Therapy using perturbation and effect of physiological systems

The disclosure herein relates generally to methods and devices for use in treating a patient by utilizing a response index associated with a selected organ. The selected organ may be assessed by comparing the response index to psychological parameters monitored in response to one or more perturbations. For example, the status of the selected organ may be assessed by comparing the relationship between one or more perturbations (e.g., electrical stimulation of the efferent vagus nerve) and one or more effects associated with the one or more perturbations (e.g., electrical activity effect of the vagus nerve, effect on the sympathetic efferent pathways, etc.). Information derived by the comparisons using the response index may be used to initiate or adjust therapy delivered by medical devices such as an implantable medical device (IMD) (e.g., a pacemaker (PM), an implantable cardioverter defibrillator (ICD), neuromodulation system, etc.).