CA2219310A1

CA2219310A1 - Electromagnetic noise detector for implantable medical devices

Info

Publication number: CA2219310A1
Application number: CA002219310A
Authority: CA
Inventors: Patrick J. Paul; David Prutchi
Original assignee: Individual
Current assignee: Intermedics Inc
Priority date: 1995-06-07
Filing date: 1996-05-31
Publication date: 1996-12-19
Also published as: WO1996041203A1; EP0862743A1; US5697958A; JP2001515366A

Abstract

Disclosed are apparatus and method for detecting electromagnetic interference (EMI), or noise, that may disrupt the proper operation of medical devices (10) implantable in patients, such as cardiac stimulators. Circuitry of the detector of the invention is independent of other circuitry of the medical device. EMI is magnetically induced on an antenna (40) that may be within the metal housing (252) of the device (10) in a receiver circuit (40), and the EMI signals are output to the noise detector (66, 68). A variety of alert signals may be provided to the medical device circuitry to warn of the presence of EMI so that appropriate responses may be taken to insure the safety of the patient dependent on the device. The detector may share the telemetry antenna of the medical device, or utilize a separate, dedicated antenna to receive EMI. Alternative antennas external to the metal housing of the medical device include leads from the device to the heart of the patient, and a dedicated antenna (260) in the nonmetal header (254) of the device.

