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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an acceleration transducer. More
particularly, the present invention relates to an electromagnetic
accelerometer. Still more particularly, the present invention relates to
an electromagnetic accelerometer for use in implantable medical devices.
2. Description of the Related Art
Acceleration transducers, commonly refer to as accelerometers, generate an
electrical output signal corresponding to an acceleration experienced by
the transducer caused by an externally applied force. Typically, the
magnitude of the voltage of the electrical output signal is portional to
the acceleration. Thus, by monitoring the magnitude of the voltage of the
transducer's output signal, a measure of the acceleration experienced by
the transducer can be determined.
Accelerometers are used in a variety of applications such as in some
implantable medical devices including pacemakers and defibrillators
(collectivity referred hereafter simply as pacemakers). Pacemakers use
accelerometers for several reasons. First, an accelerometer may be
included as a body activity sensor within the pacemaker's housing or "can"
which encloses and seals the electronic circuitry of the pacemaker. An
accelerometer-based body activity sensor provides an output signal that is
proportional to the overall motion of the patient's body. The output
signal from such a body activity sensor can be processed to distinguish a
sleeping patient, for example, from a patient engaging in strenuous
exercise with a high level of body motion. Rate-responsive pacemakers
increase or decrease the rate of pacing (i.e., the rate at which the
pacemaker emits electrical pulses to force a chamber of the heart to beat)
in response to the measured index. A body activity accelerometer can be
used to provide a control signal for a rate-responsive pacemaker keyed to
body motion. Using a body activity accelerometer, a rate-responsive
pacemaker can determine when the patient is engaging in strenuous
exercise, and accordingly, increase the rate of pacing to meet the
increased metabolic load of the patient during exercise. By contrast, the
same rate-responsive pacemaker reduces the rate of pacing when the patient
exhibits little motion such as during sleep when the patient's heart
preferably beats less often. Examples of accelerometers used as body
activity sensors are disclosed in U.S. Pat. Nos. 5,014,700, 5,031,615, and
5,044,366.
Accelerometers also are used for other purposes in pacemaker systems.
Several attempts have been made at incorporating an accelerometer in a
pacemaker lead which couples the pacemaker to the heart. A pacemaker lead
typically is a thin flexible cable including one or more electrical
conductors. One end of the lead couples to a header plug on the pacemaker
can and the other end of the lead includes one or more conductive
electrodes. A lead-based accelerometer can be used to measure the
acceleration of the wall of the heart to which the lead is coupled. It is
also known that lead accelerometers can be used to determine various
physiological parameters such as contractility and ejection fraction.
Further, an accelerometer incorporated into a lead can also be used to
determine if the chamber of the heart in which an electrode is implanted
has contracted in response to a pacing pulse generated by pacemaker. This
determination is referred to as "capture verification."
U.S. Pat. No. 5,304,208 discloses a lead-based accelerometer coupled to an
electronic pre-amplification network. This accelerometer, however,
requires an extra pair of conductors to be included within the pacemaker
lead to couple the accelerometer and associated electronics to the
pacemaker can. It is generally recognized that there is a risk that once
implanted a conductor within a lead may break disrupting the operation of
the pacemaker and requiring surgical repair. Such surgery obviously
increases risk and discomfort of the patient. Further, the risk of
conductor breakage increases as the number of conductors in a lead
increases. Reducing the number of conductors in a cardiac lead thus is
highly desirable to improve the reliability of a pacemaker system.
Moreover, the risk of conductor breakage in a lead is reduced if fewer
conductors are included in a lead.
Many accelerometers, such as the accelerometer of U.S. Pat. No. 5,304,208,
require electrical power for their operation. Implantable pacemakers and
defibrillators typically operate from batteries housed within the can of
the pacemaker. Because batteries store only a finite amount of electrical
energy, it is highly desirable for pacemakers to use as little power as
possible. Thus, pacemakers are designed for minimum power consumption.
