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EGM RECORDING SYSTEM FOR
IMPLANTABLE MEDICAL DEVICE
FIELD OF THE INVENTION
The present invention generally relates to implantable medical device and more particularly to a method and apparatus for recording EGM sequences in response to potential lead integrity failures in sensing leads associated with such devices.
BACKGROUND OF THE INVENTION
By way of definition, in the field of automatic implantable arrhythmia control devices, e.g. implantable cardioverter/ defibrillators (ICDs) and implantable pacemaker/ cardioverter/defibrillators (PCDs) the term "cardioversion" or "cardioverter" refers to the process of and device for discharging relatively high energy electrical shocks into or across cardiac tissue to arrest a life threatening tachyarrhythmia. The delivery of cardioversion shocks may or may not be synchronized with a cardiac depolarization or rhythm and may be applied to arrest a malignant ventricular tachycardia or ventricular fibrillation with a selectable or programmable shock energy. In practice, the arrest of atrial or ventricular tachycardia or fibrillation by such shocks delivered in synchrony with a cardiac depolarization is typically referred to as "cardioversion". Similarly, the arrest of atrial or ventricular fibrillation by a shock delivered without such synchronization is typically referred to as "defibrillation". In the following description and claims, it is to be assumed that these terms are interchangeable, and that use of one term is inclusive of the other device or operation, unless specific distinctions are drawn between them.
ICD systems have been implanted in patients over the preceding 15 years for detecting fibrillation or abnormal high rate tachycardia in a heart chamber and providing a fibrillation shock in the attempt to terminate the detected arrhythmia. Cardiac depolarizations of the particular heart chamber are sensed by sense amplifiers having inputs coupled to sense electrodes typically attached to the heart chamber as sensed events. The intervals between the sensed events are measured and compared to threshold fibrillation intervals. When detection criteria are satisfied, the defibrillation shock is delivered to the heart chamber.
Current arrhythmia control implantable pulse generators (IPGs) and associated lead systems for the treatment of tachyarrhythmias, e.g. the MEDTRONIC Model 7217 PCD IPG and associated leads, provide sensing of tachyarrhythmias and programmable staged therapies including antitachycardia pacing regimens and cardioversion energy and defibrillation energy shock regimens in order to terminate the sensed tachyarrhythmia with the most energy efficient and least traumatic therapies (if possible). The Model 7217 PCD IPG provides a programmable energy, single polarity wave form, shock from the discharge of a high voltage output capacitor bank through a pair of defibrillation electrodes disposed in relation to the heart. The Model 7217 PCD IPG also provides programmable single chamber bradycardia pacing therapies through the pace/sense electrodes.
In recent years, dual chamber cardiac pacemakers have also been proposed for incorporation into PCDs. The atrial and ventricular pacing pulse generators, sense amplifiers and associated timing operations are proposed to be incorporated into the system with atrial and ventricular pace/sense leads and electrodes. Various pacing modes may be programmed for recognizing and providing bradycardia and tachycardia pacing regimens.
Atrial or ventricular tachyarrhythmias are typically diagnosed in such systems by detecting a sustained series of short R-R or P-P intervals corresponding to an average high rate indicative of tachyarrhythmia or an unbroken series of
5 short R-R or P-P intervals between R-waves or P-waves corresponding to such a high rate. The suddenness of onset of the detected high rates (short intervals), the stability of the high rates, or a number of other factors known to the art may also be measured at this time. Appropriate ventricular tach
j0 yarrhythmia detection methodologies measuring such factors are described in U.S. Pat. No. 4.830.006, incorporated herein by reference in its entirety. An additional set of tachycardia recognition methodologies is disclosed in the article "Onset and Stability for Ventricular Tachyarrhythmia
15 Detection in an Implantable Pacer-CardioverterDefibrillator" by Olson et al.. published in Computers in Cardiology. Oct. 7-10.1986. IEEE Computer Society Press, pages 167-170. also incorporated herein in its entirety. Other atrial tachycardia, fibrillation and flutter detection
20 methodologies are disclosed in the article "Automatic Tachycardia Recognition", by Arzbaecher et al.. published in PACE, Vol. 7, May-June 1984. part H. pages 541-547 and in PCT Application No. US92/02829. Publication No. WO 92/18198 by Adams et al.. both incorporated herein by
25 reference in their entireties. In the PCT application, careful synchronization of the high voltage atrial defibrillation pulse to the ventricles to avoid induction of ventricular tachycardia or fibrillation is also discussed.
