CA1329238C - Arrhythmia type determination by gain control and reference level crossing detector - Google Patents
Arrhythmia type determination by gain control and reference level crossing detectorInfo
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- CA1329238C CA1329238C CA000616630A CA616630A CA1329238C CA 1329238 C CA1329238 C CA 1329238C CA 000616630 A CA000616630 A CA 000616630A CA 616630 A CA616630 A CA 616630A CA 1329238 C CA1329238 C CA 1329238C
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Abstract
ABSTRACT OF THE DISCLOSURE
An implantable cardiac stimulator for detecting and treating cardiac arrhythmias includes a sense amplifier responsive to sensed cardiac signals for detecting and distinguishing normal and abnormal cardiac activity within the sensed signals. The sense amplifier includes an automatic gain control amplifier, a filter and quad comparator having a pair of signal channels for processing the sensed signals according to different frequency bandpass characteristics to establish sensing thresholds, margins and signal gain. The sense amplifier has a feedback loop containing a microprocessor which implements preselected algorithms in conjunction with the outputs of the quad comparator to variably adjust the amplifier gain and to programmably control the sensing margin.
An implantable cardiac stimulator for detecting and treating cardiac arrhythmias includes a sense amplifier responsive to sensed cardiac signals for detecting and distinguishing normal and abnormal cardiac activity within the sensed signals. The sense amplifier includes an automatic gain control amplifier, a filter and quad comparator having a pair of signal channels for processing the sensed signals according to different frequency bandpass characteristics to establish sensing thresholds, margins and signal gain. The sense amplifier has a feedback loop containing a microprocessor which implements preselected algorithms in conjunction with the outputs of the quad comparator to variably adjust the amplifier gain and to programmably control the sensing margin.
Description
IMPLANTABLE CA~DIAC STINULATOR WITH AUTOMATIC GAIN
CONTROL AND BANDPASS FILTERIN~ IN FEEDBACK LOOP
This application is related to the sub~ect matter of U.S. Patent No. 4,830,006 titled "IMPLANTABLE CARDIAC
STINULATOR FOR DETECTION AND TREATMENT OF VENTRICULAR
ARRNYT~NIAS n ~ a~signed to the same assignee as is the present application.
FIELD OF THE INVENTION
The present invention relates generally to implantable cardiac 6timulators ~uch as pacemaXer~ and defibrillators or combinations thereof, and more particularly to automatic gain control for such implantable devices for the purpose of enhancing tbe capability of the device to sense an arrhyt~mia for which therapy is to be applied.
BACKGROU~D OF THE lNvE~lON
Arrhythmias are variations in heart rate from the normal cinus rate range of approximately 60 to 120 beats per minute ~bpm) prevalent in healthy adult humans with normally functioninq hearts. Bradycardia i6 characterized by rates below 60 bpm, whereas for tachycardia the rates generally range upwardly fro~ 120 bpm. Typically, tachycardia is experienced as a result of 6uch factors as phy~ical stress texercise), emotional stress (excitement), consumption of alcoholic or caffeinated beverages, ingestion of certain drugs such as nicotine, and ~o forth. The heart rate of a healthy person Yill gradually return to the 6inus rate after removal of the tachycardia-inducing factor (8). However, arrhythmias reguire 6pecial medical treatment. For exa~ple, fibrillation i6 a high rate arrhythmia (tachyarrhythmia) characterized by completely uncoordinated contractions oS
sections of conductive cardiac ti~sue of the affected chamber of the heart, resulting in a complete 106s of synchronous contraction of the overall mass of tis6ue. As a consequence, the cha~ber ceases to pump blood effectively and, in the case of ventr~cular f~brillat~oa~ the lack of oxygenated blood to the t~ssues w~ll lead to death wlt~ln ~lnute~.
Implantatlon of a c~rdiac pacemaker has been a typlcal procedure of choice for treatment of bradycardia patients.
The pacemaXer pulse generator i6 implanted in a pouch beneath the 6kin in the patient'6 chest and deliverc electrical impulses to electrodes positioned at the patient'6 ~eart via one or more catheter leads, to stimulate the heart to beat at a desired rate in the normal 6inus range. Cardiac pacinq has also found increasing u6age in tbe ~anagement of tachyarrhythmias. In essence, the heart may be artificially stimulated at a faster than normal pacing rate to terminate a tachycardia or to suppress premature atrial or ventricular contractions which could otherwise lead to 6upra-ventricular or ventricular tachycardia, flutter, or fibrillation. T~e pulses delivered to tbe heart for pacing therapy need only be of sufficient magnitude to stimulate tbe excitable myocardial tissue in the immediate vicinity of the pacing electrode.
More recently, the automatic defibrillator has been proposed for i~plantation in cardiac patients prone to suffer ventricular tachycardia and/or fibrillation. The device i~
adapted to 6hock the heart with electrical pul~es of considerably higher energy content than i~ delivered $n pacing pul~es. Vpon detecting fibrlllation, one or ~ore hiqh energy 'counter-shoc~ are applied to the heart to overwhelm the chaotic contractions of individual ti6sue sections and re-establis~ organized cpreading of action potential from cell to cell of the ~yocardium, thereby restoring the 6ynchronized contraction of t~e mass of tissue. In general, prior art implantable defibrillatorc have been implemented to detect fibrillation from the patient's electrogram waveform and/or t~e absence of a ~echanical~ function 6uch as rhythmic contractions or pul6atile arterial pressure, and, in response, to deliver a fixed therapy typically con6isting of one or ~ore shocking pulse6 of preset wavefor~ and energy content.
For example, detection of fibrillation activity by monitorlng the electrogram signal ha6 been achleved wit~
conventional ~en~e amplifier configuration6 involving fixed ~en~e ~argin, flxQ~ or ~el-~tiY-ly ad~u~ted galn and flxed bandpa6s characteri~tlc~. Thi~ type of detectlon tends to neqlect the characterlstic di~ference6 between QRS complexes and fibrillation waYeforos. Yibrillatlon waveforms are characterized by erratic a~plitude Dodulated sinusoids of relatively low frequency in a narrow band (3-12 ~z~. On the other hand, QRS complexes have relati~ely con~tant amplitude, charp peaks ~ith 6ignificant high freguency content and thus a broader frequency spectrum.
The aforementioned U.S. Patent No. 4,830,006 discloses an improved medical device for treating ventricular tachycardia and defibrillation, which uses sophisticated techniques for detecting ~nd distinguisbing the arrhythmia from normal high rates, and a h}erarcbical approacb to tbe aggressiveness and delivery of tberapies to terminate tbe arrhythmia. Tbe functions of bradycardia and anti-tachycardia pacing-type tberapies, and cardioversion and debifrillation shock-type therapies, are inteqrated to provide a coordinated approach to the ~anagement and treatment of ventricular arrhythmias. A
6ignificant aspect of t~at approach i6 to provide flexible seguencing among the therapies, ~ith appropriate regard to hemodynamic tolerance ~or intolerance) of the patient to the detected arrhythmi~, and sophi6ticated detection of arrhyt~mias together with Deans for distinguishing those episodes for ~hich treatment is required tsuch as re-entrant tachycardias) fro~ those which are not associated with cardiac or other di6ease (suc~ as exercise-qenerated sinus tachycardias). A multiplicity of hierarchical detection algoritbms and therapeutic ~odalities are selectively available to t~e physician to detect and treat classes of ventricular tachycardia according to their respect~ve po6ition6 in the heart rate continuu~, and thus according to hemodynamic tolerance or $ntolerance of the patient to the tachycardia, ~ith backup capabilltle~ of defibrillation and bradycardia paclng for cardiac arrhythmia6, lncluding po6t-6hock bradycardia.
In an embodimQnt describQd in U.S. Patent No.
4,830,006 the cardiac stimulator permits ~elective partitioning of the .
~eart rate cont~nul~ lnto a plurality Or contiguou~
tachycardia cla~sel~ of progres~vely h1ghar rate r~nges, the lowest and highest of the6e classe~ belng bounded respectively by reg~on6 of the contlnuu~ denotlng ~inu6 rate and fibrillation. ~ach of the rate range6 and the latter req~ons may be program~ed by the phy6ic~an, a8 may be necessary to me~t the particular needs of the patient'6 di60rder and the flexibility of t~e therapy regimens to be prescribed. The ~ierarcbical detection sy~tem employed in the 6ti~ulator for detecting cardiac episodes ~ndlcative of arrhyt~mia and for distingui6bing between noroal and abnormal tac~ycardias am3ng the detected Qpisodes uses criteria of greater or lesser stringency depending ~n tbe location of the epi60de in the rate contlnuum. The device permits programming of the various detection criteria for tacbycardias and fibrillation, and includas plural co~binations of basic detection criteria in each category.
The arrbyth~ia detection ~ystem ai w losed in that application also utili2ed signal processing including amplification with automatic gain control and bandpass filtering according to tbe present invention, for enbancing sensltivity to the differences bet~een tbe electrical and phy~ical cbaracteri~tic~ of ~inu~ rbytbm~ and arrhyt~ia ~aveforms.
It ~ould be noted that the prior ~rt ~as ~ugqested the ~naly~ie of vavefor~ norphology for fibrillation detection.
Moreov~r, it is ~nown to use auto~atic g~in control in defibrillation systems. However, the prior art does not suggest a system and method for arrhythmia detection which recogniseQ and takes advantage of the characteristic differences between sinus rhythm and arrhythmias, including those mentioned above, utilizing automatic gain control and bandpass filtering in a feedback loop.
A significant ob~ective is to provide an implantable cardiac stimulator utilizing automatic gain control in interaction with arrhythmia detection, as well as with special bandpass characteristics and bradycardia pacing.
A further important aim is to provide an approach for dealing with interactions between gain and pacing in an implantable device capable of treating bradyarrhythmias as well as tachyrhythmias.
Yet another objective is to provide an implantable device which implements the foregoing ob~ectives in treating tachycardia and fibrillation, and which is adapted also to treat post-shock bradycardia.
Still another objective is to provide a cardiac arrhythmia detection system and method employing automatic gain control in conjunction with bandpass filtering in a feedback loop, wherein the margin of sensitivity to the various waveforms encountered varies to enhance the capability and speed of detection of the arrhythmias of interest, such that an appropriate therapy is quickly applied to return the heart to normal sinus rhythm.
SUNMARY OF THE INVENTION
According to the invention there is provided an implantable apparatus for dQtecting cardiac arrhythmias including electrode means for sensing electrical activity of a patient~s heart and processing means responsive to the sensed electrical activity for determining whether sensed events are indicative of arrhythmias~ The processing means includes target means establi~hing at least one reference level against which the amplitudes of predeterminQd sensed events are compared, gain control means controllably varying sensitivity of the processing means according to the amplitude of each sensed event relative to said at least one reference level, and comparison means responsive to said gain control means being ad~usted in sensitivity in one direction immediately following an ad~ustment in the other direction for counting the number of times the sensed event cros~es the reference level as an indication of the nature of the arrhythmia.
The sense amplifier of a cardiac stimulator is provided with automatic gain control (AGC) as well as bandpass filtering and comparison of sensed signal (sensed event) amplitudes against selected target levels in a system and method for detecting cardiac arrhythmias. Interactions between gain, bandpass and pacing are utilized in a manner not found in prior art techniques employed to detect and treat cardiac arrhythmias. In essence, the invention addresses bandpass filtering and its effe~t on sensing margins; varies the sensing margin with the shape of the waveform under analysis; minimizes noise and T wave sensing while QRS complexes are present; and maximizes sensing of fibrillation waveforms in the face of widely varying amplitudes.