Accelerometers, such as that used in U.S. Pat. No. 5,304,208, that require
electrical power for their operation are not desirable for this reason.
The output signal from an accelerometer typically requires amplification
and filtering to condition the accelerometer's output signal for use by
the pacemaker. Amplification and filtering circuitry requires electrical
power to operate, thereby imposing an additional power drain on the
pacemaker's batteries. Thus, it is desirable for transducers associated
with a pacemaker, such as an accelerometer, to produce an electrical
output signal that requires as little amplification and/or filtering as
possible.
Accordingly, an accelerometer is needed, especially for use in implantable
medical devices, that solves the shortcomings discussed above. Such an
accelerometer should be small enough to be included in a typical cardiac
lead coupling a pacemaker to the heart for capture verification or for
determining various hemodynamic parameters. If incorporated into a lead,
such an accelerometer preferably should require a minimal number of
conductors in the lead. Further, such an accelerometer should require
little, if any, operational power, thereby causing little or no drain on
the pacemaker's battery. Battery drain could be further minimized if the
accelerometer generates an output signal requiring little amplification
and filtering.
SUMMARY OF THE INVENTION
Accordingly, there is herein provided an electromagnetic accelerometer that
includes a rigid shell with a cavity in which two magnets are fixed at
either end of the rigid shell and a third magnet is allowed to oscillate
(reciprocate) between the fixed magnets. The three magnets are arranged so
that, when the accelerometer is not being accelerated, the movable magnet
is held suspended between the fixed magnets by magnetic forces from the
fixed magnets. A coil of wire surrounds the shell and magnets. A change in
velocity of the accelerometer causes a displacement of the movable magnet
within the cavity. As a result, an electrical current is induced in the
coil of wire. The voltage in the coil of wire is proportional to the
acceleration experienced by the accelerometer. The electromagnetic
accelerometer is particular useful in implantable pacemaker or
defibrillator and can be included in either a lead or the housing of the
pacemaker. Further, the accelerator generates its own voltage in response
to acceleration and the voltage can be used as a power source.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the invention will become apparent upon
reading the following detailed description and upon reference to the
accompany drawings, wherein:
FIG. 1 is a cross-sectional view of the accelerometer in accordance with a
preferred embodiment;
FIG. 2 is a perspective view of a pacemaker and lead which incorporates the
accelerometer of FIG. 1;
FIG. 3 is a schematic wiring diagram of a pacemaker lead including an
accelerometer in accordance with a preferred embodiment;
FIG. 4a is a cross-sectional view of the accelerometer of FIG. 1
incorporated into a pacemaker lead with an adapter for receiving a
steering stylette;
FIG. 4b is an alternative cross-sectional view of the accelerometer with a
hollow longitudinal portion for receiving a steering stylette;
FIG. 5 is a block diagram of the electronics of a pacemaker constructed in
accordance with the preferred embodiment for processing acceleration
signals from two accelerometers and using the acceleration signal from one
accelerometer as a body motion compensator and power source; and
FIG. 6 is a method for calibrating the accelerometers of FIG. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, an electromagnetic accelerometer 100 constructed
in accordance with a preferred embodiment generally includes a shell or
housing 102 and a cavity 104 in which three magnets 106, 110, and 114 are
housed. Magnets 106, 110, and 114 are arranged colinearly as shown. A coil
of conductive wire is disposed about cavity 104. The shell 102 preferably
is constructed of a rigid, hermetically-sealed material, such as ceramic,
and is appropriately sized to fit either within an implantable lead or a
pacemaker can. Coil 116 may impregnate shell 102. Alternatively, shell 102
may include a space between the shell and the cavity through which the
coil is wound.
The coil of wire 116 preferably is constructed of fine insulation-coated
conductive wire axially wound about cavity 104 in the matter shown in FIG.