In such PCD and ICD systems, and in pacing systems of
30 all types, the integrity of the pace/sense leads and/or defibrillation leads, and the integrity of the connections of the proximal lead connector elements with IPG terminals, is of great importance. Lead insulation failures, interior lead conductor wire fracture or fractures with other lead parts.
35 and loose, intermittent connections with the IPG connector terminals, e.g. loose set screws, can occur and are collectively referred to herein as lead integrity failures. Lead integrity failures can result in lead sensing failures as follows.
40 When pace/sense lead integrity is compromised, the lead impedance may increase or decrease, depending on the nature of the failure, affecting the sensing of cardiac signals (as well as the delivery of adequate energy to the heart during cardioversion/defibrillation and/or pacing therapies).
45 If pace/sense lead integrity is not maintained, oversensing of artifacts generated in the lead and resembling high rate P-waves or R-waves can occur, resulting in a mis-diagnosis of a non-existent tachyarrhythmia. Or. undersensing due to the failure to conduct intrinsic high rate R-waves or P-waves
50 to the sense amplifier can occur, resulting in a failure to diagnose an actual tachyarrhythmia. In the event that an atrial or ventricular tachyarrhythmia is mis-diagnosed, the applied anti-tachyarrhythmia pacing therapies or cardioversion/defibrillation shock therapies may themselves
55 provoke a tachyarrhythmia episode. And. the tachyarrhythmia will not be treated in the event of a failure to recognize it In the context of bradycardia pacing, inappropriate or insufficient pacing may be caused by undersensing or oversensing failures.
60 Consequently, a great deal of attention is paid to maintaining pace/sense lead integrity in the first place and to detecting pace/sense lead integrity failures before harm can occur to the patient. A pace/sense lead failure of the types described above may be a gradual process, and collected
65 lead impedance data may signify an impedance change trend suggesting an impending failure that may be monitored more closely or may result in replacement of the lead or
re-positioning of the lead electrode. An improvement in this process is described in the commonly assigned, co-pending U.S. patent application Ser. No. '08/346.661 filed Nov. 30. 1994 for AUTOMATIC LEAD RECOGNITION FOR AUTOMATIC IMPLANTABLE DEVICE and in an abstract 5 by R. Mead M.D.. et al.. entitled "Evaluation and Potential Applications of a New Method for Measuring Pacing System Lead Impedance" PACE. April. 1995. Part n. p. 817. As shown therein, pace/sense lead integrity failures are detected by entering a test routine and directly injecting sub-threshold voltage pulses into a pair of the IPG connector terminals coupled to a pair of pace/sense lead connector elements (or into one such terminal/lead connector element and the IPG can electrode) and measuring current flow during delivery of the voltage pulse. The impedance of the circuit including the J5 pace/sense lead pair impedance is determined as a simple function of the voltage divided by the current. A high variance from an impedance range specifications of the pace/sense lead(s) provide an indication of either a fracture in a pace/sense lead body or a connection failure of the lead 2Q connector end element with the IPG connector terminal. A low impedance variance from the lead impedance range specifications is indicative of an electrical short which may be present in a bipolar lead body due to a lead insulation failure. With respect to bipolar or multipolar leads, each lead 25 conductor and associated electrode is tested in the same manner.
In cardiac pacemaker IPGs, the lead integrity check may also be undertaken during delivery of a pacing pulse. Pacing pulses are not perceptible, and therefore the patient is not 30 aware that the testing is taking place, however, pacing may be inhibited because of oversensing due to a pace/sense lead integrity failure. (Consequently, it is preferred to conduct such testing at regular intervals independently from pacing as described in the above-referenced '661 application. The 35 collected lead impedance data can be stored within IPG memory for transmission out to an external programmer through uplink telemetry on receipt of an interrogation command from the programmer.
This approach assumes that the lead integrity failures 40 relatively gradually affect the pace/sense lead impedance. Moreover, it assumes that the failure mode affects lead impedance relatively constantly and the impedance change is not so transitory that it would be missed by the periodic testing. This may be the case with lead insulation failure 45 modes, but may not be the case in other failure modes resulting in transient signals that may be detected and cause oversensing.
