Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2002-01-11
2004-07-20
Getzow, Scott M. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06766197
ABSTRACT:
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 output (as may be reported in terms of pulse duration, pulse amplitude, pulse energy, pulse current or current density) at which consistent 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, varies significantly 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.
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 occurred. If a loss of capture is detected by such “capture-verification” algorithms, the pacemaker output is automatically increased until capture is restored. A threshold test is then performed by the cardiac stimulation device in order to re-determine the threshold and automatically adjust the stimulating pulse output. While a primary parameter to vary for adjusting the stimulation pulse output is the voltage, it should be clear that other parameters could be adjusted as well, including pulse duration, energy, charge, and/or current density.
This approach, referred to as “automatic capture,” improves the cardiac stimulation device performance in at least four ways: 1) by verifying that the stimulation pulse delivered to the patient's heart has been effective, 2) by maintaining the stimulation pulse output at the lowest level possible, thus 3) greatly increasing the device's battery longevity by conserving the energy used to generate stimulation pulses, yet 4) always protecting the patient by providing a significantly higher output back-up pulse in the setting of loss of capture associated with the primary pulse.
One implemented technique for verifying capture automatically by an implantable stimulation device involves monitoring the intra-cardiac electrogram signal, also referred to as EGM or IEGM, received on the cardiac stimulation and sensing 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 response” may be detected by special evoked response detection circuitry. The evoked response is the intracardiac atrial or ventricular depolarization that is observed as the P-wave or R-wave, respectively, on the surface ECG associated with a stimulus output. Detection of an evoked response indicates electrical activation of the respective cardiac tissue by the stimulating pulse. The depolarization of the heart tissue in response to the heart's natural pacing 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. A very short blanking period, or period of absolute refractoriness, following the stimulation pulse is applied to the evoked response sensing circuit immediately following the stimulation pulse to minimize or block out the stimulation pulse artifact.
This blanking period is followed by a special evoked response detection window, commonly 15 to 60 ms in duration, wherein the evoked response sensing circuit looks for an evoked response. For example, if a stimulation pulse is applied to the ventricle, an R-wave sensed by a special evoked response detection circuit of the pacemaker immediately following application of the ventricular stimulation pulse evidences capture of the ventricles.
If no evoked response is detected, a high-energy back-up stimulation pulse is delivered to the heart very shortly after the primary ineffective stimulus, typically within 60-100 ms of the primary pulse, in order to maintain the desired heart rate. If back-up stimulation pulses are required on two successive cycles, the system automatically begins to increase the stimulation output associated with the primary pulse until capture is restored, again for two consecutive cycles. Once capture is regained, an automatic threshold test is performed to re-determine the minimum pulse energy required to capture the heart at that time and adjust stimulation pulse output as needed.
An exemplary automatic threshold determination procedure is performed by progressively reducing the output from the functional output in 0.25 Volt steps until loss of capture occurs. With each loss of capture, a higher output back-up pulse is delivered in order to maintain the desired heart rate. Once loss of capture is achieved, the system increases the output in 0.125 Volt steps until stable capture is restored. Stable capture is defined as capture occurring on two consecutive primary pulses. Thus, reliable capture verification is of utmost importance in proper determination of the threshold.
Normally, capture threshold should be stable after the initial postoperative healing period. Frequent fluctuations in threshold can occur later, however, if a stimulating lead becomes dislodged, fractured, or its insulting sheath becomes discontinuous. Fluctuations in threshold may also reflect a change in clinical condition or the effects of a pharmacological agent. The automatic capture feature responds to such fluctuations by repeating a threshold test whenever the threshold rises enough to cause a loss of capture at the existing output setting. Threshold tests may also be repeated on a periodic basis to ascertain if a decrease in threshold has occurred. This automatic feature protects the patient by ensuring adequate stimulation pulse energy despite fluctuating threshold.
It is also desirable to store threshold test results on a frequent basis. Having a record of threshold changes over time will alert a medical practitioner to a possible lead failure or a change in the clinical condition of the patient, both of which warrant further medical evaluation. Such a feature is also ref
Getzow Scott M.
Pacesetter Inc.
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