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

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

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

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.

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.

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.