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`HTC Ex. 1009
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`U.S. Patent
`
`Sep.28,1999
`
`Sheet 7 of 11
`
`5,957,957
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`Page 8 of 21
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`U.S. Patent
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`Sep.28,1999
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`Sheet 8 of 11
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`5,957,957
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`HTC v. Uniloc
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`U.S. Patent
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`Sep.28,1999
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`Sheet 9 of 11
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`5,957,957
`
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`5,957,957
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`1
`RATE RESPONSIVE CARDIAC PACEMAKER
`引TITH TILT SENSOR
`
`REFERENCE TO RELATED APPLICATION
`
`This application is a division of application Ser. No. 5
`08/668,524 filed Jun. 28, 1996 which application is now:
`U.S. Pat. No. 5,725,562, which is a continuation in part of
`application Ser. No. 08/413,733 filed Mar. 30, 1995, now
`abandoned.
`
`2
`striking the exterior of the pulse generator case. Activity
`sensor configurations employing integrated circuit, AC
`accelerometers on an IC chip inside the pacemaker are also
`being employed in the EXCEL”VR pacemaker sold by
`Cardiac Pacemakers, Inc., and in similar rate responsive
`pacemakers sold by other manufacturers. The AC acceler(cid:173)
`ometer is formed of a silicon beam mass suspended on the
`IC that swings or moves in response to shock waves caused
`by body motion and provides an output signal having a
`10 magnitude dependent on the rate of movement.
`The relative virtues and weaknesses of piezoelectric crys(cid:173)
`BACKGROUND OF THE INVENTION
`tal and AC accelerometer activity sensors and associated
`1. Field of the Invention
`pacemakers are reported widely， ι.g. in the article “ Activity(cid:173)
`The present invention relates to rate responsive cardiac
`Based Pacing: Comparison of a Device Using an Acceler(cid:173)
`pacemakers and more particularly to the use of a DC
`ometer Versus a Piezoelectric Crystal”, by Bacharach et al.
`accelerometer for detection of patient posture and activity 15
`(PACE，协115, pp.188-196, February 1992). As indicated in
`level, particularly to provide appropriate pacing rates during
`that article, the pacing rate responses of these pacemakers
`stair climbing and descending.
`strapped on patients with normal hearts who were subjected
`to various stress tests were measured and compared to each
`2. Description of the Prior Art
`Rate responsive pacing has been widely adopted for 20 other and to the patien侣’ average actual heart rates. The tests
`adjusting pacing rate to the physiologic needs of the patient
`conducted included stair ascending or climbing and
`in relatively recent years. Early single chamber cardiac
`descending tes毡， and conclusions were drawn to the effect
`pacemakers provided a fixed rate stimulation pulse generator
`that the AC accelerometer performed superiorly to the
`that could be res己t, on demand, by sensed atrial or ventricu-
`piezoelectric sensor in certain respects. Higher cardiac out-
`lar contractions recurring at a rate above the fixed rate. Later, 25 put is required in ascending a flight of stairs than in walking
`dual chamber demand pacemakers became available for
`at the same rate or in descending the flight of stairs at the
`implantation in patients having an intact atrial sinus rate but
`same rate as indicated by the patien侣’ heart rates. The
`no AV conduction, so that ventricular pacing could be
`reported AC accelerometer induced pacing rate during stair
`synchronized with the atrial sinus rate, and backup fixed rate
`climbing more closely matched the required cardiac output
`ventricular pacing could be provided on failure to sense 30 as indicated by the test subjec侣’ average heart rates. During
`atrial d己polarizations. In addition, rate programmable pace-
`stair descending, the AC accelerometer induced pacing rate
`makers became available wherein the base pacing rate could
`did not appreciably fall and exceeded the patien毡’ actual
`be selected by a physician to provide a compromise fixed
`heart rate. The reported piezoelectric sensor induced pacing
`rate that did not interfere with patient rest and provided
`rate during stair climbing fell below the required cardiac
`adequate cardiac output at moderate levels of exercise.
`35 output as indicated by the test subjec侣’ average heart rates.
`Such fixed rate pacing, particularly for patients not having
`During stair descending, the piezoelectric crystal induced
`an adequate atrial sinus rate to allow synchronous pacing,
`pacing rate increased from the rate achieved during ascend-
`left most patients without the ability to exercise, lift objects
`ing and also exceιded th己 patien侣’ heart rate.
