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Claims  |
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What is claimed is:
1. An implantable system for detecting and discriminating among cardiac
arrhythmias and for providing therapeutic electrical stimulation to
cardiac tissue in response to detected cardiac arrhythmias, said
implantable system comprising:
an implantable lead including electrode means for delivering therapeutic
electrical stimulation pulses to said cardiac tissue and for transmitting
a signal indicative of cardiac electrical activity to said implantable
system, said implantable lead further including a cardiac wall
acceleration sensor for providing a signal indicative of cardiac wall
accelerations;
acceleration signal analyzing means for receiving said signal indicative of
cardiac wall acceleration and for using said signal to detect and
discriminate among cardiac arrhythmias; and
pulse generating means for generating said therapeutic electrical
stimulation pulses for delivery to said cardiac tissue by said electrode
means in response to cardiac arrhythmias detected by said cardiac wall
acceleration signal analyzing means.
2. The implantable system of claim 1, further comprising:
electrical activity analyzing means for receiving said signal indicative of
cardiac electrical activity transmitted by said electrode means and for
using said signal indicative of cardiac electrical activity to detect and
discriminate among cardiac arrhythmias; and
control means for enabling said pulse generating means to respond primarily
to one of said acceleration signal analyzing means and said electrical
activity analyzing means.
3. The implantable system of claim 1, wherein said cardiac wall
acceleration sensor comprises an accelerometer.
4. The implantable system of claim 3, wherein said accelerometer includes a
central axis, said accelerometer being responsive to cardiac wall
accelerations along said central axis and to accelerations perpendicular
to said central axis.
5. The implantable system of claim 3, wherein said implantable lead
comprises a plurality of conductors for connecting said accelerometer to
said acceleration signal analyzing means and for connecting said electrode
means to said pulse generating means.
6. The implantable system of claim 3, wherein said implantable lead
comprises a flexible patch, and said accelerometer and said electrode
means are disposed within said patch.
7. The implantable system of claim 3, wherein:
said implantable lead comprises a substantially inflexible myocardial
electrode mount, said electrode mount having an active-fixation helix
protruding therefrom; and
said accelerometer is secured to said electrode mount.
8. The implantable system of claim 3, wherein:
said implantable lead comprises an elongated lead body for transvenous
placement within a patient, the lead body having a distal end;
said electrode means comprises a tip and a ring electrode disposed at a
distal end of said lead body, wherein said ring electrode comprises an
interior chamber; and
said accelerometer is disposed within said interior chamber of said ring
electrode.
9. The implantable system of claim 8, wherein said ring electrode includes
a first end and a second end, wherein said ring electrode further
comprises:
a plug at said first end of said ring electrode and a hermetic feedthrough
at said second end of said ring electrode for hermetically sealing said
accelerometer within said ring electrode.
10. A method of detecting and discriminating among cardiac arrhythmias
using a signal indicative of cardiac wall accelerations provided by a
cardiac wall acceleration sensor delivered to a cardiac wall using an
implantable lead, and for providing therapeutic electrical stimulation
pulses to cardiac tissue in response to detected cardiac arrhythmias, said
method comprising the steps of:
generating said signal indicative of cardiac wall acceleration;
analyzing said signal indicative of cardiac wall acceleration to detect and
discriminate among cardiac arrhythmias;
generating said therapeutic electrical stimulation pulses in response to
detected cardiac arrhythmias; and
delivering said therapeutic electrical stimulation pulses to cardiac tissue
using an electrode delivered to said cardiac tissue by said implantable
lead.
11. The method of claim 10, wherein said cardiac wall acceleration sensor
comprises a cantilever beam having a central axis, and said step of
generating said signal indicative of cardiac wall acceleration comprises:
generating a signal indicative of accelerations of said cardiac wall along
said central axis and perpendicular to said central axis.
12. The method of claim 11, further comprising the steps of:
sensing cardiac electrical activity to provide a signal indicative of said
cardiac electrical activity; and
detecting a cardiac arrhythmia using a primary indicating signal, said
primary indicating signal being a selected one of said signal indicative
of cardiac wall accelerations or said signal indicative of cardiac
electrical activity.
