Numerical studies of blood shear and washing in a continuous flow ventricular assist device

The third prototype of a continuous flow ventricular assist device (CF3) is being developed and tested for implantation in humans. The blood in the pump flows through a fully shrouded four bladed impeller (supported by magnetic bearings) and through small clearance regions on either side of the impeller. Computational fluid dynamics (CFD) solutions for this flow have been obtained by using TascFlow, a software package available from AEA Technology, UK. These flow solutions have been used to estimate the shear stresses on the blood in the pump and, hence, to minimize hemolysis. In addition, the solutions are informative for achieving a design that will provide good washing of the blood to minimize the possibility of stagnation points that can lead to thrombosis. This study presents numerical studies of these phenomena in the CF3. The calculated shear rate results are compared with values published in the open literature. The comparisons indicate that hemolysis will not be a problem with CF3, which is in agreement with preliminary experimental measurements. Flow studies are being conducted to determine the optimal size of the clearance regions.     

Design of a continuous flow centrifugal pediatric ventricular assist device

Thousands of pediatric patients suffering from cardiomyopathy or single ventricular physiologies secondary to debilitating heart defects may benefit from long-term mechanical circulatory support due to the limited number of donor hearts available. This article presents the initial design of a fully implantable centrifugal pediatric ventricular assist device (PVAD) for 2 to 12 year olds. Conventional pump design equations, including a nondimensional scaling approach, enabled performance estimations of smaller scale versions (25 mm and 35 mm impeller diameters) of our adult support VAD. Based on this estimated performance, a computational model of the PVAD with a 35 mm impeller diameter was generated. Employing computational fluid dynamics (CFD) software, the flow paths through the PVAD and overall performance were analyzed for steady state flow conditions. The numerical simulations involved flow rates of 2 to 5 LPM for rotational speeds of 2750 to 3250 RPM and incorporated a k-epsilon fluid turbulence model with a logarithmic wall function to characterize near-wall flow conditions. The CFD results indicated best efficiency points ranging from 25% to 28%, which correlate well with typical values of blood pumps. The results further demonstrated that the pump could deliver 2 to 5 LPM at 70 to 95 mmHg for desired physiologic conditions in resting 2 to 12 year olds. Scalar stress levels remained below 300 Pa, thereby signifying potentially low levels of hemolysis. Several flow regions in the pump exhibited signs of vortices, retrograde flow, and stagnation points, which require optimization and further study. This CFD model represents a reasonable starting point for future model enhancements, leading to prototype manufacturing and experimental validation.     

Readmissions after continuous flow left ventricular assist device implantation

Continuous flow left ventricular assist device (CF-LVAD) therapy has improved the survival of patients with advanced heart failure. However, the readmission rate of CF-LVAD patients is still relatively high. A total of 90 patients who received CF-LVADs between April 2011 and March 2016 at our institute and were discharged home were analyzed retrospectively. They were followed up through March 2017. Clinical data, including frequency, length and etiology of readmission, were obtained from medical records. The mean observation period after initial discharge was 713 ± 322 days. In total, 73 patients (81%) had 236 readmissions, 214 unplanned and 22 planned. The overall and unplanned readmission rates were 1.34 and 1.22 per patient-year, respectively. The rate of freedom from unplanned first readmission at 1 year after initial discharge was 39%. The median interval between the previous hospital discharge and first and second readmissions was 311 and 213 days, respectively (log-rank test, p = 0.117). The rate of readmission after more than three readmissions was significantly higher than that of first or second readmission (log-rank test, p < 0.001). The most common etiology of readmission was driveline infection (DLI) (36%), followed by stroke (9%). The median length of hospital stay due to DLI was 23 days. The patients with repeated unplanned readmissions had significantly lower EuroQol 5 dimensions questionnaire utility score than those with no or just one readmission. Readmission was common in CF-LVAD patients, and the most common etiology of readmissions was DLI. The interval to the next readmission seemed shorter for patients with repeated readmissions.     

