An implantable cardiac device and method monitors a patient's body impedance so as to diagnose medical problems, such as fluid balance problems in congestive heart failure patients. The device monitors the patient's position and determines an appropriate time for collecting body impedance data when the patient is in repose. The device then obtains a series of measurements of thoracic impedance by transmitting a signal on at least one lead, which may be a standard pacemaker lead, and sensing a signal received at a different point. The series of measurements are combined to obtain desired representations of thoracic impedance. In an embodiment, impedance is represented by an extracellular water (ECW) resistance value and an intracellular water (ICW) resistance value determined using a Cole calculation method.
