Defibrillation shock polarity

The polarity in which an ICD shock is delivered is programmable in most conventional devices (initial or reversed polarity). Selecting a shock polarity comes into play when testing shock efficacy. One polarity may be successful when the other is not. Thus, when successful VF defibrillation is not accomplished to an adequate safety margin with one shock polarity a reversal of polarity is usually attempted first. This simple maneuver may yield an appropriate shock safety margin and negate the need to implant further hardware. Also, successful defibrillation is more likely to occur when the amount of left ventricular myocardium in the shock pathway is maximized. To this end some pathways and lead coiffigurations may work better than others. 

Even with an appropriately positioned dual coil system and attempts using both shock polarities the DPT may rarely remain unacceptably high. In this instance the implanting physician has the option of taking the proximal coil out of the defibrillation circuit and adding a subcutaneous array or patch as a means of improving the DPT. 

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