WO1992019319A1

WO1992019319A1 - Apparatus and method for an implanted cardiac pacemaker with multiuse electrode with potential restoration

Info

Publication number: WO1992019319A1
Application number: PCT/US1992/000502
Authority: WO
Inventors: Edward A. Schroeppel
Original assignee: Intermedics Inc
Current assignee: Intermedics Inc
Priority date: 1991-05-06
Filing date: 1992-01-21
Publication date: 1992-11-12
Anticipated expiration: 1993-11-06
Also published as: US5228437A; DE69217932D1; EP0583246A1; JPH06506840A; DE69217932T2; EP0583246B1; ES2101087T3

Abstract

An apparatus for measuring an electrical potential in a human body by sampling the potential during a selected interval and storing the value of that potential at the end of the interval. The stored value of the potential will be restored as an initial value at the end of a second interval. The apparatus includes a timing circuit to identify appropriate intervals. Means are provided for sampling and storing the value of the electric potential at a given time and for restoring the measured potential to an electrode at a second selected time.

Description

APPARATUS AND METHOD FOR AN IMPLANTED CARDIAC PACEMAKE WITH MULTIUSE ELECTRODE WITH POTENTIAL RESTORATION

TECHNICAL FIELD

My invention relates to implantable therapeutic devices having electro carried on leads, and in particular to implantable cardiac pacemakers. M specifically, my invention relates to a cardiac pacemaker having an electrode on implanted lead adapted to perform multiple functions through the same electro for example, the function of pacing the heart and the function of measurin parameter indicative of the physiologic condition of the patient or the patient's he

BACKGROUND ART

Many therapeutic devices use electrodes to measure electrical potential selected locations in a human body. Electrodes have been used to measure elec signals relating to chemical concentrations, nerve activity, respiration rate, or card rhythms, as examples. Cardiac pacemakers, for example, stimulate the heart electrical impulses to induce a heartbeat. Pacemakers may also sense the condit of the heart so that stimuli may be applied in an appropriate manner. Electrodes a lead of a cardiac pacemaker may be used to measure features of the intracard electrogram, lead or tissue impedance, pH of the blood and other parameters. addition to the desired, information carrying signals, the electrode may det unwanted signals such as remote or far field signals, noise, muscle activity potenti and so on. The sense electrode or another electrode may be used to stimulat physiological mechanism by an appropriate signal; for example, stimulating the he to induce a heartbeat. Such a stimulation can result in a residual polarizati voltage which masks the desired signal.

Accurate signal detection through an implanted electrode continues present difficulties for the designer of an implanted therapeutic device. In so applications, it may be desired to detect one or more input signals and to apply or more stimulating or output signals. At the same time, it may be inadvisable employ a dedicated electrode for each operation. The desired size of a lead limit the number of conducting wires which can practically be connected to separ electrodes or the number of electrodes themselves may be limited. Moreo interelectrode cross-talk between separate electrodes may limit the usefulness separate electrodes.

To solve these problems, time multiplexing of an electrode has heretof been employed. The same electrode used to output a stimulating pulse durin first interval may be employed to sense cardiac signals during another interval a noise or some other parameter during yet another interval. Bandpass filtering h been used to discriminate between desirable signals and noise. These metho however, are not effective in all cases. Unwanted signals may mask desired sign in certain applications. In other applications, a signal of interest may mask anoth signal of interest, requiring the rejection of the first signal in order detect the secon

With the foregoing in mind, it is an object of my invention, therefore, provide an apparatus and method for an implanted therapeutic device having electrode which can be used for multiple functions.

It is also an object of my invention to provide an apparatus and electro which can be used for both input and output functions.

Another object of my invention is to provide an implanted apparatus whi can extract information from a background signal during a selected interval despi the presence of a primary signal which tends, during another time interval, influence the electrode potential to destroy the electrode's ability to sense t background signal.

DISCLOSURE OF INVENTION

I have invented an apparatus which measures an electrical potenti characteristic of a parameter during a selected interval and stores the value of th potential at the end of the interval. During a successive interval, an intervenin event may occur to seriously disrupt the potential. This intervening event ma either an intrinsic potential caused by the human body or an external stimulation example, a stimulus from a pacemaker. If such an intervening event occu during the second time interval, the stored value of the measured potential wil restored as an initial value at the end of the second interval.

Apparatus, according to my invention, includes a timing circuit to ide appropriate intervals. Control circuitry or software is provided to determi conditions have occurred which require potential restoration. Means are provi for sampling and storing the value of the electric potential at a given time and restoring the measured potential to an electrode at a second selected time. means may include electrical circuitry or software and may employ either an circuitry or analog to digital conversion with digital registers or memory circu Algorithms to establish appropriate responses to detected noise or other classe signals may also be provided.