`or even walk up stairs without suffering loss of breath due
`As a result, while the authors suggest that the AC accel-
`to insufficient cardiac output. However, the introduction of 40 己rometer is superior in certain respects to the piezoelectric
`crystal sensor, the test data also indicat“ that the AC
`the Medtronic( Activitrax(
`pacemaker provided patients
`with the a pulse generator having a rate responsive capabil-
`accelerometers do not adequately distinguish between stair
`ity dependent on the level of patient activity. A piezoelectric
`ascending and descending or walking at the same rate on a
`crystal bonded to the interior of the implantable pulse
`fiat surface to set an appropriate pacing rate. Neither the AC
`generator can or case is employed in that pacemaker and 45 accelerometer nor the piezoelectric sensor can inherently
`distinguish these patient activities. If an appropriate rate for
`successor models to provide a pulse output signal related to
`the pr己ssure wave generated by a patient’s footfall and
`an individual patient is set for stair climbing, for example,
`that rate may only be triggered by the frequency of recur-
`conducted through the body to the crystal. Thus, low fre-
`quency activity signals recurring at the patient’s rate of
`rence of the patient footfalls and consequ己ntly may be too
`walking or running could be sensed and processed to derive so high a rate for either stair descending or level walking at the
`same speed.
`a pacing rate appropriate to the level of activity. The activity
`sensor and its operation is described in commonly assigned
`Like the piezoelectric crystal sensor, there is no signal
`U.S. Pat. No. 4,428,378 to Anderson.
`output from the AC accelerometer in the absence of body
`Since the introduction of the Activitrax( pacemaker, a
`motion and related to body position or attitude. In other
`great many rate responsive pacemakers employing a wide 55 words, when a patient is at rest, neither activity sensor
`variety of activity sensors and other physiologic sensors
`provides any indication as to whether the patient is upright
`have been proposed and marketed. A comprehensive listing
`and awake and resting or lying down and presumably
`of such rate responsive pacemakers, sensors and sensed
`sleeping or resting. Other sensors for sensing physiologic
`physiologic parameters is set forth in commonly assigned
`parameters induced by high levels of exercise have been
`U.S. Pat. No. 5,226,413 to Bennett et al., incorporated herein 60 proposed to detect the physiologic changes accompanying
`by reference. However, the activity sensor of the type
`exercise, rest and sleep to trigger appropriate rates. To lower
`employed in the Activitrax(
`pacemaker continues to be
`the pacing rate during sleep, the inclusion of a real time
`used in successor single and dual chamber, rate responsive
`clock to establish a Circadian rhythm pacing rate has also
`pacemaker models and remains the most widely used physi-
`been proposed. None of these proposed sensors or systems
`ologic sensor.
`65 are capable of determining a patie时’S position or posture.
`A mechanical sensor has been proposed in the article “A
`As mentioned above, this piezoelectric crystal sensor is
`responsive to pressure waves generated by patient footfalls
`New Mechanical Sensor for Detecting Body Activity and
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`5,957,957
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`4
`problems identified above resulting in the same or a higher
`Posture, Suitable for Rate Responsive Pacing” by Alt et al.
`pacing rate being developed during stair descending than
`(PACE, Vol.11, pp. 1875-81, November, 1988, Part II) and
`in U.S. Pat. No. 4,846,195 that involves use of a multi-
`during stair climbing.
`Despite the weaknesses reported with respect to the
`contact, tilt switch. This switch employs a mercury ball
`within a container that is proposed to be fixed in the pulse s piezoelectric sensors and solid state accelerometers, they
`generator case, so that if the pulse generator is implanted ~t
`remain favored over the other physiologic s己nsors that have
`a c忘rtain orientation, and stays in that orientation, certam
`been proposed or are in clinical use due to their relative
`contacts are closed by the mercury ball when the patient is
`simplicity, reliability, predictability, size, and low cost.