1. Conductors 107 and 108 provide the output signal from accelerometer 100
and couple directly to opposite ends of the coil of wire 116. Coil 116
preferably includes two, spaced-apart coil sections 116a and 116b
connected by conductor 109. Coil section 116a is wrapped about cavity 104
generally near the adjacent ends 106a and 110a of magnets 106 and 110,
respectively. Coil section 116b is wrapped about cavity 104 generally near
adjacent ends 110b and 114a of magnets 110 and 114, respectively. The
placement of the coil sections 116a and 116b and conductor 109, the size
of the wire comprising the coil, the material from which the coil is made,
and the number of winds of the coil is determined to maximize the output
voltage signal of the accelerometer 100 as will be discussed below in more
detail. Instead of a coil of wire, coil 116 may be constructed from
electrodeposition of conductive traces on an insulating substrate in a
coil-like arrangement.
Referring still to FIG. 1, magnets 106 and 114 are fixed within cavity 104
at opposite ends of the cavity. Fixed magnets 106, 114 are bonded to
opposite ends of cavity 104 using epoxy or suitable other adhesive. Magnet
110, however, is not fixed to cavity 104 and thus is movable and allowed
to reciprocate or oscillate within the cavity 104 in response to
externally applied forces on the accelerometer 100. Cavity 104 is lined
with friction-reducing material, such as Teflon or other fluoropolymer.
Fixed magnets 106, 114 and movable magnet 110 preferably are manufactured
from sintered rare earth ceramic, or similar materials.
As is commonly known, a magnet has a south pole and a north pole. The south
and north poles of magnets 106, 110, 114 are shown in FIG. 1 with S
denoting the south pole and N denoting the north pole of each magnet. The
magnets 106, 110, 114 are arranged within cavity 104 so that like poles
coincide between adjacent magnets. Thus, the north pole of fixed magnet
106 is adjacent the north pole of movable magnet 110 and the south pole of
fixed magnet 114 is adjacent the south pole of movable magnet 110.
Alternatively, the poles of the magnets could be reversed so that the
south poles of magnets 106 and 110 are adjacent each other and the north
poles of magnets 110 and 114 are adjacent. As commonly understood, like
poles between two magnets create opposing magnetic forces exerted on the
magnets. By contrast opposite poles attract. Therefore, because the north
pole of movable magnet 110 is adjacent the north pole of fixed magnet 106,
a force results tending to push movable magnet 110 away from fixed magnet
106, and towards fixed magnet 114. Similarly, because the south pole of
movable magnet 110 is adjacent the south pole of fixed magnet 114, another
force results tending to push movable magnet 110 away from fixed magnet
114 and towards fixed magnet 106. If the two opposing forces on movable
magnet 110, created from the magnetic interaction between fixed magnets
106, 114 and movable magnet 110, are equal in magnitude (and opposite in
direction) moving magnet 110 will be forced to the center position in
cavity 104 as shown in FIG. 1. Thus, the movable magnet is held stationary
within cavity 104 by fixed magnets 106, 114 when the accelerometer is not
subjected to acceleration forces.
It is also known that a changing magnetic field in close proximity to a
coil of wire will induce an electrical current in the coil of wire. A
static magnetic field, that is, a magnet magnetic field whose amplitude
and direction do not change, does not induce an electrical current in a
coil of wire. Thus, with movable magnet 110 stationary and centered in the
cavity 104 as shown in FIG. 1, no electrical current is induced in coil of
wire 116, and thus the accelerometer produces a zero voltage output signal
on conductors 107 and 108.
Referring still to FIG. 1, if a force is applied to one end or the other of
accelerometer 100 along longitudinal axis 119, the accelerometer 100
experiences an acceleration in the direction of the externally applied
force. The inertia of moving magnet 110, however, causes it to resist the
acceleration experienced by accelerometer 100, resulting in movement of
movable magnet 110 within cavity 104. Thus, if a force F.sub.1 is applied
to the left side 103 of the accelerometer 100, accelerometer 100 is forced
to the right in response to the applied force. Movable magnet 110,
however, resists the motion of accelerometer 100 thus creating a relative
motion for movable magnet 110 to the left, i.e. in the opposite direction
of the applied force F.sub.1. Similarly, accelerometer 100 will move to
the left within the cavity if a force F.sub.2 is applied to the right side
109 of accelerometer 100.