13. The method of claim 12, further comprising the step of:
confirming a cardiac arrhythmia detected by said primary indicating signal
using a secondary indicating signal, said secondary indicating signal
being different from said primary indicating signal, said secondary
indicating signal being a selected one of said signal indicative of
cardiac wall accelerations or said signal indicative of cardiac electrical
activity.
14. The implantable system of claim 5, wherein said accelerometer
comprises:
a mounting surface attached to the electrode means;
a first cantilever beam having a free end, an end affixed to said mounting
surface, and a planar surface, said first cantilever beam comprising a
material having an electrical characteristic that varies measurably when a
mechanical stress or strain is exerted on said material to provide said
signal indicative of said accelerations; and
a first mass disposed on said free end of said first cantilever beam for
inducing a mechanical stress or strain in said material of said first
cantilever beam when said first means is accelerated.
15. The implantable system of claim 14, wherein:
said first cantilever beam has a first and a second axis, said first axis
extending from said fixed end to said free end of said first cantilever
beam, said second axis being perpendicular to said planar surface of said
first cantilever beam;
said first mass is disposed on said free end of said first cantilever beam
so as to be offset with respect to said planar surface, so that a
mechanical stress or strain is exerted on said material in response to
accelerations along said first axis and in response to accelerations along
said second axis; and
said plurality of conductors comprises at least two wires for conducting
said signal indicative of cardiac wall accelerations to said implantable
cardiac stimulation device.
16. The implantable system of claim 5, wherein:
said electrode means includes a tip and a ring electrode;
said accelerometer has an output terminal and a ground terminal, said
ground terminal being electrically connected to said ring electrode;
said plurality of conductors includes a first wire connected to said output
terminal, and a second and third wire for connecting said tip and ring
electrodes, respectively, said third wire being a shared wire between said
ring electrode and said ground terminal of said accelerometer. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to cardiac stimulating devices and particularly to
implantable cardiac stimulating devices, including implantable cardiac
pacemakers and implantable cardiac defibrillators, as well as implantable
cardioverters and cardioverter/defibrillators. More particularly, this
invention relates to implantable leads for such cardiac stimulating
devices, which incorporate cardiac wall motion sensors that provide
signals indicative of cardiac mechanical activity.
Implantable cardiac stimulating devices for providing therapy in response
to a variety of pathological cardiac arrhythmias are known. For example,
an implantable cardiac stimulating device may be capable of detecting a
pathological cardiac arrhythmia, and responding to the detected arrhythmia
by providing therapeutic electrical stimulation. Implantable cardiac
stimulating devices may be capable of providing "tiered therapy," in which
the type of electrical stimulation provided by the device is determined in
accordance with the severity of the arrhythmia, with more aggressive
therapies being applied in response to more severe arrhythmias. For
example, an implantable cardiac stimulating device may respond to a
relatively less severe occurrence of tachycardia by delivering
antitachycardia pacing pulses of about 25 microjoules to about 30
microjoules in a sequence known to interrupt such an arrhythmia. In
response to a relatively more severe occurrence of tachycardia, the
implantable cardiac stimulating device may deliver a low energy shock on
the order of about 2 joules to about 5 joules, either in combination with,
or as an alternative to, antitachycardia pacing pulses. In response to an
occurrence of an even more severe arrhythmia, for example, ventricular
fibrillation, the implantable cardiac stimulating device may deliver a
high energy "defibrillation" shock on the order of about 10 joules to
about 40 joules.
Implantable cardiac stimulating devices for providing pacing pulses to
cardiac tissue to maintain a heart rate at a physiologically acceptable
rate (i.e.--to provide "bradycardia pacing support") are also known.
Bradycardia pacing support may be provided by a dedicated pacemaker, or by
a device that is also capable of providing other forms of therapy, such as
tiered therapy.
