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Claims  |
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What is claimed is:
1. Analytical apparatus comprising:
waveguide means defining an axially-elongated energy path having an inlet
aperture receptive to an incident energy signal within a predetermined
wavelength range and an emission aperture for emitting an emitted energy
signal, said waveguide means allowing passage of and being substantially
transparent to energy within eh predetermined wavelength range;
an axial segment of said waveguide means comprising a block of fixed axial
length of a solid material which is substantially transparent to energy
within the predetermined wavelength range and permeable to a gaseous
component that has an energy absorption peak in the predetermined
wavelength range, the remainder of said waveguide means being
substantially impermeable to the gaseous component;
whererin, when said axial segment is exposed along said fixed length to a
fluid containing the gaseous component; the gaseous component permeates
said segment and absorbs the incident energy signal along said fixed
length to thereby reduce the intensity of the emitted energy signal in
proportion to the concentration of the gaseous component in the segment,
whereby differnces between the intensities of the incident and emitted
energy signals may provide an indication of the concentration of the
gaseous component in the fluid.
2. The apparatus of claim 1, wherein
said waveguide means comprises an optical fiber having a distal end with
said block disposed along its axial length.
3. The apparatus of claim 2, wherein
said block has first and second ends, wherein said first end is disposed at
the distal end of said optical fiber and said second end contacts a
reflective surface.
4. The apparatus of claim 1, wherein
said block comprises a solid polymeric material.
5. The apparatus of claim 4 for measuring the concentration of carbon
dioxide in a fluid, wherein
said optical fiber is made of a fluoride glass and said block is made of
silicone and said fiber and block are substantially transparent to a
predetermined wavelength range of from about 4.1 to about 4.4 micrometers.
6. Analytical apparatus comprising:
a catheter body having a distal end positionable in vivo,
waveguide means carried by the catheter body and defining an
axially-elongated energy path having an inlet aperture for receiving an
incident energy signal within a predetermined wavelength range and an
emission aperture for emitting an emitted energy signal, said waveguide
means allowing passage of and being substantially transparent to energy
within the predetermined wavelength range;
an axial segment of said waveguide means comprising a block of fixed axial
length of a solid material which is substantially transparent to energy
within the predetermined wavelength range and permeable to a gaseous
component that has an energy absorption peak in the predetermined
wavelength range, the remainder of said waveguide means being
substantially impermeable to the gaseous component;
wherein, when said axial segment is exposed along said fixed length to a
body fluid containing the gaseous component, the gaseous component
permeates said segment and absorbs the incident energy signal along said
fixed length to thereby reduce the intensity of the emitted energy signal
in proportion to the concentration of the gaseous component in the
segment, whereby differences between the intensities of the incident and
emitted energy signals may provide an indication of the concentration of
the gaseous component in the patient's body fluid.
7. The analytical apparatus of claim 6, wherein
the waveguide means comprises an optical fiber having a distal end with
said block disposed along its axial length.
8. the analytical apparatus of claim 7, wherein
said block has first and second ends, wherein said first end is disposed at
the distal end of the fiber and said second end contains a reflective
surface.
9. The analytical apparatus of claim 8 for measuring the concentration of
carbon dioxide in the blood, wherein
said optical fiber is made of a fluoride glass and said block is made of
silicone and said fiber and block are substantially transparent to a
predetermined wavelength range of from about 4.1 to about 4.4 micrometers.
10. The analytical apparatus of claim 6, wherein
said catherter has an aperture adjacent its distal end and said block is
disposed adjacent said aperture so as to allow contact with body fluid.
11. A system for measuring the concentration of a gaseous component in a
fluid by absorption, the gaseous component having an energy absorption
peak lying within a predetermined wavelength range, said system
comprising:
waveguide means defining an axially-elongated energy path having an inlet
aperture receptive to an incident energy signal within the predetermined
wavelength range and an emission aperture for emitting an emitted energy
signal, said waveguide means for allowing passage of and being
substantially transparent to energy within the predetermined wavelength
range;
an axial segment of said waveguide means comprising a block of fixed axial
length of a solid material which is substantially transparent to energy
within the predetermined wavelength range and permeable to a gaseous
component to be measured, the remainder of said waveguide means being
substantially impermeable to the gaseous component;
wherein, when said axial segment is exposed along said fixed length to a
fluid the gaseous component to be measured will permeate said segment and
absorb an incident energy signal along said fixed length to thereby reduce
the intensity of an emitted energy signal in proportion to the
concentration of the gaseous component in the segment;
energy source means for directing to said inlet aperture the incident
energy signal;
a detection means and means for directing the emitted energy signal from
said emission aperture to said detection means, said detection means being
constructed and arranged to provide an indication of the concentration of
the gaseous component in the fluid as a function of the intensity of the
emitted every signal.