`upright and others are closed or none are closed when the
`patient is prostrate, i.e.， ιither prone or supine. During
`Problems to be Solved by the Invention
`movement of the body, the mercury ball is expected to jiggle 10
`In view of the demonstrated advantages of the piezoelec-
`randomly and the number of contacts made per unit of time
`tric and AC accelerometer type activity sensors, it would be
`d叫吵l~ to 已mploy solid state sensors responsive to patient
`may be used as a measure of the level of activity. Similar
`activity ma similar manner that would also distinguish stair
`sensors have been proposed in U.S. Pat. Nos. 4,869,251,
`5.010.893. 5.031.618 and 5.233.984.
`or steep incline climbing from other activities in order to
`The use of elemental mercury is generally not favored and 15 provide an appropriate rate r叫onse to provide adequate
`would increase environmental problems related to disposal
`cardiac output.
`of the pulse generators after use. Long term contact con-
`SUMMARY OF IBE INVENTION
`tamination and bridging issues would also arise, particularly
`given the extremely small size of the switch for confinement
`within modern pulse generator cases.
`Presumably, the multi-contact tilt switch sensor would
`also not necessarily be able to distinguish between stair
`climbing and descending at the same stepping rate. Given
`the necessary small size of the tilt switch, it would be
`difficult to accurately position the pacemaker pulse genera- 25
`tor so that consiste时， reproducible signal outputs from the
`sets of contacts bridged while stooped forward or rearward
`would be achieved in a given patient over time. Moreover
`’
`
`In view of the above, it is an object of the present
`20 invention to provide a rate responsive pacemaker employing
`a body position sensor to distinguish stair climbing from
`other activities, e.g. stair descending or walking on a level
`surfac忘， and to provide an appropriate pacing rate increase
`from a rest rate during stair climbing.
`It is yet a further particular object of the present invention
`to provide such pacing rate setting capabilities to provide a
`higher pacing rate for a patient ascending stairs or a steep
`incline than descending stairs or walking on a relatively
`level surface.
`
`~~~~i：~~：fm~~：~oe~ ~；u~~~~a~~h~：i：~.e；：~:i~，s~~~~~l！~fi 30
`
`There is provided in accordance with the prese~t
`
`invention, a rate responsive pacemaker for pacing a patient’s
`of pacemaker pulse generators using such a tilt switch have
`heart at a pacing rate dependent on patient activity and
`been reported.
`posture particularly during stair climbing, at least including
`More recently, the use of a solid state position sensor in
`the form of a DC accelerometer is proposed in U.S. Pat. No. 35 the m~ans for and steps of:
`5.354.317. The DC accelerometer disclosed in the ’317
`denving a body posture tilt signal having a characteristic
`patent is fabricated in hybrid sen山onductor IC form as a
`:rarying with the degree to which the patient posture is
`polycrystalline silicon, square plate, suspended at its four
`m an upright stance or leaning forward;
`corners above a well in a single silicon crystal substrate, and
`detecting patient footsteps;
`associated low pass filter circuits are formed on the same 40
`deriving a patient activity signal having a signal level
`dep已ndent on the frequency of patient footsteps recur-
`substrate. The suspended plate structure moves between
`stationary positions with respect to the well on the suspen-
`ring over a time unit;
`sion arms in response to earth gravity, depending on its
`deriving a rate control signal from the body posture tilt
`signal and the patient activity signal correlated to the
`orientation to the gravitational field. The plate also vibrates
`on the suspension arms similar to the AC accelerometer in 45
`physiologic demand on the patie时’s heart;
`response to acceleration movements of the patie时’s body.
`defining physiologic escape intervals as a function of the
`The single DC accelerometer of the ’ 317 patent is ori-
`rate control signal to establish a physiologic pacing
`ented to be sensitive to the anterior-posterior axis of the
`rate;
`patient so that the upright, supine and prone body positions
`generating pacing pulses at the physiologic pacing rate;
`can be discriminated, and separate base pacing rates can be 50
`and
`set. Rate cha吨巳s from the base pacing rates d~pendent on
`applying the pacing p由“ to the patient’s heart.
