The use of embedded software is advancing in modern medical devices, so does its capabilities and complexity. This paradigm shift brings many challenges such as an increased rate of medical device failures due to software faults. In this letter, we present a rigorous correct by construction approach for the trustworthy development of hemodialysis machines, a sub-class of active medical devices. We show how informal requirements of hemodialysis machines are modeled and analyzed through a rigorous process and suggest a generalization to a larger class of active medical devices.