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FIELD OF OUR INVENTION
Our invention relates to cardiac pacemakers and other cardiac stimulators
which monitor the operation of the heart and stimulate the heart tissue as
required to maintain the proper operation of the heart, including
implantable cardioverters and defibrillators. In particular, our invention
relates to an implantable cardiac stimulating system with the capability
of alerting or warning a patient of certain conditions or situations,
including, without limitation, battery depletion, lead malfunction, or the
eminent delivery of therapy.
BACKGROUND OF OUR INVENTION
It has long been known that the heart muscle provides its pumping function
in response to electrical events which occur within the atrium and
ventricle of the heart. Conductive tissue connects the atrium and the
ventricle and provides a path for electrical signals between the two
areas. In a normal heart, a natural atrial event spontaneously occurs in
the atrium and a corresponding ventricular event occurs later in the
ventricle. Synchronized electrical events occurring naturally in the
atrium and ventricle cause the heart muscle to rhythmically expand and
contract and thereby pump blood throughout the body.
In a diseased heart, atrial and ventricular events may not naturally occur
in the required synchronized manner and the pumping action of the heart is
therefore irregular and ineffective to provide the required circulation of
blood. The required synchronized activity of such diseased hearts can be
maintained by any implanted cardiac pacemaker which applies synchronized
stimulating pulses to either the atrium or ventricle or both.
A diseased heart may also beat unusually quickly, a condition known as
tachycardia, or may lapse into a rapid, disorganized quivering known as
fibrillation. The former condition is undesirable; the latter condition
may be fatal. To correct these conditions, implantable cardioverters and
defibrillators have been proposed. Like the related cardiac pacemaker,
these devices monitor the electrical condition of the heart and provide a
corrective electrical therapy to correct the improper heart function. The
three functions of pacing, cardioverting and defibrillating, or any of
them, may be incorporated into a single device, generically, an
implantable cardiac stimulator.
Cardiac stimulators am battery powered and, consequently, have a finite
life before battery depletion may be expected. In addition to the battery,
other components of the cardiac stimulation system may fail, such as
leads, electrodes, or other system components. As an example of another
type of change, the sensitivity of a patient's heart to electrical
stimulation may change over time, altering the so-called threshold level
for electrical stimulation. Such change of condition requires adaptation
of the therapy delivered by the implantable cardiac stimulator, either
automatically or by intervention by the attending physician. In any of
these situations, or others, it may be deemed desirable to alert the
patient to a changed condition so that action may be taken. For example, a
pacemaker may detect the approaching end of life of its battery, in a
known manner. It is desirable to alert the patient to this condition.
Moreover, in the case of implantable defibrillators, delivery of therapy
can be traumatic. It is sometimes deemed important to alert the patient to
the prospect of eminent delivery of therapy.
Cardiac stimulators which alert or warn the patient of such conditions are
known in the art. For example, such a device is described by Dutcher, et
al. in U.S. Pat. No. 4,140,131. In the device described by Dutcher, et
al., a device-controlled switch is activated to enable a specialized
electrode adjacent the pacemaker to stimulate the patient's muscles to
twitch. The nature of the electrode is not described in detail, but
Ferek-Petric, in U.S. Pat. No. 5,076,272, described the electrode of
Dutcher, et al., as an auxiliary electrode surrounded by the indifferent
electrode and fixed on the pacemaker can. In contrast, Ferek-Petric
describes a cardiac stimulator with patient warning with an electrode
affixed to the header of the stimulator. Another electrode is described in
U.S. patent application Ser. No. 08/426,949, filed Apr. 21, 1995, by some
of us (Paul and Prutchi), also assigned to Intermedics, Inc.
SUMMARY OF OUR INVENTION
We have found that an elongated electrode mounted near the pacemaker can is
effective in providing reliable stimulation for warning. The electrode has
an extended length and a short width. The length is preferably at least
double the width and more preferably at least four times the width. This
extended length increases the probability that contact with the
surrounding tissue will be achieved. The short width, on the other hand,
increases the probability that a high enough current density will be
achieved, causing stimulation to occur. Moreover, an edge profile further
concentrates the current. An "edge" may be produced by, for example,
either a raised area or bump or by a conjunction of two surfaces, as at a
corner. The electrode may be mounted directly on the can or header of the
cardiac stimulator or may be part of a separate pin electrode, as
described in our U.S. patent application Ser. No. 08/532,961, now U.S.
