It is generally necessary to provide some means for adjusting the pacing rate in an artificial heart pacemaker so as to tailor the operation of the device to the needs of the patient. In the case of an A-V sequential pacemaker, it is also generally necessary to provide some means for adjusting the delay interval between the atrial and ventricular stimulation pulses. Implantable pacemakers present their own problems in achieving these adjustment functions, and numerous techniques have been developed in the prior art to meet these needs. In the case of external heart pacemakers, in which the generating and control circuitry is in a device external of the body connected by electrode leads which pass through the skin to the patient's heart, control of pacing rate, A-V delay and possibly other operating parameter is generally accomplished by control knobs on the front panel of the device. Through the use of these controls the physician can adjust the timing parameters of the pacemaker to meet the patient's needs. However, external A-V sequential pacemakers in the prior art are subject to certain unwanted interactions between the control for rate and the control for A-V delay, in that changing the control knob setting for one control not only changes the intended timing function, but also causes an unwanted change in the other parameter.
It has long been recognized in the prior art that the basic heart beat rate to be maintained by an A-V pacemaker must be with reference to the ventricular beats, either spontaneous or stimulated, and not by reference to the atrial stimulation. This has caused certain complications in the design of satisfactory rate and A-V delayed generating circuits. If it were possible simply to reference basic rate to the atrial pulses, it would be a simple matter to provide circuits to time out a first interval for the atrial-to-atrial rate, and a second shorter interval following an atrial pulse to provide the A-V delay interval, and the two time intervals could easily be independently controllable. However, it has been recognized that such a system would lead to variable or erratic heart rate, because of the uncertainty in practice as to whether the ventrical may depolarize following a stimulated atrial depolarization but before the pacemaker would provide a ventricular stimulation output pulse. Since this situation can change from beat to beat, the time interval between successive ventricular depolarizations would vary by a certain amount under such a system, and this variation would not be acceptable.
Accordingly, generating circuits have been designed in the prior art to control the basic heart rate according to the ventricular depolarizations, either spontaneous or stimulated, while providing means for generating the atrial stimulation pulse at the proper time interval prior to the ventricular pulse. Generally these systems have used a wave or ramp generating circuit and a pair of variable voltage divider references, one for the atrial pulse and one for the ventricular pulse. Obviously, changing the control for the ventricular repetition rate completely changes the A-V interval, which would necessitate readjustment and monitoring of both quantities each time one was to be changed. To minimize this problem, it has been proposed in the prior art to use a pair of precision ganged potentiometers having a fixed resistance difference between them to operate both references simultaneously for rate, with a separate control for interval. However, prior art devices according to this approach are subject to the disadvantages of the relatively high cost of precision potentiometers and inaccuracies resulting from even slight mismatching of the precision potentiometers.
The present invention solves these and other problems by providing an improved rate and A-V delay generator in which the separate controls for ventricular rate and A-V delay interval are independent and have no unwanted couplings or interferences with each other's operation. This permits the physician to operate one control knob to vary the rate, while having no effect upon the A-V delay interval; and to adjust the delay knob to control the A-V delay while having no effect upon the basic ventricular rate. The independence of these controls results in quicker and easier adjustments by the physician with less chance of error. This independent adjustment feature is achieved accurately, efficiently, and without the cost and accuracy problems associated with matched potentiometers.