Numerical solution for blood flow in a centrifugal ventricular assist device

A very small centrifugal pump, fully supported by magnetic bearings, is being developed for use as a ventricular assist device to be implanted in humans. In this paper, we apply computational fluid dynamics to model the blood flow to aid in the design of the ventricular assist device. The flow of blood through the pump has been modeled using computational fluid dynamics (CFD) software that is commercially available from AEA Technology, UK. The flow regions modeled in version 3 of the Continuous Flow Ventricular Assist Device (CF3) are the fully shrouded four bladed impeller and the two clearance regions around the impeller that are bounded by the pump hub and shroud. This paper describes the geometry and computational grids developed for the flow regions, and the equations of motion for the blood flow are developed. The overall numerically-evaluated flow rates and head rise have similar trends to the flow parameters experimentally measured, indicating that future pump designs can be effectively modeled numerically before being constructed and tested. Numerical solutions are presented and compared with experimentally-obtained overall pump performance results. These solutions are used to predict shear stress levels to be experienced by the blood flowing through the pump, and it is predicted that hemolysis will be insignificant. The solutions also indicate no regions of flow stagnation that can be a source of thrombosis in pumps. The calculations provide a viable design method to achieve improved efficiency in future versions of this pump.     

Modeling and simulation of blood flow in a sac-type left ventricular assist device

Left ventricular assist devices (LVADs) are among the most important mechanical artificial hearts in medical equipment industry. Since the need for heart transplantation is on the rise, there is a requirement for implantable LVADs, which can be safely used for long-term purposes. One of the most promising kinds of these devices is the sac-type LVAD (ST-LVAD) that has the ability to generate pulsatile flow. In this study and for the first time, three different models of ST-LVAD are analyzed numerically. In the first model, the motion of the elastic membrane wall is simplified, while in the second model, the motion is assumed to be wavy. The pressure boundary conditions are added to the second model to allocate for the effect of pressure on the flow pattern, and hence, form the third model. The simulation results of the analyzed models show that in this particular type of LVAD, the viscous term of the applied stress from the fluid on the moving wall is negligible, compared with the pressure term. Additionally, it can be concluded that the motion pattern of the moving wall does not affect the blood flow pattern in a great deal. Furthermore, the inclusion of the fluid pressure in the boundary conditions does not have a major influence on the blood flow pattern.     

Micro-pressure sensor for continuous monitoring of a ventricular assist device

We have been involved in the development of a clinical ventricular assist device (VAD) system. Here, we report our investigation of in vitro and in vivo stability and sensibility of pressure microsensors. The sensors were mounted in the in-flow and out-flow cannulae wall to measure the left atrial and aortic pressures during VAD pumping. The pressure sensitive surface of the piezoresistive effect absolute pressure sensor was coated with a thrombo-resistant polymer, as was the inner surface of the cannulae of the VAD. In the in vitro and chronic animal experiments which were of more than a month duration, reliable stability and sensitivity, without any thrombus formation on the blood contacting surface of the sensors, and high sensitivity were observed. After chronic experiments, the sensitivity of sensors was reevaluated in the mock circulatory system as compared to reference values. The relationship between the output of the micro-sensors and the reference value was linear and correlated well.     

Design and transient computational fluid dynamics study of a continuous axial flow ventricular assist device

A ventricular assist device (VAD), which is a miniaturized axial flow pump from the point of view of mechanism, has been designed and studied in this report. It consists of an inducer, an impeller, and a diffuser. The main design objective of this VAD is to produce an axial pump with a streamlined, idealized, and nonobstructing blood flow path. The magnetic bearings are adapted so that the impeller is completely magnetically levitated. The VAD operates under transient conditions because of the spinning movement of the impeller and the pulsatile inlet flow rate. The design method, procedure, and iterations are presented. The VAD's performance under transient conditions is investigated by means of computational fluid dynamics (CFD). Two reference frames, rotational and stationary, are implemented in the CFD simulations. The inlet and outlet surfaces of the impeller, which are connected to the inducer and diffuser respectively, are allowed to rotate and slide during the calculation to simulate the realistic spinning motion of the impeller. The flow head curves are determined, and the variation of pressure distribution during a cardiac cycle (including systole and diastole) is given. The axial oscillation of impeller is also estimated for the magnetic bearing design. The transient CFD simulation, which requires more computer resources and calculation efforts than the steady simulation, provides a range rather than only a point for the VAD's performance. Because of pulsatile flow phenomena and virtual spinning movement of the impeller, the transient simulation, which is realistically correlated with the in vivo implant scenarios of a VAD, is essential to ensure an effective and reliable VAD design.     

