Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-06-29
2001-07-03
Getzow, Scott M. (Department: 3767)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06256535
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to implantable cardioverter defibrillators, and, more particularly, to a method for delivering cardiac therapy (e.g., defibrillation/shock therapy), and a cardiac therapy device (e.g., an ICD) for implementing the same.
Implantable cardioverter defibrillators (ICDs) are sophisticated medical devices which are surgically implanted (abdominally or pectorally) in a patient to monitor the cardiac activity of the patient's heart, and to deliver electrical stimulation as required to correct cardiac arrhythmias which occur due to disturbances in the normal pattern of electrical conduction within the heart muscle. Cardiac arrhythmias can generally be thought of as disturbances of the normal rhythm of the heart beat. Cardiac arrhythmias are broadly divided into two major categories, namely, bradyarrhythmia and tachyarrhythmia. Tachyarrhythmia can be broadly defined as an abnormally rapid heart rate (e.g., over 100 beats/minute, at rest), and bradyarrhythmia can be broadly defined as an abnormally slow heart rate (e.g., less than 50 beats/minute). Tachyarrhythmias are further subdivided into two major sub-categories, namely, tachycardia and fibrillation. Tachycardia is a condition in which the electrical activity and rhythms of the heart are rapid, but organized. Fibrillation is a condition in which the electrical activity and rhythm of the heart are rapid, chaotic, and disorganized. Tachycardia and fibrillation are further classified according to their location within the heart, namely, either atrial or ventricular. In general, atrial arrhythmias are not life-threatening, because the atria (upper chambers of the heart) are only responsible for aiding the movement of blood into the ventricles (lower chambers of the heart), whereas ventricular arrhythmias are life-threatening, because if the ventricles become arrhythmic, the heart's ability to pump blood to the rest of the body is impaired. The most serious and immediately life-threatening type of cardiac arrhythmia is ventricular fibrillation, in which the electrical activity of the ventricles becomes so random and chaotic that the heart rapidly becomes unable to pump sufficient blood to sustain life.
In general, an ICD continuously monitors the heart activity of the patient in whom the device is implanted by analyzing electrical signals, known as electrograms (EGMs), detected by endocardial (intracardiac) sensing electrodes positioned in the right ventricular apex and/or right atrium of the patient's heart. More particularly, contemporary ICDs include waveform digitization circuitry which digitizes the analog EGM produced by the sensing electrodes, and a microprocessor and associated peripheral ICs which analyze the digitized EGM in accordance with a diagnostic algorithm implemented by software stored in the microprocessor. Contemporary ICDs are generally capable of diagnosing the various types of cardiac arrhythmias discussed above, and then delivering the appropriate electrical stimulation/therapy to the patient's heart, in accordance with a therapy delivery algorithm also implemented in software stored in the microprocessor, to thereby correct or terminate the diagnosed arrhythmia.
In this connection, contemporary ICDs are capable of delivering various types or levels of electrical therapy. The first type of therapy is bradycardia and antitachycardia pacing, in which a low level of electrical energy (generally between millionths to thousandths of a joule) is delivered to the patient's heart in order to correct detected episodes of bradycardia or tachycardia, respectively. The second type of therapy is cardioversion, in which an intermediate level of electrical energy (generally between 1-5 joules) is delivered to the patient's heart in order to terminate a detected episode of ventricular tachycardia (e.g., a detected heart beat in the range of 130-190 beats/minute) or an ongoing episode of tachycardia that antitachycardia pacing has failed to correct or terminate. The third type of therapy is defibrillation, in which a high level of electrical energy (generally above 15 joules) is delivered to the patient's heart in order to abort a detected episode of ventricular fibrillation or an episode of ventricular tachycardia which has degraded into ventricular fibrillation due to failure of cardioversion therapy.
The provision of the above-described different types or levels of therapy is generally referred to in the art as “tiered therapy”. In this regard, contemporary ICDs which are capable of delivering tiered therapy are sometimes referred to as combination pacemakers/defibrillators or as implantable cardioverter-defibrillators. As used herein, the terminology “implantable cardiac defibrillator” (ICD) is intended to encompass these and other forms and types of implantable cardiac therapy devices.
Current-generation ICDs which are capable of delivering tiered therapy provide several advantages over previous-generation ICDs which were only capable of delivering high energy defibrillation therapy. Namely, ICDs which are capable of delivering tiered therapy are generally more energy-efficient, since they can deliver much lower energy therapy, such as antitachycardia pacing and cardioversion, to terminate many arrhythmia events before they degrade into a ventricular fibrillation event. The much higher energy defibrillation therapy is only necessary when these lower energy therapies fail to abort the arrhythmia. Thus, tiered therapy conserves the energy stored in the battery(ies) of the device, thereby extending the longevity of the device, and also enables a significant portion of potential ventricular fibrillation events to be aborted with lower energy therapy which is much less painful and uncomfortable to the patient.
A primary goal in the design and further development of ICDs is to ensure delivery of effective therapy with a minimum expenditure of energy. Reduction of the total energy required to deliver effective therapy enables the size of the batteries and capacitors used in the ICDs to be reduced, thereby enabling a commensurate reduction in the size of the ICD. The benefits to the patient are two-fold. First, the use of lower voltage cardioversion and defibrillation therapy reduces patient pain and discomfort during delivery of such therapy, and second, the reduction in the size of the ICD decreases patient discomfort due to the physical pressure exerted by the ICD within the patient's body. A further benefit is that the longevity of the device can be extended for a given power supply. Additionally, the smaller the ICD, the easier it is to implant the device using minimally invasive surgery, thereby decreasing the cost of implantation. In this regard, it is highly preferable that the ICD be at least small enough to be implanted pectorally, rather than abdominally, without sacrificing functionality, because pectoral implantation requires much less invasive surgery than abdominal implantation. Consequently, pectoral implantation is both much less costly and much more comfortable to the patient (both at the time of implantation and thereafter), than abdominal implantation.
One of the major areas of ongoing R&D within the field of ICDs is the development of increasingly sophisticated diagnostic and therapy delivery algorithms, which enable the above-stated primary ICD design goal to be realized by optimizing the therapeutic efficacy of the device. More particularly, in accordance with the diagnostic algorithm, the microprocessor and associated peripheral ICs continuously monitor the digitized EGMs in order to sense or detect various features thereof, e.g., waveform slope (dv/dt), waveform minima and maxima, intervals between specified cardiac events, etc., which are indicative of various prescribed cardiac events, e.g., (P)QRS complexes, depolarization, repolarization, tachycardia, bradycardia, fibrillation, etc.
When a specified cardiac event is detected, the microprocessor, under the control of the therapy delivery algorithm,
Pless Benjamin D.
Province Rose
Getzow Scott M.
Mitchell Steven M.
Pacesetter Inc.
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