A Compliant Model of the Ventricular Apex to Study Suction in Ventricular Assist Devices.

Ventricular suction is a frequent adverse event in patients with a ventricular assist device (VAD). This study presents a suction module (SM) embedded in a hybrid (hydraulic-computational) cardiovascular simulator suitable for the testing of VADs and related suction events. The SM consists of a compliant latex tube reproducing a simplified ventricular apex. The SM is connected on one side to a hydraulic chamber of the simulator reproducing the left ventricle, and on the other side to a HeartWare HVAD system. The SM is immersed in a hydraulic chamber with a controllable pressure to occlude the compliant tube and activate suction. Two patient profiles were simulated (dilated cardiomyopathy and heart failure with preserved ejection fraction), and the circulating blood volume was reduced stepwise to obtain different preload levels. For each simulated step, the following data were collected: HVAD flow, ventricular pressure and volume, and pressure at the inflow cannula. Data collected for the two profiles and for decreasing preload levels evidenced suction profiles differing in terms of frequency (intermittent vs. every heart beat), amplitude (partial or complete stoppage of the HVAD flow), and shape. Indeed different HVAD flow patterns were observed for the two patient profiles because of the different mechanical properties of the simulated ventricles. Overall, the HVAD flow patterns showed typical indicators of suctions observed in clinics. Results confirmed that the SM can reproduce suction phenomena with VAD under different pathophysiological conditions. As such, the SM can be used in the future to test VADs and control algorithms aimed at preventing suction phenomena.

Influence of left ventricular assist device pressure-flow characteristic on exercise physiology: Assessment with a verified numerical model.

Current left ventricular assist devices are designed to reestablish patient's hemodynamics at rest but they lack the suitability to sustain the heart adequately during physical exercise. Aim of this work is to assess the performance during exercise of a left ventricular assist device with flatter pump pressure-flow characteristic and increased pressure sensitivity (left ventricular assist device 1) and to compare it to the performance of a left ventricular assist device with a steeper characteristic (left ventricular assist device 2). The two left ventricular assist devices were tested at constant rotational speed with a verified computational cardiorespiratory simulator reproducing an average left ventricular assist device patient response to exercise (EXE↑) and a left ventricular assist device patient with no chronotropic and inotropic response (EXE→). According to the results, left ventricular assist device 1 pumps a higher flow than left ventricular assist device 2 both at EXE↑ (6.3 vs 5.6 L/min) and at EXE→ (6.7 vs 6.1 L/min), thus it better unloads the left ventricle. Left ventricular assist device 1 increases the power delivered to the circulation from 0.63 W at rest to 0.67 W at EXE↑ and 0.82 W at EXE→, while left ventricular assist device 2 power shows even a minimal decrease. Left ventricular assist device 1 better sustains exercise hemodynamics and can provide benefits in terms of exercise performance, especially for patients with a poor residual left ventricular function, for whom the heart can hardly accommodate an increase of cardiac output.

The Effect of LVAD Pressure Sensitivity on the Assisted Circulation Under Consideration of a Mitral Insufficiency: An In Vitro Study.

Current left ventricular assist devices (LVADs) differ with respect to their pump characteristics as described by the pump characteristic curve (also called HQ-curve). Pressure sensitive LVADs depict a flat characteristic curve while most available LVADs have a steep, less pressure sensitive characteristic curve. This in vitro study investigated the effect of LVAD pressure sensitivity with a focus on the afterload of the right ventricle (RV) which is one out of many factors influencing right heart failure (RHF). To this end, two laboratory pumps differing in pressure sensitivity were tested as LVAD in an established, active mock circulation loop (MCL). The MCL represented patients with left heart failure and mitral insufficiency as another contributing factor to RV afterload. The results show that the pressure-volume loop (PV-loop) of the left ventricle (LV) undergoes a leftward and thus somewhat of a downward-shift for highly pressure sensitive support. Consequently, the LV is unloaded to a higher degree at comparable arterial blood pressure and identical cardiac output, pulmonary and systemic vascular resistance and ventricular contractility. This causes a concomitant decrease of RV afterload. This effect seems to be due to increased unloading during systole. In case of a severe concomitant mitral insufficiency and looking at left atrial pressure, the difference is 18.5%. Without mitral insufficiency, the difference is reduced to 10.2%.

Multivariate Assessment of the Effect of Pump Design and Pump Gap Design Parameters on Blood Trauma.

Pump gaps are the most critical regions in a rotary blood pump when it comes to blood trauma in the form of hemolysis, protein destruction, and platelet activation. This study investigated six pump design parameters affecting the flow in a radial pump gap. A multivariate approach was employed to determine individual and quantitative parameter effects on blood trauma as well as parameter interactions. To consider the effect of shear stress and blood cell residence time, a validated numerical Lagrangian particle tracking approach was used. Based on the results, small-diameter pumps can be as blood compatible, if not more blood compatible, as large-diameter pumps as long as identical circumferential velocities and clearance gaps are maintained. Furthermore, the results indicate that an eccentric rotor position in the casing is not harmful and that a pressure difference generating washout flow and thereby reducing the cell residence time is of significant importance.

Hydraulic and hemodynamic performance of a minimally invasive intra-arterial right ventricular assist device.