Description

W O 96/41203 PCT/U',~ 178 Descrirtion Electrom~n~tic Noi.~t- Detector for Imrlantable Mel1ir~l Devices T.orhnir~l Field The present invention relates to techni-luP~ for tllqteeting electrt-m:~gn~tir fields which may cause 5 hllclr~lcllce to implantable medical devices, and particularly to methofl~ and a~aldlus for idellliryillg electrom~gnrtir noise so that a~pl~l;alc responses can be effected by the implantable devices to avoid m~lfimrti--n thereof caused by the noise. The present invention finds particular application to implantable n~u~ l-ccul~r stim~ tQrs and implantable cardiac stimlll~t-)rs such as p~rPm~kers and defibrillators, inrlllding ;~ ""~ir implantable cardioverter defibrillators (AICD), and provides Pnh:~nred noise ifl~-./irif~linn for the proper l~:~ol~se of such devices when exposed to various sources of illlclrelcllce~
13ar~round of the Inverlfi- n The p~lrollllallce of i...~ .hlc medical devices 5,~ li",e5 suffers due to ill~clr~lellce caused by electr ~m~gnPtir noise. In impl~nf~hle p~cem~kers and defibrillators, for example, such illlclr~ellce may cause the device to operate so as to put the patient's life at risk. For example, ~ir~ Cl~ gnf-fir noise may cause inhibition of a parPm~ker in a p~rem~k~or~ .I patient, or may be recognized as an ;~lhylllll~id by a defibrillator, causing it to hla~l~lialcly shock the patient. Sources of electrom~gnlofir hlLclr~lcllce (EMI) are c.. -.-.. , and cannot always be avoided by patients with pacpm~kerc or j",l,l~,.l l l.o defibrillators, for example. Some e~mI)les of such sources of hll~lr~lellce are anti-theft article surveillance devices in stores, cellular telellhon~-~, power l,al~çullll~s, welding ç~ .l and certain 20 medical equipment such as m:lgn~fir ncul~ r sfimlll~fors, rli~fhrrmyil~llulllcllL~ele~;llu~ulg~
devices and m~gnefir resonance imaging (MRI) units.
Electrom~gnrfir iul~clrcl~llccc may enter an i~ lallLdble pacing or defibrillator device either directly, a~eàlillg on the sensitive electronic cil~;uilly through electr~-m~gn.ofir. induction for which the typical titanium casing of the device does not provide snfflri~ont .chil~lrling, or indirectly through the 25 electrodes or leads to the device, acting as ~I~~c, wif~h high frequency noise bypassing the high fie~lu~ ;y noise protection circuits by taking erratic pdlhwdy~. Although p~rlom~kPr and defibrillator circuits are dç~ignPd to reject unwanted freq~lPnri~, it is difficult, if not impossible, to reject hl~t:l~lellcc signals having the same or similar rl~.a~ ics as the signals produced by the heart. In such cases, even speci~li7rd noise detection ~ih-;uilly based on repetition rate dis~;iilllillalion may be fooled by EMI.
3 0 It is ull~lerule advantageous and desirable to provide an independent means for the detection and confirm~tion of the presence of near-field electrom:-gnPtic noise in the management of ill~elr~l~nc~
conditions in impl~nt~hle cardiac medical devices. The present invention provides such an independent means, using a m:~gnPtir~lly coupled circuit, for example, for the wideband detPctir~n and co..r....~1 iOll of near-field electrom~gnPtic noise that may be induced directly or indirectly into the circuitry of an implantable cardiac ~timnl:~tor~ with the EMI ~l~otpction and c- - .r.. ~, ion circuitry of the present invention unrelated to sensing cil~;uiLly of the implantable medical device.
U. S. Patent No. 5,383,912, issued January 24, 1995, and assigned to the ~ gnPe of the present invention, discloses a method by which an external device c~--",~lir~s to an il"l,l,.lll l)le medical device by sending to the antenna of the irnplantable device electr m~gnPtir energy which is det~cted by a non-linear device, such as a diode, and stored as elec~ lic energy. After a delay period ~ cscllLillg data to be commnnir~t~l, the stored energy is released and Ll,.ll~ d by way of tbe antenna as ele~ gnf~tir energy back to the external device. U. S. Patent No. 5,383,912 shows one possible form of antenna circuit with which the present invention may be employed.
While embodiments of the present invention are presented herein in the context of implantable medical devices such as cardiac .ctim~ tors, the term "medical device" as used herein is int~nfl--d to include any type of h~Ll.llll~ irJn that is impl~nt~ble within a living site; the term "patient" as used herein is int~n~lrd to include any type of living being, both human and nl-lllllllll,,ll The present invention is Lll,lcr~lc applicable to any type of instrllmf~nt~ti~m that is implantable within any type of living being.
Di.~rlosure of the Jnvention The present invention provides a~,~ald~us and method for ~l~tecting the ~lC~llce of clc~,LI~ gnf~tir iulLclr~l~"lce in an implantable medical device, inflPpen-l-ont of other ~;h~;uiLly of the device, and .cign~ling the device of the presence of hlLclrclellce so that ~ liaLc steps may be taken in lc~l,ol.se.
An elecLl~ l lrl ir hlt~,lf~ llce detector accoldillg to the present invention utilizes an antenna that may be the antenna utilized by a tCl~lllcLIy Cil~uiLly of the medical device, or which may be ~ ir~trd to 2 0 EMI reception. In the latter c~e, the antenna of the detector may be a coil within the c~ing of the device, such as a printed coil, a ferrite core coil or an air core coil, for example, or the antenna of the detector may extend beyond the metal case into the plastic header of the medical device so that the antenna is exposed to the electrical component of the electr )m~gn~tir. hlLelrclcilce and not just to the m:~gn-otir flux of the hlLclr~lcllce inside the case. Another alternative antenna for use by a detector according to the 2 5 present invention is one or more leads between the medical device and the patient.
Signal processing ~ ;uiLly is provided to process received and detected hlLclrcl~llce signals and provide a signal to the medical device depending on the presence of hlLclrclcllce. The signal processing ~;hcuiLly may include a threshold detector that selects those portions of the ~letectrd hlLclrclcllce signals that are above a de~ign:~trd threshold value and provides a signal to the medical device in-lir~ring whether hlLtlrclellce above the threshold value is present. The signal processing ~;h~;uiLIy may include an analog-to-digital converter which converts the detected hlLclrcicllce signals to digital form and provides signals to the medical device inf1ir~ting the strength of hlLclrtlcllce c7etl~çtecl The signal processing cil~;uilly may include CilcuiLly for C~ aliilg fletect~d hlltlrclellce with a signal of physiological origin, for example, received by the medical device from the patient by way of a 3 5 lead, such as circuitry for sampling the hl~clr~ lce and the signal from the lead, for example, threshold WO 96/41203 PCT/U~ 178 detectors for selecting those portions of the illLelre~ ce and the signal from the lead that are above a ~lecign~ttod threshold value, and a coinrirlenre detector for culll~dlillg the portions of the h~k;lrelellce and the signal from the lead that are above the ~lPcign~ted threshold value and providing a signal to the medical device intiir~ting whether a coinr~ qnre exists between the co~ aled portions of the i--Lt:-r~,r~"lce and the signal from the lead. When used, sampling ~;h~;uiLl~ may include two amplifiers that sample the illLelr~l~nce signal and the signal from the lead, respectively, and, whenever a sampled signal circuit is used, a timing circuit directs the sampling by the two amplifiers to occur at the same rate and in unison.
In a method of the invention an electrom~gnetir illLtlrel~;llce detector is provided that is independent of the 1il~;Uill,y of the medical device. Illl~lrtlcince signals are received on the antenna of a receiver and tletectecl~ then ~luce~ed to provide a signal to the medical device depending on the piest;.lce of illt~lrèlellce~ The antenna may be provided as a system of multiple coils oriented in dirre-c -- directions, or as a single coil having turns oriented along two or more dirrt l~ directions. Ploce.,~illg the i llelf~ llce signals may include colll~alillg them to a ~lecign~ted threshold, and the step of providing a signal to the medical device that in-lir~te,c whether hllelrtl~;llce above the threshold is present. Ploces~,i..g the illlelrelellCe signals may include converting them to digital form from analog form, and providing a signal to the medical device that in-lir~tes the strength of illl~lrèl~nce ~letec~<l Processing the hllc~lr~ llce signals and providing a signal to the medical device may include sampling detected i~ lrelellce and sa~ lillg a physiological signal from the patient at the same sampling rate and in unison with the sampling of the hllelrelcllce~ comparing the sampled hlle-rel~llce and the 2 0 sampled physiological signal from the patient with a ~iecign~terl threshold value and s~lPcting those portions of the sampled illl~lrelc:llce and the signal from the patient that are above the ~l~sign~ted threshold value, and cu..,l.,.. ;..g the portions of the illlelrel~llce and the physiological signal from the patient that are above the flecign~ted threshold value and providing a signal to the medical device in-lir~ting whether a coinr~ nre exists between the cu.ll~al~d portions of the h~ r~ lc~;t and the physiological signal from the 2 5 patient.
~riefDescrir~tion of Dlawi-~
Fig. 1 is a block diagram of an implantable cardiac stimnl~tor incorporating an electrom:~gnl tic noise detector according to the present invention; Fig. 2 is â block diagram of a preferred embodiment of a noise detector according to the present invention, utilizing the telemetry antenna of an implantable medical device to detect electrom~gnetic noise; Fig. 3 iS a sr-hem~tic diagram of another preferred embodiment of a noise detector according to the present invention, utilizing a ~erlir~fe~l ~nt~-nn~; Fig. 4 O is a srhr~ c diagram of still another embodiment of a noise detector according to the present invention, providing fullwave rectification of the EMI signal; Fig. 5 is a srhrm~tir diagram of a noise detector according to the present invention, showing details of signal processing cil~;uilly; Fig. 6 is a graph of a 3 5 hypothetical twelve-hour peak EMI exposure profile for a patient having an implantable medical device W O 96/41203 PCT/U~ 178 with a noise detector according to the present invention; Fig. 7 is a flowchart of process steps for use in an impl~nt~hle cardiac .stimnl:~t-r in c~"-j"".~ with a noise detector according to the present invention;.