In the PCD and ICD system context, when undersensing or oversensing appears to be present, based on a patient's 50 description of failure to provide a therapy when an arrhythmia is perceived or of therapies delivered in the absence of a perceived arrhythmia, respectively, impedance measurements may be undertaken. One difficulty with measuring lead impedance and using the results to gauge lead integrity 55 lies again in the transitory nature of many lead failures or failures in the secure attachment of the lead connector element in the IPG connector block terminal. The condition may not be present when lead impedance is measured on a periodic basis. 60
With respect to oversensing. transient signals due to pace/sense lead integrity failures and to electromagnetic interference (EMI) have to be present in the pacing cycle when the sense amplifier is not blanked and when sense amplifier sensitivity is set to a suitable threshold level. Sense 65 amplifier sensitivity is typically established in a patient work-up in the physician's facilities. Such sensitivity set
tings may be inappropriate when the patient is exposed to EMI sources at home, in the work place or elsewhere, leading to transitory oversensing episodes. When the patient reports the delivery of the therapy (which also may be stored in the IPG memory with a time stamp), it is difficult to ascertain the source of the problem. The testing of lead impedance may prove inconclusive. Physicians are averse to programming the therapies off to simply record the details of the sensed events and attempt to correlate them in time to a patient location and to then to physically locate the source of the problem.
When the impedance testing does not prove conclusive and there is no apparent sense amplifier sensitivity error or source of EMI. one other approach that has been undertaken is to provide the patient with an external monitor for recording the patient's far-field, external skin electrode. ECG in the hope of recording an oversensing or undersensing episode and establishing a diagnosis from it. In such PCD IPGs having "Marker Channel" capabilities for transmitting out sensed event markers and also having internal EGM recording and transmitting capabilities, the external monitor operates interactively with the PCD IPG to trigger transmission and recording of all three signals. The resulting recordings may be analyzed to determine the nature or suspected cause of oversensing and undersensing episodes.
Such an episode of oversensing is depicted in the strip from such a recording depicted in FIG. 1. The oversensing is evidenced by the additional sense marker pulses that are not synchronized with the ECG and EGM tracings of R-wave peaks. The additional sense event marker pulses evidence the sense amplifier response to other signals that are randomly generated and not associated with the patient's intrinsic ventricular heart depolarizations. The sources of such extra-ventricular sense events may lie in a lead integrity failure (as defined above) a sense amplifier sensitivity problem or a sense amplifier response to EMI. Given the rate at which these sense events are generated, their number, and the suddenness of onset, the PCD IPG may interpret the sequence as a high rate tachycardia or fibrillation and deliver an inappropriate shock therapy which at the least is extremely uncomfortable and frightening to the patient.
The external monitor is useful, but is practical only in the case where the patient has already experienced the frightening and painful delivery of a shock or the failure tc respond to a tachyarrhythmia. In addition, the system disclosed in the above-referenced '661 application, while providing useful data, does not provide waveforms exhibiting the sense amplifier behavior.
Finally, it should be noted that many proposed anc existing PCD IPGs have the capability of storing EGM dab and sense detect data in relation to confirmed episodes ol tachyarrhythmias that are treated by delivery of a therapy, a; disclosed, for example U.S. Pat. Nos. 4.223.678 and 4.295 474. The data storage also may include counts of confirmee tachyarrhythmias, therapies delivered and dates of detectioi and delivery. Depending on the available memory, one oi more episodes, including 5 seconds or more of the EGV preceding and following the confirmation of the tach yarrhythmia and delivery of the therapy, may be digitizec and stored in memory.
The storage of such data may. at times, be useful ii detecting oversensing. However, it is only triggered 01 delivery of the therapy with the attendant patient alarm an< discomfort. Moreover, the rate, onset and other criteri; employed in the detection algorithms for detecting thi tachyarrhythmia may not respond to oversensing. The higl
rate threshold for counting intervals as potential high rate tachycardia or fibrillation is typically on the order of 188-250 bpm. corresponding to high rate tachycardia and fibrillation intervals of about 240-320 ms. Once such a high rate tachycardia, fibrillation or flutter occurs, the intervals 5 are too long for many exhibited oversensing episodes due to EMI or lead integrity failures.
In addition, even if sporadic episodes of oversensing due to EMI or lead integrity failures occur from time to time, initial episodes may not last long enough so that enough are to counted in order to satisfy the detection criteria. Particularly, when lead integrity failures progress to the point that enough oversense events occur within the sensing intervals to satisfy a detection criteria, the lead may be in a failure mode that leads to continuous detection and delivery of a number of is therapies which could prove dangerous to the patient. Many such PCD IPGs have a limit on the number, e.g. 4. of therapies that could be delivered in response to the continued detection of the arrhythmia following a preceding delivered therapy. However, the limit may in fact then make 20 the PCD IPG non-responsive to a genuine tachyarrhythmia provoked by the delivery of the therapies.
