Abstract: A heart beat detector for use in emergencies and especially under adverse conditions of noise and vibration, of the type adapted to continuously monitor the ECG, extracting the QRS complexes and signalling the occurrence of heart beats by easily perceptible, light signals. The detector includes an inter-electrode impedance sensor; for automatically ascertaining whether adequate conductivity exists between the body of the patient and the electrodes of the detector applied to said body. According to a specific embodiment of the invention the device comprises a sub-system adapted to indicate the activity of the batteries of the instrument, the state of the electrodes and of the other circuits.