The AGC amplifier includes an amplifier section having a gain that may be altered as often as necessary to maintain a set of predefined conditions, a set of bandpass amplifiers with different characteristics for AGC and sense target comparators, and a dual set of comparators to convert the processed analog information into digital information to be used by a microprocessor for making decisions about the need ,.
. . . ,, ~ , . .
for cbanges ln e~e gain of ~he arpl1r1er ~nd ~en61ng act~vlty.
T~e present in~ention take~ advantDge of the d~fferences between waveforcs or 6ignal6 repre~ent~t~ve of c~rdiac arrhythmias and tho~e indicat~ve of norc~l 6~nus rhyth~ to provide rapid detection. For exa~ple, differences in t~e c~aracteristics of fibrillation waveforms and standard QRS
co~plexes include wide and frequent variation in the amplitude of a fibrillation if set to a gain to assure ~ensing o~er the neces6arily wide a~plitude range of ~nterest (e.g., 0.3 xv to ten~ of mill~volts), the device could ~ense and respond to false signals ~ncluding the unwanted noise ~ignal~ whic~ ~re often present in t~e a~sence of fibrillation at typically low ampl~tude level~ (e.g. T-~aves).
Accordingly, a fibrillation detection method and system may be provided for a cardiac stimulator capable of automatic delivery of defibrillating therapy, in which signal amplification with automatic gain control and bandpass filtering is utili2ed for reliably sensing wide and frequent amplitude variation of fibrillation waveforms.
Automatic gain control may be provided in the signal analysis path for fibrillstion detection, to avoid falsely sensing noise signals as fibrillation and consequent undesirable delivery of unnecessary defibrillating shocks to the patient.
A set of bandpass amplifiers may be provided to produce different bandpass characteristics between peak and sensing comparator targets. Without the differential bandpass characteristic, the sansing margin would be the same for all electrogram waveforms, regardless of their frequency content.
In such a system there is a difficult tradeoff between the large sensing margin that would achieve reliable sensing of erratic fibrillation wa~eforms and a small sensing margin that would sense QRS complexes but re~ect T-waves. With the addition of the differential bandpass characteristic, fibrillation waveforms (which do not contain the higher frequency component~ that ar~ charact ri~tic Or QRS co~plcx~s) are ~electively enhanc~d at the sensing target This elQctlve enhancement provides the hlg~er senslng ~argin requlred to reliably ~ense thi6 erratic ~gnal But by not enhancing the higher frequency ~ignal components of QRS complexes, the lower margin needed to re~ect T-~aves i~ ~till obt~ined BRIEF DESCRIP~IO~ OF THE DRAWI~GS
The abo~e and ~till further ob~ectc, features and attendant advantages of ~he present invention ~ill become apparent from a consideration of th~ following detailed description of a presently preferred embodi~ent, t~ken in conjunction ~ith the accompanying draw~ngs, in which M G 1 is a bl~ck circuit diagr~m of t~e therapy generator of an implantable cardiac ~timulator, utilizing a ~ense amplifier havinq AGC, b~ndpass filter~ and quad comparator~ according to the presently preferred embodiment;
FIG 2 ~ a Dore detailed circuit diagram of the ~icroprocessor and ~ense amplifier portion of the therapy generator circuit of FIG l;
FIG 3 i~ a ~c~ematic circuit diagra~ of the AGC
amplifier ~ection of the ~ense amplifier;
FIG ~ ic a 6chematic circuit diagra~ of t~e bandpass amplifier cection of the cense amplifier;
~ IG S ~ che~atic circuit diagra~ of the guad comparator section of tbe cense amplifier; and FI~S 6 t~rough 9 are waveforms u~eful for describing the operation of t~e cense a~plifier according to t~e presently preferred embodi~ent DETAILE~ DESCRIPTION OF T~ ESEN~ REFERRED ENBODIMENT
The invention herein will be described in the environment of an implantable cardiac stimulator of the type disclosed in U S Patent No 4,830,006, but it is not limited to that usage and it will be understood as this description progresses that the invention may be utilized as well in more restricted anti-tachycardia pacemakers or bradycardia pacemakers The therapy generator (~IG. 1) unit of the ~timulator (akin to the pulse generator unlt of an implantable pacemaker) i6 adapted to detect preselecte~ aspects of the patient'6 cardiac activity, and to respond to arrhythmias by generating and ~anaging the delivery of pacing and 6hock therapies. It will be evident, then, th~t despite its name the therapy generator does ~ore than 8i~ply generate therapies, ~ust ac the pulse generator of A pacer does more than 6imply generate pulse~; and in particular tbat the generator incorporates c~rcuitry for ~ensing cardiac activity. Among other things, the generator bas a self-contained power ~ource, and is asse~bled and housed in a ~etal case w~ich is inert to body tissue and fluids.
Lead/electrode assemblies for use in ~ensing cardiac activ~ty and delivering the respective pacing and 6hock impulses to t~e patient'~ ~eart are 6ep~rably connectable to the therapy generator, in a manner analogous to leads of a cardiac pacemaker. Toqether, then, the therapy generator and the lead~electrode a6se~blies constitute the cardiac st~mulator.
T~e therapy generator includes a digital control 6ection for storinq and executing 60ftware in6truction~ and for storing and processing the data for all digital functions of the de~ice (a~ide from those function6 ~hich, for purposes of conserving memory capacity, are re~dily consigned to an external progra~mer unit). Digital function6 of the device ~ay include the phy6ician-programmable a6pects, ~uc~ as provicion for programming rate boundaries to selectively partition the rate contlnuum, desired therapy 6tructure6 and delivery ceguence~, and detection and redetection algorithms;
and, as ~ell, the variou~ proce6sing, ti~ing, switching, control and other functions, all of which are de~cribed in detail in U.S. Patent No. 4,830,006 and many of which need not be repeated here inasmuch as they are not essential to an understanding of the present invention.
The therapy generator al~o include~ an analog ~y~tem portion for functions including those of the pre~ent in~ention in~olving ~onitoring of the patient's ECG ~ignal ., 9 inforeation over ach card~ cycle and nhanclng that ~lgnal infor~ation while reducing noi~e and other lnter~erence througb eignal filterlng and auto~atic gain control. Other analog functions of tbe generator include developing the re6pectl~e i~pul~e wavefor~s to be dellvered for the pacing and shock therapies, transmitting data between the dev~ce and external units sucb as the progra~mer and transtelephonic ~onitoring equipment, and protecting again6t overloads. ~180 included are the battery cell6, and voltage regulation and priority power seguencing section, for supplying power to the other ~ections of the overall generator.
With reference now to ~IG. 1, a central microprocessor and associated ~e~ory ~ection 10 of tbe therapy generator processes and ~tores the dat~ for rate boundaries within the ~eart rate continuum, therapy ~tructures, detection algoritbms, and other features. Bidirectionally coupled to ~icroprocessorJDe~ory 6ection 10 i6 a programming and data transmission section 14 for transmitting data to and from the external programmer and~or to receivinq and ~onitoring equipment, via the antenna 1~. A cry6tal oscillator 20 i6 electrically coupled to section 10 to provide precise timing cignal~ for ~yste~ operation. A reed 6w$tch 22 allows limited external control ~y tbe patient of pbysician-~elected progra~nable function6, by Dean~ of placing an external ~agnet (not 6bown) in proxi~ity to the 6witch for actuation thereof.
A ~ense amplifier 6ect$0n 25 of tbe pre~ent invention, described in detail below, i6 coupled to the nicroprocessor/memory ~ection lo to furnish electrogram signal infor~ation and to receive control 6ignal6 from the Dicroprocessor. The ~ense amplifler al60 6upplies electrogra~ signal information directly to data transmission ~ection 1~ for telemetry to tbe externàl monitoring equipment. A quad co~parator within sense amplifier 25 6er~es a6 tbe llnk to con~ert electrogram, ~ense signal information fro~ 6en6ing electrode(6) (not sbown) attacbed to tbe patient'~ heart into digital infornation for use by tbe ~lcroproces~or. Tbe ~icroproces~or 10 iB dlspo6ed ln the ~ 10 i329238 feedb~ck loop of tlle en~e ~plifler 25 tor auto~atic galn control.
The sen6e a~plifler 25 enhance6 the lectrogr~ slgnal6 to enable tracking the rapidly ~arylng a~plltude, of fibrillation ~ignals. Preferably, tbe ~en6e ~plifier 25 has a range of ga~n of the order of 30:1. A6 wlll be explained ~n more detail below, AGC with bandpas6 filtering is employed to provide the du~l function of reducinq the amplitude of 6ignal6 outside the fresuency b~nd of interest, ~nd further amplifying the relatively low freguency te.g., fibr~llation) 6ignal6 in the absence of nor~al R-waves.
The power ~ource section 28 of the therapy generator includes hiqh rate battery cell6, a voltage regulator and a priority power 6equencer none of which are 6hown and all of which are conventional. The cells are capable of delivering 6ufficient energy to charge capacitors (not 6hown) in the output bigh voltage ~ection ~0 within a period of 20 ~econds or les6. The ~olt~ge regulator circuit provides voltage reduction tdivi6ion) depending on the number and connection of cells e~ployed, to i~prove power source efficiency. The priority power ~equencer assures that adeguate power is ~ade a~ailable to es6ential circuit function6, including the contro;l logic, during period6 of high current drain on the cell6, 6uch as during charge-up of the high voltage capacitor~ in preparatlon for deli~ery of defibrillatinq 8hocks.
A pacing cection 31 in the therapy generator includes a ~oltage ~ultiplier and output section (neitber of which is shown and both of which are conventional). The rultiplier ecales up the regulated supply ~oltage fro~ power source ~ection 28 by ~ultiple~ of one, two or three. The output ~ection provide6 output 6witcbing from tbi6 ~caled voltage to deli~er p~cing ~ti~uli to tbe p~tient's heart via the pacing electrodes ~not shown), in the ~anner of a pulse generator, under tbe control of tbe ~icroprocessor 10. An analog r~te li~lt circuit 35 controllably li~it~ the paclng ratc, to prevent p~cem~ker run~w~y ln the event of failure of the cry6tal o~cillator circuit 20. The ~icroproce~sor 10 auto~atlc~lly dis~bles tbe rate l~iter 35 whenever bigh rate pacing pul~e- are requlred~o be dellv-red, ~uch a~ ~or a bur6t pacing therapy.
The lead6 (not shown~ for the p~cing and cen6ing electrodes (also not ~hown) are electrlcally nonltored by an i601ation/protection circuit 3~ to protect low ~oltage, low power component6 of the cardiac sti~ulator from being sub~ected to tbe hlgh voltages of the defibrillating 6hocXs generated by tbe stimulator or applied by an external defibrillator that night be used in the course of emergency medical procedures on the patient.
The cardioverter/defibrillator shock therapy portion of the therapy generator includes an i601ated ~igh voltage ~enerator ~nd output 6ection 40, in ~hich an osc~llator charges output capacitors to the required level through an isolation transformer, under the control of the ~icroprocessor. A low power analog-to-digital (A/D~
converter in 6ection ~o ~onitor~ the voltage on the capacitor~ to ~ssure ~ microprocessor ~etting of the desired biqh ~oltage output level ~ccording to the proqram~ed content of the ~hock therapy. ~onitoring of the capacitor voltaqe provides the Dicroprocessor with a mea~ure of the residual charqe on the capacitor~, after delivery of each output pulse, by which to e6ti~ate the ~nount of energy consu~ed in the delivery for lead iDpedance measurement. In ~ddition, the A~D converter input circuit may be ~witched by the Dicroproces60r for connection to the power 60urce 6ection 28 to Donitor the battery voltage, and thereby determine the present condition of the cells.
