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Contents
1. The method of claim 17 wherein the heart stimu lating device comprises a pacemaker 26 The method of claim 21 wherein the altering step comprises selecting a pacing rate from a set of prede fined pacing rates 27 The method of claim 17 wherein the device com prises a defibrillator 5 251 622 19 20 28 The method of claim 27 further comprising the waveform comprises pulses and wherein the altering step of comparing stored TDR signals step includes altering the shape of each pulse of the 29 The method of claim 27 further comprises means waveform for telemetering stored TDR signals to an external re 31 The method of claim 17 further comprising the ceiver 5 step of storing the generated TDR signals amp g 30 The method of claim 17 wherein the generated 10 15 20 25 30 35 45 50 55 652. This would include the pacemak er s internal wiring to the pacing electrode connectors the electrical connection between the connectors and the pacing electrode and the entire length of the pacing wire terminating in the portion placed in heart tissue The amplitude of the reflected voltage is a function of the electrode impedance admittance conductance and the applied pulse and therefore can be interpreted in dB or in millirho which is a function of impedance Circuitry for time domain reflectometers is well known and in isolation do not form the present invention In general a TDR comprises an I O controller a digital timebase an analog timebase and a pulse generator As described further below and shown in FIG 8 the TDR generates a short fast rise time output pulse After a predetermined amount of time has passed the TDR base time the pulse as reflected back by the wire elec trode is monitored This comprises means for deferring the storing of the TDR signal until the amount time specified by the TDR base time signal has elapsed after the generation of a TDR incident pulse At a specific time as determined by the digital timebase a portion or slice of the reflected wave is stored in an analog timebase This value is then converted to a digital value by an analog to digital converter and stored in memory The pulse generating wave storing process is repeated except that the time period between the pulse genera t
3. 11 sends an signal to pulse selector unit 26 which causes the electrode leads to be switched from an electrical connection with output section 13 to the TDR 1 and control section 23 During normal pacemaker operation TDR I O and control 23 is insu lated by pulse selector 26 from the stimulating pulses to minimize the possibility that the relatively large cur rents and voltages of the stimulating pulses will harm the TDR circuitry Also as described below pulse selector unit 26 may also operably connect TDR 23 with either X 134 Y 135 or Z 136 motion axis probes Logic and control 11 then sends a signal to TDR I O and control 23 which comprises means for transmitting an electrical signal to the electrode receiving means commanding the TDR to generate an incident pulse 104 see FIG 8 on the selected electrode lead In one embodiment of the invention logic and con trol section 11 may include in the signal it sends to TDR 1 O 23 a signal representing a impedance through which the TDR pulse should be sent Ideally the impe dance equals the impedance of the electrode Accord ingly TDR I O 23 may include an internal array of source resistors of various impedances through which an incident pulse may be transmitted and be connected to a multiplexor to select which resistor the pulse should be transmitted This provides a preferred TDR reflection waveform After generation of incident pulse 104 TDR 1 0 waits the amount of time repr
4. As maximum stroke volume continues to increase TDR reading portion 128 will continue to rise Once the TDR readings increase a 5 251 622 15 sufficient amount logic and control unit 11 may be used to cause the heart stimulating waveform to be altered such as by increasing the pacing rate Also when a decrease is detected the pacing rate may be reduced or the waveform otherwise altered Detection of a change in electrode tip resistance obviously requires that TDR readings be stored so that they may be compared to current TDR readings This may be accomplished by logic and control unit 11 storing digital TDR readings in RAM 22 In one embodiment of the invention a TDR reading may be taken during the maximum stroke volume approximately every ten seconds and thirty readings may be stored in RAM This may provide logic and control until 11 with sufficient historical data to determine when a change in maximum heart stroke volume has occurred A decision on when to alter the heart stimulating waveform may be made on any number of factors and the specific method used does not constitute the inven tion per se Algorithms for adjusting a heart stimulating waveform based on a change in heart stroke volume are known the art For example after a pacemaker has been implanted and the patient is sufficiently healthy to undertake a stress test a physician may monitor the TDR readings during the maximum stroke volume have telemetering the r
5. U S Patent Oct 12 1993 Sheet 7 of 7 5 251 622 Fig 13 Fig 14 Fig 15 1 RESPONSIVE PACEMAKER WITH TIME DOMAIN REFLECTOMETER AND METHOD OF USE CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation in part of application Ser No 07 866 850 filed Apr 10 1992 now U S Pat No 5 231 987 FIELD OF THE INVENTION This invention relates to a device system and method used in medical equipment and in particular to a re sponsive system for adjusting the waveform such as the pacing rate of a pacemaker in response to changes in physiological activity of the user BACKGROUND OF THE INVENTION _ When a patient is provided with a pacemaker the pacemaker may be designed to generate heart stimulat ing pulses continually or only when the patient s natu heart rate falls below a predetermined rate or inter nal threshold In either case the generated pulses will occur at a predetermined rate the In addi tion some pacemakers are rate responsive which means that they automatically adjust the pace rate if the patient s suspected physical activity increases or de creases They are many different systems used in pace makers to predict when a patient s physical activity has increased and therefore when the pacing rate should be increased Several systems for varying the pacing rate of a pace maker work on the assumption that increased physical motion means that t
6. the physician has specified the desired elec trode configuration values the physician may store them in the pacemaker so that they do not have to be reprogrammed each time a TDR reading is taken This is done by depressing the up or down arrows until the Store Configuration to Pacemaker option is selected The Select button is then depressed which causes all of the displayed electrode information to be transmitted to the pacemaker by RF transceiver 27 and stored in RAM 22 If the pacemaker provides means for connecting to second electrode or wire such as for multiple leads includes two leads for single bipolar electrode pulse cathode and a ground anode then the pace maker RAM 22 may be configured with sufficient mem ory to store a separate setting for each electrode or lead It should also be noted that a singe electrode can com prise a multi filament wire In addition pulse selector will include switch means for selecting whether the time domain reflectometer is operably connected to the means for connecting the first implantable electrode or the means for connecting to the second implantable 30 35 40 45 55 60 65 electrode The location for each storage will be desig nated by the Electrode No option in FIG 2 If a pacemaker having capability for storing only one set of electrode readings receives an instruction and data to store electrode settings for an electrode other than