Arterial pulsatility improvement in a feedback-controlled continuous flow left ventricular assist device: An ex-vivo experimental study

Continuous flow left ventricular assist devices (CF-LVADs) reduce arterial pulsatility, which may cause long-term complications in the cardiovascular system. The aim of this study is to improve the pulsatility by driving a CF-LVAD at a varying speed, synchronous with the cardiac cycle in an ex-vivo experiment. A Micromed DeBakey pump was used as CF-LVAD. The heart was paced at 140 bpm to obtain a constant cardiac cycle for each heartbeat. First, the CF-LVAD was operated at a constant speed. At varying-speed CF-LVAD assistance, the pump was driven such that the same mean pump output was generated. For synchronization purposes, an algorithm was developed to trigger the CF-LVAD each heartbeat. The pump flow rate was selected as the control variable and a reference model was used for regulating the CF-LVAD speed. Continuous and varying-speed CF-LVAD assistance provided the same mean arterial pressure and flow rate, while the index of pulsatility doubled in both arterial pressure and pump flow rate signals under pulsatile pump speed support. This study shows the possibility of improving the pulsatility in CF-LVAD support by regulating pump speed over a cardiac cycle without compromising the overall level of support.     

Assessment of ocular blood flow in continuous-flow ventricular assist device by laser speckle flowgraphy

Journal of Artificial Organs - Although the influence of continuous-flow left ventricular assist device (CF-LVAD) support on peripheral circulation has been widely discussed, its monitoring...     

一种经皮植入式左心辅助装置的泵流量测试

目的 研究设计一种能用于心血管急危重症的经皮植入式左心辅助装置（血泵）。方法 根据机翼理论,设计一种经皮植入的左心辅助装置,通过测量3种不同参数（叶片旋转角度、血泵出水口距离、血泵出水口长度）的血泵所能产生的流量,最终选择最优化的血泵设计。结果 经过简易流量测定装置测量,当血泵采取单叶设计,血泵叶片的旋转角度为720°时,或血泵出水口与叶片的距离为0 mm时,血泵出水口长度为4 mm时,血泵流量最大。结论 选择能产生最大流量的参数值,研制出一种可在体外正常运转的经皮植入式左心辅助装置,为最终研制一种可用于临床的经皮植入式左心辅助装置提供理论和数据支持。     

心室辅助下切应力对红细胞形态的影响

目的心室辅助是晚期心衰最有效的治疗方法之一。但是,心室辅助装置产生的高流体切应力会对红细胞造成损伤。目前,大多数研究仅利用心室辅助装置造成的溶血值评价其对血液的损伤程度,却忽略了对还未破裂的红细胞受到损伤的关注。本文重点探究流体切应力所造成的红细胞形态的变化。方法首先,利用数值计算软件FLUENT,采用拉格朗日离散模型计算出心室辅助装置在一个心动周期内作用于红细胞的流体切应力曲线;然后,根据数值计算得出的切应力范围,对不同切应力下和不同暴露时间下的血液样本进行实验,并设立无受力对照组减小实验误差;最后,以实验后血样的游离血红蛋白含量作为判断血样溶血情况参数,并以血涂片中非常态红细胞的数量占总体的百分比作为衡量红细胞受损伤但未破裂状态的参数。结果非常态红细胞数目和切应力的相关系数是0.725,P=0.027(0.05)。实验结果显示,血泵产生切应力会导致非常态红细胞出现,并且暴露时间一定,非常态红细胞数目随着切应力增大而增大。结论在心室辅助下,血泵流场除了会产生溶血这一极端现象,同时也会导致非常态红细胞数量的增多,降低血液质量,影响人体血液功能。     

In vitro laser Doppler anemometry of pulsatile flow velocity and shear stress measurements downstream from a jellyfish valve in the mitral position of a ventricular assist device

Thrombus formation and hemolysis have both been linked to the dynamic flow characteristics of heart valve prostheses. To enhance our understanding of the flow characteristics past the mitral position of a jellyfish (JF) valve in the left ventricle under physiological pulsatile flow conditions, in vitro laser Doppler anemometry (LDA) measurements were carried out. The hydrodynamic performance of the JF valve was compared with that of a Bjork-Shiley tilting-disk valve (BS mono). The results indicated that both valves created disturbed flow fields and turbulence shear stress levels in the immediate vicinity and up to 1D (diameter of the valvering) downstream from the valve that were capable of causing lethal damage to blood elements. At a location further downstream, the JF valve showed better hydrodynamic performance than the BS in terms of back flow properties and velocity and turbulence stress characteristics. However, any imperfection in the manufacturing of the valve structure, particularly membrane thickness, adversely affected the performance of the JF valve.     

