• Difference between the defibrillation threshold estimates (voltage & energy) for 2.5, 3.5 and 4.5 ms membrane time constant based biphasic defibrillation waveforms
  

• NYHA Class
  
• High Voltage Impedance
  
• QT Interval
  
• Cardiac Dimension
  
• Antiarrhythmic Drug Therapy
  

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 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%.