This tracing allows demonstrating a characteristic pattern of lead dysfunction with oversensing of rapid, anarchic signals, at the limit of the programmed refractory period. In the early stages of dysfunction, the device memory usually features multiple episodes of non-sustained VT without abnormality of lead measurements. In a second step, a clear break in the different impedance, threshold and right ventricular sensing curves can be observed. In the presence of a suspected lead dysfunction, various exams and measurements must be performed:

1. chest X-ray: radiographic abnormalities are not systematic and a typical pattern of lead fracture is not observed in over 50% of cases;
2. repetitive pacing and defibrillation impedance measurements: the latest generations of ICDs perform periodic (daily) impedance measurements with good correlation between sub-threshold measurements and actual measurements on an effectively-delivered high amplitude shock. The presence of an abnormal value or significant variations in the daily measurements (break in the impedance curve) may reveal a lead dysfunction albeit with moderate sensitivity. Indeed, a significant number of patients present a lead dysfunction revealed by the presence of oversensing episodes without abnormal impedance measurements or abrupt variations in values. A low impedance value is suggestive of an insulation break (current leakage) while a high value is suggestive of a conduction wire break (loss of continuity of the defibrillation circuit);
3. evaluation of the sensing and pacing thresholds: the alteration in standard pacing parameters is often delayed; the sensitivity relative to a decrease in ventricular sensing or an increase in pacing thresholds in predicting lead fractures is therefore very low;
4. analysis of the different endocardial electrograms: the pattern of endocardial EGMs associated with a lead fracture is suggestive but non-specific: intermittent sensing of disorganized, rapid, non-physiological cardiac cycles with possible saturation of the amplifiers (conductor wire break) or low amplitude (sensing of myopotentials due to insulation break). These signals exhibit significant variability in both amplitude and frequency, are intermittent in the cardiac cycle and are most often recorded in the VF zone with values at the limit of the post-ventricular refractory period. They can affect the sensing channel and/or the far-field channel depending on the fracture site and only may become apparent after an electrical shock has been delivered on an actual VF episode.

In this asymptomatic patient, remote monitoring allowed an early diagnosis and thus avoided the occurrence of inappropriate therapies. Early diagnosis also avoids the succession of inappropriate charging of the battery which, even if deviated, are energy-consuming and can greatly reduce the life of the device. Explantation of the lead showed an insulation break with friction between the pulse generator and the damaged portion of the lead.