7. tion of TDR 1 23 from output section 13 by pulse selector 26 guards against any damage to the circuitry of TDR I O 23 from stimulating pulses generated by output section 13 Thereafter the generation of stimu lating pulses may resume If the number of readings averaged parameter is greater than one then the TDR reading process may be repeated either immediately if the time until the next stimulating pulse to be generated is sufficiently long or else after the next stimulating pulse is generated Taking muitiple TDR readings and averaging them reduces any noise that may be inherent in a single reading For averaged readings instead of storing the each set of individual TDR waveform readings to the same RAM address the digital values may be added to the previ ously stored values After the total number of TDR readings specified by the of Readings Averaged parameter has been completed the each sum may be divided by the number of readings comprising the sum to obtain a composite reading namely the average Alternatively it is envisioned that merely the raw TDR readings may be transmitted to programmer 28 as de scribed below and programmer 28 perform the averag ing of the readings It will be appreciated from the description of the foregoing embodiment that the time domain reflectome ter the system for generating incident pulses and storing the reflected wave form comprises a logic and control system as i
8. 1986 What de These Pagers Have In Tektronix 1502c Metallic Time Domain Reflectometer Operator Manual May 1990 Genesis Cardiac Pacing System Model 285 Techni cal Manual Pacesetter Systems Inc 1985 9190420 001 Ventek P AICD TM Model 1600 Physician s Manual Automatic Implantable Cardioverter Defibrillator 1991 Cardiac Pacemaker Inc Hewlett Packard Application Note 62 TDR Funda mentals Apr 1988 Hewlett Packard Application Note 62 1 Improving Time Domain Network Analysis Measurements Apr 1988 1 Hewlett Packard Application Note 62 3 Advanced TDR Techniques May 1990 Primary Examiner William E Kamm Attorney Agent or Firm Baker amp Daniels 57 ABSTRACT Responsive heart stimulating device and method for operating the device which includes a heart stimulating waveform generator for transmitting a waveform to an electrode A time domain reflectometer TDR is also connectable to the electrode for generating TDR sig nals A logic and control unit processes TDR readings from the electrode and is adapted to alter the heart stimulating waveform in response to the TDR readings The TDR may be connected to a pacemaker pacing electrode to detect changes in heart stroke volume or cardiac output or to one or more probes capable of detecting motion of the patient and the logic and con trol unit may adjust the pacing rate accordingly 31 Claims 7 Drawing Sheets S
9. 8 4 OL 15 05 ASIGOW 41138 300419313 NOLLVYENDIANOO 193135 NOUS NOLLVENDIANOOD AVIY Sheet 3 of 7 5 251 622 Oct 12 1993 U S Patent a HUWS NVISISAHd Sid 1037135 OL 193145 1 OL 3 000 0 SWALSAS 3113530 00 100 66 SOIGAWYSLNI 00 142 OVIGHVO 0001 142 5 OVIGYVO 103136 01 1997135 1 OL SSAYd MO 6 40 LNALLVd 1 60 HLNOW 9 26 10 20 dN MOSHO 4 180 26 10 20 3490419373 0001 SOINOH LHOSL 26 10 10 0001 5 LNVIdWi aud 26 10 40 U S Patent Oct 12 1993 Sheet 4 of 7 5 251 622 Sheet 5 of 7 5 251 622 Oct 12 1993 U S Patent 9 AL SWLL JWL 9093 901 801 501 3 WAYALNI ONDINV18 YAL 1 4 201 801 101 ADVLIOA Sheet 6 of 7 5 251 622 Oct 12 1993 U S Patent gol ALG AONVLSIG vol Ait 243 26 Ait
10. United States Patent Robson AAR US005251622A Patent Number 45 Date of Patent 5 251 622 Oct 12 1993 54 RESPONSIVE PACEMAKER WITH TIME DOMAIN REFLECTOMETER AND METHOD OF USE 75 Inventor Jack Robson Beech Grove Ind 73 Assignee Random Technologies Inc Indianapolis Ind 21 Appl No 971 281 22 Filed Nov 4 1992 Related U S Application Data 63 Continuation in part of Ser No 866 850 Apr 10 1992 Pat No 5 231 987 5 AGIN 1 362 52 U S 607 19 58 Field of Search 128 419 PG 419 PT 56 References Cited U S PATENT DOCUMENTS 3 922 914 12 1975 Fuchs 73 290 4 466 288 8 1984 Grynberg et al 73 654 4 786 857 11 1988 Mohr et al 324 58 5 BO 4 843 234 7 1989 Berthold et 250 227 4 893 895 1 1990 Berthold et al 350 96 4 960 989 10 1990 Liebenrood et al 250 227 5 033 826 7 1991 350 355 OTHER PUBLICATIONS Tektronix 1502c Metallic Time Domain Reflectometer Service Manual Tektronix Inc Jul 1991 pp 5 1 to 5 11 Quantum II Intermedics Cardiac Pulse Generator Physician s Model Models 253 25 and 254 30 Jan 1990 Pacesetter 8 Technical Manual APSII Model 3000 Programmer with Model 3030 Function 1988 Cordis Corp
11. a patient a tube configured with an embed ded electrical conductor running the length of the tube and connectable to a TDR and which will break if the tube breaks is also included within the definition of an electrode as used herein It will also be appreciated that although the in the embodiment described above a TDR reading is output through RAM and an RF antenna other transceiver or output means are within the spirit and scope of the invention For example virtually any other electromag netic wave communication means may be used at any desired frequency including optical frequencies or wire leads may be used to transmit a TDR reading Alternatively the system is useful for non telemetering non RF systems such as pacing system analyzers used during implants of cardiac pulse generator or AICD s Moreover the TDR reading described above is ob tained by generating multiple incident pulses and stor ing a small portion of each reflected pulse It is contem plated that with the development of faster electronic and CPU circuitry that TDR reading may possibly be generated by storing the reflected wave from a single incident pulse and such systems included within the definition of a TDR It will also be appreciated that in the embodiment described above a user selected num ber of multiple raw TDR readings may be averaged to produce a composite reading to eliminate noise associ ated with individual readings The averaging fun
12. cause a TDR reading to be generated as described above and superimposed over the archived TDR reading By highlighting and adjusting the Max Millirho Alarm option the physician may specify a millirho value predetermined threshold by which if a TDR reading deviates in a relevant portion an indicating output warning signal such as a flashing light buzzer or DEFECTIVE screen display is generated High and low limits from the electrode portion of the display waveform may be represented as horizontal lines on the graphical display Specifically minimum warning line 64 and maximum warning line 65 as shown in FIG 67 define the boundaries in which the entire electrode portion of the waveform is expected to fall If a wave form deviates from these limits DEFECTIVE legend 66 may be displayed on the screen preferably in a highly contrasting color and accompanied by an audible alarm FIG 6 shows a representative superimposed TDR reading in which archived reading 61 shows an elec trode in good condition while current reading 62 shows that the electrode has broken Specifically reading 62 includes waveform rise 63 indicating that the impe dance of the electrode has risen This may be caused by for example a filament becoming cracked or com pletely severed current reading 62 exceeds maximum alarm level 65 DEFECTIVE legend 66 is displayed on the screen alerting the physician that the electrode may be def
13. ction could easily be transferred from the internal implantable device to external programmer 28 Moreover other manipulations of raw TDR readings such as by averag ing multiple readings would not necessarily alter their definition as being a TDR reading Other functions contemplated to be performed by the pacemaker could also be performed by external programmer 28 For example the storage of TDR readings could be per formed on external programmer 28 and optionally may be indexed by a unique key such as patient social secu rity number or pacemaker serial number to distinguish between stored readings from different patients Also graphical display of programmer 28 may include means for displaying the amplitude differential between the incident pulse and a selected portion of a TDR reading An important feature of the present invention is that the TDR may be used not only to test the integrity of an 10 20 25 30 35 40 45 14 electrode but also to provide a responsive heart stimu lating system This is done by performing additional analysis of the TDR readings from an implanted elec trode which detects an increase in heart stroke volume Alternatively or in addition the TDR may also be operatively connected to TDR probes in an enclosed chamber containing a flowable substance which oper ates as a motion detector The conclusions reached from analyzing the implanted electrode and or motion sensor TDR readings