Movement of movable magnet 110 within coil of wire 116 causes a changing
magnetic field which induces an electrical current in coil of wire 116.
Further, the amount of electrical current induced in coil 116 is
proportional to the magnitude of the acceleration experienced by
accelerometer 100. The voltage across conductors 107 and 108 thus is
proportional to the acceleration experienced by accelerometer 100. The
opposing magnetic forces caused by fixed magnets 106, 114 force movable
magnet to its center position after the externally applied force is
removed and the acceleration ceases. The magnetic forces caused by fixed
magnets 106, 114 should be large enough so that movable magnet 110 does
not contact either fixed magnet while the accelerometer 100 experiences
acceleration.
It is preferred that two coil sections 116a, 116b are used to wrap cavity
104, rather than one continuous coil of wire from one end of the
accelerometer to the other. If one continuous coil of wire were used, the
magnetic flux lines (an exemplary flux line is shown as line 117) would
cut through opposite ends of the coil in opposite directions generating
oppositely directed currents in either end of the coil, thereby generating
a net current of zero amperes. Coil section 116a preferably is wound in an
opposite direction around cavity 104 from coil wire 116b. Winding the two
coils section in opposite directions allows the electrical current induced
in one coil section to contribute to the electrical current induced in the
other coil section, and not cancel the electrical current induced in the
other coil of wire as would be the case if the two coil of wire were wound
in the same direction. Alternatively, the coil sections 116a, 116b may be
wound in the same direction in such a way, as known to those of ordinary
skill, to allow the currents induced in both coil sections to add together
rather than cancel each other.
It is important for creating a larger output signal (i.e., voltage), as is
desirable, that moving magnet 110 be allowed to move as freely as possible
within cavity 104. To allow moving magnet 110 to move freely, it may be
necessary to remove all of the air within cavity 104. That is, a vacuum
would be drawn and the cavity sealed with the air pressure in the cavity
preferably less than 0.1 atmosphere. Alternatively, an axial slot may be
made in can 102 along one side of the cavity to provide an air path around
the moving magnet.
In operation the electromagnetic accelerometer 100 of FIG. 1 preferably
produces a zero output voltage when no external force is applied to the
accelerometer 100. When accelerometer 100 experiences an acceleration
resulting from an external force having a component along longitudinal
axis 119, the output voltage on conductors 107 and 108 becomes positive
(or negative depending on the direction of the applied force resulting in
the acceleration). If an oscillatory force is applied to accelerometer
100, moving the accelerometer back and forth along longitudinal axis 119,
the output voltage on conductors 107 and 108 will oscillate between
positive and negative voltage values. In this case, the acceleration can
be determined by measuring the peak-to-peak voltage across conductors 107
and 108.
The magnitude of the output voltage is proportional to the magnitude of the
magnetic field of moving magnet 110. Thus, if higher voltage output
signals are desired, the strength of the magnetic field associated with
magnet 110 should be increased. The magnetic field of magnet 110 generally
can be increased by increasing the mass of the magnet (i.e., making the
magnet 110 larger) or any other technique known to those of ordinary
skill. A larger moving magnet may force the overall size of electromagnet
accelerometer 100 to be increased.
Additionally, the accelerometer's output voltage can be increased by
including more than one coil of wire around cavity 104. Two or more coils
can be coaxially wrapped with one coil wrapped abut the other coil around
the cavity with the current induced in each coil contributing to the
currents induced in the other coil(s).
It should also be recognized that the electromagnetic accelerometer of the
preferred embodiment is a single axis device meaning it generates maximum
output signals for accelerations along longitudinal axis 119.
Nevertheless, the accelerometer will still produce an output signal,
albeit at a lower voltage level, for accelerations at angles (other than
90.degree.) to axis 119. Two or three accelerometers mounted at 90.degree.
angles to each other can be used to provide two or three axis
accelerometer devices.
