U.S. patent application number 11/441583 was filed with the patent office on 2007-05-10 for medical sensor and technique for using the same.
Invention is credited to Michael P. O'Neil, David B. Swedlow.
Application Number | 20070106134 11/441583 |
Document ID | / |
Family ID | 38004717 |
Filed Date | 2007-05-10 |
United States Patent
Application |
20070106134 |
Kind Code |
A1 |
O'Neil; Michael P. ; et
al. |
May 10, 2007 |
Medical sensor and technique for using the same
Abstract
A sensor is provided that is appropriate for transcutaneous
detection of tissue or blood constituents. A sensor for tissue
constituent detection may include a gas collection chamber with a
conduit to a sensing component and a conduit from the sensing
component to the chamber. A sensor as provided may also include a
barrier layer to prevent water from infiltrating the sensor.
Inventors: |
O'Neil; Michael P.;
(Pleasanton, CA) ; Swedlow; David B.; (Danville,
CA) |
Correspondence
Address: |
Michael G. Fletcher;FLETCHER YODER
P.O. Box 692289
Houston
TX
77269-2289
US
|
Family ID: |
38004717 |
Appl. No.: |
11/441583 |
Filed: |
May 26, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60735621 |
Nov 10, 2005 |
|
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|
Current U.S.
Class: |
600/309 |
Current CPC
Class: |
A61B 5/097 20130101;
A61B 5/083 20130101 |
Class at
Publication: |
600/309 |
International
Class: |
A61B 5/00 20060101
A61B005/00 |
Claims
1. A system comprising: at least one gas collection chamber into
which a tissue constituent is able to diffuse, wherein the gas
collection chamber is adapted to be placed proximate to a tissue;
an efferent conduit adapted to transfer the tissue constituent from
the gas collection chamber to at least one sensing component,
wherein the sensing component is adapted to provide a signal
related to the tissue constituent; an afferent conduit adapted to
transfer the tissue constituent from the sensing component to the
gas collection chamber; and a motive force structure adapted
circulate the tissue constituent through the system, wherein the
motive force structure is adapted to be operatively connected to at
least one of the efferent conduit, the afferent conduit, or the
sensing component.
2. The system, as set forth in claim 1, wherein the tissue
constituent comprises carbon dioxide, oxygen, ethanol, or carbon
monoxide.
3. The system, as set forth in claim 1, wherein the tissue
constituent comprises a volatile anesthetic agent, a volatile
product of metabolism, or a volatile xenobiotic.
4. The system, as set forth in claim 1, comprising a calibration
element adapted to provide at least one signal related to at least
one physical characteristic of the gas collection chamber.
5. The system, as set forth in claim 4, wherein the calibration
element comprises a coded resistor or an electrically erasable
programmable read-only memory.
6. The system, as set forth in claim 1, comprising a barrier layer
defining at least part of a surface of the gas collection
structure, wherein the barrier layer is substantially impermeable
to water.
7. The system, as set forth in claim 1, comprising a barrier layer
defining at least part of a surface of the gas collection
structure, wherein the barrier layer is selectively permeable to
the tissue constituent.
8. The system, as set forth in claim 1, comprising a
water-permeable conduit that is impermeable to the tissue
constituent, wherein the water-permeable conduit is adapted to
direct water away from gas collection structure.
9. The system, as set forth in claim 1, wherein the sensing
component comprises a non-optical transducer, an optical
transducer, a chemical indicator, a spectroscopic transducer, or an
electrochemical transducer.
10. The system, as set forth in claim 1, wherein the gas collection
structure comprises a tube, wherein at least a portion of the tube
adapted to be placed proximate to the tissue is permeable to the
tissue constituent.
11. The system, as set forth in claim 1, wherein the sensing
component comprises a calibration element adapted to provide a
signal related to the calibration characteristics of the sensing
component.
12. The system, as set forth in claim 1, wherein the motive force
structure comprises a pump, a one-way valve, a kinetic motion
structure, or a piezoelectrically powered structure.
13. The system, as set forth in claim 1, comprising an agent
adapted to be placed proximate to the tissue, wherein the agent is
adapted to increase blood flow to the tissue or wherein the agent
is adapted to increase the tissue's permeability to the tissue
constituent.
14. The system, as set forth in claim 13, wherein the agent
comprises an electrical heating element, a chemical heating
element, nicotinic acid, or salicylic acid.
15. A monitoring device comprising: a monitor; and a system adapted
to be coupled to the monitor, the system comprising: at least one
gas collection structure adapted to be placed proximate to a
tissue; and an efferent conduit adapted to transfer gas from the
gas collection structure to a sensing component, wherein the
sensing component is adapted to provide a signal related to a
tissue constituent; and an afferent conduit adapted to transfer gas
from the sensing component to the gas collection structure.
16. The monitoring device, as set forth in claim 15, wherein the
tissue constituent comprises carbon dioxide, oxygen, ethanol, or
carbon monoxide.
17. The monitoring device, as set forth in claim 15, wherein the
tissue constituent comprises a volatile anesthetic agent, a
volatile product of metabolism, or a volatile xenobiotic.
18. The monitoring device, as set forth in claim 15, comprising a
calibration element adapted to provide at least one signal related
to at least one physical characteristic of the gas collection
chamber.
19. The monitoring device, as set forth in claim 18, wherein the
calibration element comprises a coded resistor or an electrically
erasable programmable read-only memory.
20. The monitoring device, as set forth in claim 15, comprising a
barrier layer defining at least part of a surface of the gas
collection structure, wherein the barrier layer is substantially
impermeable to water.
21. The monitoring device, as set forth in claim 15, comprising a
barrier layer defining at least part of a surface of the gas
collection structure, wherein the barrier layer is selectively
permeable to the tissue constituent.
22. The monitoring device, as set forth in claim 15, comprising a
water-permeable conduit that is impermeable to the tissue
constituent, wherein the water-permeable conduit is adapted to
direct water away from gas collection structure.
23. The monitoring device, as set forth in claim 15, wherein the
sensing component comprises a non-optical transducer, an optical
transducer, a chemical indicator, a spectroscopic transducer, or an
electrochemical transducer.
24. The monitoring device, as set forth in claim 15, wherein the
gas collection structure comprises a tube, wherein at least a
portion of the tube adapted to be placed proximate to the tissue is
permeable to the tissue constituent.
25. The monitoring device, as set forth in claim 15, wherein the
sensing component comprises a calibration element adapted to
provide a signal related to the calibration characteristics of the
sensing component.
26. The monitoring device, as set forth in claim 15, wherein the
motive force structure comprises a pump, a one-way valve, a kinetic
motion structure, or a piezoelectrically powered structure.
27. The monitoring device, as set forth in claim 15, comprising an
agent adapted to be placed proximate to the tissue, wherein the
agent is adapted to increase blood flow to the tissue or wherein
the agent is adapted to increase the tissue's permeability to the
tissue constituent.