Fig. 8 is an illustration of an antenna system inrln-ling three coils oriented mutually orthogonally for m~imnm directional noise .c~ul~ivc-~ess for use as part of a noise detector according to the present invention; and Fig. 9 is a side elevation of an implantable medical device, ill~ ,,./;"g another type of independent, ~lP-lir~ted antenna which may be utilized by a noise detector according to the present invention.
l~Pt~iled Descriptions of thP Plcsc~llly Preferred FmhotlimPnt~
A noise detector accol-lhlg to the present illvcllLioll may be utilized in collju~lcLion with any type of implantable medical device, inrlll-ling, but not limited to, nCu~ c~l,.l ctimlll:~t~rc and cardiac .~timnl~t-)rs, such as p~rem~ker.~ and defibrillators, and implantable drug delivery devices, for example.
To this end, the present ill~ iull is shown in Fig. 1 inrlurlPd in a generic impl~nt~hle cardiac stim~ t -r which leplcscllL~ all forms of such devices, for ~uu,~oses of illll~ . rather than limit~tinn Further, the noise detector of the present invention is in~lir~tPd in Fig. 1 srhP-m~tir~lly only, and is infPnflPd to rcAu.~sc,,L therein the present invention in general, as well as the several preferred embo-limPntc of the present invention r1iccllc.~ed more fully below. In Fig. 1 an implantable cardiac ctimlll~t- r is shown generally at 10, and includes all of the culll~ol1~.lL~ of any such device. A llli~lu~?luce~ol and lll~;;lllUl,y 12 provides control and culll~ facilities for the device 10, as well as memory c~iliLy. Memory for the device 10 can be ,uruvided in whole or in part in a section separate from the lllicloplucessor. Also, it will be d~l,lcciaL~d that other forms of cil-,uiL~y, such as analog or discrete digital ~;hcuiLly, can be used in place of the microprocessor 12.
Sensing ~;hcuilly 14 and stimulus circuitry 16 are c~-nnPctPd to the Illiclu~ulucessor and memory 12 by lines 18 and 20, respectively. The sensing CilcuiLly 14 may lC~UlCScll~ amplifier sections for atrial sensing and/or ventricular sensing, according to the specific form the device 10 assumes. Likewise, the stiml~ll-c cilclliLly 16 may be of various forms, depending on the nature of the device 10. For example, if the device 10 is a p~Pm~kPr, the stimulus circuitry 16 may be an atrial pacing signal gcllcl~Lul, or may be a vçntrirlll~r pacing signal generator, or may lc~lcScllL both atrial and ventricular pacing signal generators. If the device 10 is a defibrillator, the stimulus cilcuiLIy 16 may include a high voltage generator for producing shock signals utilized to defibrillate the heart of the patient. In a combination system inr~ ing a p~rrm~k~-r and a defibrillator, the stimulus ~:h-;uiLly 16 may lc~lescllL multiple ge~ for producing the various stim--l~ti~-n signals required to shock or pace the heart, for example.
The sensing circuitry 14 is shown cullllec~cd to the heart of the patient, in whom the device 10 is implanted, by a lead system 22 that ends in one or more electrodes ~,u"uloplidtc for the device 10. For example, if the device 10 is a dual chamber p~rPm~krr that senses the conditions in both atrial and 3 5 ventricular chambers, the lead system 22 may include two lines ending in two electrodes, with one _~_ electrode positioned in an atrial chamber and the other electrode po~hion.od in a ventricular chd~llber.
Otherwise, if the device 10 is to sense heart function in only one clldlll'L/el, the lead system 22 may end in a single electrode ~,u~lo~ul;alely p~itioned relative to the heart. In any event, the sensing cheuiLly 14 detects signals in the ~lP~ign~tt-d area or areas of the heart by way of the lead system 22 as i"dicd~ive of the heart function, amplifies the sensed signals and conveys them to the llliclul,locessor and memory section 12 over the line 18. It will be appreciated that heart signal sensors are not the only type of sensors utilized by implantable medical devices. Other types of sensors include oxygen sensors, for example, and the present invention may be utilized with medical devices employing such sensors as well.
The stimulus Uil~;Ui~ly 16 is shown c- I,llP~led to the heart of the patient by a lead system 24 that ends in one or more electrodes d~lu,ul;ate for the device 10. For example, if the device 10 is a ~l~fihrill~tûr, the lead system 24 may include multiple leads and high voltage patch electrodes for applying shock signals to the heart. If the device 10 is a cu~ hldLion cardiac stim~ tor, the lead system 24 may e~ .éll~ a defibrillator electrode system as well as a pacer electrode system. In the event that the device 10 is a p:~c.om~ker, the two lead systems 22 and 24 may be culll'o;"cd to provide electrodes used for both ~rlllliring sensed signals from the heart to the sensing circuitry 14 as well as cullveyillg pacing signals from the stimulus circuitry 16 to the selected one or more cl,al,,bcl~. of the heart. Timing Cil~;uiLly 26 is shown c~.lll.Ptle-l to the miclv~uluce~.sul and lll~;lllUly section 12 by a line 28 to provide ti~ming signals necessary for t'ne ~ela~iOll of the device 10. For example, the timing circuitry 26 may provide pacing interval timing for a p~r~m~ker, and a clock for any other required timing signals for the upe~ of the device. The line 28 is shown ~ sel,lh~g a capability for timing signals to be conveyed to the ,,,iclo~,ucessor 12, and a capability for triggering signals, for example, to be conveyed from the microprocessor to the timing ;Ui~l,~/.
A telemetry section 30 is connected to the microprocessor and memory 12 by a line 32, and is equipped with an antenna 34. The telemetry system 30 may receive h~rol"ldLion from the microprocessor 2 5 12 cullcelllillg the functioning of the heart as well as the condition of the device 10, for example, by means of the line 32, and transmit such i"rull"a~ion over the antenna 34 to an a~,u,u~l;ale receiver outside the patient. Also, the telemetry system 30 may receive pro~;ldll.~hlg and comm~nrl~ from an ~ttf n-ling physician, for example, by way of dpp~o~,iate comml-nie~tion signals received over the antenna 34, and convey such information over the line 32 to the microprocessor and memory 12. In this way, various 3 0 timing controls and thresholds may be adjusted, for example, to ~Jroplid~ ly control the operation and responses of the device 10 to meet the needs of the patient. Typically, the telemetry antenna 34 takes the form of a multi-turn coil inside the housing, or case, of the device 10.
In practice, EMI that is a concern addressed by the present application may be received by leads to the heart such as the lead systems 22 and 24 of the device 10, and conveyed to the circuitry thereof.
Also, EMI may penetrate the case coll~ g the device 10, although such case is typically constructed WO96/41203 PCT~US96/08178 of ~ .i.,.,, and hrrmt-fic~lly sealed. In particular, such noise invading the case may be m~gnrtir~lly induced on the coil antenna 34, for exarnple, and thereby introduced into the cil~;uilly of the device 10.
The present invention provides a trr'fmi~le for ~letrcting such EMI that may be iu.~ .ed on the ~ih-;uilly of the device 10 regardless of the avenue by which the noise has reached the circuitry, and for identifying the noise as such to the device which can then respond so as to not endanger or nnnpcec~rily stress the patient. A noise detector according to the present invention is shown in 15 Fig. 1 to include an EMI
~l.orrction section 36, coupled to the microprocessor and memory 12 by a line 38, and equipped with an antenna 40. Details of the cc,l-~.Ll.l.;lion and ~laLiull of noise detectors according to the present hlvt~
are provided below. In general, the detector circuitry 36 in Fig. 1 receives EMI by means of the antenna 1 0 40. As explained more fully below, the antenna 40 of the noise detector may be the same as the antenna 34 of the telemetry section or may be a sep~r~tr, ~lr~lir~ted ~ntenn~ or may be a lead in one or the other of the lead systems 22 or 24. However, beyond such antenna pl~JVi~.iOll., the cil~;uilly of a noise detector of the present invention functions independently of other Cil~,uilly of the implantable medical device with which the noise detector is used, and particularly is inflrprn-lrnt of the ciluuilly of the medical device used 1 5 to detect and process sensed signals from the patient, whether such signals are physiological, that is, that relate to phy.ciologir~l functions of the patient, or n~ hy~.iological signals, such as signals in~lir~ting movement by the patient.
In one form of a ~!lGr~ ed embodiment of the present invention the antenna of an implantable medical device that is used for c~ )nc and telemetry is also used as part of a circuit to detect the presence of electr )m:~gnrtir fields that may cause the device to m~lfimrti~m, or that may adversely in~nrnre the behavior of the device. In particular, the circuit associated with the telemetry coil antenna is enh~nred to enable the detection of wideband electrom~gn~tir signals, such as with Colll~Jl~llL'. in a range from 0.5 HZ to 2 GHZ, even of relatively low level. In the event that clccL~ g~.Ptir noise signals are induced in the telemetry ~ntrnn~, they are detected and d~loplial~ly processed, resulting in the 2 5 ~ ;ldLiC)ll of a warning signal to alert the microprocessor of the ~ lce of EMI, for example. Such EMI
processing may include, but is not limited to, amplification, filtering, demodulation, integration and CuullLillg, for example. Fig. 2 is a block diagram of a form of telemetry l;h~:uiLly, and of an embodiment of noise ~letPctirn and processing circuitry according to the present invention, all shown generally at 50, for use in an implantable medical device such as a cardiac stimnl~tor, for example. It is nn-lrr.stood, 3 0 however, that a noise detector according to the present invention may utilize any ~y~ liaL~: form of receiver circuit, and any ~ LJlial~ form of antenna. Telemetry cil~;uilly and noise detection circuitry, according to the present invention, shown generally at 50 in Fig. 2 include an antenna circuit col~ .mg the " ~- .ll il. ~ . " coil antenna 52 of the cardiac stimnl~tr,r as described above, a capacitor 54, and a diode 56.
A switch 58 is positioned in parallel with the diode 56, and is associdl~d with a switch control circuit 60 3 5 that selectively closes the switch to effect ll,. ~ of data from the cardiac stimlll~ror over the antenna ~ =