`the exercise level of the patient in each posit10n are sug-
`Preferably, the posture of the patient is determined
`gested. When changes in patient position are detected in the
`through the use of a solid state, DC accelerometer mounted
`absence of physical exercise, the base pacing rate change is
`within the pacemaker pulse generator case having a sensitive
`smoothed between the old and new rate to avoid a sudden 55 axis aligned with the pacemaker case and the patient’s
`step cha吨巳．
`ar阳io叩osterior (A刊 body axis. The DC accelerometer
`The signal processing of the output signal from the single
`provides an output signal due to the force of gravity which
`DC accelerometer of the ’ 317 patent includes signal level
`has a polarity and magnitude dependent on the degree to
`calibration for each individual patient to account for di旺ιr-
`which the sensitive axis is tilted forward or rearward from
`ences in the angle of orientation of the DC accelerometer 60 the direction of earth’s gravity. Forward lean or tilt of the
`patient while upright accompanied by a recurring series of
`plate resulting from the implantation angle of the pulse
`generator case in the patie时’s body. However, this calibra-
`footfalls can be distinguished from an upright stance and a
`tion is not suggested in order to distinguish body positions
`similar level of footfalls to thereby distinguish stair climbing
`having a more or less common angular relation of the
`from other activities in the same stepping rate range and
`movable plate to the gravitational field.
`65 provide an appropriate pacing rate for each activity.
`In addition, the ’317 patent does not appear to suggest any
`The DC accelerometer is preferably mounted into an IC
`discrimination of stair climbing that would alleviate the
`chip with a second and optionally a third DC accelerometer
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`5,957,957
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`6
`from the output signal of the DC accelerometer of FIG. 2
`oriented along its sensitive axis in the A-P direction:
`FIG. 4 is a detailed flowchart of the stair climbing
`discrimination step of the flowchart of FIG. 3;
`FIG. 5 is a detailed flowchart of the discrimination rat 已
`
`5
`so that their sensitive axes are aligned with the three axes of
`axes of the pulse generator case. The physician can implant
`and stabilize the pulse generator case in the proper orienta(cid:173)
`tion to the patient’s thorax to align the with the superior-
`inferior (S-1), anterio叩osterior (A刊， and lateral-medial 5
`(L-M) ax已s of th已 ch已
`l已V已ls ar已 d已V已lop已d by 已ach DC acc已l已rom已t已r in 已ad1
`FIG. 6 is a graph illustrating the calculation of the
`posture position due to the effect of gravity on the sensitive
`axis of each serr山onductor element. From these signal
`approp.riate pacing rates related to the degree of body tilt of
`levels, the posture of the patient can be determined for 10 an active patient in walking, climbing and descending a
`flight of stairs;
`providing additional pacing rates appropriate to the other
`determined body positions and the activity level of the
`FIGS. 7-9 are graphs illustrating the tilt deviation distri-
`patient.
`butions resulting from of tests conducted on test subjects
`Advantageously, one or more of the DC accelerometers
`employing the stair climbing discrimination algorithm of
`can be used to derive the level of patient activity from the 1s FIGS. 3 5;and
`FIG. 10 is a graph illustrating the delivery of the appro-
`number of changes in signal levels exceeding a certain
`threshold occurring in a given sampling time period, as is
`priate pacing rates related to the degree of body tilt of an
`conventional in use of the piezoelectric and AC accelerom-
`active patient in walking, climbing and descending a flight
`of stairs.
`eter activity sensors described above.
`FIG. 11 is a graph of time versus signal level for the
`The present invention may also be implemented employ- 20
`ing other forms of body position or tilt sensors having a
`anterior-posterior tilt signal for one patient.
`sensitive axis in the A-P direction, particularly the sensor
`disclosed in the above-referenced ’ 984 patent.
`It should be noted that the DC accelerometer of the
`above-referenced ’ 317 patent is a bulk micromachined IC 25
`The present invention is preferably implemented in multi-
`structure that has a sensitive axis normal to the plane of the
`programmable DDDR pacemakers of types widely known in
`movable plate and provides the +1, -1 and 0 static output
`the prior art. However, the invention could be implemented
`signal levels depending on the orientation of the sensitive
`in simpler, single chamber pacemakers. As described above
`axis to the vertical gravitational force. If such a DC accel-
`with respect to other medical devices, the invention may also
`erometer is used in the practice of the present invention, the 30 be implemented in other medical devices for providing other
`orthogonally arranged DC accelerometers would provide
`therapies and/or for monitoring physiologic parameters in
`similar signal responses as long as the sensitive axes are
`the various body positions the patient may assume where
`oriented in the same manner as described above.