28. The monitoring device, as set forth in claim 27, wherein the
agent comprises an electrical heating element, a chemical heating
element, nicotinic acid, or salicylic acid.
29. The monitoring device, as set forth in claim 15, comprising a
multi-parameter monitor.
30. A method comprising: transferring a tissue constituent in a gas
collection chamber to at least one sensing component not located in
the gas collection chamber, wherein the sensing component is
adapted to provide a signal related to the tissue constituent.
31. The method, as set forth in claim 30, comprising contacting the
tissue constituent with a barrier layer defining at least part of a
surface of the gas collection structure, wherein the barrier layer
is substantially impermeable to water.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional
Application No. 60/735,621, filed Nov. 10, 2005, the disclosure of
which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to medical devices
and, more particularly, to sensors used for sensing physiological
parameters of a patient.
[0004] 2. Description of the Related Art
[0005] This section is intended to introduce the reader to various
aspects of art that may be related to various aspects of the
present invention, which are described and/or claimed below. This
discussion is believed to be helpful in providing the reader with
background information to facilitate a better understanding of the
various aspects of the present invention. Accordingly, it should be
understood that these statements are to be read in this light, and
not as admissions of prior art.
[0006] In the field of medicine, doctors often desire to monitor
certain physiological characteristics of their patients.
Accordingly, a wide variety of devices have been developed for
monitoring many such characteristics of a patient. Such devices
provide doctors and other healthcare personnel with the information
they need to provide the best possible healthcare for their
patients. As a result, such monitoring devices have become an
indispensable part of modern medicine.
[0007] Physiological characteristics that physicians may desire to
monitor include constituents of the blood and tissue, such as
oxygen and carbon dioxide. For example, abnormal levels of carbon
dioxide in the blood may be related to perfusion problems. Thus,
assessment of carbon dioxide levels may be useful for diagnosing a
variety of clinical states related to the circulation. Carbon
dioxide and other blood constituents may be directly measured by
taking a blood sample, or may be indirectly measured by assessing
the concentration of those constituents in the tissue or
respiratory gases. For example, carbon dioxide in the bloodstream
equilibrates rapidly with carbon dioxide in the lungs, and the
partial pressure of the carbon dioxide in the lungs approaches the
amount in the blood during each breath. Accordingly, physicians
often monitor respiratory gases during breathing in order to
estimate the carbon dioxide levels in the blood.
[0008] However, estimation of carbon dioxide by respiratory gas
analysis has certain disadvantages. It is often inconvenient to
measure carbon dioxide in samples collected from an intubation tube
or cannula. Although these methods are considered to be
noninvasive, as the surface of the skin is not breached, the
insertion of such devices may cause discomfort for the patient.
Further, the insertion and operation of such devices also involves
the assistance of skilled medical personnel.
[0009] Carbon dioxide in the blood that diffuses into the tissue
may also be measured transcutaneously by sensors placed against a
patient's skin. While these sensors are easier to use than
respiratory gas sensors, they also have certain disadvantages. For
example, these sensors may be sensitive to the infiltration of
water or bodily fluids, particularly when applied to a mucosal
surface.
SUMMARY
[0010] Certain aspects commensurate in scope with the originally
claimed invention are set forth below. It should be understood that
these aspects are presented merely to provide the reader with a
brief summary of certain forms of the invention might take and that
these aspects are not intended to limit the scope of the invention.
Indeed, the invention may encompass a variety of aspects that may
not be set forth below.
[0011] There is provided a system that includes: at least one gas
collection chamber into which a tissue constituent is able to
diffuse, wherein the gas collection chamber is adapted to be placed
proximate to a tissue; an efferent conduit adapted to transfer the
tissue constituent from the gas collection chamber to at least one
sensing component, wherein the sensing component is adapted to
provide a signal related to the tissue constituent; an afferent
conduit adapted to transfer the tissue constituent from the sensing
component to the gas collection chamber; and a motive force
structure adapted circulate the tissue constituent through the
system, wherein the motive force structure is adapted to be
operatively connected to at least one of the efferent conduit, the
afferent conduit, or the sensing component.
[0012] There is also provided a monitoring device that includes: a
monitor; and a system adapted to be coupled to the monitor, the
system including: at least one gas collection structure adapted to
be placed proximate to a tissue; and an efferent conduit adapted to
transfer gas from the gas collection structure to a sensing
component, wherein the sensing component is adapted to provide a
signal related to a tissue constituent; and an afferent conduit
adapted to transfer gas from the sensing component to the gas
collection structure.
[0013] There is also provided a method that includes: transferring
a tissue constituent in a gas collection chamber to at least one
sensing component not located in the gas collection chamber,
wherein the sensing component is adapted to provide a signal
related to the tissue constituent.
[0014] There is also provided a sensing system component that
includes: at least one gas collection chamber into which a tissue
constituent is able to diffuse, wherein the gas collection chamber
is adapted to be placed proximate to a tissue; a first conduit in
communication with the gas collection chamber comprising a
connector located distally from the gas collection; and a second
conduit in communication with the gas collection chamber comprising
a connector located distally from the gas collection chamber.
[0015] There is also provided a method of manufacturing a sensing
system component that includes: providing at least one gas
collection chamber into which a tissue constituent is able to
diffuse, wherein the gas collection chamber is adapted to be placed
proximate to a tissue; providing a first conduit in communication
with the gas collection chamber comprising a connector located
distally from the gas collection; and providing a second conduit in
communication with the gas collection chamber comprising a
connector located distally from the gas collection chamber.
[0016] There is also provided a sensor that includes: a sensor body
comprising at least one gas collection chamber adapted to be placed
proximate to a tissue; a sensing component disposed on the sensor
body adapted to provide a signal related to a tissue constituent;
and a barrier layer defining at least part of a surface of the gas
collection chamber, wherein the barrier layer is substantially
impermeable to water.
[0017] There is also provided a system that includes: a monitor;
and a sensor adapted to be coupled to the monitor, the sensor
including: a sensor body comprising a gas collection chamber
adapted to be placed proximate to a tissue; a sensing component
disposed on the sensor body adapted to provide a signal to the
monitor related to a tissue constituent; and a barrier layer
defining at least part of a surface of the gas collection chamber,
wherein the barrier layer is substantially impermeable to
water.
[0018] There is also provided a method of measuring a tissue
constituent that includes: diffusing a tissue constituent through a
barrier layer that is substantially impermeable to water, wherein
the barrier layer defines at least part of a surface of a gas
collection chamber; and providing a signal related to the tissue
constituent with a sensing element disposed on the gas collection
chamber.
