Absolute intrathoracic impedance based scheme to stratify patients for risk of a heart failure event

A method of operation of a medical system for determining whether a patient is at risk of exhibiting excessive fluid or dehydration, the method comprising:

• (a) acquiring from an implantable medical device memory a patient's absolute intrathoracic impedance data over a time period;
• (b) determining, through a processor, an average of the intrathoracic impedance data over the time period;
• (c) defining, through the processor, a target impedance range bounded by a first range and a second range, the first range being a lower value intrathoracic electrical impedance and the second range being a higher value of intrathoracic electrical impedance;
• (d) determining, through the processor, whether the average of the intrathoracic impedance data is outside one of the first and second ranges;
• (e) in response to step d, the processor determines a level of heart failure (HF) event risk based upon whether the average of impedance is outside the targeted impedance range;
• and (f) in response to step d, adjusting therapy delivered to a patient;
• wherein the system comprises a cardiac pacemaker and wherein the therapy is adjusted by switching to one of fusion pacing, biventricular pacing, and multisite pacing and using the pacemaker to deliver the adjusted therapy.