Heart failure is a progressive disease that decreases the pumping action of the heart. This may cause a backup of fluid in the heart and may result in heart beat changes. Using very low electrical pulses that travel across the inside of the chest cavity, thoracic fluid status monitoring (OptiVol™) can measure the level of resistance to the electrical pulses, which indicates the level of fluid in the chest. The purpose of this study is to collect information related to fluid build up in the chest of subjects with heart failure.