U.S. patent application number 11/881809 was filed with the patent office on 2008-04-24 for method and system for remote hemodynamic monitoring.
Invention is credited to Joseph M. Ruggio, Mark J. Zdeblick.
Application Number | 20080097227 11/881809 |
Document ID | / |
Family ID | 32825210 |
Filed Date | 2008-04-24 |
United States Patent
Application |
20080097227 |
Kind Code |
A1 |
Zdeblick; Mark J. ; et
al. |
April 24, 2008 |
Method and system for remote hemodynamic monitoring
Abstract
A cardiac sensor system includes implanted cardiac sensor
assemblies and an external controller which receives information
from the implanted sensors. The sensors permit direct measurement
of a number of physiologic parameters. The external controller
permits calculation of a variety of performance values based on the
measured physiological parameters. Optionally, patient oxygen
consumption can be measured externally and combined with the
internally measured physiologic parameters in order to calculate a
variety of unique performance values.
Inventors: |
Zdeblick; Mark J.; (Portola
Valley, CA) ; Ruggio; Joseph M.; (Laguna Hills,
CA) |
Correspondence
Address: |
BOZICEVIC, FIELD & FRANCIS LLP;(PROTEUS BIOMEDICAL, INC)
1900 UNIVERSITY AVENUE, SUITE 200
EAST PALO ALTO
CA
94303
US
|
Family ID: |
32825210 |
Appl. No.: |
11/881809 |
Filed: |
July 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10764125 |
Jan 23, 2004 |
7267649 |
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11881809 |
Jul 27, 2007 |
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60442376 |
Jan 24, 2003 |
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Current U.S.
Class: |
600/486 |
Current CPC
Class: |
A61B 5/0205 20130101;
A61N 1/36843 20170801; A61B 5/1107 20130101; A61B 5/02158 20130101;
A61B 5/1473 20130101; A61B 5/029 20130101; A61N 1/3684 20130101;
A61N 1/36585 20130101; A61B 5/02028 20130101; A61N 1/3702 20130101;
A61N 1/36514 20130101; A61N 1/3627 20130101; A61B 5/6882
20130101 |
Class at
Publication: |
600/486 |
International
Class: |
A61B 5/0215 20060101
A61B005/0215 |
Claims
1. A cardiac sensor system implantable in a heart, said system
comprising: means for measuring a cardiac characteristic at least
one point in the cardiac cycle; means for measuring a myocardial
characteristic at least one point in the cardiac cycle; and means
for transmission of the data from an implanted location in the
heart to an external location.
2. A cardiac sensor system as in claim 1, wherein the means for
measuring a cardiac characteristic comprises a sensor adapted to
measure pressure, differential pressure, volume, temperature, pH
hemocrit, oxygen concentration, regurgitant flow, and cardiac valve
area.
3. A cardiac sensor system as in claim 1, wherein the means for
measuring a myocardial characteristic comprises a sensor adapted to
measure myocardial displacement, myocardial compliance, myocardial
dimensions such as thickness, myocardial strain, myocardial
expansibility, myocardial contractility, myocardial density,
myocardial temperature, myocardial thermal conductivity, myocardial
electrical conductivity, myocardial acoustic velocity, myocardial
force, and myocardial stress.
4. A cardiac sensor system as in claim 1, further comprising a
flame, wherein the cardiac characteristic measuring means and the
myocardial characteristic measuring means are locked on the
frame.
5. A cardiac sensor system as in claim 4, wherein the frame is
implantable in tissue.
6. A cardiac sensor system as in claim 5, wherein the frame is
implantable across a cardiac wall.
7. A cardiac sensor system as in claim 1, further comprising a
power source control circuitry, and a transmitter, wherein the
transmitter is adapted to transmit signals from the sensors to an
external receiver.
8. A cardiac sensor system as in claim 7, wherein the power source
includes a coil for receiving externally generated power.
9. A cardiac sensor system ad in claim 7, wherein the power source
includes a battery.
10. A cardiac sensor system as in claim 7, further comprising a
receiver adapted to receive signals generated externally and
communicate with the control circuitry to modify or initiate
function of at least one of the sensors.
11. A cardiac sensor assembly implantable across a cardiac wall,
said assembly comprising: means for spanning the cardiac wall to
provide surfaces on each side of the wall; and at least one sensor
on each surface.
12. A cardiac sensor as in claim 11, wherein the spanning means
comprises a pair of anchors joined by a tether.
13. A cardiac sensor as in claim 12, wherein the anchors contact
the cardiac wall over a surface area in the range from 1 mm.sup.2
to 100 mm.sup.2.
14. A cardiac sensor as in claim 12, wherein the tether comprises
electrical conductors coupling the anchors together, wherein at
least some of the sensors are coupled to some of the wires.
15. A cardiac sensor as in claim 11, wherein at least one of the
sensors measures a cardiac characteristic selected from the group
consisting of pressure, differential pressure, volume, temperature,
pH, hemocrit, oxygen concentration, regurgitant flow, cardiac
output, and cardiac valve area.
16. A cardiac sensor as in claim 15, wherein at least another of
the sensors measures a myocardial characteristic selected from the
group consisting of myocardial displacement, myocardial dimensions
such as thickness, myocardial strain, myocardial compliance,
myocardial expansibility, myocardial contractility, myocardial
density, myocardial temperature, myocardial thermal conductivity,
myocardial electrical conductivity, myocardial acoustic velocity,
myocardial force, and myocardial stress.
17. A cardiac sensor as in claim 11, further comprising a power
source control circuitry, and a transmitter, wherein the
transmitter is adapted to transmit signals from the sensors to an
external receiver.
18. A cardiac sensor as in claim 17, wherein the power source
includes a coil for receiving externally generated power.
19. A cardiac sensor as in claim 17, wherein the power source
includes a battery.
20. A cardiac sensor as in claim 17, further comprising a receiver
adapted to receive signals generated externally and communicate
with the control circuitry to modify or initiate function of at
least one of the sensors.
21. A cardiac sensor system implantable across a cardiac wall, said
system comprising means for measuring displacement of opposite
surfaces of the cardiac wall over time.
22. A cardiac sensor system as in claim 21, wherein the
displacement measuring means comprises a first position locator
positionable on one surface of the cardiac wall and a second
position locator positionable on the other surface of the cardiac
wall and means for measuring the displacement between the two
locations.
23. A cardiac sensor system as in claim 22, further comprising
means for spanning the cardiac wall, wherein the position locators
on opposite surfaces of the cardiac wall are connected by the
spanning means.
24. A cardiac sensor system as in claim 22, wherein the position
locators are independently implantable in the opposite surfaces of
the cardiac wall.
25. A cardiac sensor system implantable across a cardiac wall, said
system comprising means for measuring expansibility across the wall
over time.
26. A cardiac sensor system as in claim 25, further comprising
means for spanning the cardiac wall, wherein the expansibility
measuring means is disposed on or in the spanning means.
27. A cardiac sensor system as in claim 26, wherein the
expansibility measuring means comprises a strain gauge mounted on
the wall spanning means, wherein the opposite ends of the wall
spanning means are anchored on opposite surfaces of the cardiac
wall so that an expansive force exerted by the wall applies a
tensile force on the wall spanning means which is measured by the
strain gauge.
28. A cardiac sensor system implantable on a cardiac wall, said
system comprising means for measuring muscular contractility over a
surface of the wall over time.
29. A cardiac sensor system as in claim 28, wherein the muscular
contractility measuring means comprises a planar strain gauge.
30. A cardiac sensor system as in claim 29, wherein the planar
strain gauge has a circular configuration.
31. A cardiac sensor system as in claim 29, wherein the planar
strain gauge has an orthogonal configuration.
32. A cardiac sensor system implantable on or across a cardiac
wall, said system comprising means for measuring myocardial
compliance.
33. A cardiac sensor system as in claim 32, wherein the compliance
measuring means comprises a probe which pushes against the
myocardium to measure stiffness as a ratio offeree and displacement
of the probe.
34. A system for assessing cardiac status of a patient, said system
comprising: a first interface adapted to receive data from cardiac
sensors implanted in a patient and produce a plurality of outputs
corresponding to said data; a second interface adapted to receive
external data selected from the group consisting of ambient
pressure, patient oxygen consumption data, and patient carbon
dioxide production data from a breath analyzer; and a processor
adapted to receive data from both interfaces and to calculate one
or more cardiac performance values from the received data.
35. A system as in claim 34, wherein the first interface is adapted
to receive cardiac characteristic data transmitted from an
implanted cardiac sensor and selected from the group consisting of
pressure, differential pressure, volume, temperature, pH, hemocrit,
oxygen concentration, regurgitant flow, cardiac output, and cardiac
valve area.