Right ventricular assistance is still in the early phase of development compared to left ventricular assist device (LVAD) technology. In order to provide flexible pulmonary support and potentially reduce the known complications, we propose a minimally invasive right ventricular assist device (MIRVAD), located in the pulmonary artery (PA) and operating in series with the right ventricle (RV). The MIRVAD is an intra-arterial rotary blood pump containing a single axial impeller, which is not enclosed by a rigid housing but stent-fixed within the vessel. The impeller geometry has been designed with the assistance of analytical methods and computational fluid dynamics (CFD). The hydraulic performance of the impeller was evaluated experimentally with a customized test setup using blood synthetic medium (HES). The blade-tip clearance (BTC) was varied between 0.25-4.25 mm to evaluate the effect of different PA sizes on impeller performance. Furthermore, the Langrangian particle-tracking method was used to estimate the level of hemolysis and generate numerical blood damage indexes.The impeller design generated 25.6 mmHg for flow rates of 5 lpm at a speed of 6,000 rpm at the baseline condition, capable of providing sufficient support for the RV. The BTC presented a significant effect on the static pressure generation and the efficiency, but the operational range is suitable for most vessel sizes. The numerical results demonstrated a low risk of blood damage at the design point (mean Lagrangian damage index 2.6*10(-7)). The preliminary results have encouraged further impeller optimization and development of the MIRVAD.

Evaluation of hydraulic radial forces on the impeller by the volute in a centrifugal rotary blood pump.

In many state-of-the-art rotary blood pumps for long-term ventricular assistance, the impeller is suspended within the casing by magnetic or hydrodynamic means. For the design of such suspension systems, profound knowledge of the acting forces on the impeller is crucial. Hydrodynamic bearings running at low clearance gaps can yield increased blood damage and magnetic bearings counteracting high forces consume excessive power. Most current rotary blood pump devices with contactless bearings are centrifugal pumps that incorporate a radial diffuser volute where hydraulic forces on the impeller develop. The yielding radial forces are highly dependent on impeller design, operating point and volute design. There are three basic types of volute design--singular, circular, and double volute. In this study, the hydraulic radial forces on the impeller created by the volute in an investigational centrifugal blood pump are evaluated and discussed with regard to the choice of contactless suspension systems. Each volute type was tested experimentally in a centrifugal pump test setup at various rotational speeds and flow rates. For the pump's design point at 5 L/min and 2500 rpm, the single volute had the lowest radial force (∼0 N), the circular volute yielded the highest force (∼2 N), and the double volute possessed a force of approx. 0.5 N. Results of radial force magnitude and direction were obtained and compared with a previously performed computational fluid dynamics (CFD) study.

Description of a flow optimized oxygenator with integrated pulsatile pump.

Extracorporeal membrane oxygenation (ECMO) is a well-established therapy for several lung and heart diseases in the field of neonatal and pediatric medicine (e.g., acute respiratory distress syndrome, congenital heart failure, cardiomyopathy). Current ECMO systems are typically composed of an oxygenator and a separate nonpulsatile blood pump. An oxygenator with an integrated pulsatile blood pump for small infant ECMO was developed, and this novel concept was tested regarding functionality and gas exchange rate. Pulsating silicone tubes (STs) were driven by air pressure and placed inside the cylindrical fiber bundle of an oxygenator to be used as a pump module. The findings of this study confirm that pumping blood with STs is a viable option for the future. The maximum gas exchange rate for oxygen is 48mL/min/L(blood) at a medium blood flow rate of about 300mL/min. Future design steps were identified to optimize the flow field through the fiber bundle to achieve a higher gas exchange rate. First, the packing density of the hollow-fiber bundle was lower than commercial oxygenators due to the manual manufacturing. By increasing this packing density, the gas exchange rate would increase accordingly. Second, distribution plates for a more uniform blood flow can be placed at the inlet and outlet of the oxygenator. Third, the hollow-fiber membranes can be individually placed to ensure equal distances between the surrounding hollow fibers.

Improving oxygenator performance using computational simulation and flow field-based parameters.

Current goals in the development of oxygenators are to reduce extrinsic surface contact area, thrombus formation, hemolysis, and priming volume. To achieve these goals and provide a favorable concentration gradient for the gas exchange throughout the fiber bundle, this study attempts to find an optimized inlet and outlet port geometry to guide the flow of a hexagonal-shaped oxygenator currently under development. Parameters derived from numerical flow simulations allowed an automated quantitative evaluation of geometry changes of flow distribution plates. This led to a practical assessment of the quality of the flow. The results were validated qualitatively by comparison to flow visualization results. Two parameters were investigated, the first based on the velocity distribution and the second calculated from the residence time of massless particles representing erythrocytes. Both approaches showed significant potential to improve the flow pattern in the fiber bundle, based on one of the parameters of up to 66%. Computational fluid dynamics combined with a parameterization proved to be a powerful tool to quickly improve oxygenator designs.

Investigation of the influence of volute design on journal bearing bias force using computational fluid dynamics.

Hydrodynamic fluid film bearings represent an optimal possibility for rotary blood pump (RBP) miniaturization and wear-free operation. Size is a key parameter in the development of ventricular assist devices (VADs) as smaller patients and the pediatric population become eligible for the device. In order to maintain rotor suspension, radial journal bearings have been widely used in industrial applications as well as in some VADs. A main influence on the performance of such a bearing is the applied hydraulic bias force. This study combines numerical and analytical approaches to determine the bias force of different impeller-volute configurations and the resulting eccentricity for the hydraulic design point and also for off-design operation. Significant differences occur for different impeller-volute configurations, with the circular volute displaying the most beneficial properties for a stable impeller suspension. Moreover, an analytical prediction of eccentricity was found to be incorrect for the relatively small forces that occur in RBPs.