Hospital Emergency Departments (EDs) are one of the more complex units of the healthcare system, that require coordination of both medical personnel and other resources to manage situations effectively. This article establishes the basic principles to design primitives of an Agent Based Modeling and Simulation (ABMS) modular system, that allows the creation of computational models that, used as Decision Support Systems (DSS), allows the EDs to achieve the highest possible level of quality of service, given the resources available. Inspired in the modularity of Lego® blocks, this ABMS system has attempted to shift from a monolithic approach to an adjustable system. This means that the system allows the description of the metasystem and agent box used to build the computational models (simulator) that can be employed as DSS.