Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-10-23
2004-02-03
Layno, Carl (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06687545
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an implantable cardiac stimulation device. More specifically, the present invention is directed to an implantable cardiac stimulation device with automatic capture verification capabilities made possible during bipolar stimulation by monitoring for the presence of anodal stimulation.
BACKGROUND OF THE INVENTION
In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave, and resulting atrial chamber contractions. The excitation pulse is further transmitted to, and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system, causing a depolarization known as an R-wave and resulting ventricular chamber contractions.
Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired pacing output (amplitude and pulse width) and rate. A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
A stimulation pulse delivered to the myocardium must be of sufficient energy to depolarize the tissue, thereby causing a contraction, a condition commonly known as “capture.” In early pacemakers, a fixed, high-output pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery charge and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.
The capture “threshold” is defined as the lowest stimulation pulse output at which capture occurs. By stimulating the heart chambers at, or just above capture threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery charge. Capture threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Furthermore, capture threshold will vary over time within a patient as, for example, fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state.
Hence, techniques for monitoring the cardiac activity following delivery of a stimulation pulse have been incorporated in modern pacemakers in order to verify that capture has indeed occurred. If a loss of capture is detected by such capture-verification algorithms, a threshold test is performed by the cardiac pacing device in order to re-determine the threshold and automatically adjust the stimulating pulse output. This approach, called “automatic capture”, improves the cardiac stimulation device performance in at least two ways: 1) by verifying that the stimulation pulse delivered to the patient's heart has been effective, and 2) significantly increasing the device's battery longevity by conserving the battery charge used to generate stimulation pulses.
Commonly implemented techniques for verifying capture involve monitoring the intracardiac electrogram (EGM) signals received on the cardiac sensing electrodes. When a stimulation pulse is delivered to the heart, the EGM signals that are manifest concurrent with the depolarization of the myocardium are examined. When capture occurs, detection of an “evoked response,” observed as the intracardiac P-wave or R-wave on the EGM, indicates contraction of the respective cardiac tissue. The depolarization of the heart tissue in response to the heart's natural pacemaking function is referred to as an “intrinsic response”. Through sampling and signal processing algorithms, the presence of an evoked response following a stimulation pulse is determined. For example, if a stimulation pulse is applied to the ventricle, an R-wave sensed by ventricular sensing circuits of the pacemaker immediately following application of the ventricular stimulation pulse evidences capture of the ventricles.
If no evoked response is detected, typically a high-output back-up stimulation pulse is immediately delivered to the heart in order to provide backup support to the patient. An automatic threshold test is next invoked in order to re-determine the minimum pulse output required to capture the heart. An exemplary automatic threshold determination procedure is performed by first increasing the stimulation pulse output level to a relatively high predetermined testing level at which capture is certain to occur. Thereafter the output level is progressively decremented until capture is lost. The stimulation pulse output is then set to a level safely above the lowest output level at which capture was attained. Thus, reliable capture verification is of utmost importance in proper determination of the threshold.
Sensing an evoked response, however, can be difficult for several reasons. The greatest difficulty encountered is probably that of lead polarization. Lead polarization is commonly caused by electrochemical reactions that occur at the lead-tissue interface due to application of an electrical stimulation pulse across the interface. A lead-tissue interface is that point at which an electrode of the pacemaker lead contacts the cardiac tissue. If the evoked response is sensed through the same electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to as “afterpotential”, formed at the electrode can corrupt the evoked response signal that is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation output and other variables, many of which are continually changing overtime.
A false positive detection of an evoked response may lead to missed heartbeats, a highly undesirable and potentially life-threatening situation. Failure to detect an evoked response that has actually occurred will cause the pacemaker to unnecessarily invoke the threshold testing function in a chamber of the heart.
The importance of the problem of lead polarization is evident by the numerous approaches that have been proposed for overcoming this problem. For example, specially designed electrodes with properties that reduce the polarization effect have been proposed. When additional electrodes are available for sensing, polarization can be avoided by sensing the EGM signals using a different pair of electrodes than that used for stimulation.
Another problem that prevents reliable evoked response sensing during bipolar stimulation is the presence of anodal stimulation. Typically cathodal stimulation of the myocardium is recommended. Cathodal stimulation produces a negative pulse that acts to reduce the capacitance of the cell membrane allowing depolarization to occur. Anodal stimulation, that is a positive pulse, can also cause cell depolarization by first hyperpolarizing the cell and then, as the cell repolarizes, an overshoot causes depolarization. However, anodal stimulation generally requires higher stimulation output then cathodal stimulation, thus increasing the battery current drain. Anodal stimulation has been thought to increase the risk of arrhythmoge
Layno Carl
Pacesetter Inc.
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