Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-04-12
2001-12-11
Kamm, William E. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06330477
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to implantable medical devices (IMDs), particularly implantable cardioverter/defibrillators (ICDS) for detecting and treating tachyarrhythmias, and more particularly to a method and apparatus for alleviating or preventing hemodynamic collapse under certain conditions precedent to or during delivery of an anti-tachyarrhythmia therapy by the ICD.
BACKGROUND OF THE INVENTION
By way of definition, the term “tachyarrhythmia” refers to any fast, abnormal rhythm of a heart chamber which may be amenable of conversion to a normal sinus rhythm by “cardioversion” or “defibrillation” or the application of certain anti-tachycardia pacing therapies to the heart chamber. Such tachyarrhythmias include ventricular tachycardia (VT), supraventricular tachycardia (SVT), ventricular flutter or fibrillation (VF), atrial tachycardia (AT), and atrial flutter or fibrillation (AF).
In the field of automatic implantable arrhythmia control devices, particularly ICDs (also referred to as pacemaker/cardioverter/defibrillators or PCDs), the terms “cardioversion” or “cardioverter” and “defibrillation” and “defibrillator” generally refer to the processes of and devices for discharging relatively high energy electrical shocks into or across cardiac tissue to arrest a life threatening tachyarrhythmia. In practice, the conversion of AT or VT or low rate AF or VF to normal sinus rhythm by a relatively low amplitude cardioversion shock delivered in timed synchrony with a sensed atrial or ventricular cardiac depolarization (P-wave or R-wave) is typically referred to as “cardioversion”. The conversion of malignant AF or VF by the same or higher energy shock delivered without such synchronization is typically referred to as “defibrillation”.
Implantable cardiac pacing functions are also currently incorporated into ICDs to supplant some or all of an abnormal heart's natural pacing function by delivering appropriately timed electrical pacing pulses to cause the chamber or chambers of the heart to contract or “beat”, i.e., to “capture” the heart. Either single chamber (atrial or ventricular) pacing functions or dual chamber (atrial and ventricular) pacing pulses are applied to the atria and/or the ventricles in response to a detected bradycardia or dissociation of the atrial and ventricular heart rates at a pacing rate to restore cardiac output that is appropriate to the physiologic requirements of the patient. Moreover, anti-tachycardia bursts of pacing pulses or single overdrive pacing pulses are applied to the atria and/or the ventricles to counter and convert certain low rate AT or VT episodes to normal sinus rates. Such pacing pulses have well-defined amplitude and pulse width characteristics which can be adjusted by remote programming and telemetry equipment to meet physiologic needs of the particular patient and device power conservation requirements.
It is known in the ICD art to provide a “tiered” therapy with regard to the termination of sensed tachyarrhythmias. The term “tiered” therapy has been used typically to describe the different tachyarrhythmia rate zones, such as low rate ventricular tachycardia (VT Low), high rate tachycardia (VT High), and VF. The term “tiered therapy” also has been used to describe the increasing degree of aggressiveness within each rate zone. For example, within each zone the physician may program the number and types of therapies to be delivered, the intervals between applied therapies, the energy level of the therapy, etc. Thus, based upon the detected rate of the arrhythmia, the ICD will continue to increase the level of aggressiveness until such arrhythmia is terminated.
Cardioversion is often thought of as being “low energy” since the heart has been known to successfully convert the arrhythmia with electrical shocks in the range of 0.1 to 10 joules. However, it is still classified as cardioversion if the arrhythmia is a ventricular tachycardia even if the energy required to convert the arrhythmia goes up to the maximum value that the ICD is capable of delivering. (e.g., 40 joules). Defibrillation, on the other hand, is often thought of as being “high energy” since the heart has been successfully defibrillated with energy shocks in the range of 11 to 40 joules. In contrast, the pacing pulses that are applied to a heart during normal or anti-tachycardia pacing are typically of much lower energy (e.g., between 50 and 200 micro joules). The principal difference between the types of therapy provided by an ICD supporting tiered therapy is that of arrhythmia detection and the programmed level of aggressiveness with the therapy typically starting by applying the lowest energy stimulation for that detected arrhythmia and working its way up to high energy shock therapy, as required.
To detect and classify tachyarrhythmias, one or two basic strategies are generally followed which start with identifying atrial and/or ventricular sensed events and deriving atrial and/or ventricular event intervals and/or rates therefrom. The event intervals are compared to programmed fibrillation detection interval ranges and tachycardia detection interval ranges and to suddenness of onset criteria and rate variability criteria to distinguish various tachyarrhythmias from one another. In the first strategy, the event intervals are compared with a preset group of criteria which must be met as precedent to detection or classification. As the episode progresses, the first set of criteria to be met results in detection and diagnosis of the tachyarrhythmia. The second strategy is to define a set of criteria for events, event intervals and/or rates which is generally indicative of a group of arrhythmias; following the satisfaction of those criteria, preceding or subsequent events are analyzed to determine which specific tachyarrhythmia is present. The Model 7219 ICD commercially available from Medtronic, Inc., employ an arrhythmia detection and classification system, generally as disclosed in commonly assigned U.S. Pat. No. 5,342,402, incorporated herein by reference in its entirety, which uses both strategies together. Numerous other or additional detection and classification systems have been proposed as set forth in commonly assigned U.S. Pat. Nos. 5,545,186, 5,782,876, and 5,814,079, incorporated herein by reference in their entireties.
Menus of such tachyarrhythmia detection criteria and anti-tachyarrhythmia therapies that are tailored to a range of detected tachyarrhythmias are typically provided in such current ICDs. The above-incorporated patents and U.S. Pat. Nos. 4,830,006, 4,726,380, and 4,587,970, all incorporated herein by reference in their entireties, illustrate such detection criteria and therapy menus. The particular therapies that are to be provided upon detection of particular tachyarrhythmias in an individual patient are remotely programmed into ICD memory by the physician. For example, upon sensing a low rate AT or VT, an ICD may attempt to terminate such a tachycardia by first applying a prescribed anti-tachycardia pacing therapy to the atria or ventricles, respectively. Such anti-tachycardia pacing therapies typically includes burst pacing, ramp pacing, adaptive pacing and/or scanning pacing, as is known in the art. See, e.g., U.S. Pat. Nos. 4,427,011 and 4,541,430 (burst pacing); 4,398,536 (ramp pacing); and 5,103,822 (scanning pacing); which patents are incorporated herein by reference in their entireties. Upon re-detection of the AT or VT, a more aggressive anti-tachycardia pacing therapy may be scheduled.
If the anti-tachycardia pacing therapy is unsuccessful or inappropriate, the ICD may be programmed to apply a low energy, cardioversion energy shock in an attempt to cardiovert the heart. In order to apply a cardioversion or defibrillation shock to a heart chamber, it is first necessary to charge one or more high voltage (HV) output capacitors of the ICD device with to a voltage providing the programmed energy upon discharge through the cardioversion/defibrillation electrodes. When the HV output capacitors are charg
Kamm William E.
Medtronic Inc.
Wolde-Michael Girma
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