The objective of this study is to compare the ULV (Upper Limit of Vulnerability)/DFT (Defibrillation Threshold) efficacy between the 2.5, 3.5 and 4.5 ms membrane time constant based biphasic defibrillation waveforms. This comparison will result in identifying the optimal membrane time constant when programming the "tuned" defibrillation waveform.

Sudden cardiac death (SCD) continues to be a significant cause of cardiac mortality with annual deaths ranging from 250,000 to 400,000 in the United States. Accordingly, implantable cardioverter-defibrillators (ICD's) have proven to be an important therapeutic option for patients susceptible to SCD. Successful therapy and generator longevity are greatly dependent on proper defibrillation threshold (DFT) determination.

All modern ICD's utilize a biphasic waveform for defibrillation. It has been clearly shown that biphasic waveforms reduce the energy required for internal defibrillation of the heart. However, there is no consensus on which pulse widths are best for defibrillation.

St. Jude Medical ICD's (implantable cardioverter defibrillators) have programmable pulse widths, which allow the physician multiple options in dealing with ICD patients. By implanting ICD's with programmable pulse widths, this study utilizes the Tissue RC Resistance/Capacitance) model to try to identify the optimal pulse widths.

To determine the efficacy of an optimal membrane time constant estimate for the "tuned" waveform, defibrillation testing must be performed. Upper limit of vulnerability (ULV) has been proposed as an alternative means of predicting the DFT and it has been shown that ULV guided DFT testing can achieve a defibrillation success rate of 95%.

Inclusion Criteria:

• Patient is a candidate for ICD/CRT-D implantation.
• Patient has a compatible transvenous defibrillation lead system.
• Patient has had an echocardiogram, MUGA, or cath procedure within 6 months of ICD/CRT-D implant.
• Patient is able to tolerate ULV guided DFT testing.

Exclusion Criteria:

• Patient has a mechanical valve in the tricuspid position.
• Patient has epicardial defibrillation electrodes.
• Patient is pregnant.
• Patient is less than 18 years old.