W O 96/41203 PCT/U~,G/'~3l78 52 to outside the body of the patient. A c~-nnPrtir7n62 extends from the junction between the capacitor 54 and the diode 56 in parallel with the switch 6O, to a pulse receiver 64 of the telemetry and c~ ir~ n circuit as well as an amplifier 66 in the noise detection section of the circuit 50. The output from the amplifier 66 is c~ tPd to signal processing ~;h~;uiLly 68, and the processed signals from that ch~;uilly are cu~ irAtPd to decision ~,h~;uilly70. Output from the pulse receiver 64iscommlmirAtPd to a decoder 72, which has an output co~ e~ d to a L,An~",illr- control logic circuit 74. The output from the logic circuit 74 signals the switch control circuit 60. The noise detection section of the cardiac .sfim~ t-lr circuitry cc,...~,.ises circuits 66-7O, and shares the use of the antenna circuit, culu~ ,ing parts 52-56, with the rPmAin-ler of the telemetry and c-"""~ irAtion section cu---~ ,i -g circuit~s 60,64,72 and 10 74, but is otherwise intlPpPn~Pnt of tne telemetry and ~ "i, Alir,n section in col~,Ll.l~;Lion and funrtirn The operation of the antenna circuit 52-56, with the switch 58, in the receipt of cl~mml-nir.~ti~ n.c to the cardiac stimnlAtor, and in the L.A,-.~ inn of data from the cardiac stimnlAtrlr, is ~iiccllcced in U.
S. Patent No. 5,383,912. In that context, cu-----~ irAtion between the cardiac .srim~1Atc,r and an external receiver/L.An.~ lPl through the antenna circuit 52-56 and the switch 58 is by way of pulses of electr-mAgnPtir energy. The telemetry and c~-"""~"irAtion section, shown generally here in Fig. 2, COIIIIIIIIII;~AI~S with additional cil-;uill~ of the impl~nt~hle device, such as sensing and .$tim~lnc cil~;uiLly 14 and 16, respectively, and a ~- i~ -uct:sso- such as 12, as shown in Fig. l, for example, sending data to the additional ~;h~;uiLl y at 76 and receiving data therefrom at 78 in Fig. 2.
With the switch 58 open, electrom~gn~-tir energy pulses from an external L1A~ I are received 2 o by the antenna coil 52. The resulting Lldl~ llL electric current produced in the antenna 52is converted with the use of the diode 56 as a non-linear component to a non-zero average electric current which charges the ca~,a.,iL()r 54 with ele~;L~u~,L~ic energy. The receipt of the clc~t-----.Agn~tir pulses is sensed by the pulse receiver 64 by means of an analog signal along line 62. Upon detection of the receipt of an electrl~mAgnPtir pulse, the pulse receiver 64~.ullllllulliCdLt~S a binary pulse to the decoder 72. The incoming pulse signals to the antenna 52 from an external l1A~ III;IIeI may be in the forrn of pulse trains that are pulse-position mn.~ AtP~I The decoder tracks the timing of the pulses and c~...",~"";r~tPc binary signals as data out to the microprocessor and other .;ii1uiLlyof the cardiac srimnlAtQr at 76.
The cardiac stimnlAt~r generates selected hlrulllldLion signals in binary form regarding the condition and functioning of the heart of the patient, and the condition of the device itself, and conveys 3 0 these signals as data 78 to the L1AI~ I controI logic circuit 74. An output from the decoder circuit 72 provides hlrolllldLion of the timing of the inroming pulse signals from the external l1A~ 1 to the logic circuit 74so that the L~ "-i.~.~ion of signal pulses by the cardiac $rim~ tor to the external receiver may be a~u.~,L,.idL~ly coordinated with the incoming pulse timing. The time-plArPmPnt of the signal pulses to be IlA~ rd by the cardiac ~$timlllAt~r7 with the incoming pulses, carries the data regarding the condition 3 5 and functioning of the patient heart and of the cardiac $timnl~tor to the external receiver. The logic circuit WO 96/41203 PCT/U~C,'~I~178 74 sends a pulse to the switch control circuit 60 each time an electrnmAgnPtir pulse is to be tlA~ d over the antenna 52 to the external receiver, and the control circuit closes switch 58 in response for the time dictated by the signal from the logic circuit. Closing the switch 58 allows a ~ hdlge of the energy stored in the capacitor 54 by the original pulse or pulses sent to the antenna 52 from the external l1A~ II;IIP1~ with the result that current flows through the antenna for the time lG~luhcd to produce an electromAgn.-ti~ pulse according to the pulse conveyed by the logic circuit 74. In this way, the cardiac ;IIIIIIAI~1 uses energy received from an external IIA~I~III;IIr~ for the pro~llrti~m of electromAgnPtir- signals l1A~I~III;IIr(~ by the cardiac .stimlll~r to an external receiver. In addition to the electr mA~n~otir signals from the external l1A~ rI~ EMI may be induced on the antenna 52 and c~ .,ir~lrcl to the pulse receiver 64, there to confuse the h~ ,Alinn pulse train signals, or to be hlL~ ed as possible c .
signals, for example.
By further processing the signals received on the antenna 52 from the external L1A'.'';III;IIrl and detect-o~l, it is possible to detect the prt~;llce of EMI and also mAintAin the full fimrti~nAIity of the telemetry system. The same signals induced on the coil antenna 52 that are c~ lrd through the diode 56 to the pulse receiver 64 are also c-.llllll~lli~AIrd to the AmplifiPr 66. The analog signals from the antenna section 52-56 are amplified at 66, and the amplified signals selectively processed at 68 to detect any EMI present. A decision is made at 70 based on the pl~sellce and strength of EMI, and an a~
signal is c~llllllllll;r~te(i at 80 to the llli~,lu~loce~ol of the implantable device. The processing and hAn-llin~
of signals as at 68 and 70 are ~li.~i,;~lcsecl in detail below.
2 0 Although a particular type of telemetry and c~ -l l " l l~ n section is ~ s~ d herein for ~-ul~ses of illustration only, a noise detector accoldillg to the present illv~ ioll may be employed with any type of telemetry system c.~ lll with the re4uh~lllGllL~ of an implantable medical device.