Effective delivery of therapy from an implantable cardiac stimulating
device depends upon accurate measurement of intrinsic cardiac activity. In
the case of an implantable cardiac stimulating device capable of providing
tiered therapy, the device must not only be capable of detecting the onset
of an arrhythmia, but must also be capable of discriminating among various
types of arrhythmias in order to deliver an appropriate form of electrical
stimulation therapy. For example, if ventricular fibrillation is
incorrectly diagnosed by the device as a relatively less severe
arrhythmia, valuable time may be lost if an inappropriate, less aggressive
therapy, such as antitachycardia pacing, is applied. If tachycardia is
incorrectly diagnosed as ventricular fibrillation, the patient may
consciously experience high energy defibrillation shocks, which may be
ineffective in terminating the tachycardia, in addition to being extremely
uncomfortable.
Measurement of intrinsic cardiac activity is also desirable for implantable
cardiac stimulating devices capable of providing bradycardia pacing
support. Typically, the delivery of bradycardia pacing pulses from such
devices is inhibited by spontaneous, hemodynamically effective, cardiac
contractions occurring at a predetermined rate. For example, if the
intrinsic heart rate of a patient during a particular time interval is
greater than a predetermined threshold rate, delivery of pacing pulses may
be inhibited during that time interval. Pacing pulses would be provided
when the intrinsic heart rate falls below the threshold rate. Pacing pulse
inhibition is desirable because it extends battery life by avoiding
delivery of unnecessary stimulation pulses. In order for a device to be
capable of inhibiting delivery of pacing pulses, it must be capable of
detecting intrinsic cardiac activity.
Many implantable cardiac stimulating devices that detect and discriminate
among cardiac arrhythmias monitor heart rate, which is usually
accomplished by measuring cardiac electrical activity--i.e., the
intercardiac electrogram (IEGM). The IEGM is typically sensed by
electrodes that are also used to deliver electrical stimulation therapy to
the cardiac tissue. However, under many circumstances, it is difficult to
sense the IEGM. For example, the device may not be able to discern the
IEGM over noise or other physiological electrical activity, or perhaps
even external interference. As a result, an implantable cardiac
stimulating device may have difficulty detecting the onset of an
arrhythmia. As another illustration, implantable cardiac stimulating
devices capable of providing bradycardia pacing support may be inhibited
from sensing cardiac electrical activity during a period of time
immediately following the delivery of a pacing pulse, due to the presence
of a pulse-induced after-potential.
Other known implantable cardiac stimulating devices use hemodynamic signals
to detect cardiac arrhythmias. For example, U.S. Pat. No. 4,774,950 to
Cohen refers to a system that may detect cardiac arrhythmias by measuring
mean pressure at a variety of locations (e.g., mean arterial pressure,
mean right ventricle pressure, mean left atrial pressure, mean left
ventricle pressure or mean central venous pressure). For a selected mean
pressure, a short term current mean pressure is compared to a long term
mean baseline pressure, and if they differ by a predetermined valve, the
patient may be deemed to be experiencing a cardiac arrythmia. The mean
pressure data may also be used in combination with heart rate measurements
to detect arrhythmias.
Another example of a device that uses hemodynamics to detect cardiac
arrhythmias is described in U.S. Pat. No. 4,967,748 of Cohen. In that
patent, blood oxygen level is measured at a particular site in the
circulatory system of a patient. A comparison is made between a short term
sensed blood oxygen level and a baseline blood oxygen level, and if they
differ, the patient may be deemed to be experiencing a cardiac arrhythmia.
Unfortunately, the use of hemodynamic indicators such as mean pressure and
blood oxygen level may have certain associated drawbacks. One possible
drawback is that hemodynamic indicators may not respond rapidly to the
onset of an arrhythmia. Thus, an implantable cardiac stimulating device
that relies on such hemodynamic signals to detect cardiac arrhythmias may
not deliver therapy as rapidly as desired.
In view of the deficiencies associated the use of the IEGM or certain
hemodynamic indicators, it would be desirable to provide an improved
sensor for detecting and discriminating among various cardiac arrhythmias,
and for determining the intrinsic heart rate of a patient. Ideally, such a
sensor would provide a signal that rapidly responds to the onset of an
arrhythmia, and is not subject to electrical interference from external
sources or from pacemaker-induced after potentials.