Output section ~0 al~o contains level 6hifter6 and i601ation tran6formers (not shown) to convert the ~icroproce6sor-supplied low level logic control slgnals to the control cignal level~ required to drive the output switche~ of that ~ection~ The switche~ are of low 'on' impedance and capable of handling the high voltage6 and current- being generated to control the deli~ery and polarity of each output pulse. A short circuit protection clrcuit ~ay be employed to open the output circuit in the event that the current therethrough rise~ above a predeter~ined level, to prev-nt de~tructive capacitor discharge into a very low l~pedance lf, for exampl~ e de~lbrillator patch l-ctrode6 Yere 6horted.
Sense amplifier 25 and it6 relation6hlp to the ~icroprocessor 10 are lllu6trated in qreater detail ln FIG.
CONTROL AND BANDPASS FILTERIN~ IN FEEDBACK LOOP
This application is related to the sub~ect matter of U.S. Patent No. 4,830,006 titled "IMPLANTABLE CARDIAC
STINULATOR FOR DETECTION AND TREATMENT OF VENTRICULAR
ARRNYT~NIAS n ~ a~signed to the same assignee as is the present application.
FIELD OF THE INVENTION
The present invention relates generally to implantable cardiac 6timulators ~uch as pacemaXer~ and defibrillators or combinations thereof, and more particularly to automatic gain control for such implantable devices for the purpose of enhancing tbe capability of the device to sense an arrhyt~mia for which therapy is to be applied.
BACKGROU~D OF THE lNvE~lON
Arrhythmias are variations in heart rate from the normal cinus rate range of approximately 60 to 120 beats per minute ~bpm) prevalent in healthy adult humans with normally functioninq hearts. Bradycardia i6 characterized by rates below 60 bpm, whereas for tachycardia the rates generally range upwardly fro~ 120 bpm. Typically, tachycardia is experienced as a result of 6uch factors as phy~ical stress texercise), emotional stress (excitement), consumption of alcoholic or caffeinated beverages, ingestion of certain drugs such as nicotine, and ~o forth. The heart rate of a healthy person Yill gradually return to the 6inus rate after removal of the tachycardia-inducing factor (8). However, arrhythmias reguire 6pecial medical treatment. For exa~ple, fibrillation i6 a high rate arrhythmia (tachyarrhythmia) characterized by completely uncoordinated contractions oS
sections of conductive cardiac ti~sue of the affected chamber of the heart, resulting in a complete 106s of synchronous contraction of the overall mass of tis6ue. As a consequence, the cha~ber ceases to pump blood effectively and, in the case of ventr~cular f~brillat~oa~ the lack of oxygenated blood to the t~ssues w~ll lead to death wlt~ln ~lnute~.
Implantatlon of a c~rdiac pacemaker has been a typlcal procedure of choice for treatment of bradycardia patients.
The pacemaXer pulse generator i6 implanted in a pouch beneath the 6kin in the patient'6 chest and deliverc electrical impulses to electrodes positioned at the patient'6 ~eart via one or more catheter leads, to stimulate the heart to beat at a desired rate in the normal 6inus range. Cardiac pacinq has also found increasing u6age in tbe ~anagement of tachyarrhythmias. In essence, the heart may be artificially stimulated at a faster than normal pacing rate to terminate a tachycardia or to suppress premature atrial or ventricular contractions which could otherwise lead to 6upra-ventricular or ventricular tachycardia, flutter, or fibrillation. T~e pulses delivered to tbe heart for pacing therapy need only be of sufficient magnitude to stimulate tbe excitable myocardial tissue in the immediate vicinity of the pacing electrode.
More recently, the automatic defibrillator has been proposed for i~plantation in cardiac patients prone to suffer ventricular tachycardia and/or fibrillation. The device i~
adapted to 6hock the heart with electrical pul~es of considerably higher energy content than i~ delivered $n pacing pul~es. Vpon detecting fibrlllation, one or ~ore hiqh energy 'counter-shoc~ are applied to the heart to overwhelm the chaotic contractions of individual ti6sue sections and re-establis~ organized cpreading of action potential from cell to cell of the ~yocardium, thereby restoring the 6ynchronized contraction of t~e mass of tissue. In general, prior art implantable defibrillatorc have been implemented to detect fibrillation from the patient's electrogram waveform and/or t~e absence of a ~echanical~ function 6uch as rhythmic contractions or pul6atile arterial pressure, and, in response, to deliver a fixed therapy typically con6isting of one or ~ore shocking pulse6 of preset wavefor~ and energy content.
For example, detection of fibrillation activity by monitorlng the electrogram signal ha6 been achleved wit~
conventional ~en~e amplifier configuration6 involving fixed ~en~e ~argin, flxQ~ or ~el-~tiY-ly ad~u~ted galn and flxed bandpa6s characteri~tlc~. Thi~ type of detectlon tends to neqlect the characterlstic di~ference6 between QRS complexes and fibrillation waYeforos. Yibrillatlon waveforms are characterized by erratic a~plitude Dodulated sinusoids of relatively low frequency in a narrow band (3-12 ~z~. On the other hand, QRS complexes have relati~ely con~tant amplitude, charp peaks ~ith 6ignificant high freguency content and thus a broader frequency spectrum.
The aforementioned U.S. Patent No. 4,830,006 discloses an improved medical device for treating ventricular tachycardia and defibrillation, which uses sophisticated techniques for detecting ~nd distinguisbing the arrhythmia from normal high rates, and a h}erarcbical approacb to tbe aggressiveness and delivery of tberapies to terminate tbe arrhythmia. Tbe functions of bradycardia and anti-tachycardia pacing-type tberapies, and cardioversion and debifrillation shock-type therapies, are inteqrated to provide a coordinated approach to the ~anagement and treatment of ventricular arrhythmias. A
6ignificant aspect of t~at approach i6 to provide flexible seguencing among the therapies, ~ith appropriate regard to hemodynamic tolerance ~or intolerance) of the patient to the detected arrhythmi~, and sophi6ticated detection of arrhyt~mias together with Deans for distinguishing those episodes for ~hich treatment is required tsuch as re-entrant tachycardias) fro~ those which are not associated with cardiac or other di6ease (suc~ as exercise-qenerated sinus tachycardias). A multiplicity of hierarchical detection algoritbms and therapeutic ~odalities are selectively available to t~e physician to detect and treat classes of ventricular tachycardia according to their respect~ve po6ition6 in the heart rate continuu~, and thus according to hemodynamic tolerance or $ntolerance of the patient to the tachycardia, ~ith backup capabilltle~ of defibrillation and bradycardia paclng for cardiac arrhythmia6, lncluding po6t-6hock bradycardia.
In an embodimQnt describQd in U.S. Patent No.
4,830,006 the cardiac stimulator permits ~elective partitioning of the .
~eart rate cont~nul~ lnto a plurality Or contiguou~
tachycardia cla~sel~ of progres~vely h1ghar rate r~nges, the lowest and highest of the6e classe~ belng bounded respectively by reg~on6 of the contlnuu~ denotlng ~inu6 rate and fibrillation. ~ach of the rate range6 and the latter req~ons may be program~ed by the phy6ic~an, a8 may be necessary to me~t the particular needs of the patient'6 di60rder and the flexibility of t~e therapy regimens to be prescribed. The ~ierarcbical detection sy~tem employed in the 6ti~ulator for detecting cardiac episodes ~ndlcative of arrhyt~mia and for distingui6bing between noroal and abnormal tac~ycardias am3ng the detected Qpisodes uses criteria of greater or lesser stringency depending ~n tbe location of the epi60de in the rate contlnuum. The device permits programming of the various detection criteria for tacbycardias and fibrillation, and includas plural co~binations of basic detection criteria in each category.
The arrbyth~ia detection ~ystem ai w losed in that application also utili2ed signal processing including amplification with automatic gain control and bandpass filtering according to tbe present invention, for enbancing sensltivity to the differences bet~een tbe electrical and phy~ical cbaracteri~tic~ of ~inu~ rbytbm~ and arrhyt~ia ~aveforms.
It ~ould be noted that the prior ~rt ~as ~ugqested the ~naly~ie of vavefor~ norphology for fibrillation detection.
Moreov~r, it is ~nown to use auto~atic g~in control in defibrillation systems. However, the prior art does not suggest a system and method for arrhythmia detection which recogniseQ and takes advantage of the characteristic differences between sinus rhythm and arrhythmias, including those mentioned above, utilizing automatic gain control and bandpass filtering in a feedback loop.
A significant ob~ective is to provide an implantable cardiac stimulator utilizing automatic gain control in interaction with arrhythmia detection, as well as with special bandpass characteristics and bradycardia pacing.
A further important aim is to provide an approach for dealing with interactions between gain and pacing in an implantable device capable of treating bradyarrhythmias as well as tachyrhythmias.
Yet another objective is to provide an implantable device which implements the foregoing ob~ectives in treating tachycardia and fibrillation, and which is adapted also to treat post-shock bradycardia.
Still another objective is to provide a cardiac arrhythmia detection system and method employing automatic gain control in conjunction with bandpass filtering in a feedback loop, wherein the margin of sensitivity to the various waveforms encountered varies to enhance the capability and speed of detection of the arrhythmias of interest, such that an appropriate therapy is quickly applied to return the heart to normal sinus rhythm.
SUNMARY OF THE INVENTION
According to the invention there is provided an implantable apparatus for dQtecting cardiac arrhythmias including electrode means for sensing electrical activity of a patient~s heart and processing means responsive to the sensed electrical activity for determining whether sensed events are indicative of arrhythmias~ The processing means includes target means establi~hing at least one reference level against which the amplitudes of predeterminQd sensed events are compared, gain control means controllably varying sensitivity of the processing means according to the amplitude of each sensed event relative to said at least one reference level, and comparison means responsive to said gain control means being ad~usted in sensitivity in one direction immediately following an ad~ustment in the other direction for counting the number of times the sensed event cros~es the reference level as an indication of the nature of the arrhythmia.
The sense amplifier of a cardiac stimulator is provided with automatic gain control (AGC) as well as bandpass filtering and comparison of sensed signal (sensed event) amplitudes against selected target levels in a system and method for detecting cardiac arrhythmias. Interactions between gain, bandpass and pacing are utilized in a manner not found in prior art techniques employed to detect and treat cardiac arrhythmias. In essence, the invention addresses bandpass filtering and its effe~t on sensing margins; varies the sensing margin with the shape of the waveform under analysis; minimizes noise and T wave sensing while QRS complexes are present; and maximizes sensing of fibrillation waveforms in the face of widely varying amplitudes.
The AGC amplifier includes an amplifier section having a gain that may be altered as often as necessary to maintain a set of predefined conditions, a set of bandpass amplifiers with different characteristics for AGC and sense target comparators, and a dual set of comparators to convert the processed analog information into digital information to be used by a microprocessor for making decisions about the need ,.
. . . ,, ~ , . .
for cbanges ln e~e gain of ~he arpl1r1er ~nd ~en61ng act~vlty.
T~e present in~ention take~ advantDge of the d~fferences between waveforcs or 6ignal6 repre~ent~t~ve of c~rdiac arrhythmias and tho~e indicat~ve of norc~l 6~nus rhyth~ to provide rapid detection. For exa~ple, differences in t~e c~aracteristics of fibrillation waveforms and standard QRS
co~plexes include wide and frequent variation in the amplitude of a fibrillation if set to a gain to assure ~ensing o~er the neces6arily wide a~plitude range of ~nterest (e.g., 0.3 xv to ten~ of mill~volts), the device could ~ense and respond to false signals ~ncluding the unwanted noise ~ignal~ whic~ ~re often present in t~e a~sence of fibrillation at typically low ampl~tude level~ (e.g. T-~aves).