14. ctrode manufac turer or manually programmed into the programmer With a known the vertical gridlines or divisions represent a specific length from the TDR I O output to the end of the electrode The vertical axis of the wave form represents millirhos which is directly convertible into impedance Thus a rise in the waveform represents 5 251 622 11 increased resistance along the electrode while a fall in the waveform represents a short circuit between the electrode and the pacemaker ground Accordingly for the representative waveform shown in FIG 7 waveform rise 52 represents an increase in resistance which in this representative case is attributa ble to the internal pacemaker wiring connection be tween the application specific integrated circuit on which pacemaker circuitry is connected and the wires connected to the pacing leads 14 15 and 16 and 17 Second waveform rise 53 is attributable to the intercon nection between electrode receptacle and the electrode plug Thereafter the waveform is flat indicating a con stant impedance throughout the length of the electrode with no breaks or shorts FIG 5 shows a representative TDR reading in which the electrode has a partial short and is in need of re placement Such a short may be caused for example by defective insulation between the leads wires of a bipo lar electrode or by the exterior insulation of the elec trode becoming worn by for example excessive
15. dings for probes indicating respectively no moderate and heavy movement as sensed by a single probe The tech niques used to analyze the relevant TDR reading of each probe signal may be the same used to analyze accelerometer or piezoelectric motion detector signals Specifically the portions of the TDR readings repre senting the probe portion within enclosed chamber 130 over 0 5 2 minutes may be processed using well known analysis of variance techniques such as those described in Bendat and Piersol Random Data Analysis and Mea surement Procedures Wiley Interscience 1971 or Spiegel Schaum s Outline of Theory and Problems of Probability and Statistics McGraw Hill 1975 which are incorporated herein by reference When such analy sis indicates that a sufficient level of movement has occurred logic and control unit 11 can cause a change in the pacing rate In order to obtain a more accurate indication of the total movement and to filter out noise in readings all three probes may be processed to gener ate a composite variance reading Of course the specific thresholds appropriate to justify a change in pacing rate will vary with materials used and sizes of the probes the conductivity of enclosed chamber 130 flowable material 137 and the pacing levels appropriate for the particular patient as determined by a qualified physician It will further be appreciated by those of skill in the art that the heart stroke volume detection sy
16. e TDR reading process begins Also some sensing electrodes such as those used to monitor heart activity may have a rhythmic voltage on them generated by an internal organ Such electrodes are used for example to moni tor cardiac activity and logic and control unite 11 sec tion of the pacemaker is capable of determining at any point in time the status of the rhythmic activity For such electrodes it is desirable to time each TDR read 5 251 622 9 ing to begin at the same time in the rhythmic cycle so that each TDR reading is taken at the same time of the rhythmic cycle and therefore less subject to noise For a cardiac sensing electrode TDR blanking interval 103 may begin after completing of physiological event such as the atrial beat as sensed by logic and control 11 and _ last for 300 milliseconds After the 300 millisecond blanking interval the TDR reading or readings may be made as further described below and the readings may be completed before the ventricular beat begins This method in combination of the minimal current needed to generate a TDR incident pulse minimizes the likelihood of causing an irregular heartbeat This system constitutes an anti coincidence detector adapted to pre vent a stimulating signal or physiological event from interfering with the incident pulse signal generated by the time domain reflectometer and its reflected wave After the TDR blanking interval has passed logic and control system
17. e sensing device such as a thermistor to sample the temperature of the body The thermistor is built into an electrode of the pacemaker The resistance of the thermistor varies as a function of temperature so that the device can generate an electrical signal that corre sponds to the sensed temperature This signal may be translated into a pre programmed activity level used to set the pace rate However body temperature sensitive systems also result in many of the problems that occur in motion sensitive rate responsive systems as changes in body temperature may occur without regard to physical ac tivity Thus the system may attempt to filter out such extraneous temperature changes Yet such a filtering system poses the risk that changes in temperature that should be used to vary the pacing rate will be ignored Further because the thermistor is built into a special ized electrode that electrode can only be used with a pacemaker sold by a particular manufacture thereby limiting the physician s and patient s choice of pace makers Due to cost and insurance regulations it is not normally feasible to replace a previously implanted electrode or pacemaker Thus temperature responsive systems have the shortcomings of being poor predictors of a change in physiological activity and of having only a limited choice of pacemakers from which to select Other systems referred to as QT systems adjust the pace rate by determining the activit
18. eadings taken during a stress level test may choose to program which portion of the TDR reading should be subject to pro cessing to determine a change in stroke volume For example assuming a TDR processor is used that breaks up each reading into 256 individual reading examina tion of readings may indicate that readings 150 200 are most representative of the portion of the electrode sub ject to stretching during the stroke volume period In another patient or when a different electrode is used readings 180 210 might be most appropriate Such pa rameters may be programmable into the pacemaker using a programmer software and telemetering tech niques well known in the art It will also be appreciated that the present invention may be used with either endocardial or epicardial elec 20 25 40 45 55 60 65 16 trodes When epicardial electrode is used physi cian should ensure that the electrode is implanted against the exterior wall of the heart having the greatest movement during the heartbeat In order to minimize the likelihood of incorrectly interpreting TDR readings the above waveform alter ing system may also incorporate motion sensor read ings Combining motion sensor readings with readings from other physiological readings to determine when an adjustment in a pacing rate is warranted is also known in the art However the present invention allows mo tion sensor readings to be taken by a TDR which
19. eadings to an external programmer as described above Of course the programmer would include an option to take TDR readings at the approxi mate maximum stroke volume By comparing TDR readings with the level of work performed by the pa tient the physician may easily identify thresholds of electrode ends appropriate to cause an increase or de crease in the pacing rate and may program such param eters into the pacemaker using an external programmer Such a programmer may be used to specify a predeter mined set of pacing rates such as 60 75 90 and 105 beats per minute Alternatively a pacemaker may be programmed to automatically increase or decrease the pacing rate based on the extent to which the maximum heart stroke volume is between its levels when the pa tient is at rest and when the patient is engaged in strenu ous physical activity For example the pacing rate could continuously vary between a baseline and maxi mum level based on a baseline and maximum maximum heart stroke volume to thereby provide a pacemaker that is responsive to a patient s heart stroke volume It will also be appreciated that the specific portion of the TDR waveform that is responsive to changes in maximum stroke volume will vary with parameters such as the brand and model of electrode used and size of the patient s heart and the exact position in the heart where the electrode is implanted Therefore the physi cian after viewing the results of TDR r