SUMMARY OF THE INVENTION
The present invention is directed to implantable leads for an implantable
cardiac stimulating device, which incorporate cardiac wall motion sensors
that provide signals indicative of cardiac mechanical activity. Broadly,
the implantable leads of the present invention include a carrier that is
adapted for contacting cardiac tissue, a cardiac wall motion sensor
delivered to cardiac tissue by the carrier, and a connector that connects
the carrier to an implantable cardiac stimulating device. The carrier
typically includes conductors disposed therein for conducting the signal
provided by the cardiac wall motion sensor to the implantable cardiac
stimulating device.
The implantable leads of the present invention may be provided in a number
of configurations, depending upon the needs of a particular patient. For
example, a cardiac wall motion sensor may be disposed within a flexible
patch, a myocardial active-fixation lead, an endocardial lead, or other
leads suitable for use with an implantable cardiac stimulation device. A
myocardial active-fixation lead is disclosed in copending application
entitled "Implantable Myocardial Stimulation Lead with Sensors Thereon,"
filed concurrently herewith, which is hereby incorporated herein by
reference. Although the implantable leads of the present invention
typically include an electrode for delivering therapeutic electrical
stimulation to cardiac tissue, a cardiac wall motion sensor may be
delivered to cardiac tissue by a dedicated cardiac wall motion sensor
lead. A dedicated cardiac wall motion sensor lead may be advantageous when
it is desirable to measure cardiac wall motion at a region remote from the
cardiac tissue locations intended to receive electrical stimulation.
In a preferred embodiment, the implantable leads of the present invention
incorporate one or more cardiac wall motion sensors that are
accelerometer-based. The cardiac wall motion sensors transduce
accelerations of cardiac tissue to which the leads are attached, so as to
provide one or more signals indicative of cardiac mechanical activity.
Preferably, the cardiac wall motion sensors of the present invention are
sensitive to accelerations along at least two perpendicular axes, and may
be sensitive to accelerations along three perpendicular axes.
In another aspect of the invention, a method of fabricating implantable
leads incorporating cardiac wall motion sensors is provided. The method of
the present invention may be used to fabricate leads in a variety of
configurations, depending on the needs of a particular patient.
The present invention also provides an implantable system that uses a
signal provided by a cardiac wall motion sensor delivered to cardiac
tissue by an implantable lead, to detect and discriminate among various
cardiac arrhythmias. The implantable system of the present invention
applies therapeutic electrical stimulation to cardiac tissue when a
cardiac arrhythmia is detected. The signal from the cardiac wall motion
sensor may be used by the implantable system as a primary indicator of
potentially malignant cardiac arrhythmias. Alternatively, the cardiac wall
motion sensor signal may be used by the implantable system in combination
with, for example, conventional R-wave detection circuitry that relies on
an IEGM for measuring cardiac activity. In either mode, the use of output
from a cardiac wall motion sensor of the present invention overcomes known
difficulties associated with relying solely on an IEGM for detecting and
discriminating among various cardiac arrhythmias.
