The purpose of this study is to determine whether there is a relationship between serial B-natriuretic peptide (BNP) and implantable device measured intrathoracic impedance in symptomatic and asymptomatic heart failure patients over a 1-year period. Furthermore, this study will establish whether implantable device measured intrathoracic impedance trends with the distance walked on a six-minute hall walk (6MHW), an exercise-based acute measure that has also been associated with heart failure clinical status.

This is a prospective non-randomized longitudinal study. One hundred subjects will participate in 6 monthly and 3 bi-monthly serial assessments of plasma-based BNP, implantable device measured intrathoracic impedance, and clinical evaluation of heart failure status. Each patient will be enrolled in the study for a period of 12 months.

Inclusion Criteria:

• Baseline BNP > 80 pg/ml
• Meets current guidelines for ICD implantation with or without CRT
• Receiving a Medtronic InSync Sentry, Concerto, Virtuoso, or subsequently approved Medtronic ICD with the OptiVol feature.
• Walk < 450 meters during 6 minute hall walk test

Exclusion Criteria:

• Less than 18 years of age
• Life expectancy < 6 months
• History of non-compliance with clinic visits or study-related tasks which may decrease chance of completing of the study
• Physical disability which limits exercise

Patients receiving with an implantable cardioverter defibrillator (ICD) with or without cardiac resynchronization therapy (CRT).