Pat. No. 5,549,653, and in a continuation-in-part application filed on the
same date as this application. As described in the last two mentioned
applications, a pin electrode can be mounted in one of two or more
standard connector sockets in the header of a dual chamber pacemaker or
multi-function cardiac stimulator to provide the necessary stimulus to the
skeletal muscles of the patient to produce an effective twitch. A hood
configuration can be used to surround at least part of the header,
reducing rotation of the pin electrode. By making only a portion of the
surface of the pin electrode conductive, and particularly edges or
corners, an elongated electrode, as described herein, is formed which
produces a higher electric current density and more efficient stimulation.
It is an object of our invention, therefore, to provide an elongated
electrode for use in a patient warning system in an implantable medical
device. It is also an object to provide means whereby a cardiac
stimulator, capable of being programmed, may be modified to include a
patient warning apparatus. It is a further object of our invention to
provide an auxiliary electrode for the purpose of providing patient
warning signals by stimulating excitable tissue of the patient, for
example, nerve ends or voluntary muscles. It is a further object of our
invention to provide for an effective implantable cardiac stimulation
system with a reliable patient warning apparatus. With the foregoing in
mind, we will now describe the preferred embodiment of our invention with
respect to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective drawing of a dual chamber cardiac stimulator and
warning electrode according to our invention.
FIG. 2 is a perspective drawing of a warning electrode according to our
invention.
FIG. 3 is a block diagram of the cardiac stimulator of FIG. 1.
FIG. 4 is a perspective drawing of an alternative embodiment of the cardiac
stimulator of FIG. 1.
FIG. 5 is a perspective drawing of an alternative embodiment of the cardiac
stimulator with a pin-type warning electrode.
FIG. 6 is an exploded perspective view of a pin-type warning electrode.
FIG. 7 is a perspective view of the pin electrode of FIG. 6 with conductive
corners.
FIG. 8 is a perspective view of the electrode of FIG. 6 with conductive
corner and edge.
FIG. 9 is a perspective view of the pin electrode of FIG. 6 with partially
exposed corners.
DETAILED DESCRIPTION OF OUR PREFERRED EMBODIMENT
FIG. 1 is a perspective drawing illustrating a cardiac stimulator,
generally designated 10, according to our invention. We have illustrated
our invention in connection with a dual chamber pacemaker, but our
invention is equally applicable with other implantable cardiac stimulators
such as cardioverters and defibrillators, as are known in the art. The
cardiac stimulator 10 comprises a hermetically sealed case or can 12
which, in a known fashion, contains batteries and electrical circuitry. A
header 14, attached to the can 12, has two sockets 16, 18 to which leads
can be mechanically and electrically connected. Leads are commonly used to
place the cardiac stimulator 10 in electrical communication with the heart
or other body tissues. Electrical conductors 22, 24 provide an electrical
connection between the sockets 16, 18 and the circuitry inside the can 12
through a feedthrough 20.
A warning electrode 26 is mounted on the cardiac stimulator 10, preferably
on the header 14. An electrical conductor 28 connects the warning
electrode 26 to the circuitry inside the can 12 through the feedthrough
20. A preferred embodiment of the warning electrode 26 is more
particularly illustrated in FIG. 2. As shown in FIG. 2, the warning
electrode comprises at least one stimulating surface 30, and may comprise
a plurality of such surfaces, such as the illustrated surfaces 30, 32.
Each stimulating surface 30 has a length 34 and a width 36 orthogonal to
said length 34. The length may be curved as shown, which we believe is
advantageous because it tends to produce a point or small area of contact
between the surface 30 and the surrounding tissue. The length may also,
however, be made linear. Similarly, the width may be linear or curved. It
is important, however, that the length be greater than the width,
preferably at least twice the width, and more preferably at least four
times the width. The length makes it likely that the stimulating surface
will contact adjacent tissue when the cardiac stimulator 10 is implanted
in the body of a patient. The relatively narrow width and relatively sharp
profile, however, makes it likely that only a limited portion of the
stimulating surface will contact adjacent tissue, thereby minimizing the
actual conducting surface area and increasing the current density. The
rounded or relatively sharp profile of the width forms an "edge", which
also appears to concentrate the current density. Increased current density
makes it more likely that effective stimulation will occur and that the
patient will perceive the warning signal.
