspirin nonresponders.     

Implantation of the HeartWare left ventricular assist device

Left ventricular assist devices (LVADs) are the treatment of choice for advanced heart failure that is refractory to medical therapy for both Bridge to Transplantation and Destination Therapy in appropriately selected patients. The newer continuous flow LVADs are more reliable and durable and have resulted in significant size reduction compared to pulsatile flow LVADs. This "miniaturization" of the LVAD has potential advantages including less surgical trauma for implantation. The HeartWare HVAD is a new continuous flow LVAD, currently in trials, that is designed to be implanted and contained completely within the pericardial space.     

Hemodynamic controller for left ventricular assist device based on pulsatility ratio

Hemodynamic control of left ventricular assist devices (LVADs) is generally a complicated problem due to diverse operating environments and the variability of the patients: both the changes in the circulatory and metabolic parameters as well as disturbances that require adjustment to the operating point. This challenge is especially acute with control of turbodynamic blood pumps. This article presents a pulsatility ratio controller for LVAD that provides a proper perfusion according to the physiological demands of the patient, while avoiding adverse conditions. It utilizes the pulsatility ratio of the flow through the pump and pressure difference across the pump as a control index and adjusts the pump speed according to the reference pulsatility ratio under the different operating conditions. The simulation studies were performed to evaluate the controller in consideration of the sensitivity to afterload and preload, influence of the contractility, and effect of suction sensitivity. The controller successfully adjusts the pump speed according to the reference pulsatility ratio, and supports the natural heart under diverse pump operating conditions. The resulting safe pump operations demonstrate the solid performance of the controller in terms of sensitivity to afterload and preload, influence of the contractility, and effect of suction sensitivity.     

High-speed visualization of ingested,ejected, adherent,and disintegrated thrombus in contemporary ventricular assist devices

Biocompatibility of ventricular assist devices (VADs) has been steadily improving, yet the rate of neurological events remains unacceptably high. Recent speculation for elevated stroke rates centers on ingestion of thrombi originating upstream of the pump, such as in the ventricle or left atrial appendage. These thrombi may be ejected by the VAD or become deposited within the blood flow pathway, presenting serious complications to the patient. This study was performed to visualize and quantify the degree of disruption, adherence, and disintegration of thrombi that are ingested by the three most implanted VADs: the HeartMate II, HeartMate 3, and HVAD. Clot analogs of varying microstructure compositions (red, white) and sizes (0.5, 1, 2 cm³) were synthesized in vitro based on clinical explant data. These were introduced individually into an in vitro flow loop with a transparent replica of the HMII, HM3, and HVAD operated at nominal steady flow (2.3-4.0 L/min). High-speed videography (up to 10 000 fps) revealed the ingestion, disruption, ejection, and adherence of thrombus fragments. Thromboemboli of varying compositions and sizes were observed mechanically attaching to components in all 3 VAD models. In some instances, ingested thrombi physically obstructed portions of the blood flow path; 18% (3 of 17 total) of red thrombi adhered to the inflow straightener of the transparent HMII. In the HVAD model, fewer than 4% of clots were adherent or trapped within the pump, irrespective of microstructure or initial volume. In comparison, 100% (4 of 4 total) of 1-cm³ white (fibrin) clots became lodged within the transparent HM3 while, in contrast, less than 5% of macerated red clots (3 of 63 total) of the same volume were adherent inside the pump. A significant proportion of ingested thrombi were macerated into infinitesimal fragments; 84% and 74% of 2-cm³ red thrombi in the HVAD and HM3 models, respectively, were found to have disintegrated upon ingestion. However, large emboli were also discharged from both centrifugal VADs; these fragments, ranging from 0.01 to 0.29 cm³ regardless of microstructure and original volume, may be capable of occluding an intracranial vessel. Therefore, ingested thrombus may explain, in part, elevated stroke rates in contemporary blood pumps in the absence of adherent pump thrombosis.     

