Why low-voltage shock impedance measurements fail to reliably detect insulation breaches in transvenous defibrillation leads.

Background: Implantable cardioverter-defibrillators (ICDs) use low-voltage measures of shock impedance (LVSZ) to monitor integrity of leads.

Objective: To determine the separation distance between conductors required for LVSZ to detect insulation breaches that produce short circuits during shocks, causing failed defibrillation.

Methods: We simulated in-pocket insulation breaches between the ICD generator (CAN) and cables to the distal coil of 10 leads from 2 manufacturers. The ICD and lead were placed in an electrolyte bath. Polystyrene sheets were used to control the breach-CAN separation. We determined both the maximum lead-CAN separation for shorts during 800 V shocks and the shock strength at which shorts occurred for a fixed separation. We also calculated breach impedance and measured it using a low-voltage instrument.

Results: The maximum breach-CAN separation for shorting was 350-500 μm for all leads. The minimum shock strength to short varied from 650 to 771 V (24-32 J). LVSZ never triggered a warning, even with no separation between the cable's inner insulation and the CAN. Using low-voltage pulses, breach impedance was measured at approximately 500-1000 Ω.

Conclusion: LVSZ is insensitive to insulation breaches that cause life-threatening, shorted shocks. The explanation likely relates to impedance differences between ionic conduction during LVSZ measurements and free-electron conduction in plasma discharges.