Implantable defibrillators impedance measurement using pacing pulses versus shock delivery with intact and modified high voltage lead system.

At present the only method for measuring the high voltage system lead impedance in patients with an ICD is to deliver a low energy test shock. This is painful, requires sedation, and carries a risk of ventricular fibrillation induction. We sought to assess the shock lead and electrode function by calculating IMP using low voltage pacing pulses, and compared it to the measured impedance of a shock through the same lead. This was performed in both an intact and a modified lead system in order to mimic common clinical scenarios that alter lead system IMP (e.g., lead fracture). In an anesthesized canine model (n = 12) a standard (S) transvenous defibrillation lead (TDL), a modified (M) TDL (two-thirds of coil covered with heat-shrunk tubing), an active can (AC), and a M epicardial patch (EP) (two of four coils were disconnected) were used. Three configurations (C) were tested: C1:S/TDL-->AC, C2:M/TDL-->AC, and C3:M/TDL-->MEP. A measured IMP was obtained by an ICD using a 5-J shock as control. IMP was calculated using a 5-J shock, pacing pulses of 10-, 5-, 2-, and 1-V amplitude, as well as from a square wave drive train of low amplitude/high frequency signals (1 and 0.2 V, at 10 kHz) in all Cs. Ohm's law (V = IR) was utilized for measuring calculated IMP. As the surface area of the high voltage lead system decreased, the mean measured IMP (control) increased from C 1 to 3 (63 +/- 10, 95 +/- 4, and 127 +/- 20 omega, respectively). The correlation of calculated IMP from all Cs to measured impedance (control) remained high throughout the IMP range (range of correlation coefficient (r): 0.921-0.981). Calculated IMP using delivery of pacing pulses is highly correlated to IMP measured during shock delivery. This correlation remains high over a clinically significant range of high voltage lead system IMP changes. This study suggests that pacing pulses can be used to predict the IMP changes in the high voltage lead system which may occur clinically, reducing the need to deliver a shock for IMP measurement.