Accordingly, a fibrillation detection method and system may be provided for a cardiac stimulator capable of automatic delivery of defibrillating therapy, in which signal amplification with automatic gain control and bandpass filtering is utili2ed for reliably sensing wide and frequent amplitude variation of fibrillation waveforms.
Automatic gain control may be provided in the signal analysis path for fibrillstion detection, to avoid falsely sensing noise signals as fibrillation and consequent undesirable delivery of unnecessary defibrillating shocks to the patient.
A set of bandpass amplifiers may be provided to produce different bandpass characteristics between peak and sensing comparator targets. Without the differential bandpass characteristic, the sansing margin would be the same for all electrogram waveforms, regardless of their frequency content.
In such a system there is a difficult tradeoff between the large sensing margin that would achieve reliable sensing of erratic fibrillation wa~eforms and a small sensing margin that would sense QRS complexes but re~ect T-waves. With the addition of the differential bandpass characteristic, fibrillation waveforms (which do not contain the higher frequency component~ that ar~ charact ri~tic Or QRS co~plcx~s) are ~electively enhanc~d at the sensing target This elQctlve enhancement provides the hlg~er senslng ~argin requlred to reliably ~ense thi6 erratic ~gnal But by not enhancing the higher frequency ~ignal components of QRS complexes, the lower margin needed to re~ect T-~aves i~ ~till obt~ined BRIEF DESCRIP~IO~ OF THE DRAWI~GS
The abo~e and ~till further ob~ectc, features and attendant advantages of ~he present invention ~ill become apparent from a consideration of th~ following detailed description of a presently preferred embodi~ent, t~ken in conjunction ~ith the accompanying draw~ngs, in which M G 1 is a bl~ck circuit diagr~m of t~e therapy generator of an implantable cardiac ~timulator, utilizing a ~ense amplifier havinq AGC, b~ndpass filter~ and quad comparator~ according to the presently preferred embodiment;
FIG 2 ~ a Dore detailed circuit diagram of the ~icroprocessor and ~ense amplifier portion of the therapy generator circuit of FIG l;
FIG 3 i~ a ~c~ematic circuit diagra~ of the AGC
amplifier ~ection of the ~ense amplifier;
FIG ~ ic a 6chematic circuit diagra~ of t~e bandpass amplifier cection of the cense amplifier;
~ IG S ~ che~atic circuit diagra~ of the guad comparator section of tbe cense amplifier; and FI~S 6 t~rough 9 are waveforms u~eful for describing the operation of t~e cense a~plifier according to t~e presently preferred embodi~ent DETAILE~ DESCRIPTION OF T~ ESEN~ REFERRED ENBODIMENT
The invention herein will be described in the environment of an implantable cardiac stimulator of the type disclosed in U S Patent No 4,830,006, but it is not limited to that usage and it will be understood as this description progresses that the invention may be utilized as well in more restricted anti-tachycardia pacemakers or bradycardia pacemakers The therapy generator (~IG. 1) unit of the ~timulator (akin to the pulse generator unlt of an implantable pacemaker) i6 adapted to detect preselecte~ aspects of the patient'6 cardiac activity, and to respond to arrhythmias by generating and ~anaging the delivery of pacing and 6hock therapies. It will be evident, then, th~t despite its name the therapy generator does ~ore than 8i~ply generate therapies, ~ust ac the pulse generator of A pacer does more than 6imply generate pulse~; and in particular tbat the generator incorporates c~rcuitry for ~ensing cardiac activity. Among other things, the generator bas a self-contained power ~ource, and is asse~bled and housed in a ~etal case w~ich is inert to body tissue and fluids.
Lead/electrode assemblies for use in ~ensing cardiac activ~ty and delivering the respective pacing and 6hock impulses to t~e patient'~ ~eart are 6ep~rably connectable to the therapy generator, in a manner analogous to leads of a cardiac pacemaker. Toqether, then, the therapy generator and the lead~electrode a6se~blies constitute the cardiac st~mulator.
T~e therapy generator includes a digital control 6ection for storinq and executing 60ftware in6truction~ and for storing and processing the data for all digital functions of the de~ice (a~ide from those function6 ~hich, for purposes of conserving memory capacity, are re~dily consigned to an external progra~mer unit). Digital function6 of the device ~ay include the phy6ician-programmable a6pects, ~uc~ as provicion for programming rate boundaries to selectively partition the rate contlnuum, desired therapy 6tructure6 and delivery ceguence~, and detection and redetection algorithms;
and, as ~ell, the variou~ proce6sing, ti~ing, switching, control and other functions, all of which are de~cribed in detail in U.S. Patent No. 4,830,006 and many of which need not be repeated here inasmuch as they are not essential to an understanding of the present invention.
The therapy generator al~o include~ an analog ~y~tem portion for functions including those of the pre~ent in~ention in~olving ~onitoring of the patient's ECG ~ignal ., 9 inforeation over ach card~ cycle and nhanclng that ~lgnal infor~ation while reducing noi~e and other lnter~erence througb eignal filterlng and auto~atic gain control. Other analog functions of tbe generator include developing the re6pectl~e i~pul~e wavefor~s to be dellvered for the pacing and shock therapies, transmitting data between the dev~ce and external units sucb as the progra~mer and transtelephonic ~onitoring equipment, and protecting again6t overloads. ~180 included are the battery cell6, and voltage regulation and priority power seguencing section, for supplying power to the other ~ections of the overall generator.
With reference now to ~IG. 1, a central microprocessor and associated ~e~ory ~ection 10 of tbe therapy generator processes and ~tores the dat~ for rate boundaries within the ~eart rate continuum, therapy ~tructures, detection algoritbms, and other features. Bidirectionally coupled to ~icroprocessorJDe~ory 6ection 10 i6 a programming and data transmission section 14 for transmitting data to and from the external programmer and~or to receivinq and ~onitoring equipment, via the antenna 1~. A cry6tal oscillator 20 i6 electrically coupled to section 10 to provide precise timing cignal~ for ~yste~ operation. A reed 6w$tch 22 allows limited external control ~y tbe patient of pbysician-~elected progra~nable function6, by Dean~ of placing an external ~agnet (not 6bown) in proxi~ity to the 6witch for actuation thereof.
A ~ense amplifier 6ect$0n 25 of tbe pre~ent invention, described in detail below, i6 coupled to the nicroprocessor/memory ~ection lo to furnish electrogram signal infor~ation and to receive control 6ignal6 from the Dicroprocessor. The ~ense amplifler al60 6upplies electrogra~ signal information directly to data transmission ~ection 1~ for telemetry to tbe externàl monitoring equipment. A quad co~parator within sense amplifier 25 6er~es a6 tbe llnk to con~ert electrogram, ~ense signal information fro~ 6en6ing electrode(6) (not sbown) attacbed to tbe patient'~ heart into digital infornation for use by tbe ~lcroproces~or. Tbe ~icroproces~or 10 iB dlspo6ed ln the ~ 10 i329238 feedb~ck loop of tlle en~e ~plifler 25 tor auto~atic galn control.
The sen6e a~plifler 25 enhance6 the lectrogr~ slgnal6 to enable tracking the rapidly ~arylng a~plltude, of fibrillation ~ignals. Preferably, tbe ~en6e ~plifier 25 has a range of ga~n of the order of 30:1. A6 wlll be explained ~n more detail below, AGC with bandpas6 filtering is employed to provide the du~l function of reducinq the amplitude of 6ignal6 outside the fresuency b~nd of interest, ~nd further amplifying the relatively low freguency te.g., fibr~llation) 6ignal6 in the absence of nor~al R-waves.
The power ~ource section 28 of the therapy generator includes hiqh rate battery cell6, a voltage regulator and a priority power 6equencer none of which are 6hown and all of which are conventional. The cells are capable of delivering 6ufficient energy to charge capacitors (not 6hown) in the output bigh voltage ~ection ~0 within a period of 20 ~econds or les6. The ~olt~ge regulator circuit provides voltage reduction tdivi6ion) depending on the number and connection of cells e~ployed, to i~prove power source efficiency. The priority power ~equencer assures that adeguate power is ~ade a~ailable to es6ential circuit function6, including the contro;l logic, during period6 of high current drain on the cell6, 6uch as during charge-up of the high voltage capacitor~ in preparatlon for deli~ery of defibrillatinq 8hocks.
A pacing cection 31 in the therapy generator includes a ~oltage ~ultiplier and output section (neitber of which is shown and both of which are conventional). The rultiplier ecales up the regulated supply ~oltage fro~ power source ~ection 28 by ~ultiple~ of one, two or three. The output ~ection provide6 output 6witcbing from tbi6 ~caled voltage to deli~er p~cing ~ti~uli to tbe p~tient's heart via the pacing electrodes ~not shown), in the ~anner of a pulse generator, under tbe control of tbe ~icroprocessor 10. An analog r~te li~lt circuit 35 controllably li~it~ the paclng ratc, to prevent p~cem~ker run~w~y ln the event of failure of the cry6tal o~cillator circuit 20. The ~icroproce~sor 10 auto~atlc~lly dis~bles tbe rate l~iter 35 whenever bigh rate pacing pul~e- are requlred~o be dellv-red, ~uch a~ ~or a bur6t pacing therapy.
The lead6 (not shown~ for the p~cing and cen6ing electrodes (also not ~hown) are electrlcally nonltored by an i601ation/protection circuit 3~ to protect low ~oltage, low power component6 of the cardiac sti~ulator from being sub~ected to tbe hlgh voltages of the defibrillating 6hocXs generated by tbe stimulator or applied by an external defibrillator that night be used in the course of emergency medical procedures on the patient.
The cardioverter/defibrillator shock therapy portion of the therapy generator includes an i601ated ~igh voltage ~enerator ~nd output 6ection 40, in ~hich an osc~llator charges output capacitors to the required level through an isolation transformer, under the control of the ~icroprocessor. A low power analog-to-digital (A/D~
converter in 6ection ~o ~onitor~ the voltage on the capacitor~ to ~ssure ~ microprocessor ~etting of the desired biqh ~oltage output level ~ccording to the proqram~ed content of the ~hock therapy. ~onitoring of the capacitor voltaqe provides the Dicroprocessor with a mea~ure of the residual charqe on the capacitor~, after delivery of each output pulse, by which to e6ti~ate the ~nount of energy consu~ed in the delivery for lead iDpedance measurement. In ~ddition, the A~D converter input circuit may be ~witched by the Dicroproces60r for connection to the power 60urce 6ection 28 to Donitor the battery voltage, and thereby determine the present condition of the cells.
Output section ~0 al~o contains level 6hifter6 and i601ation tran6formers (not shown) to convert the ~icroproce6sor-supplied low level logic control slgnals to the control cignal level~ required to drive the output switche~ of that ~ection~ The switche~ are of low 'on' impedance and capable of handling the high voltage6 and current- being generated to control the deli~ery and polarity of each output pulse. A short circuit protection clrcuit ~ay be employed to open the output circuit in the event that the current therethrough rise~ above a predeter~ined level, to prev-nt de~tructive capacitor discharge into a very low l~pedance lf, for exampl~ e de~lbrillator patch l-ctrode6 Yere 6horted.
Sense amplifier 25 and it6 relation6hlp to the ~icroprocessor 10 are lllu6trated in qreater detail ln FIG.
2. T~e ECG waveform components detected by the 6ens~ng electrodes (not 6hown) are applied to an AG~ amplifier 60 via an input circuit 63. The gain of amplifier 60 i6 automatically controlled by a feedback loop 65 containing a portion 68 of the ~icroprocessor/~emory 6ection 10 (~IG. 1).
