Abstract: An implantable rate responsive pacemaker uses the physiological demand parameter to determine the classification of the patient (i.e., his degree of congestive heart failure). For this purpose, the parameter is monitored for extended time periods (i.e., several days) to determine the levels for said parameter corresponding to different levels of physical activity, such as maximum physical activity or rest. The variation in these levels is then translated into corresponding classification (0-IV) using a preselected criteria.