[0019] There is also provided a method of manufacturing a sensor
that includes: providing a sensor body comprising a gas collection
chamber adapted to be placed proximate to a tissue; providing a
sensing component disposed on the sensor body adapted to provide a
signal related to a tissue constituent; and providing a barrier
layer defining at least part of a surface of the gas collection
chamber, wherein the barrier layer is substantially impermeable to
water.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Advantages of the invention may become apparent upon reading
the following detailed description and upon reference to the
drawings in which:
[0021] FIG. 1 illustrates a perspective view of a patient using a
sensor for detection of a physiological tissue constituent
according to the present invention;
[0022] FIG. 2 is a schematic cross-sectional view of the sensor of
FIG. 1;
[0023] FIG. 3A illustrates a schematic view of an embodiment of a
sensor according to the present techniques;
[0024] FIG. 3B illustrates a view of an exemplary disposable
portion of the sensor of FIG. 3A;
[0025] FIG. 4 is a flow chart of a method of operating a sensor
according to the present invention;
[0026] FIG. 5A-5B illustrate an alternate configuration of a tissue
constituent collection portion of a sensor according to the present
techniques;
[0027] FIG. 7 illustrates a perspective view of a patient using a
sensor including a barrier layer for detection of a physiological
tissue constituent according to the present invention;
[0028] FIG. 8 is a schematic cross-sectional view of the sensor of
FIG. 7;
[0029] FIG. 9 is a sensor including multiple collection chambers
for tissue constituent detection; and
[0030] FIG. 10 illustrates a physiological constituent detection
system coupled to a multi-parameter patient monitor and a sensor
according to embodiments of the present invention.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
[0031] One or more specific embodiments of the present invention
will be described below. In an effort to provide a concise
description of these embodiments, not all features of an actual
implementation are described in the specification. It, should be
appreciated that in the development of any such actual
implementation, as in any engineering or design project, numerous
implementation-specific decisions must be made to achieve the
developers' specific goals, such as compliance with system-related
and business-related constraints, which may vary from one
implementation to another. Moreover, it should be appreciated that
such a development effort might be complex and time consuming, but
would nevertheless be a routine undertaking of design, fabrication,
and manufacture for those of ordinary skill having the benefit of
this disclosure.
[0032] A sensor and/or sensing system is provided herein that may
assess a tissue constituent content with a sensing component that
is adapted to provide a signal related to the tissue constituent.
In certain embodiments of the invention, the sensing system may
include a collection chamber placed against the tissue and a
sensing component that is connected to the chamber by a conduit
with a return conduit between the sensing component and the
collection chamber. The collection chamber is able to capture a
volume of volatile tissue constituents as they diffuse out of the
tissue. When the concentration of the tissue constituents in the
gas collection chamber and throughout the sensing system is
substantially equal to the concentration of those constituents in
the tissue, the sensing system is equilibrated
[0033] Such a system may provide multiple advantages. By separating
the sensing component from the tissue constituent collection
chamber, a sensor may be more versatile. For example, a sensing
component may be easily exchanged for an alternate sensing
component without disrupting the collection of the tissue
constituent. This may be advantageous when a sensing component
needs to be maintained or serviced, and the healthcare provider
does not wish to disrupt physiological monitoring while replacing
the sensing component. Additionally, the separation of the sensing
component from the tissue constituent collection chamber may help
reduce water infiltration into the sensing component.
[0034] Sensors according to the present techniques may
transcutaneously sense tissue gases or other tissue constituents in
a tissue layer and provide an electrical and/or visual signal. For
example, carbon dioxide and other constituents in the bloodstream
may diffuse through the tissue and may dissolve into any liquids
that may be found at the surface of the tissue. Thus, the levels of
carbon dioxide in the tissue may serve as a surrogate marker for
carbon dioxide levels in the bloodstream. A sensor according to the
present techniques placed proximate to a tissue surface may capture
and measure carbon dioxide that would otherwise diffuse into the
airstream or other surrounding airspace.
[0035] Generally, it is envisioned that sensors according to the
present techniques are appropriate for use in determining the
presence or levels of tissue constituents in a variety of tissues.
The sensor may be placed against the tissue, either manually,
mechanically, adhesively, or otherwise, forming a seal to prevent
the carbon dioxide from diffusing away. For example, a sensor may
be used in the upper respiratory tract or the gastrointestinal
tissue, including the oral and nasal passages. These passages may
include the tongue, the floor of the mouth, the roof of the mouth,
the soft palate, the cheeks, the gums, the lips, the esophagus and
any other respiratory or gastrointestinal tissue. Further, a sensor
as described herein is appropriate for use adjacent to or proximate
to any mucosal surface, i.e., patient surfaces that include a
mucous membrane or surfaces that are associated with mucus
production. In addition to the respiratory tract, mucosal surfaces
may include vaginal, rectal, or gastrointestinal surfaces.
[0036] Sensors as provided by the present techniques may be
disposable, reusable, or partially disposable. In addition, the
sensors may be appropriate for short-term or for longer-term
monitoring. When used for long-term monitoring, the sensor may be
applied to the patient's tissue either by mechanical clamping or by
a suitable adhesive, such as a mucoadhesive, or by any other
suitable holding device, such as a clip.
[0037] In additional to carbon dioxide monitoring, sensors and
sensing systems as provided herein may be used to monitor oxygen,
carbon monoxide, ethanol, or anesthetic gases (such as isoflurane,
halothane, desflurane, sevoflurane and enflurane) that may diffuse
transcutaneously. Additionally, these sensors and/or sensing
systems may be used to monitor volatile products of metabolism
(such as ketones, alcohols, lactones, terpenes, furans, dimethyl
sulfone, pyrrole, and allyl isothiocyanate), as well as volatile
xenobiotics and their metabolites. Further, these sensors may be
useful in monitoring the levels of parenterally administered or
enterally administered therapeutic agents.
[0038] For example, FIG. 1 illustrates the placement of a gas
collection chamber 12 of a sensor 10 on a buccal surface in order
to assess a tissue gas, for example carbon dioxide, in the tissue,
blood or interstitial fluid. Specifically, FIG. 1 shows an
embodiment of a sensor 10 including a gas collection chamber 12 and
a conduit 14 in communication with a sensing component 16. The
conduit 14 may be adapted to transport gases from the gas
collection chamber 12 to a distal sensing component 16. The
collected gases may diffuse through the efferent conduit 14a that
is connected to the collection chamber, and the gases may then be
further assessed and/or measured by the sensing component 16,
discussed in more detail below. The collected gases may then
circulate back to the gas collection chamber 12 through the
afferent conduit 14b. The gas collection chamber 12 may be suitably
sized and shaped such that a patient may easily close his or her
mouth around the sensor with minimal discomfort.
[0039] The gas collection chamber 12 is secured to the mucosal
tissue 18 such that the area covered by the gas collection chamber
12 creates a seal 13 to prevent environmental air flow out or into
of the gas collection chamber 12, thus preventing tissue gases at
the gas collection chamber 12 placement site from dissipating into
the airstream or being diluted, which may lead to inaccurate
measurements. Further, the gas collection chamber's 12 tissue seal
may also prevent respiratory gases or oral fluids from entering the
sensor 10A.