My invention may be employed with electrodes generally, inclu physiological, chemical, physical, or other electrodes. It may also be used to the initial potential conditions of one electrode with reference to a detected condi at another electrode.

These and other objects and features of my invention will be apparen those skilled in the art from the following detailed description of my prefer embodiment, made with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is perspective view of a cardiac pacemaker system according to present invention with a cardiac pacemaker and a lead, showing placement of lead in a cutaway section of a human heart.

FIG. 2 is a block diagram of a heart pacemaker employing appar according to my invention. FIG. 3 is a logic diagram for describing timing of switches according to present invention.

FIG. 4 is a graph of a potential varying over time.

FIG. 5 is the potential of FIG. 4 sampled at selected intervals.

FIG. 6 is a graph of a sampling of the potential of FIG. 4 with the potenti forced to zero at the beginning of each interval.

FIG. 7 is a sampling of the potential of FIG. 4 with the terminal conditions each sampled interval imposed on the succeeding sample interval as initi conditions.

FIG.8 is a graph of an electric potential comprising a potential similar to th shown in FIG. 4 combined with an intrinsic cardiac depolarization signal.

FIG. 9 Is a graph of the potential of FIG. 8 combined with stimulating cardia pacing pulses.

FIG. 10 is a potential including the potential of FIG. 9 combined wit restoration of initial conditions according to my present invention.

FIG. 11 is the potential of FIG. 10 with the cardiac pulses and stimulatin pulses eliminated.

BEST MODE FOR CARRYING OUT THE INVENTION

I will now describe my preferred embodiment of my invention. In referring t the accompanying figures, like numerals will be used to refer to like parts throughou this description. Although I have chosen to describe my invention in terms of implanted cardiac pacemaker, those skilled in the art will recognize that my inventio can be applied to measure electric potentials in many applications. ln FIG. 1, a perspective view of a cardiac pacemaker system , gener designated 10, is shown in perspective view. A cardiac pacemaker 12 is provi having the capacity to sense electrical artifacts at sense electrodes within a he A lead 14 is shown connected to the pacemaker 12. In the illustrated embodim the lead is illustrated as having a distal electrode 16 at a distal end of the lead. is known in the art, fixation means should generally be provided. I have illustra tines 21. The distal electrode 16 may be used for both sensing and stimulating heart in accordance with my present invention. An additional ring electrode 1 shown within the atrium of the heart. The ring electrode 18 may also be used multiple purposes and employed with the apparatus of my present invention. It be apparent that the number of electrodes employing the invention is not limite

FIG. 2 illustrates a block diagram of the cardiac pacemaker 12 showing t apparatus of my present invention. The lead 14 and the distal tip 16 are illustrat Additional connections may be provided for additional electrodes, such as electro 18. Timing and control circuitry 22 control pacing output circuitry 24 to provi stimulating pulses to the heart and, at appropriate times, receive signals fr sensing input circuitry 26 from the heart. Multiplexing between pacing and sensi are known in the pacemaker art and need not be described in greater detail here Sensed signals may comprise the heart's QRS complexes, polarization potentials the electrode 16, noise from electromagnetic sources or from muscles of the bo or various physiological parameters such as respiration, pH and so on. Timing a control circuitry 22 may be implemented by those skilled in the art in either electri hardware or a combination of hardware and software.

Under the control of the timing and control circuit 22, a first switch 28 clos briefly after an output pulse is delivered from the pacing output 24. Closing of t first switch 28 connects the electrode 16 to ground or charge dump 29. The timi and control circuitry 22 may then begin to time out a physiological parame sampling interval for a predetermined length of time. At the beginning of t sampling interval, the timing and controlling circuit closes a restore switch 30 whi connects a charge restore circuit 32 to the electrode 16. The charge restore circ 32 is effective to restore the electrical condition of the electrode to the condition that electrode at the end of the last preceding sampling interval. The charge co circuit 32 may be implemented by a storage capacitor and analog circuitry o digital circuitry with a digital to analog converter.

The timing and control circuitry 32 then activates the sensing input 2 measure the desired electrical potential at the electrode 16 during the samp interval. At the end of the sampling interval, the timing and control circuit 22 bri closes a charge store switch 34 and stores the value of the potential of the electr 16 in charge store means 36. As with the charge restore means 32, the cha store means may comprise analog or digital circuitry. Any anticipated losses to electrode potential occurring in either potential measurement, charge storage restoration are compensated in transferring the charge storage means 36 to charge restore means 32.

