Detection of a discontinuity, a disconnection or loss of conductivity, of cables connected to a physiological signal recorder, in particular a Holter recorder of ECG signals by measuring an impedance of the line(s). To measure the impedance of the line comprising the connecting cable connecting an external electrode placed on a patient to a signal terminal (12) of the recorder (10), one generates a current impulse, applies this impulse to the line, measures the variation of voltage resulting on the terminal from the signal during the application duration of the current impulse, and determines the impedance of the line based upon the voltage variations thus measured. The current impulse is a biphasic impulse including two successive cycles of opposite polarities, the durations and the amplitudes of these two cycles being selected so as to define approximately equal respective loads and of contrary signs. A switch (36) connects the base of the pull-down resistance (32) of the signal terminal (12) to the output of a voltage or a current generator (38) during the impedance measurement, and connects it to the potential ground (34) during the remainder of time.