Another preferred embodiment of a noise detector according to the present invention utilizes an antenna separate from the telemetry antenna of the implantable medical device in which the noise detector is employed. A detection circuit using one or more zero-bias Schottky detector diodes, such as diodes in the HSMS-285X Series of Hewlett-Packard Company, pclr,lllls detection of wideband ele~ gm~-tir. noise signals with virtually no power consulll~lion from the power source of the implantable device. Fig. 3 illllct-~t~ s such a noise detector, shown generally at 90, inrln-ling a wideband antenna circuit, Colll~ illg 3 0 a ~ At.od coil antenna 92 and a capacitor 94, and an impe~lAnr~m~t~hin~ network. The antenna 92 is a mnltihlrn, ll~ idlult; coil which can be printed by lithographic t.-rhni-ln~c, for example, on one of the wiring layers of the ceramic ~ lr on which circuitry of the i~ ldllLdl~le medical device is constructed.
A micro-strip l1A~ 11 line 96 of the impedance-m~trhing n~Lwolh is in parallel with the coil antenna 92 and the capacitor 94, and an inductor 98 is connected to the junction of those three 3 5 C~ )011t;11L;. A second micro-strip 11AII~III;C~ II line 100 is c-)nnrctrd to the other end of the inductor 98, g and a zero-bias Schottky diode 102, such as HSMS-2850, is c. ~."~ d to this latter l~ sion line. An integrating capacitor 104 is cnnm~cted between the output 106 of the diode 102 and ground. The impe~i~nre-m~tl~hing network comprises parts 96-102. Output 106 from the circuit 90 is c~ ;r~l~d to processing circuitry, as tliccncced below, for d~ hlillg the nature of the EMI signals and cign~ling 5 the Illi-,lu~lùcessor of the imrl~nt~hle medical device accordingly. The network 96-102, along with the antenna circuit 92,94, can be tuned to vary the frequency response of the detector 90, particularly by SPl~c,ting the capacitor 94, the inductor 98, and the two tl~ ion lines 96 and 100. For example, the circuit 90 may be tuned to have high s~l~iLivily at specific frecln~nri~c, such as the frequency range at which digital cellular phones operate. Also, variation of the frequency l~ ullse alters the bandwidth within which the detector 90 is crr~ ivt .
In general, wideband EMI detectors such as the detector 90 of Fig. 3 may be constructed to take advantage of the high llrteçtinn se~ ivi~y of zero-bias Schottky diodes. Specifications and uses of the HSMS-285X Series of diodes, inrlntling power transfer and c.. "~.. ir~lions methods, are reported in "Surface Mount Zero Bias Schotthy Detector Diodes Technical Data" publication #5963-2333E of Hewlett-Packard Company, 1994, which discloses the following ~lrt~ctinn scnsiLivilies for those diodes:
40 mVt,uW at 915 MHz, 30 mV/,uW at 2.45 GHz, and 15 22 mV/~W at 5.80 GHz.
Fig. 4 shows a detector circuit 110 for use in an implantable medical device, inrln~1ing a tuned 2 0 antenna circuit 112 having an antenna of any type, and an impetl~nre-l ~ i "g network 114. Output from the n~lwcJlh 114 is co,..."..,.;~ d to a fullwave rectifier, C~ li',illg two zero-bias Schotthy diodes 116 and 118, such as HSMS-2852 diodes. The rectifier 116,118 is followed by an intl~gr~ting capacitor 120.
Output from the detector 110 is cll,---,---llirc.l~d at 122 for fiurther ~iuces~,iulg. Where exposure of zero-bias Schottky diodes to very high RF fields may cause power coupling into the Schotthy diode rectifier, such 2 5 power coupling may be prevented by use of a PIN diode pair such as a HSMS-3822.
Signal processing circuitry for an EMI detector according to the present invention may implement a number of different filnrti~n.c. Fig.5 shows a noise detector according to the present invention with signal ~IOCes~iUlg (;il~;Ui~ /, and sensing ~;h~;uill.y of an associated implantable cardiac stimnl~tor, for example, all shown generally at 130, for providing several options to the cardiac .5timnl~tor for utilizing detected 3 o EMI signals from the noise detector.
An electrom~gn,-ti~ signal receiver is in~lir~trtl in Fig. S within the dashed-line border 132, and includes a coil antenna 134, a storage capacitor 136, a detector diode 138 and a resistor 140. A signal rectified by the diode 138 is demodulated by the low pass filter formed by the capacitor 136 and the resistor 140, and is output to an amplifier 142. The rrCict~nre 140 could be the input iull~edance of the amplifier 142. The signal processing (.;il~;Ui~l,y illustrated in Fig. 5 could be utilized with any type of electrom~gnPti~ signal receiver, using the telemetry antenna of the impl~nt~hle device or another antenna of any suitable type, and the signal receiver 132 is lc~!JlcscllL~livc of any type of such circuit.
The rectified and demodulated noise signal is amplified by the amplifier 142 and al?~lu~lialcly filtered at 144. In one type of signal processing the output of the filter 144 is used by decision cilcuilly 146 COlll~ 7illg a c(.. -~ , ful and logic latching cil~;uiLly to Aett-rminP whether the detected electrom~gnP-tic field exceeds a certain pro~.,..l....;lble threshold level. The threshold detector 146 may measure the noise signal dl~liLude, duration, repetition rate, frequency, or any other feature of the signal.
As used herein, therefore, the terms "level", "threshold level" and "threshold value" are understood to refer to any feature of a signal, and a threshold detector such as 146 (:X)llll?~ICS such a feature of the signal input thereto to a dPcign~tPd level, or quantity, etc., of the feature. The threshold detector 146 generates an "EMI present" flag at 148 to the llliClU~lUCci.77ol of the ;~ hle device whenever the threshold is eY~eçAPA The presence of such an electrom~nPti~ field may also be causing the sensing Cil~,Ui~ly of the impl~nt~hle device to be c~cnaLillg hlcollc-;lly. The lllic-up-ùcessor is alerted by the "EMI p.c~,c..l" flag of this possibility, and can then take a~p-u~lialc steps to insure the safety of the patient. The threshold detector 146 receives the threshold level setting and a latch reset signal from the llliulu~lùcci-7i-70l at 150 and 152, respectively.
The analog signal output from the filter 144 may also be ~lvccssed to allow the micluplucc~ ol to ~lu~nliry t'ne strength of the EMI field, and to construct a log, or profile, of the exposure of the 10 i.. l.l~.. l;.hle device to EMI over a selected time period, for example. The filtered signal is co.. ir~tPd to an analog-to-digital COu~cl~cL 154 whose digital output signal is c.. ;. ~l~d to the llliClO~luc~ ,o.
as EMI pl~;,cllce and strength data, to be collected and stored, as a function of time. The ~f~cllm~ ti- n of such stored data may be con~lllctPd over a selected time period, such as a day, for example. The stored data may then be downloaded on cu-----~ from the implantable medical device to an external receiver by way of the telemetry system of the implantable device as a profle of peak CA~0.7UlC to EMI throughout 2 5 the selected time period, with the data parceled in short time periods, such as of fifteen minutes each, for example.
Fig. 6 illll~tr~tPs such a profile of EMI experience for a hypothetical case, in~hl~ling peak exposure readings every fifteen minutes for a twelve hour period. The exposure data may be provided as average EMI values over each fifteen minute ~ - -~ . .. ;. .g period, or as pealc EMI values over each such mP~cnring 3 0 period. Such an EMI cA~JOi7ulc profile would provide the phyi7icidll treating the patient who is the recipient of the irnplantable device with valuable h~llllalion for pro~lall.ulillg the sensing and response parameters of the implantable device, for example. Further, correlating the time scale of the profile with actual behavior of the patient during the selected data g~thPring time period would provide hlr~ aLion about areas or activities that should be avoided during the patient's daily life.
3 5 An EMI exposure profile such as shown in Fig. 6 may also be used in conjunction with a patient W O 96/41203 PCT~US96/08178 activity profile col~Lluuled from actual ,llea~u~ llL~ of the activity. For example, the i~ Jl"l~ 'e medical device with which the detector of the present invention is used may include an accel~lulll~Lc. which senses movements by the patient and provides data signals accu~ ly. Pluct~illg the accelel~ll~l~l data signals can provide an activity time profile, say for the same time period as an EMI exposure profile as shown 5 in Fig. 6 is obtained for the same patient. Additionally, various physiological pararneters of the patient, such as heart beat, blood P1C~S~ oxygen col~ulllluLion, etc. may be sensed over the same time period and graphs obtained similar to the EMI exposure profile and the patient activity profile, for example. In this way, valuable information relative to the patient's health and exposure to possible danger may be obtained and studied toward treating the patient and/or reducing the patient's exposure to dangerous 10 .;il~ s.
Another variation in the formation and use of an EMI exposure profile such as illllctr~t~d in Fig.
6, as well as profiles of lll~,a~uled patient activity and physiological pdldlll~Lel~, for example, involves recording the data on which such a profile is constructed only upon the oc~;ullellce of some .specified event. For example, the EMI peak hlrc,lllldlion obtained with the use of the output 156 of the A/D
15 converter 154 can be free running in general, and frozen, or saved, only when some event occurs, such as the EMI strength ex~ee-ling a selected threshold. Then, the microprocessor can save the EMI data for the next twelve hours, for example, and cul~llu.,l a profile that is t;A~ il,le as a graph as shown in Fig.
6, for example.
Alternatively, EMI data from the output 156 can be saved cnntimlollcly, and, after startup, every 2 o twelve hours the data for the twelve hour period beginning twenty-four hours ago and ending twelve hours ago erased. Then, when a triggering event occurs, such as EMI .-Y~ee~ling the selected threshold value, EMI data for the twelve hour time period imm~ rely preceding the event can be saved along with data for the next twelve hour period following the event and presented as a twenty-four hour EMI exposure profile, for example. Profiles of patient activity and other parameters for the same time periods may be 25 obtained as ~iiecllcced above for cu~ ali~oll and analysis in cunj-L~l.,Lion with the use of EMI exposure profiles. Variations of such save-and record profile patterns can be utilized with noise detectors according to the present invention.
Other events which may be utilized to trigger collecting EMI exposure data and constructing an exposure profile, for example, would include the occurrence of a specified number of EMI signals above 3 0 a selected threshold within a given period of time. For example, if the patient ~elicllces ~ o~ule to EMI