Control of the switches 28, 30 and 34 can also be understood with refere to the logic diagram of FIG. 3. Under the control of the timing in control circuit all switches are normally open 38. Before attempting to restore the electri condition of the electrode 16, as, for example, after a stimulating pulse from pacing output 24, first switch 28 closes 40, connecting the electrode to ground a then quickly reopens. Just before the sampling interval and after switch 28 been closed, the charge restore switch 30 closes 42, allowing charge rest circuitry 32 to charge the electrode 16 to the predetermined electrical condition. T electrical condition of the electrode is relatively quickly restored and then switch again reopens. Just before the end of the sampling interval, the charge store swit 34 is closed 44 connecting the charge store means 36 to the electrode. T electrical condition of the electrode is rapidly sampled over a very short interval a the switch 34 is again opened.

I will now explain the operation of my invention with reference to the grap of FIGS. 4 through 11. FIG. 4 illustrates an exemplary electrical potential 46 whi varies in amplitude over time. The electrical potential is the signal which is to measured or detected by an electrode. This does not imply, however, that t potential 46 is the only electrical signal present. There may be other signa extraneous noise, or, in the case of a heart pacemaker, output signals or stimulati pulses which mask or make the measurement of the potential 46 more difficult.

FIG. 5 shows an idealized measurement of the potential 46 by a multiplex electrode. As the electrode 16 is multiplexed to its sensing function, poten segments 48 are sensed during selected intervals 50. In the desired situation, t potential segments 48 closely approximate the potential 46 and meaning information about the potential 46 can be extracted from the potential segments With uncompensated multiplexing alone, however, the wave form of FIG. 5 is seld if ever retrieved. This is because the initial conditions of the segments 48 in rea do not correspond to the initial conditions of each segment as shown in FIG. Rather, each segment begins at some other value and only approaches the tr value for the segment. Such a condition is illustrated in FIG. 6. At the beginning each time segment 50 the value of the measured potential is constrained to be ze Such a condition might occur if the electrode were grounded immediately before t beginning of the time segment 50. Those skilled in the art will recognize, of cour that the zero initial condition is chosen only as an example because of its simplic and clarity. Any other initial condition, constant or varying, which was not relat to the desired signal 46, would have the same negative affect. Because of t inherent time constants of the system, during each measurement 50 a measur segment of potential 52 will only begin to approximate the true value of the poten 46. A great deal of information about the potential 46 could be lost, resulting potentially erroneous responses by an implanted device such as a cardi pacemaker.

In a device with apparatus and methods according to my invention, the wa form of FIG. 7 would be recovered. During each time segment 50, the magnitu of the measured potential would more closely approximate the true potential 46. the end 56 of a potential segment 54, the magnitude or other parameter of t potential would be stored. At a beginning 58 of the next succeeding segment the ending value at 56 would be restored to the electrode. Measurement wo then begin and the potential on the electrode would begin to approach the tr value of the potential 46, constrained by the inherent time constants of the particu system. Since it is likely, however, that the value at the end of the last measur period more closely approximates the true value at the beginning of the n succeeding period than would an arbitrary value such as zero, the measuremen potential, as shown in FIG. 7, would more closely approximate the ideali measurements of FIG. 5 and thus be a better representation of the true poten shown in FIG. 4.

FIGS. 8 through 11 illustrate an application of my invention in the field cardiac pacing. FIG. 8 shows a signal 60 similar to the potential 46 of FIG. 4 with the addition of an intrinsic intracardiac QRS complex 62 at periodic interv Such a signal 62 would be detected on an electrode in the right ventricle of a typi healthy heart. The intrinsic QRS signals 62 are essentially added to the underlyi baseline potential 46 shown in FIG. 4. To extract the baseline potential a low pa filter or sampling method analogous to that illustrated in FIG. 5 might be employ Under such circumstances filtering and sampling would be adequate to extract t desired information.

If a cardiac pacemaker is present, however, additional complications a introduced. FIG. 9 illustrates the result of cardiac pacing on a prior art bandpa and sampling electrode. A pacing pulse 64 and resulting depolarization disrupts t measurement of the signal 60. Charge dumping, a known technique used to redu lead polarization potential, tends to force the voltage at the electrode to zero aft each pulse. The sampling of the underlying baseline potential occurs during t intervals between pulses. Because of the pacing, the sampling intervals 50 m tend to occur irregularly over time, being associated with a delay after the paci pulse. Moreover, charge dumping removes the baseline information and distorts t sample values. Over time, of course, the baseline will return to its natural value b during the time period surrounding a series of pacer pulses, significant amounts information will be lost.