12. A fiber optic sensor for measuring the concentration of a gaseous
component in a fluid by absorption, the gaseous component having an energy
absorption peak lying within a predetermined wavelength range, said sensor
comprising:
an optical fiber which is substantially impermeable to the gaseous
component and substantially transparent to the predetermined wavelength
range, said fiber being axially-elongated and having a proximal end and a
distal end, the proximal end being coupled to means for inputting an
incident energy signal within the predetermined wavelength range and being
further arranged to emit an emitted energy signal;
a block of fixed axial length disposed at the distal end of said fiber,
said block being made of a solid material which is substantially
transparent to the predetermined wavelength range and permeable to the
gaseous component, said block having a first end coupled to said distal
end of said fiber and a second end having an inwardly reflective surface,
wherein, when said block is exposed to the fluid along said fixed length
the gaseous component permeates said block and absorbs the incident energy
signal as it travels along said fixed length to thereby reduce the
intensity of the emitted energy signal in proportion to the concentration
of the gaseous component in the block, and
means for detecting the emitted signal for measuring differences between
the intensities of the incident and emitted signals and for providing an
indication of the concentration of the gaseous component in the fluid
based on said differences.
13. The fiber optic sensor of claim 12 for measuring the concentration of
carbon dioxide in a fluid wherein
said optical fiber and block are substantially transparent to a
predetermined wavelength range which corresponds to an absorption peak for
carbon dioxide.
14. The fiber optic sensor of claim 13, wherein
said fiber and block are substantially transparent to a predetermined
wavelength range of from about 4.1 to about 4.4 micrometers.
15. The fiber optic sensor of claim 14, wherein
said fiber is made of a material selected from the group consisting of
fluoride glass, chalcogenide glass, chloride glass, silver halide, and
potassium halide; and
said block is made of a material selected from the group consisting of
silicone, polystyrene, polyurethane, polyethylene, cellulose,
polybutadiene, poly(methylmethacrylate), and polycarbonate.
16. The fiber optic sensor of claim 15, wherein
said fiber is a heavy metal fluoride glass and said block is silicone.
17. The fiber optic sensor of claim 16 for measuring a partial pressure of
carbon dioxide of from about 10 to about 100 mm Hg, wherein
said block has a length of from about 0.5 to about 2 mm.
18. The fiber optic sensor of claim 17, wherein
said block has a length of about 1 mm.
19. The fiber optic sensor of claim 12, wherein
said second end of said block is covered with a reflective metal coating.
20. The fiber optic sensor of claim 12, wherein
substantially the entire outer surface of said block, other than said first
end, is at least partially covered by a reflective metal coating.
21. The fiber optic sensor of claim 12, wherein
said block comprises a cylinder coaxially adhered to the distal end of the
fiber.
22. The fiber optic sensor of claim 12, wherein
said block comprises a cylinder of silicone coaxially adhered to the distal
end of the fiber.
23. The fiber optic sensor of claim 12, wherein
said fiber and block are disposed within a flexible catheter adapted for
insertion in a body cavity for making an in vivo determination of the
concentration of a gaseous component in a body fluid.
24. The fiber optic sensor of claim 12, wherein
said fiber and block are disposed within a flexible catheter adapted for
insertion in the bloodstream of a patient for making an in vivo
determination of the concentration of a gaseous component in the blood.
25. A method for detecting a gaseous component in a fluid by absorption,
the gaseous component having an energy absorption peak lying within a
predetermined wavelength range, said method comprising the steps of:
directing an incident energy signal within said predetermined wavelength
range to an inlet aperture of an axial waveguide means, the waveguide
means allowing passage of and being substantially transparent to energy in
the predetermined wavelength range and being substantially impermeable to
the gaseous component except for a fixed-length axial segment thereof
comprising a block of solid material which is permeable to the gaseous
component;
exposing said axial segment along the fixed length thereof to a fluid
containing the gaseous component, wherein the gaseous component permeates
the segment;
passing the incident energy signal through said waveguide means, wherein
the incident energy signal is absorbed by the gaseous component along the
fixed length of the axial segment to thereby reduce the intensity of the
emitted energy signal in proportion to the concentration of the gaseous
component in the segment;
detecting the emitted energy signal from an emission aperture of said
waveguide means; and determining the concentration of the gaseous
component from the detected emitted energy signal.
26. The method of claim 25 for measuring the concentration of a gase having
an energy absorption peak in the infrared region, wherein
an incident energy signal having a wavelength in the infrared region is
directed to the inlet aperture.
