 Theoretical analysis has shown that difference between the tuned and 50 tilt waveforms is amplified at high impedance levels with tuned waveforms being superior unpublished. Accordingly, this study has been designed to prospectively compare the DFT estimates with tuned and 50 tilt waveforms in left-sided, active pectoral defibrillation lead systems when the SVC coil has been turned OFF or is not part of the shocking circuit. 