Apparatus for assessing the electrical properties of patient-electrode interfaces has a carrier signal source injecting two carrier signals comprising an AC signal with a DC offset to the electrodes. The carrier signals are out of phase. The outputs from the electrodes are formed into electrocardiographic lead signals in a pre-amplifier circuit. Signal processing circuit is coupled to the pre-amplifier circuit and provides a first signal comprising the AC carrier signal contained in an ECG lead signal and a second signal containing a DC offset signal. The first and second signals are provided to a microprocessor to obtain an output indicative of the electrical properties of electrode interfaces for the ECG lead signal.