Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2001-10-16
2004-05-04
Getzow, Scott M. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06731985
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an implantable cardiac stimulation system using an automatic capture feature. More specifically, the present invention relates to an implantable cardiac stimulation system in which an external programmer controls an automatic capture calibration routine and displays pertinent information regarding the feasibility of performing automatic capture.
BACKGROUND OF THE INVENTION
Implantable cardiac stimulating devices including pacemakers, cardioverters and defibrillators, detect and treat incidents of cardiac arrhythmias. Such devices are coupled to a patient's heart through transvenous leads that are used to sense electrical signals from the heart and deliver both low voltage and high voltage electrical therapy to the heart. The device circuitry generally includes sensing circuitry for sensing cardiac electrical activities in order to detect intrinsic electrical depolarizations of the cardiac tissue that cause contraction of the respective heart chambers.
In the atria, detection of a P-wave indicates atrial contraction, and in the ventricles detection of an R-wave, also known as a QRS complex, indicates ventricular contraction. If detection of an intrinsic P-wave or an R-wave does not occur within a given interval of time, generally referred to as the “escape interval,” the heart rate is determined as being too slow. A stimulation pulse is then generated by the pacemaker circuitry and delivered to the appropriate heart chamber at the end of the escape interval in order to stimulate the muscle tissue of the heart to contract, thus maintaining a minimum heart rate. The duration of the escape interval corresponds to some base pacing rate, for example an escape interval of 1,200 msec would maintain a base pacing rate of 50 heart beats per minute.
The electrical depolarization caused by the delivery of a stimulation pulse is known as an “evoked response.” An evoked response will only occur when the stimulating pulse is of sufficient energy to cause depolarization of the cardiac tissue, a condition known as “capture.” The minimum stimulating energy required to capture a chamber of the heart is known as “threshold.”
Modern pacemakers often include a feature known as “automatic capture.” When the automatic capture feature is implemented, the pacemaker circuitry detects the evoked response following the delivery of a stimulation pulse in order to verify that capture has occurred. If no evoked response is detected, the stimulation pulse may have been of insufficient energy to capture the heart; therefore, a high-energy back-up pulse is quickly delivered to the heart in order to maintain the desired heart rate. A threshold detection algorithm is then invoked in order to re-determine what minimum energy is required to capture the heart.
The stimulating pulse energy is automatically adjusted to this new threshold value plus some safety margin. As long as an evoked response is detected following a stimulation pulse, that is, as long as capture is verified, pacing will continue at the set pulse energy.
Hence, the automatic capture feature improves pacemaker performance in at least two ways: 1) it verifies that the stimulation therapy delivered has been effective in causing the heart chamber activation, and 2) it improves battery energy longevity by determining the lowest stimulation energy needed to effectively capture the heart.
However, one problem with capture detection is that the signal sensed by the ventricular and/or atrial sensing circuits immediately following the application of a stimulation pulse may not be an evoked response. Rather, it may be noise, either electrical noise caused, for example, by electromagnetic interference (EMI), or myocardial noise caused by random myocardial or other muscle contractions (muscle “twitching”). Alternatively, the signal sensed by the ventricular and/or atrial sensing circuits may be a natural R-wave or P-wave that just happens to occur immediately following the application of a non-capturing stimulation pulse.
Another problematic condition is “fusion”. Fusion occurs when a pacing pulse is delivered such that the evoked response occurs coincidentally with an intrinsic depolarization. The evoked signal may be absent or altered preventing correct capture detection by the pacemaker's capture detection algorithm. A loss of capture may be indicated when capture is in fact present, which is an undesirable situation that will cause the pacemaker to unnecessarily deliver a high-energy back-up pacing pulse and to invoke the threshold testing function in a chamber of the heart. Frequent delivery of back-up pacing pulses or execution of threshold tests defeats the purpose of the energy-saving features of autocapture. If fusion continues during a threshold test, the pacing energy output may be driven to a maximum level, quickly depleting the battery energy.
The incidence of fusion can be particularly problematic in patients with intermittent or intact atrio-ventricular conduction being treated by dual chamber pacing. In dual chamber pacing, both atrial and ventricular activity are monitored. A P-wave detected in the atria is followed by an AV/PV interval which is the desired delay between an atrial depolarization and a ventricular depolarization. If an intrinsic R-wave is not detected prior to expiration of the AV/PV delay, a Vpulse is delivered to pace the ventricles. Since the AV conduction time may vary, an intrinsically conducted R-wave may occur at different times and therefore may occur approximately the same time that a ventricular pacing pulse is delivered. Furthermore, the AV/PV interval may be programmed inappropriately leading to increased likelihood of fusion events. Fusion masquerading as loss of capture will cause the pacemaker to initiate frequent threshold tests and may drive the pacemaker to its maximum pacing output.
Yet another signal that interferes with the detection of an evoked response is associated with lead electrode polarization. Lead electrode polarization is caused by electrochemical reactions that occur at the lead/tissue interface due to the application of the electrical stimulation pulse across such interface. The lead polarization signal is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy, and many other variables.
The evoked response is monitored within 3 to 80 msec of the stimulation pulse. During the early portion of this time, the lead polarization signal voltage is still relatively high. In order to minimize lead polarization voltage, low polarization materials can be used in manufacturing the electrode. Still, since the evoked response and polarization signal occur simultaneously, if the polarization signal is very high, it may not be possible to reliably detect an evoked response. The result may be a false positive detection of the evoked response. Such false positive detection leads to a false capture indication, which, in turn, can lead to missed heartbeats, a highly undesirable situation.
Variation in the lead polarization signal can be significant from patient to patient depending on implanted lead configurations and other factors. Therefore, calibration methods are generally required to determine a threshold for detecting the evoked response and distinguishing it from the lead polarization signal.
Different parameters or characteristics of the evoked response have been proposed in automatic capture calibration and automatic capture detection schemes, including impedance change, voltage differential (dV/dt), signal polarity reversal, and peak negative amplitude.
Typically, evoked response sensing occurs between the tip and ring of a bipolar lead connected to the device sensing circuits. The evoked response may also be monitored by sensing between the ring electrode and device housing. In either event, a bipolar pacing lead has generally been required in order to detect the evoked response. These configurations have been selected since they reduce the likelihood of false positive capture detection. Such reduc
Bornzin Gene A.
Bradley Kerry
Florio Joseph J.
Poore John W.
Sloman Laurence S.
Getzow Scott M.
Pacesetter Inc.
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