The effect of shock configuration and delivered energy on defibrillation impedance.

Shock impedance is an important determinant of defibrillation efficacy. Lead configuration, shock polarity, and delivered energy can affect shock impedance, but these variables have not been studied in active can lead systems. The present study was a prospective evaluation of 25 patients undergoing initial transvenous defibrillator implantation. In all patients, a dual coil lead and pectoral emulator were placed and three lead configurations were tested in random order: Lead (distal to proximal coil), unipolar (distal coil to can), and triad (distal coil to can + proximal coil). Shock energies of 0.1- to 15-J shock were evaluated. Impedance increased a mean of 21% as delivered energy was decreased (P < 0.001), an effect independent of lead configuration. At all delivered energies, impedances in the unipolar configuration were about 40% higher than triad, while the lead configuration was about 20% higher than triad (ps < 0.001). Polarity did not affect impedance. These results indicate that transvenous lead configurations and delivered energy, but not polarity, significantly influence shock impedance. The magnitude of the increase of impedance at low energies is independent of the shocking pathway. This effect has important implications for low energy shocks used to terminate atrial fibrillation or ventricular tachycardia.