The effect of aortic valve incompetence on the hemodynamics of a continuous flow ventricular assist device in a mock circulation

There is evidence that the incidence of aortic valve incompetence (AI) and other valvular pathologies may increase as more patients are submitted to longer periods of ventricular assist device (VAD) support. There is a need to better understand the mechanisms associated with the onset of these conditions and other possible complications related to the altered hemodynamics of VAD patients. In this study, the effect of AI on the hemodynamic response of continuous flow VAD (C-VAD) patients was measured in a mock loop over a range of pump speeds and level of native cardiac function. Our results showed that, in the presence of sufficient ventricular function, decreasing the C-VAD speed can allow a transition from series to parallel flow. Our study demonstrated that AI reduces the aortic pressure and flow when system impedance is unchanged. AI produces wasteful recirculation that substantially increases the pump work and decreases systemic perfusion, in particular during series flow conditions coupled with higher C-VAD speeds. The hematologic consequence of this regurgitant flow is a much higher exposure to shear for the blood, increasing the likelihood of hemolysis and thrombosis. While a certain degree of AI can be tolerated by a heart with good cardiac function, the consequences of AI for patients with VADs and poor cardiac function are much greater. Valve dysfunction in VAD patients may be related to structural changes in the tissue induced by altered biomechanics and excessive stress.     

Particle image velocimetry assessment of stent design influence on intra-aneurysmal flow

Endovascular stenting appears to be an appealing treatment modality to selected complex intracranial aneurysms. However, stents currently used for endovascular treatment are not specifically designed for the cerebrovasculature. Stent parameters, such as porosity and filament size, have to be carefully optimized for long-term successful treatment. We investigated the influence of the stent filament size on the intra-aneurysmal flow dynamics in a sidewall aneurysm model in vitro. Three helical stents with 76% porosity but different filament sizes of 178, 153, and 127 m were studied using particle image velocimetry. Twenty-four pulsatile flow conditions were investigated. The results show that stenting significantly reduces intra-aneurysmal vorticity and the mean circulation inside the aneurysm is reduced to less than 3% of its value before stenting. For constant porosity, a further reduction of the mean circulation, up to 30% can be obtained by reducing the filament diameter. For a constant Womersley number, this further reduction is accentuated with increase in the peak Reynolds number. Further reduction in the mean circulation inside the aneurysm was not achieved for the 127 m stent. With further reduction in filament diameter, the helical stent filaments positioned against the aneurysm neck started wavering with the flow transferring added momentum into the aneurysm. For stents of smaller filament diameter, a supporting ultrastructure is required. © 2002 Biomedical Engineering Society. PAC2002: 8719Uv, 8780Rb, 8719La, 0630Gv     

Particle image velocimetry in the investigation of flow past artificial heart valves

Full-field measurement of instantaneous velocities in the flow field of artificial heart valves is vital as the flow is unsteady and turbulent. Particle image velocimetry (PIV) provides us the ability to do this as compared to other point measurement devices where the velocity is measured at a single point in space over time. In the development of a PIV system to investigate the flow field of artificial heart valves, many problems associated with the project arose and were subsequently resolved. Experience gained in the setting up of an environment conducive for PIV studies of artificial heart valves; from seed particle selection to refractive index matching, and the evolution of computer algorithms to satisfy the varied flow conditions in artificial heart valves are presented here. Velocity profiles and distributions are computed and drawn for a porcine tissue heart valve based on measurements with the PIV system developed.     

Precise quantification of pressure flow waveforms of a pulsatile ventricular assist device

Unreliable quantification of flow pulsatility has hampered many efforts to assess the importance of pulsatile perfusion. Generation of pulsatile flow depends upon an energy gradient. It is necessary to quantify pressure flow waveforms in terms of hemodynamic energy levels to make a valid comparison between perfusion modes during chronic support. The objective of this study was to quantify pressure flow waveforms in terms of energy equivalent pressure (EEP) and surplus hemodynamic energy (SHE) levels in an adult mock loop using a pulsatile ventricle assist system (VAD). A 70 cc Pierce-Donachy pneumatic pulsatile VAD was used with a Penn State adult mock loop. The pump flow rate was kept constant at 5 L/min with pump rates of 70 and 80 bpm and mean aortic pressures (MAP) of 80, 90, and 100 mm Hg, respectively. Pump flows were adjusted by varying the systolic pressure, systolic duration, and the diastolic vacuum of the pneumatic drive unit. The aortic pressure was adjusted by varying the systemic resistance of the mock loop EEP (mm Hg) = (integral of fpdf)/(integral of fdt) SHE (ergs/cm3) = 1,332 [((integral of fpdt)/(integral of fdt))--MAP] were calculated at each experimental stage. The difference between the EEP and the MAP is the extra energy generated by this device. This difference is approximately 10% in a normal human heart. The EEP levels were 88.3 +/- 0.9 mm Hg, 98.1 +/- 1.3 mm Hg, and 107.4 +/- 1.0 mm Hg with a pump rate of 70 bpm and an aortic pressure of 80 mm Hg, 90 mm Hg, and 100 mm Hg, respectively. Surplus hemodynamic energy in terms of ergs/cm3 was 11,039 +/- 1,236 ergs/cm3, 10,839 +/- 1,659 ergs/cm3, and 9,857 +/- 1,289 ergs/cm3, respectively. The percentage change from the mean aortic pressure to EEP was 10.4 +/- 1.2%, 9.0 +/- 1.4%, and 7.4 +/- 1.0% at the same experimental stages. Similar results were obtained when the pump rate was changed from 70 bpm to 80 bpm. The EEP and SHE formulas are adequate to quantify different levels of pulsatility for direct and meaningful comparisons. This particular pulsatile VAD system produces near physiologic hemodynamic energy levels at each experimental stage.     

