Patent ID: 7207947

Claim:
In an implantable medical device for implant within a patient, a method for detecting a circadian state of the patient, the method comprising: detecting changes in one or more blood carbon dioxide (CO 2 ) parameters; determining the circadian state of the patient based upon the changes in the one or more blood CO 2 parameters; detecting minute ventilation levels of the patient and wherein the step of determining the circadian state of the patient additionally takes into account the detected minute ventilation levels; determining a ratio of minute ventilation to blood CO 2 levels of the patient and wherein the step of determining the circadian state of the patient additionally takes into account the determined ratio; and detecting activity levels of the patient; wherein detecting changes in selected blood CO 2 parameters is performed to detect changes in etCO 2 levels and changes in Δ cycle CO 2 ; and wherein the step of determining the circadian state of the patient takes into account a combination of Δ cycle CO 2 levels, etCO 2 levels, activity levels and minute ventilation levels.