The system of the present invention that uses a signal provided by a
cardiac wall motion sensor to detect and discriminate among cardiac
arrhythmias operates based on knowledge that cardiac wall motion
associated with normal sinus rhythm follows a regular, identifiable
pattern. Cardiac wall motion associated with potentially malignant
arrhythmias, such as tachycardia or ventricular fibrillation, is typically
rapid, chaotic or both. In a patient experiencing bradycardia, cardiac
wall motion is not rapid or chaotic, but is typically distinguishable from
cardiac wall motion associated with normal sinus rhythm. By affixing an
implantable lead incorporating a cardiac wall motion sensor to selected
regions of cardiac tissue, cardiac wall motion is experienced and
transduced by the sensor, and the resulting signal may be used by the
implantable system of the present invention to distinguish between normal
and pathological cardiac rhythms, and to discriminate among various known
arrhythmias.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and advantages of the invention will be
apparent upon consideration of the following detailed description, taken
in conjunction with the accompanying drawings, in which like reference
characters refer to like parts throughout, and in which:
FIG. 1 is a graph of a signal from a cardiac wall motion sensor disposed
within an endocardial lead attached to right ventricular endocardial
tissue, a surface electrocardiogram and an aortic pressure signal, all
plotted versus time, each indicative of a subject in normal sinus rhythm;
FIG. 2 is a graph of a signal from a cardiac wall motion sensor disposed on
a myocardial patch electrode attached to left ventricular myocardial
tissue, a surface electrocardiogram and an aortic pressure signal, all
plotted versus time, each indicative of a subject transitioning from
tachycardia to ventricular fibrillation;
FIG. 3 is a partial cutaway view of a preferred embodiment of a flexible
patch electrode having a two-terminal bifurcated lead and incorporating a
cardiac wall motion sensor in accordance with the principles of the
present invention;
FIG. 4 is a partial cutaway view of another preferred embodiment of a
flexible patch electrode having a one-terminal in-line lead and
incorporating a cardiac wall motion sensor in accordance with the
principles of the present invention;
FIG. 5 is a partial cutaway view of another preferred embodiment of a
flexible patch electrode having a two-terminal bifurcated lead and
incorporating two cardiac wall motion sensors in accordance with the
principles of the present invention;
FIG. 6 is a partial cutaway view of another preferred embodiment of a
flexible patch electrode having a three-terminal bifurcated lead and
incorporating three cardiac wall motion sensors in accordance with the
principles of the present invention;
FIG. 7 is a partial cutaway view of a preferred embodiment of a myocardial
active-fixation lead incorporating a cardiac wall motion sensor in
accordance with the principles of the present invention;
FIG. 8 is a partial cutaway view of a preferred embodiment of an
endocardial lead incorporating a cardiac wall motion sensor in accordance
with the principles of the present invention;
FIG. 9 is a cross-sectional view taken along line 9--9 of the endocardial
lead shown in FIG. 8;
FIG. 10 is a cross-sectional view taken along line 10--10 of the
endocardial lead shown in FIG. 8;
FIG. 11 is a cross-sectional view taken along line 11--11 of the
endocardial lead shown in FIG. 8;
FIG. 12 is a perspective view of a preferred embodiment of a cardiac wall
motion sensor in accordance with the principles of the present invention;
FIG. 13 is a cross-sectional view taken along line 13--13 of the cardiac
wall motion sensor shown in FIG. 12, showing a cantilever beam of the
cardiac wall motion sensor in a resting state;
FIG. 14 is a cross-sectional view taken along line 13--13 of the cardiac
wall motion sensor shown in FIG. 12, showing an upward deflection of a
cantilever beam of the cardiac wall motion sensor in accordance with the
principles of the present invention;
FIG. 15 is a cross-sectional view taken along line 13--13 of the cardiac
wall motion sensor shown in FIG. 12, showing a downward deflection of a
cantilever beam of the cardiac wall motion sensor in accordance with the
principles of the present invention;
FIG. 16 is a schematic diagram of a preferred embodiment of local
electronics for the cardiac wall motion sensor shown in FIG. 12 in
accordance with the principles of the present invention;
FIG. 17 is a perspective view of another preferred embodiment of a cardiac
wall motion sensor in accordance with the principles of the present
invention;
FIG. 18 is a schematic block diagram of an implantable cardiac stimulating
device constructed in accordance with the principles of the present
invention;
FIG. 19 illustrates a preferred configuration of an implantable system for
delivering therapeutic electrical stimulation to cardiac tissue that uses
two bipolar endocardial leads incorporating cardiac wall motion sensors in
accordance with the principles of the present invention and a subcutaneous
patch electrode; and
FIG. 20 is another preferred configuration of an implantable system for
delivering therapeutic electrical stimulation to cardiac tissue that uses
two patch electrodes and a myocardial active-fixation lead, each
incorporating a cardiac wall motion sensor in accordance with the
principles of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 and 2, two cardiac wall motion sensor signals 40 and
50 provided from two cardiac wall motion sensors (not shown; described
below) in accordance with the principles of the present invention are
described and compared to two electrocardiograms 42 and 52 and two aortic
pressure signals 44 and 54. In FIG. 1, the cardiac wall motion sensor
signal 40, the electrocardiogram 42 and the aortic pressure signal 44 were
recorded at a chart speed of 100 mm/sec from a subject in normal sinus
rhythm. An accelerometer-based cardiac wall motion sensor disposed within
an endocardial lead (not shown; described below) was used to provide the
cardiac wall motion sensor signal 40. In FIG. 2, the cardiac wall motion
sensor signal 50, the electrocardiogram 52 and the aortic pressure signal
54 were recorded at a chart speed of 5 mm/sec from a subject transitioning
from an epinephrine-induced tachycardia into ventricular fibrillation. An
accelerometer-based cardiac wall motion sensor disposed on a patch
electrode (not shown) was used to provide the cardiac wall motion sensor
signal 50.