The stimulating surface 30 may also curve around the cardiac stimulator 10
from a front side 38 across an edge 40 to a back side 42 of the stimulator
10. Where more than one stimulating surface 30, 32 are provided, the
length of each stimulating surface 30, 32 is preferably parallel to the
lengths of the other stimulating surfaces. In our preferred embodiment,
the two stimulating surfaces 30, 32 are connected by a saddle 44. With the
two stimulating surfaces connected mechanically and electrically, it is
easier to mount the warning electrode 26 on the cardiac stimulator, more
specifically in the header. The warning electrode 26 may be pre-cast into
the header, or the header may be formed by casting epoxy or another
suitable material around the warning electrode and the sockets 16, 18. The
header is usually non-conductive and preferably covers the saddle 44,
leaving only the stimulating surfaces 30, 32 exposed.
In the can 12 of the cardiac stimulator 10, a microprocessor 46 preferably
provides control and computational facilities. It will be appreciated that
other forms of circuitry, such as analog or discrete digital circuitry,
can be used in place of the microprocessor 46. However, a microprocessor
is preferred for its miniature size and flexibility, both of which are of
critical importance for the implantable systems in which it is envisioned
our invention will find use. A particularly energy efficient
microprocessor which is designed specifically for use in pacemakers is
fully described in Gordon, et al, U.S. Pat. No. 4,404,972, which is
assigned to the assignee of our invention. The disclosure thereof is
incorporated herein by reference.
The microprocessor 46 has input/output ports connected in a conventional
manner via a bi-directional bus 48 to memory 50, and interval timers 52,
54. Memory 50 preferably includes both ROM and RAM. The microprocessor 46
may also contain additional ROM and RAM as described in Gordon, et al.,
above. Generally, the pacemaker operating routine is stored in ROM or
EPROM memory. RAM stores various programmable parameters and variables
used in conjunction with the pacemaker operation. The interval timers 52,
54 may be external to the microprocessor 46, as illustrated, or internal
thereto, as described in Gordon, et al., above. The timers 52, 54 are
conventional up or down counters of a type initially loaded with count
value and count up to or down from the value and output a roll-over bit on
completing the programmed count. If the stimulator is used as a dual
chamber pacemaker, the interval timers would be used to time AV and VA
intervals. If the stimulator is used as a single chamber pacemaker, a
timer would be used to time an A/A or VN interval, depending on the
chamber of the heart being sensed and paced.
The microprocessor 46 preferably has an input/output port connected to a
telemetry interface 56. The implanted cardiac stimulator 10 is thus able
to receive pacing, rate control, or other parameters from an external
programmer through an antenna 58 and to send data to an external receiver
if desired. Many suitable telemetry systems are known to those skilled in
the art. One such system and coding arrangement is described in Calfee, et
al. U.S. Pat. No. 4,539,992 which is also assigned to the assignee of our
invention. That description is incorporated therein by reference.
Microprocessor output ports are connected to the input of a stimulus pulse
generator 60, used to stimulate the atrium, by a control line 62.
Similarly, a stimulus generator 64, ordinarily used to stimulate the
ventricle, is connected to the microprocessor by a control line 66. The
microprocessor 46 transmits pulse parameter data, such as pulse amplitude
and width, as well as enable/disable and pulse initiation codes to the
stimulus generators 60, 64 along their control lines 62, 66 respectively.
The stimulus generators 60, 64 are connected to the heart 68 by leads 70,
72 with electrodes 74, 76 and will, under the control of the
microprocessor, stimulate the atrium or the ventricle as determined by the
pacemaker programming. Another stimulus generator 78, on the other hand,
is controlled by control line and is connected to excitable tissue 82 by
the conductor 28 and the warning electrode 26. The excitable tissue could
be skeletal muscle, a nerve ending, or other tissue capable of a
perceptible physiologic reaction in response to electrical stimulation.