36. A system as in claim 34, wherein the first interface is adapted
to receive myocardial characteristic data transmitted from an
implanted cardiac sensor and selected from the group consisting of
myocardial displacement, myocardial dimensions such as thickness,
myocardial strain, myocardial compliance, myocardial expansibility,
myocardial contractility, myocardial density, myocardial
temperature, myocardial thermal conductivity, myocardial electrical
conductivity, myocardial acoustic velocity, myocardial force, and
myocardial stress.
37. A system as in claim 34, wherein the first interface comprises
a radiofrequency receiver adapted to receive plurality of different
signals from different implanted sensors and input corresponding
data to the processor.
38. A system as in claim 34, wherein the second interface is
adapted to receive at least inhalation and exhalation volumes,
oxygen concentrations, and ambient pressure.
39. A system as in claim 38, wherein the second interface comprises
a mouthpiece, a pressure transducer, and analysis and measurement
circuitry adapted to input corresponding data to the processor.
40. A system as in claim 34, wherein the processor is adapted to
calculate a cardiac hypertrophy value by performing the following
steps: determining a cardiac output value based on oxygen
consumption data received from the second interface and blood
oxygen concentration data received from the first interface;
determining a myocardial thickness change at two points in the
cardiac cycle based on data received from a muscle displacement
sensor implanted in the patient's heart through the first
interface; and determining the hypertrophy value based at least in
part on the ratio of the cardiac output value and the myocardial
thickness change.
41. A system as in claim 34, wherein the processor is adapted to
calculate a ventricular performance value by performing the
following steps: determining a cardiac output value based on oxygen
consumption data received from the second interface and blood
oxygen concentration data received from the first interface;
determining a change in ventricular pressure at two points in the
cardiac cycle based on data received from a pressure sensor
implanted in a patient's heart through the first interface;
determining a change in a myocardial contraction force at
corresponding points in the cardiac cycle based on data received
from a muscle contraction force sensor implanted in the patient's
heart; and determining the ventricular performance value based at
least in part on the ratio of the determined changes in ventricular
pressure and myocardial contraction force.
42. A system as in claim 34, wherein the processor is adapted to
calculate a cardiac efficiency value by performing the following
steps: determining a maximum pressure difference between a right
ventricle or atrium and a left ventricle or atrium based on
pressure data received from pressure sensors in the right and left
ventricle or atrium through the first interface; determining a
myocardial thickness change at two points in the cardiac cycle
based on data received from a muscle displacement sensor implanted
across the myocardium through the first interface; determining a
myocardial contraction force difference at a location on the
myocardium based on data received from a muscle contraction force
sensor in the patients heart through the first interface;
determining a cardiac output value based on oxygen consumption data
received from the second interface and blood oxygen concentration
data received from the first interface; and determining a cardiac
efficiency value based at least in part on the cardiac output
value, the determined maximum pressure difference, the determined
myocardial thickness change, and the determined myocardial
difference contraction force.
43. A method for measuring a cardiac performance value, said method
comprising: measuring a cardiac characteristic at least one point
in the cardiac cycle; measuring a myocardial characteristic at
least one point in the cardiac cycle; and determining the cardiac
performance value based on a ratio of the measured cardiac
characteristic and the measured myocardial characteristic.
44. A method as in claim 43, wherein the cardiac characteristic is
selected from the group consisting of intracardiac pressure,
intracardiac differential pressure, intracardiac volume,
temperature, pH, hemocrit, oxygen concentration, intracardiac
regurgitant flow, cardiac output, and cardiac valve area.
45. A method as in claim 43, displacement, myocardial dimensions
such as thickness, myocardial strain, myocardial compliance,
myocardial wherein the myocardial characteristic is selected from
the group consisting of myocardial contractility, myocardial
density, myocardial temperature, myocardial thermal conductivity,
myocardial electrical conductivity, myocardial acoustic velocity,
myocardial force, and myocardial stress.
46. A method as in claim 43, wherein the cardiac characteristic and
the myocardial characteristic are measured at the same point on the
cardiac cycle.
47. A method as in claim 43, wherein at least one of the cardiac
characteristic and the myocardial characteristic is a difference in
values measured at two points on the cardiac cycle.
48. A method as in claim 43, wherein both the cardiac
characteristic and the myocardial characteristic are differences in
values measured at the same two points on the cardiac cycle.
49-99. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the priority benefit of U.S.
Provisional Patent Application No. 60/442,376 (Attorney Docket No.
21308-000700US), filed Jan. 24, 2003, the full disclosure of which
is hereby incorporated by reference. The present application is
related to U.S. patent application Ser. Nos.: 10/______ (Attorney
Docket No. 21308-000810US); and 10/______ (Attorney Docket No.
21308-001110US); both of which are filed concurrently with the
present application, and both of which are hereby incorporated
fully by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to medical devices
and methods. More particularly, the present invention relates to
medical devices, systems, and methods for determining cardiac
performance characteristics based on data obtained from implanted
cardiac sensors.
[0004] Intravascular and intraluminal interventions and monitoring
have become essential in modern cardiology and other medical
fields. Of particular interest to the present invention, a variety
of implantable sensors, intravascular catheters, and other devices
and systems have been developed for monitoring cardiac performance
both in and outside of a medical facility.
[0005] The ability to adequately treat patients suffering from or
at risk of cardiac disease can be greatly enhanced by frequent, or
better still real time continuous, monitoring of cardiac function.
For example, patients suffering from congestive heart failure could
titrate dosages of certain medications if more information were
available and/or information were available more often relating to
cardiac function and how it has responded to drug treatment.
Additionally, the need for surgical intervention could also be
better assessed if better cardiac performance data were
available.
[0006] For these reasons, it would be desirable to provide improved
devices, systems, and methods for monitoring cardiac performance
both in and outside of medical facilities. Such improved devices,
systems, and methods should allow for measuring a variety of
mechanical, biological, and chemical parameters related to cardiac
performance and further to analyze calculated cardiac performance
values based on such measured performance characteristics.
Preferably, the devices and apparatus will include implantable
sensors which transmit data to allow for periodic or continuous
collection of in situ cardiac performance data. The devices and
systems should further include external components for collecting
the internally transmitted data and for optionally obtaining
external patient data. The systems may then calculate secondary
cardiac performance parameters based on the measured internal and
external performance data which has been collected. At least some
of these objectives will be met by the inventions described
hereinafter.
[0007] 2. Description of the Background Art
[0008] Catheters and implantable sensors capable of measuring
various physiologic parameters in the heart and/or vasculature are
described in U.S. Pat. Nos. 5,814,089; 6,328,699 B1; 6,438,408 B1;
U.S. Patent Publication Nos. 2001/0053882 A1; 2001/0047138 A1;
2002/0077568 A1; 2002/0111560 A1; 2002/0151816 A1; 2002/0156417;
2002/0169445; and PCT Publication WO 02/065894 A2. The full
disclosures of each of these patents and patent publications are
incorporated herein by reference.
BRIEF SUMMARY OF THE INVENTION
[0009] According to the present invention, apparatus, systems, and
methods are provided for monitoring cardiac performance. Such
monitoring can be performed on a continuous basis, such as
ambulatory monitoring, where the performance data are transmitted
and collected for real time or subsequent analysis. Alternatively,
the monitoring can be performed periodically, for example, while
the patient is at a medical office. The data collected at a medical
facility can also be collected and analyzed immediately or at some
subsequent time.
[0010] The apparatus and systems of the present invention will rely
at least in part on a plurality of sensors implanted in or on a
patient's heart. The sensors will be capable of directly measuring
certain cardiac data including both "cardiac characteristics" and
"myocardial characteristics." Cardiac characteristics include those
characteristics which can be directly measured by an implanted
sensor and which are characteristic of the overall performance of
the heart. Exemplary cardiac characteristics include pressure, such
as ventricular pressure or atrial pressure; differential pressure,
such as pressure differences measured at different times at a same
location, at different locations at the same time, or combinations
hereof; volume, typically heart chamber volume; systemic vascular
resistance; pulmonary vascular resistance; blood oxygen
concentration; regurgitant flow, such as regurgitant flow through
the mitral valve; and cardiac valve area.
[0011] In contrast to cardiac characteristics, myocardial
characteristics are typically characteristic of a localized region
in or on the myocardium of the heart. Exemplary myocardial
characteristics include myocardial displacement, typically changes
in wall or septal thickness during the cardiac cycle; myocardial
compliance; myocardial expansibility; myocardial contractility;
myocardial density; myocardial temperature; myocardial thermal
conductivity; myocardial electrical conductivity; myocardial and
acoustic velocity. Force, typically the amount of force exerted by
the myocardium against blood; myocardial stress, which is an
intrinsic force per unit area normal to the myocardial surface;
myocardial strain, which is the change in myocardial thickness
divided by myocardial thickness; and myocardial modulus, which is
the ratio of myocardial stress over myocardial strain.
