Abstract: A multi-pole pacing/sensing lead is used to measure the instantaneous impedance in the right ventricle of a patient's heart. The resulting impedance waveform is signal processed to obtain a measure of the time interval beginning with the occurrence of a paced beat or a spontaneous QRS complex (systole marker) and ending with the point where the impedance versus time signal crosses the zero axis in the positive direction following the paced or spontaneous QRS complex initiating the interval or some other predetermined point along the positive-going waveform. The resulting time interval is inversely proportional to the contractility of the heart and is found to decrease with exercise and the introduction of catecholamines. Thus, it can be used as a control parameter for a demand-type cardiac pacemaker. A means for obviating changes in pacing rate due to long-term drift in the impedance sensing circuitry is also disclosed.

Abstract: A cardiac pacer of the type having an implantable pulse generator and electrical leads deployed in the right ventricular chamber of the heart. The lead has a stimulating electrode as well as a plurality of sensing electrodes coupled to the pulse generator. Certain control circuitry associated with the implantable pacer provides for measurement of the impedance variations occasioned by the pumping action of the heart and a differentiating circuit operatively coupled to the impedance sensing circuit for developing a control signal corresponding to the rate of change of the impedance with respect to time. This control signal may be coupled to the timing circuit of the implantable pulse generator, allowing the pacing rate to be altered in accordance with the level of physiologic activity of the person in whom the pacer is implanted.