[0040] Tissue constituents 22 may be transferred through a conduit
14, which may include tubes or tube segments. The conduit 14 may
include, for example, medical grade catheter tubing, polyethylene,
polypropylene or vinyl. The efferent conduit 14a and the afferent
conduit 14b may be disposed on any appropriate location on the gas
collection chamber 12. For example, the efferent conduit 14a and
the afferent conduit 14b may be parallel or perpendicular to each
other. Generally, the conduit 14 may be relatively impermeable to
the tissue constituent. This may be accomplished by selecting a
conduit 14 made from an appropriate material or by applying a
sealing coat to the conduit 14. The conduit 14 may include
gas-impermeable plastics such as PET. Appropriate gas-impermeable
coatings may include Funcosil.RTM. (available from Remmers,
Loeningen, Germany). Such coatings may be applied to the conduit 14
in any appropriate manner.
[0041] A cross-sectional view of the sensor 10A is shown in FIG. 2.
The housing 20 is formed to provide a surface that is suitably
shaped to be secured against a mucosal tissue 18. The housing 20
may be any suitable material that is generally suited to the
aqueous environment of the mucous membrane. For example, the
housing 20 may be formed from polypropylene, polyethylene,
polysulfone or similar polymers. Generally, the housing 20 should
be substantially impermeable to tissue constituents, shown by
arrows 22, such that the sensor 10A may collect tissue constituents
22, such as tissue gases, for a sufficient period of time to allow
for detection and measurement. Hence, it may be advantageous to
coat the sensor 10A with additional sealants to prevent leakage of
the tissue constituents 22. The housing 20, once secured to the
tissue, forms a collection chamber 12 that traps tissue
constituents 22 that diffuse through the mucosal tissue 18. The
trapped tissue gas 22 may then be transferred to the sensing
component 16, which is coupled the gas collection chamber 12 by the
efferent conduit 14a.
[0042] In the depicted embodiment, the sensing component 16 is not
located on or within the gas collection chamber 12. When the
housing 20 is contacted with a tissue sensor site, blood or tissue
constituents 22 perfuse through the tissue and enter the collection
chamber 12. The sensing component 16 is adapted to respond to the
presence of the blood or tissue constituents 22 collected in the
gas collection chamber 12 and to provide a signal, as discussed in
more detail below. The sensing component 16 is sensitive to the
presence of a tissue constituent 22 and may be capable of being
calibrated to give a response signal corresponding to a given
predetermined concentration of the tissue constituent. In certain
embodiments, the signal may be related to the concentration or
level of the tissue constituent 22, or the partial pressure of the
tissue constituent 22.
[0043] In certain embodiments, the gas collection chamber 12 may
include materials that function as a barrier layer 28 that are
hydrophobic or otherwise water-resistant, but that are permeable to
carbon dioxide. For example, a barrier layer 28 may form a contact
surface of the sensor 10A that prevents water from entering the
sensor 10A. In such an embodiment, carbon dioxide in the tissue can
perfuse through the contact surface to enter the gas collection
chamber 12. In one embodiment, it is envisioned that the ratio of
water permeability to carbon dioxide permeability of a barrier
layer 28 may be less than 1:1, and in certain embodiments, the
ratio may be less than 1:10. Suitable materials include polymers,
such as polytetrafluorethylene (PTFE). Other suitable materials
include microporous polymer films, such as those available from the
Landec Corporation (Menlo Park, Calif.). Such microporous polymer
films are formed from a polymer film base with a customizable
crystalline polymeric coating that may be customized to be highly
permeable to carbon dioxide and relatively impermeable to water. In
one embodiment, the barrier layer 28 may be a relatively thin PTFE
material such as plumber's tape (0.04 mm). In other embodiments,
the barrier layer 28 may be a PTFE material such as Gore-Tex.RTM.
(W. L. Gore & Associates, Inc., Newark, Del.) or plumber's
tape. Alternatively, the barrier layer 28 may be formed from a
combination of appropriate materials, such as materials that are
heat-sealed or laminated to one another. For example, the barrier
layer 28 may include a PTFE layer with a pore size of 3 microns and
a second PTFE layer with a pore size of 0.1 microns. Additionally,
in certain embodiments, a sensor 10A may also include a porous
substrate 29 that is permeable to a wide variety of tissue
constituents 22. As a barrier layer 28 may be quite thin, the
porous substrate may be advantageous in providing rigidity and
support to the barrier layer 28 film. The porous substrate may be
adhered, laminated, or otherwise attached to the barrier layer 28.
In certain embodiments, the porous substrate may be disposed on the
tissue-contacting side of the barrier layer 28. Suitable materials
for the porous substrate include paper, plastics, or woven
materials.
[0044] In certain embodiments, the barrier layer 28 or porous
substrate 29 may include a tissue irritant or other agent or
structure that increases blood flow to the tissue at the gas
collection chamber 12 placement site. The agent may include a
counterirritant, such as a mixture of methyl salicilate and menthol
(12% methyl salicilate, 9% menthol) in a cream base is applied to
the patient's skin at the chosen sensor site. A cream of this type
is sold in retail drug stores under the trademark ICY HOT. Other
contemplated agents of this type may include heaters, such as
mechanical or chemical heaters, that increase blood perfusion in
response to lowered tissue temperatures.
[0045] The sensing component 16 may be disposed on or within any
appropriate substrate that provides a suitable contact area with
which the tissue constituent 22 may interact, react, or otherwise
come into the proximity of the sensing component 16. For example,
in embodiments in which the sensing component includes a chemical
indicator, it may be appropriate to include, as part of a holder
for the sensing component 16, a transparent viewing window for the
healthcare provider to view a change in color of the indicator. In
embodiments in which the sensing component includes an optical
detection system, it may be appropriate to dispose the sensing
component in a chamber.
[0046] Although the tissue constituent 22 may diffuse and circulate
through the conduit 14 to the sensing component 16 without a
drawing force, such a process may be lengthy. Thus, it may be
advantageous to provide a motive device, such as a pump, within the
sensor. FIG. 3 is a schematic diagram of the exemplary sensor 10A
that provides a pumping and sensing component system 33 to draw the
tissue constituent 22 to the sensing component 16. A flow regulator
32, which may be a valve or any other suitable device, and pump 34
are connected into or between segments of conduit 14 to maintain a
desired flow velocity of the stream of tissue constituent 22 to the
sensing component 16. As shown, the flow regulator 32 is connected
to a pump 34. The pump 34 is in turn interposed between sections of
the conduit 14, which is connected to the sensing component 16.
[0047] The ends of the conduit 14 segments may be secured to
connectors 30, as shown in FIG. 3, which may be clamped to prevent
leaking of the tissue constituent 22 to the outside. Connectors 30a
and 30b are exemplary, and it should be understood that the
connectors 30 and conduit 14 may be arranged along the sensor in
any manner that is convenient for the user. For example, it may be
advantageous to provide additional connectors 30 in order to allow
a healthcare provider to easily swap out to service, clean, or
exchange any part of the sensor 10A, including the flow regulator
32, pump, 34, or sensing component 16.
