EOEA
`
`United States Patent 1
`5,593,431
`[11] Patent Number:
`Jan. 14, 1997
`[45] Date of Patent:
`Sheldon
`
`[54] MEDICAL SERVICE EMPLOYING
`MULTIPLE DC ACCELEROMETERS FOR
`PATIENT ACTIVITY AND POSTURE
`SENSING AND METHOD
`
`[75]
`
`Inventor: Todd J. Sheldon, Eagan, Minn.
`
`[73] Assignee: Medtronic, Inc., Minneapolis, Minn.
`
`[21] Appl. No.: 413,736
`
`[22] Filed:
`
`Mar. 30, 1995
`
`“Airbags Boom When IC Accelerometer Sees 50G”, Elec-
`tronic Design, Aug. 8, 1991.
`
`“Monolithic Accelerometer with Signal Conditioning”, Rev.
`O, published Jun. 1993 by Analog Devices, Inc.
`
`Primary Examiner—William E. Kamm
`Assistant Examiner—Scott M. Getzow
`Attorney, Agent, or Firm—Reed A. Duthler; Harold R.
`Patton
`
`[57]
`
`ABSTRACT
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3/1981 McDonald .
`4,257,423
`2/1983 Markowitz .
`4,374,382
`1/1984 Anderson .
`4,428,378
`12/1985 Thompson .
`4,556,063
`9/1989 Lekholm .
`4,869,251
`4/1991 Sholder .
`5,010,893
`7/1991 Mullett .
`5,031,618
`7/1992 Wyborny .
`5,127,404
`7/1993 Bennett .
`5,226,413
`8/1993 Thompson .
`5,233,984
`6/1994 Barreras .
`5,318,596
`7/1994 Legay .
`5,330,510
`8/1994 Alt.
`5,342,404
`5,354,317 10/1994 Alt .
`
`(51 ]
`Tint, Cheecaccescsssceessssseessncecsssecsssseecseees AGIN 1/365
`
`A method of and apparatus for determining the physical
`[SZ]WAS.Checeacscsssssesesscsccssscssssnsssssssenesccccsssnsesssesceseeeees 607/19
`posture ofa patient’s body, having a superior-inferior body
`[58] Field of Search oo... ..cssssssssssseseccsssssees 607/2, 19, 18,
`607/17
`axis, an anterior-posterior body axis and a lateral-medial
`body axis, in relation to earth’s gravitational field. A medical
`device havingfirst, second and, optionally, third DC accel-
`erometers having sensitive axes mounted orthogonally
`within an implantable housing is adapted to be implanted
`with the sensitive axes generally aligned with the patient’s
`body axes. Each DC accelerometer generates DC acceler-
`ometer signals having characteristic magnitudes and polari-
`ties on alignment of the sensitive axis with, against or
`normal to earth’s gravitational field and DC accelerometer
`signals of varying magnitudes and polarities when not so
`aligned. Body position may be determined through com-
`parison of the magnitudes andpolarities of the DC acceler-
`ometersignals with the characteristic magnitudes andpolari-
`ties. A patient activity signal may also be determined from
`the frequency of body movements recurring overa time unit
`effecting magnitude changes in the DC accelerometer sig-
`nals within a certain range of magnitude and frequency. The
`activity and body position signals may be stored and/or used
`to monitor and effect the delivery of a therapyto the patient,
`e.g. by controlling the pacing rate of a rate responsive
`pacemaker.
`
`OTHER PUBLICATIONS
`
`Bacharachet al., “Activity-Based Pacing: Comparison of a
`Device Using an Accelerometer Versus a Piezoelectric Crys-
`tal”PACE,vol. 15, pp. 188-196, Feb. 1992.
`Alt et al., “A New Mechanical Sensor for Detecting Body
`Activity and Posture, Suitable for Rate Responsive Pacing”,
`PACE,vol. 11, pp. 1875-1881, Nov., 1988, Part II.
`
`29 Claims, 15 Drawing Sheets
`
`
`
`
` SUPERIOR—
`INFERIOR
`
`
`LATERAL-MEDIAL
`
`76
`
`74
`
`ANTERIOR—
`
`POSTERIOR
`
`50
`
`APPLE 1110
`
`APPLE 1110
`
`1
`
`

