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

A patient-specific echogenic soft robotic left ventricle embedded into a closed-loop cardiovascular simulator for advanced device testing

Cardiovascular medical devices undergo a large number of pre- and post-market tests before their approval for clinical practice use. Sophisticated cardiovascular simulators can significantly expedite the evaluation process by providing a safe and controlled environment and representing clinically relevant case scenarios. The complex nature of the cardiovascular system affected by severe pathologies and the inherently intricate patient-device interaction creates a need for high-fidelity test benches able to reproduce intra- and inter-patient variability of disease states. Therefore, we propose an innovative cardiovascular simulator that combines in silico and in vitro modeling techniques with a soft robotic left ventricle. The simulator leverages patient-specific and echogenic soft robotic phantoms used to recreate the intracardiac pressure and volume waveforms, combined with an in silico lumped parameter model of the remaining cardiovascular system. Three different patient-specific profiles were recreated, to assess the capability of the simulator to represent a variety of working conditions and mechanical properties of the left ventricle. The simulator is shown to provide a realistic physiological and anatomical representation thanks to the use of soft robotics combined with in silico modeling. This tool proves valuable for optimizing and validating medical devices and delineating specific indications and boundary conditions.

Computational Fluid-Structure Interaction Study of a New Wave Membrane Blood Pump

PURPOSE

Wave membrane blood pumps (WMBP) are novel pump designs in which blood is propelled by means of wave propagation by an undulating membrane. In this paper, we computationally studied the performance of a new WMBP design (J-shaped) for different working conditions, in view of potential applications in human patients.

METHODS

Fluid-structure interaction (FSI) simulations were conducted in 3D pump geometries and numerically discretized by means of the extended finite element method (XFEM). A contact model was introduced to capture membrane-wall collisions in the pump head. Mean flow rate and membrane envelope were determined to evaluate hydraulic performance. A preliminary hemocompatibility analysis was performed via calculation of fluid shear stress.

RESULTS

Numerical results, validated against in vitro experimental data, showed that the hydraulic output increases when either the frequency or the amplitude of membrane oscillations were higher, with limited increase in the fluid stresses, suggesting good hemocompatibility properties. Also, we showed better performance in terms of hydraulic power with respect to a previous design of the pump. We finally studied an operating point which achieves physiologic flow rate target at diastolic head pressure of 80 mmHg.

CONCLUSION

A new design of WMBP was computationally studied. The proposed FSI model with contact was employed to predict the new pump hydraulic performance and it could help to properly select an operating point for the upcoming first-in-human trials.

Exercise physiology in left ventricular assist device patients: insights from hemodynamic simulations

Left ventricular assist devices (LVADs) assure longer survival to patients, but exercise capacity is limited compared to normal values. Overall, LVAD patients show high wedge pressure and low cardiac output during maximal exercise, a phenomenon hinting at the need for increased LVAD support. Clinical studies investigating the hemodynamic benefits of an LVAD speed increase during exercise, ended in inhomogeneous and sometimes contradictory results. The native ventricle-LVAD interaction changes between rest and exercise, and this evolution is complex, multifactorial and patient-specific. The aim of this paper is to provide a comprehensive overview on the patient-LVAD interaction during exercise and to delineate possible therapeutic strategies for the future. A computational cardiorespiratory model was used to simulate the hemodynamics of peak bicycle exercise in LVAD patients. The simulator included the main cardiovascular and respiratory impairments commonly observed in LVAD patients, so as to represent an average hemodynamic response to exercise. In addition, other exercise responses were simulated, by tuning the chronotropic, inotropic and vascular functions, and implementing aortic regurgitation and stenosis in the simulator. These profiles were tested under different LVAD speeds and LVAD pressure-flow characteristics. Simulations output showed consistency with clinical data from the literature. The simulator allowed the working condition of the assisted ventricle at exercise to be investigated, clarifying the reasons behind the high wedge pressure and poor cardiac output observed in the clinics. Patients with poorer inotropic, chronotropic and vascular functions, are likely to benefit more from an LVAD speed increase during exercise. Similarly, for these patients, a flatter LVAD pressure-flow characteristic can assure better hemodynamic support under physical exertion. Overall, the study evidenced the need for a patient-specific approach on supporting exercise hemodynamics. In this frame, a complex simulator can constitute a valuable tool to define and test personalized speed control algorithms and strategies.

