Lead fracture

This example illustrates the successive steps typically observed during a lead dysfunction. Initially, the device memory reveals multiple episodes of non-sustained VT without anomaly of the lead measurements. In a second step, a clear break can be observed in the different impedance, threshold and right ventricular sensing curves. Finally, the duration of the oversensing episodes is lengthened leading to the occurrence of multiple electrical shocks. This patient had been completely lost to follow up without visual or remote monitoring which precluded early diagnosis; the prevention of inappropriate therapies is one of the major advantages of remote monitoring. This type of episode also demonstrates the value of limiting the therapies to 6 shocks in the VF zone for a single episode. Indeed, the occurrence of successive inappropriate shocks constitutes a particularly difficult and traumatic experience for the patient concerned.

The lead constitutes the weak link of the defibrillation system with a variable percentage of dysfunction depending on the models. When in the presence of a suspected lead fracture, different exams and measurements must be performed:

1. a chest X-ray: radiographic abnormalities are not systematic and a typical pattern of lead fracture is not observed in over 50% of cases;
2. repeated measurements of pacing and defibrillation impedances: the latest generations of ICDs perform periodic (daily) impedance measurements. The presence of an abnormal value or significant variations in daily measurements (abrupt change of the impedance curve) may reveal a lead dysfunction with however only moderate sensitivity. Indeed, a significant number of patients present with lead dysfunction revealed by the presence of oversensing episodes without abnormal impedance or abrupt variation in pacing values. A low impedance value is suggestive of an insulation break (current leakage), a high value suggestive of a conductor wire break (loss of continuity of the defibrillation circuit);
3. evaluation of the sensing and pacing thresholds: the alteration of the standard pacing parameters is often delayed; the sensitivity relative to a decrease in ventricular sensing or an increase in pacing thresholds in predicting lead rupture is therefore very low;
4. analysis of the various electrograms: the pattern of the EGMs associated with a lead fracture is suggestive but non-specific: intermittent sensing of sudden, rapid, non-physiological cardiac cycles with possible saturation of the amplifiers (conductor wire break) or low amplitude (detection of myopotentials due to insulation break). These signals display substantial 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-sense ventricular blanking period. These abnormal signals can affect the near-field channel and/or the far-field channel depending on the site of the fracture and may only become apparent after the delivery of a shock on an actual VF episode.