`

`U.S. Patent
`
`Jan
`
`14, 1997
`
`Sheet 1 of 15
`
`5,593,431
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`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 2 of 15
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`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 3 of 15
`
`5,593,431
`
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`
`100
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`U.S. Patent
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`Jan. 14, 1997
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`U.S. Patent
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`Jan. 14, 1997
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`U.S. Patent
`
`Jan. 14, 1997
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`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 7 of 15
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`5,593,431
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`

`

`Jan. 14, 1997
`
`Sheet 8 of 15
`
`5,593,431
`
`U.S. Patent
`
`200
`
`SAMPLE OUTPUT OF
`DC ACCELEROMETER
`
`DETERMINE
`202
`s ACTIVITY COUNTS
`
`PACING RATE
`
`204
`
`206
`}
`
`208
`
`DETERMINE
`
`BODY POSITION
`
`SELECT
`TARGET RATE
`
`PACE AT
`
`FIG. 8
`
`9
`
`

`

`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 9 of 15
`
`5,593,431
`
`MEASURE A-P, S—I, AND
`L-M DC ACCELERATION
`
`210
`
`No OQ -
`
`212
`
`
`
`ARE O OR MORE THAN 1
`YES|UNKNOWN
`
`
`ACCELEROMETER AXES MEASUREMENT
`POSITION
`
`>.707g or>—.707g
`
`NO
`216
`
`218
`
`214
`
`292
`
`226
`
`230
`
`234
`
`238
`
`NO
`
`NO
`
`NO
`
`220
`
`224
`
`228
`
`NO
`
`232
`
`NO
`
`236
`
`FIG. 94 0
`
`10
`
`

`

`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 10 of 15
`
`5,593,431
`
`MEASURE A-P AND S-I
`DC ACCELERATION
`
`310
`
`204
`a
`
`312
`
`N
`
`A-P>.707g
`
`?
`a
`
`YES
`
`314
`
`0 —.707g>A-P>+.707q
`
`
`
`
`
`?
`
`YES
`
`N
`
`316
`0 A-P<-.707g
`
`?
`.
`
`YES
`
`318
`
`520
`
`322
`
`YES
`330
`
`ERROR
`
`YES
`332
`
`YES
`334
`
`ERROR
`
`326
`
`328
`
`
`<'707g>S-I>+.707g
`
`—.707g>S—I>+.707q
`
`
`
`—.707g>S—I>+.707g
`
`
`
`NO
`
`324
`
`
`
`
`
`
`
`
`
`YES
`PRONE
`
`YES
`LYING RIGHT
`OR LEFT SIDE
`
`YES
`
`SUPINE
`336
`
`344
`
`346
`
`ERROR
`
`340
`
`988
`
`ERROR
`
`UPSIDE DOWN
`
`342?
`
`FIG. 10
`
`11
`
`11
`
`

`

`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 11 of 15
`
`5,593,431
`
`MEASURE S—I AND L—M
`DC ACCELERATION
`
`310°
`
`312’
`NO
`
`YES
`
`— .707g>S—I>+.707q
`
`
`
`
`YES
`
`318"
`
`NO
`
`320°
`
`NO
`
`YES
`
`330’
`
`YES
`
`332'
`
`204
`—
`
`316’
`
`NO
`
`YES
`
`322’
`
`NO
`
`YES
`
`334"
`
`ERROR
`
`LEFTSIDE
`
`ERROR
`
`326’
`324’
`
`
`
`.707g>L—M>+.707q
`
`
`
`
`
`
`328’
`
`YES
`UPSIDE DOWN
`
`.707g>L—M>+.707g
`
`YES
`SUPINE OR
`PRONE
`
`
`
`.107g>L—M>+.707g
`
`NO
`
`:
`
`YES
`
`336°
`
`958"
`
`344°
`
`346’
`
`RIGHTSIDE
`
`349")
`
`FIG. I
`
`12
`
`12
`
`