The electroqram ~ignal6 processed by AGC amplifier 60 are further enhanced by a filter 6ection 70 having a primary hiqh gain bandpass amplifier 73 to amplify 6ignals witbin the band. The output of amplifier ~3 i8 ~plit and applied to 6eparate bandpass amplifiers 75 and 76, tbe former being diqitally controlled by ~icroprocessor portion 68. The output derived from amplifier and filter 6tages 60 and 70 is applied to a guad comparator 80 (to be described in greater detail below with reference to ~IG. 5), comprising a ~et of sensing target comparator6 82 and a ~et of peak target comparators 83, 85, which develop three inputs to tbe microprocessor ~n the feedback loop.
The primary purpose of the invent~ve 6y~tem i6 to track the 6ignal6 of intere6t. The sen6e a~plifier with AGC, filter and feedbac~ ~y6te~ of ~e pre~ent l m ention i6 i~ple~ented to 601ve the difficult problem of ~en6ing the low frequency, ~arying amplitude whic~ i6 characteri6tic of ventricular fibrillation. It will be recognized that under ordinary circu~stance~ a 106s of sen6ing ~ay be indicative of fibrillation, reguiring tbat the gain of the 6en~e a~plifier be increased to enable better detection. If, however, the loss of 6ensing i6 attributable, for example, to inter~ittent heart block rather than to fibrillation, a return of 6en6e signal likely ~ould be over-a~plified, ~ith a con6eguent 6eriou6 pertur~ation of the entire 6y6te~.
Tbe 6ystem of ~IG. 2 pro~ides dual signal path6 or channel6 ba~ing different bandpa66 characteri6tic6. Tbe 6ignal pat~ through amplifier 75 to t~e sense comparators 82 deter~ine~ the sen6in~ point ~nd the 6ignal path through amplifier 76 to the AGC comparators 83 portion is part of the feedback loop tbat incl~4es the ~icroproces60r 68, wh~ch determines tbe gain of AGC aDpllfier 60. The ~en61ng ~arg~n 18 defined a6 the ratio of peak cignal size at the input~ to lnner comparators 82 and thelr sen~lng thre6hold. A ratio greater than one ru~t be cho6en in order to avold 105s of sensing as a consequence of a reduction ln 61gnal amplitude.
~or example, the use of a 2-to-1 margin would Dean that the 6ignal amplitude would have to be reduced by one-half to lose 6ensing. The goal of the AGC sy~tem i8 to ~aintain a preset 6ensing margin. ~or a given thre~hold level on the sensing comparator, this may be achieved by ad~usting the gain of the AGC amplifier ~o t~at the peak voltaqe seen by the senslng comparator~ remains ~ore or less con~tant. The ~icroprocessor 68 6a~ples the output of the peak co~parators 83 on a cycle by cycle basis. In essence, lf the waveform peak exceeds the t~reshold of the AGC the Dicroprocessor 68 will reduce the gain a 6mall amount. ~If the ~aveform peak does not exceed the threshold of the AGC, it will increase the gain a small amount (the increase/decrease decision process will be dlscussed ln more detail later).
The bandpass of the signal path to the sensing comparator i6 6haped 80 that frequencies below 25 Hz are attenuated. This give~ the ~esirable effect of attenuating the lower frequency T-vave relative to the QRS complex.
However, it has the undesirable effect of attenuating fibrillation 61qnal6. But the bandpas6 of the 6ignal path to the AGC comparator i~ 6haped 60 that the frequencies in the 3 to 12 HZ range are attenuated even more than they are in the signal path to the sensing comparator. The AGC ~ystem vill compensate for t~e attenuatlon of a fibrillation 6ignal by increa6ing the gain of the aGc ampllfler. However, becau6e the flbrillatlon 6ignal to the AGC comparator i6 attenuated ~ore than t~e ~ignal to the sen6ing target, after tbe ~icroproce660r ~a~ lncrea6ed t~e galn the AGC voltage that appearc at the ~en6ing comparator wlll be higher than tbe AGC voltage tbat appear6 at the senslng comparator when QRS complexes are being sen6ed. Thus tbe ~argin ~8 hlgher when a flbrillation waveforn 1B present.
~ ith resp~ct to paclng-~nt-ractlon, in t~e ca-e of a bradycardia patlent the gain ~cttlng mu~t be appropr1ate to allow senslng of ~ubseguent fibrlllation. Also, the T-wave window ~ust be ~et to ~ense T-waves evoked ~y the p~cemaker.
In thl6 particular ca6e, the galn 1~ ad~usted by the ~icroproces60r ~ that tbe T-wave lc at approxi~ately the level of the 6ensing target. The a6su~ption6 are that evoked T-waves are larger than intrin6ic T-waves, and that the QRS
complex i6 ~uch larger than T-wave~. Such a gain 1~
rea60nably sufficient to sen6e fibrillation or a QRS complex.
Referring to FIG. 3, AGC a~plifier 60 of the 6an6e amplifier 6ection 25 provides initial bandpa~s filterlng by ~eans of a c~rcuit con6i6ting of re6istor6 110 and 111 and capacitor6 112 and 113. The AGC amplifier gain i8 controlled by varying the gate voltage of ~n N-cbannel ~unction field effect transi6tor (JFET) ~00 which acts a~ a voltage controllQd input resi6tor to a non-inverting amplifier 101.
The ~icroprocessor 68 (FIG. 2) control~ the on/off duty cycle of ~witches 103 and 10~, to 6et the gate voltage of JFET 100 by charging and discharging capacitor 106 to a voltage between V~ and Vc. This technique i6 used to obtain a gain range of 30:1 a~ deter~ined by the re6i6tance of resi6tor 107 ~nd the on impedance of JFET 100. Resistor 115 allows proper bia6inq of t~e JEET circuit. Although switche6 103 and 10~
are 6chematically depicted a6 ~echanical devices in the AGC
amplifier circuit of FIG. 3 and in 60me of the other circuit diagra~s, lt wlll be understood that in practice electronic 6wltches t~uch a~ field effect tran~istors) typically would be employed.
Turning now FIG. ~, the output of the AGC ~cplifier ~ection 60 16 6upplied to bandpass amplifier 6ection 70 of 6ense aoplifier 25~ Section 70 has a progr~able sensing ~argln feature and ~pecl~l bandpass charactcrl~tic~ whlch ald in tracklng the varlable ampl~tude fibrlllation lgnal. The overall bandpa~s ~pllfier lncludes two actl~e bandp~s f~lter ~pllflers 130 and 131, a progr~mable gain DC
aopllfler 135, and a passl~e high pasc filter compri6ing capacitor 137 and resistor 138. Thls circuit reduces the a~plitude of the ~lgnal components outside the ~reguency band ~ 15 ^- 1329238 of intere~t, and ~n,crea~e~ n~ing ~ar~in for tb- lov freguency fibrlllation ignal~ The u1croproces~or 68 ~etc the ~Agn~tude of resi~tance 140, and therefore can et the gain of DC amplifier 135 and, thus, the galn to the ~en6~ng target comparator6 ceparately fro~ the gain to the AGC target comparators In this manner, the ~en6ing margln (which is proportional to the ratio between the ~ensing and AGC
tarqet6) i8 prograDmable, being selectable by the ~icroprocessor Alternatively, the effecti~e ratio of the target6 ~ and, therefore, the sen~ing ~argin) could be programmably changed by varying the qain of bandpass filter amplifier 131 through changes of the value of re6i6tor 142, or by changing the target reference voltages them6elves The guad comparator 80 of ~ense a~plifier 25 i6 6hown in FIG 5, and an exemplary input signal and logic outputs of the comparator are illu6tr~ted in n G 6 Tbe comparator has two comparator pair6 comprising ~ensing target comparator6 150 and AGC target comparators 151 The logic output~ Ll and L2 (upper and lower, respectively) of the ~ensing target comparator6 are used by tbe ~icroproces60r as valid sen~e input signal6 The loqical '0~ eitber po~it~ve or negati~e) of the AGC target co~parator6 1~ u~ed by tbe ~icroprsce6sor to evaluate the need for increa6ing or decrea6inq the AGC a~plifier gain The ~enslng t~rget6 represent a sen~ing t~resh~ld, and the AGC targets represent a sensed 6ignal peak amplitude target During ~inus r~yt~, the a~plitude of the QRS co~plex dictate6 the gain 6etting of the AGC amplifier bec~u~e of the relatively large amplitude and high freguency content of tbe QRS The ~a~e situation ex~6t6 during 6en6ing of a tachycardia Howe~er, in the pre~ence of a lower freguency fibr$11ation ~ignal there i~ nearly a doubling of the sensing ~argin, becau6e of the bandpa6~ fllters 1~1 and 1~3 (FIG ~), ther~by per~itting more reliable tracking of the variable a~plitude fibrillation ~$gnal ln this re~pect, it Yill be recogni~ed that the 6eries re6istance-capacitance path in parallel Yith the resl~tance path of each of circuit6 1~1 and 143 provldes an input iDpedanc- to the re~pectlve operatlonal ampllflers 130 and 131 ~uch that the gain at h~gh 132923~
frequencies ~111 be ~ore t~ n the galn at lov frequencles.
Thus, the frequency plot for the input ~lgnal to the AGC
target6 con~titutes a bandpas6 (FIG. 7) ~hlch will p~6S Q~S
complexes faithfully, while T-waves will be relat$vely attenuated to Dlninize double 6en~ing. Fibrillation 6ignal6 will be passed accurately, although relatively attenuated, but will boost the gain of tbe AGC to over-compensate.
By way of an example of operation, in the presence of a QRS complex which i6 being Donitored by tbe sense amplifier with lt6 sen~ing and AGC target~, the nicroproces~or 68 ~eeks to maintain the gain in such a way that the peak of the 6ignal in the QRS co~plex approximately crosses the AGC
target, as shown in ~IG. 8. Such gain maintenance i6 achieved in the following manner. Any ~ignal that crosses the sensing target6 i~ considered to be a sense event. If a sense event occur6 in a cardiac cycle, then 60meti~e after it i8 sensed (whicb may, for example, occur during the refractory period, although the precise time is not significant) a flag is checked which indicates whether or not the AGC target ~as crossed by that sense event. If t~e AGC
target was cros~ed, it i8 an indication that the gAin is too high and should be decreased. At that point, the microprocessor decrea6es the gain. In the presently preferred embodiment, the gain i6 decreased by a certain percentage. A~ an alternative, however, the gain Day be decreased by a fixed amount.
The microprocessor also counts cycles in whic~ 6ense events have occurred but there has been no crossing o~ the AGC target. If two consecutive cycles are detected in which the 6enfiing target is crossed but the AGC target ~8 not, the microprocessor recogni~es that the gain i~ too low and ~ust be increased. Here again, the increase may be effected as a fixed percentage or a fixed amount.
If ~ fixed amplitude QRS complex is detected, a repeated pattern of alternately increasing and decreasing the sense amplifier gain takes place, controlled by the ~lcroprocessor in the previou61y described ~anner. When the gain i6 increased, the cignal crosses the AGC target. The gain i6 then decreased,is ~aint~ined low for two cycle~, and i6 then increased agaln, all as con eguence of the ~lxed a~plltude of the incoming ignal and the operation of the AGC, ~ilter and comparator circuit6 in con~unctlon with the ~lcroproces60r.
Tbi6 process i6 referred to as ~dithering' in whlch ~he peak amplitude is dithered around the thre6hold of the- AGC
comparator in the presence of nor~al 6ensed act~vity. In essence, the amplitude of the 6ensed a~plified s$gnal is increased and decreased by small amounts in such a way that the peak of the signal i~ approxi~ately at the level of the AGC tarqet.