20. ected to have numerous follow up visits with the physician during which the integrity of the implanted electrode may be evaluated This may be done using the same programmer 28 described above After the programmer is turned on wand 27 is positioned over the patient s pacemaker and the TDR option is selected the screen shown in FIG 2 may appear The physician choose to first retrieve a copy of the archived TDR reading from when the electrode was first implanted This may be done by using up 44 and down 43 arrows to highlight the Review Archived TDR Readings option and depressing Select This will cause programmer 28 to send a signal to the pacemaker instructing logic and control section 11 to transmit the archive number date comment and physician portions of each archived TDR reading to the programmer An archived TDR display window as shown in FIG 3 is then displayed Using up 44 and down 43 arrows the physician may highlight an archived TDR reading which will normally be the baseline reading or first reading archived after implanta tion Depressing the Select portion of the screen causes programmer 28 close the window and to command the pacemaker to transmit the selected archived TDR read ing including the electrode sampling and display op tions to the programmer where they are displayed The physician may then depress the up arrow to high light the Obtain TDR Reading menu option then press Select 50 This will
21. ective The current Configuration parameters 40 41 and 42 shown in FIG 2 as mentioned above may adjusted With regard to Display Options 42 these parameters effect how a particular TDR reading is displayed on the 5 251 622 13 screen In particular different electrodes from different manufactures and for different purposes will generate TDR readings have different impedance baselines and lengths In order to scale a TDR reading on the graphi cal display the Vertical and Horizontal refefence points and scales may be adjusted These values will also be stored along with an archived TDR reading so that when the reading is retrieved it will be initially dis played using the same viewing parameters as when it was stored It will be appreciated to those of skill in the art that may changes could be made in the foregoing represen tative embodiment without departing from the spirit and scope of the invention For example the present invention may be used with virtually any type of im plantable electrode such as ventricular rate sensing morphology high voltage mapping sensor temporary ablation and angio artheretomy electrodes or patient cables The invention may also be used in connection with wires used in connection with devices such as implanted insulin pumps and such wires are within the scope of the term electrode as used herein and in the claims In addition in instances where a tube or stint is implanted in
22. er 3 The device of claim 2 wherein the heart stimulating waveform comprises heart pacing pulses 4 The device of claim 3 wherein the means for alter ing comprises means for altering the rate of the heart pacing pulses 5 The device of claim 4 wherein the means for alter ing the rate of the heart pacing pulses comprises means for selecting a pacing rate from a set of predefined pac ing rates 6 The device of claim 3 further comprising a pacing electrode connected to the electrode receiving means and wherein the means for altering is responsive to the conductance of the electrode 7 The device of claim 6 further comprising means for determining when a heart is at a predeter mined point in a heart cycle and wherein the generating means includes means for generating a TDR signal when the heart is at the predetermined point in the heart cycle 8 The device of claim 7 wherein the predetermined point in the heart cycle comprises the maximum stroke volume point 9 The device of claim 3 further comprising an enclosed chamber comprising an interior surface a flowable substance within the enclosed chamber a plurality of conductive contact points on the inte rior surface the conductive contact points on the interior surface the conductive contact points being contactable by the flowable substance and operably connected to the TDR such that a TDR signal from a contact point changes when the en closed chamber moves 10 T
23. esented by TDR Base time 105 Normally this amount of time will be selected to represent the amount of time it will take for a reflected pulse to be detected by TDR 1 23 and may be on the order of 1 10 000 nanoseconds depending on the elec trical characteristics and length of the electrode After TDR Base Time 105 passes the TDR stores analog voltage detected 106 on the electrode in an analog time base Voltage 106 represents only a small portion of the entire reflected waveform 107 This analog voltage value is then converted to digital format by an analog to digital converter in TDR I O 23 and then transmit ted to logic and control section 11 for storage in output device such as RAM 22 After a predetermined amount of time such as 200 microsecond from the initiation of the first incident pulse TDR I O 23 generates second TDR pulse 108 The above process is repeated numer ous e g 256 times except the time at which an analog voltage reading is stored in the analog timebase is incre mented slightly with each cycle As a result RAM 22 has stored in it a raw TDR reading representing the reflected waveform 20 25 30 35 40 45 60 65 10 After the TDR reading has been generated logic and control section 11 sends a signal to pulse selector 26 causing the electrode connectors 14 15 and or 16 17 to be electrically reconnected to the output section 13 and electrically disconnected from TDR I O 23 The isola
24. for endocardial lead systems i e those residing within the heart but not for epicardial lead systems Therefore selection of a pacemaker that uses a specialized electrode is limited Though stroke volume is a physiological phenomenon which accu rately reflects the pacing requirements it is desirable to provide a rate responsive system using stroke volume 20 25 30 35 40 45 50 55 60 65 4 which is not dependent upon the critical placement of multipolar specialized electrode It is also desirable to provide a rate responsive system which works in con junction with both endocardial and epicardial lead sys tems OBJECTS OF THE INVENTION Accordingly it is one object of the present invention to provide a method to adjust the pacing rate of the pacemaker in response to physical activity level of the patient It is another object of the present invention to pro vide a system that measures the physical activity level of the user through a physiological phenomenon to avoid generating unnecessary pacing signals It is another object of the present invention to vide a pacing system that requires little power to oper ate It is still another object of the present invention to provide a pacing system that may use universal elec trodes to insure user flexibility It is yet another object of the present invention to provide a pacing system which operates with both en docardial and epicardial lead sy
