Medical expert systems are useful since they assist the doctors in making decisions on treatment plan. Such a system can be developed by applying effective design techniques in order to improve the performance. Since medical expert systems are domain dependent, requirements analysis must be carried out before software design. In the object oriented design, use case diagram play a major role. The analysis of use case diagrams can be carried out by analysing the class hierarchy more effectively. Use case analysis with slicing technique will identify the details of one level of a class hierarchy. Therefore, a new use case slicing based approach is used to design software for developing a medical expert system. This work focuses on providing assistance to asthma and breathing related diseases. For this purpose, a knowledge based system is designed and implemented in this work which uses rules for making effective decisions. This knowledge based system is assisted with a feature selection module and a classification module. From the experiments conducted in this research work, it had been observed that the proposed expert system provides suitable decisions and recommendations on asthma treatment.