Dither flags are 'set~ or cleared at various times during the perfor~ance of the AGC decision process. Two flags are utilized, one of which is set when a detected event crosses the AGC target and change6 the gain, and the other of which i6 set when two consecutive 6ensed events fail to cross the AGC target. These are referred to as 'dither up~ and 'dither down,' respectively. If, for example, the patient is in fibrillation, the detected 6iqnal is very 6mall and the gain of tbe sense amplifier is very high. After delivery of ~hock therapy and consequent defibrlllation, the detected cardiac 6ignal now appears to be very large because the gain of the a~plifier i6 no longer appropriate. The de6ire 16 to avoid rate counting any target crossings to determine whether an arryth~ia ic present until t~e gain has been restored to an appropriate level. A deteroination is ~ade that the gain ls appropriate when the dither flags are ~et,' which may, for example, take place when the qain is increased. At that point, the detection sy6tem ~s locked into the 6~gnal. Upon setting of both bits the dither criterion i6 achieved, and valid tachycardia detection or fibrillat~on detect~on ~ay then validly proceed. The 6ituation, and the need to reestabli~h the appropriate gain, will change only after a pace event or after an anti-tachycardia therapy.
Consideration ru6t be gi~en to the Danner in which the gain is to be ad~usted during bradycardia paclng, where, for example, a lack of censed event~ ~ay be attributable to elther a 810w heart rate or an inadeguate gain setting of the ~en~o u~plifier. To deter~ino which of these i~ re6pon6ible, ~-n-ing of the relatively lower frequency post-pace T-wave~
1- perforned. If no T-wave_l~ cen~ed at the -n~ing target co~parators ln a preset tlDe vlndow followlng a p~ce event, tbe AGC galn 1B lncrea~ed. Thl~ 16 contlnue~ untll the a~plltude of a T-wave exceed~ the en~lng target and 18 thereby ~en6ed, a~ lndlcated ln PIG. 9, or untll the previou~ly undetected heart rhyth~ i6 ~en~ed. m e ~a~or dlfference6 between tbe gain control ba6ed on post-pace T-waves and the gain control based on QRS and flbrlllatlon signal6 are the tl~e window and that tbe T-wave amplitudes are controlled about tbe cen~ing target ratber than the AGC
target. Tbl6 tendc to as6ure tbat the a~plifier gain 1~ not set too high ln the event that an intrinsic QRS 6ignal i6 sensed; that is, to avoid double sen6ing of QRS fo~lowed by a T-wave.
In the case of bradycardia, then, the sense amplifier will see pace events ln6tead of a QRS complex. If capture i~
achleved, ~ T-wave will follow each pace event. It 16 known that T-w~ves are of lower a~plitude tban QRS complexe6, and tbat post-paced T-waves are of greater a~plitude t~an intrin~ic T-waves. Tbe lnvention take6 advantage of the6e cbaracteri6tic6 $n that the window ~ndicated ln FIG. 9 i6 opened a predetercined delay interval after a pace -- for exa~ple, an interval of 200 to ~00 ~illl6econd6 after tbe pace -- and a T-wave ls looked for durlng the wlndow period.
If a T~vave i6 ~en6ed, the qain 16 aa~u6ted by ~icroprocessor control to dither the a~plltude of that wave around tbe 6en~ing target, a6 sbown ln FIG. 9, ln a ~anner corresponding to that previou61y de6cribed for dithering about tbe AGC target.
T~e a66u~ption i6 ~ade that after a pace event the patlent'e beart ~ay be ~n flbrlllatlon or bradycardia ~or the gain ~ay be too low). If the galn 18 set to ~u6t barely ~ense ~o~ethlng in tbe window perlod, and fibrillation 16 pre6ent, tbere generally will be ignals wbicb occur after the T-wave window and before tbe next pace. In tbat ca6e, lt 1~ recognl~ed tbat tbe next cycle wlll not be a pace cycle, and there lc no ne~d to look for a T-wave. A different algorltb~ 1~ followed to reduce tbe tl~e requlred to detect deflbrlllatlon ln tbe~e clrcu~stances. If, after two ~ 19 1329,238 consecutive pace~ noth1ng ~ ~en~d in the ~-wavQ interval, tbe gain 1B lncrea~ed by a relatively large a~ount (l.e., the sensitivity of the cense amplifier ic considerably increased) ln an attempt to detect a fibrillatlon slgnal.
Ihe process is repeated if, after increasing the gain, there i6 6till no detection of a T-wave or fibrillation following two consecutlve paces. In this way, the gain undergoes rapid change to seek ~ome kind of signal. The programmable maximum gain is nominally the ~aximum gain of the device. In the presently preferred e~bodiment, ~aximum gain can be programmed suc~ that the devlce is sufficiently ~ensitive to detect signals as low a6 O. 3 to 0.5 millivolts.
For bradycardia pacing, the AGC and sense targets are set at two-to-one because t~e area of lnterest is the relationship between the QRS and the T-wave. Howe~er, in eituations w~ere one Dorphology of waveforo ~ay be present for a while and an abrupt 6hift Day take place to another morphology, a greater sense margin is reguired.
Witb the above considerations in rind, the following AGC
algorithms are i~ple~ented in the presently preferred embodi~ent of t~e invention:
1. Reduce gain by a small amount after one ~igh gain ~ense event. Increase qain by a small amount after two 6eguential low galn cense e~ent6, ~nd each 6ubseguent low gain ~enee event.
The rationale for an initial ratio of two low gain sense events to lncrease gain to one high gain event to decrease gain i6 that, in 60 doing, the very high R-waves are allowed to dominate the AGC ad~ustment until the R-waves are dithering about t~e AGC target. In t~at case, if the sense margin 16 correctly cet, T-waves 6hould not be ~en~ed.
Eguivalently, the margin must be set 60 that an R-wave and it~ T-wave cannot bot~ be sensed a6 low gain events.
However, the reguire~ent of two 6equential low gain sense event6 makes t~e galn lncrease portion of thi6 algorithm somewhat erratic in t~e presence of flbrillation, because of the po6sibllity that a run of two cequential low gain ~ense events ~ay not be found in fibrillation. For example, the equence ~ight b high gainf~o~ galn/no-~-n~ ollo~-d by a bradycardia pace 2 ~or a bradycardia pace even~ ~a) 1~ a T-wave crossed the AGC target (in the T-wave wlndow), decroa~e the gain by a s~all a~ount (b) if a T-wave crossed the ~en6ing target but not the AGC target, and there has been a 6tep increa~e ln gain but there has not yet been an R-wave 6ense event, decrease the gain by a ~mall amount; (c) if a T-wave ha6 not crossed the ~ensing target, increase the gain by twice tbe 's~all decrease~ a~ount; (d) if there are two ~equential brady p~ce~no T-w~ve event6, increase the gain by a large ~tep and clear tbe ditber flag , and step lncrea6e the gain again on each following con~ecutive brady pace/no T-wave event A T-wave Day not be 6ensed after a brady pace event for ~ny of the folloYing reason6 (i) the T-wave did not exceed t~e ensing target, ~ii) tbe pacing too~ place during fibrillation, (iii) a fusion beat wa6 followed by an abnormal T-w~ve, ~iv~ there wa~ a failure to capture, or ~v) there was a failure of the ~ensing ~yste~
3 If ~rady pacinq cau6es a step increase in gain, clear tbe dither flag~ Double tbe incremental ~tep cize used to increase and decrease the qain on R-wave events until th- gain dither~
T~e rationale ~ere co~e~ fro~ dog 6tudies in wbicb fusion beats were found to produce wavefor~s witb ~bnormally ~all post-pace T-waves, tbereby causing step increa~e6 in gain ~hich/ in turn, resulted in seriou6 over-a~plification Tberefore, in the presently preferred e~bodi~ent, the dither flags are cleared after a ~tep lncrease in gain, and rhythm classification i~ not resu~ed until the gain has dithered w~ich will not occur until any over-a~plification has been corrected Because thi~ tends to increase fibrillation detection tine, vhich i~ contrary to the reason for the step lncrea6e in gain, the incre~ental ~tep ~i~e i6 doubled when the dither flag~ are cleared Thereby, the ~earch ti~e i6 decre~sed without leading to ~ignificant under- or over-~ensing ~ . Att~r ~n antl-ar~ythnlc tb-rapy, cl-ar t~- dlther flags. ln all ca~el-, do not atte-pt to clas~lfy tb- cardiac r~ythm until the galn bas lncrea~d (i.-., aa a re~ult of two ~eguential low galn vent~).
Tbe rationale here is that rhytbm cla~61ficatlon probably should not begln untll tbe galn i8 locked on the event but, slnce thi6 can lnordinately lengthen tbe tl~e for detection of fibrillatlon, a reasonable compromise i8 to co~mence clas6ification as soon a~ it can be ascertained that the 6y6tem is not over-a~plified, which i6 after two con6ecutl~e low sense ~vents.
5. If gain ls driven to the ~aximu~ by R-wave 6en~ing and the R-wave ha6 not yet cro~ed the AGC target, ~et tbe dit~er flags. If gain i6 drl~en to t~e ~ini~um by any 6equence of events, set tbe ditber flag~.
6. If noise is 6ensed durlng quiet time, and the noi6e cro66ed the AGC target, reduce the gain by a small ~mount.
If the noi6e doe6 not cross tbe AGC target, do not ad~u6t the gain.
7. If a progra~mable nu~ber of consecutive gain decreases occur, tben inbibit rhyt~n clas6ification until a gain increase occurs. T~i~ reduce~ tbe li~elibood of inapproprlate detection due to over-a~plification of rhyt~ms v~ic~ change ~pontaneou~ly or due to external influences (e.g. cardiover610n).
Although a presently preferred embodlment of t~e inventlon has been ~e6crlbed herein, it vill be apparent to tho6e ~illed in the field to vhich ehe invention pertaln~
that the variation6 and modification6 of the di~clcsed e~bodiDent ~ay be i~ple~ented vithout departing fro~ the true ~pirit and ~cope of the invention. accordingly, it i6 intended that t~e invention ~all be li~ited only to the extent reguired by the appended clai~6 and ehe relevant principle~ of the applicabl- lav.
The electroqram ~ignal6 processed by AGC amplifier 60 are further enhanced by a filter 6ection 70 having a primary hiqh gain bandpass amplifier 73 to amplify 6ignals witbin the band. The output of amplifier ~3 i8 ~plit and applied to 6eparate bandpass amplifiers 75 and 76, tbe former being diqitally controlled by ~icroprocessor portion 68. The output derived from amplifier and filter 6tages 60 and 70 is applied to a guad comparator 80 (to be described in greater detail below with reference to ~IG. 5), comprising a ~et of sensing target comparator6 82 and a ~et of peak target comparators 83, 85, which develop three inputs to tbe microprocessor ~n the feedback loop.
The primary purpose of the invent~ve 6y~tem i6 to track the 6ignal6 of intere6t. The sen6e a~plifier with AGC, filter and feedbac~ ~y6te~ of ~e pre~ent l m ention i6 i~ple~ented to 601ve the difficult problem of ~en6ing the low frequency, ~arying amplitude whic~ i6 characteri6tic of ventricular fibrillation. It will be recognized that under ordinary circu~stance~ a 106s of sen6ing ~ay be indicative of fibrillation, reguiring tbat the gain of the 6en~e a~plifier be increased to enable better detection. If, however, the loss of 6ensing i6 attributable, for example, to inter~ittent heart block rather than to fibrillation, a return of 6en6e signal likely ~ould be over-a~plified, ~ith a con6eguent 6eriou6 pertur~ation of the entire 6y6te~.