The electrical condition of the heart must also be sensed and that
condition must be transmitted to the microprocessor 46. For this purpose,
ventricular and atrial sense amplifiers 84, 86 are connected between the
leads 72, 70 and the microprocessor 28. The ventricular sense amplifier 84
detects occurrences of R waves. An atrial sense amplifier 86 detects the
occurrence of P waves.
An alternative embodiment of our invention is shown in FIG. 4. In this
embodiment, the warning electrode 26 does not pass over the header 14 as
shown in FIG. 1, but rather extends along the header, substantially
parallel to the sockets 16, 18. As above, the electrode can extend from
the front side 38 of the cardiac stimulator 10, across the edge 40 and to
the back side 42.
Our invention may also be configured as a pin electrode, of a form more
particularly described in U.S. patent application Ser. No. 08/532,961 and
the continuation-in-part thereof, attorney docket number ITM-310-CIP, the
disclosure of which is incorporated herein by reference. A pin electrode
90 utilizing our invention is illustrated affixed to the cardiac
stimulator 10 in FIG. 5 and in exploded perspective view in FIG. 6. The
pin electrode 90 comprises a cylindrical metal shaft 92 which is
configured to make electrical contact with connections inside the socket
16. An insulating sheath 94 surrounds the shaft 92. The sheath 94
preferably has circumferential ridges 96 which help exclude body fluids
from the socket 16. A warning electrode 98 connects to the shaft. In use,
the shaft 92 and sheath 94 are inserted into the socket 16 of the
stimulator 10 such that the warning electrode 98 lies against the header
14. This prevents the pin electrode 90 from rotating in the socket 16 and
helps to avoid inadvertent disassembly of the pin electrode 90 from the
socket 16.
The warning electrode 98 comprises a front face 100 which lies generally
perpendicular to an axis of the shaft 92. The shaft 92 may protrude into
the face 100 and be welded thereon. In our preferred embodiment, the face
100 has a curved upper edge 102 and a generally straight lower edge 104.
From the upper edge 102, a lip 106 extends backwardly, partially
surrounding the shaft 92. The face 100 and lip 106 form a hood shape which
fits around a portion of the header 14, shown in FIG. 5 as a part of the
cardiac stimulator 10. Provision may be made in the hood to accommodate
methods for attaching the pin in the header. For example, a sidelock
attachment apparatus is described by Frey et al. in U.S. Pat. No.
4,860,750 to accommodate such an attachment mechanism, a notch 108 may be
provided on one side of the lip 106.
To provide a stimulating surface having a length and width as described
above, only a portion of the head or stimulating electrode is exposed, and
particularly only corners or edges are exposed. This is illustrated
particularly in FIGS. 7 through 9. It is known in the pacemaker art to
coat the cans of pacemakers with parylene, forming a nonconductive coating
on the pacemaker, and then to etch or otherwise remove a portion of the
parylene, exposing a small part of the pacemaker can which acts as an
indifferent electrode. Similar processes can be used on the pin of our
invention to coat the hood or electrode 98. As illustrated in FIG. 7, a
parylene layer 108 would coat both the face 100 and lip 106 of the
hood-like electrode. This would render most of the electrode
nonconductive. Selected portions of the front face 96 and the lip 106 can
then be exposed by, for example, etching to provide a relatively small,
well defined electrode area which would be both likely to come in contact
with adjacent tissue and would support a high electric current density to
stimulate that tissue. In particular, the corner 102 can be so exposed.
An alternative embodiment is illustrated in FIG. 8. In addition to an
exposed edge or corner 102, a rear edge 110 of the lip 106 has also been
exposed. A final embodiment is illustrated in FIG. 9 wherein only selected
portions of the corner 102 have been exposed forming a series of point
electrodes 112 along the corner. Such a configuration could also be
employed on an edge of the hood electrode 98. The effect of the series of
point electrodes 112 is to form a stimulating surface with an effective
length longer than an effective width of the surface.
Our invention may be embodied in other specific forms without departing
from the spirit or essential characteristics thereof. Present embodiment
is therefore considered in all respects to be illustrative and not
restrictive, the scope of our invention being indicated by the appended
claims whether by the foregoing description and all changes which come
within the meaning and range of equivalency of the claims are therefore
intended to be embraced therein.
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