Medical computerised systems which have a major effect on human lives (e.g. those used for diagnosis, therapy, surgery, in the intensive care units, etc) are considered as safety critical systems. Such systems are sometimes responsible for major damages and injuries due to unpredicted malfunction. Misleading user requirements, errors in the specification and in the implementation are the usual reasons responsible for non-safe systems. This paper advocates the use of an integrated formal framework based on a computational machine (X-Machine), in the development of safety critical medical systems. This formal framework gives the ability to intuitively as well as formally model a system, then automatically check if the produced model has all the desired properties, and finally test if the implementation is equivalent to the specification by applying a complete set of test cases. Therefore, the use of this framework in the development of systems in safety critical medical domains can assure that the final product is valid with respect to the user requirements by revealing errors during the whole development life cycle and subsequently add to the confidence of their use. The proposed framework is accompanied by an example, which demonstrates the use of X-Machines in specification, testing and verification.