Tbe 6ystem of ~IG. 2 pro~ides dual signal path6 or channel6 ba~ing different bandpa66 characteri6tic6. Tbe 6ignal pat~ through amplifier 75 to t~e sense comparators 82 deter~ine~ the sen6in~ point ~nd the 6ignal path through amplifier 76 to the AGC comparators 83 portion is part of the feedback loop tbat incl~4es the ~icroproces60r 68, wh~ch determines tbe gain of AGC aDpllfier 60. The ~en61ng ~arg~n 18 defined a6 the ratio of peak cignal size at the input~ to lnner comparators 82 and thelr sen~lng thre6hold. A ratio greater than one ru~t be cho6en in order to avold 105s of sensing as a consequence of a reduction ln 61gnal amplitude.
~or example, the use of a 2-to-1 margin would Dean that the 6ignal amplitude would have to be reduced by one-half to lose 6ensing. The goal of the AGC sy~tem i8 to ~aintain a preset 6ensing margin. ~or a given thre~hold level on the sensing comparator, this may be achieved by ad~usting the gain of the AGC amplifier ~o t~at the peak voltaqe seen by the senslng comparator~ remains ~ore or less con~tant. The ~icroprocessor 68 6a~ples the output of the peak co~parators 83 on a cycle by cycle basis. In essence, lf the waveform peak exceeds the t~reshold of the AGC the Dicroprocessor 68 will reduce the gain a 6mall amount. ~If the ~aveform peak does not exceed the threshold of the AGC, it will increase the gain a small amount (the increase/decrease decision process will be dlscussed ln more detail later).
The bandpass of the signal path to the sensing comparator i6 6haped 80 that frequencies below 25 Hz are attenuated. This give~ the ~esirable effect of attenuating the lower frequency T-vave relative to the QRS complex.
However, it has the undesirable effect of attenuating fibrillation 61qnal6. But the bandpas6 of the 6ignal path to the AGC comparator i~ 6haped 60 that the frequencies in the 3 to 12 HZ range are attenuated even more than they are in the signal path to the sensing comparator. The AGC ~ystem vill compensate for t~e attenuatlon of a fibrillation 6ignal by increa6ing the gain of the aGc ampllfler. However, becau6e the flbrillatlon 6ignal to the AGC comparator i6 attenuated ~ore than t~e ~ignal to the sen6ing target, after tbe ~icroproce660r ~a~ lncrea6ed t~e galn the AGC voltage that appearc at the ~en6ing comparator wlll be higher than tbe AGC voltage tbat appear6 at the senslng comparator when QRS complexes are being sen6ed. Thus tbe ~argin ~8 hlgher when a flbrillation waveforn 1B present.
~ ith resp~ct to paclng-~nt-ractlon, in t~e ca-e of a bradycardia patlent the gain ~cttlng mu~t be appropr1ate to allow senslng of ~ubseguent fibrlllation. Also, the T-wave window ~ust be ~et to ~ense T-waves evoked ~y the p~cemaker.
In thl6 particular ca6e, the galn 1~ ad~usted by the ~icroproces60r ~ that tbe T-wave lc at approxi~ately the level of the 6ensing target. The a6su~ption6 are that evoked T-waves are larger than intrin6ic T-waves, and that the QRS
complex i6 ~uch larger than T-wave~. Such a gain 1~
rea60nably sufficient to sen6e fibrillation or a QRS complex.
Referring to FIG. 3, AGC a~plifier 60 of the 6an6e amplifier 6ection 25 provides initial bandpa~s filterlng by ~eans of a c~rcuit con6i6ting of re6istor6 110 and 111 and capacitor6 112 and 113. The AGC amplifier gain i8 controlled by varying the gate voltage of ~n N-cbannel ~unction field effect transi6tor (JFET) ~00 which acts a~ a voltage controllQd input resi6tor to a non-inverting amplifier 101.
The ~icroprocessor 68 (FIG. 2) control~ the on/off duty cycle of ~witches 103 and 10~, to 6et the gate voltage of JFET 100 by charging and discharging capacitor 106 to a voltage between V~ and Vc. This technique i6 used to obtain a gain range of 30:1 a~ deter~ined by the re6i6tance of resi6tor 107 ~nd the on impedance of JFET 100. Resistor 115 allows proper bia6inq of t~e JEET circuit. Although switche6 103 and 10~
are 6chematically depicted a6 ~echanical devices in the AGC
amplifier circuit of FIG. 3 and in 60me of the other circuit diagra~s, lt wlll be understood that in practice electronic 6wltches t~uch a~ field effect tran~istors) typically would be employed.
Turning now FIG. ~, the output of the AGC ~cplifier ~ection 60 16 6upplied to bandpass amplifier 6ection 70 of 6ense aoplifier 25~ Section 70 has a progr~able sensing ~argln feature and ~pecl~l bandpass charactcrl~tic~ whlch ald in tracklng the varlable ampl~tude fibrlllation lgnal. The overall bandpa~s ~pllfier lncludes two actl~e bandp~s f~lter ~pllflers 130 and 131, a progr~mable gain DC
aopllfler 135, and a passl~e high pasc filter compri6ing capacitor 137 and resistor 138. Thls circuit reduces the a~plitude of the ~lgnal components outside the ~reguency band ~ 15 ^- 1329238 of intere~t, and ~n,crea~e~ n~ing ~ar~in for tb- lov freguency fibrlllation ignal~ The u1croproces~or 68 ~etc the ~Agn~tude of resi~tance 140, and therefore can et the gain of DC amplifier 135 and, thus, the galn to the ~en6~ng target comparator6 ceparately fro~ the gain to the AGC target comparators In this manner, the ~en6ing margln (which is proportional to the ratio between the ~ensing and AGC
tarqet6) i8 prograDmable, being selectable by the ~icroprocessor Alternatively, the effecti~e ratio of the target6 ~ and, therefore, the sen~ing ~argin) could be programmably changed by varying the qain of bandpass filter amplifier 131 through changes of the value of re6i6tor 142, or by changing the target reference voltages them6elves The guad comparator 80 of ~ense a~plifier 25 i6 6hown in FIG 5, and an exemplary input signal and logic outputs of the comparator are illu6tr~ted in n G 6 Tbe comparator has two comparator pair6 comprising ~ensing target comparator6 150 and AGC target comparators 151 The logic output~ Ll and L2 (upper and lower, respectively) of the ~ensing target comparator6 are used by tbe ~icroproces60r as valid sen~e input signal6 The loqical '0~ eitber po~it~ve or negati~e) of the AGC target co~parator6 1~ u~ed by tbe ~icroprsce6sor to evaluate the need for increa6ing or decrea6inq the AGC a~plifier gain The ~enslng t~rget6 represent a sen~ing t~resh~ld, and the AGC targets represent a sensed 6ignal peak amplitude target During ~inus r~yt~, the a~plitude of the QRS co~plex dictate6 the gain 6etting of the AGC amplifier bec~u~e of the relatively large amplitude and high freguency content of tbe QRS The ~a~e situation ex~6t6 during 6en6ing of a tachycardia Howe~er, in the pre~ence of a lower freguency fibr$11ation ~ignal there i~ nearly a doubling of the sensing ~argin, becau6e of the bandpa6~ fllters 1~1 and 1~3 (FIG ~), ther~by per~itting more reliable tracking of the variable a~plitude fibrillation ~$gnal ln this re~pect, it Yill be recogni~ed that the 6eries re6istance-capacitance path in parallel Yith the resl~tance path of each of circuit6 1~1 and 143 provldes an input iDpedanc- to the re~pectlve operatlonal ampllflers 130 and 131 ~uch that the gain at h~gh 132923~
frequencies ~111 be ~ore t~ n the galn at lov frequencles.
Thus, the frequency plot for the input ~lgnal to the AGC
target6 con~titutes a bandpas6 (FIG. 7) ~hlch will p~6S Q~S
complexes faithfully, while T-waves will be relat$vely attenuated to Dlninize double 6en~ing. Fibrillation 6ignal6 will be passed accurately, although relatively attenuated, but will boost the gain of tbe AGC to over-compensate.
By way of an example of operation, in the presence of a QRS complex which i6 being Donitored by tbe sense amplifier with lt6 sen~ing and AGC target~, the nicroproces~or 68 ~eeks to maintain the gain in such a way that the peak of the 6ignal in the QRS co~plex approximately crosses the AGC
target, as shown in ~IG. 8. Such gain maintenance i6 achieved in the following manner. Any ~ignal that crosses the sensing target6 i~ considered to be a sense event. If a sense event occur6 in a cardiac cycle, then 60meti~e after it i8 sensed (whicb may, for example, occur during the refractory period, although the precise time is not significant) a flag is checked which indicates whether or not the AGC target ~as crossed by that sense event. If t~e AGC
target was cros~ed, it i8 an indication that the gAin is too high and should be decreased. At that point, the microprocessor decrea6es the gain. In the presently preferred embodiment, the gain i6 decreased by a certain percentage. A~ an alternative, however, the gain Day be decreased by a fixed amount.
The microprocessor also counts cycles in whic~ 6ense events have occurred but there has been no crossing o~ the AGC target. If two consecutive cycles are detected in which the 6enfiing target is crossed but the AGC target ~8 not, the microprocessor recogni~es that the gain i~ too low and ~ust be increased. Here again, the increase may be effected as a fixed percentage or a fixed amount.
If ~ fixed amplitude QRS complex is detected, a repeated pattern of alternately increasing and decreasing the sense amplifier gain takes place, controlled by the ~lcroprocessor in the previou61y described ~anner. When the gain i6 increased, the cignal crosses the AGC target. The gain i6 then decreased,is ~aint~ined low for two cycle~, and i6 then increased agaln, all as con eguence of the ~lxed a~plltude of the incoming ignal and the operation of the AGC, ~ilter and comparator circuit6 in con~unctlon with the ~lcroproces60r.
Tbi6 process i6 referred to as ~dithering' in whlch ~he peak amplitude is dithered around the thre6hold of the- AGC
comparator in the presence of nor~al 6ensed act~vity. In essence, the amplitude of the 6ensed a~plified s$gnal is increased and decreased by small amounts in such a way that the peak of the signal i~ approxi~ately at the level of the AGC tarqet.
Dither flags are 'set~ or cleared at various times during the perfor~ance of the AGC decision process. Two flags are utilized, one of which is set when a detected event crosses the AGC target and change6 the gain, and the other of which i6 set when two consecutive 6ensed events fail to cross the AGC target. These are referred to as 'dither up~ and 'dither down,' respectively. If, for example, the patient is in fibrillation, the detected 6iqnal is very 6mall and the gain of tbe sense amplifier is very high. After delivery of ~hock therapy and consequent defibrlllation, the detected cardiac 6ignal now appears to be very large because the gain of the a~plifier i6 no longer appropriate. The de6ire 16 to avoid rate counting any target crossings to determine whether an arryth~ia ic present until t~e gain has been restored to an appropriate level. A deteroination is ~ade that the gain ls appropriate when the dither flags are ~et,' which may, for example, take place when the qain is increased. At that point, the detection sy6tem ~s locked into the 6~gnal. Upon setting of both bits the dither criterion i6 achieved, and valid tachycardia detection or fibrillat~on detect~on ~ay then validly proceed. The 6ituation, and the need to reestabli~h the appropriate gain, will change only after a pace event or after an anti-tachycardia therapy.
