Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-10-25
2004-03-23
Getzow, Scott M. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06711438
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to implantable cardiac stimulation devices, such as pacemakers or implantable cardioverter-defibrillators (“ICDs”) and, in particular, to techniques for processing electrical cardiac signals detected using combipolar sensing.
BACKGROUND OF THE INVENTION
A pacemaker is a medical device, typically implanted within a patient, which recognizes various dysrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the dysrhythmias. An ICD is a device, also implantable into a patient, which additionally recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation.
Pacemakers and ICD's carefully monitor characteristics of the heart such as the heart rate to detect dysrhythmias, discriminate among different types of dysrhythmias, identify appropriate therapy, and determine when to administer the therapy. The heart rate is tracked by the device by examining electrical signals that are manifest concurrent with the contraction and expansion of the chambers of the heart. The contraction of atrial muscle tissue is manifest by the generation of a P-wave. The contraction of ventricular muscle tissue is manifest by the generation of an R-wave (sometimes referred to as the “QRS complex”). Expansion of the ventricular tissue is manifest as a T-wave. Expansion of the atrial tissue usually does not result in a detectable signal. The sequence of electrical events that represent P-waves, followed by R-waves (or QRS complexes), followed by T-waves are sensed using sensing leads implanted inside the heart, e.g., sensing leads.
One commonly used type of sensing lead is the unipolar lead, which includes a single electrode at its tip. The device detects electrical voltage differentials between the electrode and the external body of the device itself. Typically, one unipolar lead is inserted within the atria and another within the ventricles, from which the device derives separate atrial and ventricular channel cardiac signals. Another commonly employed type of sensing lead is the bipolar lead wherein the lead includes two electrodes mounted near its tip. The device detects electrical voltage differentials between the two electrodes. Again, typically, one lead is inserted within the atria and another within the ventricles, from which the device derives separate atrial and ventricular channels of cardiac signals.
A common problem with unipolar leads is that, because the device is sensing voltage differentials between the tip of the lead and the body of the device, significant far-field electrical signals are detected along with the intended atrial or ventricular cardiac signals. A “far-field” signal is a signal originating far from the sensor of the sensing lead, but detected by the sensing lead nonetheless. For example, the atrial cardiac signal derived from the atrial lead will typically include significant ventricular signals. A significant advantage of the bipolar lead is that, because electrical voltage differentials are detected only between two electrodes located closely adjacent to one another at the end of the lead, far-field sensing is significantly reduced. However, bipolar leads are more expensive and are generally perceived as being less reliable than unipolar leads and hence are not preferred by all physicians.
In an attempt to provide the advantages of bipolar sensing using unipolar leads, some state-of-the-art devices employ combipolar sensing techniques. With combipolar sensing, a pair of unipolar leads are mounted within the heart, one in the atria and one in the ventricles. A ventricular channel cardiac signal is generated in the same manner as with conventional unipolar sensing wherein electrical voltage differentials are detected between the tip of the ventricular lead and the body of the device. However, the atrial channel of the cardiac signal is generated by detecting voltage differentials between the electrodes at the tips of the atrial and ventricular leads. For a more complete description of combipolar systems, see U.S. Pat. No. 5,522,855 (Hognelid), incorporated herein by reference.
With combipolar sensing, because the atrial channel is derived based upon voltage differentials between the tips of the two unipolar leads, improved detection of atrial signals is achieved as compared with systems which require the relatively weak atrial electrical signals to be detected based upon voltage differentials generated between the tip of the atrial lead and the body of the device. Ventricular electrical signals are typically much greater in magnitude than atrial signals, hence, with the combipolar sensing technique, it is sufficient to sense the ventricular signals based upon voltage differentials generated between the tip of the ventricular lead and the body of the device. Hence, an overall improvement in the sensitivity of the detection of atrial signals is achieved using combipolar sensing, yet the perceived benefits of unipolar leads are retained, namely that the leads are less expensive and more reliable.
Thus, combipolar sensing provides many advantages. One disadvantage, however, is that, because the atrial channel is detected based upon voltage differentials between the tips of the atrial and ventricular leads, ventricular signals are sensed as “near-field” signals. As a result, ventricular signals may have a greater magnitude on the atrial channel than the atrial signals. Hence it may be difficult to filter the ventricular signals from the atrial channel. (The ventricular channel, because it is detected based upon voltage differentials between the tip of the ventricular lead and the body of the device, may also pick up far-field atrial signals, but these are typically very weak as compared to the ventricular signals and hence can easily be filtered out.)
Regardless of the electrode configuration being used, there is a need for the implanted device to be able to readily and reliably distinguish between various electrical events such as P-waves, R-waves and T-waves. 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 problematic when an combipolar electrode configuration is being used because, as noted, P-waves, R-waves, and T-waves may be sensed as being of the same order of magnitude on the atrial channel. This problem exacerbated during an automatic mode switch (AMS), e.g., when switching the device from a DDD mode to a VVI or DDI mode. DDD, VVI and DDI are standard device codes which identify the mode of operation of the device. DDD indicates a device which senses and paces in both the atria and the ventricles and is capable of both triggering and inhibiting functions based upon sensed events. VVI indicates that the device is capable of pacing and sensing only within the ventricle and is only capable of inhibiting the functions based upon sensed events. DDI is identical to DDD except that the device is only capable of inhibiting functions based upon sensed events, rather than triggering functions. Numerous other device modes of operation are possible, each represented by standard abbreviations of this type.
One technique commonly employed for processing the atrial or ventricular channel signals to eliminate unwanted signals uses “blanking intervals”. With a blanking interval, the device does not process electrical signals during a predetermined interval of time either for all device functions (absolute blanking) or for selected device functions (relative blanking). As one example of absolute blanking, upon detection of an R-wave on the ventricular channel, the device will not detect any signals on the atrial channel during a post ventricular atrial blanking (PVAB) interval. The atria
Bornzin Gene A.
Florio Joseph J.
McClure Kelly H.
Getzow Scott M.
Pacesetter Inc.
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