The electromagnetic accelerometer of the preferred embodiment can be used
advantageously in an implantable medical device, such as a pacemaker or
defibrillator (both referred to hereafter as a pacemaker). Referring now
to FIG. 2, a pacemaker 50 includes a lead 60 in which one end 62 of the
lead couples to pacemaker 50 (referred to as the "proximal" end) and the
other end 63 (the "distal" end) couples to the patient's heart 40. The
distal end of the lead coupled to the patient's heart includes one or more
electrodes. The lead 60 in FIG. 2 includes a tip or cathode electrode 70
and ring or anode electrode 80. Electrodes 70 and 80 couple to pacemaker
50 via conductors (not shown) within lead 60. An electromagnetic
accelerometer 100 preferably is disposed within lead 60 between electrodes
70 and 80, rigidly affixed to electrode 70, or with electrode 70 being a
portion of the outer surface of accelerometer 100.
The mass of the suspended magnet 110 and the spring like action of magnets
106 and 114 together with the damping from sliding, or an functional
forces or magnetic braking of coil 116 forms a mechanical filter which
shapes the frequency response of the acceleration signals developed. The
design is chosen preferably to enhance motions in the frequency range 3-30
Hz.
In accordance with standard implantation techniques, tip electrode 70 is
inserted into the myocardium of a chamber of the heart. As the wall of the
heart in which tip electrode 70 is inserted moves, such as when that
chamber of the heart contracts, accelerometer 100 senses the heart wall
acceleration and generates an output voltage proportional to the
acceleration of the chamber wall. The output voltage from accelerometer
100 is transmitted through lead 60 to pacemaker 50 which measures and
processes the voltage output by accelerometer 100. Pacemaker 50 may use
the accelerometer's output voltage to determine capture verification,
measure various physiological parameters, and the like.
Referring now to FIG. 3, a schematic wiring diagram of lead 60 is shown
including the anode 80, electromagnetic accelerometer 100, and cathode 70.
Conductor 61 couples anode 80 to the pacemaker through lead 60. Similarly,
conductor 63 couples cathode 70 to the pacemaker. By connecting conductor
107 from accelerometer 100 to conductor 61, only one extra conductor 108
is required in lead 60. The accelerometer 100 constructed in accordance
with a preferred embodiment thus advantageously shares one of the existing
conductors in the lead, and that requires only one extra conductor in lead
60. Moreover, in requiring only one extra conductor in a cardiac lead, the
increased risk of conductor breakage within a lead is kept to a minimum.
Referring again to FIG. 2, a steering stylette 67 is axially disposed
through lead body 60. Steering stylettes are commonly used when implanting
leads to provide the surgeon the ability to steer the distal end of the
lead through an appropriate path from the pacemaker to the chamber of the
heart in which the electrode in the distal end of the lead is to be
implanted. Once implanted, the steering stylette 67 is removed and the
surgeon then connects lead 60 to pacemaker 50. Accordingly, it may be
preferred to include an adapter within the accelerometer to provide a
mating surface against which the stylette contacts. Referring now to FIG.
4a, accelerometer 100 is shown provided in a cardiac lead 60. Stylette
adapter 130 preferably is included as part of accelerometer 100. Stylette
adapter 130 includes a contact surface 132 having an opening 131 for
receiving the distal end 68 of stylette 67. Once the lead is properly
implanted in the appropriate cardiac tissue, the surgeon removes the
stylette, and the accelerometer 100 and stylette adapter 130 remain in the
lead 60.
It may also be preferred for the outer surface of shell 102 (FIG. 1) of the
accelerometer to include a conductive portion thereby allowing the outer
surface of shell 102 to serve as one of the electrodes in the lead. For
example, the outer surface of shell 102 may be used as the anode electrode
80, thereby physically combining the anode and the accelerometer and
reducing the complexity of the cardiac lead. In this example, shell 102
would preferably be made of titanium or IROX-coated titanium.