Consideration ru6t be gi~en to the Danner in which the gain is to be ad~usted during bradycardia paclng, where, for example, a lack of censed event~ ~ay be attributable to elther a 810w heart rate or an inadeguate gain setting of the ~en~o u~plifier. To deter~ino which of these i~ re6pon6ible, ~-n-ing of the relatively lower frequency post-pace T-wave~
1- perforned. If no T-wave_l~ cen~ed at the -n~ing target co~parators ln a preset tlDe vlndow followlng a p~ce event, tbe AGC galn 1B lncrea~ed. Thl~ 16 contlnue~ untll the a~plltude of a T-wave exceed~ the en~lng target and 18 thereby ~en6ed, a~ lndlcated ln PIG. 9, or untll the previou~ly undetected heart rhyth~ i6 ~en~ed. m e ~a~or dlfference6 between tbe gain control ba6ed on post-pace T-waves and the gain control based on QRS and flbrlllatlon signal6 are the tl~e window and that tbe T-wave amplitudes are controlled about tbe cen~ing target ratber than the AGC
target. Tbl6 tendc to as6ure tbat the a~plifier gain 1~ not set too high ln the event that an intrinsic QRS 6ignal i6 sensed; that is, to avoid double sen6ing of QRS fo~lowed by a T-wave.
In the case of bradycardia, then, the sense amplifier will see pace events ln6tead of a QRS complex. If capture i~
achleved, ~ T-wave will follow each pace event. It 16 known that T-w~ves are of lower a~plitude tban QRS complexe6, and tbat post-paced T-waves are of greater a~plitude t~an intrin~ic T-waves. Tbe lnvention take6 advantage of the6e cbaracteri6tic6 $n that the window ~ndicated ln FIG. 9 i6 opened a predetercined delay interval after a pace -- for exa~ple, an interval of 200 to ~00 ~illl6econd6 after tbe pace -- and a T-wave ls looked for durlng the wlndow period.
If a T~vave i6 ~en6ed, the qain 16 aa~u6ted by ~icroprocessor control to dither the a~plltude of that wave around tbe 6en~ing target, a6 sbown ln FIG. 9, ln a ~anner corresponding to that previou61y de6cribed for dithering about tbe AGC target.
T~e a66u~ption i6 ~ade that after a pace event the patlent'e beart ~ay be ~n flbrlllatlon or bradycardia ~or the gain ~ay be too low). If the galn 18 set to ~u6t barely ~ense ~o~ethlng in tbe window perlod, and fibrillation 16 pre6ent, tbere generally will be ignals wbicb occur after the T-wave window and before tbe next pace. In tbat ca6e, lt 1~ recognl~ed tbat tbe next cycle wlll not be a pace cycle, and there lc no ne~d to look for a T-wave. A different algorltb~ 1~ followed to reduce tbe tl~e requlred to detect deflbrlllatlon ln tbe~e clrcu~stances. If, after two ~ 19 1329,238 consecutive pace~ noth1ng ~ ~en~d in the ~-wavQ interval, tbe gain 1B lncrea~ed by a relatively large a~ount (l.e., the sensitivity of the cense amplifier ic considerably increased) ln an attempt to detect a fibrillatlon slgnal.
Ihe process is repeated if, after increasing the gain, there i6 6till no detection of a T-wave or fibrillation following two consecutlve paces. In this way, the gain undergoes rapid change to seek ~ome kind of signal. The programmable maximum gain is nominally the ~aximum gain of the device. In the presently preferred e~bodiment, ~aximum gain can be programmed suc~ that the devlce is sufficiently ~ensitive to detect signals as low a6 O. 3 to 0.5 millivolts.
For bradycardia pacing, the AGC and sense targets are set at two-to-one because t~e area of lnterest is the relationship between the QRS and the T-wave. Howe~er, in eituations w~ere one Dorphology of waveforo ~ay be present for a while and an abrupt 6hift Day take place to another morphology, a greater sense margin is reguired.
Witb the above considerations in rind, the following AGC
algorithms are i~ple~ented in the presently preferred embodi~ent of t~e invention:
1. Reduce gain by a small amount after one ~igh gain ~ense event. Increase qain by a small amount after two 6eguential low galn cense e~ent6, ~nd each 6ubseguent low gain ~enee event.
The rationale for an initial ratio of two low gain sense events to lncrease gain to one high gain event to decrease gain i6 that, in 60 doing, the very high R-waves are allowed to dominate the AGC ad~ustment until the R-waves are dithering about t~e AGC target. In t~at case, if the sense margin 16 correctly cet, T-waves 6hould not be ~en~ed.
Eguivalently, the margin must be set 60 that an R-wave and it~ T-wave cannot bot~ be sensed a6 low gain events.
However, the reguire~ent of two 6equential low gain sense event6 makes t~e galn lncrease portion of thi6 algorithm somewhat erratic in t~e presence of flbrillation, because of the po6sibllity that a run of two cequential low gain ~ense events ~ay not be found in fibrillation. For example, the equence ~ight b high gainf~o~ galn/no-~-n~ ollo~-d by a bradycardia pace 2 ~or a bradycardia pace even~ ~a) 1~ a T-wave crossed the AGC target (in the T-wave wlndow), decroa~e the gain by a s~all a~ount (b) if a T-wave crossed the ~en6ing target but not the AGC target, and there has been a 6tep increa~e ln gain but there has not yet been an R-wave 6ense event, decrease the gain by a ~mall amount; (c) if a T-wave ha6 not crossed the ~ensing target, increase the gain by twice tbe 's~all decrease~ a~ount; (d) if there are two ~equential brady p~ce~no T-w~ve event6, increase the gain by a large ~tep and clear tbe ditber flag , and step lncrea6e the gain again on each following con~ecutive brady pace/no T-wave event A T-wave Day not be 6ensed after a brady pace event for ~ny of the folloYing reason6 (i) the T-wave did not exceed t~e ensing target, ~ii) tbe pacing too~ place during fibrillation, (iii) a fusion beat wa6 followed by an abnormal T-w~ve, ~iv~ there wa~ a failure to capture, or ~v) there was a failure of the ~ensing ~yste~
3 If ~rady pacinq cau6es a step increase in gain, clear tbe dither flag~ Double tbe incremental ~tep cize used to increase and decrease the qain on R-wave events until th- gain dither~
T~e rationale ~ere co~e~ fro~ dog 6tudies in wbicb fusion beats were found to produce wavefor~s witb ~bnormally ~all post-pace T-waves, tbereby causing step increa~e6 in gain ~hich/ in turn, resulted in seriou6 over-a~plification Tberefore, in the presently preferred e~bodi~ent, the dither flags are cleared after a ~tep lncrease in gain, and rhythm classification i~ not resu~ed until the gain has dithered w~ich will not occur until any over-a~plification has been corrected Because thi~ tends to increase fibrillation detection tine, vhich i~ contrary to the reason for the step lncrea6e in gain, the incre~ental ~tep ~i~e i6 doubled when the dither flag~ are cleared Thereby, the ~earch ti~e i6 decre~sed without leading to ~ignificant under- or over-~ensing ~ . Att~r ~n antl-ar~ythnlc tb-rapy, cl-ar t~- dlther flags. ln all ca~el-, do not atte-pt to clas~lfy tb- cardiac r~ythm until the galn bas lncrea~d (i.-., aa a re~ult of two ~eguential low galn vent~).
Tbe rationale here is that rhytbm cla~61ficatlon probably should not begln untll tbe galn i8 locked on the event but, slnce thi6 can lnordinately lengthen tbe tl~e for detection of fibrillatlon, a reasonable compromise i8 to co~mence clas6ification as soon a~ it can be ascertained that the 6y6tem is not over-a~plified, which i6 after two con6ecutl~e low sense ~vents.
5. If gain ls driven to the ~aximu~ by R-wave 6en~ing and the R-wave ha6 not yet cro~ed the AGC target, ~et tbe dit~er flags. If gain i6 drl~en to t~e ~ini~um by any 6equence of events, set tbe ditber flag~.
6. If noise is 6ensed durlng quiet time, and the noi6e cro66ed the AGC target, reduce the gain by a small ~mount.
If the noi6e doe6 not cross tbe AGC target, do not ad~u6t the gain.
7. If a progra~mable nu~ber of consecutive gain decreases occur, tben inbibit rhyt~n clas6ification until a gain increase occurs. T~i~ reduce~ tbe li~elibood of inapproprlate detection due to over-a~plification of rhyt~ms v~ic~ change ~pontaneou~ly or due to external influences (e.g. cardiover610n).
Although a presently preferred embodlment of t~e inventlon has been ~e6crlbed herein, it vill be apparent to tho6e ~illed in the field to vhich ehe invention pertaln~
that the variation6 and modification6 of the di~clcsed e~bodiDent ~ay be i~ple~ented vithout departing fro~ the true ~pirit and ~cope of the invention. accordingly, it i6 intended that t~e invention ~all be li~ited only to the extent reguired by the appended clai~6 and ehe relevant principle~ of the applicabl- lav.
Claims (2)
1. Implantable apparatus for detecting cardiac arrhythmias, comprising:
electrode means for sensing electrical activity of a patient's heart, and processing means responsive to the sensed electrical activity for determining whether sensed events are indicative of arrhythmias, including target means for establishing at least one reference level against which the amplitudes of predetermined sensed events are to be compared, gain control means for controllably varying the sensitivity of the processing means according to the amplitude of the sensed event relative to said at least one reference level, and comparison means responsive to an adjustment of sensitivity by said gain control means in one direction immediately following an adjustment of sensitivity in the other direction for counting the number of times the sensed event crosses the reference level as an indication of the nature of the arrhythmia.
electrode means for sensing electrical activity of a patient's heart, and processing means responsive to the sensed electrical activity for determining whether sensed events are indicative of arrhythmias, including target means for establishing at least one reference level against which the amplitudes of predetermined sensed events are to be compared, gain control means for controllably varying the sensitivity of the processing means according to the amplitude of the sensed event relative to said at least one reference level, and comparison means responsive to an adjustment of sensitivity by said gain control means in one direction immediately following an adjustment of sensitivity in the other direction for counting the number of times the sensed event crosses the reference level as an indication of the nature of the arrhythmia.
2. The invention according to claim 1, further comprising:
means responsive to determination of the arrhythmia for delivering a therapy to the heart specifically for the treatment of that arrhythmia to restore normal sinus rhythm.
means responsive to determination of the arrhythmia for delivering a therapy to the heart specifically for the treatment of that arrhythmia to restore normal sinus rhythm.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000616630A CA1329238C (en) | 1988-06-07 | 1993-05-11 | Arrhythmia type determination by gain control and reference level crossing detector |
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/203,322 | 1988-06-07 | ||
| US07/203,322 US4880004A (en) | 1988-06-07 | 1988-06-07 | Implantable cardiac stimulator with automatic gain control and bandpass filtering in feedback loop |
| CA000596532A CA1325661C (en) | 1988-06-07 | 1989-04-12 | Implantable cardiac stimulator with automatic gain control and bandpass filtering in feedback loop |
| CA000616630A CA1329238C (en) | 1988-06-07 | 1993-05-11 | Arrhythmia type determination by gain control and reference level crossing detector |
Related Parent Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000596532A Division CA1325661C (en) | 1988-06-07 | 1989-04-12 | Implantable cardiac stimulator with automatic gain control and bandpass filtering in feedback loop |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1329238C true CA1329238C (en) | 1994-05-03 |
Family
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000616630A Expired - Fee Related CA1329238C (en) | 1988-06-07 | 1993-05-11 | Arrhythmia type determination by gain control and reference level crossing detector |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1329238C (en) |
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1993
- 1993-05-11 CA CA000616630A patent/CA1329238C/en not_active Expired - Fee Related
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