Low Voltage Direct Current Delivered Through Unipolar Transvenous Leads: An Alternate Method for the Induction of Ventricnlar Fibrillation

The induction of VF during testing of an ICD may not always be possible using either burst pacing or high energy T wave shocks. The purpose of this study was to evaluate the effectiveness of low energy DC stimulation for inducing VF in a porcine model. The VFT was measured using constant voltage stimuli and a step-up method in ten anesthetized pigs (25-30 kg). Stimuli of different durations (0.5, 1.0, 2.0 s) were delivered (unsynchronized) between a right ventricular apical coil and a subcutaneous test can. Current was measured from the voltage drop across a series resistor (10 omega). With anodal stimulation, VF required 6.4 +/- 0.2 V compared to 13.8 +/- 0.6 V with cathodal stimulation (P < 0.001). The current required to induce VF (measured 10 ms after the stimulus onset) was 58.3 +/- 2.2 mA with anodal stimulation and 119.3 +/- 4.7 mA with cathodal stimulation (P < 0.001). Stimulus duration did not significantly influence the voltage or current required for VF induction. In 6 of the 10 pigs, synchronizing a 0.5-second stimulus to the R wave did not significantly alter the VFT compared to same stimulus synchronized to mid-upslope of the T wave. The results indicate that VF can be consistently induced through transvenous electrodes by passing unsynchronized DC for 0.5-2 seconds. The induction of VF required about 50% less current and voltage with anodal stimulation. It should be possible to induce VF with the DC voltage available from the internal battery source of an ICD.