25. gain depressed causing programmer 28 to transmit the TDR waveform along with the Sampling Options Display Options and textual information including the date which comprises a means for associating each stored output signal with a time reference indicating when the output signal is gen erated through wand antenna 27 to antenna 20 of the pacemaker accompanied by a command instructing the pacemaker logic and control 11 to store the information in RAM 22 This stored reading may be used as a base line TDR reading against which future TDR readings may be compared to assist in evaluating electrode integ rity In one embodiement RAM 22 has sufficient capac ity to store up to 512 TDR readings and associated information Logic and control 11 stores in RAM 22 an incremental counter indicating the total number of read 10 15 20 25 30 35 40 45 50 55 60 ings that have been stored in RAM 22 and the address of 65 the next subsequent reading to be stored Assuming the initial TDR reading is acceptable the physician may proceed with implantation of the pace 12 maker and electrode Following implantation but prior to closing the surgical incision in the patient the physi cian may take a second TDR reading to ensure that no damage to the pacemaker or electrode occurred during implantation Assuming the TDR reading is acceptable the physician may close the incision Following implantation the patient can be exp
26. he device of claim 1 wherein the TDR is opera tively connected to the electrode receiving means 11 The device of claim 1 wherein the device com prises a defibrillator 12 The device of claim 1 wherein the device further comprises 15 20 25 30 35 40 45 55 65 18 an electrode connected to the electrode receiving means such that the TDR signals and generated pacing waveforms may be transmitted to the elec trode 13 The device of claim 1 wherein the generated waveform comprises pulses and wherein the altering means comprises means for altering the shape of each pulse of the waveform 14 The device of claim 13 further comprising means for comparing stored TDR signals 15 The device of claim 13 further comprises means for telemetering stored TDR signals to an external re ceiver 16 The device of claim 1 further comprising a means for storing the TDR signals 17 A method for altering a waveform of a heart stimulating device the stimulating device including time domain reflectometer TDR means for generating TDR signals processor means operatively connected to the time domain reflectometer for processing the TDR signals means for generating a heart stimulating wave form means for altering the waveform the method comprising generating a TDR signal processing the TDR signal altering the heart stimulating waveform based upon the processing of the TDR signal 18 The method of claim 17 where
27. he patient is engaging in physical activity and that the pacing rate must therefore be increased For example the Models 7074 7075 7076 and 7077 manufactured by Medtronic Inc of Minneapolis Minn may be programmed to vary the pacing rate in response to detected changes in body motion A sensor within the device typically a piezo electric crystal placed on the inside wall of the pace maker detects pressure waves within the body caused by body motion The device then converts these pres sure waves into electrical signals The pacing rate is set in proportion to the frequency and amplitude of these electrical signals Other systems such as the Relay TM Models 293 03 and 294 03 manufactured by Interme dics Inc of Freeport Tex use an accelerometer in stead of a piezoelectric crystal to detect physical mo tion The accelerometer computes acceleration by mea suring the force exerted by restraints that hold a mass in a fixed position The accelerometer may either be elec trically excited or self generating using a piezoelectric crystal as discussed above Some systems also include multiple accelerometers oriented in different axes so that movement in different directions can be discerned and used to reduce to obtain are more accurate indica _ tion of the extent of physical movement However the theory behind motion sensitive rate responsive systems is fundamentally flawed because physical motion does not necessa
28. heet 1 of 7 5 251 622 Oct 12 1993 U S Patent 41 H E asvo 7 2 a vl YOLVTIIOSO 8 TWLSAYD ANY BOVLIOA AYWLSWS TAL 92 a T AL 4 Sheet 2 of 7 5 251 622 Oct 12 1993 U S Patent Sid OHY 008 5 LV OHY 26 0 0 1 LNSYYND WO 9 HLONAT NOISIAIG WIN 02 31 06 WLNOZIHOH WO 9 HLONA AT 5 WW 0 39 343338 WLNOZINOH NOLLISOd 5 1 NOISIAIG OHYW 081 31 06 00 WOLLYSA 22 AONVLSISAY 5 820 9 40 WO SI HLONAT WOISAHd OHY SE 0 06 0001 5 2 SLNAWV1id 006 ONDINVIE YAL YVIOdINN ALIWVIOd 539 SNIDOVAY 30 000 SOINOYLOAIAL SNOILdO L NOLLVHNDIANOD NI JAIHOYY 6 Pay AN YAL 180 15 3499993 128319
29. in the processing step includes the step of comparing the generated TDR signal to a reference TDR signal 19 The method of claim 17 wherein the TDR is operatively connected to an electrode receiving means 20 The method of claim 19 wherein the altering step comprises altering the rate of pulses of the heart stimu lating waveform 21 The method of claim 20 wherein the stimulating device further comprises a pacing electrode operatively connected to the TDR and wherein the altering step is responsive to the conductance of the electrode 22 The method of claim 20 further the steps of determining when a heart is at a predetermined point in a heart cycle and wherein the generating step comprises generating a TDR signal when the heart is at its approximate maxi mum stroke volume 23 The method of claim 22 wherein the predeter mined point in the heart cycle comprises the maximum stroke volume 24 The method of claim 20 further comprising the step of providing an enclosed chamber comprising an interior surface a flowable substance within the enclosed chamber a plurality of conductive contact points on the inte rior surface the conductive contact points being contactable by the flowable substance and are op erably connected to the TDR and wherein the generated TDR signal is transmitted to at least one of the conductive contact points such that a TDR signal from the contact point changes when the enclosed chamber moves 25
30. ion an when a portion of the reflected wave is stored is increased slightly causing a different slice of the reflected wave to be stored After a sufficient number of samples e g 256 have been collected a compilation of the stored waveform readings a TDR reading pro vides a view of the entire reflected wave A representa tive TDR pulse may comprise a 300 mV amplitude into a 50 ohm load with a 25 microsecond pulse duration and the reflected rise may be detected in less than 200 picoseconds In general the present invention operates in the fol lowing manner Logic and control 11 is designed to periodically send pacing signals via output line 24 to output section voltage multiplier 13 Logic and control section 11 is programmed to cause output section volt age multiplier to generate cardiac stimulating pulses of predetermined amplitude duration and frequency ac cording to parameters stored in RAM 22 A typical cardiac pacemaker generates stimulating pulses at fre quencies of 0 5 to 3 per second at amplitudes from 2 5 V to 8 5 V and at durations of 0 15 to 2 3 milliseconds Accordingly there is a substantial time gap of at least 300 milliseconds between pulses As a complete TDR pulse and reflection reading time can be accomplished with pulse repetition rate of 200 microseconds it is possible to take an entire set of 256 readings in well under 60 milliseconds Thus a complete TDR reading can be generated between the
31. ions after it has been implanted and also allows the pace maker to be reprogrammed without surgery For exam ple the device can be reprogrammed to generate stimu lating pulses on the pacing electrode at set rate or at a varying rate depending on cardiac activity Other pa rameters such as the pulse width and pulse amplitude can also be specified after the pacemaker has been im planted These operating parameters are stored in ran dom access memory RAM 22 while the control pro gram is stored in read only memory ROM 21 Repro gramming is accomplished through the use of an exter nal system programmer 28 having an RF transceiver wand 27 although a convention serial data port with lead connectors extending through the skin of the pa tient may also be used The invention also includes time domain reflectome ter TDR I O control 23 which includes the circuitry necessary to generate a TDR pulse on the electrodes and to detect the resulting reflected voltage A TDR applies a narrow pulse of current with a fast rise time 20 25 30 35 40 45 50 55 60 65 6 typically by a tunnel diode to the electrode and moni tors the resulting reflected voltage on the electrode over a period of time A stored reflected voltage wave form comprises a raw TDR reading If the electrode has a known propagation velocity the time delay to a particular reflection may be interpreted in distance from the pulse generator