As shown in FIG. 1, the cardiac wall motion sensor signal 40 from a subject
in normal sinus rhythm exhibits relatively low frequency amplitude
fluctuations that are substantially periodic. FIG. 2 shows that the
cardiac wall motion sensor signal 50 from a subject experiencing
tachycardia is chaotic, and that the frequency is relatively high. Upon
the onset of ventricular fibrillation, the amplitude of the signal
fluctuations substantially decreases, while the frequency remains
relatively high.
Transitions in the cardiac wall motion sensor signals 40 and 50 are
coincident with transitions in the electrocardiograms 42 and 52 and the
aortic pressure signals 44 and 54. Thus, it is shown that the cardiac wall
motion sensor signals 40 and 50 may be used to discriminate among various
cardiac arrhythmias in a manner traditionally accomplished by analyzing
the electrocardiograms 42 and 52 or the aortic pressure signals 44 and 54.
An implantable cardiac stimulating device (not shown; described below) may
be constructed to receive a cardiac wall motion sensor signal (which is
indicative of cardiac mechanical activity) and an IEGM (which is
indicative of cardiac electrical activity), and may be configured to use
either form of information, or both forms of information in combination,
to detect and discriminate among various types of cardiac arrhythmias and
to determine intrinsic heart rate.
A cardiac wall motion sensor in accordance with the principles of the
present invention (which is preferably accelerometer-based) may be
delivered and affixed to cardiac tissue using a variety of leads known to
be suitable for use with an implantable cardiac stimulating device. The
described embodiments of the present invention are merely illustrative
examples of such leads, and the principles of the present invention may be
applied to other suitably configured leads. For instance, when it is
desirable to measure cardiac wall motion at regions remote from areas
normally contacted by a stimulating lead, a dedicated cardiac wall motion
sensor lead may be used.
Referring now to FIG. 3, a preferred embodiment of a flexible epicardial
patch electrode incorporating a cardiac wall motion sensor suitable for
use with an implantable cardiac stimulating device is described. An
epicardial patch electrode 60 includes an electrically conductive wire
mesh 64 substantially enclosed within a carrier 62. Preferably, the wire
mesh 64 is made from titanium wire or a titanium sheet, and the carrier 62
is made from silicone sheeting reinforced with synthetic polyester fibers
(commonly known by the trademark DACRON, owned by E. I. du Pont de Nemours
& Company). The side of the carrier 62 that is intended for contact with a
region of the cardiac wall (not shown) includes a plurality of windows 66,
as shown in FIG. 3. The windows 66 permit the wire mesh 64 to make
electrical contact with a region of the cardiac wall when the patch
electrode 60 is sutured to the epicardium (not shown), so that the patch
electrode 60 can deliver therapeutic electrical stimulation when so
required.
The patch electrode 60 further includes a cardiac wall motion sensor 68
embedded therein. When the patch electrode 60 is sutured to the
epicardium, the cardiac wall motion sensor 68 will experience the motion
of a region of the cardiac wall to which the patch electrode 60 is
attached. Motion experienced by the cardiac wall motion sensor 68 will
cause the cardiac wall motion sensor 68 to generate an electrical signal
that is indicative of the motion of a region of the cardiac wall.
Preferably, the cardiac wall motion sensor 68 is within a hermetically
sealed enclosure.
The cardiac wall motion sensor 68 is electrically connected to an
implantable cardiac stimulating device (not shown) by two wires 70 and 72,
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