Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-02-25
2001-05-29
Evanisko, George R. (Department: 3737)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S009000
Reexamination Certificate
active
06238422
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to cardiac pacing systems with the capability of preventing dangerous cardiac conditions, for example, implantablc pacing systems that provide preventive therapy for minimizing the risk of myocardial infarction, especially during morning hours.
BACKGROUND OF THE INVENTION
Modern cardiac pacing systems have, in recent years, sought to incorporate at least some capability of detecting and dealing with various arrhythmias. For example, pacemaker designers have attempted to provide various special modes of pacing to control the effects of atrial and/or ventricular arrhythmias, particularly tachycardia and fibrillation. However, relatively little has been done to provide for preventive pacemaker therapy. For example, there is no known pacemaker therapy for preventing, or minimizing the risk of myocardia infarction (MI).
An early procedure adopted in pacemaker design for dealing with cardiac arrhythmias generally was simply to switch into an asynchronous mode, providing ordinary fixed rate asynchronous ventricular pacing. Such a response is a help, but is recognized to have limitations. Other more sophisticated response schemes have been adopted, with varying degrees of success. However, rather than simply trying to control the heart after the onset of a dangerous arrhythmia, it is to be recognized that it is preferable to anticipate when a patient may be potentially subject to a dangerous cardiac condition; and to take preventive action which aims to deal with the circumstances which give rise to the condition and thereby minimize its probability of occurrence. Relatively little work has been done in this area of preventive pacing therapies.
This invention is directed particularly toward preventing and/or responding to ventricular arrhythmias which have an increased incidence in the morning, after awakening. It is known from the literature that malignant arrhythmias, and types of acute myocardial infarctions, have a greater incidence in certain patients shortly after awakening. It is suggested that there is a circadian variability in the incidence of sudden cardiac death, with a peak in the early morning hours. See “Insights Into the Pathogenesis of Sudden Cardiac Death From Analysis of Circadian Fluctuations of Potential Triggering Factors,” Hohnloser and Klingenheben, PACE, Vol. 17, March 1994, Part II, pp. 428-433. More specifically, as this reference indicates, there has been interest in exploring the relation between disturbances of ventricular repolarization, or prolonged QT interval, and sudden cardiac death. The distribution of occurences of MI over the 24-hour day reveals an increased risk shortly after the time of awakening and arising.
Other investigators likewise have discussed the potential correlation of such malignant arrhythmias and abnormal myocardial repolarization, or prolonged QT interval. It is known that the QT interval prolongs during sleep, and this prolongation extends into the arousal period. It is suggested that this QT interval prolongation may play an important role with respect to the diurnal variation of some ventricular arrhythmias. See, for example, “Prolongation of the Q-T Interval In Man During Sleep, “Browne et al,.
The America Journal of Cardiology
, July 1983, Vol. 52, pp. 55-59. See also “Electrocardiographic Repolarization During Stress From Awakening on Alarm Call,” Tiovonen et al., JACC, Vol. 30, No. Sept. 3, 1997, pp. 774-779. It is known that the QT interval can vary independently of rate, and particularly that QT interval prolongs during sleep independent of any change of the RR interval. The cause of such QT variation, and the corresponding effect on cardiac rate stability, are the subject of considerable debate and analysis. However, there does appear to be a consensus that QT interval variability reaches a peak shortly after awakening, and that the time of this peak corresponds to the period of increased vulnerability to MI and ventricular tachycardias. Further, there is an inertia in adaptation of the QT interval during the period after awakening, i.e., the arousal period. By inertia it is meant that the QT interval does not directly shorten, but remains relatively prolonged, as patient heart rate increases during the time of arousal.
It is a basis of this invention that the delay in QT shortening, which results in a longer than normal QT interval after awakening, can result in an insufficient diastolic filling time for certain patients. Thus, if ventricular repolarization is extended relative to the patient's RR interval, the time available for diastolic filling is shortened. This in turn would lead to the result that insufficient blood is made available to flow through the myocardium, leaving the patient vulnerable to myocardial infarction.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an implantable pacemaker device and system which obtains and utilizes data reflective of patient QT variations, and in particular circadian variations in the QRS-T signal which are indicative of reduced diastolic filling time at and shortly after patient awakening. The object of the invention is to provide an implanted device for providing preventive therapy, which device is particularly useful for patients known to have a prolonged QT interval, to be subject arrhythmias, or patients who already have myocardial ischemia or infarction. The pacemaker device of the invention provides therapy in the form of overdrive pacing to slightly increase pacing rate before the patient awakening time, so as to manage shortening of the QT interval and better provide sufficient diastolic filling time during the dangerous period following awakening.
In accordance with the above objects, the pacemaker of this invention provides for determining, each night, an expected start of daytime, or awakening of the patient. A preventive therapy routine is initiated at a predetermined time, e.g., 30 minutes, prior to awakening, suitably by slowly increasing patient rate above the underlying intrinsic rate, in order to obtain a healthy QT/R ratio for the patient by time awakening arrives. In other words, the pacemaker's pre-awakening therapy anticipates delayed QT prolongation during awakening which would otherwise occur, and programs an increase in paced heart rate so as to achieve a desired target QT or QT/R ratio, by time of awakening. In addition, the pacemaker monitors QT and R for the remainder of a predetermined morning period after awakening, and if desired QT/R criteria are not met, further responsive action is taken, e.g., inhibiting synchronous tracking of atrial signals and limiting the pacemaker rate response feature.
In a first simple version of the invention, the patient's data is either inputted by the physician or collected continuously and used to determine a target QT interval, or QT
targ
, which is a measure of the minimum acceptable diastolic filling time at the time of awakening. This value is used as the criteria for increasing rate before awakening and also for limiting synchronous tracking and rate response. In another version, the pacemaker computes a value of diastolic filling time (DFT) as a function of QT interval, AV interval and RR interval, and modifies pacing rate before and even after awakening to achieve, within predetermined tolerances, the computed value.
The preferred embodiments of the invention are illustrated in terms of utilizing the QT interval as an indirect measure of diastolic filling time. The invention also embraces the use of other data from the QRS-T signal for obtaining data indicative of diastolic filling time, and hence controlling the rate variation during the morning.
REFERENCES:
patent: 5065759 (1991-11-01), Begemann et al.
Djordjevic, Milan, et al., “Circadian Variations of Heart Rate and STIM-T Interval: Adaptation for Nighttime Pacing,”PACE, vol. 12, Nov. 1989, pp. 1757-1762.
Toivonen, Lauri et al., Electrocardiographic Repolarization During Stress From Awakening on Alarm Call, JACC, vol. 30, No. 3, Sep. 1997:774-779.
Hohnloser, Stefan
Evanisko George R.
Jaro Michael J.
Medtronic Inc.
Patton Harold
Woods Thomas F.
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