We describe design and prototyping efforts for a Personal Health Management Assistant for heart failure patients as part of Project HealthDesign. An assistant is more than simply an application. An assistant understands what its users need to do, interacts naturally with them, reacts to what they say and do, and is proactive in helping them manage their health. In this project, we focused on heart failure, which is not only a prevalent and economically significant disease, but also one that is very amenable to self-care. Working with patients, and building on our prior experience with conversational assistants, we designed and developed a prototype system that helps heart failure patients record objective and subjective observations using spoken natural language conversation. Our experience suggests that it is feasible to build such systems and that patients would use them. The system is designed to support rapid application to other self-care settings.