Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-06-18
2003-04-08
Getzow, Scott M. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06546288
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to implantable cardiac electrical stimulation devices. More specifically, the present invention is directed to a cardiac electrical stimulation device possessing automatic capture with a high threshold response and patient notification method.
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 the 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 the resulting ventricular chamber contractions.
Disruption of this natural pacing 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 energy 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-energy pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery energy 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 energy at which capture occurs. By stimulating the heart chambers at or just above threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery energy. 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, 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.
Modern pacemakers have incorporated techniques for monitoring the cardiac activity following delivery of a stimulation pulse 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 energy. 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) greatly increasing the device's battery longevity by conserving the energy used to generate stimulation pulses.
Commonly implemented techniques for verifying capture involve monitoring the internal myocardial electrogram (EGM) signals received on the implanted cardiac electrodes. When a stimulation pulse is delivered to the heart, the EGM signals that are manifest concurrent with depolarization of the myocardium are examined. When capture occurs, an “evoked,” which is the intracardiac P-wave of R-wave that indicates depolarization of the respective cardiac tissue, may be detected. The depolarization of the heart tissue in response to the heart's natural pacing function is referred to as “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 higher-energy, back-up stimulation pulse is delivered to the heart very shortly after the primary ineffective stimulus, commonly in the order of 60-100 ms, in order to maintain the desired heart rate.
The output of the primary pulse is then progressively increased to restore stable capture. This is followed by an automatic threshold test to determine the minimum pulse required to capture the heart at that time. Threshold tests may also be performed on a periodic basis, for example three times a day, daily or weekly. 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 after which the output is progressively increased in small steps until capture is reestablished. The stimulation pulse energy is then set to a level above the lowest output level at which capture was attained. This additional working margin above the measured threshold allows for small fluctuations in threshold to occur without risk of loss of capture and frequent delivery of both back-up pulses and initiation of the threshold testing sequence. A safety margin for the patient is provided by a fixed significantly higher output of the back-up pulse. Thus, reliable capture verification is of utmost importance in proper determination of the threshold.
Such automatic methods for verifying and maintaining capture are currently implemented in cardiac stimulation devices utilizing bipolar sensing and unipolar stimulation. In commercially available cardiac pacing devices with automatic capture verification capabilities, a fixed maximum stimulation pulse energy is set, at which the autocapture feature becomes disabled. The advantage of setting a maximum stimulation pulse energy limit is to minimize patient discomfort should the output be increased to the maximum allowed setting in the situation of rising thresholds. This maximum value is higher than the default output value for most pacemakers recently introduced to the market.
Automatic capture routines thus improve pacemaker performance as long as the capture threshold remains within a normal range of stimulation pulse energies. However, if a pacing-dependent patient undergoes an unexpected, massive increase in threshold, for example if an electrode shifts acutely or the threshold rises on a chronic basis due to progression of disease or as a side-effect of a new pharmacologic agent, the fixed maximum stimulation pulse energy allowed by Autocapture may not effectively capture the heart. Therefore, in these cases, further increases in pulse energy may be needed.
Allowance of high-energy output will afford the patient greater protection against ineffective stimulation. Therefore, it would be desirable to allow exceptional conditions to supercede the present autocapture maximum output limit. With automatic capture enabled, the frequency of follow-up evaluations may have been reduced, but the patient will be protected by the automatic capture algorithm in the presence of a rising capture threshold.
However, a rising output energy requirement could deplete the pacemaker battery energy more quickly than under normal stimulation conditions resulting in device failure prior to the next scheduled follow-up evaluation. Notifying the patient that a change in stimulation conditions has occurred that warrants medical evaluation
Getzow Scott M.
Pacesetter Inc.
LandOfFree
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