A health care system acquires data determines whether a patient is at risk of hypervolemia or hypovolemia. The method comprises (a) acquiring from a device memory a patient's absolute intrathoracic impedance data over a pre-specified time period, (b) determining a running average of the intrathoracic impedance data over the pre-specified time period, and (c) determining by the system whether the running average of the intrathoracic impedance data over the pre-specified time period exceeds one of a first and second range, the first range being a higher value boundary of intrathoracic electrical impedance and the second range being a lower value boundary of intrathoracic electrical impedance.