[0048] In certain embodiments, it may be advantageous to exchange a
first sensing component 16 adapted to sense carbon dioxide for a
second sensing component 16. The second sensing component 16 may
also sense carbon dioxide, but may operate by a different sensing
mechanism. Alternatively, the second sensing component 16 may be
adapted to sense a different tissue constituent, such as carbon
monoxide or oxygen. Further, in an alternate embodiment (not
shown), the sensor 10A may have multiple sensing components 16 in
series.
[0049] In certain embodiments, the pump 34 and flow regulator 32
may be adjusted so that the flow is maintained at the desired rate.
One suitable flow regulator is orifice/needle valve model
F-2822-41-B80-55 available from Air Logic, Racine, Wis., which can
be adjusted to obtain a desired gas flow rate in the range of up to
40-60 m/min. One suitable pump is model NMP 05 diaphragm micro
pump, available from KNF Neuberger, Inc, Princeton, N.J., which has
a free flow capacity of 0.4 L/min. The pump 34 and flow regulator
32 may be located anywhere in the flow stream of the sensor 10A.
Generally, the pumping system is substantially sealed to prevent
leaking of the tissue constituent 22 to the outside or dilution by
entraining of fresh gas. In other embodiments, any suitable motive
force structure may be appropriate for use with the present
techniques. For example, suitable motive force structures include
gravity pumps, one-way valves, kinetic motion pumps, or
piezoelectric pumps.
[0050] In certain embodiments, the pump 34, flow regulator 32, and
sensing component 16 may be connected to a processor 40. The
processor may be part of a monitor or multi-parameter monitor, as
discussed in detail below. The processor 40 may receive signals
related to the output signals from sensing component 16,
corresponding to the tissue constituent 22 concentration or partial
pressure concentration. Additionally, the processor 40 may control
the flow of the vacuum and sensing component system 33. It should
be understood that the processor 40 may be adapted to determine a
suitable equilibration time of the sensor 10A by comparing the
equilibration time to stored equilibration curves that may be
empirically obtained. Additionally, in certain embodiments, the
concentration of the tissue constituent 22 may be extrapolated from
a concentration curve obtained by the sensor 10A during the
pre-equilibration period, as such a curve will start to plateau as
it approaches the equilibrated state.
[0051] In certain embodiments, the gas collection chamber 12, the
conduit 14, or the connectors 30 may include a calibration element
42, such as a coded resistor or EEPROM or other coding devices
(such as a capacitor, inductor, PROM, RFID, a barcode, parallel
resonant circuits, or a colorimetric indicator) that may provide a
signal to the processor 40 related to the volume and other
characteristics of the gas collection chamber 12 that may allow the
processor 40 to determine the appropriate calibration
characteristics for the sensor 10A. Generally, such a calibration
element 42 may be located on a disposable portion of the sensor
10A, shown in FIG. 3B, that may include the gas collection chamber
12, the conduit segments 14 attached to the gas collection chamber
12, or any connectors 30 proximate to the gas collection chamber
12. In such an embodiment, for example when the calibration element
is disposed on the connector 30 as shown, the connector 30, conduit
segment 14, and the calibration element 42 may be constructed as a
unitary assembly such that the calibration element 42 may be
inseparable from the gas collection chamber 12. Further, the
calibration element 42 may include encryption coding that prevents
a disposable part of the sensor 10A from being recognized by a
processor 40 that is not able to decode the encryption. Such
encryption coding is described in U.S. Pat. No. 6,708,049, which is
hereby incorporated by reference in its entirety.
[0052] The sensing component 16 may also include a calibration
element (not shown) that provides information to the processor 40
that may include the type of tissue constituent 22 that is being
analyzed or other characteristics of the sensing component 16. For
example, such a sensing component 16 calibration element may send a
signal to the processor 40 to employ a certain correction algorithm
for calculating the concentration of the tissue constituent 22.
Such a correction algorithm may be appropriate when the sensing
component 16 includes a chemical indicator that consumes a
percentage of the tissue constituent 22 while actively measuring
it. As the consumption of the tissue constituent 22 by the sensing
component 16 may alter the equilibration state of the sensor 10A,
the correction algorithm may mitigate such effects on the sensing
component 16 output signal. In an alternative embodiment, a
correction algorithm may also be employed if a sensing component 16
generated the tissue constituent 22 during measurement. Further, a
correction algorithm may account for any minimal leakage of tissue
constituent 22 in the system.
[0053] The collection chamber 12 is part of a substantially closed
environment, as the conduit 14, and the sensing component 16 are
generally impermeable to the tissue constituent 22 of interest. The
sensor 10A is permeable to the tissue constituent 22 where the
collection chamber 12 contacts the tissue 18. When the partial
pressure of the tissue constituent 22 in the sensor 10A is
substantially equal to the partial pressure of the tissue
constituent in the tissue 18, the sensor 10A is equilibrated. The
sensor 10A is arranged to provide circulating flow of the tissue
constituent 22 through the sensor 10A. Thus, the tissue constituent
22 may equilibrate throughout the sensor 10A while being
transferred from the gas collection chamber 12 through the efferent
conduit 14a to contact the sensing component 16, and may return to
the gas collection chamber 12 through the afferent conduit 14b.
Such an embodiment may be advantageous when a tissue constituent 22
is being continuously or regularly monitored. As the initial
application of the gas collection chamber 12 to the mucosal tissue
26 may involve waiting for 5-10 minutes before the tissue
constituent 22 equilibrates in the gas collection chamber prior to
being analyzed, it is desirable to keep the sensor in the
equilibrated state. In such an embodiment, the vent 38 to the
outside may be closed during equilibration and tissue constituent
22 monitoring. The vent 38 may be opened when necessary in order to
flush out the sensor with room air or purge gas.
[0054] The equilibration time of the sensor 10A may be influenced
by certain factors. Generally, equilibration times may be in the
range of substantially instantaneous, i.e., real time, to less than
5 minutes. In certain embodiments, the response time is in the
range of 5 seconds to 30 minutes. For example, in certain
embodiments, the thickness of a barrier layer 28 may be modified in
order to achieve the desired rate of carbon dioxide perfusion and
sensing component 16 response time. Where a very rapid response is
desired, a thin film of the barrier layer 28, for example less than
0.2 mm in thickness, may be used. Additionally, the barrier layer
28 may be formed with small pores that increase the carbon dioxide
permeability. In other embodiments, the response time may be
influenced by the volume of the gas collection chamber 12 or the
length and diameter of the conduit 14. It is envisioned that the
volume of the gas collection chamber 12 may be optimized to be
large enough to allow sufficient tissue constituents 22 to be
collected to obtain accurate measurements while being small enough
to provide rapid response times. For example, in certain
embodiments, the total volume of the gas collection chamber may be
0.2-5.0 cubic centimeters. It may be appropriate to use a
relatively smaller, e.g., 0.2-0.8 cubic centimeters, gas collection
chamber on a neonate. In certain embodiments, the total volume of
the sensor 10A, including the conduit 14, sensing component 16, and
pumping system 33 may be 2-500 cubic centimeters. Generally,
smaller sensor 10A volumes are associated with faster equilibration
times.
