In safety critical systems, certain safety constraints must be satisfied before an operation can be performed. Such constraints typically depend on the state of the system at the instant of invocation of the operation. Further, for a specific version in a family of related systems, its intended application and the individual user profiles may play a role in determining the constraints. To promote reuse while allowing customizability, a good design should decouple the operations from their associated constraints. Also, the increased flexibility should not entail significant execution time and memory overheads. The paper presents one such design of an object oriented framework for constraint verification and applies it to our motivational problem in the context of implantable cardiac devices. It also serves a broader objective of cataloging the experience gained in developing safety critical systems. Such a knowledge base will be of practical value to the developer community.