In the example of FIG. 1, the some of the oversense events recur at intervals of about 130 ms. Some of the oversense events could be sensed within a fibrillation detection win- 25 dow. If the episode continued long enough, the detection criteria could be satisfied resulting in delivery of a therapy to the heart.
Accordingly, a need exists for a simple fully automatic and implantable system for detecting and recording such 30 abnormal sense amplifier responses to transitory lead integrity failures or due to exposure to EMI. Such a system is needed from which the gradual deterioration of lead integrity can be determined well before the patient experiences a failure to deliver or inappropriate delivery of such a therapy. 35
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a practical and durable system incorporated into a PCD IPG w for recording intervals of oversensing for later analysis that is not dependent on a delivered tachyarrhythmia therapy.
It is a further object of the present invention to provide such a system incorporated into a PCD IPG for recording intervals of oversensing for later analysis before the over- 45 sensing increases in frequency to the level that tachyarrhythmia detection criteria are satisfied by the oversense events.
These and other objects of the invention are realized in a method and apparatus implemented in a PCD IPG and operable in an EGM recording mode for recording an EGM 50 epoch in an epoch window that precedes and follows the oversense episode upon satisfaction of short interval and accumulated oversense event criteria. In the practice of the present invention, a count of oversense events that fall within an oversense short interval (SI) accumulates over a 55 prolonged period. When a programmable short interval counter (SIC) count is exceeded within the prolonged period, EGM data storage is triggered of the EGM (nearfield or far-field) epoch associated with one or more oversense events. In a preferred embodiment, each time a 60 multiple of the initial SIC count is exceeded, the storage of the EGM epoch related to the oversense event is stored. The count number and a date/time stamp may also be recorded with the particular EGM epoch.
Preferably the EGM epoch associated with the first over- 65 sense epoch to occur is stored and remains in storage until the stored data is read out in response to an interrogation
command and telemetered out to the external programmer so that the beginning date/time may be ascertained. The more recent n EGM epochs and associated data may be stored on a FIFO basis in N stages of a rolling buffer.
The present invention may be implemented in dual chamber PCD systems for both the atrial and ventricular channels. In the ventricular chamber certain oversense events falling within the SI are excluded, namely a second oversense event detected following a ventricular pace. Such events typically result from a premature ventricular contraction (PVC) following a post-pace T-wave of an amplitude that is sensed as an oversense event. Such events are excluded because of their probable origin.
The stored near-field and/or far-field EGM epochs provide useful information to distinguish lead integrity failures from other possible causes of the frequently occurring oversense events, e.g.. T-wave oversensing. R-wave double sensing. P-wave oversensing or the like. The physician may program the oversense recording algorithm including the number of short interval counter counts to trigger EGM epoch storage, the prolonged time period for accumulating the counts, and the short interval duration. From the resulting data, the physician may initiate a further external 24 hour monitor test or proceed directly to test and re-tighten a loose connection or replace the affected pace/sense lead, if necessary.
Patient environments may be tested for EMI by having the algorithm programmed on and all counters cleared shortly before entering the. suspect environment. If EMI is present and being sensed, the SIC count will increase rapidly, and the date/time of the first stored epoch can be related to the entry of the patient into the environment or a particular location of the patient in the environment. Moreover, if both near-field and far-field EGM epochs are stored, the comparison of the waveshapes may be of use in confirming EMI.
The present invention provides the physician with a tool for determining the likelihood of lead integrity or EMI affects well before they cause inappropriate delivery of therapies.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, advantages and features of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures thereof and wherein:
FIG. 1 is an external monitor tracing illustrating oversensing of ventricular sensed events from a ventricular pace/sense lead;
FIG. 2 is a schematic illustration of an exemplary atrial and ventricular chamber pacemaker/cardioverter/ defibrillator IPG implanted in a patient's chest with am IPG can electrode and endocardial leads transvenously introduced into the RA. CS and RV of the heart wherein oversensing due to lead integrity failure or EMI may occur;
FIG. 3 is a block diagram of the PCD IPG of FIG. 2 in which the present invention may be practiced for storing near-field and/or far-field EGM epochs for both atrial and ventricular channels; and
FIG. 4 is a flow chart of the EGM epoch recording algorithm of the present invention for the atrial channel:
FIG. 5 is a timing diagram illustrating the sequence of sensed events following a ventricular pace event that signifies a PVC; and