`stair climbing discrimination may be important.
`Advantages of the Invention
`The DC output signal of a DC accelerometer can be 35
`FIG. 1 is block level diagram of such a pacemaker
`processed to detect body forward tilt, while the patient
`implantable pulse generator or IPG and lead set 12 and 14
`moves at a walking pace, and thereby employed to discrimi-
`which sets forth the structures required to incorporate the
`nate stair climbing from other activities and to develop an
`invention into a DDDR pacemaker. In the drawing, the
`appropriate pacing rate, solving the problems associated
`patient’s heart 10 has an atrial pacing lead 12 passed into the
`with the prior art rate responsive pacemakers employing 40 right atrium and a ventricular lead 14 passed into the right
`activity sensors. The DC accelerometer and associated cir-
`ventricle. The atrial lead 12 has an atrial electrode array 16
`cuitry can be easily incorporated into a pacemaker pulse
`which couples the pulse generator 30 to the atrium. The
`generator at low cost. The ease of use, and the reproduc-
`ventricular lead 14 has a ventricular electrode array 18 for
`ibility and consistency of results attained will lead to accept-
`coupling the pulse generator 30 to the ventricle of the
`ability within the medical community.
`45 patient’s heart 10. Atrial and ventricular leads 12 and 14 are
`depicted as bipolar leads coupled to a bipolar IPG 30,
`although unipolar leads could be employed with a suitable
`IPG.
`These and other objec侣， advantages and features of the
`The IPG circuit 30 of FIG. 1 is divided generally into a
`present invention will be more readily understood from the 50 pacing circuit 32 coupled to a battery power supply 50, an
`followi吨 detailed desc句tion of the preferred embod叩ιnts
`activity sensor 60 of the type desc由己d below, a telemetry
`coil 4S and a microcomputer circuit 34. The pacing circuit
`thereof, when considered in conjunction with the drawmgs,
`in which like reference numerals indicate ~dentical structures
`32 includes the atrial and v已ntri
`throughout th已 s已V已ral vi已ws, and wh已r已m:
`36 and s已ns已 amplifi已rs 38 that ar已 coupl已d to th已 atrial and
`FIG. 1 is block level diagram of a DDDR pacemaker 55 ventricular leads 12 and 14, respectively, the digital
`capable of implementing at least one of three possible,
`controller/timer circuit 40 and other associated components
`mutually orthogonal DC accelerometers as activity and
`described below. The output circuit 36 and sense amplifier
`patient posture sensors particularly to detect forward tilt;
`circuit 38 may contain atrial and ventricular pulse generators
`FIG. 2 is a schematic illustration of the orientations of the
`and sense amplifiers corresponding to any of those presently
`S-I, L-M, and A-P sensitive axes of three DC accelerometers 60 employed in commercially marketed dual chamber cardiac
`mounted orthogonally with respect to a hybrid circuit sub-
`pacemakers.
`strat已 moun
`ar已 con且rm已d by th已 atrial s已ns已 amplifi已r ar已 communi­
`FIG. 1 and th已 markings on th已 housing for ori已nting th已
`cated to the digital controller/timer circuit 40 on the ASE
`puls已 g已n已rator with th已 pati已nt body axes;
`FIG. 3 is a rate response overview flowchart of the 65 line. Similarly, ventricular depolarizations (V-SENSE) or
`algorithm incorporated into the pacemaker of FIG. 1 for
`R-waves that are confirmed by the ventricular sense ampli-
`deriving a physiologic pacing rate related to stair climbing
`fier are communicated to the digital controller/timer circuit
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
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`5,957,957
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`7
`sensor derived pacing escape interval is adjusted propor-
`40 on VSE. The sensitivity control block 42 adjusts sensi-
`tivity of each sense amplifier in response to control signals
`tionally. A timed interrupt, e.g., every two seconds, may be
`provided in order to allow the microprocessor 54 to analyze
`provided by digital controller/timer 40 that are in turn stored
`the output of the activity c山uit (PAS) 62 and update the
`in memory in microcomputer circuit 34.
