Abstract: A pacemaker is disclosed with a pacing rate that is restricted within a range defined by variable upper and lower rate limits that change as a function of the patient's detected level of activity. An indication such as the nonambient temperature of blood is sensed in the right ventricle of the heart to indicate detected activity level. Other activity indications, such as blood oxygen concentration and body motion, either singly or in combination are also disclosed. A control circuit of the pacemaker varies the upper and lower rate limits according to the patient's detected level of activity. The control circuit also varies the atrial and ventricular refractory periods along with the AV delay period in response to detected activity level changes.

Abstract: Method and appartaus are disclosed for stimulating the heart with a pacemaker in response to a nonambient temperature of a body, which is sensed, for example, in the right ventricle of the heart. The pacemaker stimulates the heart at one of at least three different nominal rates which have been selected by the physician for the various activity levels of the patient. When the heart is stimulated at an interim rate for mild or brief exercise conditions, the activity level of the patient is sensed using a motion sensor. A control circuit terminates a predetermined time period selected for the pacemaker to stimulate at the interim rate when the sensed level is less than a predetermined activity level.