[0012] Optionally, the apparatus, systems, and methods of the
present invention will further provide for measuring cardiac
performance characteristics external to the patient. In particular,
certain of the methods and systems described hereinafter will rely
on breath analyzers or other external equipment for determining
patient oxygen consumption. Such external analyzers may also be
used for acquiring other patient data, such as heart rate, vascular
blood pressure, and body temperature, as well as ambient data, such
as ambient pressure, temperature, oxygen concentration, carbon
dioxide concentration, and the like.
[0013] The apparatus, systems, and methods of the present invention
are particularly useful for calculating cardiac performance values
based on at least two of the measured characteristics, more usually
based on at least one cardiac characteristic and at least one
myocardial characteristic. Such cardiac performance values are
particularly useful since they are able to detect changes, usually
deterioration, of local regions of the myocardium. The comparison
of a local myocardial performance value with other performance
value(s) which are characteristics of other regions in the
myocardium or of the heart as a whole, are able to more rapidly
detect and predict localized deterioration which can lead to heart
failure. For example, comparison of a measurement of a cardiac
performance value such as pressure to a myocardial performance
value which is characteristic of the force-generating capability of
a region of myocardium would allow direct tracking of deterioration
of the heart due to an ischemic event. The treating physician may
then be alerted to take whatever steps are necessary to determine
the source of the damage, such as catheterization of the
patient.
[0014] Monitoring of the myocardium in order to detect ischemia and
other deleterious events can be achieved by the present invention
by sampling the cyclical change in force exerted by the myocardium
with an implanted force sensor and comparing it to the cyclical
change in pressure of the ventricle. An ischemic event on the
ventricular wall will cause a shift in this ratio. By computing the
difference between the maximum and minimum force applied by the
myocardium, and dividing by the area of the sensor, it is possible
to calculate myocardial stress. Particular methods for computing
cardiac performance values for the detection of ischemia are
discussed in more detail below. In one method, for example, cyclic
changes in wall thickness are compared to cyclic changes in
pressure to detect ischemia.
[0015] In a first specific aspect of the present invention, a
cardiac sensor system implantable in a heart comprises a first
sensor or other means for measuring a cardiac characteristic at
least one point in the cardiac's cycle and a second sensor or other
means for measuring a myocardial characteristic at least one point
in the cardiac cycle. Typically, the system will further include a
transmitter or other means for communication of data from the
measuring sensor(s) to an external location. The sensor or other
means for measuring a cardiac characteristic will be adapted to
measure at least one of the cardiac characteristics listed above.
Similarly, the sensor or other means for measuring a myocardial
characteristic will comprise a sensor adapted to measure any of the
myocardial characteristics set forth above. Typically, the systems
will further comprise a frame, base, or other support structure,
which carries at least one of the sensors, usually carrying at
least two of the sensors, and more usually carrying at least one
cardiac characteristic sensor and at least one myocardial
characteristic sensor. The frame will be implantable in or on
tissue, and may optionally be implantable across a cardiac wall,
such as a ventricular or atrial septum, where the attached sensors
may be carried on either or both sides of the cardiac wall.
[0016] Preferred cardiac sensor systems will further comprise a
power source, control circuitry and a transmitter, where the
transmitter is adapted to transmit signals from the sensors to an
external receiver. The power source may include a coil for
receiving externally generated power, a battery, or optionally both
a coil and a battery where the energy transmitted in through the
coil can recharge the battery. Often, the cardiac sensor system
will further comprise a receiver adapted to receive signals
generated externally and communicate such signals with the control
circuitry to modify or initiate function of at least one of the
implantable sensors.
[0017] In a second aspect of the present invention, a cardiac
sensor implantable across a cardiac wall includes means for
spanning the cardiac wall to provide surfaces on each side of said
wall. The sensor assembly further includes at least one sensor on
each surface. Typically, the spanning means may comprise a pair of
anchors joined by a tether or other shaft or member joining the
anchors together. The anchors are placed on opposite surfaces of
the cardiac wall, such as the ventricular or atrial septum, and the
anchors are then tensioned to using the tether. Typically, the
anchors will each contact a cardiac wall segment having a surface
area in the range from 1 mm.sup.2 to 200 mm.sup.2, preferably from
1 mm.sup.2 to 100 mm.sup.2, often from 5 mm.sup.2 to 50 mm.sup.2.
The tether or other connector may comprise electrical conductors
coupling the anchors and any sensors present thereon together, and
optionally to control circuitry, transmitters, receivers, and the
like, as described hereinafter. The cardiac sensors present on the
assembly may include one or more sensors selected from the group of
cardiac sensors listed above as well as one or more sensor selected
from the group of myocardial sensors listed above. In one
embodiment, the anchors may be withdrawn to facilitate removal of
the implant in case of infection or for other reasons.
[0018] The cardiac sensor assembly may include receivers, power
sources, transmitters, control circuitry, and the like, all as
generally described above in connection with the earlier
embodiments of the present invention. Cardiac sensors which are
expandable are particularly suitable for measuring myocardial
displacement, which may be measured by sensors including a first
position locator on one surface of the device and a second position
locator positioned on the other surface of the device, so that said
locators are located on opposite sides of the cardiac wall. By then
measuring relative displacement of the position locators, for
example by monitoring elongation or shortening of the tether
holding the two ends of the sensor together, or by monitoring
inductive coupling between anchors positioned on the opposite
surfaces of the cardiac wall, the change in thickness of the wall
or septum can be measured.
[0019] A second exemplary cardiac sensor system constructed in
accordance with the principles of the present invention comprises a
sensor or other means for measuring muscular contractility over a
surface of the cardiac wall. For example, it may comprise planar
strain gauge, having a circular or orthogonal configuration.
[0020] A third exemplary cardiac sensor system according to the
principles of the present invention comprises a sensor or other
means for measuring myocardial compliance. For example, the
compliance measuring sensor may comprise a probe which is adapted
to push against the myocardium to measure stiffness as a ratio of
applied force to displacement of the probe.
[0021] In a further aspect of the present invention, a system for
assessing cardiac status of a patient comprises a first interface,
a second interface, and a processor adapted to receive data from
both interfaces. A first interface is adapted to receive data from
cardiac sensor implanted in a patient, typically cardiac sensors
implanted in the heart, and to produce a plurality of outputs
corresponding to the received data. The second interface is adapted
to receive external data generated with respect to the patient,
particularly oxygen consumption data from breath analyzer and
external data, such as ambient pressure, ambient oxygen
concentration, patient pressure, and the like. Such systems will
further comprise a processor adapted to receive data from both
interfaces and to calculate one or more cardiac performance values
from the received data.
[0022] In a first preferred aspect of the system for assessing
cardiac status, the first interface is adapted to receive cardiac
characteristic data transmitted from an implanted cardiac sensor
and selected from the group of cardiac characteristic listed above.
The first interface will usually comprise a radiofrequency receiver
adapted to receive a plurality of different signals from different
implanted sensors and to input corresponding data to the processor
of the system. The second interface of the system is adapted to
receive at least inhalation and exhalation volumes, oxygen
concentrations, ambient pressures and the like. In particular, the
second interface surface may comprise a mouthpiece, a pressure
transducer, and analysis and measurement circuitry adapted to
process and input corresponding data to the processor.
[0023] The system for assessing cardiac status may be particularly
adapted to calculate a cardiac hypertrophy value by programming the
processor to determine a cardiac output value based on oxygen
consumption data received from the second interface and blood
oxygen concentration data received from the first interface.
Cardiac output value may be calculated based on the relative
changes in inspired oxygen and blood oxygen. Myocardial thickness
change may then be determined at two points in the cardiac cycle
based on data from a muscle displacement sensor implanted in the
patient's heart, where the data is received through the first
interface. The hypertrophy value is then determined at least in
part based on the ratio of the cardiac output value to the
myocardial thickness change.
[0024] The system for assessing cardiac status may further be
adapted to calculate a ventricular performance value by programming
the processor to perform the following steps. First, a cardiac
output value is determined generally as described above. A change
in ventricular pressure at two points in the cardiac cycle is then
determined based on data received from a pressure sensor planted in
the patient's heart, where the data is received through the first
interface. A change in myocardial contraction force would also
determine that corresponding points in the cardiac cycle based on
data received from a muscle contraction force sensor implanted in
the heart, where the data is received through the first interface.
The processor is programmed to determine the ventricular
performance value based at least in part on the ratio of the
determined changes in ventricular pressure and myocardial
contraction force.
[0025] The system for assessing cardiac status of a patient may
still further be adapted to calculate a cardiac myopathy value (M)
characteristic of the heart's efficiency where the processor is
programmed to perform the following steps. A maximum pressure
difference between a right ventricle or atrium and a left ventricle
or atrium is determined based on pressure data received from
pressure sensors in the right and left ventricles or atriums
through the first interface. A change in myocardial thickness is
then determined at two points in the cardiac cycle based on data
received from a muscle displacement sensor implanted across the
myocardium through the first interface. A difference in myocardial
contraction force is then determined at a location on the
myocardium based on data received from a muscle contraction force
sensor in the heart through the first interface. Cardiac output
value is then determined based on oxygen consumption and blood
oxygen concentrations as generally described above. The cardiac
myopathy value is then determined based at least in part on a ratio
between the cardiac output value, the determined maximum pressure
difference, the determined myocardial thickness change, and the
determined myocardial difference in contraction force. A decrease
in cardiac myopathy value (M) is an indication that conversion of
myocardial work by the heart into hemodynamic work is decreasing.