27. The method of claim 26 wherein said gaseous component is selected from
the group consisting of carbon dioxide, water vapor, nitrous oxide,
halogenated hydrocarbon, ethyl alcohol, and anesthetic gases, and wherein
the incident energy signal is selected to have a wavelength corresponding
to an absorption peak of the selected gas.
28. The method of claim 27, for the measurement of the carbon dioxide
concentration in blood, wherein
an incident energy signal having a wavelength of from about 4.1 to about
4.4 micrometers is directed to the inlet aperture. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to a sensor for determining the concentration of a
gaseous component in a fluid by absorption and, in the preferred
embodiment, to a fiber optic probe for making in vivo measurements of the
partial pressure of carbon dioxide in the blood.
Blood gas analysis is performed on nearly every hospital patient both
during and after surgery. The analysis is concerned primarily with three
parameters--the partial pressures of oxygen and carbon dioxide, the
PO.sub.2 and PCO.sub.2, and the negative logarithm of hydrogen ion
activity, the pH. These three parameters give a good indication of the
patient's cardiac, respiratory and circulatory functioning and of
metabolism. Monitoring the level of carbon dioxide in the blood alone
gives a good indication of proper functioning in all of these systems
because carbon dioxide, a waste product of metabolism, travels through the
circulatory system and is removed through the respiratory system.
Several sophisticated blood gas analyzers are commercially available for
analyzing blood samples after the blood is extracted from the patient (in
vitro). The withdrawal and subsequent analysis of a blood sample is both
cumbersome and time-consuming and does not allow for continuous monitoring
of the concentrations of gases dissolved in a patient's blood. There has
been a need for many years for a system which would enable blood gas
measurements to be made directly in the patient (in vivo), thereby
avoiding the difficulties and expense inherent in the in vitro techniques.
Among the suggestions in the prior art was the use of indwelling electrode
probes for continuous monitoring of the blood gas. The in vivo electrode
probes have not been acceptable. Two principal disadvantages of the
electrode probes are the danger of using electrical currents in the body
and the difficulty in properly calibrating the electrodes.
Also among the suggested techniques for in vivo measurement have been the
use of fiber optic systems. In a fiber optic system, light from a suitable
source travels along an optically conducting fiber to its distal end where
it undergoes some change caused by interaction between the light and a
component of the medium in which the probe is inserted or interaction with
a material contained in the probe tip which is sensitive to (i.e.,
modified by) a component in the medium. The modified light returns along
the same or another fiber to a light-measuring instrument which interprets
the return light signal.
Fiber optic sensors appear to offer several potential benefits. A fiber
optic sensor is safe, involving no electrical currents in the body. The
optical fibers are very small and flexible, allowing placement in the very
small blood vessels of the heart. The materials used, i.e., plastic,
metal, and glass, are suitable for long-term implantation. With fiber
optic sensing, existing optical measurement techniques could be adapted to
provide a highly localized measurement. Light intensity measurements could
be processed for direct readout by standard analogue and digital circuitry
or a microprocessor. However, although the potential benefits of an
indwelling fiber optic sensor have long been recognized, they have not yet
been realized in a viable commercial product. Among the principal
difficulties has been in the development of a sensor in a sufficiently
small size which is capable of relatively simple and economical
manufacture so that it may be disposable.
One type of in vivo fiber optic blood gas sensor proposed in the prior art
involves the transmission of light directly into the blood. Light travels
down the fiber and is allowed to leave the fiber at the distal end to
interact directly with the blood and to report back via the return signal
some characteristic spectroscopic property of the blood. An absorption
sensor of this type is described in U.S. Pat. No. 4,509,522 to Manuccia et
al., wherein absorption occurs as an incident light beam travels through
the blood flowing between the ends of two chopped fibers, or as the beam
travels through the blood flowing between the distal end of a fiber and a
spaced mirror. These devices are complex and difficult to manufacture.
A similar fiber optic sensor for measuring the concentration of bilirubin
in the blood is described in Coleman et al., "Fiber Optic Based Sensor For
Bioanalytical Absorbance Measurements," 56 Anal. Chem. 2246-2249 (1984).
The authors state that a sample chamber having a well-defined optical path
length is necessary to obtain true absorbance values, and they propose a
sensor having a single optical fiber in order to achieve a very small
size. The Coleman et al. sensor consists of an optical fiber disposed in a
needle and spaced from the distal end thereof, an aluminum foil reflector
disposed at the end of the needle, and apertures in the needle to allow
blood to flow into the chamber defined within the needle between the
distal end of the fiber and the reflector. The needle and fiber assembly
is then inserted into a larger gauge needle. Again, this device is
difficult to construct and use of the rigid needle prohibits advancing the
same through the blood vessels.
