Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-11-25
2001-06-19
Jastrzab, Jeffrey R. (Department: 3737)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S019000
Reexamination Certificate
active
06249700
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to implantable medical interventional devices which provide a range of pacing, cardioversion, and defibrillating functions to preserve the life and stamina of the patient, and more particularly to an implantable defibrillator that exhibits improved hemodynamic response and enhanced myocardial stability to vastly improve the quality of life of the patient. As used herein, the terminology “implantable defibrillator” is intended to refer broadly to a device which is adapted to perform a variety of essential cardiac interventional functions, as is typically the case in actual medical device practice, and not merely limited to defibrillation therapy.
Administering therapy from implantable defibrillators has proven to be highly effective in preventing sudden cardiac death. Nonetheless, many patients provided with defibrillators suffer from myocardial failure attributable to serious underlying disease that contributes to electrical instability and reduced myocardial function. The determinants of cardiac output (the volume of blood discharged from the ventricle per minute), especially during exercise, are volume (the volume of blood discharged from the ventricle with each contraction) and heart rate (cardiac output=stroke volume×heart rate). While the normal heart is capable of increasing its stroke volume by a factor of 50% when the patient goes from conditions of rest to exercise, the majority of patients who are candidates for an implantable defibrillator lack that degree of contractile reserve. For such patients it is essential that the implanted device adapt the heart rate to closely if not precisely match the limited cardiac output to the needs of the patient's body.
While a healthy person or a patient who may be only slightly myocardially compromised has mechanisms that enable his or her cardiac output to adapt to a wider variation of stroke volume, the typical defibrillator patient lacks any such mechanism by which to adapt, and instead predominantly adjusts cardiac output by means of a modification of heart rate. But if the patient's heart rate is too low for a given exercise load, an increase in endiastolic left ventricular filling pressure is experienced. In essence, the heart is simply incapable on its own of pumping sufficient blood into systemic circulation, which results in congestion of the pulmonary system and reduced oxygen pressure, and also affects the stability of the myocardium. Increased endiastolic pressures cause an increased stress to the myocardial wall which is a factor in the triggering of ventricular extrasystoly (i.e., premature ventricular contraction or PVC). Although the malady is commonly experienced in otherwise relatively healthy adults who engage in heavy smoking or experience severe emotional excitement, it is most of ten encountered to be of multifocal origin in cases of organic heart disease or digitalis intoxication, and can lead to ventricular tachycardia, and ultimately, ventricular fibrillation.
In the past, a wide variety of sensors has been proposed for potential control of ventricular rate. But not all of the potential sensor signals are suitable for heart rate control in patients needing an implantable defibrillator. Control that produces a heart rate which is either too slow or too fast in terms of the patient's metabolism, is inappropriate. A sensor that produces these types of improper responses, for example because of its sensitivity to environmental noise sources or to other phenomena which are not matched to the body metabolism, is unsuitable for use in implantable defibrillators or other cardiac interventional devices.
It is therefore a principal aim of the present invention to provide an implantable defibrillator with improved hemodynamic response, and which provides greater myocardial stability. The desire is to achieve these results by use in the medical interventional device of a particularly suitable and effective rate control signal, so that the frequency of device intervention by delivery of either cardioverting or defibrillating shocks will be substantially reduced. Ultimately, although the device is intentionally implemented to deliver such therapy repeatedly despite its battery-operated nature, a marked reduction in the number of times the patient will receive shocks from the device by virtue of a more circumspect hemodynamic response of the device will substantially lessen duress on the patient's myocardial function and other aspects of his physiology, including orthopedic distress, for example, and with it, considerably less pain and general discomfort to the patient. Furthermore, reducing the number of shocks that must be generated by the device is effective to conserve energy and will thereby prolong the useful life of the device. A more appropriate rate control can also serve to increase the patient's capacity for exercise, and with it, improve the patient's quality of life.
SUMMARY OF THE INVENTION
The present invention provides means for limiting the tendency of a diseased heart to undergo pathologic increases in heart rate—such as extrasystole, reentrant tachycardia, and so forth—so that less need will exist for treating accelerated heart rate by means of debilitating shocks to the patient's heart. With this emphasis on what might be termed prevention rather than cure, not only will the patient experience less injury to cardiac and other body tissue, but battery life of the implanted device will increase and the frequency of subjecting the patient to surgical procedures for device replacement will decrease. Since state of the art automatic implantable cardioverter/defibrillators (sometimes referred to in short as “AICDs” or “ICDs”) possess the capability to provide all of the conventional pacing functions as well as to provide the therapies necessary for antitachycardia, cardioversion and defibrillation, the potential of fered by the pacing function is conveniently examined in the effort to reduce dependence on cardioverting and defibrillating functions of the device.
Another aspect of the invention is to prolong the battery life of the device by reducing the pacing rate at prolonged resting periods such as during nighttime hours. While a low pacing rate such as 40 or 50 beats per minute (bpm) certainly is too low to provide adequate hemodynamics during daytime hours with activity, the accelerometer or other sensor controlled rate adaptation allows therefore an adequate increase with physical or emotional exercise. The in pacing rate can be either linked to a clock function inside the device or adjusted to periods of activity or inactivity detected by the sensor. In case of prolonged periods of inactivity and clearly identified periods of activity, the device can define day and night phases over the course of a certain time window. A combination of the clock function and sensed activity or rest is used to refine the pacing rate including further reducing it in those prolonged periods of inactivity.
In one of its aspects the present invention seeks to optimize a match between the pacing functions of the implanted device and the patient's metabolism. By achieving a better match of the pacing function to the heart rate of a normal healthy subject, within all of environmental conditions the patient is likely to encounter in the pursuit of a healthy lifestyle, the cardiac function will be improved in at least two critical ways. First, matching decreases endiastolic filling pressure to reduce the chances of intrinsic rhythm disorders by reducing the stress factors, and second, matching operates to prevent extrasystoly by overdrive suppression. Overdrive suppression may be used when the patient suffers a pathologic ventricular tachycardia, for the purpose of establishing a heart rate that exceeds the resting heart rate by, say 10 to 40 bpm. This shortens the Q-T interval and the ventricle's refractory period. The goal is to prevent ventricular ectopy (i.e., an aberrant impulse that has its orig
Blank Rome Comisky & McCauley LLP
Jastrzab Jeffrey R.
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