`

`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 12 of 15
`
`5,593,431
`
`310"
`
`MEASURE A-P AND L-M
`DC ACCELERATION
`
`204
`—
`
`<'707g>A-P>+.707¢ YES
`
`YES
`
`318”
`
`NO
`
`320”
`
`NO
`
`YES
`
`330"
`
`YES
`
`332"
`
`322"
`
`NO
`
`YES
`
`334°
`
`324"
`.107g>L—M>+.707q
`
`
`
`
`
`£.707g>L—-M>+.707q
`NO
`
`
`
`
`
`.707g>L-M>+.707g
`
`NO
`
`326"
`
`328°
`
`
`
`YES
`
`UP RIGHT OR
`UPSIDE DOWN
`
`336"
`
`338
`
`RIGHTSIDE
`
`540”
`
`346"
`
`FIG. (2
`
`13
`
`13
`
`

`

`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 13 of 15
`
`5,593,431
`
`PATIENT ASSUMES SUPINE
`POSTITION
`
`
`
`
`DETERMINE DC ACCELERATION FOR
`THE GIVEN POSTURE USING 2
`OR MORE DC ACCELEROMETERS.
`
`
`
`
`
`
`
`
`
`CREATE POSTURE CONFIDENCE
`INTERVAL BY ADDING +/-—.25g TO
`
`THE DC ACCELERATION OF EACH
`
`AXES FOR THE GIVEN POSTURE.
`
`
`
`404
`
`
`
`REPEAT PROCEDURE FOR THE
`
`FOLLOWING POSTURES: PRONE,
`
`UPRIGHT, RIGHT, LEFTSIDE.
`
`
`406
`
`14
`
`

`

`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 14 of 15
`
`5,593,431
`
`MEASURE A-—P, S-—I, AND
`L-M DC ACCELERATION
`
`410
`
`204
`
`412
`422
`
`
`ARE A-P, S-I AND L-M
`YES
`
`
`
`ACCELERATIONS WITHIN SUPINE
`INTERVAL
`
`
`?
`NO
`
`SUPINE
`
`414
`
`
`
`
`ARE A-P, S-I AND L-M
`
`
`
`ACCELERATIONS WITHIN PRONE
`INTERVAL
`
`
`?
`NO
`
`416
`
`YES
`
`424
`
`PRONE
`
`
`
`
`ARE A-P, S—-I AND L-M
`
`ACCELERATIONS WITHIN RIGHT SIDE
`INTERVAL
`
`?
`
`
`
`426
`
`YES
`
`RIGHTSIDE
`
`NO
`
`418
`
`
`
`
`ARE A-P, S-I AND L-—M
`
`ACCELERATIONS WITHIN LEFT SIDE
`INTERVAL
`
`?
`NO
`
`
`
`420
`
`
`
`
`
`ARE A-P, S-I AND L-M
`
`ACCELERATIONS WITHIN UPRIGHT
`INTERVAL
`
`
`
`?
`NO
`
`FIG. 14
`°
`
`15
`
`YES
`
`YES
`
`428
`
`LEFTSIDE
`
`430
`
`UPRIGHT
`432
`
`UNKNOWN
`
`15
`
`

`

`U.S. Patent
`
`Jan. 14, 1997
`
`Sheet 15 of 15
`
`5,593,431
`
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`

