Formal methods have emerged as an alternative approach to ensuring quality and correctness of highly critical systems, overcoming limitations of traditional validation techniques such as simulation and testing. We propose a refinement-based methodology for complex medical systems design, which possesses all the required key features. A refinement-based combined approach of formal verification, model validation using a model-checker and refinement chart is proposed in this methodology for designing a high-confidence medical device. Furthermore, we show the effectiveness of this methodology for the design of a cardiac pacemaker system.