`In order to trigger generation of a ventricular pacing or s basic V-A escape interval employed in the pacing cycle. In
`VPE pulse, digital controller/timer circuit 40 generates a
`the DDDR mode, the V-Aescape interval may be selected as
`trigger signal on the V-trig line. Similarly, in order to trigger
`the variable pacing rate establishing interval, but the A-V
`an atrial pacing or APE pulse, digital controller/timer circuit
`interval and the atrial and ventricular refractory periods may
`40 generates a trigger pulse on A-trig line.
`also vary with the V-A escape interval established in
`Crystal oscillator circuit 44 provides the basic timing 10 response to patient activity.
`clock for the pacing circuit 30, while battery 50 provides
`Preferably, two separate lower rate V-A interval timer
`power. Reference mode circuit 48 generates stable voltage
`functions are provided. The first is set by the physician when
`reference and current levels for the analog circuits within the
`the base pacing rate is selected. This DDD V-A time interval
`pacing circuit 30 from the battery voltage and current.
`starts from the occurrence of a VPE or VPE, and provided
`Power-on-reset circuit 46 responds to initial connection of 15 neither an ASE nor a VSE occurs during the V-A time
`interval, an APE is generated after the expiration of the V-A
`the circuit 30 to the battery 50 for defining an initial
`operating condition and also resets the operating condition
`time interval. The duration of the second lower rate time
`in response to detection of a low battery energy condition.
`interval is a function of the measured patient activity
`Analog to digital converter (ADC) and m山忡mr circuit 叫uired by the activity sensor 21. Typically, this DDDR,
`52 digitizes analog signals and voltage to provide real time 20 V-A time interval begins with a VSE or VPE and has a time
`telemetry of ASE and VSE cardiac signals from sense
`duration reflecting patient activity. In this art, such structures
`are well known, and a variety of techniques can be used to
`amplifiers 38, for uplink transmission via RF transmitter and
`receiver circuit 47. Voltage reference and bias circuit 48,
`implement the required timer functions.
`ADC and multiplexor 52, power-on-reset circuit 46 and
`Digital controller/timer circuit 40 starts and times out
`crystal oscillator circuit 44 may correspond to any of those 25 these and other intervals employed over a pacing cycle
`presently used in current marketed implantable cardiac
`comprising a successive A-V and V-A interval in a manner
`pacemakers.
`well known in the art. Typically, digital controller/timer
`Data transmission to and from an external programmer
`circuit 40 defines an atrial blanking interval following
`(not shown) is accomplished by means of the telemetry
`delivery of an atrial pacing pulse, during which atrial
`antenna 45 and the associated RF transmitter and receiver 30 sensing is disabled, as well as ventricular blanking intervals
`47, which serves both to demodulate received downlink
`following atrial and ventricular pacing pulse delivery, during
`telemetry and to transmit uplink telemetry. For example,
`which ventricular sensing is disabled. Digital controller/
`circuitry for demodulating and decoding downlink telemetry
`timer circuit 40 also defines the atrial refractory period
`may correspond to that disclosed in U.S. Pat. No. 4,556,063
`(ARP）由ring which atrial sensing is disabled or the ASE is
`issued to Thompson et al. and U.S. Pat. No. 4,257,423 issued 35 ignored for the pu叩ose of resetting the V-A escape interval.
`to McDonald et al., while uplink telemetry functions may be
`The ARP extends from the beginning of the A-V interval
`provided according to U.S. Pat. No. 5,127,404 issued to
`following either an ASE or an A-trig and until a predeter-
`Wyborny et al. and U.S. Pat. No. 4,374,382 issued to
`mined time following sensing of a ventricular depolarization
`Markowitz. Uplink telemetry capabilities will typically
`or triggering the delivery of a VPE pulse. A post-ventricular
`include the ability to transmit stored digital information as 40 atrial refractory period (PVARP) is also defined following
`well as real time or stored EGMs of atrial and/or ventricular
`delivery of a VPE pulse. The durations of the ARP, PVARP
`electrical activity ( accordi吨 to the teachi吨 of the above-
`and VRP may also be selected as a programmable parameter
`cited Wyborny patent), as well as transmission of Marker
`stored in the microcomputer 34. Digital controller/timer
`Channel pulses indicating the occurrence of sensed and
`circuit 40 also controls the pulse widths of the APE and VPE
`paced depolarizations in the atrium and ventricle, as dis- 45 pacing pulses and the sensitivity settings of the sense
`amplifiers 38 by means of sensitivity control 42. Digital
`closed in the cited Markowitz patent.
