Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-11-17
2001-11-27
Kamm, William E. (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
Reexamination Certificate
active
06324422
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to implantable cardiac therapy devices, and more particularly, to improved methods for sensing arrhythmias in a pacemaker/defibrillator, and a pacemaker/defibrillator configured or programmed to implement 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.
A depolarization signal, which is a small electrical impulse, triggers contraction of the myocardial tissue of the human heart. In this regard, the beating of a human heart is manifested by depolarization signals corresponding to the contraction of the atria, referred to as P-waves, and to the contraction of the ventricles, referred to as R-waves. The complex of depolarization signals produced by a normal heart beat is commonly referred to as the PQRS or QRS complex. The sequence of PQRS complexes produced by a beating heart constitutes an electrogram or electrocardiogram signal (depending upon whether the signal is detected within or outside of the heart, respectively) that can be monitored by appropriate electrical circuitry to determine the condition of the heart.
In general, an implantable pacemaker includes sensing circuitry which monitors the heart by analyzing electrograms (EGMs) detected by endocardial (intracardiac) sensing electrodes positioned in or adjacent to the patient's heart, and pacing circuitry that delivers anti-bradycardia pacing pulses to the heart upon detection of bradycardia, in order to thereby stimulate or pace the heart back into a normal sinus rhythm. More particularly, if the heart does not beat naturally (on its own) within a prescribed time period, (i.e., if an intrinsic heart beat is not detected within a prescribed time period), then an electrical stimulation pulse (pacing pulse) is provided to force the heart muscle tissue to contract, thereby assuring that a prescribed minimum heart rate is maintained. Dual-chamber pacemakers are capable of detecting either atrial or ventricular bradycardia, and delivering the appropriate atrial and/or ventricular anti-bradycardiapacing pulses as required.
In general, an ICD continuously monitors the heart activity of the patient in whom the device is implanted by analyzing electrograms (EGMs) detected by endocardial (intracardiac) sensing electrodes positioned in the right ventricular apex and/or right atrium of the patient's heart. 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 to thereby correct or terminate the diagnosed arrhythmia. As used herein, the terminology “implantable cardioverter defibrillator” (ICD) is intended to encompass these and other forms and types of implantable cardiac therapy devices.
It is common in implantable cardiac stimulation devices such as pacemakers and ICDs to employ “refractory periods” during which the sensing circuits of the device are inhibited in order to prevent false detection of a cardiac depolarization. More particularly, refractory periods are necessary in such implantable cardiac stimulation devices in order to prevent “oversensing”. Oversensing is a phenomenon in which a normal cardiac event associated with a depolarization, such as the repolarization of cardiac tissue, referred to as the T-wave, or an afterpotential generated by a paced depolarization, is sensed and incorrectly determined to be a separate and natural depolarization. Thus, the “refractory period” is defined as the period of time immediately following a natural or induced depolarization during which sensing is inhibited in order to prevent oversensing.
U.S. Pat. No. 3,648,707, issued to Greatbatch, discloses a dual-chamber rate responsive pacemaker which is adapted to operate in an atrial synchronous mode. This type of pacemaker is generally referred to in the art as a “VDD” pacemaker. The Greatbatch pacemaker includes electrodes for sensing contractions of the atrium and ventricle, and a pulse generator for pacing the ventricle. After sensing a contraction of the ventricle or pacing the ventricle, a lower rate timer is restarted. If this timer expires, it triggers generation of a ventricular pacing pulse. The Greatbatch pacemaker also includes an A-V interval timer, initiated in response to the sensing of an atrial contraction. On expiration of the timer, the ventricular pacing pulse is triggered. The Greatbatch pacemaker also includes a third timer, defining an upper rate interval initiated following ventricular pacing or sensing of a ventricular contraction. During the upper rate interval, time out of the A-V interval will not trigger a ventricular pacing pulse. This allows for inhibition of the ventricular pulse generator in the event that a natural atrial contraction follows a ventricular contraction. The Greatbatch pacemaker also uses a maximum synchronous pacing rate corresponding to the upper rate interval. If the atrial rate exceeds this rate, the pacing rate is lowered to the higher of one-half of the sensed atrial rate or the rate determined by the lower rate timer. In this way, pacemaker induced or mediated tachycardias (PMTs) are prevented.
U.S. Pat. No. 4,059,116, issued to Adams, discloses a VDD dual-chamber rate responsive pacemaker which, rather than preventing generation of a ventricular stimulus in response to time out of the A-V interval during the upper rate interval, instead delays the ventricular stimulus until the expiration of the upper rate interval. In addition, the Adams pacemaker utilizes a post-ventricularatrial refractory period (PVARP) after each ventricular pacing pulse and each sensed ventricular contraction, during which an atrial contraction does not initiate timing of the A-V interval. Because of these features, the Adams pacemaker exhibits an improved response to atrial contractions occurring at intervals less than the upper rate interval. The Adams pacemaker was programmed to generate ventricular stimulation pulses separated by the upper rate interval, displaying gradually lengthening A-V intervals until an atrial contraction fell within the post-ventricular atrial refractory period. The Adams pacemaker would then resynchronize on the next subsequent atrial contraction, mimicking the natural condition known as Wenckebach behavior. In commercially marketed pacemakers employing the Adams invention, the behavior of the pacemaker in the presence of high natural atrial rates is referred to as “Pseudo-Wenckebach” upper rate behavior.
Numerous other dual-chamber pacemakers have been proposed which vary their post-ventricular atrial refractory periods (PVARPs) in an attempt to prevent PMTs. For example, U.S. Pat. No. 4,920,965, discloses a dual-chamber pacemaker in which a post-ventricular atrial refractory interval is calculate
Olson Timothy
Williams Michael O.
Kamm William E.
Mitchell Steven M.
Pacesetter Inc.
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