Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-09-15
2001-03-27
Getzow, Scott M. (Department: 3737)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
Reexamination Certificate
active
06208899
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to implantable cardioversion devices (ICDs) which sense a dangerous cardiac arrhythmia and, in response, provide therapy to the patient's heart to revert it to a normal sinus rhythm. More specifically, the present invention pertains to a device which includes a bandpass filter with a variable center frequency for detecting and categorizing intrinsic cardiac activity.
BACKGROUND OF THE INVENTION
As used herein, the term “arrhythmia” refers to any abnormal heart rhythm that may be dangerous to the patient and specifically includes fibrillation, tachycardias, supraventricular tachycardias (SVT), ventricular tachycardias (VT), ventricular fibrillation and flutter (VF), and bradycardia. As further used herein, the term “therapy”
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refers to any means used by the ICD device to restore normal heart rhythm, such as defibrillation, cardioversion, anfitachycardia pacing and drug infusion. The disclosed invention has application to ICD devices which treat tachyarrhythmias (abnormally high heart rates).
A modem conventional bradycardia pacemaker has a sensing mechanism to enable the device to inhibit pacing when the heart is beating normally. Implantable tachyarrhythmia devices must also sense the heart's intrinsic electrical activity, known as the electrocardiogram (ECG), to determine whether the patient needs treatment. ECGs exhibit highly variable amplitudes and frequencies as the cardiac activity changes from normal sinus rhythms (NSR) to other abnormal rhythms such as ventricular tachycardia and ventricular fibrillation.
U.S. Pat. No. 4,184,493 to Langer et al., which issued on Jan. 22, 1980, and is entitled “Circuit for Monitoring a Heart and for Effecting Cardioversion of a Needy Heart”, describes a sensing circuit that automatically adjusts to the amplitude of the heart's electrical signal using an automatic gain control (AGC) system.
Another form of sensing system with AGC is described in U.S. Pat. No. 4,903,699 to Baker et al., which issued on Feb. 27, 1990, and is entitled “Implantable Cardiac Stimulator With Automatic Gain Control”. The Baker et al. patent uses a system of comparators and adjustable thresholds to optimally detect the ECG signal.
Both of the foregoing patents describe systems which also filter the ECG signal to remove low frequency noise and artifacts. In addition, high pass filtering is used in these patents to reduce sensing of T waves during normal sinus rhythm (NSR). However, high frequency filtering also attenuates VF signals and makes it difficult to detect the same, especially since VF signals have low amplitudes as compared to signals during NSR.
Another control scheme is disclosed in commonly assigned U.S. Pat. No. 5,395,393, incorporated herein by reference. This patent discloses an ICD in which a different sensing circuit is provided for sensing tachyarrhythmia (including supraventricular tachycardia), ventricular tachycardia or ventricular fibrillation, and for differentiating these conditions from normal sinus rhythm. More specifically, a sensor circuit includes an amplifier, a bandpass filter having a fixed center frequency, a dual dynamic threshold detector and a classifier for detecting tachyarrhythmia using a clustering algorithm. This type of control is commonly referred to as automatic threshold control (ATC) and while it generally works well to differentiate the various types of tachyarrhythmia from random noise, it is generally not effective in detecting tachyarrhythmia in the presence of electromyography signals (EMG) generated by muscles in the chest area, or noise due to standard 50 or 60 Hz power sources.
SUMMARY OF THE INVENTION
In view of the above mentioned disadvantages of the prior art, it is an objective of the present invention to provide an implantable cardioversion device (ICD) with an improved sensing circuit that can detect arrhythmias quickly and accurately.
A further objective is to provide an ICD which is capable of detecting dangerous cardiac conditions in the presence of extraneous signals such as noise from standard power supplies or muscular activity.
Yet a further objective is to provide a system which can be implemented easily with minimal changes to existing sensing circuits.
Other objectives and advantages of the invention shall become apparent from the following description.
Briefly, an implantable cardiac stimulator constructed in accordance with this invention includes a sensor for sensing intrinsic cardiac activity in the heart of a patient and a detector for detecting an abnormal condition of the heart based on this sensed intrinsic activity. The stimulator further includes a therapy generator for automatically applying a preselected therapy when the abnormal condition is detected to cause the heart to revert quickly and reliably to a normal sinus rhythm. Preferably the sensor includes an amplifier for amplifying the signals from the heart and a filter for filtering the signals prior to providing the same to the detector. Importantly, the filter has a programmable or adjustable center frequency fc. A controller is also provided to control the center frequency fc of the filter.
It has been found that intrinsic signals sensed during normal sinus rhythm and during ventricular fibrillation have different characteristic frequency spectra. More specifically, the spectrum for normal sinus rhythm has a center frequency which is higher than the center frequency of the spectrum characterizing ventricular fibrillation (fvf).
Therefore, during a normal sinus rhythm period, the center frequency of the filter in embodiments of the present invention is set to correspond to the center frequency (fnsr) of sinus rhythm R waves. If ventricular fibrillation is suspected, for example, because R waves characteristic of a normal sinus rhythm are not detected, the center frequency fc is changed to fvf. Thus, the filter is tuned automatically so that it is optimized to detect ventricular fibrillation waves while rejecting or at least de-emphasizing extraneous noise signals produced, for example, by muscle contractions in the chest area and/or noise from external 50 or 60 Hz power supplies. After the heart chamber reverts to normal sinus rhythm, the center frequency of the filter is returned to its original value.
In this manner, the condition of the patient's heart is quickly determined and therefore appropriate therapy can be applied effectively.
Preferably, the center frequency fc is changed gradually from one value to the other.
In one embodiment of the invention, an automatic threshold control (ATC) scheme is used. In this embodiment, the filtered output is fed to a comparator for comparison with a threshold T which is a time varying parameter. More particularly, the threshold T is also changed at the same time that the center frequency fc of the filter is changed. Preferably, the threshold T is decreased when fc is decreased.
In another embodiment of the invention an automatic gain control (AGC) scheme is used. In this embodiment, the gain of the input amplifier is raised as the center frequency fc of the filter is lowered.
REFERENCES:
patent: 4184493 (1980-01-01), Langer et al.
patent: 4381786 (1983-05-01), Duggan
patent: 4903699 (1990-02-01), Baker, Jr. et al.
patent: 4967747 (1990-11-01), Carroll et al.
patent: 5188117 (1993-02-01), Steinhaus et al.
patent: 5339820 (1994-08-01), Henry et al.
patent: 5395393 (1995-03-01), Wickham
patent: 5404880 (1995-04-01), Throne
patent: 5431685 (1995-07-01), Alt
patent: 5492128 (1996-02-01), Wickham
patent: 5558097 (1996-09-01), Jacobson et al.
patent: 93108031 (1993-05-01), None
patent: PCT/US92/04648 (1992-06-01), None
Getzow Scott M.
Pacesetter Inc.
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