Clinical Outcomes of Patients Treated With Pulmonary Vasodilators Early and in High Dose After Left Ventricular Assist Device Implantation

Right ventricular failure (RVF) is common after left ventricular assist device (LVAD) implantation and a major determinant of adverse outcomes. Optimal perioperative right ventricular (RV) management is not well defined. We evaluated the use of pulmonary vasodilator therapy during LVAD implantation. We performed a retrospective analysis of continuous‐flow LVAD implants and pulmonary vasodilator use at our institution between September 2004 and June 2013. Preoperative RVF risk was assessed using recognized variables. Sixty‐five patients (80% men, 50 ± 14 years) were included: 52% HeartWare ventricular assist device (HVAD), 11% HeartMate II (HMII), 17% VentrAssist, 20% Jarvik. Predicted RVF risk was comparable with contemporary LVAD populations: 8% ventilated, 14% mechanical support, 86% inotropes, 25% BUN >39 mg/dL, 23% bilirubin ≥2 mg/dL, 31% RV : LV (left ventricular) diameter ≥0.75, 27% RA : PCWP (right atrium : pulmonary capillary wedge pressure) >0.63, 36% RV stroke work index <6 gm‐m/m²/beat. The majority (91%) received pulmonary vasodilators early and in high dose: 72% nitric oxide, 77% sildenafil (max 200 ± 79 mg/day), 66% iloprost (max 126 ± 37 μg/day). Median hospital stay was 26 (21) days. No patient required RV mechanical support. Of six (9%) patients meeting RVF criteria based on prolonged need for inotropes, four were transplanted, one is alive with an LVAD at 3 years, and one died on day 35 of intracranial hemorrhage. Two‐year survival was 77% (92% for HMII/HVAD): transplanted 54%, alive with LVAD 21%, recovery/explanted 2%. A low incidence of RVF and excellent outcomes were observed for patients treated early during LVAD implantation with combination, high‐dose pulmonary vasodilators. The results warrant further investigation in a randomized controlled study.     

Use of Left Ventricular Assist Device (HeartMate II): A Singapore Experience

Recent advances in medical and device therapies in heart failure have improved the survival of patients with heart failure. However, due to the limited availability of suitable heart donors, left ventricular assist devices (LVADs) have become an important tool as a bridge‐to‐heart transplantation for patients with refractory heart failure in Singapore. We report our experience with the HeartMate II (HMII) LVAD (Thoratec Corporation, Pleasanton, CA, USA) as a bridge‐to‐heart transplant in our center from 2009 to 2012. This was a retrospective review of 23 consecutive patients who underwent HMII LVAD implantation in our center between May 2009 and December 2012. All patients were classified as Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) levels 1 to 3 and underwent LVAD implantation as a bridge‐to‐heart transplant. There were 17 male and 6 female patients. The mean age was 43.6 years old (range 14 to 64). The etiologies of heart failure included ischemic heart disease [8], idiopathic dilated cardiomyopathy [11], viral myocarditis [2], and chemotherapy‐induced cardiomyopathy [2]. Nine patients were INTERMACS level 1, 12 patients level 2, and two patients level 3. All patients successfully underwent HMII LVAD implantation. There was no mortality within the first 30 postoperative days. Postoperative complications included stroke with full neurological recovery (21.7%), mediastinal infection (21.7%), cardiac tamponade or mediastinal collection requiring reopening of the chest (39.1%), cardiac arrhythmia (13.0%), and pump thrombosis with pump replacement (4.3%). All patients were discharged from hospital after LVAD implantation. Three patients experienced driveline infections during outpatient follow‐up. There were 19 readmissions due to the following conditions: sub‐therapeutic anticoagulation (13.0%), gastrointestinal bleeding (13.0%), suspected pump thrombosis (13.0%), transient ischemic attack (8.7%), arrhythmia (8.7%), congestive cardiac failure due to severe aortic regurgitation (8.7%), right ventricular failure (4.3%), rhabdomyolysis (4.3%), and hematuria (4.3%). Post‐LVAD implantation, 20 patients were functionally New York Heart Association (NYHA) class I, while 3 reported NYHA III symptoms. Three patients were successfully bridged to heart transplantation. One patient was successfully explanted 11 months after LVAD implantation. There were two mortalities during the follow‐up period. The average duration of LVAD support was 522 days (range 47 to 1316 days). The HeartMate II LVAD has proven to be effective in our Asian population. Driveline infection rate remains low even in the tropical hot, humid climate in Singapore. With more patients ending up on extended periods of LVAD support, increased emphasis in the detection and management of long‐term complications of ventricular assist devices will be needed.     

