Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-03-14
2003-04-22
Dolinar, Andrew M. (Department: 3747)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S025000, C607S028000
Reexamination Certificate
active
06553259
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an implantable cardiac stimulation device capable of delivering both high and low voltage therapies for treating bradycardia, tachycardia, and fibrillation. The present invention relates more specifically to an implantable cardiac stimulation device possessing automatic sensitivity control and beat-by-beat automatic capture.
BACKGROUND OF THE INVENTION
Implantable cardiac stimulating devices include pacemakers and cardioverter defibrillators (ICDs). A primary function of pacemakers is to detect and treat incidents of a slow heart rate, known as bradycardia, or no heart rate, known as asystole. A primary function of ICDs is to detect and treat incidents of an excessively high heart rate, known as tachycardia, or incidents of fibrillation.
Combined pacemaker/cardioverter defibrillators are commercially available for treating both bradycardia, and tachycardia or fibrillation. Such a combined cardiac stimulating device is coupled to the patient's heart through transvenous leads which are used to sense electrical signals from the heart, and deliver both low voltage and high voltage electrical therapy to the heart.
The pacemaker circuitry generally includes sensing circuitry for sensing cardiac electrical activities in order to detect intrinsic electrical depolarizations of the cardiac tissue that cause contraction of the respective heart chambers. In the atria, detection of a P-wave indicates atrial contraction, and in the ventricles detection of an R-wave, also known as a QRS complex, indicates ventricular contraction.
If detection of an intrinsic P-wave or an R-wave does not occur within a given interval of time, generally referred to as the “escape interval,” the heart rate is determined as being too slow. A stimulation pulse is then generated by the pacemaker circuitry and delivered to the appropriate heart chamber at the end of the escape interval in order to stimulate the muscle tissue of the heart to contract, thus maintaining a minimum heart rate. The duration of the escape interval corresponds to some base pacing rate, for example an escape interval of 1,200 msec would maintain a base pacing rate of 50 heart beats per minute.
The electrical depolarization caused by delivery of a pacing pulse is known as an “evoked response.” An evoked response occurs when the stimulating pulse is of sufficient energy to cause depolarization of the cardiac tissue, a condition known as “capture.” The minimum stimulating energy required to capture a chamber of the heart is known as “threshold.”
Modern pacemakers often include a feature known as “automatic capture.” When automatic capture is implemented, the pacemaker circuitry detects the evoked response following delivery of a pacing pulse in order to verify that capture has occurred. If no evoked response is detected, the pacing pulse may have been of insufficient energy to capture the heart; therefore, a high-energy back-up pacing pulse is quickly delivered to the heart in order to maintain the desired heart rate. A threshold detection algorithm is next invoked in order to re-determine what minimum energy is required to capture the heart. The pacing pulse energy is then automatically adjusted to this new threshold value plus some safety margin. As long as an evoked response is detected following a pacing pulse, that is as long as capture is verified, pacing will continue at the set rate and pulse energy. Hence automatic capture improves pacemaker performance in at least two ways: 1) it verifies that the stimulation therapy delivered has been effective in pacing the heart chamber, and 2) it improves battery energy longevity by determining the lowest stimulation energy needed to effectively capture the heart.
The cardioverter defibrillator circuitry of an implantable cardiac stimulating device monitors the electrical activity of the heart to detect when the intrinsic heart rate exceeds a defined upper rate limit. In the case of tachycardia, a high energy stimulation pulse is usually delivered in synchrony with the heart's QRS wave in an attempt to terminate the tachycardia, a treatment known as “cardioversion.” Synchronized delivery of the high-energy pulse prevents stimulating the heart during the T-wave portion of the P-QRS-T cardiac cycle. During the T-wave portion of the cardiac cycle, the ventricular tissue is re-polarizing and delivery of any kind of stimulation pulse during this time could accelerate the heart rhythm into a faster tachycardia or even into fibrillation.
A serious form of tachycardia is ventricular fibrillation, which is usually fatal if not treated within a few minutes of occurrence. During fibrillation, disorganized depolarizations occur throughout the heart tissue (myocardium) causing the heart chamber to contract in a chaotic way, i.e., fibrillate, resulting in ineffective ejection of blood from the heart chamber. These disorganized depolarizations, also referred to as fibrillation waves, are typically low amplitude signals that occur at an irregular rate. When the cardioverter-defibrillator circuitry detects fibrillation, a high energy shocking pulse is delivered in an attempt to re-coordinate the depolarization of all (or most of) the individual muscle fibers and thus regain coordinated cardiac contractions.
In order to allow detection of both higher amplitude R-waves and low amplitude fibrillation signals, implantable cardioverter defibrillators commonly include automatic gain control or automatic sensitivity control for detecting both high amplitude R-waves and low-amplitude fibrillation signals. Reference is made to U.S. Pat. No. 5,685,315 to McClure et at. for a more detailed description of the use of automatic sensitivity control in cardiac arrhythmia detection, herein incorporated by reference. Automatic gain or sensitivity control allows straightforward detection of cardiac events based on event amplitude crossing of the sensing threshold. An initially higher sensing threshold is applied starting at the end of a refractory period that follows a detected P-wave or R-wave or a delivered pacing pulse. As the gain or sensitivity decays, detection of low amplitude fibrillation waves is possible. The rate at which the detected events occur allows classification of the detected rhythm into bradycardia, normal sinus, low rate tachycardia, high rate tachycardia, or fibrillation.
However, a problem exists for patients having a combined pacemaker cardioverter defibrillator in that the electrogram signal from the ventricular fibrillation may be so low in amplitude that neither the ICD nor the pacemaker sensing circuits sense anything, thus causing the pacemaker portion of the system to release a stimulation pulse. Upon releasing the stimulus, the automatic sensitivity feature the sensing circuits of the stimulation device, if enabled, incrementally increases its sensitivity to its most sensitive setting, in an attempt to sense an R-wave. If a failure to sense an R-wave persists, the diagnosis is “true asystole,” and the stimulation device will continue to release stimulation pulses at its programmed base pacing rate. However, if the rhythm is truly ventricular fibrillation with an electromyogram signal that is too low to be sensed by either the stimulation device, the stimulation pulses will not be effective. However, the stimulation device does not recognize the ineffectiveness of the stimulation pulses, and will continue to deliver such ineffective stimuli.
Therefore, what is needed is a combined stimulation device or system wherein a proper response to an alleged asystole can occur, and wherein the device can ascertain whether or not a given stimulation pulse is effective, i.e., whether it “captures” the heart. The automatic capture feature is therefore also desirable in a combined ICD/pacemaker in order to ensure effective stimulation therapy and to increase device longevity by conserving battery energy. The importance of providing automatic capture is described in U.S. Pat. No. 5,350,401, to Levine, which is incorporated herein by reference.
One pr
Amely-Velez Jorge N.
Badelt Steven W.
Isaac George I.
Mouchawar Gabriel A.
Dolinar Andrew M.
Pacesetter Inc.
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