Damage to electrode leads in implanted cardiac pacemakers is commonly deduced from diminished and distorted stimulus artefacts. These changes may not occur and do not discriminate between fractured wire and ruptured insulation. Moreover, amplitudes vary from other causes and diagnosis from shape changes requires experience and oscilloscope equipment. The two types of lead damage cause opposite changes in the loading of the stimulus generator. For a few generators the stimulus duration and rate are load-sensitive, although the circuit is otherwise very stable, and simple measurement of these two temporal features can indicate and classify a failure, often in a latent stage before pacing failure has occurred.