32. lighted in reverse video To select a default electrode configuration from the database the physician presses down arrow 43 to cause the Select Configuration from Electrode Database option to be highlighted The physician then depresses the Select button 50 on the screen This causes an overlapping window to be dis played on the screen as shown in FIG 4 displaying a list of electrode manufacturers and model numbers The physician may repeatedly depress the down arrow until the electrode to be implanted is highlighted then de press the Select option 50 on the touch screen This will close the display window and cause the Electrode Sampling and Display options to be set to the default values recorded in the database for the particular elec trode While in this window or any other window which may be opened at any time prior to depressing the Select option 50 the physician may depress the Escape 47 portion on the screen which will close the window and cause the display to revert to its previous status A representative window and set of electrode default information values is shown in FIG 1 Should 20 25 8 the physician desire to change any of the default values the physician may repeatedly depress the down arrow until the value to be changed is highlighted The physi cian may then depress the left 48 and or right 49 ar rows which will cause the highlighted values to be decremented or incremented respectively After
33. ly displayed TDR reading for an electrode have a short in it FIG 8 is a representative time graph showing the trailing end of a stimulating pulse on the electrode a TDR incident pulse and reflective pulse FIGS and 9b are views of a heart in which a pacemaker is implanted in which the heart is at its re spective minimum and maximum stroke volumes FIGS 10a and 10 are representative TDR readings from an electrode taken at times when the heart is at its respective minimum and maximum stroke volumes FIGS 11 and 12 are side and top sectional views respectively of an enclosed chamber having liquid and 29 5 251 622 5 TDR probes therein and which acts as motion sensor component FIGS 13 15 are representative TDR readings from the TDR probes in FIGS 11 and 12 SUMMARY OF THE INVENTION In one embodiment the invention is a responsive heart stimulating device and method for operating the device which includes a heart stimulating waveform generator for transmitting a waveform to an electrode A time domain reflectometer TDR is also connectable to the electrode for generating TDR signals A logic and control unit processes TDR readings from the elec trode and is adapted to alter the heart stimulating waveform in response to the TDR readings The TDR may be connected to a pacemaker pacing electrode to detect changes in heart stroke volume or cardiac out put or to one or more probes capable of detecting mo tio
34. may already be a component of a pacemaker thus making additional specialized circuitry unnecessary Moreover as TDR readings require much less energy than conven tional resistance based or opto electric based readings battery life is extended Referring to FIG 11 there is shown a vertical cross sectional view of a TDR motion sensor in accordance with the present invention Specifically enclosed cham ber 130 which is preferably formed from a substantially non conductive material includes two slotted lugs 131 and 132 for physical connection to a pacemaker header can by screws 133 Alternatively enclosed chamber may be implanted separately from pacemaker and con nected thereto by electrodes Enclosed chamber 130 includes conductive contact points namely three metal TDR probes namely X axis probe 134 Y axis probe 135 and Z axis probe 136 each of which is sealingly engaged in the wall of enclosed chamber 130 and con nected to a separate wire Within enclosed chamber 130 is a flowable substance such a saline solution 137 In one embodiment solution 137 occupies approximately 35 of the volume of enclosed chamber 130 It will be appreciated that as enclosed chamber 130 moves solu tion 137 will move within chamber 130 and contact different portions of probes 134 135 and 136 As differ ent portions of these probes are contacted their con ductive properties as detected by a TDR reading will change FIGS 13 14 and 15 are TDR rea
35. may be used to automatically adjust the waveform of a heart stimulating device by for exam ple increasing or decreasing the pacing rate of a pace maker Referring to FIG 9a there is shown a heart 120 dur ing its minimum stroke volume in which a pacing elec trode 121 is implanted As heart 120 beats it continually expands and contracts FIG 95 shows heart 120 at its maximum stroke volume as evidenced by the increased size of lower heart portion 122 It is known that when a person undertakes physical activity their maximum heart stroke volume normally increases This is gener ally true even for persons suffering from chronotropic incompetence whose pulse rate does not sufficiently increase with increased physical activity Thus detec tion of a change in maximum heart stroke volume can used in increase or decrease a pacemakers pace rate or to otherwise alter the waveform generated by a heart stimulating device In one embodiment of the present invention a change in heart stroke volume can be detected by generating a TDR reading from the implanted electrode timed from a physiological event or stimulation pulse or when the heart is at or near its maximum stroke volume Detec tion of when a heart is at or near its maximum stroke volume by monitoring implanted electrode is known in the art and is used for example in existing bipolar heart stroke volume detectors A change in heart stroke volume using a TDR read ing is
36. n of the patient One feature of the invention is that because cardiac chemistry affects the cardiac substrate which increases or decreased how hard the heart mus cle contracts or relaxes such changes can be detected and the a pulsing waveform may be modified accord ingly DETAILED DESCRIPTION Referring to FIG 1 there is shown a representative block diagram of one embodiment of the present inven tion In this embodiment the invention is located in an implantable multi programmable pacemaker which includes logic and control unit 11 which includes a CPU and appropriate software to carry out the func tions described herein rate limit section 12 and means for generating a heart stimulating waveform namely output section voltage multiplier 13 Conventional mi crocircuitry and preferably and application specific integrated circuit is used to package the TDR and other components in the implantable case The pace maker is designed to provide periodic pulse to two implantable pacing electrodes through electrode receiv ing means namely connectors 14 and 15 and 16 and 17 respectively However the invention may also be used with a device connected to a single electrode Con nected to logic and control unit 11 is a telemetry system comprised of telemetry transmitter 18 and program receiver 19 both of which on connected to common antenna 20 The telemetry system allows the pacemaker to be interrogated to determine its operating condit
37. no 1 the number information may be ignored and the val ues replaced by the received values After this information has been specified and prior to implantation the physician may take an initial TDR reading This is done by using the up and down arrows to highlight the Obtain TDR Reading option the depressing the Select option 50 on the touch screen This action causes programmer 28 to transmit a com mand to the pacemaker commanding the pacemaker to take a TDR reading according to the parameters stored in RAM 22 When the pacemaker receives an instruction to take a TDR reading the pacemaker waits until no stimulating pulse is present on the electrode Referring to FIG 8 normally if stimulating pulses are being generated on a periodic basis logic and control unit 11 will wait until the trailing edge of stimulating pulse 101 has been gen erated Because stimulating pulse 101 may cause noise to be present on the electrode for a short time period after the pulse is generated no action is taken during the time previously specified as TDR Blanking Interval 103 This system comprises means for deferring genera tion of the TDR incident pulse until the amount of time specified by the TDR blanking interval has elapsed after the transmission of a stimulating pulse on the elec trode or the detection of an identified physiological event For stimulating pulse electrodes this allows volt age on the electrode to completely drain until th
