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 card ioversion shock that is a lower voltage pulse of the same morphology a nd tilt as its defibrillation pulse. We investigated the internal elec tric field resulting from an ICD low voltage shock to determine whethe r its field characteristics matched those of the internal electric fie ld of a high voltage shock. We attached epicardial patch electrodes, f or shock delivery, to five fresh pig hearts placed in a diluted, hepar inized saline bath. We inserted two plunge electro des into the myocar dium to measure an internal voltage proportional to the electric field . Monophasic 20-msec shocks, from a 140-mu F capacitor, ranging from 0 .1-30 joules, were delivered through the patches. We measured the curr ent, external voltage, and infernal 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 el ectrodes. At this 65% tilt time position, we measured the pulse widths and calculated the infernal tilt from the internal voltage. We found that the initial internal voltage for the 30-joule shock was 173 +/- 4 0 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 5 6 +/- 14 volts compared to 2 +/- 1 volts for the 0.1-joule shock. Thus , the infernal 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 cardioversio n shock (0.1 J), on the other hand, has a significantly higher interna l tilt. The higher internal tilt of low strength, tilt-based shocks sh ould be investigated as a possible factor in the proarrhythmia of card ioversion therapy.