The cardiac myopathy value (M) may also increase, indicating an
improvement in efficient conversion of myocardial work to
hemodynamic work, as a result for example of heart valve
replacement, bypass or other procedures which improve blood flow to
the myocardium; retiming of a biventricular pacing device; or the
like.
[0026] In a further aspect of the present invention, methods for
measuring a cardiac performance value comprise measuring a cardiac
characteristic and a myocardial characteristic. The characteristics
are typically measured at least one point in the cardiac cycle,
typically at the same point, and are often measured at least two or
more points in the cardiac cycle, typically at the same multiple
points in the cardiac cycle, usually end-diastole and end-systole.
The cardiac performance value may thus be determined as a ratio
between the measured cardiac characteristics and the measured
myocardial characteristics. The ratio may be a simple ratio, but
will frequently be a complex ratio as described in more detail
below. Useful cardiac characteristics and myocardial
characteristics have all been set forth above. The cardiac
performance values may be measured at successive times, either
continuously or periodically, in order to monitor changes in the
cardiac performance value.
[0027] A first method according to the present invention is useful
for calculating a ventricular performance value. The method
comprises measuring a change in ventricular pressure at two points
in the cardiac cycle, measuring a change in myocardial contraction
force at the corresponding two points in the cardiac cycle, and
determining the ventricular performance value based at least in
part on a ratio of the measured changes in ventricular pressure and
myocardial contraction force. Typically, the changes in ventricular
pressure and myocardial force are measured in the left ventricle,
but they can also be measured in the right ventricle or either of
the atriums. The ventricular pressure is typically measured with at
least one pressure transducer implanted in a ventricular wall. The
myocardial contraction force is typically measured across a
ventricular septum typically using at least one strain gauge
implanted in or across the septum or other location in the
myocardium. The changes in both ventricular pressure and myocardial
contraction force are preferably measured with implanted sensors,
typically on a common implanted device, frame, or other
structure.
[0028] Methods of the present invention may also be used to
calculate a hypertrophy value characteristic of a patient's heart.
Cardiac output is determined and a change in myocardial thickness
is measured at two points in the cardiac cycle usually diastole and
systole. The hypertrophy value may then be based at least in part
on a ratio of the cardiac output value and the measured change in
myocardial thickness. Typically, the cardiac output value is stroke
volume. Cardiac output is determined by measuring a quantity or
rate of air breathed, a change in oxygen concentration between
inhaled air and exhaled air, a blood oxygen concentration in the
left ventricle, a blood oxygen concentration in the oxygen volume
needed per right ventricle, and a pulse rate. Stroke volume may
then be calculated by determining the oxygen volume needed per
stroke to oxygenate sufficient blood to account for the oxygen
removed from the breathed air. In particular, the stroke volume is
the ratio of mean cardiac output over pulse rate, where mean
cardiac output is calculated as oxygen consumed by the patient
(i.e., the quantity of air breathed times the change in oxygen
concentration) divided by the change in blood oxygen concentration
between the left and right ventricles. The stroke volume is
preferably a mean stroke volume calculated as cardiac output
divided by the pulse rate measured over one second to one minute.
The change in myocardial thickness is usually the maximum change in
thickness measured in a single heart cycle. The mean myocardial
thickness change is the average of the myocardial thickness
measured over a time from one second to one minute. The myocardial
thickness change may be measured by a sensor implanted across the
myocardial wall. The hypertrophy value is preferably the ratio of
the mean stroke volume over the cube of the mean myocardial
thickness change.
[0029] The present invention still further provides a method for
calculating a cardiac myopathy value (M). The method comprises
determining a cardiac output value, typically as described above,
measuring a maximum pressure difference between a right atrium and
a left ventricle, measuring a myocardial thickness change at two
points in the cardiac cycle, preferably diastole and systole,
determining a difference in myocardial contraction force at a
location on the myocardium, and determining the cardiac myopathy
value based at least in part on each of the determined and measured
values.
[0030] The maximum pressure difference is usually measured as the
difference between maximum left ventricular pressure and the
minimum right ventricular pressure during a cardiac cycle. The
maximum left ventricular pressure and the minimum right ventricular
pressure are preferably measured with pressure transducer present
simultaneously in the left and right ventricles, where the pressure
transducers are preferably implanted in a ventricular wall or
across the ventricular septum. The change in myocardial thickness
is preferably measured by a sensor assembly implanted across the
myocardial wall, and the change in myocardial contraction force is
preferably the difference between a maximum force at a location and
a minimum force at the same location. Such maximum and minimum
contraction forces are preferably determined with a force
transducer implanted in or on the myocardium. Alternatively, the
differences in myocardial contraction force may be determined using
a myocardial stiffness sensor and a myocardial thickness sensor
implanted in or on the myocardium. In particular, the cardiac
myopathy value may be determined as a ratio of a first product of
the cardiac output value times the maximum pressure difference and
a second product of the change in myocardial contraction force and
the change in myocardial thickness.
[0031] The present invention still further provides methods for
calculating a cardiac elasticity value. Such methods comprise
measuring a change in myocardial thickness between two points in a
cardiac cycle. A change in myocardial contraction force is measured
between the same two points in the cardiac cycle, and the cardiac
elasticity value is then based at least in part on a ratio of the
changes in myocardial thickness and contraction force. The change
in myocardial thickness and myocardial contraction force may be
measured by the methods set forth in more detail above. Both the
myocardial thickness and myocardial contraction force are
preferably measured with implanted sensors, more preferably by
sensors implanted on a common device, frame, or other structure.
Preferably, cardiac elasticity value is then calculated as the
ratio of a first product of the myocardial force change and an
average myocardial thickness over a second product of the average
myocardial force and the change in myocardial thickness.
[0032] Methods of the present invention may also be used to
calculate a myocardial power (MyP) value characteristic of a
patient's heart. This parameter represents the instantaneous power
output of the myocardial muscle tissue. It is calculated by taking
a first measurement of force, F.sub.1 and thickness, D.sub.1 at
time t.sub.1 and shortly thereafter taking a second measurement of
force, F.sub.2 and thickness, D.sub.2 at time t.sub.2. Myocardial
power is then defined by the equation: MyP = F 2 .times. D 2 - F 1
.times. D 1 t 2 - t 1 ##EQU1##
[0033] Methods of the present invention may also be used to
calculate a ventricular power (LVPo for the left ventricle, RVPo
for the right ventricle) value characteristic of a patient's heart.
This parameter represents the instantaneous power output of the
heart. It is calculated by taking a first measurement of pressure,
P.sub.1 and thickness, D.sub.1 at time t.sub.1 and shortly
thereafter taking a second measurement of pressure, P.sub.2 and
thickness, D.sub.2 at time t.sub.2. Thickness D.sub.1 is translated
into volume V.sub.1 and thickness D.sub.2 is translated into volume
V.sub.2 using the calibration method referred to earlier.
Instantaneous cardiac power is then defined by the equation: VPo =
P 2 .times. V 2 - P 1 .times. V 1 t 2 - t 1 ##EQU2## where for the
left ventricle, P is LVP, V is Left Ventricular Volume, and for the
right ventricle, P is RVP, and V is Right Ventricular Volume.
[0034] Methods of the present invention may also be used to
calculate left ventricular power efficiency (LVPOE) value, which is
the ratio of left ventricular power over the product of myocardial
power and left ventricular surface area, and right ventricular
power efficiency value (RVPOE), which is the ratio of right
ventricular power over the product of myocardial power and right
ventricular surface area. Ventricular surface area may be
determined using catheters described in copending U.S. patent
application Ser. No. 10/734,490 (Attorney Docket No.
21308-000510US), the full disclosure of which is incorporated
herein by reference, and U.S. patent application Ser. No. 10/______
(Attorney Docket No. 21308-000810US), filed concurrently herewith
and also fully incorporated herein by reference. Ventricular
surface area may also be estimated using external ultrasound
imaging equipment. Ventricular surface area is the ratio of the
diastolic ventricular surface area over the area of the myocardial
tissue sampled by the force sensor. In between catheterizations, it
is assumed to be constant, but may be modified using the
hypertrophy parameter to estimate the increase in surface area of a
ventricle.
[0035] Methods of the present invention may also be used to
calculate systolic ventricular power (SLVP for systolic left
ventricular power and SRVP for systolic right ventricular power).
These parameters represent the power exerted by the respective
ventricle during systole and may be calculated using the equation:
SLVP = .intg. SEP .times. PV .times. .times. d t SEP ##EQU3## where
SEP is the well know systolic ejection period, and P is LVP and V
is left ventricular volume, determined using D and a calibration
table.