38. ount of power which reduces the life of the battery powering the pacemaker Addi tionally the system requires special electrodes that may lock the physician into the selection of a particular system Rate responsive systems are also available which depend upon a patient s respiration to alter the pacing rate These systems assume that increased air volume in the lungs means a patient is breathing deeper suggest ing that the patient is engaged in increased physical activity Alt et al Function and Selection of Sensors for Optimum Rate Modulated Pacing New Perspec tives In Cardiac Pacing ed Barold et al 1991 p 189 196 Specifically such a system measures the respi ratory rate by detecting the electrical impedance be tween an auxiliary electrode lead and the pacemaker can Because the electrical conductivity of lung tissue decreases with inspiration breathing can be detected by monitoring changes in electrical resistance The resis tance can be measured between different points within the system One can apply a current between the pace maker can and an anodal ring of a bipolar electrode so that the system measures the resistance change between the pacemaker can and the tip of the bipolar electrode However one disadvantage of these respiration sys tems is their sensitivity to movement For example a respiration dependent system is likely to detect high impedance changes if the pacemaker user moves his arms or has ches
39. possible in part because as the heart continually expands and contracts the end 123 of a pacemaker electrode is stretched and contracted When the elec trode end 123 is stretched this results in a slightly lower conductance higher resistance in the stretched portion of the electrode As stroke volume increases the stretching of electrode tip 123 will correspondingly increases as will its resistance When the electrode tip resistance during the stroke volume has increased by a certain level an adjustment the heart stimulating waveform such as an increased pacing rate may 50 55 60 65 performed Also when the electrode tip resistance dur ing the stroke volume has decrease by a certain level the waveform may also be altered such as by ing the pacing rate As shown in FIG 100 TDR reading from an im planted electrode taken during a heart s minimum stroke volume when electrode tip 123 is not stretched shows a fairly even resistance at TDR reading portion 124 When TDR reading is taken during the maximum stroke volume as shown in FIG 106 there is a slight increase in resistance at TDR reading portion 125 as evidenced by the higher waveform portion 126 As a patient undertakes increased physical activity the maxi mum heart stroke volume will increase stretching elec trode end 123 more and more This will result in in creased resistance of electrode end 123 as shown by TDR reading portion 127
40. ptions into the pacemaker external programmer 28 is first turned on and the telemetry head of wand antenna 27 is positioned over the pacemaker The telem etry head generates a magnetic field which activates reed switch 25 inside the pacemaker This switch causes logic and control unit 11 to activate program receiver 19 and to receive instructions from programmer 28 In one embodiment of the invention programmer 28 has a touch screen and various options are selected by touch ing the indicated portion of the screen The physician will initially step through the prompts displayed on programmer 28 to transmit the desired pacemaker set tings e g stimulation rate pulse amplitudes sensitivi ties and mode into the pacemaker The physician may then select a TDR option on programmer 28 which will cause programmer 28 permit the TDR parameters to be specified and displayed such as through the TDR options screen shown in FIG 2 The physician will initially wish to specify the default TDR values to be stored in the pacemaker Ideally programmer 28 will include a database of electrode manufacturers and models with default electrode sam pling and display options for each electrode model The database may be periodically updated by programmer 28 manufacturer via a floppy disk with information concerning new electrodes on the market When the physician first enters the TDR menu the top Read Configuration for Pacemaker option will be high
41. rily mean that a pa tient s physical activity has increased For example motion based responsive systems will increase the pac ing rate when a patient is driving down a bumpy road even though there is no increase in physical activity Likewise a patient may undertake significant physical activity that does not involve movement of the motion 5 251 622 10 20 25 35 40 45 2 sensor such as when performing bench presses with heavy barbells In such circumstances no physical mo tion will be detected so the pacemaker will not increase the pacing rate even though an increase would be ap propriate In short motion based responsive pacemak ers can not distinguish between motions that relate to increased physical activity and those that do not Fur thermore if unnecessary pacing is activated by these devices battery power consumption results in a short ened battery life Shortened battery life may require a patient to undergo a surgical procedure to replace the battery sooner than may otherwise be required It is also desirable to develop a rate responsive system that re quires little power to operate so as to extend pacemaker battery life Due to the shortcomings of motion based rate re sponsive sensors other rate adjusting systems re sponsive to certain physiological conditions of a patient Some pacemaker systems vary the pace rate based on changes in body temperature These systems use a tem peratur
42. rub bing against a bone pacemaker case or other structure The short is evident by waveform fall 60 indicating the impedance of the electrode at that point has fallen At anytime while a TDR Reading is displayed the physician may depress the cursor left 45 or cursor right 46 arrows below the display to cause graphical cursor 48 to move left or right At the point where cursor 48 intersects waveform 49 the distance of the electrode circuit and impedance of the waveform are shown in displays 54 and 55 Thus cursor 48 and displays 54 and 55 comprise means for superimposing a distance scale measurement corresponding to the length of the im planted electrode on the graphical display After the physician has obtained and displayed the composite TDR reading the reading may be transmit ted back to the pacemaker and stored in RAM 22 which comprises means for storing multiple TDR out put signals As shown in FIG 2 this may be done by depressing the down arrow until the Archive Current TDR Reading in Pacemaker menu option is high lighted The Select button is then depressed This causes programmer 28 display a dialogue box on the screen in which the physician may type a short descriptive sum mary of the reading using a keyboard connected to programmer 28 such as Pre implant readings w Tech nitronics 1000 and in which the physician may enter his or her name After this option information has been entered the Select button is a