[0055] Referring to FIG. 4, a flow chart 44 illustrates how a
tissue constituent 22 may be analyzed by the sensor 10A. A gas
collection chamber 12 is applied to a patient's mucosal tissue
(block 46) and the tissue constituent 22 diffuses into the gas
collection chamber 12. The pump 34 and/or flow regulator 32 is then
activated, either by the healthcare provider or by a processor 40
(block 49 and the tissue constituent 22 equilibrates while being
pumped through the sensor 10A. The tissue constituent 22 is drawn
into proximity with sensing component 16, and the sensing component
16 provides a signal related to the tissue constituent 22 (block
50). The tissue constituent 22 may be circulated through the sensor
(block 54) back to the gas collection chamber 12 in order to
maintain the equilibrated state.
[0056] In certain embodiments, it may be advantageous to provide a
sensor with a gas collection portion with a large surface area that
contacts the tissue. Such a sensor may equilibrate more rapidly, as
tissue constituent 22 may diffuse more rapidly into the gas
collection portion. FIGS. 5A-5B illustrate sensors 10B with
alternative gas collection configurations. In FIG. 5A, a sensor 10B
may include a gas collection portion in the form of a coiled tube
80, which may be coiled in the manner of a garden hose, that is
permeable to the tissue constituent 22. The coiled tube 80 may
increase the available surface area of the gas collection portion
of the sensor 10B, as it may be adapted to lay flat against a
tissue 18. The tissue constituent 22 diffuses into the coiled tube
80 and is drawn into the efferent conduit 14a. The tissue
constituent 22 may be circulated through conduit 14b. Although the
coiled tube 80 is permeable to the tissue constituent 22, the
tissue constituent is able to equilibrate in the sensor as the
coiled tube 80 may adapted to be substantially surrounded by
mucosal tissue 18. For example, the coiled tube 80 may be placed
sublingually. In an alternate embodiment, the coiled tube 80 may be
adapted to be permeable only on one side by applying a tissue
constituent impermeable coating (not shown) to certain portions of
the coiled tube 80. Thus, once the tissue constituent 22 diffuses
into the coiled tube 80, the partial pressure of the tissue
constituent 22 in the sensor 10B may equilibrate with the tissue 18
without leaking out the portion of the coiled tube 80 not in
contact with the tissue 18. Such a configuration may be appropriate
for use on buccal tissue. In an alternate embodiment, shown in FIG.
5B, the tissue constituent permeable collection portion may assume
a zigzag configuration 82 connected to the conduit 14. Exposed
portion of the substrate 84, i.e., portions not in contact with the
tissue 18, may be coated with a tissue constituent impermeable
coating 86 to prevent leaking. It should be understood that the
configurations shown are merely exemplary, and the gas collection
portions of the sensor 10B may take any suitable shape, such as a
helix, a coiled coil, or other configurations. Appropriate
permeable materials from which the permeable gas collection
portions may be formed may include Silastic.RTM. silicone rubber,
available from Dow Coming (Midland, Mich.).
[0057] FIG. 6 illustrates a tissue constituent permeable coiled
tube 80 connected to a pumping and sensing component system 33 by
efferent conduit 14a and afferent conduit 14b. Efferent conduit 14a
is adapted to draw the tissue constituent 22 to the sensing
component 16. The system may include a calibration element 81 as
described herein that is adapted to communicate with the processor
40 and provide information related to the characteristics of the
disposable portion of the sensor 10D, which may include the tissue
constituent permeable coiled tube 80 and certain segments of the
conduit 14. It is envisioned that any suitable tissue constituent
permeable assembly as described herein may be connected to the
pumping and sensing component system 33 as shown.
[0058] The sensors as provided herein may prevent water
infiltration into a sensing component by arranging a sensor such
that the sensing component is removed from the tissue and thus is
removed from bodily fluids. However, in certain embodiments it may
be advantageous to provide a unitary sensor configuration including
a gas collection chamber on which or within which the sensing
component is disposed. Such an arrangement may be easier to for a
healthcare worker to apply and operate, as it does not involve a
motive device. Additionally, such a sensor may be smaller and
lighter, providing certain transportation and storage advantages.
In such an embodiment, water infiltration into the sensor may be
reduced by providing a sensor that includes a water barrier layer.
FIG. 7-FIG. 8 illustrate an alternate embodiment of a tissue
constituent sensor 10D in which the sensor body 55 includes a
sensing component disposed proximate to a gas collection chamber
57. FIG. 7 shows the sensor applied to a patient. FIG. 8 shows a
cross-sectional view of the sensor 10D. As depicted, water is
prevented from infiltrating the sensor 10D by a barrier layer 58 as
described herein that forms at least part of a surface of the
sensor 10D that contacts the tissue. In an alternate embodiment
(not shown), the sensor 10D may be configured to prevent water
infiltration by a structure that absorbs and/or redirects water
away from the sensing components. For example, the sensor 10B may
include a water vapor permeable backflush tube that is selectively
permeable to water vapor to allow water vapor to be absorbed and
evaporate away from the sensing components without infiltrating the
sensor. Such a tube may include a material such as Nafion
(available from DuPont, Wilmington, Del.). The barrier layer 58 is
connected to a housing 56 that, when applied to the mucosal tissue
28, forms a collection chamber that traps a tissue constituent 22
that diffuses through the barrier layer 58. It should be understood
that the sensor 10D may include any sensing component as described
herein. For example, sensing component may be an optical
transducer. In such an embodiment, the trapped tissue constituent
22 may be irradiated by an emitter 60, and the emitted light that
passes through the tissue constituent may be detected by a detector
62. The emitter 60 and the detector 62 are electrically coupled to
a cable 64 by wires 68. The wavelength of the light emitted by the
emitter 60 and the detection range of the detector 62 may be
selected to detect a wide range of tissue constituents 22. For
example, the emitter 60 may also include a filter, for example a
4.26 micron wavelength filter. Such a filter may be appropriate for
use in an embodiment where carbon dioxide is measured.
[0059] In some embodiments, the sensor 10D is arranged to operate
in transmission mode, and casings for the emitter and detector may
be formed in the housing 56 on opposite sides of the sensor 10D. In
an alternate embodiment, the emitter 60 and the detector 62 may be
arranged to operate in reflectance mode (not shown), and can be
located on the same side of sensor 10D. In such an embodiment (not
shown), a mirror may be placed on the opposite side of the housing
56 to reflect the radiation emitted from the emitter 60 back to the
detector 62. When employing optical sensing components 16, it may
be advantageous to dispose an opaque or reflective layer on the
tissue-contacting surface of the sensor 10D to prevent signal
artifacts as a result of the absorption of a portion of the emitted
light by the tissue 18.
