Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1998-07-02
2001-08-07
Shay, David M. (Department: 3739)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S017000, C607S019000
Reexamination Certificate
active
06272381
ABSTRACT:
This invention relates to rate-responsive pacemakers, and more particularly to closed-loop control, rate-responsive pacemakers.
In order to satisfy the metabolic needs of a patient, it is advantageous to implant a rate-responsive pacemaker. Such a device responds to some rate control parameter (RCP) which is indicative of the body's need for cardiac output. The measured value of the rate control parameter (“MRCP”) is used to adjust the pacing rate. In the copending application of Frank Callaghan entitled “Rate Responsive Pacing Using the Magnitude of the Depolarization Gradient of the Ventricular Gradient,” Ser. No. 810,877 filed on Dec. 18, 1985, which application is hereby incorporated by reference, there are described numerous rate control parameters which may be used. The particular parameter which is the focus of that application is the depolarization gradient—the integral of the QRS segment of an evoked potential. The magnitude of the depolarization gradient has been found to be an excellent indicator of cardiac output needs.
One of the most formidable problems in designing a rate-responsive pacemaker is to devise an algorithm which relates the MRCP to pacing rate—even assuming that the MRCP is measured correctly. It would be highly advantageous to provide a closed-loop control system for a rate-responsive pacemaker. Such a negative feedback system would allow the control of pacing rate automatically. Instead of having to derive or look up in a table the value of pacing rate which is to be set for each MRCP, a closed-loop control system would simply change the rate in the direction which tends to keep the MRCP constant. If the MRCP tends to change in either direction, the rate adjusts in a direction which returns the MRCP to its value before the change.
What makes the depolarization gradient an excellent rate-control parameter is that increased stress (including both emotional stress and exercise) causes the depolarization gradient to decrease while an increased heart rate causes the depolarization gradient to increase. It is the opposite effects which stress and heart rate have on the depolarization gradient that permits a closed-loop control system to be effected. An increase in stress causes the MRCP to decrease. In the case of increased stress, what is desired is an increased rate. Thus the pacemaker is made to respond to a decrease in MRCP by increasing its rate. But when the rate increases, so does the MRCP; the MRCP increase is in a direction which is opposite to the original MRCP decrease. When the increase in MRCP due to faster pacing cancels the original decrease, the pacing rate stops increasing. The governing rule is that when there is a change in MRCP, the rate is changed so that MRCP moves in the opposite direction, until MRCP is restored. This is a negative feedback system, and it avoids the need for a complex relationship between a measurement value and the way in which the pacing rate should change.
Reference is made to an article entitled “Central Venous Oxygen Saturation for the Control of Automatic Rate-Responsive Pacing,” Wirtzfeld et al, PACE, Vol. 5, Page 829, November-December 1982. The thesis of this article is that central venous oxygen saturation represents the only rate-control parameter which is suitable for the realization of a closed feedback loop. The thesis is incorrect because the depolarization gradient is another parameter which allows closed-loop control. One of the major advantages which the depolarization gradient has over central venous oxygen saturation is that an additional sensor is not required. The cardiac signal which appears on the pacemaker lead can be processed such that the depolarization gradient is determined without requiring the provision of an additional sensor.
Unfortunately, simply selecting a rate-control parameter which theoretically lends itself to closed-loop control is not enough. The object of a closed-loop control system is to maintain a control parameter constant. In the case of a rate-responsive pacemaker, it would appear that it is the MRCP which must remain constant: any change in MRCP caused by stress controls a change in rate which returns the MRCP to the desired (constant) value. (A particular MRCP is suitable, of course, only if maintaining the MRCP constant indeed provides the desired pacing rates for all metabolic needs to be handled.) The problem is that rate control parameters can change not only with stress, but also due to other factors. The most important of these is perhaps drugs. Many rate control parameters are affected by the taking of drugs. Thus if an MRCP increases due to the patient having taken a drug, and no change has otherwise taken place in his metabolic needs, it is not desirable for the pacing rate to change in such a way that the MRCP will be returned to its previous value. Furthermore, the operation of a mechanical or chemical sensor may change with time. Even when measuring the depolarization gradient and using it as a rate-control parameter, if for one reason or another the lead shifts in position it is possible for a shift to appear in the MRCP. In such a case, without some way to compensate for a non-stress change in MRCP, a closed-loop control system would effect a permanent shift in pacing rate. The Wirtzfeld et al closed-loop control system did not provide compensation for this type of shift in the rate-control parameter; as far as we know, the Wirtzfeld et al pacemaker has not been commercialized. It is anything but a simple matter to compensate for drifts in a rate-control parameter which are unrelated to stress.
It is a general object of our invention to compensate for the effects of lead maturation, drug therapy and other non-stress related phenomena in a closed-loop rate-responsive pacemaker.
Stated briefly, in accordance with the principles of our invention the pacing rate is not changed in a direction which tends to keep MRCP constant. Instead, the pacing rate is adjusted in accordance with a parameter denominated as (MRCP-target), where target is a value indicative of changes in MRCP due to non-stress and non-rate factors (such as changes which result from lead maturation, drugs, etc.). Exactly how the value of target is derived requires careful analysis, although once the concept is grasped it will be seen that there are only three simple rules which must be employed. How target is derived will be described in detail below.
In the illustrative embodiment of the invention, the depolarization gradient is the integral of the QRS waveform of an evoked potential. Not every QRS waveform must be processed, but QRS waveforms must be integrated often enough to allow MRCP up-dating to follow metabolic changes. Since it is evoked potentials which are measured in the illustrative embodiment of the invention, this means that periodically stimulated beats must take place, as opposed to intrinsic beats. Some way must be found to pace the heart periodically even if pacing pulses are not otherwise required. This is accomplished by increasing the pacing rate on an individual beat basis to just above the intrinsic rate when an MRCP sample is required.
Because the heart is paced in the illustrative embodiment of the invention approximately every fourth beat in order that an MRCP sample be taken, it is especially important to use the lowest possible energy in each pulse in order to extend battery life and to minimize distortion of MRCP due to lead polarization. It is therefore particularly important to provide improved automatic output regulation (control of output energy), a type of self-adaptation. This involves testing for the lowest possible pulse output energy which results in heart capture, a concept not unknown in pacing but one which has not achieved high grades for successful implementation.
What often confounds an automatic output regulation circuit is a fusion beat. A fusion beat is a combined intrinsic and paced event; the pacemaker does not have enough time between start of the intrinsic beat and timeout of the escape interval to inhibit generation of a stimulus.
Callaghan Francis J.
Malkin Robert A.
Vollmann William
Gottblieb, Rackman & Reisman, P.C.
Pacesetter Inc.
Shay David M.
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