Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-12-22
2002-08-13
Evanisko, George R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06434424
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of external and implantable cardiac pacing systems incorporated into cardiac pacemakers or implantable cardioverter/defibrillators (ICDs), particularly atrial and ventricular synchronous pacing systems, wherein ventricular heart rate is stabilized or regularized during atrial tachyarrhythmias, particularly atrial fibrillation.
BACKGROUND OF THE INVENTION
Episodes of atrial tachyarrhythmias, including atrial fibrillation (AF), are experienced by some heart patients. Although AF episodes may not be immediately life threatening, they may be associated with extreme symptoms, a reduced quality of life, and a reduced cardiac output. During AF episodes, the ventricular intervals may vary substantially from one ventricular cycle to the next if such patients have intact AV conduction. See R. J. Cohen et al., “Quantitative Model For Ventricular Response During Atrial Fibrillation”,
IEEE Transactions on Biomedical Engineering
Volume 30, pages 769-782 (1983). Ventricular rate irregularity is undesirable because it is uncomfortable for the patient, may increase susceptibility to ventricular tachyarrhythmias, and can decrease cardiac output.
Many patients are not symptomatic during AF. However, symptomatic AF patients are typically treated with drugs, e.g., amiodarone, to suppress the AF and maintain sinus rhythm. These “rhythm control” drug therapies are frequently ineffectual and/or have undesirable side effects. Alternatively, physicians prescribe drugs to reduce the ventricular heart rate (“rate control”). In some patients, it is not possible to achieve rate control due to ineffective drugs, side effects, contraindications, or lack of compliance by the patient. Physicians treat patients who fail rhythm control or rate control drug therapies with AV nodal ablation and pacemaker implantation. AV node ablation is undesirable because it causes irreversible destruction of the AV node, results in the patient being pacemaker dependent, and is associated with an increased risk for sudden cardiac death.
Many dual chamber pacing systems have been proposed or clinically used to provide bradycardia pacing and respond to AF in a variety of ways to reduce patient symptoms. For example, a variety of mode switching features have been disclosed which respond to an excessively rapid atrial rhythm by causing the pacing system to switch from an atrial synchronized pacing mode, such as DDD/DDDR, to a non-synchronized mode such as VVI/VVIR or DDI/DDIR. Such mode switching features are disclosed in U.S. Pat. No. 5,144,949, by Olson, U.S. Pat. No. 5,318,594, by Limousin et al., U.S. Pat. No. 4,944,298, by Sholder, U.S. Pat. No. 5,292,340, by Crosby et al., and U.S. Pat. No. 4,932,406 by Berkovits, all incorporated herein by reference in their entireties. In such pacing systems, the primary purpose of the mode switch is to prevent the pacing system from tracking a non-physiologic atrial rate. Unfortunately, these mode switching mechanisms do not address the problem of inappropriately high, irregular ventricular rhythms that often result during AF. Ventricular pacing is commonly inhibited because the intrinsic Ventricular rate of the AF patient is commonly faster than the rate of the pacemaker.
Various ventricular pacing regimens have been proposed to attempt to stabilize or regularize the ventricular heart rate and to avoid AV nodal ablation. Wittkampf et al., (“Rate Stabilization by Right Ventricular Pacing in Patients With Atrial Fibrillation”,
PACE,
Vol. 9, November-December, 1986, Part II, pp. 1147-1153 and “Effect Of Right Ventricular Pacing On Ventricular Rhythm During Atrial Fibrillation”,
JACC
11:539-545, 1988) proposed VVI pacing at a rate that results in a substantial percentage of depolarizations resulting from ventricular pacing, and a smaller percentage of depolarizations resulting from intrinsic conduction. The pacing rate was increased each time a ventricular sense event restarted the pacing escape interval and was decreased each time the pacing escape interval timed out and a ventricular pace was delivered. The articles state that this methodology provides stable ventricular rates in the presence of AF with only a moderate increase in overall ventriculiar heart rate.
Lau et al., “A New Pacing Method for Rapid Regularization and Rate Control in Atrial Fibrillation”,
Am Journal of Cardiology
, Vol. 65, pp. 1198-1203, May 15, 1990 attempted to stabilize the ventricular heart rate by triggering a ventricular pace after every ventricular sense. The average triggered pacing interval was about 230 ms. It is believed that this approach doesn't effectively smooth out the ventricular rate and that it may be proarrhythmic in that the ventricular pace pulse may be delivered during the vulnerable period of ventricular repolarization.
Other DDD/DDDR pacing systems have been proposed that undergo mode switching in response to detected AF and attempt to stabilize the ventricular heart rate. Such pacing systems are disclosed in U.S. Pat. Nos. 5,480,413 and 5,591,215, both issued to Greenhut et al., and in U.S. Pat. No. 5,893,882 issued to Peterson et al., all incorporated herein by reference, which detect the presence of atrial tachyarrhythmia and switch from a dual chamber DDD/DDDR pacing mode or a VDD/VDDR pacing mode to a DDI/DDIR or VDI/VDIR pacing mode that effectively monitors the AF condition while pacing in the ventricle. In the '413 and '215 patents, the intrinsic ventricular heart rate irregularity is measured, and ventricular pacing rate is incremented and decremented based on the measured irregularity. However, pacing rate adjustments based on rate irregularity criteria require the time and energy consuming process of measuring the irregularity. The irregularity criterion also requires programming to set an appropriate target and an inappropriate irregularity criterion may result in ineffective pacing therapy. In the '882 patent, upon mode switching in response to detected AF, the ventricular pacing rate is modulated based upon preceding ventricular intervals such that the current pacing interval is set equal to the preceding intrinsic or paced interval, with an increment if the preceding interval is less than the desired or target pacing interval, as typically will be the case, or with a decrement if the preceding interval is greater than the desired pacing interval. This approach may result in a sudden increase in pacing rate after a very fast intrinsic rate, which commonly occurs during AF.
An alternative approach that attempts to smooth the ventricular heart rate employing a physiologic rate or “phys_rate” and a “flywheel” rate is disclosed in U.S. Pat. No. 5,792,193 to Stoop, incorporated herein by reference, in the context of a DDD/DDDR mode switching pacing system. During AF, phys_rate “tracks” the average ventricular rate by incrementing a small amount when a ventricular sense occurs, and decrementing when a ventricular pace occurs. The escape pacing rate, or the flywheel rate, is set to be slightly slower than the phys_rate. If the flywheel interval associated with the flywheel rate times out, a ventricular pace is delivered. In this way, ventricular pacing occurs at a rate slightly slower than the mean rate, or phys_grate. However, depending on how quickly pacing rate is incremented after a ventricular sense, and how quickly pacing rate is decremented after a ventricular pace, this feature may result in pacing at an inappropriately fast rate. For example, if the '193 feature increases pacing rate by 2 bpm after a ventricular sense and decreases by only 0.5 bpm after a ventricular pace, the feature may inappropriately pace at the upper pacing rate during an atrial flutter with 3:2 conduction.
What the art has not yet shown is a simple system that does not require complex computations of ventricular rate irregularity and stabilizes ventricular rate without increasing mean rate inappropriately.
SUMMARY OF THE INVENTION
The objective of the present invention is to regularize heart rate while avoiding pac
Betzold Robert A.
Igel David A.
Kleckner Karen J.
Markowitz H. Toby
Schroetter Jeremy A.
Evanisko George R.
Medtronic Inc.
Wolde-Michael Girma
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