[0060] In certain embodiments (not shown), the gas collection
chamber 57 may include a calibration element 66 or other transducer
that may provide a signal related to the volume and other
characteristics of the gas collection chamber 57. Such a
calibration element 66 may allow a downstream processor or monitor
to determine a suitable amount of time to allow the sensor 10B to
equilibrate (i.e. to allow the tissue constituent 22 to diffuse
into the gas collection chamber 57) before obtaining accurate
measurements related to the tissue constituent 22. Additionally,
the calibration element 66 may be a coded resistor or EEPROM or any
other suitable device as described herein that provides information
related to the calibration of any optical sensing components. Such
a calibration element 66 may be advantageous in increasing
manufacturing yield of the sensor 10D. For example, a sensor 10D
including such a calibration element 66 that provides information
about the emission wavelength or wavelength range of the emitter 60
may be able to be more accurately calibrated for a wider range of
potential emission wavelengths than a sensor lacking such a
calibration element 66.
[0061] It may be advantageous to provide a sensor 10E as a
dipstick-like device with a holder 88 that has a familiar and
comfortable shape that is easy to use. For example, water-resistant
sensors as provided herein may be used in vivo by a patient much
like an oral thermometer. FIG. 9 illustrates a cross-sectional view
of a sensor assembly 10E according to the present techniques. Such
a sensor 10E may be adapted to assess one or more tissue
constituents 22, as illustrated. A barrier layer 90, as described
herein, may reduce water infiltration into the sensor 10E. As
illustrated, the sensor 10E includes multiple gas collection
chambers, each of which may include a different sensing component
16. For example, sensing components 16a, 16b, 16c, and 16d may be
adapted to each sense a different tissue constituent 22. In certain
embodiments, the different tissue constituents may be carbon
dioxide, carbon monoxide, oxygen, and other diffusible gases or
volatile compounds. Each of the sensing components 16 may be
electrically coupled to a display 94 by wires 92. The display may
then indicate the concentrations of the tissue constituents 22 as
measured by the sensing components 16. In an alternate embodiment,
the sensor 10E may include electrical input and output wires (not
shown) that may extend along the holder 88 to couple to a cable,
which may be connected to a patient monitor. In another alternate
embodiment (not shown), such a sensor 10E may be adapted to include
distal sensing components 16 as described herein and a motive force
structure to draw the tissue constituent 22 into the distal sensing
components. Further, in another alternate embodiment (not shown),
the barrier 90 may include a series of selectively permeable
barriers specific for a variety of tissue constituents 22. Thus,
each of the gas collection chambers 12 may only be permeable to a
particular tissue constituent.
[0062] The sensor 10E may be inserted into the oral passage and
placed adjacent to a mucosal tissue 18. The sensor 10E may be
suitably sized and shaped such that a patient may easily close his
or her mouth around the holder 88 with minimal discomfort. In
certain embodiments, the sensor 10E may be adapted to be held
against the cheek or any other mucosal tissue. The holder 88 may
also include a handle portion that is accessible from outside the
mouth and may be manipulated by the patient or a healthcare worker
in order to properly position the sensor assembly 10E within the
mouth.
[0063] Sensors as described herein may include any appropriate
sensing component for assessing a tissue constituent, including
chemical, electrical, optical, non-optical, quantum-restricted,
electrochemical, enzymatic, spectrophotometric, fluorescent, or
chemiluminescent indicators or transducers. In certain embodiments,
the sensing component may include optical components, e.g. an
emitter and detector pair that may be of any suitable type. For
example, the emitter may be one or more light emitting diodes
adapted to transmit one or more wavelengths of light in the red to
infrared range, and the detector may one or more photodetectors
selected to receive light in the range or ranges emitted from the
emitter. Alternatively, an emitter may also be a laser diode or a
vertical cavity surface emitting laser (VCSEL). An emitter and
detector may also include optical fiber sensing components. An
emitter may include a broadband or "white light" source, in which
case the detector could include any of a variety of elements for
selecting specific wavelengths, for example reflective or
refractive elements or interferometers. These kinds of emitters
and/or detectors would typically be coupled to the rigid or
rigidified sensor via fiber optics. Alternatively, a sensor may
sense light detected from the tissue is at a different wavelength
from the light emitted into the tissue. Such sensors may be adapted
to sense fluorescence, phosphorescence, Raman scattering, Rayleigh
scattering and multi-photon events or photoacoustic effects. It
should be understood that, as used herein, the term "light" may
refer to one or more of ultrasound, radio, microwave, millimeter
wave, infrared, visible, ultraviolet, gamma ray or X-ray
electromagnetic radiation, and may also include any wavelength
within the radio, microwave, infrared, visible, ultraviolet, or
X-ray spectra.
[0064] Alternatively, the sensing component may include an active
ingredient of the indicating element, for example the active
ingredient involved in providing the required response signal when
exposed to a given concentration of carbon dioxide or other
constituents. The active ingredient may be any indicator that is
sensitive to the presence of carbon dioxide and that is capable of
being calibrated to give a response signal corresponding to a given
predetermined concentration of carbon dioxide. The signal may be
visual, e.g. a change in color, or electrical. Indicators which
provide a color change in a presence of carbon dioxide may include
chromogenic pH-sensitive indicators and oxidation/reduction
indicators.
[0065] A chromogenic pH-sensitive indicator may provide a color
change upon exposure to a given concentration of carbon dioxide or
other metabolites in the presence of other ingredients of the
element that provide the appropriate chemical conditions to induce
the required color change. For such an indicator to be capable of
giving a determination of carbon dioxide, it is typically used in
combination with a suitable base that provides an alkaline
solution. The hydroxyl ions or amine residues present in the
alkaline solution react chemically with carbon dioxide to produce a
carbonate, bicarbonate and/or carbamate moiety. The resulting
reaction depletes the hydroxyl ion or amine at the interface and
thus lowers the pH at the surface of the component impregnated with
the indicating element. The lowering of the pH causes a color
change in the indicator.
[0066] Chromogenic pH-sensitive indicators according to the present
techniques may include metacresol purple, thymol blue, cresol red,
phenol red, xylenol blue, a 3:1 mixture of cresol red and thymol
blue, bromthymol blue, neutral red, phenolphthalein, rosolic acid,
alpha-naphtholphthalein and orange I. Examples of other indicators
which may be used include bromcresol purple, bromphenol red,
p-nitrophenol, m-nitrophenol, curcumin, quinoline blue,
thymolphthalein and mixtures thereof. Suitable bases include sodium
carbonate, lithium hydroxide, sodium hydroxide, potassium
hydroxide, potassium carbonate, sodium barbitol, tribasic sodium
phosphate, dibasic sodium phosphate, potassium acetate,
monoethanolamine, diethanolamine and piperidine.
[0067] The sensing component may include a semi-conductive sensing
element, such as an ion-sensitive field-effect transistor (ISFET).
An ISFET may include a silicon dioxide gate for a pH selective
membrane. Such a sensor may be adapted to sense downstream changes
in hydrogen ion concentration in response to changes in carbon
dioxide or other tissue constituent concentrations.