Modeling and control of a brushless DC axial flow ventricular assist device

This article presents an integrated model of the human circulatory system that incorporates circulatory support by a brushless DC axial flow ventricular assist device (VAD), and a feedback VAD controller designed to maintain physiologically sufficient perfusion. The developed integrated model combines a network type model of the circulatory system with a nonlinear dynamic model of the brushless DC pump We show that maintaining a reference differential pressure between the left ventricle and aorta leads to adequate perfusion for different pathologic cases, ranging from normal heart to left heart asystole, and widely varying physical activity scenarios from rest to exercise.     

Effects of continuous flow left ventricular assist device support on skin tissue microcirculation and aortic hemodynamics

Continuous flow ventricular assist devices (CFVADs) are thought to be the next generation of circulatory assist devices. With many now in various stages of development or clinical trial, it is important that the physiologic aspects of these pumps be critically analyzed. In this study, 15 calves were divided into two groups. One group received a CFVAD, and the other a sham implant. Two additional animals were used in an acute study to examine aortic blood flow patterns from a CFVAD. Tissue perfusion was measured on all animals before surgery and then weekly thereafter. Before surgery, there was no difference in hemodynamics or tissue perfusion between studied animals. Postoperatively, CFVAD animals had statistically significant increased diastolic pressure. Significantly decreased pulse pressure, pulse index, and tissue perfusion were also observed in CFVAD animals. Results from the flow pattern studies suggested that at moderate levels of pump support (40-75%), the amount of blood flow distal to the outflow graft anastomosis decreased approximately 25% because of increased regurgitant blood flow in the aorta. These results suggest that the diminished tissue perfusion is likely due to changes in aortic hemodynamics and provide some insight into the distribution of flow from CFVADs.     

Neurocognitive function in patients with ventricular assist devices: a comparison of pulsatile and continuous blood flow devices

The effect of successful ventricular assist device (VAD) implantation on neurocognitive function in terminal heart failure is uncertain. Additionally, the different impact of continuous versus pulsatile blood flow devices is unknown. A total of 29 patients (mean age 53 years), surviving implantation of a ventricular assist device as bridge to transplantation were prospectively followed (continuous flow: Micromed DeBakey, n = 11; pulsatile flow: Thoratec and Novacor, n = 18). Normative data were obtained in 40 age- and sex-matched healthy subjects (mean age 54 years). Neurocognitive function was objectively measured by means of cognitive P300 auditory evoked potentials before operation (baseline), at intensive care unit (ICU) discharge, and at the 8-week and 12-week follow-up. Before implantation of the VAD, cognitive P300 evoked potentials were impaired (prolonged) compared with age- and sex-matched healthy subjects (p < 0.001). After successful VAD implantation, P300 evoked potentials markedly improved compared with before operation (ICU discharge, p = 0.007; 8-week follow-up, p = 0.022; 12-week follow-up, p < 0.0001). Importantly, there was no difference between continuous and pulsatile VADs (before operation, p = 0.676; ICU discharge, p = 0.736; 8-week follow-up, p = 0.911 and 12-week follow-up, p = 0.397; respectively). Nevertheless, P300 peak latencies did not fully normalize at 12-week follow-up compared with healthy subjects (p = 0.012). Successful VAD implantation improves neurocognitive impairment in patients with terminal heart failure. Importantly, this effect is independent of the type of VAD (pulsatile vs. continuous blood flow).