signals above a decign~t~d level ten times in a twenty minute period, the save-and-record profile process might be triggered. The triggering signal may be initiated by the behavior of some other parameter, inr~ iing a physiological p~r~m~t~r of the patient, such as a rise in blood pressure above a specified value.
The patient may also choose to initiate saving and collecting EMI exposure data into a profile as graphed in Fig. 6, for example. U. S. Patent No. 5,304,206, issued April 19, 1994, discloses an implantable W O 96/41203 PCT~US96/08178 nGu~o~ tor that can be initiated manually by the patient applying selected pLcS~iulc to the implanted device.
Still another implemP-nt~ti~ n of the signal pll)Cc~i:iillg circuitry of an EMI detector accol~ lg to the present invention involves the use of a coinri~lpnre detector to C~ ...lly monitor for coi--~ re of sensed events with those coming from the EMI detector. In Fig. 5 a sensmg lead 158 extends from a b~
filter 160 to the patient's heart to acquire heart signals, such as atrial sense signals, for example. The lead 158 may also convey sensed signals to sensing circuitry (not shown) of the cardiac sfim~ tc r as ~ ed above in connection with Fig. l, for example. The output from the filter 160 is co.. l.. ;r~l~cl to an amplifierl62. The ~mplifir-d EMI signal from the amplifier 142iS c.,.. i. ;~ d to another ~mp1ifir-rl64.
The analog signals input to the two ~mplifier.c 162 and 164 are suitable amplified by the lc:,~e~;~ivG
amplifiers. The amplifier circuits 162 and ]64 may also include signal filtering cil.iuilly. Also, the amplifiersl62 and 164 may be linear amplifiers with filters, or they may be switched-r~ mpiifiPrc, for example. If sampled signal ~mplifir-r.c or filters are used, the c~mpling rate is ~ d by a co....~
timing signal from a common timing circuit 166 to insure that both amplifiers are s~mpling at the same rate and in unison.
The signal output from the amplifier 164iSC~--IIIII-II-i~-l~d to a phase delay circuit 168 which shifts the phase of the signal to cancel the effect of any prior phase shifting experiPnred by the tiPtPctP-l EMI, such as due to low pass filtering in the receiver circuit 132, for example.
The output signals from the amplifier 162 and the phase delay circuit 168 are c.. ir~lPd to 2 0 sense and threshold detector circuits 170 and 172, respectively. In each of the circuits 170 and 172 the input signal is rectified and cu~ al~d to the same pred~pt~prminp~l~ programmable threshold, utilizing a c~....l.,..,.lol. The feature of the signals cu~ c;d to a threshold in the detectors 170 and 172 may be any signal feature, as ~liccuc~ed above in connection with the threshold detector 146. Those portions of the signals above the threshold level are output from the detector circuit 170 to one input of a coincidence detector 174, and from the detector circuit 172 to another input of the same coinri~lpnre detector. The coinri~Pnre detector 174 COlll~JdlG~ same-time signal samples, for example, from the detected EMI and the sensed signals obtained by way of the lead 158 to ~1ptermin~o whether these two signals exhibit coinri~lPnre. If coinri~iPnre is noted between the EMI signal and the sensed signal from the lead 158, for example, the coinri~lenre detector 174c(~llllllllllir~lrs a signal to a coincidence counter 176. The output 3 0 signal from the coinri~lPnre detector 174 to the counter 176 in-lir~tf s the p-GsGIl~e of coinri~lpnre detected between the two input signals from the detectors 170 and 172. The counter 176~ rllllill~s the rate at which the PIG~G~1CG of coinri~ nrPc are signaled to it by the detector 174, as well as the duration of each such coincidence. When the counter 176deLGIlllilles that a coinritlrnre exists between the ~letpctp~l EMI
and the sensed signal from the lead 158, either in duration beyond a specified threshold time period, or 35 in repetition rate, the counter produces a coil.~ re flag signal at 178 to the microprocessor of the WO 96/41203 PCT/U'r-~178 medical device. The counter 176 receives the threshold time, or repetition rate, setting and a latch reset signal from the lllicluplocessor at 180 and 182, respectively.
The nature of the sensor associated with the lead 158 in Fig. 5 will depend on the type of i"~ hle medical device with which the noise detector circuitry of Fig 5 is utilized. In general, any type 5 of patient pdlalllc~ , whether physiological or not, may be intPntlPd to be sensed by way of the lead 158, such as patient movement, blood pressure, etc. Regardless of the nature of the pd,,....~lel to be sensed by the sensor associated with the lead 158 in Fig. 5, the actual signal conveyed by that lead to the filter 160 may be allywhclc from 0% noise to 100% noise. In the latter case, of course, the signal does not include any patient palalllc~cMIlr.~ , there having been no patient-related event to sense, either physiological 1 0 or not, and the signal on the lead will most likely exhibit coinri~lPnre with the EMI signal conveyed to the amplifier 164, for example. However, even if the signal to the filter 160 is only partially noise, coinri~lPnre with the detected EMI may be detected at 174.
The various signal processing modes for utilizing detected EMI in an imrl~nt~hle medical device clll~loyillg a noise detector accol.lillg to the present invention, as ~ ed above in co~ ... with Figs.
5 and 6, for example, may be exploited in various ways by the imrl~nt~hle device. Fig. 7 illllctr~tPc one such method, in~lir~ted generally at 200, of exploiting the hlrullllalion obtained and forwarded to the ~iclu~loce~o~ of an implantable medical device from an EMI detector as shown in Fig. 5, for example.
In the method 200 of Fig. 7 the mi-,lupluce~sor of a demand p~rPm~kPr, for e~mphP, lc~ol~ls to the message that EMI has been detected in the same manner that it would respond if it sensed that the 2 0 heart of the patient has failed to perform an çxrectPd event, that is, by producing a pacing signal to pump the chamber in which heart signals are being sensed, for example. Upon the start 210 of the illustrated process 200 the llliclu~lucessor initiates an escape interval at 212, that is, a time interval during which the pz~rPm~krr waits for the heart tû perform an event, such as producing a pacing signal, for the pacemaker to sense and during which the p~cPm~kt r will not send a pacing signal to the heart. At 214 the p~rem~kPr 2 5 waits for the escape interval to expire, or the p~rPTn~kPr to sense an event at the heart, whichever occurs first.
The microprocessor inquires at 216 whether a heart event was sensed by the expiration of the escape interval. If the answer is ilrr.. ~ive, the microprocessor proceeds from 216 to fi~tPTTninr at 218 whether it has received an "EMI present" flag, or an EMI coinridçnre flag, for example, from the noise 3 0 detector of the present invention during the time period since the start of the escape interval at 212. If no in-lir~tir)n of EMI was received during that time, the microprocessor starts a refractory time period at 220, usually of a few millisecond duration, during which the p~rPm~k~r is blind to any heart activity. The microprocessor waits for the expiration of the refractory period at 222. A new escape time interval is initiated at the end of the refractory period at 224, and the process returns to step 214 to continue. As an 3 5 alternative, the new escape interval may be started with the start of the refractory period at 220, provided CA 022l93l0 l997-l0-24 the new escape time interval is e~ten~lrd by an amount equal to the length of the refractory time period.
If, at 216, the microprocessor ~L~ s that the pacrm~kor has not sensed a heart event by the expiration of the current escape interval, this ch....... ~ e is taken to mean the patient needs ~ ;mre so that the process proceeds to 226 wherein the p~rem:~krr gelleldLt~S a single pulse to stim~ tlo the heart.
The process then moves to 220 to start a refractory period as ~li.ccllcce~l above, followed by the steps 222 and 224, with a snhse~lllçnt return to 214 to continue the process. Again, if the p~r~-m~krr has not timely sensed a heart event as inquired at 216, a single stimulus pulse will be generated and ~ ".i~ l to the heart at 226. Such single pulses will continue to be provided to the heart as long as step 216is reached without a heart event being sensed durmg the then-current escape interval. This is the normal response of the p~r~m~krr in question to a failure of the heart to .. ;.,i;.;.. a proper rhythm of self-generated stirnulus signals in the chamber at which the sensing electrode is situated.
As the process diagram 200 shows, whenever a heart event is not timely sensed, the llliclu~locessor will cause the p~r~-m~kPr to provide a stimulus pulse to the heart at 226, followed by steps 220-224, without inquiring at 218 whether EMI has been (l~teçtr~l As long as the heart is showing the need for ~Ccict~nre in the form of a stimulus signal, this normal operation of the p~rPm~k~or will be followed, regardless of the p-~,s~llce in the p~r~m~k~r of EMI; conceq~lf ntly, under such ~ r~c no inquiry need be made lc~;~dillg the ~ lce of EMI. However, if the p~rrm~k~-r has received a sense signal intlir~ting the timely genPr~ti--n of a heart signal by the heart itself, inquiry is made at 218l~;gal-1iug the presence of EMI. Then, if EMI is not present, a conclusion is reached that the p~r~m~krr is functioning properly and the sensed signal is a true in/lir~tion of the occurrence of the heart event.
However, if EMI is ~IPterminf~c~ at 218 to be present, the llli~ ocessor cannot know whether the signal sensed by the p~rçm:~kf~r was actually the result of a heart event or the result of EMI in the p~ç~n~k.or electronics. As noted above, a sensed signal may include al.ywh~lG from 0% noise to lQ0% noise, and not include any signal sensed as a result of an actual heart event. Therefore, if EMI is ~f trrmin~ocl at 218 to be present, the p~rrm~k~r proceeds as if the patient needed ~Ccict~nre The p~r~m~k~r waits at 228 for the current escape time interval to be compl~t~-l if it has not yet been cr)ml lerf~d (since the llliclu~luce~ul arrived at step 218 only because a sense signal was detected at 216 to have been received by or before the expiration of the escape interval), and generates a stimulus pulse at 226 followed by steps 220-224, etc.
The process steps 200 of Fig. 7 are only one example of utilization of a noise detector according 3 0 to the present invention. Processes for other and more sophictir:~trd implantable devices, utilizing noise detectors according to the present invention, are equally possible. Likewise, such processes may be provided in conjunction with any of the various techniqll~ s for processing the EMI signals, such as tliccllc.ce~l 5 in connection with Fig. 5, for example. The inquiry 218 in the process 200 as detailed in Fig.