Referring to FIG. 4b, it may be preferred for the accelerometer 100 to
include a hollow longitudinal center portion 129 for receiving the
steering stylette 67. As such, the steering stylette may thus be pushed
through the accelerometer 100 and couple to a tip electrode. In this
embodiment, the magnets 106, 110, 114 are toroidal or do-nut shaped to
allow for the stylette to pass through the center whole of the magnets. An
inner rigid tube is disposed between the stylette and the inner surface of
the magnet's holes to prevent the stylette from interfering with the
magnets.
Providing an accelerometer in accordance with the preferred embodiment in
the distal end of the lead 60 is useful for capture verification, and
determining various hemodynamic parameters. Alternatively, or
additionally, an accelerometer of the present invention can be included
within the can of the pacemaker, thereby providing, for example, a body
activity sensor. It may be desired, in fact, to include both an
accelerometer in the lead and another accelerometer in the can.
Referring now to FIG. 5, a block diagram 400 of a pacemaker processes
signals from two accelerometers 200 and 300. Accelerometer 200 preferably
is included in a cardiac lead and accelerometer 300 preferably is included
within the pacemaker's can. Accelerometers 200, 300 are identical to
accelerometer 100 previously described with reference to FIG. 1. The block
diagram 400 generally includes a filter and peak-to-peak detector 402, 406
for each accelerometer 200, 300, a processor 410, amplifiers 412 and 420,
power conditioner 430, and power storage device 435. Filter and
peak-to-peak detectors 402, 406 provide low pass filtering and determine
the peak-to-peak voltage range from the output signals from each
accelerometer in accordance with the known techniques. Each filter and
peak-to-peak detector 402, 406 includes an analog-to-digital converter
403, 407. The output of each analog-to-digital converter 403, 407 is
provided to processor 410. The analog output of filter and peak-to-peak
detector 402 is provided on line 418 to amplifier 420. The analog output
of filter and peak-to-peak detector 406 is provided on line 409 to
amplifier 412. The output signal from amplifier 412 is provided as an
input signal to amplifier 420. Amplifier 420 amplifies the difference
between the signals on lines 418 and 419 and provides an output signal on
line 421.
The analog-to-digital converters 403, 407 digitize the analog output
signals from filter and peak-to-peak detectors 402, 406 and provide the
digitized data on digital lines 404, 408. Processor 410 interprets the
digital values on digital lines 404 and 408 from analog to digital
converters 403, 407 and, in response, controls the gain of amplifier 412
via lines 411 in accordance known techniques.
The pacemaker 400 of FIG. 5 is particularly useful for avoiding the
detection of false positive signals in which a pacemaker confuses an
acceleration signal from a lead-based accelerometer as a heart wall
acceleration (i.e., contracting chamber) when the signal, infact, was the
result of general body activity and not a contracting heart chamber. It
should be apparent that both lead accelerometer 200 and can accelerometer
300 will experience motion of the patient's body. Thus, general body
activity causes an acceleration that is common to both accelerometers.
Accordingly, amplifier 420 amplifies only the difference between the
output signals from accelerometers 200 and 300, thereby canceling the
common body activity acceleration experienced by both accelerometers 200,
300. The output signal from amplifier 420 on line 421 thus represents the
acceleration experienced by lead accelerometer 200 resulting only from
heart wall accelerations and not general body activity.
It should be recognized that the output voltage from lead accelerometer 200
and can accelerometer 300 will have different magnitudes for a variety of
reasons. First, each accelerometer only detects the acceleration along its
longitudinal axis 119 shown in FIG. 1. In general, lead accelerometer 200
and can accelerometer 300 will be included in the patient's body in
different orientations and thus, even if the two accelerometers were
identical, the magnitude of their output signals will be different.
Further, it may be desired to use the can accelerometer 300 as a power
source, as will be explained in more detail below. If desired as a power
source, the can accelerometer 300 may be constructed so that it's output
voltage and current are larger than the output voltage from lead | | |
