 Potential candidates with criteria for implantable cardiac resynchronization therapy who have or will undergo implantation of a Medtronic CRT-D device with thoracic impedance measurement capabilities, will be eligible to enter the study if they meet all of the inclusion criteria and none of the exclusion criteria. Patients will be enrolled in the study either at the time of device implantation or if they have already undergone device implantation will be enrolled at next routine outpatient clinic visit. 