Surgery – Diagnostic testing – Cardiovascular
Utility Patent
1998-10-28
2001-01-02
Evanisko, George R. (Department: 3737)
Surgery
Diagnostic testing
Cardiovascular
C607S009000, C128S901000
Utility Patent
active
06169918
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to a cardiac rhythm management system with cross-chamber “soft blanking” for sensing desired signals and reducing unwanted noise.
BACKGROUND
When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias is via drug therapy. Drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, or pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart. Heart contractions are initiated in response to such pace pulses. By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardioverters, and defibrillators, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
Modern cardiac rhythm management systems are also typically capable of sensing intrinsic electrical heart activity signals. These electrical heart activity signals include cardiac depolarizations that are autonomously produced by the heart. The depolarizations initiate the contractions of the muscle tissue of the heart. These contractions, in turn, pump blood through the circulatory system.
One example of a cardiac rhythm management system that senses intrinsic electrical heart activity signals would be a dual-chamber implantable pacer (or pacer/defibrillator) that includes a pulse generator implanted subcutaneously, such as in the abdomen or in the pectoral region of the patient's upper chest near the clavicle bone. The pulse generator includes a power source, such as a battery, as well as sensing, therapy, and timing circuits. The pulse generator is coupled to the heart via endocardial leadwires. Such leadwires are typically routed from the pulse generator to the heart through blood vessels. Electrodes are located at the terminal end of these leadwires, distal from the pulse generator. The electrodes are typically positioned in the right atrium and right ventricle of the heart. Pacing pulses are delivered to the heart via the electrodes. Moreover, the intrinsic electrical heart activity signals are received at the electrodes and communicated to the pulse generator via the leadwires. At the pulse generator, atrial and ventricular sensing circuits detect atrial and ventricular depolarizations in the respective atrial and ventricular electrical heart activity signals. The pulse generator adjusts the therapy delivered according to the sensed atrial and ventricular depolarization information. In one mode of operation, for example, the pulse generator withholds delivery of a scheduled pacing pulse when it detects that the heart is already contracting as the result of an intrinsic cardiac depolarization.
Sensing intrinsic heart activity signals in each of the atrium and ventricle is complicated by the fact that an electrode placed in one chamber may receive not only intrinsic heart activity signals from that chamber of the heart, but the electrode may also receive unwanted “cross-chamber” intrinsic heart activity signals, that is, far-field noise signals resulting from depolarizations in the other chamber of the heart. For example, this is particularly problematic in using the atrial electrode to detect atrial depolarizations. Because ventricular depolarizations typically have a larger signal amplitude than atrial depolarizations, the atrial electrode is likely to detect the ventricular depolarization as well as the atrial depolarization.
At least two problems are caused by cross-chamber sensing of heart activity signals originating in a heart chamber other than that in which the electrode is located. First, in the above example, the pulse generator may erroneously recognize the cross-chamber ventricular depolarization as being an atrial depolarization. As a result, the pulse generator may improperly withhold delivery of a pace pulse to the atrium because it erroneously thought the atrium was already contracting. Second, the occurrence of ventricular depolarization may obscure detection of an atrial depolarization that occurs at about the same time as the ventricular depolarization. Thus, cross-chamber sensing of a ventricular depolarization by the atrial electrode results in a noisy signal at the atrial electrode that makes it more difficult for the atrial sensing circuit to detect an actual atrial depolarization. Conversely, cross-chamber sensing of an atrial depolarization by the ventricular electrode makes it more difficult for the ventricular sensing circuit to detect an actual ventricular depolarization.
Cross-chamber noise problems have been described above with respect to sensed intrinsic cross-chamber heart activity signals. However, the delivery of a pacing pulse to the heart by the pulse generator also causes similar noise problems, making intrinsic heart activity signals difficult to detect, both in the chamber of the heart to which the pace pulse was delivered, as well as at the electrode located in the opposite chamber of the heart.
One technique for dealing with cross-chamber sensing of unwanted far-field signals is referred to as cross-chamber blanking. In one example of this technique, the occurrence of a ventricular event (i.e., a sensed ventricular depolarization or a delivered ventricular pace pulse) triggers a lengthy time period, referred to as a blanking or refractory period. During the blanking period, electrical heart activity signals received at the atrial electrode are simply ignored. In this way, the atrial sensing circuit isn't confused by unwanted cross-chamber electrical signals resulting from ventricular depolarizations or paces. Similarly, the occurrence of an atrial event (i.e., a sensed atrial depolarization or a delivered atrial pace pulse) triggers a blanking period during which time the electrical heart activity signals received at the ventricular electrode are ignored. In this way, the ventricular sensing circuit isn't confused by unwanted cross-chamber signals resulting from atrial depolarizations or paces.
One limitation of the above-described cross-chamber blanking technique is that it requires the use of relatively long blanking periods. For example, it is not uncommon to ignore signals at the atrial electrode for a blanking period of approximately betwe
Haefner Paul A.
Seim Gary Thomas
Cardiac Pacemakers Inc.
Evanisko George R.
Schwegman Lundberg Woessner & Kluth P.A.
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