Techniques for determining whether one or more leads are not adequately connected to a patient, e.g., for ECG monitoring, are described. The techniques involve injection of an integrated signal (which includes a test signal) into one lead, and monitoring the driven lead and the response at the other leads, including the common mode and the difference between the other leads. These “lead-off” detection techniques may be provided by an external defibrillator that provides three-wire ECG monitoring. Techniques for determining a type of a cable coupled to a defibrillator are also described. The cable-type identification may allow a defibrillator to, for example, operate in either a three-wire ECG monitoring mode or a therapy mode, based on whether a three-wire ECG cable or a defibrillation cable is coupled to the defibrillator.