In another type of proposed blood gas sensor, the gas component to be
measured is separated from the blood while making the spectroscopic
determination. A gas-permeable membrane is used to form a chamber at the
distal end of the fiber. The spectroscopic determination is made within
the chamber either directly with the diffused component or with an
intermediate reagent contained in the chamber. For example, U.S. Pat. No.
4,201,222 to Haase describes an optical catheter having at its distal end
an absorption chamber in which a direct absorption measurement is made,
formed by a cylindrical housing and a distensible semipermeable diaphram.
The diaphram is silicon [sic]rubber which permits diffusion of oxygen and
carbon dioxide into the chamber. A reflective coating of vacuum deposited
or evaporated gold or aluminum is applied to the interior surface of the
diaphram to prevent light from escaping. Two sources of light, one visible
red for absorption by oxygen and the other infrared for absorption by
carbon dioxide, are alternately pulsed down the fiber. The resiliently
deformable diaphram also allows monitoring of blood pressure and pulse
rate. Again however, the rigid sample chamber is difficult to construct in
small size.
A fiber optic PCO.sub.2 sensor having an intermediate reagent is described
in G. Vurek, et al., "A Fiber Optic PCO.sub.2 Sensor," 11 Annals of
Biomedical Engineering 499-510 (1983). The sensor is made with plastic
fibers and has at its distal end a silicone rubber tube filled with a
phenol red-KHCO.sub.3 solution. The ambient PCO.sub.2 controls the pH of
the bicarbonate buffer solution which influences the optical transmittance
of the phenol red. The resulting electrical signal is said to be linearly
related to the PCO.sub.2 over a certain range. However, a problem with
shifts in the calibration curve is noted due to deformation of the
flexible silicone tube.
A PO.sub.2 sensor probe utilizing a fluorescent intermediate reagent is
described in U.S. Pat. No. 4,476,870 to Peterson et al. The probe, which
operates under the principle of oxygen quenching of dye fluorescence,
includes two plastic fibers ending in a section of porous polymer tubing
serving as a jacket for the fibers. The tubing is packed with a
fluorescent light-excitable dye placed on a adsorptive polymeric beads.
The polymeric adsorbent is said to avoid the problem of humidity
sensitivity found with inorganic adsorbents such as silica gel. Again, it
is difficult to construct this jacket and bead configuration in a small
size.
Still another approach is suggested in U.S. Pat. Nos. 4,399,099 and
4,321,057, to Buckles. In Buckles apparatus for biochemical analysis, the
optical fiber itself serves as the medium in which the spectroscopic
change occurs. The fiber, which is permeable to the blood gas of interest,
absorbs the gas. The absorbed gas affects the light that emerges from the
exit end of the fiber in proportion to the concentration of the gas in the
sample fluid in contact with the fiber. However, because the fiber is
permeable to the gas there is no way to control the path length over which
absorbance occurs. Even if a nonpermeable coating covers all but a fixed
portion of the fiber, there is no way to prevent the analyte from
permeating along the length of the fiber thereby creating an indeterminate
length of measurement. Without knowledge of the path length, an accurate
absorbance measurement cannot be made. In other embodiments, the
spectroscopic change occurs in one or more sheaths surrounding the optical
fiber which may contain an intermediate reagent.
Thus, in spite of the great need for an in vivo fiber optic sensor, none of
the proposed sensors has met with commercial success. Generally, they are
either not reliable or not adapted for production manufacturing
techniques. Devices involving sample chambers are difficult if not
impossible to make in a miniature size required for use in the blood
vessels. Other sensor probes are not flexible enough to be threaded
through the narrow blood vessels.
It is an object of this invention to provide an in vivo sensor for the
continuous real time monitoring of the concentration of a gaseous
component in a body fluid, such as the carbon dioxide concentration of the
blood.
It is another object of this invention to provide a very small and flexible
fiber optic catheter that can be easily advanced through the small blood
vessels and cavities of the body.
Still another object is to provide a fiber optic sensor which is both
reliable and adapted for production manufacturing techniques.
A still further object is to provide a sensor having a fixed path length
for absorption in order to obtain reliable absorbance measurements.