[0036] Methods of the present invention may also be used to
calculate systolic myocardial power (SMyP) which represents the
power generated by the myocardial tissue surrounding a ventricle.
This parameter may be calculated as SMyP = .intg. SEP .times. FD
.times. .times. d t SEP ##EQU4## where F is the force exerted by
the myocardium on the sensor, D is the thickness of the myocardial
tissue and SEP is the systolic ejection period.
[0037] These various power measurements are important extensions of
the well-known parameter dP/dt, which is the maximum rate of
increase in pressure in a ventricle and has been used as a proxy
for contractility of the heart. dP/dt, however, is affected by at
least four well known parameters, which reduces the parameter's
predictive ability. Power is important because of its connection to
metabolism and consumption of nutrients including oxygen. A heart
under two different medications may perform the same amount of
work, but performs that work under one medication in a shorter
period of time, and therefore at higher power. The higher power
heart condition should deplete the intra-cyclic concentration of
more nutrients and be more susceptible to ischemia. The ability of
a heart to deliver more power, conversely, is also a sign of
myocardial health. In addition, the power output of a ventricle
normalized by the power output of the myocardium produces a
parameter that should be the same for all healthy hearts, but is
likely to decrease for diseased hearts. Thus, this power efficiency
parameter should be a useful monitor for the contribution of any
intervention or medication.
[0038] Methods of the present invention may also be used to
calculate a tamponade value (TV), that is useful for the diagnosis
of tamponade. During tamponade, the pericardial fluid causes the
septum to bulge from the right ventricle towards the left. This
increases the effective volume of the right ventricle and decreases
the effective volume of the left ventricle. To generate the
tamponade parameter, the stroke volume (SV) is measured as
described above, as is the thickness of the septum at end diastole
(D.sub.end-diastole) and the thickness of the septum at end systole
(D.sub.end-systole). From these thickness values, end systolic and
end-diastolic left and right ventricular volumes (LVESV, LVEDV,
RVESV, and RVEDV, respectively) are determined using the
calibration table created during the most recent catheterization.
In addition, left regurgitant volumes and right regurgitant volumes
are determined from another calibration table also prepared during
the most recent calibration catheterization. From these volumes,
the calculated ejection volumes for the left and right ventricles
may be compared to the stroke volume (SV). Left ejection volume
LEV=LVEDV(D.sub.end-diastole)-LVESV(D.sub.end-systole). Left
predicted stroke volume (LPSV)=LEV-MRV(D.sub.end-diastole)
MRV(D.sub.end-systole), where MRV are parameters derived during the
catheterization calibration to estimate mitral regurgitation.
Similarly, right ejection volume (REV) may be calculated as
RVEDV(D.sub.end-diastole)-RVESV(D.sub.end-systole). Right predicted
stroke volume (RPSV) may be calculated as
REV-TRV(D.sub.end-diastole) TRV(D.sub.end-systole), where TRV are
parameters derived during the catheterization calibration to
estimate tricuspid regurgitation. End diastolic and end systolic
volumes may also be determined from two look-up tables, the first
between end-diastolic pressure and volume and the second between
end-systolic pressure and volume, both created during the most
recent catheterization.
[0039] If the regurgitant flow is unchanged from calibration and
there is no tamponade, then the ejection volumes for the left side
should be the same as the right. Regurgitant flow is compensated
for using a compensation algorithm implemented during the
calibration catheterization (compensates for the level of
regurgitant flow that existed at the time of the calibration). Once
the regurgitation is compensated for then the actual Stroke Volume
should equal both Predicted Stroke Volumes, as defined above.
[0040] If there is Tamponade, then the new right ventricle would be
larger and the "new" left ventricle would be smaller than at
calibration. Thus, a change in septal thickness would correspond to
a change greater than that predicted for the right ventricular
volume and a change less than that predicted for the left
ventricular volume. The actual stroke volume would be less than the
predicted stroke volume. This is due to the ejection volume of the
right ventricle being greater than the ejection volume of the left
ventricle, while blood gradually pools in the lungs (which is what
eventually leads to death, if left untreated). Thus the left
predicted stroke volume is greater than the actual stroke volume of
the left ventricle. The parameter tamponade (T) is thus simply the
ratio of left predicted stroke volume over actual stroke volume.
When T increases abruptly, it implies a tamponade condition may be
occurring. Alternatively, if T increases very, very gradually, it
implies a gradual increase in mitral regurgitation. Thus, T is
defined by the ratio LPSV/SV, where LPSV and SV are as defined
above.
[0041] Methods of this invention may also be used to calculate a
re-synchronization correlation, .rho.p,d. A re-synchronization
correlation, .rho.p,d, is calculated by simultaneously measuring
the pressure in the left ventricle and the thickness of part of the
myocardium. In a synchronized heart, regardless of where one
measures the thickness, the maximum thickness typically occurs
simultaneously with the maximum pressure. Using one or more
sensors, the thickness of the myocardium may be measured at
multiple locations. The re-synchronization correlation is the
correlation between the thickness measurement and the pressure
measurement and is defined as follows: .rho. p , d = Cov .function.
( P , D ) .sigma. p .sigma. d ##EQU5## where .times. : .times. - 1
< .rho. p , d < 1 ##EQU5.2## and .times. : ##EQU5.3## Cov
.function. ( P , D ) = 1 n .times. i = 1 n .times. .times. ( P i -
.mu. p ) .times. ( d i - .mu. d ) ##EQU5.4## and .sigma. is the
standard deviation and .mu. is the mean value.
[0042] Thus, for a set of data points taken during a cycle, the
synchronization between when the muscles are contracting and when
the pressure generated is maximized when the re-synchronization
index is closest to 1. This may be used to optimize a pacing
therapy, for example, by varying the various delays
(interventricular delay and the atrial-ventricular delay) or the
location of the stimulating electrodes to maximize the
resynchronization index, R. This optimization could be done
automatically by the implanted pacing system or by an external
optimization system as part of a clinic visit.
[0043] Another similar variable with the same purpose is the
resynchronization phase, .THETA..sub.R. This parameter is defined
as ratio of the time between when the maximum thickness occurs and
when the maximum pressure occurs and the time between successive
pressure maxima .THETA. R = T P .times. .times. max - T D .times.
.times. max T P .times. .times. max .times. .times. i - T P .times.
.times. max .times. .times. i - 1 ##EQU6##
[0044] Synchronization of the contraction of the myocardium is
maximized when the resynchronization phase is equal to zero.
[0045] A related parameter is the resynchronization parameter, R. R
is the ratio between the cyclic pressure gain and the cyclic
thickness change. The cyclic pressure gain is the maximum pressure
in a single cardiac cycle minus the minimum pressure in the same
cardiac cycle. Similarly, the cyclic thickness change is the
maximum thickness in a cardiac cycle minus the minimum thickness in
the same cardiac cycle. Typically, the cyclic pressure gain
measures the left ventricular pressure.
[0046] Thus, an optimization system would vary the various timing
delay between the various electrodes and the various electrode
positions by first bringing .THETA..sub.R as close to zero as
possible, and then maximizing R and finally maximizing
.rho.p,d.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1 illustrates a cardiac performance monitoring system
constructed in accordance with the principles of the present
invention.
[0048] FIG. 2 is a schematic illustration of the system according
to FIG. 1 including sensor interfaces.
[0049] FIG. 3 illustrates a patient's heart with a plurality of
implanted sensors according to the principles of the present
invention.
[0050] FIG. 4 illustrates a first exemplary implantable sensor
structure carrying multiple sensors and constructed in accordance
with the principles of the present invention.
[0051] FIG. 5 illustrates a second exemplary implantable sensor
having a paired anchors and carrying coils to permit measurement of
displacement in accordance with the principles of the present
invention.
[0052] FIG. 6 illustrates a polarographic oxygen sensor.
[0053] FIG. 7 illustrates an array of the oxygen sensors of FIG.
6.
[0054] FIG. 8 illustrates a sensor intended to measure myocardial
elasticity or contractility.
DETAILED DESCRIPTION OF THE INVENTION
[0055] The present invention is directed at apparatus, systems, and
methods for their use in monitoring a patient condition,
particularly in monitoring a patient's cardiac performance. The
apparatus of the present invention will usually comprise individual
components, and the systems will comprise two or more components
arranged to function together to permit patient monitoring. At a
minimum, the systems of the present invention will include one or
more implantable devices which will include one or more sensors
adapted to measure one or more physiologic parameters of the
patient. The systems will also include external component(s),
referred to generally hereinafter as an external controller, which
are capable of acquiring data generated by the implantable devices
representative of the physiologic parameters being monitored.
