Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-12-03
2001-11-20
Getzow, Scott M. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06321115
ABSTRACT:
FIELD OF THE INVENTION
The present invention is generally directed to an implantable medical device, e.g., a cardiac stimulation device and is particularly directed to a noise detection system and method for use in such a device.
BACKGROUND OF THE INVENTION
Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers which provide stimulation pulses to cause a heart, which would normally or otherwise beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators which detect when the atria and/or the ventricles of the heart are in fibrillation and apply cardioverting or defibrillating electrical energy to the heart to restore the heart to a normal rhythm. Implantable cardiac stimulation devices may also include the combined functionalities of a pacemaker and a defibrillator.
As is well known, implantable cardiac stimulation devices sense cardiac activity for monitoring the cardiac condition of the patient in which the device is implanted. By sensing the cardiac activity of the patient, the device is able to provide cardiac stimulation therapy when it is required.
Unfortunately, noise within the sensing channel bandwidth can interfere with the sensing function. In a pacemaker, for example, noise can be mistaken by the device for a legitimate cardiac event causing stimulation inhibition. This can lead to long periods of asystole. In an implantable defibrillator, noise can cause mistaken diagnosis of fibrillation resulting in inappropriate therapy delivery.
Common sources of noise are either external, such as power line noise, or internal, such as noise from muscle activation. These sources present a particular problem because they generally pass through the sensing bandpass filter with little or no attenuation.
Noise detection systems for use in implantable cardiac stimulation devices are known in the art. However, prior art noise detection systems generally contemplate the use of long refractory periods wherein cardiac events occurring during the refractory period are not acted upon by the device because of the refractory period extension. This can cause the devices in which such prior art noise detection systems are employed to miss normal activations or cardiac events which occur simultaneously with the noise. The present invention addresses this and other deficiencies of prior art noise detection systems.
SUMMARY OF THE INVENTION
The present invention provides a system for detecting noise in an electrogram signal generated by an implantable cardiac stimulation device. The noise detection system includes a threshold circuit that generates an event signal when the electrogram signal exceeds a threshold, a timer that times a time period responsive to an event signal, and a zero crossing detector that generates a zero crossing signal when the electrogram signal transitions between positive and negative values. The noise detection system further includes a counter that counts the zero crossing signals during the time period and a comparator that determines if the counter reaches a predetermined count during the time period. When the comparator determines that the zero crossings in the electrogram signal exceed a predetermined number of zero crossings during the time period, the detection of noise is declared and the device ignores the event signal as a legitimate cardiac event.
Zero crossing detection, in accordance with one embodiment of the present invention, may be accomplished by noting sign changes in the electrogram signal. Alternatively, the zero crossing detection may be accomplished by detecting when the electrogram signal transitions from above a positive threshold to below a negative threshold or from below the negative threshold to above the positive threshold.
In accordance with other aspects of the present invention, the noise detection time period initiated upon the electrogram signal exceeding a threshold may be on the order of 90 to 250 milliseconds. Further, the noise detection time period may be a refractory period comprising first and second refractory period portions of equal duration. In accordance with this embodiment of the present invention, noise detection is declared when the number of zero crossings occurring during each of the first and second refractory period portions is greater than a predetermined number of zero crossings.
The present invention further provides a method of detecting noise in an electrogram signal for use in an implantable cardiac stimulation device. The method includes the steps of generating an event signal when the electrogram signal exceeds a threshold, detecting zero crossings in the electrogram signal during a time period following an event signal, and determining if the zero crossings in the electrogram signal exceed a predetermined number of zero crossings during the time period.
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Causey, III James D.
Knight Curtis A.
Morgan Wayne A.
Mouchawar Gabriel A.
Shieh Mae-Mae
Getzow Scott M.
Pacesetter Inc.
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