`

`5,593,431
`
`1
`MEDICAL SERVICE EMPLOYING
`MULTIPLE DC ACCELEROMETERS FOR
`PATIENT ACTIVITY AND POSTURE
`SENSING AND METHOD
`
`REFERENCE TO RELATED APPLICATION
`
`Reference is made to commonly assigned co-pending
`U.S. patent application Doceket No. P-3270 entitled RATE
`RESPONSIVE CARDIAC PACEMAKER FOR DIS-
`CRIMINATING STAIR CLIMBING FROM OTHER
`ACTIVITIESfiled on even date herewith.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present inventionrelates to the use of an array of DC
`accelerometers for detection of patient posture and activity
`level for medical monitoring and/or the delivery of thera-
`pies, including cardiac pacing.
`2. Description of the Prior Art
`In the field of medical device technology, patient moni-
`toring of physiologic parameters e.g. heart rate, temperature,
`blood pressure and gases and the like are well known. In
`addition, the delivery of various therapies including drugs
`and electrical stimulation by implanted or invasive medical
`devices is well known. Factors that may be appropriately
`taken into account during monitoring or delivery of thera-
`pies include patient position or posture and activity level.
`Both may have an effect on the other parameters monitored
`and in the decision process for setting an appropriate
`therapy. Particularly in the field of cardiac pacing, patient
`activity level can be correlated to the need for cardiac
`output.
`Rate responsive pacing has been widely adopted for
`adjusting pacing rate to the physiologic needs of the patient
`in relatively recent years, Early single chamber patient in
`relatively recent years. Early single chamber cardiac pace-
`makers provideda fixed rate stimulation pulse generator that
`could be reset, on demand, by sensed atrial or ventricular
`contractions recurring at a rate above the fixed rate. Later,
`dual chamber demand pacemakers became available for
`implantation in patients having an intact atrial sinus rate but
`no AV conduction, so that ventricular pacing could be
`synchronized with the atrial sinus rate, and backup fixedrate
`ventricular pacing could be provided on failure to sense
`atrial depolarizations. In addition, rate programmable pace-
`makers became available wherein the base pacing rate could
`be selected by a physician to provide a compromise fixed
`rate that did not interfere with patient rest and provided
`adequate cardiac output at moderate levels of exercise.
`Such fixed rate pacing, particularly for patients not having
`an adequate atrial sinus rate to allow synchronous pacing,
`left most patients without the ability to exercise, lift objects
`or even walk up stairs without suffering loss of breath due
`to insufficient cardiac output. However, the introduction of
`the Medtronic® Activitrax® pacemaker provided patients
`with the a pulse generator having a rate responsive capabil-
`ity dependentonthe level ofpatient activity. A piezoelectric
`crystal bonded to the interior of the implantable pulse
`generator can or case is employed in that pacemaker and
`successor models to provide a pulse outputsignal related to
`the pressure wave generated by a patient’s footfall and
`conducted through the body to the crystal. Thus, low fre-
`quency activity signals recurring at the patient’s rate of
`walking or running could be sensed and processed to derive
`
`2
`a pacing rate appropriate to the level of activity. The activity
`sensor andits operation is described in commonly assigned
`U.S. Pat. No. 4,428,378 to Anderson.
`Since the introduction of the Activitrax® pacemaker, a
`great many rate responsive pacemakers employing a wide
`variety of activity sensors and other physiologic sensors
`have been proposed and marketed. A comprehensivelisting
`of such rate responsive pacemakers, sensors and sensed
`physiologic parameters is set forth in commonly assigned
`US. Pat. No. 5,226,413 to Bennettet al., incorporated herein
`by reference. However,
`the activity sensor of the type
`employed in the Activitrax® pacemaker continues to be
`used in successor single and dual chamber, rate responsive
`pacemaker models and remains the most widely used physi-
`ologic sensor.
`As mentioned above, this piezoelectric crystal sensor is
`responsive to pressure waves generated by patient footfalls
`striking the exterior of the pulse generator case. Activity
`sensor configurations employing integrated circuit, AC
`accelerometers on an IC chip inside the pacemakerare 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-
`ometer is formed ofa 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
`magnitude dependent on the rate of movement.
`Like the piezoelectric crystal sensor, there is no signal
`output from the AC accelerometer in the absence of body
`motion and related to body position or attitude. In other
`words, when a patient is at rest, neither activity sensor
`provides any indication as to whether the patient is upright
`and awake and resting or lying down and presumably
