Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-07-15
2003-03-25
Bockelman, Mark (Department: 3737)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06539259
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed towards an implantable stimulation device, such as a cardiac pacemaker or an implantable cardioverter-defibrillator device, which has a system that automatically and reliably adjusts its sensitivity for sensing cardiac signals and for reliably discriminating between P-waves, R-waves and T-waves, even in the presence of noise and complete heart block.
BACKGROUND OF THE INVENTION
Every modern implantable stimulation device includes sensing capability, whether in one or two chambers of the heart. Typically, sensing of the low amplitude cardiac signals is achieved by a pre-amplifier and a comparator which detects if a predetermined threshold have been exceeded. The “sensitivity” is generally thought of as a measure of the level, in millivolts, in which a cardiac signal must exceed in order to be detected by the sensing circuitry. If the sensitivity is too low (i.e., too insensitive), then some cardiac events will not be detected. If the sensitivity is too high, then erroneous sensing of noise or undesired cardiac signals may result (e.g., the double sensing of T-waves may occur, noise may be mistaken for P-waves, and far-field R-waves may be mistaken as P-waves, etc.).
Systems for automatic sensitivity control in an implantable stimulation device have long been plagued by the inability to reliably detect the low amplitude P-waves signals (typically about 4 mV) in the presence of noise and myopotentials, and by the inability to discriminate P-waves and R-waves from T-waves. Although a physician may be able to readjust the sensitivity of the device, many patients are unable to see their physicians frequently enough. This problem is compounded when several other factors may also affect the device's sensitivity as a function of time, such as fibrotic tissue growth, drugs, dislodgment, arrhythmias, changes due to defibrillation shock, etc. Thus, it is desirable to have a system that can automatically and reliably adapt itself as the patient's needs change.
Many schemes have been attempted which require additional sensing amplifiers and hardware, which in turn require more real estate on the IC's and more current drain. For example, Hamilton et al. (U.S. Pat. No. 4,708,144) discloses a system for measuring a peak R-wave and deriving a long term average to determine the gain setting and threshold of the amplifier. This configuration requires a conventional amplifier and a comparator, in addition to, an attenuator, a filter, an A/D converter, an absolute value circuit and a peak detector.
Decote, Jr. (U.S. Pat. No. 4,768,411) discloses a dual comparator system, having two different thresholds but coupled to the same input, which simultaneously adjusts each threshold until one comparator senses the cardiac signal and one does not. In addition to a conventional amplifier and comparator, this system requires a precision signal rectifier, a D/A converter, a voltage divider, and a second comparator. The reference by Schroeppel (U.S. Pat. No. 4,766,902) teaches a similar approach albeit with less specificity.
Baker, Jr. et al. (U.S. Pat. No. 4,903,699) discloses an even more complicated scheme using a quad-comparator system for separately determining sensing threshold and amplifier gain. This system requires an automatic gain control circuit, three amplifiers, and four comparators.
It is also known that low amplitude ventricular fibrillation (VF) can go undetected if the sensitivity level (in mV) is set for normal bradycardia activity. If the sensitivity level is set too high (i.e., too sensitive), however, then double sensing of T-waves can result in erroneously classifying a rhythm as VF. Also, it is known that, post defibrillation shock, the current of injury T-wave can be higher in amplitude than R-waves. The Grevis et al. reference (U.S. Pat. No. 4,940,054) discloses a system for setting the sensing circuitry to any one of a multiple sensitivities depending on the type of rhythm detected, or expected, based on the present therapy.
Different ones of the electrode configurations have proven more useful than others for sensing certain cardiac events. Briefly, the benefits of bipolar leads are well known: for example, capture detection is superior when used with bipolar electrodes. Also, it is well known that bipolar stimulation can eliminate pectoral stimulation; bipolar sensing can eliminate sensing of noise and EMI. In some instances, unipolar sensing may provide a larger cardiac signal based on the vector; and unipolar stimulation may avoid diaphragm stimulation in some patients. Implanting bipolar leads enables complete programmable polarity. Thus, if the patient needs a different electrode configuration to avoid a pacing or sensing problem, or if a wire fractures, the physician can reprogram the electrode configuration to an alternative pair of operable electrodes. The need to change sensing electrode configurations provides yet another reason for automatically determining a new sensitivity setting, and this need is magnified if the implanted device can perform this reprogramming automatically based on predefined criteria (e.g., an improper lead impedance, a change in the rhythm, etc).
The problem of accurately automatically sensing P-waves and R-waves is even more pronounced when using an “A-V combipolar” electrode configuration, that is, an electrode configuration in which the stimulation device senses cardiac signals between an atrial tip electrode and a ventricular tip electrode, and stimulates each chamber in a unipolar fashion from the respective electrode to the housing (i.e., typically referred to as the case electrode). For a more complete description of combipolar systems, see U.S. Pat. No. 5,522,855 (Hognelid), which reference is incorporated herein by reference. When such electrodes are implanted, various electrode sensing configurations are possible, e.g., atrial unipolar (Atip-case); ventricular unipolar (Vtip-case); atrial-ventricular combipolar (Atip-Vtip); ventricular unipolar ring (Vring-to-case) or atrial unipolar ring (Aring-to-case).
Regardless of the cardiac event being sensed, however, and regardless of the electrode configuration being used, there is a need for the implantable device to be able to readily and reliably distinguish between P-waves, R-waves and T-waves. This is because the implantable device, if it is to perform its intended function, must know when an atrial depolarization occurs (P-wave), and when a ventricular depolarization occurs (R-wave), and it must not be fooled by falsely sensing a T-wave or noise as a P-wave or R-wave.
For example, it is of critical importance that the device be capable of recognizing the occurrence of certain atrial arrhythmias based on the sensed atrial rate, and in determining such rate it is critically important that neither R-waves nor T-waves be falsely sensed as a P-wave. Such may be particularly noticeable when an A-V combipolar electrode configuration is being used because, in such configuration, P-waves, R-waves, and T-waves may be sensed as being of the same order of magnitude. This problem may be made even more difficult during a mode switch, e.g., when switching a pacemaker from a DDD mode to a VVI or DDI mode, because such a mode switch tends to introduce retrograde P-waves, of which occurrences may be sensed and falsely assumed to be an antegrade P-wave.
While it is well known that various blanking schemes may be used to block or blank out unwanted T-waves and retrograde P-waves by using different blanking intervals (i.e., PVARP, automatic PVARP extension, PVAB, etc.), and thereby prevent such T-waves or retrograde P-waves from being falsely sensed as P-waves, such blanking schemes (based solely on timing considerations) have proven less than satisfactory because legitimate (antegrade) P-waves (which need to be sensed) may and do occur during these blanking intervals.
Differentiation schemes based on the morphology of the sensed waveform have also been used. Such schemes are premised on the fact that P-waves, R-waves and T-waves have inh
Bornzin Gene A.
McClure Kelly H.
Weinberg Lisa P.
Bockelman Mark
Pacesetter Inc.
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