Employing my invention, however, tends to restore the information regardin the baseline 60. As seen in FIG. 10, the ending condition of one segment i restored at 66 to the next time segment. The baseline can be much more closel approximated. FIG. 11 extracts an approximation 61 of the baseline informat from FIG. 10 and omits the pacer pulses and intrinsic QRS waves. A compari between this waveform and the waveform of FIG. 8 or FIG. 4 illustrates that a clo approximation of the baseline potential can be expected.

It will be apparent to those skilled in the art that my invention can embodied in other configurations without departing from the teachings or essen characteristics thereof. The foregoing description is, therefore, to be conside illustrative and not restrictive and the scope of my invention is to be defined by following claims. All changes or variations that would come within the equivale of the claims are intended to be incorporated therein.

Claims

1. An apparatus for measuring an electrical potential comprising an electrode capable of sampling said electrical potential; and characterized by: timing means for timing a series of first and second intervals; means for measuring said electrical potential during said first intervals; means for storing the value of said electrical potential at the end of said fi intervals; and means for restoring the stored value of said electrical potential at the end said second intervals.

2. The apparatus according to claim 1 further comprising means compensating for expected losses in restoring the stored value of said electri potential.

3. The apparatus according to claim 2 further comprising means for connecti the electrode to ground prior to restoring the stored value of the electrical potenti

4. The apparatus according to claim 3 wherein the storing means, the restori means, and the ground connecting means each comprise switch means f selectively connecting and disconnecting the respective means to said electrode a wherein said timing means further comprises means for controlling said switche

5. The apparatus according to claim 4 wherein the switches are normally op thereby disconnecting the storing means, the restoring means and the grou connecting means from the electrode except when the respective means a functional.

6. The apparatus according to claim 5 further comprising means for deliverin an electrical output through said electrode during at least some of said secon intervals.

7. A heart pacemaker comprising means for sensing an electrical potential in the heart, said sensing me comprising timing means for timing a series of first and second intervals; means for measuring said electrical potential during said first interv means for storing the value of said electrical potential at the end said first intervals; and means for restoring the stored value of said electrical potential at end of said second intervals; and means responsive to said electrical potential sensing means for pacing heart as a function of said electrical potential.

8. The heart pacemaker according to claim 7 further comprising means compensating for expected losses in restoring the stored value of said electri potential.

9. The heart pacemaker according to claim 8 further comprising means connecting the sensing means to ground prior to restoring the stored value of electrical potential.

10. The heart pacemaker according to claim 9 wherein the storing means, restoring means, and the ground connecting means each comprise switch me for selectively connecting and disconnecting the respective means to an electro carried on a lead and wherein said timing means further comprises means controlling said switches.

11. The heart pacemaker according to claim 10 wherein the switches normally open thereby disconnecting the storing means, the restoring means a the ground connecting means from the electrode except when the respective me are functional.

12. The heart pacemaker according to claim 11 wherein said pacing me further comprises means for delivering an electrical output through said electro during at least some of said second intervals.

13. A method for measuring an electrical potential comprising the steps of sampling said electrical potential; timing a series of first and second intervals; measuring said electrical potential during said first intervals; storing the value of said electrical potential at the end of said first interv and restoring the stored value of said electrical potential at the end of said sec intervals.

14. The method according to claim 13 further comprising compensating expected losses in restoring the stored value of said electrical potential.

15. The method according to claim 14 further comprising connecting a sampli electrode to ground prior to restoring the stored value of the electrical potential

16. The method according to claim 15 further comprising selectively connecti and disconnecting storing means, restoring means, and ground connecting mea to said electrode.

17. A method for pacing a human heart comprising the steps of sensing an electrical potential in the heart, said sensing step comprising timing a series of first and second intervals; measuring said electrical potential during said first intervals; storing the value of said electrical potential at the end of said fir intervals; and restoring the stored value of said electrical potential at the end of sa second intervals; and pacing the heart as a function of said electrical potential.

18. The method for pacing a human heart according to claim 17 furth comprising compensating for expected losses in restoring the stored value of sa electrical potential.

19. The method for pacing a human heart according to claim 18 furth comprising connecting sensing means to ground prior to restoring the stored val of the electrical potential.

20. The method for pacing a human heart according to claim 19 furth comprising selectively connecting and disconnecting storing means, restori means, and ground connecting means to an electrode carried on a lead.

21. The method for pacing a human heart according to claim 20 wherein sa pacing step further comprises delivering an electrical output through said electro during at least some of said second intervals.