`sleeping or resting. A lower sleep pacing rate than the rest
`pacing rate while awake and upright may bedesirable for a
`given patient. Other sensors for sensing physiologic param-
`eters induced by high levels of exercise have been proposed
`to detect the physiologic changes accompanying exercise,
`rest and sleep to trigger appropriate rates. Particularly, to
`lower the pacing rate during sleep, the inclusion of a real
`time clock to establish a Circadian rhythm pacing rate have
`also been proposed. None of these proposed sensors or
`systems are capable of determining a patient’s position or
`posture.
`A mechanical sensor has been proposed in the article “A
`New Mechanical Sensor for Detecting Body Activity and
`Posture, Suitable for Rate Responsive Pacing” by Alt et al.
`(PACE, Vol. 11, pp. 1875-81, November, 1988, Part II) and
`in Alt U.S. Pat. No. 4,846,195 that
`involves use of a
`multi-contact, tilt switch. This switch employs a mercury
`ball within a container that is proposed to be fixed in the
`pulse generator case, so that
`if the pulse generator is
`implantedat a certain orientation, and stays in that orienta-
`tion, certain contacts are closed by the mercury ball when
`the patientis upright and others are closed or none are closed
`when the patient is prostrate, ie., either prone or supine.
`During movementofthe body, the mercury ball is expected
`to jiggle randomly and the numberof contacts madeperunit
`of time may be used as a measure of the level of activity.
`Similar sensors have been proposed in U.S. Pat. Nos.
`4,869,251, 5,010,893, 5,031,618 and 5,233,984.
`In the commonly assigned ’984 patent, a cubic shaped
`multi-axis position and activity sensor is employed in rate
`responsive pacing applications and in the detection of tachy-
`cardia base on the patient being supine and inactive. In the
`commonly assigned ’618 patent, a single axis position
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`Problems to be Solved by the Invention
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`sensor is employed that is employed to control the therapy
`delivered by a spinal cord stimulator. The sensors in both
`In view of the demonstrated advantages of the piezoelec-
`patents employ conductive liquids, including an electrolyte
`tric and AC accelerometer type activity sensors, it would be
`or elemental mercury.
`desirable to employ solid state sensors responsiveto patient
`The use of elemental mercury is generally not favored and
`activity in a similar manner that would also distinguish
`would increase environmental problemsrelated to disposal
`between a wide variety of patient body positions for patient
`of the pulse generators after use. Long term contact con-
`monitoring or in order to provide an appropriate therapy to
`tamination andbridging issues would also arise, particularly
`a patient. Particularly, in a multi-programmable,rate respon-
`given the extremely small size of the switch for confinement
`sive pacemaker, suchasolid state sensor is desired to derive
`within modern pulse generatorcases. To date, no implants of
`both patient activity signals and body position signals to set
`pacemaker pulse generators using such a tilt switch have
`an appropriate pacing rate providing adequate cardiac output
`been reported.
`in each position and activity level.
`Morerecently, the use of a solid state position sensor in
`the form of a DC accelerometer is proposed in Alt U.S. Pat.
`No. 5,354,317. The DC accelerometeris fabricated in hybrid
`semiconductor IC form as a polycrystallinesilicon, square
`plate, suspendedat its four corners above a well in a single
`silicon crystal substrate, and associated low passfilter cir-
`cuits are formed on the samesubstrate. The suspendedplate
`structure moves betweenstationary positions with respect to
`the well on the suspension armsin responseto earth gravity,
`depending on its orientation to the gravitational field. The
`plate also vibrates on the suspension armssimilar to the AC
`accelerometerin response to acceleration movements of the
`patient’s body.
`In the pacemakeralgorithms disclosed in the ’317 patent,
`different base pacing rates are established depending on the
`static output of the position sensor that indicate the position
`of the patient, namely the upright, supine and prone posi-
`tions, and separate base pacing rates can beset. Rate changes
`from the base pacing rates dependent on the exercise level
`of the patient in each position are suggested. Also, when
`changes in patient position are detected in the absence of
`physical exercise, the base pacing rate change is smoothed
`between the old and new rate to avoid a sudden step change.
`The rate responsive pacemaker disclosed in the 7317
`patent offers some discrimination of patient position, but
`cannot distinguish among various patient positions where
`the suspendedplate structureis aligned at the same angle to
`earth’s gravitational field. The plane of the movableplate is
