Numerical simulation of centrifugal and hemodynamically levitated LVAD for performance improvement

Left ventricular assist device (LVAD) provides an effective artificial support system to the heart patients. Despite the improved life survival rate, complications like hemolysis, blood trauma, and thrombus formation still limit the performance of the blood pump. The geometrical aspects of blood pumps majorly influence the hemodynamics, therefore these devices must be carefully engineered. In this work, several versions of centrifugal blood pumps are simulated using ANSYS-CFX to propose a best compatible design for LVAD. A hemodynamic levitation approach for the impeller is also suggested to overcome the cost and weight issue associated with the magnetic levitation. The blood flow is modeled by implementing Bird-Carreau model and its turbulence is solved using the SST turbulence model. The influence of the blade profile, blade tip width, blade numbers, and splitter blades on the hemodynamic characteristics through the pump is observed. An optimum design of centrifugal blood pump for the assistance of failed ventricle is proposed that can effectively pump the blood from the left ventricle to the ascending aorta. The proposal can be adopted by LVAD designers to have hemodynamically tuned efficient product.

A Numerical Simulation Comparing a Cavopulmonary Assist Device and VA ECMO for Failing Fontan Support

A cavopulmonary assist device (CPAD) has been developed for failing Fontan support. This CPAD pumps blood from superior/inferior vena cavae (SVC/IVC) to pulmonary artery. In this study, we compared failing Fontan support with CPAD versus veno-arterial extracorporeal membrane oxygenation (VA ECMO) in silico. A numerical lumped-parameter circulatory system model was used to simulate Fontan circulation. Failing Fontan was simulated by increased pulmonary resistance. Central venous pressure (CVP), mean pulmonary arterial pressure (mPAP), left atrial pressure (LAP), and univentricular outflow (CO) were simulated and compared with published clinical data. The CPAD and VA ECMO were simulated using 1-5 L/min pump flows. In agreement with published clinical data, the simulated failing Fontan condition had increased CVP (19 mmHg) and mPAP (18 mmHg) with decreased LAP (7 mmHg) and cardiac output (3 L/min) compared with functional Fontan condition. The CPAD achieved total Fontan assistance with pump flows higher than original CO. Veno-arterial extracorporeal membrane oxygenation provided partial Fontan assistance with low pump flows. Blood went through pulmonary circulation with CPAD whereas VA ECMO bypassed pulmonary circulation and diminished univentricular blood flow (0.8 L/min). This in silico study demonstrated that CPAD preserved heart/lung function whereas VA ECMO had very low univentricular flow, potentially leading to thrombosis or univentricular atrophy.

A paired membrane umbrella double-lumen cannula ensures consistent cavopulmonary assistance in a Fontan sheep model

OBJECTIVES

The Avalon Elite (Macquet, Rastatt, Germany) double-lumen cannula can provide effective cavopulmonary assistance in a Fontan (total cavopulmonary connection) sheep model, but it requires strict alignment. The objective was to fabricate and test a newly designed paired umbrella double-lumen cannula without alignment requirement.

METHODS

The paired membrane umbrellas were designed on the double-lumen cannula to bracket infusion blood flow toward the pulmonary artery. Two umbrellas were attached, one 4 cm above and one 4 cm below the infusion opening. Umbrellas were temporarily wrapped and glued to the double-lumen cannula body to facilitate insertion. A total cavopulmonary connection mock loop was used to test cavopulmonary assistance performance and reliability with double-lumen cannula rotation and displacement. The paired umbrella double-lumen cannula also was tested in a total cavopulmonary connection adult sheep model (n = 6).

RESULTS

The bench test showed up to 4.5 L/min pumping flow and approximately 90% pumping flow efficiency at 360° rotation and 8-cm displacement of double-lumen cannula. The total cavopulmonary connection model with compromised hemodynamics was successfully created in all 6 sheep. The cavopulmonary assistance double-lumen cannula with paired umbrellas was smoothly inserted into the superior vena cava and extracardiac conduit in all sheep. At 3.5 to 4.0 L/min pump flow, the systolic arterial blood pressure and central venous pressure returned to normal baseline and remained stable throughout the 90-minute experiment, demonstrating effective cavopulmonary assistance support. Double-lumen cannula rotation and displacement did not affect performance. Autopsy revealed well-opened and positioned paired umbrellas, and double-lumen cannulas were easily removed from the right jugular vein.

CONCLUSIONS

Our double-lumen cannula with paired umbrellas is easy to insert and remove. The paired umbrellas eliminated the strict alignment requirement and ensured consistent cavopulmonary assistance performance.

Experimental measurements of energy augmentation for mechanical circulatory assistance in a patient-specific Fontan model

A mechanical blood pump specifically designed to increase pressure in the great veins would improve hemodynamic stability in adolescent and adult Fontan patients having dysfunctional cavopulmonary circulation. This study investigates the impact of axial-flow blood pumps on pressure, flow rate, and energy augmentation in the total cavopulmonary circulation (TCPC) using a patient-specific Fontan model. The experiments were conducted for three mechanical support configurations, which included an axial-flow impeller alone in the inferior vena cava (IVC) and an impeller with one of two different protective stent designs. All of the pump configurations led to an increase in pressure generation and flow in the Fontan circuit. The increase in IVC flow was found to augment pulmonary arterial flow, having only a small impact on the pressure and flow in the superior vena cava (SVC). Retrograde flow was neither observed nor measured from the TCPC junction into the SVC. All of the pump configurations enhanced the rate of power gain of the cavopulmonary circulation by adding energy and rotational force to the fluid flow. We measured an enhancement of forward flow into the TCPC junction, reduction in IVC pressure, and only minimally increased pulmonary arterial pressure under conditions of pump support.

