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Low Voltage Shocks Have a Significantly Higher Tilt of the Internal Electric Field Than Do High Voltage Shocks

Typically, an implantable cardioverter defibrillator(ICD) uses a cardioversion shock that is a lower voltage pulse of the same morphology and tilt as its defibrillation pulse. We investigated the internal electric field resulting from an ICD low voltage shock to determine whether its field characteristics matched those of the internal electric field of a high voltage shock. We attached epicardial patch electrodes, for shock delivery, to five fresh pig hearts placed in a diluted, heparinized saline bath. We inserted two plunge electrodes into the myocardium to measure an internal voltage proportional to the electric field. Monophasic 20‐msec shocks, from a 140 μF capacitor, ranging from 0.1–30 joules, were delivered through the patches. We measured the current, external voltage, and internal voltage every 0.1 msec throughout the duration of a shock. For each shock, we calculated the time point that represented the 65% tilt position as measured across the patch electrodes. At this 65% tilt time position, we measured the pulse widths and calculated the internal tilt from the internal voltage. We found that the initial internal voltage for the 30‐joule shock was 173 ± 40 volts compared to 10 ± 2 volts for the 0.1‐joule shock. Similarly, we found that the final internal voltage for the 30‐joule shock was 56 ± 14 volts compared to 2 ± 1 volts for the 0.1‐joule shock. Thus, the internal tilt for the 30‐joule shock was 68 ± 1% versus 82 ± 3% for the 0.1‐joule shock (P < 0.05). Hence, a defibrillation shock (30 J) has an internal tilt close to its external tilt. A cardioversion shock (0.1 J), on the other hand, has a significantly higher internal tilt. The higher internal tilt of low strength, tilt‐based shocks should be investigated as a possible factor in the proarrhythmia of cardioversion therapy.