`at a fixed angle, e.g. coplanar,
`to a plane of the pulse
`generator case. Once the pulse generator is implanted in a
`patient, the movable plate plane may bealigned generally in
`parallel with the gravitational field and not detect the gravi-
`tational force (i.e., producing a zero amplitudeoutput signal
`correlated to 0 g). The output of the so-aligned DC accel-
`erometer would be the same whether a patient is standing,
`sitting or lying on either side, since the plate plane would
`remain in the same general parallel relationship to the
`gravitational field in all three positions. However, the pacing
`tates appropriate in standing,sitting or lying on a side are
`different when the patientis still.
`The signal processing of the output signal from the single
`DC accelerometer of the ’317 patent includes signal level
`calibration for each individual patient to account for differ-
`ences in the angle of orientation of the DC accelerometer
`plate resulting from the implantation angle of the pulse
`generator case in the patient’s body. However, this calibra-
`tion is not suggested in orderto distinguish body positions
`having a more or less common angular relation of the
`movable plate to the gravitational field.
`to the
`Despite the weaknesses reported with respect
`piezoelectric sensors and solid state accelerometers, they
`remain favored over the other physiologic sensors that have
`been proposed or are in clinical use due to their relative
`simplicity, reliability, predictability, size, and low cost.
`
`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 resting patient that is standing
`upright than is provided for the samepatienteithersitting or
`a lying down supine, prone or on either side.
`These and other objects of the invention are realized in a
`method of and apparatus for determining the physical pos-
`ture of a patient’s body, having a superior-inferior body axis,
`an anterior-posterior body axis and a lateral-medial body
`axis, in relation to earth’s gravitational field comprising the
`steps of and means for: implanting a multi-axis, solid state
`sensor, comprising first and second DC accelerometers
`having first and second sensitive axes, respectively, which
`respond to earth’s gravitational field to provide first and
`second respective DC accelerometer signals of a magnitude
`and polarity dependent on the degree of alignment there-
`with, in the patient’s body so that said first and second
`sensitive axes are generally aligned with a respectivefirst
`and second one of said superior-inferior, anterior-posterior
`or lateral-medial body axes; defining a first characteristic
`magnitude and polarity of said first and second DC accel-
`erometer signals on alignmentof the sensitive axes of said
`first and second DC accelerometers with earth’s gravita-
`tional field, a second characteristic magnitude and polarity
`of said first and second DC accelerometer signals on align-
`ment against earth’s gravitational field, and a third charac-
`teristic magnitude and polarity of said first and second DC
`accelerometersignals on alignment normal to earth’s gravi-
`tational field; deriving first and second DC accelerometer
`signals from said first and second DC accelerometers as the
`patient assumes various body positions moving said first or
`
`SUMMARY OF THE INVENTION
`
`it is an object of the present
`In view of the above,
`invention to provide a multi-axis, solid state position and
`activity sensor operable along at least two orthogonal axes
`to distinguish the postureor positional attitude of the patient
`at rest and at levels of exercise.
`It is a further an object of the present invention to employ
`such a sensor to record body position and activity signal
`levels derived from the output signals of such a sensor.
`It is yet a further an object of the present invention to
`employ such a sensor to employ body position and activity
`signal levels derived from the output signals of such a sensor
`in controlling the delivery of a therapyto a patient, including
`the delivery of drugs orelectrical stimulation to the patient.
`In a specific context,
`it
`is an object of the present
`invention to provide a rate responsive pacemaker with
`pacing rate setting capabilities that respond to a multi-axis
`solid state sensor operable alongat least two orthogonal axes
`to distinguish the posture or positional attitude of the patient
`at rest and at levels of exercise.
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`5,593,431
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`5
`second sensitive axes generally into alignment with earth’s
`gravitational field; and determining the body posture of the
`patient through comparison of the magnitudes andpolarities
`of said derived first and second DC accelerometer signals
`with the magnitudes andpolarities of said first, second and
`third characteristic magnitudes and polarities.
`In accordance with the preferred embodiments of the
