An ECG apparatus with lead-off detection, comprises right arm (R), left arm (L) and neutral (N) electrodes. A first amplifier circuit is connected to the L and R electrodes to provide an ECG signal on a first output and also separately to provide a signal related to the common mode noise. An inverting feedback amplifier feeds back an inverted common mode noise signal to the N electrode, the inverted common mode noise signal also being fed to a second amplifier circuit whose output changes state upon disconnection of one or both of the L and N electrodes.