Usually, the external controller will also be capable of powering
the implantable devices, controlling or altering performance of the
implantable devices, generating one or more calculated parameters
or performance values based on the acquired measured parameters,
and the like. Optionally, the systems of the present invention may
include a "repeater" which acts as an interface between the
implanted sensor devices and the external controller. The repeater
may perform a variety of functions, including power storage and
transmission, data-storage and transmission, programming storage,
and perhaps most importantly receiving and retransmitting signals
from the implanted sensor devices to and/or from the external
controller. Other functions for the repeater may also be included.
The repeater will often be implanted and may be connected or
coupled to the implanted sensors via wired or wireless links.
Examples of implanted sensors disposed on a catheter connected to
an implanted repeater are described in co-pending application Nos.
60/432,929, filed on Dec. 11, 2002 (Attorney Docket No.
21308-000500US), and 60/______ (Attorney Docket No. 21308-000800),
filed on the same day as the present application, commonly assigned
with the present application, the full disclosures of which have
previously been incorporated herein by reference.
[0056] The overall system functions may be distributed among the
various system components in a variety of ways, often dependent on
the particular intended use of the system. For example, power
transmission and storage may be provided in a variety of ways.
Usually, the implanted sensor devices will at least include a coil
or other passive means for receiving power transmissions, either
from the repeater or from the external controller. Alternatively,
the implanted sensors may also include batteries or other power
storage components which may be periodically recharged via the coil
or other power receiving device. Similarly, the repeater (if used)
will typically include at least a coil or other passive power
receiving device and will often include a battery or other storage
component as well. The repeater will also typically include power
transmission means in order to power the implanted sensor devices.
Finally, the external controller will typically include a power
transmission unit, most typically being a radiofrequency power
transmission unit, for powering the implanted sensors and/or
repeater.
[0057] In addition to such distributed power transmission and
storage capabilities, data storage and control circuitry may also
be distributed among the various system components. Even the
implanted sensor devices may include circuitry for storing data
and, in some instances, performing some level of data analysis,
although this will generally not be preferred. If used, the
repeater will also optionally include data transmission and
analysis capabilities. Usually, the repeater will include at least
data storage capabilities, permitting periodic interrogation by the
external controller to acquire data which will have been generated
over some extended time period. In virtually all instances, the
external controller will include data storage capability as well as
data analysis and system control capability. As mentioned above,
the external controller will usually further include means of
analyzing the data to produce calculated performance values or
parameters based on the measured physiologic parameters which are
collected. It will be appreciated, of course, that the latter
analysis may be performed by separate dedicated or general purpose
computers which may be interfaced with the external controller in
order to download and analyze the patient information.
[0058] Also optionally, the systems of the present invention may
include devices and components for collecting external patient and
other parameters and data. In particular, the systems may include
an analyzer adapted to measure oxygen consumption of the patient,
typically during a time when other internal physiologic parameters
are being measured. Additionally, the systems may include devices
for measuring ambient pressure, temperature, patient temperature,
patient blood pressure, patient pulse rate, and the like. All such
collected external data may be obtained while the implanted sensors
are being interrogated to obtain patient internal data. In this
way, both internal and external data can be acquired simultaneously
and used to perform a number of calculations and analyses as
described in more detail below.
[0059] Referring now to FIG. 1, an exemplary system 100 constructed
in accordance with the principles of the present invention
comprises a plurality of implantable cardiac sensor devices 102
implanted in a heart H of a patient P. The sensors 102 may be
adapted to measure a variety of physiologic parameters
characteristic of the heart function. As described above,
particular physiologic parameters include both "cardiac parameters"
which are generally characteristic of the overall heart function as
well as "myocardial characteristics" which are representative of a
more local heart function, particularly the function of a localized
region of heart tissue which may have been damaged or compromised
by myocardial infarction or other disease.
[0060] Data generated by the implantable cardiac sensor devices 102
are transmitted or otherwise delivered to the external controller
104 which collects and usually analyzes the data. While in some
cases it will be possible to directly transmit data from the
implanted cardiac sensor devices 102 to a remote or tabletop
external controller, usually it will be desirable to have at least
a scanner 106 connected to the external controller 104 (either by
wire or wireless link 107), to enhance the signal transmission
efficiency. Further optionally, a repeater 108 may be disposed
between the implanted sensors 102 and the controller 104 and/or
scanner 106. A repeater 108 may itself be implanted or could be
worn externally by the patient, e.g., on a belt, wrapped around the
waist, or by other conventional techniques. The repeater 108 will
come at a minimum, receive data generated the implanted sensors 102
(by either a wire or wireless link), and re-transmit that data,
typically at a greater power level than would be possible by the
implanted cardiac sensor devices 102 alone. A repeater 108 may
include a variety of other functions as listed above, including
power storage, power transmission, data storage, data analysis,
etc.
[0061] In addition to receiving the internal physiologic parameters
which are measured and transmitted by the implanted cardiac sensor
devices 102, the external controller 104 may receive various
external physiologic and other parameters. Perhaps most usefully, a
mouthpiece 110 or other breath collection device may be used to
monitor patient inspiration, i.e., inhalation and exhalation.
Usually, both the quantity of air inhaled and exhaled as well as
the oxygen content of the air inhaled and exhaled will be measured
and collected. Based on such collected data, patient oxygen
consumption can be readily calculated over a desired period of
time. By measuring oxygen consumption over a fixed time period
during which certain cardiac performance characteristics are also
being measured and collected, a variety of conventional and novel
cardiac performance values may be calculated, as described below in
detail.
[0062] Systems according to the present invention may take a
variety of specific forms, including both specialized and
off-the-shelf equipment. Usually, the implanted cardiac sensor
devices will be specially fabricated in accordance with the
principles of the present invention, although it may be possible,
in some instances to employ more conventional sensor devices which
may be commercially acquired now or in the future. Similarly, the
repeaters useful with the present invention will usually be
specially fabricated to function together with the other system
components, although it may be possible in the future to acquire
such implantable or wearable devices commercially. Of all system
components, it is most likely the external controller that may at
least partly comprise commercially available equipment. Often
general purpose computers and workstations may be programmed to
perform many of the functions and calculations of the systems and
methods of the present inventions.
[0063] Referring now to FIG. 2, the external components of the
system 100 illustrated in FIG. 1 are shown in more detail. In
addition to the external controller 104, scanner 106, and
mouthpiece 110, the external portions of the system may include
other input and interface devices and components. In particular,
when the external controller 104 is a general purpose computer or
workstation, it will usually be desirable to provide interface
circuitry 112 for the scanner 106 to facilitate input of internally
generated data from the implantable cardiac sensor devices 102.
Similarly, interface circuitry 114 may be provided for all
externally acquired data, including breath data acquired through
the mouthpiece 110, data obtained through ambient sensors 116,
(such as pressure, temperature, etc.), and optional further
channels 118 of such external data. Although illustrated as
separate boxes 112 and 114, the interface circuitry may of course
be packaged together in a single box and may optionally be included
physically together with the external controller 104, typically
when the controller is not an off-the-shelf computer or
workstation.
[0064] For the preferred systems 100 including breath analysis
capabilities, it will of course be necessary to provide the
mechanical, chemical, and electrical components necessary to permit
both the breath quantity as well as oxygen content to be
determined. In many instances, such information can be obtained
from commercially available breath analysis equipment, such as that
is available from the Laser Spectroscopy Oxygen Analyzer that is
available from the Oxygraf Company of Mountain View, Calif. In such
instances, the output of the breath analyzer will go directly into
the interface 114 which can condition the output. The interface 114
may further provide analogue to digital signal conversion or other
signal interfacing function necessary to allow the external
controller 104 to utilize data generated by the breath analysis
circuitry.
[0065] The external controller 104 may provide direct user
input/output capabilities, i.e., including screens, printer
interfaces, read/write data storage capabilities, etc. Optionally,
the external controller 104 may require interface with a further
computer, workstation, or other device which interfaces directly
with the user and provides the input/output capabilities. In all
cases, the external controller 104 may provide data input/output
connections shown schematically as line 120.
[0066] Referring now to FIG. 3, the implantable cardiac sensors 102
may take a variety of forms, with exemplary forms shown as
102a-102d. In the illustrated embodiments 102a-102d, the sensor
devices comprise a frame or platform which is attached to the
myocardium or other surface of the heart H as well as one or more
sensors capable of measuring particular cardiac parameters, as
described in more detail below. The frames or platforms of the
sensor devices 102 may take a variety of forms, including a simple
patch, disk, mesh, membrane, or other surface which is attached to
a myocardial or other heart surface, as shown at 102a. Such simple
platforms may be sutured, tacked, stapled, or screwed into their
desired target locations. The target locations may be endocardial
(as shown), or may be epicardial, on a septum, or elsewhere. The
sensors on the platform will be arranged to measure either
myocardial parameters, typically being directed inwardly toward the
myocardial tissue, or may be intended to measure the "cardiac"
characteristics such as blood pressure or other parameters within a
heart chamber or elsewhere.
