Traditional reasoning methods have found limited success in the medical domain due to their brittleness and lack of robust justification. Recent work has turned to model-based reasoning techniques to overcome these limitations. These techniques employ deep domain models and focus on qualitative reasoning. A discussion is presented of an ongoing project to develop a cardiovascular model with sophisticated reasoning mechanisms and robust explanation capabilities. The framework for the model combines J. DeKleer and J.S. Brown's (1984) component-oriented methods and K.D. Forbus's (1984) process-oriented ontologies. The current prototype models the electrical subsystem of the heart, and simulates normal and deviant cardiac rhythms. >