Towards Accurate and Efficient Simulations of Positive Displacement Artifical Hearts

Introduction/Motivation
For those suffering from biventricular heart failure,
where a heart transplant is not viable, a total artificial
heart is a feasible alternative. The Realheart®,
developed by Scandinavian Real Heart, mimics the
mechanics of the native heart by vertical translation of
an atrioventricular (AV) plane. Previous numerical
models of the Realheart® were not flow driven but
relied on prescribed motion of AV and semilunar (SL)
valves. This tended to overestimate device output and
limited the operating conditions that could be simulated.
This study's objective was to develop a new modelling
strategy, combining computational fluid dynamics and
fluid-structure interactions, that generalised the
pumping mechanics of the Realheart®. Initially, only
the essential and most challenging features will be
modelled before proceeding to whole device simulation.
Methods
Fluent (Ansys Inc, Canonsburg Pennsylvania, USA,
Version 2020 R2) was used to model a fluid cylinder
containing two bileaflet mechanical heart valves aligned
in series, representing the AV (upstream) and SL
(downstream) valves (figure 1a). Symmetry conditions
were imposed such that a 1/4 model was considered.
Constant pressures of 15 and 70 mmHg were applied on
the inlet and outlet respectively. A 5 mm diameter
restriction (figure 1b) represented the resistance of the
downstream vasculature. The Navier-Stokes equations
were solved assuming Newtonian rheology. Overset
meshing allowed for a refined mesh around the
boundary of the leaflet valves (figure 1c). The 6 degrees
of freedom solver computed the leaflets' rotation using
the forces and moments exerted by the fluid. A vertical
sinusoidal translation of period 0.6 s and amplitude 12.5
mm was applied to the AV overset zone, achieving both
rotational and translational motion of the AV valve. A
custom variable time-stepping scheme was used to
reduce computation time but maintain accurate capture
of leaflet motion.
Results
Variable time stepping was only slightly less accurate
than a small, fixed time step, but at 25% of the
computation time required (table 1). The SL valve
opened at 0.15 s as fluid was pushed through the domain
by the AV valve, and closed again at 0.43 s, due to
backflow through the outlet (figure 2). Instability and
fluttering in SL valve motion was seen upon opening,
due to high pressures caused by downward AV motion.
At the point that the SL valve fully opened, fluid
followed the motion of the leaflets, and was drawn from
the edge of the fluid cylinder towards the centre to form
a faster flowing central region (figure 1 d).
A modelling strategy has been successfully developed
that defines AV plane translation and AV/SL valve
rotation, producing pumping characteristics similar to
that of the Realheart®. Further work will replace the
constriction with a Windkessel model to capture
physiological conditions and improve modelling
flexibility. Additional studies will be used to understand
the interplay between stroke parameters, and efficiency
and valve leakage.