[0068] The sensing component may also include an enzyme-based
detection system. For example, one such enzyme may be carbonic
anhydrase, which is an enzyme that assists interconversion of
carbon dioxide and water into carbonic acid, protons, and
bicarbonate ions. As described above, this reaction lowers the pH
at the surface of the component impregnated with the indicating
element. The lowering of the pH may cause a color change in the
indicator. Another such enzyme-based detection system is an enzyme
linked immunosorbent assay (ELISA). For example, such an assay may
be appropriate when assessing tissue proteins. Thus, the indicator
element may include a primary antibody specific for the tissue
protein of interest, and a labeled secondary binding ligand or
antibody, or a secondary binding ligand or antibody in conjunction
with a labeled tertiary antibody or third binding ligand. The label
may be an enzyme that will generate color development upon
incubating with an appropriate chromogenic substrate. Suitable
enzymes include urease, glucose oxidase, alkaline phosphatase or
hydrogen peroxidase.
[0069] A chemical indicator may be used in conjunction with an
electrical or electronic device that is adapted to detect and
measure changes in the ambient chemical parameters induced by the
presence of critical amounts of carbon dioxide. For example,
optical fiber carbon dioxide sensors may be used to convert a
change in a chemical indicator to a quantitative measurement of
carbon dioxide in the sample. Generally, such sensors operate by
directing light of a predetermined wavelength from an external
source through the optical fiber to impinge the chemical indicator.
The intensity of the emitted fluorescent light returning along the
fiber is directly related to the concentration of carbon dioxide in
the sample, as a result of the pH-sensitive indicator material
present at the fiber tip (i.e., the pH of the indicator solution is
directly related to carbon dioxide concentration, as a result of
carbonic acid formation). The emitted light is carried by the
optical fiber to a device where it is detected and converted
electronically to a carbon dioxide concentration value. The sensor
may additionally have a reference dye present in the indicator
composition. The intensity of the light emitted form the reference
dye may be used to compensate, via rationing, the signal obtained
from the indicator. Other components may be incorporated into the
indicator composition including surfactants, antioxidants and
ultraviolet stabilizers may also be present in the indicator
composition. The sensing component may be formed from any
appropriate substrate. For example, the sensing component may be
filter paper, which may be soaked in, dipped in, or otherwise
exposed to the appropriate carbon dioxide-sensing compounds. In
certain embodiments, the filter paper may be dipped into a solution
containing the indicating compounds on only one side. The sensing
component may also be polysulfone, polyproplylene, or other polymer
substrates. The sensing component may be a thin film, or a thicker
substrate. A thicker substrate may lead to a slower response time,
which may be advantageous in situations in which a sensor is
monitoring carbon dioxide levels over a longer period of time.
Additionally, the sensing component may have pores of a variety of
sizes.
[0070] The sensing component may include an electrochemical
transducer, which may be adapted to detect and measure changes in
ambient chemical parameters induced by the presence of critical
amounts of a tissue constituent. In one embodiment, the sensing
component may include a sensor that employs cyclic voltammetry for
carbon dioxide detection. Such sensors are available from Giner,
Inc., Newton, Mass. For example, the sensing component may be a
thick film catalyst sensor utilizing a proton exchange membrane.
Such a sensing component may include thick film screen printed
electrodes and an electrochemically reversible metal oxide
catalysts. Appropriate catalysts include MO, M.sub.2O.sub.3,
MO.sub.2, where M is a metal that is any suitable metal, including
platinum ruthenium or iridium. Generally, such sensors operate by
sensing chemical reactions caused by proton dissociation from water
in which carbon dioxide is dissolved. Dissociated water protons may
electrochemically reduce a metal oxide layer of the sensor. The
electrochemical reduction of the metal oxide will result in
generation of an electrical current, which varies in response to
the degree of electrochemical reduction.
[0071] In another embodiment, the sensing component may include an
artificial nose assembly. In such an embodiment, the tissue
constituents may contact an array of electrodes coated with
polymers that have characteristic electrical properties. The
polymers change electrical resistance when contacted with specific
volatile materials.
[0072] In another embodiment, the sensing component may include
quantum-restricted components, including carbon nanotubes,
buckeyballs, or quantum dots. Generally, quantum-restricted
components may be coated or otherwise modified with a compound that
is sensitive to the tissue constituent of interest. Interaction of
the tissue constituent with the compound may affect the electrical
properties of the quantum-restricted components such that an
electrical feedback may result. In one such example, carbon
nanotubes may be coated with a carbon dioxide-sensitive compound or
polymer, such as a polyethyleneimine and starch polymer. Carbon
dioxide may combine with primary and tertiary amines in the
polyethyleneimine and starch polymer coating to form carbamates.
The chemical reaction lowers the pH of the polymer coating,
altering charge transfer to the carbon nanotubes and resulting in
an electrical signal proportional to the pH change. Other suitable
polymer coatings may be adapted to sense other tissue constituents
of interest, such as oxygen or carbon monoxide. In other
embodiments, the quantum-restricted component may include a binding
molecule, such as a receptor or an enzyme that is specific for the
tissue constituent of interest. One such molecule may include
carbonic anhydrase. Binding of the tissue constituent to its
receptor may affect a downstream response that may result in a
change in the electrical properties of a quantum-restricted
component.
[0073] The exemplary sensors, described here generically as a
sensor 10, may be coupled to a monitor 70 that may display the
concentration of tissue constituents as shown in FIG. 8. It should
be appreciated that the cable 72 of the sensor 10 may be coupled to
the monitor 70 or it may be coupled to a transmission device (not
shown) to facilitate wireless transmission between the sensor 10
and the monitor 70. The monitor 70 may be any suitable monitor 70,
such as those available from Nellcor Puritan Bennett, Inc.
Furthermore, to upgrade conventional tissue constituent detection
provided by the monitor 70 to provide additional functions, the
monitor 70 may be coupled to a multi-parameter patient monitor 74
via a cable 74 connected to a sensor input port or via a cable 76
connected to a digital communication port.
[0074] While the invention may be susceptible to various
modifications and alternative forms, specific embodiments have been
shown by way of example in the drawings and have been described in
detail herein. However, it should be understood that the invention
is not intended to be limited to the particular forms disclosed.
Indeed, the present techniques may not only be applied to
measurements of carbon dioxide, but these techniques may also be
utilized for the measurement and/or analysis of other tissue and/or
blood constituents. Rather, the invention is to cover all
modifications, equivalents, and alternatives falling within the
spirit and scope of the invention as defined by the following
appended claims. It will be appreciated by those working in the art
that sensors fabricated using the presently disclosed and claimed
techniques may be used in a wide variety of contexts. That is,
while the invention has primarily been described in conjunction
with the measurement of carbon dioxide concentration in blood, the
sensors fabricated using the present method may be used to evaluate
any number of sample types in a variety of industries, including
fermentation technology, cell culture, and other biotechnology
applications.
* * * * *