43. s already found in conventional pace mekers as well as TDR I O circuitry Moreover break in an electrode can also be detected merely by measuring the time interval between the beginning of the incident pulse and the beginning of the reflected waveform After the raw or composite TDR reading has been stored in RAM 22 logic and control section 11 trans mits the stored raw or composite waveform through TDR reading output signal means such as transceiver means comprised of telemetry transmitter 18 and an tenna 20 to wand 27 of programmer 28 In addition in the preferred embodiment logic and control section 11 will also transmit to programmer 28 the stored sam pling values used to take the TDR reading to program mer 28 This transmission assures that the correct pa rameter values may be displayed in association with the TDR reading Programmer 28 then displays the re ceived TDR reading in graphical form on a monitor in graphical display window 411 or a printer or both Preferably programmer 28 includes a Print button which when depressed causes the displayed graph and current configuration information to be printed A rep resentative TDR waveform for a working electrode is shown in FIG 7 The horizontal axis represents the time or sequential samples of the TDR reading which can be directly converted into electrode distance if the of the electrode is known As discussed above this information may be supplied by the ele
44. stem may be combined with the motion detection system to pro 5 251 622 17 vide a responsive system based upon two different pa rameters Moreover the heart stimulating waveform may altered not just by varying the pacing rate but by changing other waveform characteristics such as ampli tude pulse shape i e square triangular or sinusoidal or pulse width Moreover with any of the aforementioned TDR readings such readings may include a compos ite reading generated by averaging the values of multi ple individual TDR readings to reduce noise In addi tion to obtain a normalization of multiple TDR read ings to generate a composite reading several TDR readings may be taken using incident TDR pulses have different leading edge rise times preemphasis and the resulting waveform readings averaged What is claimed is 1 A responsive heart stimulating device comprising means for generating a heart stimulating waveform and transmitting the waveform to an electrode receiving means a time domain reflectometer TDR for generating TDR signals the TDR being operatively coupled to the means for generating a heart stimulating waveform processor means for processing the TDR signals the processor means operatively connected to the TDR and to the generating means means for altering the waveform based on the pro cessing of the TDR signals 2 The device of claim 1 wherein the heart stimulating device comprises a pacemak
45. stems It is yet another object of the invention to provide a pacing system that can also determine the physical and electrical integrity of the pacing electrode It is yet another object of the invention to provide a responsive pacing system that is responsive to heart stroke volume cardiac output physical motion or any combination of the foregoing BRIEF DESCRIPTION OF THE DRAWINGS FIG 1 shows a block diagram of one embodiment of a device of the present invention when used in connec tion with a programmable cardiac pacemaker FIG 2 a representative computer touch screen user interface for configuring and testing an implantable electrode according to the invention FIG 3 is a representative computer screen list win dow displaying an index of previous TDR readings which have been stored in the device FIG 4 is a representative computer screen list win dow displaying a partial index of electrode manufactur ers and models one of which may be selected to pro vide a set of default electrode sampling and display options for a particular electrode FIG 5 is a representative graphically displayed base line TDR reading for an electrode in good condition FIG 6 is a representative graphically displayed base line TDR reading having superimposed over it a more recent TDR reading for the same electrode showing that a break has occurred in the electrode and that the electrode is now defective FIG 7 is a representative graphical
46. stimulating pulses period ically provided to the pacing electrode However it is also within the scope of the invention to space out the TDR pulses between multiple stimulating pulses Prior to implantation of the device in body the de vice will be programmed with various default parame ters Conventional pacemakers are programmed for example to specify the stimulating pulse repetition rate 5 251 622 7 pulse amplitude positive and negative sensitivities and control mode Prior to implantation one or more pac ing electrodes will be selected and connected to pacing leads 14 15 and 16 17 of the pacemaker Each model of electrode has its own characteristics including textual model number polarity number of filaments electrical length physical length Vp and source resistance Rep resentative electrode parameters to assist in taking later TDR readings are shown as electrode menu options 40 in FIG 2 In addition each electrode will ideally have a set of default sampling 41 and display 42 options Preferably the electrode parameters sampling and dis play options may be specified by the electrode manufac turer and used to set default values in the pacemaker prior to implantation Storing these parameters into the _ pacemaker is accomplished using conventional teleme try programming equipment with appropriate software to carry out the functions described herein To program the default electrode sampling and dis play o
47. t movements Such measurements may be interpreted by the system as deep breaths that require an increased pace rate Further these system requires bipolar electrodes as one wire is needed to emit an RF wave and another wire is required to sense voltage Yet another type of rate responsive pacemaker is based on the determination of changes in the stroke volume of the heart Alt et al p 172 177 As an indi vidual increases physical activity the stroke volume of the heart increases regardless of whether the heart rate remains constant due to a condition such as chrono tropic incompetence To measure stroke volume a spe cialized electrode having two or more electrode poles is positioned with the right ventricle These impedance based rate responsive systems transmit a low amplitude AC pulse or short intermittent electrical pulses to the multipolar electrode to measure resistance between the electrode poles As resistance is affected by the amount of blood between the electrode poles stroke volume is estimated The pacing rate is increased when an in crease in heart stroke volume is detected However these systems also have limitations First only a portion of the stroke volume may be measured as the measure ments are dependent upon the position of the electrode pairs Also a multipolar rather than a unipolar elec trode is required and the electrode must be carefully placed into position Furthermore stroke volume may only be measured
48. y level as measured 50 60 65 by the QT interval measurement that is the time be tween when a pacing pulse is sent to the heart and the time the QT interval of the heartbeat begins Generally as a patient s physical activity increases the heart re sponds more quickly to a pacing pulse Therefore QT systems increase the pacing rate when they sense that this time period is reduced A particular shortcoming of QT systems is that in order to sense when pacing should be initiated or increased they must actually send a pac ing pulse to the heart regardless of the patient s intrin sic heart rate In general it is medically undesirable to send pacing pulses to the heart unless it is known that the heart actually requires a pulse In addition unneces sary pulses also ne dlessly consume battery power Another type of rate responsive system is based on measuring a patient s blood oxygen saturation levels These systems assume that when a patient increases physical activity there is a corresponding increase in the blood oxygen saturations level These systems em 5 251 622 3 ploy special electrodes equipped with a light emitting diode LED and a phototransistor which measures the occlusion or blockage between the two This block age roughly corresponds to the amount of oxygen in the blood tissue While the device is based on a physiolog ical phenomenon it has two drawbacks First the LED require a significant am
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