Patient monitoring in the operating theatre requires a high level of vigilance by anesthetists. The aim of this paper is to report the design of a clinically useful diagnostic system called real-time smart alarms for absolute hypovolemia in anesthesia monitoring (RTSAAM). The system provides decision support to the anesthetist by presenting the diagnostic results on an integrative, ergonomic display that is hoped to enhance patient safety. The performance of the system is assessed by both offline testing and real-time testing in the operation theatre. When detecting absolute hypovolemia (AHV) a satisfactory level of agreement (up to 81%) is observed between RTSAAM and the anesthetist.