The design of bug-free and safe medical device software is challenging, especially in complex implantable devices. This is due to the device's closed-loop interaction with the patient's organs, which are stochastic physical environments. The life-critical nature and the lack of existing industry standards to enforce software validation make this an ideal domain for exploring design automation challenges for integrated functional and formal modeling with closed-loop analysis. The primary goal of high-confidence medical device software is to guarantee that the device will never drive the patient into an unsafe condition, even though we do not have complete understanding of the physiological plant. To address the safety gap between ensuring the device satisfies its therapeutic requirements with the patient-in-the-loop and testing its software specifications, new approaches for closed-loop validation of the device software within the physiological context are needed. In High-Confidence Medical Device Software Development, the authors use an implantable cardiac pacemaker as a working example to demonstrate how model-based design can help improve the safety and efficacy of medical device software. It demonstrates the application of model-based design in several design activities during the development process, from the perspective of the manufacturer's design validation team.