`invention,the stored posture andactivity levels may retained
`in a monitor and/or be employedto control the delivery of
`a variety of therapies,
`including pacing, cardioversion/
`defibrillation, other body stimulation therapies, and drug
`delivery therapies.
`In the context of a pacemaker, the method and apparatus
`of the invention for pacing a patient’s heart at a pacing rate
`dependent on patient activity and the physical posture of a
`patient’s body, having a superior-inferior body axis, an
`anterior-posterior body axis and a lateral-medial body axis,
`in relation to earth’s gravitational field, comprising the steps
`of and means for: measuring the constant acceleration of
`gravity on the patient’s body in at least two of the superior-
`inferior, anterior-posterior, and lateral-medial body axes
`with first and second solid state DC accelerometer means
`aligned thereto for providing first and second DC acceler-
`ometer signals therefrom having a characteristic magnitude
`and polarity on alignment with earth’s gravitational field and
`varying magnitude and polarity depending on the degree of
`mis-alignment of said first and second solid state DC accel-
`erometer means with earth’s gravitational field; determining
`a body position signal related to the posture of the patient
`through comparison of the magnitudes and polarities of the
`first and second DC accelerometer signals with said char-
`acteristic magnitudes and polarities; determining a patient
`activity signal from the frequency of body movements
`recurring over a time unit; deriving a rate control signal from
`the body position and patient activity signals correlated to
`the physiologic demand onthe patient’s heart in the deter-
`mined body posture and level of activity; defining physi-
`ologic escapeintervals as a functionofthe rate control signal
`to establish a physiologic pacing rate; generating pacing
`pulses at the physiologic pacing rate; and applying the
`pacing pulses to a chamber of a patient’s heart.
`Preferably,
`the posture of the patient
`is determined
`through the use of two or more solid state, DC accelerom-
`eters mounted in mutual orthogonal relationship within the
`pacemaker pulse generator case to derive two or moresets
`of signals dependent on the effect of gravity on the accel-
`erometers which can be compared to derive the posture of
`the patient while standing,sitting, or prostrate in a variety of
`positions. With three DC accelerometers mounted orthogo-
`nally, the patient’s body posture at rest may be derived and
`employed to set physiologic resting pacing rates appropriate
`to the patient in each of the possible positions.
`The orthogonally mounted, DC accelerometersare pref-
`erably mounted into an IC chip so that the three sensitive
`axes are aligned with the three positional axes of the pulse
`generator housing. The physician can implant and stabilize
`the pulse generator housing in the proper orientation to the
`patient’s thorax to align the sensitive axes with the superior-
`inferior (S-D, anterior-posterior (A-P), and laterai-medial
`(L-M)body axes ofthe chest region. Asa result, distinctive
`signal levels are developed by each DC accelerometer in
`each posture position due to the effect of gravity on the DC
`accelerometer sensitive axes, so that posture of the patient
`can be correlated to the combination of the signal values.
`One or more of the DC accelerometers can also be used
`to derive the level of patient activity from the number of
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`changesin signal levels exceeding a certain threshold occur-
`ring in a given sampling time period, as is conventional in
`use of the piezoelectric and AC accelerometer activity
`sensors described above.
`
`Advantages of the Invention
`
`The use of the mutually orthogonal DC accelerometers
`and signal processing circuits and/or algorithms to deter-
`mine the postureofthe patient eliminates the limitations of
`the single DC accelerometer and does not involve accep-
`tance of unusual materials and technology in an implantable
`device. The mutually orthogonal DC accelerometers and
`associated circuits can be easily incorporated into a pace-
`maker pulse generator or other medical device at low cost.
`The ease of use, and the reproducibility and consistency of
`results attained will lead to acceptability within the medical
`community.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`These and other objects, advantages and features of the
`present invention will be more readily understood from the
`following detailed description of the preferred embodiments
`thereof, when considered in conjunction with the drawings,
`in which like reference numerals indicate identical structures
`throughout the several views, and wherein:
`FIG. 1 is block level diagram of a DDDR pacemaker
`capable of implementing the mutually orthogonal DC accel-
`erometers of the present invention as activity and patient
