Toward the Virtual Benchmarking of Pneumatic Ventricular Assist Devices: Application of a Novel Fluid-Structure Interaction-Based Strategy to the Penn State 12 cc Device

Fluid-structure interaction modelling of a positive-displacement Total Artificial Heart

For those suffering from end-stage biventricular heart failure, and where a heart transplantation is not a viable option, a Total Artificial Heart (TAH) can be used as a bridge to transplant device. The Realheart TAH is a four-chamber artificial heart that uses a positive-displacement pumping technique mimicking the native heart to produce pulsatile flow governed by a pair of bileaflet mechanical heart valves. The aim of this work was to create a method for simulating haemodynamics in positive-displacement blood pumps, using computational fluid dynamics with fluid-structure interaction to eliminate the need for pre-existing in vitro valve motion data, and then use it to investigate the performance of the Realheart TAH across a range of operating conditions. The device was simulated in Ansys Fluent for five cycles at pumping rates of 60, 80, 100 and 120 bpm and at stroke lengths of 19, 21, 23 and 25 mm. The moving components of the device were discretised using an overset meshing approach, a novel blended weak-strong coupling algorithm was used between fluid and structural solvers, and a custom variable time stepping scheme was used to maximise computational efficiency and accuracy. A two-element Windkessel model approximated a physiological pressure response at the outlet. The transient outflow volume flow rate and pressure results were compared against in vitro experiments using a hybrid cardiovascular simulator and showed good agreement, with maximum root mean square errors of 15% and 5% for the flow rates and pressures respectively. Ventricular washout was simulated and showed an increase as cardiac output increased, with a maximum value of 89% after four cycles at 120 bpm 25 mm. Shear stress distribution over time was also measured, showing that no more than [Formula: see text]% of the total volume exceeded 150 Pa at a cardiac output of 7 L/min. This study showed this model to be both accurate and robust across a wide range of operating points, and will enable fast and effective future studies to be undertaken on current and future generations of the Realheart TAH.

A Mechanistic Lumped Parameter Model of the Berlin Heart EXCOR to Analyze Device Performance and Physiologic Interactions

PURPOSE

The Berlin Heart EXCOR (BH) is the only FDA-approved, extracorporeal pulsatile ventricular assist device (VAD) for infants and children with heart failure. Clinicians control four settings on the device-systolic and diastolic drive pressures, device pump rate, and systolic time as a percentage of the pump cycle. However, interactions between BH pneumatics and the native circulation remain poorly understood. Thus, establishing appropriate device size and settings can be challenging on a patient-to-patient basis.

METHODS

In this study we develop a novel lumped parameter network based on simplified device mechanics. We perform parametric studies to characterize device behavior, study interactions between the left ventricle (LV) and BH across different device settings, and develop patient-specific simulations. We then simulate the impact of changing device parameters for each of three patients.

RESULTS

Increasing systolic pressure and systolic time increased device output. We identified previously unobserved cycle-to-cycle variations in LV-BH interactions that may impact patient health. Patient-specific simulations demonstrated the model's ability to replicate BH performance, captured trends in LV behavior after device implantation, and emphasized the importance of device rate and volume in optimizing BH efficiency.

CONCLUSION

We present a novel, mechanistic model that can be readily adjusted to study a wide range of device settings and clinical scenarios. Physiologic interactions between the BH and the native LV produced large variability in cardiac loading. Our findings showed that operating the BH at a device rate greater than the patient's native heart decreases variability in physiological interactions between the BH and LV, increasing cardiac offloading while maintaining cardiac output. Device rates that are close to the resting heart rate may result in unfavorable cardiac loading conditions. Our work demonstrates the utility of our model to investigate BH performance for patient-specific physiologies.