A key performance indicator of an artificial pacemaker lead is its ability to trigger cardiac action potentials with minimal energy. This article introduces a three-dimensional and a non-dimensional model of cardiac stimulation by an artificial pacemaker, which could be used when designing new leads. The models couple the electronics of the pulse generator to electrophysiology in the tissue through contact impedance. They are used to generate Lapicque curves which are compared to several experimental ones from sheep hearts. Computed curves are in general in qualitative agreement with the experimental data, which is a first step that contributes to the credibility of the models.