Abstract: A multichannel programmable band comparator is provided for apparatus used in cardiac surgery. A plurality of transducers supply signals relating to parameters such as the blood flow, arterial and venal pressure, body temperature and acid-base equilibrium to a multiplexer. The multiplexer supplies an analog-to-digital converter which is connected to a comparator. The comparator receives upper and lower limit values which are preset in a memory and operates a visual and acoustic alarm via a demultiplexer if any parameter is outside the band defined by the corresponding upper and lower limit values. The multichannel comparator may be used during open heart surgery, when the cardiocirculatory and respiratory functions of a patient are taken over by complex apparatus, to ensure that various physiological parameters remain within acceptable limits.