Image Number 12 for United States Patent #5031615.
An implantable activity-based cardiac pacemaker has a variable rate stimulus generator housed together with an accelermeter and associated circuitry within a case. The accelerometer is isolated from the case to detect forces of acceleration on the case while avoiding response to pressure on the case, and to generate an electrical signal representative of the magnitude of the accelerational forces. The accelerometer is integrated into hybrid electronic circuitry, such as a semiconductor chip, together with an electrical circuit which processes the signal generated by the accelerometer to vary the rate of the stimulus generator according to the magnitude of the signal. Either the accelerometer is configured as a mechanoelectrical transducer having a natural frequency response characteristic limited to a range below 4 Hz, or the electrical circuit itself includes an electrical filter having such a frequency response characteristic, so that the pacemaker is capable of distinguishing accelerational forces indicative of true physical exercise of the patient in whom the pacemaker is implanted.