Ultrasound-based estimation of remaining cardiac function in LVAD-supported ex vivo hearts

Left ventricular assist devices (LVAD) provide cardiac support to patients with advanced heart failure. Methods that can directly measure remaining LV function following device implantation do not currently exist. Previous studies have shown that a combination of loading (LV pressure) and deformation (strain) measurements enables quantitation of myocardial work. We investigated the use of ultrasound (US) strain imaging and pressure–strain loop analysis in LVAD-supported hearts under different hemodynamic and pump unloading conditions, with the aim of determining LV function with and without LVAD support. Ex vivo porcine hearts (n = 4) were implanted with LVADs and attached to a mock circulatory loop. Measurements were performed at hemodynamically defined “heart conditions” as the hearts deteriorated from baseline. Hemodynamic (including LV pressure) and radio-frequency US data were acquired during a pump-ramp protocol at speeds from 0 (with no pump outflow) to 10 000 revolutions per minute (rpm). Regional circumferential (ε_circ) and radial (ε_rad) strains were estimated over each heart cycle. Regional ventricular dyssynchrony was quantitated through time-to-peak strain. Mean change in LV pulse pressure and ε_circ between 0 and 10 krpm were −21.8 mm Hg and −7.24% in the first condition; in the final condition −46.8 mm Hg and −19.2%, respectively. ε_rad was not indicative of changes in pump speed or heart condition. Pressure–strain loops showed a degradation in the LV function and an increased influence of LV unloading: loop area reduced by 90% between 0 krpm in the first heart condition and 10 krpm in the last condition. High pump speeds and degraded condition led to increased dyssynchrony between the septal and lateral LV walls. Functional measurement of the LV while undergoing LVAD support is possible by using US strain imaging and pressure–strain loops. This can provide important information about remaining pump function. Use of novel LV pressure estimation or measurement techniques would be required for any future use in LVAD patients.     

In Vitro Hydrodynamic Analysis of Pin and Cone Bearing Designs of the Jarvik 2000 Adult Ventricular Assist Device

The Jarvik 2000 adult ventricular assist device (VAD) is a second‐generation blood pump with mechanical contact bearings. The original configuration of the pump employed a pin bearing and a more recent configuration uses a cone bearing. We compare the hydrodynamic performance of the two designs under steady‐state and pulsatile flow conditions in vitro. Furthermore, we employ the Intermittent Low Speed (ILS) Flowmaker Controller to demonstrate the effect on pulsatility index (PI) performance of both device configurations. We use an open‐loop flow system in both steady‐state and pulsatile arrangements, complete with pressure transducers and flow probes. Working fluid was a 3.6 cP blood‐analog, glycerin‐water solution. Steady‐state flow tests were carried out to determine pressure‐flow (H‐Q) performance curves. Pulsatile tests under normotensive, hypertensive, and hypotensive conditions were executed with controller speed 3 (10 710 ± 250 rpm) at 100 beats per minute. Steady‐state tests show greater capacity for pressure and flow with the cone bearing, compared with pin bearing, with best efficiency point (BEP) 68% greater for cone bearing. Pulsatile tests show the cone bearing design to yield a 20% increase in Q_avg, a 17% decrease in pulsatility index (PI_Q), and a qualitative increase in pressure responsivity. The ILS mode (for both bearing designs) decreases Q_avg by 68% and likewise increases PI_Q by 360% and pulsatility ratio (R_pul) by 200%. The ILS controller regularly reduces the flow, increasing pulsatility index during device operation. The Jarvik 2000 continuous‐flow VAD can sustain pulsatile flow under pulsating pressure conditions. The new cone bearing design yields increased flow rates over the earlier pin bearing design.     