`Control of timing and other functions within the pacing
`controller timer/logic circuit 40 also times out an upper rate
`circuit 30 is provided by digital controller/timer circuit 40
`limit interval (URL) set by a value programmed into
`which includes a set of timers and associated logic circuits
`memory in microcomput己r circuit 34. This timer is initiated
`connected with the microcomputer 34. Microcomputer 34 so by the occurrence of a VPE or VSE, and limits the upper rate
`controls the operational functions of digital controller/timer
`at which ventricular stimuli are delivered to the heart. The
`40, specifying which timing intervals are employed, and
`lower pacing rate is established by a programmed-in V-A or
`controlling the duration of the various timing intervals, via
`A-A interval value stored in memory in microcomputer
`data and control bus 56. Microcomputer 34 contains a
`circuit 34.
`microprocessor 54, associated system clock 58, and 55
`The illustrated IPG block diagram of FIG. 1 is merely
`on-processor RAM and ROM chips 64 and 66, respectively.
`己xemplary, and corresponds to the general functional orga-
`In addition, microcomputer circuit 34 includes a separate
`nization of most multi-programmable microprocessor con-
`RAM/ROM chip 68 to provide additional memory capacity.
`trolled DDDR cardiac pacemakers presently commercially
`Microprocessor 54 is interrupt driven, operating in a reduced
`available. It is believed that the present invention is most
`power consumption mode normally, and awakened in 60 readily practiced in the context of such a device, and that the
`response to defined interrupt even侣， which may include the
`present invention can therefore readily be practiced using
`A-trig, V-trig, ASE and VSE signals. The specific values of
`the basic hardware of existing microprocessor controlled
`the intervals defined are controlled by the microcomputer
`dual chamber pacemakers, as presently available, with the
`circuit 54 by means of data and control bus 56 from
`invention implemented primarily by means of modifications
`programmed-in parameter values and operating modes.
`65 to the software stored in the ROM 66 of the microcomputer
`If the IPG is programmed to a rate responsive mode, the
`circuit 34. However, the present invention may also be
`patie时’s activity level is monitored periodically, and the a
`usefully practiced by means of a full custom integrated
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`5,957,957
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`25
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`30
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`10
`9
`purposes. The effect of 1 G of gravitational force applied in
`circuit, for example, a circuit taking the form of a state
`precisely the opposite or negative direction to the sensitive
`machine as set forth in the above-cited Betzold et al. patent,
`axis provides a characteristic output voltage signal level that
`in which a state counter serves to control an arithmetic logic
`is referenced or scaled as -1. If the sensitive axis is oriented
`unit to perform calculations according to a prescribed
`transversely to the direction of the gravitational force, a bias
`sequence of counter controlled steps. As such, the present s
`voltage level output signal should be present, and that
`invention should not be understood to be limited to a
`voltage signal level is referenced or scaled as 0. The degree
`pacemaker having an architecture as illustrated in FIG. 1.
`to which the sensitive axis is oriented away or tilted from the
`FIG. 2 is a schematic illustration of embodiment of a DC
`accelerometer based forward lean sensor that may be
`direction of the gravitational force can also be detected by
`employed in the practice of the present invention. In FIG. 2, 10 the magnitude and polarity of the output voltage signal level
`three solid state, DC accelerometers, namely the S-I DC
`deviating from the bias level scaled to 0 and below the
`accelerometer 72, A-P DC accelerometer 74, and L-M DC
`output signal level values scaled to +1 and -1. The above-
`accelerometer 76, are mounted so that their s己nsitive axes
`referenced publications provide instructions for scaling the
`are orthogonally directed to the S-I, A-P and L-M axes,
`voltage signal levels to the 0, +1 and -1 static level values.
`respectively, of the pulse generator hybrid circuit substrate 15 A microprocessor interface circuit with auto calibration of
`76 and exterior case 78. In the practice of the present
`offset error and drift caused by temperature variation that
`invention, the DC output signal of theA-P DC accelerometer
`may be employed in the activity circuit 62 of FIG. 1 is also
`74 is prefer