ANABEL (ANalysis of Acid-Base status by Evaluating Lisp) is a prototype medical intelligent decision-support system aiming to assist clinicians in an Intensive Care Unit environment with the interpretation of blood-gas measurements. Its architecture is based on the merging of representations for declarative (domain-descriptive) and procedural (problem-solving) medical knowledge. The system performs diagnosis in two stages (tentative and differential) by first evaluating elementary computational units of procedural knowledge (procedures) and then abstracting their symbolic outputs in generating text. Thus, a 'semantic trace' is built which reflects the system's line of reasoning in reaching its conclusion. This paper describes the design aspects, development and clinical validation of ANABEL.