Multi‐Objective Genetic Algorithm Assisted by an Artificial Neural Network Metamodel for Shape Optimization of a Centrifugal Blood Pump

A centrifugal blood pump is a common type of the pump used as a left ventricular assist device (LVAD) in the medical industries. The reduction of the LVADs hemolysis level to reduce the blood damage is one of the major concerns in designing of such devices. Also, the enhancement of the LVADs efficiency to decrease the battery size is another design requirement. The blood damage critically depends on the state of the blood being pumped. Besides the blood state, the blood damage also depends on the pump impeller and volute geometries. In this research, a multi‐objective optimization of a centrifugal blood pump is performed. A complete 3D‐optimization platform is established for both impeller and volute of a centrifugal blood pump consisting of parametric modeling, automatic mesh generation, computational fluid dynamics (CFD) simulation, and optimization strategy. A vast number of cases with various impeller and volute shapes are numerically simulated. Three different metamodels are created using artificial neural networks (ANNs) in order to approximate the pump hydraulic efficiency, hemolysis index (HI), and pressure head. The inverse of the relative pressure head is defined as the first objective and the summation of relative hemolysis index and the inverse of the relative efficiency is assumed as the second objective. Non‐dominated Sorting Genetic Algorithm‐II (NSGA‐II) is used to find the Pareto Front. A set of optimal points is selected. Finally, for the physiological flow conditions, the optimum design that provides 11.9% HI reduction and 7.2% efficiency enhancement is selected.     

Preclinical Models for Translational Investigations of Left Ventricular Assist Device‐Associated von Willebrand Factor Degradation

Evidence suggests a major role for von Willebrand factor (vWF) in left ventricular assist device (LVAD)‐associated bleeding. However, the mechanisms of vWF degradation during LVAD support are not well understood. We developed: (i) a simple and inexpensive vortexer model; and (ii) a translational LVAD mock circulatory loop to perform preclinical investigations of LVAD‐associated vWF degradation. Whole blood was obtained from LVAD patients (n = 8) and normal humans (n = 15). Experimental groups included: (i) blood from continuous‐flow LVAD patients (baseline vs. post‐LVAD, n = 8); (ii) blood from normal humans (baseline vs. 4 h in vitro laboratory vortexer, ～ 2400 rpm, shear stress ～175 dyne/cm², n = 8); and (iii) blood from normal humans (baseline vs. 12 h HeartMate II mock circulatory loop, 10 000 rpm, n = 7). vWF multimers and degradation fragments were characterized with electrophoresis and immunoblotting. Blood from LVAD patients, blood exposed to in vitro supraphysiologic shear stress, and blood circulated through an LVAD mock circulatory loop demonstrated a similar profile of decreased large vWF multimers and increased vWF degradation fragments. A laboratory vortexer and an LVAD mock circulatory loop reproduced the pathologic degradation of vWF that occurs during LVAD support. Both models are appropriate for preclinical studies of LVAD‐associated vWF degradation.     

Differential impact on post-transplant outcomes between pulsatile- and continuous-flow left ventricular assist devices

Ventura PA, Alharethi R, Budge D, Reid BB, Horne BD, Mason NO, Stoker S, Caine WT, Rasmusson B, Doty J, Clayson SE, Kfoury AG. Differential impact on post‐transplant outcomes between pulsatile‐ and continuous‐flow left ventricular assist devices.
Clin Transplant 2011: 25: E390–E395. © 2011 John Wiley & Sons A/S. Abstract: Background: The HeartMate II (HMII) left ventricular assist device (LVAD) has proven reliable and durable and has become the preferred choice for bridge to transplant therapy (BTT) when compared with the pulsatile HeartMate XVE (XVE). In this study, we compared the post‐transplant (PTx) outcomes between XVE and HMII using a large national data registry. Methods: The Organ Procurement and Transplantation Network (OPTN)/United Network for Organ Sharing (UNOS) Thoracic Registry database was queried for all patients implanted with either an XVE or an HMII as BTT during 2004–2009. Statistical analysis between XVE and HMII were performed using Kaplan–Meier survival analysis and Cox regression analyses. Results: A total of 673 patients were implanted with the XVE and 484 with HMII. When adjusted for age, gender, ethnicity, intra‐aortic balloon pump, ventilator, inotropes, dialysis, body mass index, creatinine, bilirubin, transfusion, pulmonary capillary wedge, and pulmonary arterial pressures, the HMII had similar one‐ and three‐yr survival (hazard ratio = 0.95, CI = 0.64, 1.42) and rejection‐free survival PTx compared to XVE. The XVE group had more early incidences of allograft rejection (AR) and hospitalization for infection (HI). Conclusions: Compared to XVE, patients with HMII have similar one‐ and three‐yr survival after heart transplantation with less risk of early graft rejection and significant infection. With a strong shift toward use of continuous‐flow LVADs, PTx outcomes are expected to continue to improve.     