Development of a computational simulator of the extracorporeal membrane oxygenation and its validation with in vitro measurements

In the recent years, the use of extracorporeal membrane oxygenation (ECMO) has grown substantially, posing the need of having specialized medical and paramedical personnel dedicated to it. Optimization of the therapy, definition of new therapeutic strategies, and ECMO interaction with the cardiorespiratory system require numerous specific skills and preclinical models for patient successful management. The aim of the present work is to develop and validate a computational model of ECMO and connect it to an already existing lumped parameter model of the cardiorespiratory system. The ECMO model was connected between the right atrium and the aorta of the cardiorespiratory simulator. It includes a hydraulic module that is a representation of the tubing, oxygenator, and pump. The resulting pressures and flows within the ECMO circuit were compared to the measurements conducted in vitro on a real ECMO. Additionally, the hemodynamic effects the ECMO model elicited on the cardiorespiratory simulator were compared with experimental data taken from the literature. The comparison between the hydraulic module and the in vitro measurements evidenced a good agreement in terms of flow, pressure drops across the pump, across the oxygenator and the tubing (maximal percentage error recorded was 17.6%). The hemodynamic effects of the ECMO model on the cardiovascular system were in agreement with what observed experimentally in terms of cardiac output, systemic pressure, pulmonary arterial pressure, and left atrial pressure. The ECMO model we developed and embedded into the cardiorespiratory simulator, is a useful tool for the investigation of basic physiological mechanisms and principles of ECMO therapy. The model was sided by a user interface dedicated to training applications. As such, the resulting simulator can be used for the education of students, medical and paramedical personnel.

Haemodynamic Effect of Left Atrial and Left Ventricular Cannulation with a Rapid Speed Modulated Rotary Blood Pump During Rest and Exercise: Investigation in a Numerical Cardiorespiratory Model

PURPOSE

The left atrium and left ventricle are the primary inflow cannulation sites for heart failure patients supported by rotary blood pumps (RBPs). Haemodynamic differences exist between inflow cannulation sites and have been well characterized at rest, yet the effect during exercise with the same centrifugal RBP has not been previously well established. The purpose of this study was to investigate the hemodynamic effect of inflow cannulation site during rest and exercise with the same centrifugal RBP.

METHODS

In a numerical cardiorespiratory model, a simulated heart failure patient was supported by a HeartWare HVAD RBP in left atrial (LAC) and left ventricular cannulation (LVC). The RBP was operated at constant speed and sinusoidal co- and counter-pulse and was investigated in cardiovascular conditions of steady state rest and 80-watt bike graded exercise.

RESULTS

Cardiac output was 5.0 L min^-1 during rest and greater than 6.9 L min^-1 during exercise for all inflow cannulation sites and speed operating modes. However, during exercise, LAC demonstrated greater pressure-volume area and lower RBP flow (1.41, 1.37 and 1.37 J and 5.03, 5.12 and 5.03 L min^-1 for constant speed and co- and counter-pulse respectively) when compared to LVC (pressure-volume area: 1.30, 1.27 and 1.32 J and RBP flow: 5.56, 5.71 and 5.59 L min^-1 for constant speed and co- and counter-pulse respectively).

CONCLUSION

For a simulated heart failure patient intending to complete exercise, LVC seems to assure a better hemodynamic performance in terms of pressure-volume area unloading and increasing RBP flow.

Hemodynamic exercise responses with a continuous-flow left ventricular assist device: Comparison of patients' response and cardiorespiratory simulations

Background

Left ventricular assist devices (LVADs) are an established treatment for end stage heart failure patients. As LVADs do not currently respond to exercise demands, attention is also directed towards improvements in exercise capacity and resulting quality of life. The aim of this study was to explore hemodynamic responses observed during maximal exercise tests to infer underlying patient status and therefore investigate possible diagnostics from LVAD derived data and advance the development of physiologically adaptive LVAD controllers.

Methods

High resolution continuous LVAD flow waveforms were recorded from 14 LVAD patients and evaluated at rest and during maximum bicycle exercise tests (n = 24). Responses to exercise were analyzed in terms of an increase (↑) or decrease (↓) in minimum (Q_MIN), mean (Q_MEAN), maximum flow (Q_MAX) and flow pulsatility (Q_P2P). To interpret clinical data, a cardiorespiratory numerical simulator was used that reproduced patients’ hemodynamics at rest and exercise. Different cardiovascular scenarios including chronotropic and inotropic responses, peripheral vasodilation, and aortic valve pathologies were simulated systematically and compared to the patients’ responses.

Results

Different patients’ responses to exercise were observed. The most common response was a positive change of ΔQ_MIN↑ and ΔQ_P2P↑ from rest to exercise (70% of exercise tests). Two responses, which were never reported in patients so far, were distinguished by Q_MIN↑ and Q_P2P↓ (observed in 17%) and by Q_MIN↓ and Q_P2P↑ (observed in 13%). The simulations indicated that the Q_P2P↓ can result from a reduced left ventricular contractility and that the Q_MIN↓ can occur with a better left ventricular contractility and/or aortic insufficiency.

Conclusion

LVAD flow waveforms determine a patients’ hemodynamic “fingerprint” from rest to exercise. Different waveform responses to exercise, including previously unobserved ones, were reported. The simulations indicated the left ventricular contractility as a major determinant for the different responses, thus improving patient stratification to identify how patient groups would benefit from exercise-responsive LVAD control.