7 may utilize an "EMI present" flag 148 as illustrated in Fig. 5, so that the presence of EMI above the selected threshold level set at 150 d~ llllhles whet'ner reversion to a specified cs,;noise mode" by the W O 96/41203 PCT~US96/08178 pacemaker is a~luplidl~. Also, the detection of a It) coincidence, or multiple coinri~lenrec, or coinri~lenre~ above a certain time rate, at 178 in Fig. 5, for example, may be used to trigger the reversion to the specified "noise mode". Such a "noise mode" might be "V00", that is, vçntric~ r .stimlll~tion, no atrial or ventricular sensing, and no atrial or ventrirlll~r hlllibilioll or triggering, to achieve a~yllclllo~ us ventricular pacing at a known rate, for example.
If the signal processing cil~;uilly of a noise detector acco-diu-g to the present invention ~ e, l,~illes that a coincidence exists between sensed signals coming from the heart sensor lead of the implantable medical device, and EMI as detected by the noise detector, whether the coinri~lenre is in time or frequency of the signals in question, the implantable device will not rely on the sensed signals as inriir,~ting actual heart events, and will provide safety pulses as described above until the cil~;uilly ~1~le~ll.;l.~C that such a COi~ e~re no longer exist~s. The tletectinn of EMI in an i~ "l~hle device showing the same frequency profile as sensed heart events is an in~7ir~tion that the sensed heart signals are most probably false, and due to i.lLe~relellce from the noise rather than actual heart events. Concequ~ntly~ such a coincidence is iulltl~lcl~d as in-lir~ting EMI induced confil~i~m of the sensing circuitry, which can Ill~lcr~e not be relied on to reflect the true condition of the heart, and reversion to a specified "noise mode" is carried out to insure the well-being of the patient.
As shown in Fig. 5, both "EMI present" and EMI coinri(1enre signal processing may be utilized in the same implantable cardiac stimlll~tor, for example, with the .ctimlll:ltor safety pacing the patient's heart, for example, in response to a positive result for either type of ploces~ g of ~letrctf d EMI signals.
2 0 At the same time, a peak EMI ~yo~ule profile may also be obtained for the patient.
As noted above, the present invention may utilize various antenna configurations. EMI may pellGIIdle the metal case of an implantable device by m~gn.otir, flux intrr~rti~n with the ch-;uilly, inr~ iing whatever internal antenna may be utilized by the noise detector, for example. If the noise detector uses an internal coil ~nt~,nn~, the ability of the antenna to receive induced EMI may depend, to some extent, 2 5 on the orientation of the coil of the antenna, with the l~7lnJIISe of the antenna being greatest when the plane along which the turns of the coil are p~-~ifi--nr~l is perpen-lirul~r to the direction of the r,l~ illg m~gnetir flux of the EMI. The response of an internal coil antenna of a noise detector may be m~ximi7~d by providing coil turns that are effectively located in all possible plane orientations. This effect can be accomplished by using a three-dimensional coil antenna construction, for example.
3 0 A three-dimensional coil antenna system for use in a noise detector according to the present invention is shown s~hPm~tir~lly generally at 230 in Fig. 8. The antenna system 230 includes three mnltifllrn coils 232, 234 and 236, wound on separate ferr*e cores 238, 240 and 242, respectively. The coils 232, 234 and 236 are oriented such that a plane defined by the turns of each coil is perpPn-liclll~r to the dilt~ l of a different axis of an orthogonal axis system XYZ. Thus, coil 232 is perpen~lirnl~r to the 3 5 X axis, coil 234 is perpen-lirlll~r to the Y axis, and coil 236 is perpen-lir-nl~r to the Z axis. Each coil is thus oriented to have m~ximl-m exposure to m~gn~tir. flux changes in one of the three orthogonal axis directions. Further, the ferrite cores 238,240 and 242 enhance the signal intluctinn on the lc~cclivc coils due to exposure to cl~ g;,~g m~gnrtic flux, although the coils could be air cored.
The cullll)hlaLion of the three coils acting as a single antenna, or providing received signals to three cu~ ulldillg antenna circuits whose output is then combined, provides exposure to m:~gnPtir flux change in~lnrti~m in all directions. The coils 232,234 and 236 feature lead lines 244,246 and 248, respectively, to separate receiver circuits, for example. Alltlllalively, the three coils 232, 234 and 236 may be col~le~;Lcd in parallel, and cullll~illed to provide the coil antenna 134 in Fig. 5, for example. Thus, the orthogonal coil antenn~c can be combined to provide a single, all-dilccLiollal noise ~ntenn~) either by cl-nnl cting the antenna leads together, or by joining the received signals from the separate coils in the noise detector cil~uiLl~, for example.
The coils 232,234 and 236 of the antenna system 230 may be positioned within the case of an i.,.l.l~.,l;.hle medical device at different lor~ti-)nc, or at the same location where possible, for example. The cross-secti-n~l area of the different coils may not be the same. For example, a coil oriented in the general plane of the relatively flat case of the implantable device may have a relatively large cross section compared to the cross sections of the other two coils. The respul~,ivclless of the coils to induced noise signals may be equ~li7Pd by d~lu~ Lc variation in the respective ~ihcuiLly for the three coils, or by varying the number of turns in the dirrclellL coils, it being realized that the response of a coil to m~gnPtir flux changes increases for an increase in the number of turns of the coil.
2 0 Two of the three coils 232-236 may be wound on a single ferrite core. For two such coils 232 and 234, for example, the core is generally in the form of a cross, which may be considered as formed by combining the two cores 238 and 240.
Another possible method for ~lrt~rting the m~gn.otir colll~ullcllL of EMI is to use a solid state sensor capable of producing an e~Pctrir-~l signal proportional to a time-varying mz~gn~tir field. The use of one such sensor is described in cul~;ullcllLly-filed U. S. Patent Application Serial No. 08/475,491 titled ~l~yal~lLu~
~nfl Method for the Control of an Imrlantable Medical Device. That application f1icrlr~.crc a giant magneto resictz~nre ratio (GMR) sensor, within an implantable device's ch~uiLly, that lc~ olld~, to the m~gnrtir c~JllllJullcllL of low frequency( < lO0 MHZ) EMI fields. The :lnt~nn~ rertifir~tion and filtering functions of a noise detector according to the present invention, as presented in any of the embo~lim~ntc herein, for 3 0 example, may thus be provided by a GMR sensor that is dlu~loplidLcly excited, and the signal of which is a~u~uloplidLcly conditioned (for example, amplified and filtered) and demodulated.
An antenna outside the metal case of an implantable device can receive EMI without relying on m~gnrtir flux in-lnctirn, and therefore without needing a coil col~,Llu;Lion. The full electrom~gnrtir signal may interact with such an antenna, which can Lhclcrulc be linear. A noise detector accc.ldillg to the present invention can utilize an antenna external to the metal case of the associated imrl~nt~hle device. For W O 96/41203 PCT~US96/08178 example, one or more lead lines between the implantable medical device and the patient's heart or other body area may be used as an antenna for a noise detector. Such leads are shown as 22 and 24 in Fig. 1, and as an 158 in Fig. 5. To use a heart lead line as an antenna for a noise detector, for example, the lead line is c.~ ed to the receiver circuit 132 of Fig. 5, for example, t'nrough a filter to select out EMI from heart stimulus signals in the case of a stimulus lead line, or from heart sense signals in the case of a sensing lead, for example. To avoid the neces~,ily of di,lill~,uisllillg EMI from the stimnlll~ or sense signals, a separate, .le~li('~t.-d external antenna may be utilized for the noise detector.Fig. 9 shows an impl~nt~hle medical device 250, having a metal case to house the cil~uilly of the device, inrlllriing circuitry of a noise detector accoldillg to the present invention, and a plastic header 254.
1 0 The header 254 features two elongate bores 256 and 258 that receive the leads to and from the heart, for example. Connectors (not shown) are provided ~dj~r~llt tne bores 256 and 258 such that insertion and seating of a lead in either of the bores causes Pl~ctrir~l contact between the lead and the ~ Oplial~ circuit within the case 252. The stimulus and sensing leads (not shown) of a cardiac stimnl~tnr~ for example, are thus connected to the ~;h1uiLly of the device.
The header 254 has il.lbedded in it a conductive member as an antenna 260 which emerges from within the case ~52 and extends along the top of the header, above the bores 256 and 258, as viewed in Fig. 9. Within the case 252 the antenna 260 may be connPctPd into an antenna circuit such as 132 in Fig.
5, for example, in place of the coil 134. A separate coil may be added to the antenna loop of the circuit 132 to allow the circuit to be tuned. Allt~ ly, the antenna 260 may take the form of an elongate coil 2 0 Pxtrn-ling through the header generally as illustrated, or a short coil p~.~iti~mPd within the end of the header. Further, a three-dil,l~ ,iollal coil such as the antenna system 230 of Fig. 8, or a two-~iimPnci--n~l version thereof, may be po~itionlod within the header 254 in place of the elongate antenna 260, for example.
From the disclosure herein it will be understood that the antenna utilized with a noise detector 2 5 according to the present invention may take a variety of forms, including one or more conductive coils, or a generally linear conductive member, for example, and be located either within or outside of the metal case of the implantable medical device with which the noise detector is utilized. The antenna used by the noise detector may be shared with telemetry of the medical device, may be a lead of the medical device, or may be ~e~iir~t~d to use by the noise detector. Further, the noise detector may utilize a combination of 3 0 such types of ~ntrnn~c to receive EMI. Additionally, the present invention provides a noise detector that is independent of other circuitry of the implantable medical device. In particular, the noise detector detects and processes EMI independently of the cil~;uiLly used by the implantable device to detect and process sensed signals from the patient, either physiological or nonphysiological.

