The REFLEx Study will evaluate if ENDOTAK RELIANCE G defibrillation leads with GORE ePTFE-covered coils are as good as or better than other comparative commercially available defibrillation leads with respect to electrical performance and termination efficacy in spontaneous tachyarrhythmic episodes.

Inclusion Criteria:

• Patients who meet current ICD or CRT-D indications
• Patients who are going to be implanted with one of the aforementioned ICD or CRT-D devices
• Patients who are going to be implanted with active-fixation, dual-coil, endocardial defibrillation leads with compatible connectors (i.e., ENDOTAK RELIANCE G or study-comparative ICD leads)
• Patients who sign and date a Patient Informed Consent form prior to device implant
• Patients who remain in the clinical care of the enrolling physician in approved centers
• Patients who are at least 18 years old

Exclusion Criteria:

• Patients who have a preexisting transvenous lead system that is not completely explanted
• Patients who are hypersensitive to a single dose of approximately 1.0 mg of dexamethasone acetate
• Patients who are expected to receive a heart transplant during the duration of the study
• Patients whose estimated life expectancy is less than 12 months due to other medical conditions
• Patients who currently have or who are likely to receive a tricuspid valve prosthesis
• Patients who are likely to have an open-chest surgery in the next three months
• Patients who are currently enrolled in another investigational study that would directly impact the treatment or outcome of the REFLEx study.
• Patients who are mentally incompetent and cannot sign a Patient Informed Consent form or comply with the study
• Patients who are pregnant