A selective circuit is provided for passing only selected waveforms of a desired polarity from a complex input waveform. The circuit is particularly adapted for use in a heart ventricular arrhythmia detector. In such a detector, it is first necessary to differentiate the abnormal negative ventricular ectopic beats from the normal negative Q & S waves. After these abnormal beats are detected, they are accumulated and, if a number of them occur within a limited period of time, an alarm circuit is actuated. One embodiment also includes means for detecting bradycardia and tachycardia. A further embodiment provides means for transmitting an ECG over the telephone.