Design and initial testing of a mock human circulatory loop for left ventricular assist device performance testing

A mock circulatory loop, which simulates the human circulatory system, is needed to bench test the various versions of continuous flow (CF) left ventricular assist devices (LVADs). This article describes the design and initial testing of such a loop. The loop consists of: (1) pulsatile left and right cardiac simulators; (2) air/water tanks to model the venous and arterial compliances; (3) tygon tubes to model the venous, arterial, and other system flow resistances; and (4) a tuning clamp to model the variation in system resistance characteristics under different cardiac pressure/flow conditions. Several loop measurements were carried out without an LVAD to verify the cardiovascular modeling of a healthy person in sleep, rest, and physical activity, and in different pathological states, and compared to the data found in the literature to validate the loop performance prior to LVAD testing.     

Physiological Control of Dual Rotary Pumps as a Biventricular Assist Device Using a Master/Slave Approach

Dual rotary left ventricular assist devices (LVADs) can provide biventricular mechanical support during heart failure. Coordination of left and right pump speeds is critical not only to avoid ventricular suction and to match cardiac output with demand, but also to ensure balanced systemic and pulmonary circulatory volumes. Physiological control systems for dual LVADs must meet these objectives across a variety of clinical scenarios by automatically adjusting left and right pump speeds to avoid catastrophic physiological consequences. In this study we evaluate a novel master/slave physiological control system for dual LVADs. The master controller is a Starling‐like controller, which sets flow rate as a function of end‐diastolic ventricular pressure (EDP). The slave controller then maintains a linear relationship between right and left EDPs. Both left/right and right/left master/slave combinations were evaluated by subjecting them to four clinical scenarios (rest, postural change, Valsalva maneuver, and exercise) simulated in a mock circulation loop. The controller's performance was compared to constant‐rotational‐speed control and two other dual LVAD control systems: dual constant inlet pressure and dual Frank–Starling control. The results showed that the master/slave physiological control system produced fewer suction events than constant‐speed control (6 vs. 62 over a 7‐min period). Left/right master/slave control had lower risk of pulmonary congestion than the other control systems, as indicated by lower maximum EDPs (15.1 vs. 25.2–28.4 mm Hg). During exercise, master/slave control increased total flow from 5.2 to 10.1 L/min, primarily due to an increase of left and right pump speed. Use of the left pump as the master resulted in fewer suction events and lower EDPs than when the right pump was master. Based on these results, master/slave control using the left pump as the master automatically adjusts pump speed to avoid suction and increases pump flow during exercise without causing pulmonary venous congestion.     

Evaluation of left ventricular assist device performance and hydraulic force in a complete mock circulation loop

Centrifugal pump performance characteristics are vital in determining the ability of a prototype left ventricular assist device (LVAD) to meet the physiological circulation requirements of the cardiovascular system. These characteristics influence the static hydraulic forces encountered by the pump impeller, which determine the required load stiffness of suspension type bearings to minimize impeller touchdown. Performance investigations were conducted on an LVAD design while characterizing the impeller static hydraulic forces of various impeller/volute configurations. The pumps were inserted into a complete systemic and pulmonary mock circulation rig configured to provide suitable nonpulsatile or simulated pulsatile left heart failure environments. The single volute and closed shroud impeller configuration exhibited lowest radial (0.01 N) and axial (3 N) force at nonpulsatile design flow conditions, respectively. Normal hemodynamic conditions of 5.1 L/min at 94 mm Hg were re-established upon inserting the device into the left heart failure environment, where the pump operated along the nonpulsatile characteristic curve for 2200 rpm. The operational limits on this curve were dictated by the required pressure differential across the pump during systolic and diastolic periods. The reduction of left atrial pressure (25 to 8 mm Hg) indicated the alleviation of pulmonary congestion. The ability for the LVAD to support circulation in a left heart failure environment was successfully demonstrated in the mock circulation loop. The impeller hydraulic force characteristics attained will aid the bearing designer to select the best volute and impeller configuration to minimize impeller touchdown in magnetic, hydrodynamic or mechanical type bearing applications.     