[0067] In addition to the simple surface-mounted platforms (as
illustrated in 102a), the implantable cardiac sensor devices may
include platform pairs mounted on opposite surfaces of a heart
wall, as shown at 102b. In particular, the two platforms of device
102b need not be mechanically connected, but will typically be
arranged directly across from one another or some other
predetermined pattern relative to one another on the opposite
surfaces of the heart wall (including septums). Usually, the two
platforms of the device 102b will be intended to interact in some
predetermined fashion. A specific such structure is described and
illustrated in connection with FIG. 5 below.
[0068] Sensors 102 may also penetrate a heart wall or septum
(referred to collectively herein as "myocardial walls"). Sensor
device 102c comprises a central shaft or tether 130 (FIG. 4) having
buttons or platforms at each end. A particular structure for such a
cardiac sensor is described in connection with FIG. 4 below.
[0069] The fourth exemplary implantable cardiac sensor device 102d
is similar to 102c, except that the two buttons or platforms are
generally equal size on either side of the myocardial wall. Such
equal or at least enlarged platforms allow the device to apply
tension which may be useful in a variety of particular
measurements.
[0070] Referring now to FIG. 4, the implantable cardiac sensor
device 102d comprises a first or base platform 132 and a second or
remote platform 134 located at opposite ends of the shaft or tether
130. The shaft or tether 130 will usually include flexible wires,
optical waveguides, or other elements for interconnecting sensors
located on the respective platforms 132 and 134. The shaft or
tether itself may be rigid or flexible, optionally being at least
somewhat elastic to permit relative movement of the first and
second platforms 132 and 134 as the myocardial tissue expands and
contracts during the cardiac cycle.
[0071] Each of the platforms 132 and 134 will carry one or more
sensors intended to measure physiologic parameters or
characteristics, as generally set forth above. For example, either
of the platforms 132 or 134, or both, may carry oxygen sensors 136,
pressure sensors 138, coils 144 receiving power and/or data and
optionally transmitting data back to the repeater or external
controller interface. Additionally, the base platform 132 may have
a circular orthogonal, or other stain gauge 142 imprinted on the
surface thereof. The strain gauge 142 will thus permit measurement
of lateral deflection of the myocardial tissue in which the sensor
device 102d is implanted. Usually, the platform will include prongs
144 which secure the base platform 142 in tissue so that as the
tissue expands in the direction of arrows 146 (or contracts in the
opposite direction), the strain gauge 142 can measure the
deformation and thus the expansive force being generated by the
tissue at the location where the sensor is implanted. Additionally,
sensor 102d can include inductance coils 150 and 152 which permit
tracking elongation of the sensor along the axis of shaft or tether
130. Tracking elongation permits calculation of the expansive force
of the myocardial tissue in the direction normal to the tissue
plane. The particular uses of the data collected by the sensor 102d
(as well as all other sensor described herein) will be described in
more detail below in connection with the calculation of specific
cardiac performance values.
[0072] It should be appreciated that the sensor 102e is illustrated
to work by wireless transmission, either with an implanted or
wearable repeater or directly with the external controller.
Information and power to the sensor 102d, however, could also be
obtained through wired connections, either with a repeater or
directly with an intravascular catheter. In a particular
embodiment, the repeater could be part of a cardiac pacing system
wherein the pacing leads provide for wired connections to the
implanted cardiac sensor devices 102.
[0073] An exemplary cardiac sensor device 102b is illustrated in
FIG. 5. As shown, sensor 102b comprises a first platform 160 and a
second platform 162. The platforms 160 and 162 are independently
mounted on opposite surfaces of a heart wall, such as an outer wall
of the right ventricle, as illustrated in FIG. 3, or alternatively
on a septum or other outer heart wall. As the platforms 160 and 162
are not mechanically linked, they will "float" freely relative to
each other as the heart wall or septum deforms during the cardiac
cycle. By providing inductance coil pairs 164/166 and 168/170,
relative movement of the platforms 160 and 162 in the longitudinal
direction (z axis) and horizontal direction (x axis), the relative
movement can be monitored by the alternating current signal induced
by one coil in the other coil of the pair. By providing a third
coil pair in the orthogonal direction, the remaining axis could
also be followed and angular deformations determined. Note that the
platforms 160 and 162 will typically also comprise RF coils for
receiving power and transmitting signals, and additional sensors
could be placed on either or both of the platforms in accordance
with the general principles of the present invention. A deformable
coil may also be used to measure thickness. Such a coil would have
a variable inductance that is a function of the myocardial
thickness.
[0074] Particular sensors for directly measuring cardiac and
myocardial characteristics may take a wide variety of forms,
including many conventional sensor constructions which are known
and described in the patent and medical literature. Particular
sensors which may be utilized in the apparatus and systems of the
present invention are illustrated in FIGS. 6-8. In FIG. 6, a
polarographic oxygen sensor is illustrated. Blood oxygen enters the
sensor 180 through a permeable membrane 182, such as a PTFE
membrane. The oxygen enters a solution of concentrated potassium
chloride 184 where a redox reaction occurs generating a current I
between a silver electrode 186 and a platinum electrode 188 which
are positioned over a silicon oxide substrate 190. The generated
current I is directly proportional to the blood oxygen
concentration.
[0075] Since the chemical oxygen sensors of FIG. 6 have a finite
life, they may be arranged in an array of identical sensors 180, as
illustrated in FIG. 7. The component parts of the arrays are
hermetically sealed until it is desired to use them. Each part's
seal may be selectively dissolved, e.g. electrochemically or
thermally, so that individual sensors may be used at staggered
times. Optionally, multiple sensors can be used simultaneously in
order to reduce random errors or calibration offsets.
[0076] Referring now to FIG. 8 myocardial elasticity or
contractility may be directly sensed using a mechanical sensor
assembly 220. Sensor 220 includes a probe 222 which may be
mechanically advanced in the direction of arrow 224 so that engages
myocardial tissue. By applying a known force, and measuring the
degree of tissue displacement caused by the force, the elasticity
or contractility can be calculated. Conversely, the probe could be
moved by a known distance with the amount of required force being
tracked in order to calculate elasticity or contractility.
[0077] The apparatus and systems of the present invention are
useful primarily to collect physiologic parameters from a patient,
to store those parameters for subsequent analysis, and to calculate
myocardial performance characteristics based on such measured
physiologic parameters. Exemplary physiologic parameters that can
be directly measured using implanted and other sensors according to
the present invention are listed in Table I below. TABLE-US-00001
TABLE I Directly Measured Physiologic Parameter Symbol Pressure P
Displacement D Force F Oxygen Concentration O2 Pulse Rate BP
Quantity Air Breathed Q Volume V Myocardial Elasticity ME
[0078] A variety of sensors are available for measuring the
parameters set forth in Table I. Preferred are those sensors which
can be miniaturized to fit on the platforms described above which
are incorporated in the implantable sensor devices of the present
invention. As noted previously, the platforms will preferably have
areas in the range from 1 mm.sup.2 to 200 mm.sup.2, preferably 2
mm.sup.2 to 100 mm.sup.2, and often from 5 mm.sup.2 to 50 mm.sup.2.
After inserting the phrase, and often from 5 mm.sup.2 to 50
mm.sup.2. The large platform sizes will typically be used for radio
frequency (RF) communication with a repeater or in some instances
with a transmitter/receiver located external to the patient.
Typically, repeaters will reside on the right side of the heart or
in the pericardium. Smaller platform sizes will typically be used
for sensor placement, e.g. for sensors intended for placement on or
in the left-heart epicardium. The sensors will preferably have
footprints which are less than these available areas, typically
being from 0.001 mm.sup.2 to 1 mm.sup.2, preferably from 0.01
mm.sup.2 to 0.1 mm.sup.2. Such size limitation will, of course, not
apply to the sensors needed to measure the quantity of air
breathed, the oxygen concentration the air breathed, external pulse
rate, or the like.
[0079] Based on the measured physiologic parameters, the present
invention further provides for calculation of a number of cardiac
performance values as set forth in Table II below. TABLE-US-00002
TABLE II Calculated Cardiac Performance Value Algorithm Cardiac
Output (CO) Q .function. ( O .times. .times. 2 inhaled - O .times.
