Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-07-14
2002-02-12
Evanisko, George R. (Department: 3737)
Surgery
Diagnostic testing
Cardiovascular
C600S509000, C607S027000, C607S059000
Reexamination Certificate
active
06347245
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a device for monitoring physiological signals of a body; and, more particularly, relates to an implantable monitoring device for sensing and/or recording physiologic events with minimally invasive intrusion into the body, but which can be used with various implantable devices.
BACKGROUND OF THE INVENTION
In using implantable medical devices for recording and interpreting ECG or other physiological data, various other non-physiological signals can be recorded and used to interpret the physiological signals. For example, an automatic trigger signal used to activate data storage, or the noise present when the physiological signal is recorded can be useful in later interpreting the data record. Such non-physiologic signals can also be used to eliminate false indications of medical conditions, and to discover actual problems that would otherwise not be identified. This is particularly true when recording far-field electrogram data, since considerable noise is generally present, and the later interpretation of such signal data will be aided by storing this contemporaneous noise. For instance, in devices that utilize R-waves as a trigger event, it is of particular importance to record contemporaneous noise that may have served as a false trigger. Additionally, in devices wherein storage of signals is patient-activated, it is desirable to store and identify the patient-activation signal as the trigger event.
In the monitoring of long-term ECGs to diagnose intermittent heart irregularities, syncopal events, and other physiological conditions, minimally invasive monitors like the Reveal (TM) electrocardiogram event recorder manufactured by Medtronic, Inc. have proven to be useful. However, particularly when the device employs automatic arrhythmia detection triggers to activate the storage of a segment of the ECG, the presence of noise in the ECG signal channel may trigger activations of recordings inappropriately, causing the device memory to become full of unwanted or redundant portions of the cardiac electrogram which may be of little to no use in diagnosing the patient condition. Moreover, such noise makes interpretation and diagnose of the signal difficult.
Several problems exist with storing information related to the noise and/or trigger event associated with a physiological signal. For example, the amount of available storage must be considered. A separate memory or at least a separate location in memory from the ECG storage area may be required. Additionally, some mechanism is needed to identify which marker was associated with any given segment of ECG data storage.
An additional complexity can be found in the limitation on the nature of the data available to store electrogram data samples, especially when, for one example, the sample rate produces more electrogram features than are stored via a lossy data compression technique in long term monitoring devices, a process relied upon to save memory and achieve sufficient data storage capacity to assist the physician in evaluating a long term ECG.
Monitoring can be done using implantable pulse generators such as pacemakers and other heart stimulating devices or devices with leads in the heart for capturing physiologic parameters, including the ECG. However, the expense and risk from implanting a pacemaker or changing out one without these functions is something both patients and physicians would prefer to avoid. Such devices, in addition to performing therapeutic operations, may monitor and transmit cardiac electrical signals (e.g., intracardiac electrograms) to external diagnostic devices typically with leads fixed in the patient's heart, to observe electrical activity of a heart. It is common for implanted cardiac stimulation devices to send intracardiac ECG signals to a monitoring device, such as an external programmer, to allow a user to analyze the interaction between the heart and the implanted device. Often the user can designate that the communication from the implantable device to the programmer include a transmission of codes which signal the occurrence of a cardiac event such as the delivery of a stimulation pulse or a spontaneous cardiac depolarization.
U.S. Pat. No. 4,223,678 to Langer et al., incorporated herein by reference in its entirety, discloses an arrhythmia record/playback component within an implantable defibrillator. ECG data is converted from analog to digital (AD) form and stored in a first-in, first-out memory. When the defibrillator detects an arrhythmia event, it disables the memory so that no further ECG data is recorded in the memory until a command is received from an external monitoring device. This command requests the implantable defibrillator to transmit the stored ECG data to the monitoring device via telemetry.
U.S. Pat. No. 4,407,288 to Langer et al., also incorporated herein by reference, discloses a programmable, microprocessor based implantable defibrillator that senses and loads ECG data into a memory via a direct memory access operation. A processor analyzes this ECG data in the memory to detect the occurrence of an arrhythmia event afflicting a patient's heart. Upon such an event, the defibrillator may generate a therapy to terminate the arrhythmia event and store the ECG data sequence of the event, for transmission to an external monitoring device and later study. In normal circumstances, when no arrhythmia event is occurring, the defibrillator continuously overwrites the ECG data in the memory.
U.S. Pat. No. 4,556,063 to Thompson et al, also incorporated herein by reference, teaches a pulse interval telemetry system capable of transmitting analog data, such as sensed intracardiac electrogram signals, without converting analog data to a digital numeric value. The telemetry system is capable of sequentially transmitting both digital and analog data, individually and serially, in either an analog or a digital format, to a remote receiver. The features and capabilities of such pacemaker/defibrillator devices are now well known, but the problems in long-term monitoring for events and adequate recordation and interpretations of noisy excessively triggered records remain.
An additional implantable arrhythmia monitoring system is described in an article in the December 1992 Vol. 15 edition of PACE (15:588) by Leitch et al. In that article, a feasibility study for implantable arrhythmia monitors describes the use of subcutaneous, bipolar “pseudo-ECG” recordings.
U.S. Pat. No. 5,404,887 to Knowlan et al. describes a leadless implantable sensor for cardiac emergency warning that detects heart events through impedance measurement sensed using a coil. A similar system is disclosed in U.S. Pat. No. 5,313,953 to Yomtov et al., incorporated herein by this reference, which describes a large but leadless implant device. With sufficient hardware and connections to the body, numerous other physiologic parameters may be sensed as is disclosed in U.S. Pat. No. 5,464,434 issued to Alt and U.S. Pat. No. 5,464,431 issued to Adams et al., both incorporated herein by reference.
When using the above-described monitoring systems, it may be difficult to determine which type trigger event initiated storage of an ECG segment. The difficulties are exaggerated by the presence of interfering signals, or, in other cases, by the absence of some interfering signals that have been removed by filtering techniques or other anti-noise responses. Moreover, with subcutaneous, or far-field electrodes, ECG signal amplitude may vary greatly with mere change in patient posture, making it difficult to assess whether the recorded signal is a real arrhythmia, or an artifact of poor detection As noted above, this is particularly true when the ECG is reconstructed from a compressed electrogram data.
Therefore, there remains a need to indicate the type of noise that is present in a particular ECG segment, and to do so in an efficient manner within the constraints imposed by the limitations of inexpensive devices with limited communications capacity, battery strength, memory capac
Kane Michael R.
Lee Brian B.
Evanisko George R.
Medtronic Inc.
Wolde-Michael Girma
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