Patent ID: 7603170

Claim:
A system comprising: a medical device, including: an implantable trans-thoracic impedance measurement circuit to provide a trans-thoracic impedance signal of a subject; and a controller coupled to the trans-thoracic impedance circuit, the controller configured to: store a reference baseline value of a DC component of a trans-thoracic impedance; measure a more recent value of the DC component of a trans-thoracic impedance different from the reference baseline value; adjust a breathing volume calibration factor using the reference baseline value and the more recent measured value of a DC component of a trans-thoracic impedance, wherein the breathing volume calibration factor translates a measure of impedance to a measure of breathing volume, and wherein the adjusted breathing volume calibration factor includes a ratio of a reference DC trans-thoracic impedance to a more recent measured value of the DC trans-thoracic impedance; extract a respiration signal from the trans-thoracic impedance signal, the respiration signal including a respiration amplitude; and compute a breathing volume of the subject using the respiration amplitude scaled by the adjusted breathing volume calibration factor.