Van der Pol, Van der Mark, 1928
This version of the model has been curated by Penny Noble using COR and is known to read in to JSim. The model is also known to run in PCEnv.
The original paper reference is cited below:
The heartbeat considered as a relaxation oscillation and an electrical model of the heart, B. van der Pol and J. van der Mark, 1928, The London, Edinburgh and Dublin Physiological Magazine and Journal of Science, 6, 763-775.
This paper is a classic in the field and a Google search will yield plenty of information regarding the model (such as the Wikipedia entry).