Polarity-Dependent, Reduced or No-Output Shocks in Implantable Cardioverter-Defibrillators

Recently, polarity-dependent shock failures were reported in implantable cardioverter-defibrillators caused by structural failure in the high-voltage feedthrough. Short circuits may occur when the right ventricular coil is cathodal for phase 1 of biphasic shocks (cathodal shock). This viewpoint proposes a mechanism for observed polarity dependence and considers whether the same mechanism may apply in other shock-induced, short circuits. Implantable cardioverter-defibrillator connections to the lead traverse feedthroughs into the hermetically sealed housing ("Can"). The feedthrough comprises 2 concentric, conducting metal cylinders, the inner pin-conductor to the right ventricular coil and outer Can, separated by impermeable insulation. Shock failure depends on 3 conditions: 1) development of a fluid layer in the feedthrough, creating a conduction path in parallel with the shock pathway; 2) the radial gradient of the electric field in the fluid, so resistive heating during a shock vaporizes water to form a high-resistance gas bubble around the pin; and 3) field emission of electrons at the cathode, with rate and energy dependent on the field's strength and the cathode's potential-energy barrier to emission. For cathodal shocks, electrons emitted at the metal pin may initiate an ionization avalanche in the gas until it "breaks down" into a low-resistance plasma, resulting in a short circuit. For anodal shocks, the effective cathode is the liquid-gas interface, where the field is weaker than at the pin. Additionally, solvated electrons in aqueous solution must overcome a higher potential-energy barrier to be emitted. This permits the high-resistance gas bubble to stabilize so that the shock is completed.