Techniques are provided for use with an implantable medical device for detecting breaches in lead insulation or other lead failures. In one example, bipolar impedance is measured along single-lead vectors (i.e. intra-lead vectors) of a right atrial (RA) lead and a right ventricular (RV) leads. Impedance is also measured along various cross-lead vectors (i.e. inter-lead vectors) between electrodes of the two leads. A derived impedance value is then determined from a combination of the measured impedance values, wherein the derived impedance is sensitive to a shunt impedance arising from a breach within the RV lead. A lead breach is then detected relatively early based on the derived impedance by detecting a significant deviation in derived impedance over time. Unipolar impedance measurements are used to confirm the breach.