Abstract: A cardiac rhythm observation apparatus that separates and then provides for correlation of cardiographic and electrocardiographic signals has four electrodes for attachment to the chest of a subject. Two of the electrodes connect a high-frequency current for application to the subject. The other two electrodes disposed between the first two are connected to a channel for detecting, filtering and forming a rheocardiographic signal, which channel is connected to a heartbeat counter proper. A channel for eliminating variations in pulmonary impedance is connected to the feed electrodes and delivers a pulmonary rheographic signal to a stage in the detecting channel in which the pulmonary rheographic signal is substracted from the raw rheographic signal. A correlation stage is provided for correlation either with the separated electrocardiographic signal or with stimulation pulses.