  The heart is a vital organ that relies on the orchestrated propagation of
electrical stimuli to coordinate each heart beat. Abnormalities in the heart's
electrical behaviour can be managed with a cardiac pacemaker. Recently, the
closed-loop testing of pacemakers with an emulation (real-time simulation) of
the heart has been proposed. An emulated heart would provide realistic
reactions to the pacemaker as if it were a real heart. This enables developers
to interrogate their pacemaker design without having to engage in costly or
lengthy clinical trials. Many high-fidelity heart models have been developed,
but are too computationally intensive to be simulated in real-time. Heart
models, designed specifically for the closed-loop testing of pacemakers, are
too abstract to be useful in the testing of physical pacemakers.
  In the context of pacemaker testing, this paper presents a more
computationally efficient heart model that generates realistic continuous-time
electrical signals. The heart model is composed of cardiac cells that are
connected by paths. Significant improvements were made to an existing cardiac
cell model to stabilise its activation behaviour and to an existing path model
to capture the behaviour of continuous electrical propagation. We provide
simulation results that show our ability to faithfully model complex re-entrant
circuits (that cause arrhythmia) that existing heart models can not.