Claims

Claims What is claimed is:

1. An electromagnetic interference detector for use in a medical device (10) that is implantable within a patient, comprising:
a. a receiver (36) whereby electromagnetic interference signals are received;
b. an antenna (40) as part of the receiver and on which the interference signals are received;
c. a signal detector (66, 68) that detects the presence of the received interference signals, the detector being independent of any other circuitry of the implantable medical device; and d. signal processing circuitry (70, 80) that provides at least one signal to the implantable medical device that depends on the presence of electromagnetic interference.

2. An interference detector as defined in Claim 1 wherein the antenna is dedicated to the reception of electromagnetic interference.

3. An interference detector as defined in Claim 1 wherein the medical device includes a telemetry circuit that communicates by utilizing the antenna.

4. An interference detector as defined in Claim 3 wherein the antenna comprises a coil.

5. An interference detector as defined in Claim 1 wherein the medical device includes a metal case housing circuitry of the interference detector, and the antenna comprises a conductive member extending outside the metal case.

6. An interference detector as defined in Claim 1 wherein the antenna comprises a coil.

7. An interference detector as defined in Claim 1 wherein the medical device has least one electrical lead, and the antenna comprises one such electrical lead.

8. An interference detector as defined in Claim 1 wherein the signal processing circuitry comprises circuitry that compares detected interference with a signal received by the medical device, including:
a. threshold detectors that select those portions of the interference and the signal received by the medical device that are above a designated threshold value of a signal feature; and b. a coincidence detector that compares the portions of the interference and the signal received by the medical device that are above the designated threshold value, and provides a communication to the medical device that depends on the existence of a coincidence between the compared portions of the interference and the signal received by the medical device.

9. An interference detector as defined in Claim 8 further comprising amplifiers that amplify the interference and the signal received by the medical device.

10. An interference detector as defined in Claim 8 further comprising first and second sampling circuits that sample the interference and the signal received by the medical device, respectively, and a timing circuit that directs the sampling by the two sampling circuits to occur at the same rate and in unison.

11. An interference detector as defined in Claim 9 wherein the signal processing circuitry further comprises a counter circuit which determines the time rate at which coincidences occur between the compared portions of the interference and the signal received by the medical device, and provides a communication to the medical device that depends on the occurrence of such coincidences at a time rate at least as large as a designated time rate.

12. An interference detector as defined in Claim 1 wherein the signal processing circuitry comprises a threshold detector that selects those portions of the detected interference signals that are above a designated threshold value of a signal feature and provides a communication to the medical device indicating the presence of interference above the threshold value.

13. An interference detector as defined in Claim 1 wherein the signal processing circuitry comprises an analog-to-digital converter which converts the detected interference signals to digital form and provides signals to the medical device indicating the strength of interference detected.

14. An interference detector as defined in Claim 1 wherein the antenna comprises multiple coils oriented in different directions.

15. An interference detector as defined in Claim 1 wherein the antenna comprises multiple turns of at least one coil, with the turns oriented in at least two different directions.

16. An interference detector as defined in Claim 1 wherein the signal processing circuitry comprises an amplifier which amplifies the interference signals received be the antenna.

17. An interference detector as defined in Claim 1 wherein the medical device comprises a cardiac stimulator.

18. A method of detecting electromagnetic interference in a medical device (10) that is implantable in a patient, comprising the following steps:
a. providing a receiver circuit (36), including an antenna (40);
b. providing an electromagnetic interference detector (66, 68) that is independent of any other circuitry of the medical device;
c. detecting electromagnetic interference signals received on the antenna (40), using the detector (66, 68);
d. processing (70) the detected interference signals; and e. providing (80) a communication to the medical device that depends on the presence of interference.

19. A method as defined in Claim 18 wherein the step of providing a receiver circuit comprises providing the antenna of that receiver circuit as a system of multiple coils oriented in different directions.

20. A method as defined in Claim 18 wherein the step of processing the interference signals comprises comparing them to a designated threshold value of a signal feature, and the step of providing a communication to the medical device comprises providing a communication that indicates whether interference above the threshold value is present.

21. A method as defined in Claim 18 wherein the step of processing the interference signals comprises converting them to digital form, and the step of providing a communication to the medical device comprises providing a communication that indicates the strength of interference detected.

22. A method as defined in Claim 21 further comprising the step of providing a recording of the strength of the interference as a function of time.

23. A method as defined in Claim 22 further comprising the steps of providing a recording of at least one parameter, related to the patient, as a function of time and comparing the recording of the at least one parameter as a function of time to the recording of the strength of the interference as a function of time.

24. A method as defined in Claim 22 wherein the step of providing a recording of the strength of the interference as a function of time is initiated by a designated event.

25. A method as defined in Claim 22 wherein the step of providing a recording of the strength of the interference as a function of time is initiated by the patient.

26. A method as defined in Claim 18 wherein the steps of processing the interference signals and of providing a communication to the medical device further comprise:
a. comparing interference and a signal received by the medical device with a designated threshold value of a signal feature and selecting those portions of the interference and the signal received by the medical device that are above the designated threshold value; and b. comparing the portions of the interference and the signal received by the medical device that are above the designated threshold value and providing a communication to the medical device that depends on the existence of a coincidence between the compared portions of the interference and the signal received by the medical device.

27. A method as defined in Claim 26 further comprising the step of counting coincidences between the compared portions of the interference and the signal received by the medical device as a function of time to determine the time rate at which the coincidences occur and providing a communication to the medical device that depends on the occurrence of such coincidences at a time rate at least as large as a designated time rate.

28. A method as defined in Claim 18 further comprising the step of amplifying the interference signals received by the antenna.

29. A method as defined in Claim 18 wherein the communication to the medical device that depends on the presence of interference initiates a reversion of the medical device to a designated mode of operation.

30. A method as defined in Claim 18 wherein the medical device is a cardiac stimulator.

31. A method as defined in Claim 30 wherein the communication to the cardiac stimulator that depends on the presence of interference initiates a reversion of the cardiac stimulator to a designated noise mode.