The effect of delivered energy on defibrillation shock impedance

The impedance of internal defibrillator shocks: is an important determinant of defibrillation efficacy. To assess the effect of delivered energy on im pedance, we studied 97 patients with 4 different lead systems. The lead sys tems evaluated were two epicardial patches, a hybrid system of a patch and right atrial coil, a dual coil transvenous lead and a transvenous lead with a subcutaneous patch. impedances were measured for 6 shock energies betwee n 0.1 and 30 J. Shock impedance increased at low energies for all lead syst ems (p < 0.001), although the rate of increase varied markedly between syst ems. The energy factor (F-E), which is the ratio of impedances for the 0.1 and 10 J shocks, was least for the platinum transvenous lead (1.2 +/- 0.02) and greatest for the titanium hybrid lead (4.2 +/- 0.2). Reversing the pol arity of the hybrid lead markedly attenuated the impedance rise. These find ings indicate that there is at least a modest rise (20%) of shock impedance at very low delivered energies. The largest increases noted with titanium lead systems are primarily due to polarization. Titanium transvenous leads should be avoided when low energy shocks are utilized such as for the cardi oversion of ventricular tachycardia or atrial fibrillation.