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.     

Acute hemodynamic study of Tai-Ta left ventricular assist device in a canine model

A revised Tai-Ta centrifugal impeller pump was designed to study the interaction of the left ventricular assist device (LVAD) with the cardiovascular system in a canine model. Six healthy dogs weighing 12-16 kg were used. Blood flows in the aortic arch, the pulmonary artery (PA), and the LVAD outlet were measured simultaneously with the arterial blood pressure (ABP), the pump outflow pressure (POP), and the electrocardiograph (ECG). Normally, the blood flows in the aorta and the PA started at the S-wave of the ECG. When the LVAD was operated at a higher rotational speed (increased from 2900 to 5400 rpm), the ABP, POP, the pump flow, and the maximum rate of change of PA flow increased. However, the fluctuating amplitudes of ABP, POP, and the pump flow decreased significantly. The cardiovascular hemodynamics change with the pump speeds. For a typical 1.1-1.5 L/min cardiac output in canine, the revised LVAD was able to deliver a flow bypass ratio from 15% up to 100%. The LVAD outflow appeared to be pulsatile and matched the cardiac cycle, showing that the centrifugal impeller pump could be used as a pediatric assist device when cardiac function was impeded.     

A Numerical Method to Enhance the Performance of a Cam‐Type Electric Motor‐Driven Left Ventricular Assist Device

Pulsatile left ventricular assist devices (LVADs) driven by electric motors have been widely accepted as a treatment of heart failure. Performance enhancement with computer assistance for this kind of LVAD has seldom been reported. In this article, a numerical method is proposed to assist the design of a cam‐type pump. The method requires an integrated model of an LVAD system, consisting of a motor, a transmission mechanism, and a cardiovascular circulation. Performance indices, that is, outlet pressure, outlet flow, and pump efficiency, were used to select the best cam profile from six candidates. A prototype pump connected to a mock circulatory loop (MCL) was used to calibrate the friction coefficient of the cam groove and preliminarily evaluate modeling accuracy. In vitro experiments show that the mean outlet pressure and flow can be predicted with high accuracy by the model, and gross geometries of the measurements can also be reproduced. Simulation results demonstrate that as the total peripheral resistance (TPR) is fixed at 1.1 mm Hg.s/mL, the two‐cycle 2/3‐rise profile is the best. Compared with other profiles, the maximum increases of pressure and flow indices are 75 and 76%, respectively, and the maximum efficiency increase is over 51%. For different TPRs (0.5～1.5 mm Hg.s/mL) and operation intervals (0.1～0.4 s) in counterpulsation, the conclusion is also acceptable.     

Long‐Term Continuous Flow Left Ventricular Assist Device Support and End‐Organ Function: Prospects for Destination Therapy

Abstract Pulsatile flow left ventricular assist devices (PF‐LVADs) have successfully supported patients with severe heart failure for bridge‐to‐transplant (BTT) and destination therapy (DT). End‐organ dysfunction is often reversed, optimizing the patient's condition to enhance survival, and quality of life. Questions have been raised regarding the potential for continuous flow LVADs (CF‐LVADs) to provide the same quality of circulatory support. Prior research showing that PF is superior to continuous, non‐PF does not appear to be relevant with CF‐LVADs for BTT and DT. Under most clinical conditions, arterial pulsatility is present during CF‐LVAD support, and this type of support should not be termed “nonpulsatile.” Clinical studies have shown that renal, hepatic, and neurocognitive function is either maintained within a normal range, or is significantly improved, during CF‐LVAD support for durations up to 15 months. Results of the randomized clinical trial between the CF HeartMate II and the pulsatile HeartMate XVE (both by Thoratec Corp, Pleasanton, CA, USA) are pending final US Food and Drug Administration (FDA) review and are not yet published. Studies of microcirculation during CF‐LVAD support indicate that capillary blood flow is adequate to support cellular function. There are anecdotal cases of patients being supported with a CF‐LVAD for over seven years with preserved end‐organ function. Presently, there are no clinical reports indicating that end‐organ function is not well maintained. Current clinical evidence indicates that end‐organ perfusion and function can be well maintained for extended durations of support with a CF‐LVAD (J Card Surg 2010;25:490‐494)