  The refractory period of cardiac tissue can be quantitatively described using
strength-interval (SI) curves. The information captured in SI curves is
pertinent to the design of anti-arrhythmic devices including pacemakers and
implantable cardioverter defibrillators. As computational cardiac modelling
becomes more prevalent, it is feasible to consider the generation of
computationally derived SI curves as a supplement or precursor to curves that
are experimentally derived. It is beneficial, therefore, to examine the
profiles of the SI curves produced by different cardiac tissue models to
determine whether some models capture the refractory period more accurately
than others. In this study, we compare the unipolar SI curves of two tissue
models: the current state-of-the-art bidomain model and the recently developed
extracellular-membrane-intracellular (EMI) model. The EMI model's resolution of
individual cell structure makes it a more detailed model than the bidomain
model, which forgoes the structure of individual cardiac cells in favour of
treating them homogeneously as a continuum. We find that the resulting SI
curves elucidate differences between the models, including that the behaviour
of the EMI model is noticeably closer to the refractory behaviour of
experimental data compared to that of the bidomain model. These results hold
implications for future computational pacemaker simulations and shed light on
the predicted refractory properties of cardiac tissue from each model.