.times. 2 exhaled ) ( O .times. .times. 2 left .times. .times.
ventricle - O .times. .times. 2 right .times. .times. ventricle )
##EQU7## Stroke Volume (SV) CO/BP Left Ventricular Pressure
P.sub.left ventricle - P.sub.ambient (LVP) Right Ventricular
Pressure P.sub.right ventricle - P.sub.ambient (RVP) Ventricular
Performance Value (I.sub.1) (ischemia) ( P maximum .times. .times.
ventricular - P minimum .times. .times. ventricular F myocardial
.times. .times. maximum - F myocardial .times. .times. minimum )
##EQU8## Ventricular Performance Value (I.sub.2) (ischemia) ( P
maximum .times. .times. ventricular - P minimum .times. .times.
ventricular D myocardial .times. .times. maximum - D myocardial
.times. .times. minimum ) ##EQU9## Hypertrophy Value (H) SV ( D
myocardial .times. .times. maximum - D myocardial .times. .times.
minimum ) 3 ##EQU10## Cardiac Myopathy Value (M) SV .function. ( P
left .times. .times. .times. ventricular .times. .times. maximum -
P right .times. .times. ventricular .times. .times. minimum ) ( F
myocardial .times. .times. maximum - F myocardial .times. .times.
minimum ) ( D myocardial .times. .times. maximum - D myocardial
.times. .times. minimum ) ##EQU11## Cardiac Elasticity Value (E) (
P ventricular .times. .times. maximum - P ventriculal .times.
.times. minimum ) .times. V ventricular .times. .times. maximum (
SV ) .times. ( P ventricular .times. .times. maximum ) ##EQU12##
Myocardial Elasticity (ME) ( F mycardial .times. .times. maximum -
F mycardial .times. .times. .times. minimum F mycardial .times.
.times. maximum + F mycardial .times. .times. minimum ) .times. ( D
mycardial .times. .times. maximum + D mycardial .times. .times.
.times. minimum D mycardial .times. .times. maximum - D mycardial
.times. .times. minimum ) ##EQU13## Systemic Resistance (R.sub.s)
(P.sub.left ventricular maximum - P.sub.right ventricular
minimum)/CO Pulmonary Resistance (R.sub.p) (P.sub.right ventricular
maximum - P.sub.left ventricular minimum)/CO
[0080] A number of the above numbers are dimensionless, allowing
comparison of the values among patients, e.g. the hypertrophy value
(H) and the elasticity values (E and ME). In other instances,
however, the calculated cardiac performance values are not
dimensionless, but may be "normalized" so that the values become
dimensionless and may be compared among patients. For example, the
ventricular performance value (I) may be normalized by multiplying
the .DELTA.P/.DELTA.F value by the area of the sensors or an area
of the patient's heart or ventricle. The cardiac myopathy value (M)
may be normalized by a ratio of the area of sensors used to make
the measurements (SA/(SA.sub.LV+SA.sub.RV)) over a cardiac area of
the patient.
[0081] Usually, the algorithms set forth above for calculating the
various cardiac performance values will be programmed into the
external controller and/or a computer or workstation associated
with the external controller. Certain of the cardiac performance
values, such as cardiac output and stroke volume will be utilized
in calculation of a number of the other cardiac performance values.
It is noted that certain of the cardiac's performance values, such
as the ventricular performance value and the ventricular
elasticity, may be calculated separately for each of the left and
right ventricles. Thus, when referring to a ventricular value for
the calculation of either of these performance values, all values
should come from either the right ventricle or the left ventricle,
depending on the ventricle for which the value is to be
calculated.
[0082] The various directly measured physiologic parameters and
calculated cardiac performance values may be determined
periodically or over extended periods of time. It is noted,
however, for those values which require measurement of cardiac
output or stroke volume, measurements will usually only be
performed while the patient is wearing a mask or other hardware for
determining oxygen consumption. The values which do not require
measurement of cardiac output or stroke volume, in contrast, could
be measured continuously, even while the patient is ambulatory. For
such ambulatory measurements, the repeater or other circuitry will
usually be used to store data, and such collected data will be
periodically transferred to the external controller or other
evaluation apparatus. By periodically and/or continuously measuring
some or all of the physiologic parameters and cardiac performance
values, the health of the patient's heart can be followed over
time. After establishing a base line, deviations from the baseline
will be indicative of either deterioration of heart performance or
more hopefully stabilization or even improvement in heart
performance. Monitoring deviations from the baseline performance
can provide a more quantitative measurement of a patient's response
to medications, potentially allowing adjustment in dose levels or
changes to different medications as the cardiac performance
changes.
[0083] In addition to those physiologic parameters and calculated
cardiac performance values described above, the systems and methods
of the present invention may be utilized to allow measurement and
calculation of a variety of other parameters which have
conventionally been obtained during cardiac catherization. For
example, ventricular pressure may be measured by simply subtracting
ambient pressure from the measured ventricular absolute pressure.
Left ventricular end diastolic pressure may be determined as the
left ventricular pressure measured at the time the ventricle begins
to contract, which may be signaled for example, by an EKG or by the
pressure signal itself. Alternatively, the pressure may be measured
at the point when myocardial thickness is at a minimum, which may
be uniquely established with the methods and systems of the present
invention.
[0084] Left ventricular end systolic pressure may similarly be
measured as the left ventricular pressure at the point when the
ventricle has just contracted to eject blood. This may be
determined based on the EKG, the pressure signal itself, or
alternatively, when the myocardial thickness is at a maximum.
[0085] Other parameters measured by the system may also find use,
such as ventricular stiffness which can be the ratio of myocardial
force over myocardial thickness. This parameter may also be
directly measured using the probe device of FIG. 8.
[0086] An alternate method of determining ventricular stiffness
which is more directly correlated to the method in common use today
is to first convert a myocardial thickness measurement into a
number that represents the volume of the left ventricle. This is
done by establishing during implantation a relationship, e.g., a
look-up table between myocardial thickness and left ventricular
volume. Then, during use, the measured myocardial thickness can be
converted into a number that represents ventricular volume. This
ventricular volume number may be corrected for subsequent
hypertrophy of the left ventricle using the hypertrophy parameter,
H, defined above, which quantifies the change in the relationship
between cardiac output as measured using Fick's law (oxygen
concentration difference) and myocardial thickness change. This
corrected estimate of left ventricular volume is then compared with
the simultaneously measured left ventricular pressure. By plotting
this estimate of ventricular pressure versus ventricular volume,
the pressure/volume loop of the heart may be estimated. From this
curve, the end-diastolic pressure/volume relationship is plotted
for various end-diastolic volumes. The derivative of these plotted
points (dP/dV) becomes an estimate of the lusitropic stiffness. The
inverse, dV/dP becomes an estimate of the lusitropic compliance.
Similarly, plotting these data pairs at end-systole gives yields
the inotropic stiffness and compliance. Specifically, the
end-systolic dV/dP slope is the inotropic compliance and the
end-systolic dP/dV is the inotropic stiffness.
[0087] These measures of inotropic compliance and lusitropic
compliance may be used to predict and document the response of the
patient to inotropic agents, such as digitalis or cardiac
glycosides. In addition, since the implant provides measures of
cardiac output, left ventricle end diastolic pressure, and
stiffness (inverse of compliance) as well as pulse rate, the doctor
will be able to monitor the efficacy of a given inotropic treatment
regime as a function of dosage and pulse rate for each patient.
This could allow for precise dosage levels of substances where too
much can cause immediate problems and too low can result in long
term deterioration of the heart. For patients with pacing devices,
the ability to measure the stiffness as a function of pulse rate
would describe an optimal pulse rate for a given dosage. This
information, coupled with the data from the oxygen sensor in the
right ventricle and the pressure in the left ventricle could be
used to tailor the pulse frequency to produce a heart with improved
compliance. In addition, a biventricular or multiple lead pacing
system could have vary the timing of the electric pulses to various
leads to produce an optimal compliance at a given pulse rate. Thus,
at a low pulse rate, the optimal timing might be setting one,
producing the best possible stiffness and maximum cardiac myopathy
(efficiency) (M) values for that pulse rate, but at a higher pulse
rate necessitated by activity as measured by the oxygen sensor in
the right ventricle, the timing values might change to a different
setting to produce the best possible stiffness and efficiency (M).
The pulse rate, within limits selected by the doctor, may be set by
the blood oxygen concentration in the right ventricle.
[0088] Pulmonary Resistance, R.sub.P, is measured using the cardiac
output CO and the pressure readings LVAP and RVAP, which are the
absolute pressures of the left and right ventricles, respectively.
Most textbooks will define pulmonary impedance--approximated as a
resistance--as the maximum pressure drop between the pulmonary
artery and the pulmonary vein divided by the cardiac output. This
definition implies that all of the cardiac output is passing
through the pulmonary system at the highest pressure gradient,
which of course is not precise. Nevertheless, this approximation
serves as a reasonable proxy for mean pulmonary resistance. Using
this approximation, the maximum pulmonary pressure gradient (PPG)
is PPG=maximum (RVAP) minus minimum left ventricle pressure (LVAP)
Then, Pulmonary Resistance is simply CO/PPG.
[0089] Systemic Resistance is similar. The maximum Systemic
Pressure Gradient (SPG) is SPG=maximum (LVAP)-minimum (RVAP). The
Systemic Resistance (R.sub.S) is simply R.sub.S=CO/SPG.
[0090] While the above is a complete description of the preferred
embodiments of the invention, various alternatives, modifications,
and equivalents may be used. Therefore, the above description
should not be taken as limiting the scope of the invention which is
defined by the appended claims.
* * * * *