Dual-pump support in the inferior and superior vena cavae of a patient-specific fontan physiology

The implementation of simultaneous mechanical cavopulmonary assistance having blood pumps located in both of the vena cavae is investigated as an approach to treating patients with an ailing Fontan physiology. Identical intravascular blood pumps are employed to model the hemodynamic support of a patient-specific Fontan. Pressure flow characteristics, energy gain calculations, and blood damage analyses are assessed for each model. The performance of the dual-support scenario is compared to conditions of mechanical support in the inferior vena cava only and to a nonsupported cavopulmonary circuit. The blood pump in the superior vena cava generates pressures ranging from 1 to 22 mm Hg for flow rates of 1-4 L/min at operating speeds of 1250-2500 rpm. The blood pump in the inferior vena cava produces pressures at levels approximately 20% lower. The blood pumps positively augment the hydraulic energy in the total cavopulmonary connection circuit as a function of flow rate and rotational speed. Scalar stress levels and fluid residence times are at acceptable levels. Damage indices for the dual-support case, however, are elevated slightly above 3.5%. These results suggest that concurrent, mechanical assistance of the inferior vena cava and superior vena cava in Fontan patients has the potential to be beneficial, but additional studies are needed to further explore this approach.

Cavopulmonary assist: (em)powering the univentricular fontan circulation

Since the Fontan/Kreutzer procedure was introduced, evolutionary clinical advances via a staged surgical reconstructive approach have markedly improved outcomes for patients with functional single ventricle. However, significant challenges remain. Early stage mortality risk seems impenetrable. Serious morbidities - construed as immutable consequences of palliation - have hardly been addressed. Late functional status is increasingly linked to pathophysiologic consequences of prior staged procedures. As more single-ventricle patients survive into adulthood, Fontan failure is emerging as an intractable problem for which there is no targeted therapy. Incremental solutions to address these ongoing problems have not had a measurable impact. Therefore, a fundamental reconsideration of the overall approach is reasonable and warranted. The ability to provide a modest pressure boost (2 to 6 mmHg) to existing blood flow at the total cavopulmonary connection can effectively restore more stable biventricular status. This would impact not only treatment of late Fontan failure, but also facilitate early surgical repair. A realistic means to provide such a pressure boost has never been apparent. Recent advances are beginning to unravel the unique challenges that must be addressed to realize this goal, with promise to open single-ventricle palliation to new therapeutic vistas.

Wang-Zwische double-lumen cannula leads to total cavopulmonary support in a failing Fontan sheep model

BACKGROUND

We are developing a total cavopulmonary support system for failing Fontan physiology using the percutaneous Wang-Zwische double-lumen cannula (DLC).

METHODS

We developed a sheep model of failing Fontan physiology through a right thoracotomy in 5 sheep. An extracardiac conduit was anastomosed to the inferior vena cava and superior vena cava, and a graft was applied to connect the conduit and the right pulmonary artery (PA) to create total vena cava-to-PA diversion, excluding the right heart. The DLC (commercialized as AvalonElite, Avalon Laboratories LLC, Rancho Dominguez, CA) was coupled with a CentriMag pump (Levitronix Waltham, MA) to form a cavopulmonary support system. The DLC was inserted through the right jugular vein and the superior vena cava into the extracardiac conduit with the infusion lumen opening aligned with the right PA bridge. Blood was withdrawn from the superior vena cava and the inferior vena cava through the drainage lumen and pumped into the right PA through the infusion lumen, with flow adjusted to 4.0±0.5 L/min.

RESULTS

A successful Fontan model was created without cardiopulmonary bypass. After total venous blood diversion from the vena cava to the right PA artery, failing Fontan physiology developed, evidenced by elevated central venous pressure and dropping mean PA pressure and systolic arterial pressure. The DLC was successfully inserted, and hemodynamics were normalized in all 5 sheep for the duration of the 2-hour study.

CONCLUSIONS

We created a model of failing Fontan circulation in sheep without cardiopulmonary bypass. The DLC system achieved total cavopulmonary support for 2 hours in our failing Fontan sheep model.

Intravascular mechanical cavopulmonary assistance for patients with failing Fontan physiology

To provide a viable bridge-to-transplant, bridge-to-recovery, or bridge-to-surgical reconstruction for patients with failing Fontan physiology, we are developing a collapsible, percutaneously inserted, magnetically levitated axial flow blood pump to support the cavopulmonary circulation in adolescent and adult patients. This unique blood pump will augment pressure and thus flow in the inferior vena cava through the lungs and ameliorate the poor hemodynamics associated with the univentricular circulation. Computational fluid dynamics analyses were performed to create the design of the impeller, the protective cage of filaments, and the set of diffuser blades for our axial flow blood pump. These analyses included the generation of pressure-flow characteristics, scalar stress estimations, and blood damage indexes. A quasi-steady analysis of the diffuser rotation was also completed and indicated an optimal diffuser rotational orientation of approximately 12 degrees. The numerical predictions of the pump performance demonstrated a pressure generation of 2-25 mm Hg for 1-7 L/min over 3000-8000 rpm. Scalar stress values were less than 200 Pa, and fluid residence times were found to be within acceptable ranges being less than 0.25 s. The maximum blood damage index was calculated to be 0.068%. These results support the continued design and development of this cavopulmonary assist device, building upon previous numerical work and experimental prototype testing.