Catheter ablation procedures are performed by highly trained and experienced cardiology subspecialists. Yet the massive amount of data produced during these procedures creates a data overload problem that can impede the performance of even the best practitioners. This may be evidenced by (1) overlooking important signal features, (2) misinterpreting the signals, and (3) misinterpreting catheter locations in the heart, all of which can lead to increased procedure duration, applications of radiofrequency energy to the wrong part of the heart, or both. This article presents the first results from a project aimed at developing a model-based system for interpreting intracardiac electrograms in near real time. The system is intended to assist physicians in interpreting the enormous amounts of data recorded during catheter ablation studies. It is an extension of the Einthoven system that has been extended to account for the three-dimensional relationships in the cardiac conduction system as recorded in the various intracardiac electrograms. The new three-dimensional cardiac conduction model and the enhancements to Einthoven's reasoning algorithms are presented. The locus of this study is on interpreting the results of ventricular extrastimulus tests. Data collected for this study and the output generated by the system are presented.