`posture sensors;
`FIG.2 is a schematic illustration of the orientations of the
`S-I, L-M, and A-Psensitive axes of three DC accelerometers
`mounted orthogonally with respect to a hybrid circuit sub-
`strate mounted within the housing for the pulse generator of
`FIG.1 related to the markings on the housing for orienting
`the pulse generator with the patient body axes;
`FIG. 3 is an illustration of the implantation of the pulse
`generator of FIG. 2 in a patient’s body in substantial
`alignment with the S-I, L-M and A-P body axes;
`FIGS. 4a—4¢is a graphical depiction of the sensitive axis
`orientations and output signals of the three orthogonally
`mounted DC accelerometersin a pulse generator of FIG.2,
`implanted with the orientation shown in FIG. 2, when the
`patient is in a variety of positions;
`FIGS. 5a—5g, 6a-6g, and 7a—7¢ are graphical depictions
`of the sensitive axis orientations and output signals of three
`pairs of the three orthogonally mounted DC accelerometers
`in a pulse generator of FIG. 2, implanted with the orientation
`shownin FIG. 2, whenthepatientis in a variety of positions;
`FIG. 8 is a rate response overview flowchart of the
`algorithm incorporated into the pacemaker of FIG. 1 for
`deriving a physiologic pacing rate from the output signals of
`two or three DC accelerometers of FIG.2;
`FIG. 9 is a flowchart of a first embodiment of the
`algorithm for determining body position from the DC com-
`ponentsofthe outputsignals of all three of the DC accel-
`erometers of FIG. 2;
`FIGS. 10-12 are flowcharts of a first embodiment of the
`algorithm for determining body position from the DC com-
`ponents of the output signals of two of the three DC
`accelerometers of FIG. 2;
`FIG. 13 is a flowchart of a patient workup for deriving a
`posture confidence interval from the DC componentsof the
`output signals of any selected two orall three of the DC
`accelerometers of FIG.2;
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`5,593,431
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`7
`FIG. 14 is a flowchart of a second embodiment of the
`algorithm for determining body position from the DC com-
`ponents of the output signals ofall three of the DC accel-
`erometers of FIG. 2 employing the posture confidence
`intervals; and
`FIGS. 15 is a graph showing the DC accelerometer output
`signals obtained in different body positions.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`The present invention is preferably implementedin multi-
`programmable DDDRpacemakersof types widely known in
`the prior art. As described above with respect to other
`medical devices, the invention may also be implemented in
`other medical devices for providing other therapies and/or
`for monitoring physiologic parameters in the various body
`positions the patient may assume.
`FIG. 1 is block level diagram of such a pacemaker
`implantable pulse generator or IPG 30 and lead set 12 and
`14 which sets forth the structures required to incorporate the
`invention into a DDD/DDDRpacemaker.In the drawing,the
`patient’s heart 10 has an atrial pacing lead 12 passed into the
`right atrium and a ventricular lead 14 passed into the right
`ventricle. The atrial lead 12 hasan atrial electrode array 16
`which couples the pulse generator 30 to the atrium. The
`ventricular lead 14 has a ventricular electrode array 18 for
`coupling the pulse generator 30 to the ventricle of the
`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.
`
`The IPG circuit 30 of FIG. 1 is divided generally into a
`pacing circuit 32 coupled to a battery power supply 50, an
`activity sensor 60 of the type described below,a telemetry
`antenna 45 and a microcomputer circuit 34. The pacing
`circuit 32 includes the atrial and ventricular output amplifier
`circuit 36 and sense amplifiers 38 that are coupled to the
`atrial and ventricular leads 12 and 14, respectively,
`the
`digital controller/timercircuit 40 and other associated com-
`ponents described below. The output circuit 36 and sense
`amplifier circuit 38 may contain atrial and ventricular pulse
`generators and sense amplifiers corresponding to any of
`those presently employed in commercially marketed dual
`chamber cardiac pacemakers.
`Sensed atrial depolarizations (A-SENSE)or P-wavesthat
`are confirmed by the atrial sense amplifier are communi-
`cated to the digital controller/timer circuit 40 on the ASE
`line. Similarly, ventricular depolarizations (V-SENSE) or
`R-wavesthat are confirmed by the ventricular sense ampli-
`fier are communicated to the digital controller/timer circuit
`40 on VSE.The sensitivity control block 42 adjusts sensi-
`tivity of each sense amplifier in response to control signals
`provided by digital controller/timer 40 that are in turn stored
`in memory in microcontroller circuit 34.
`In order to trigger generation of a ventricular pacing or
`VPE pulse, digital controller/timer circuit 40 generates a
`trigger signal on the