SUMMARY OF THE INVENTION
According to the invention, apparatus is provided for measuring the
concentration of a gaseous component in a fluid by absorption, where the
gaseous component has an energy absorption peak in a predetermined
wavelength range. The apparatus includes a waveguide means, such as an
optical fiber or a hollow or liquid-filled waveguide. The waveguide means
has an inlet aperture for receiving an incident energy signal within the
predetermined wavelength range and an emission aperture for emitting an
emitted energy signal. The waveguide means is substantially transparent to
energy within the predetermined wavelength range. A segment of the
waveguide means consists of a fixed length body of solid material which is
permeable to the gaseous component. The remainder of the waveguide means
is substantially impermeable to the gaseous component. The waveguide means
is constructed and arranged so that the segment is exposed to the fluid to
enable the gaseous component to permeate the segment and absorb the
incident energy signal along the fixed length of the body to thereby
reduce the intensity of the emitted energy signal in proportion to the
concentration of the gaseous component in the fluid. The difference in
intensities of the incident and emitted energy signals is used to
determine the concentration of the gaseous component in the fluid.
In a preferred embodiment, the sensor apparatus is disposed in a flexible
catheter and is insertable into the body cavities, such as the blood
vessels, for continuous in vivo monitoring of the concentration of a
gaseous component in a body fluid. Preferably the waveguide means is a
single optical fiber which allows the catheter to be made in a very small
diameter. Preferably, the segment consists of a coaxial cylinder of a
solid polymeric material which is directly adhered to the distal end of
the fiber. A reflective coating covers the distal end of the segment for
reflecting the energy signal back through the segment and down the optical
fiber to a detector. More preferably, all outer surfaces of the segment,
except for the surface attached to the fiber, are at least partially
coated with a reflective metal coating for maintaining the light-guiding
properties of the segment yet permitting gas to permeate through the
coating into the segment.
In order to obtain an accurate absorbance measurement, it is essential that
the segment have a fixed length so that the path length over which energy
is absorbed by the gaseous component is known and fixed. Because the fiber
is nonpermeable to the gaseous component, it does not contribute to the
path length along which absorption occurs. The path length is selected
based upon the concentration to be measured and the absorption coefficient
of the gaseous component in order to attain a readily detectable change in
intensity for a change in concentration.
In the preferred embodiment, the sensor is adapted for measuring the
concentration of carbon dioxide. Carbon dioxide has a spectrally isolated
absorption peak in the infrared region with respect to other physiological
gases (e.g., oxygen and water vapor). Thus, this sensor is particularly
useful in a physiological environment such as for in vivo measurement of
the carbon dioxide concentration of the blood. The predetermined
absorption wavelength range for carbon dioxide is between about 4.1 and
4.4 micrometers, and more specifically of from about 4.16 to about 4.36
micrometers. Infrared transmitting fibers are available which are
substantially transparant at this wavelength range and which are
nonpermeable to carbon dioxide. Suitable polymeric materials are also
available to define the gas permeable segment which are both permeable to
carbon dioxide and substantially transparent to the absorption wavelength
range.
In an especially preferred embodiment of the in vivo CO.sub.2 sensor, a
single infrared transmitting fiber, such as a heavy metal fluoride glass,
is used. A solid cylindrical body of polymeric material is directly bonded
to and coaxial with the distal end of the fiber. A preferred polymer is
silicone, which has a high value of carbon dioxide permeability and is
substantially transparent at the absorption wavelength range. A reflective
metal coating is provided at the distal end of the body, and more
preferably on a substantial portion of the outer surface of the body in
order to preserve the light-guiding properties of the body. The composite
fiber and sensor is insertable within a flexible catheter for positioning
the sensor within a body cavity, such as narrow blood vessels.
The invention further includes a method for determining the concentration
of a gaseous component in a fluid by absorption. The method consists of
contacting a polymeric body with the fluid containing the gaseous
component, exposing the body to infrared radiation, and measuring the
infrared absorption in the body by the gaseous component. The body may be
any of various natural or synthetic high polymers which have a substantial
affinity for the gaseous component and which are substantially transparent
to infrared radiation at the absorption wavelength of the gaseous
component. The method is useful for determining the concentration of gases
such as carbon dioxide, water vapor, nitrous oxide, halogenated
hydrocarbons, ethyl alcohol, or anesthetic gases.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of the fiber optic sensor of this invention
and associated instrumentation.
FIG. 2 is a partial sectional view of the distal end of the fiber optic
sensor.
FIG. 3 is a graphical illustration of the energy absorption curves for the
optical fiber, the sensor body, and for carbon dioxide.
FIG. 4 is a fragmented illustration of a catheter embodying the fiber optic
sensor.
FIG. 5 is a cross-sectional view taken along section lines 5--5 of FIG. 4
showing the catheter lumens.
FIG. 6 is a partial sectional view taken along section lines 6--6 of FIG. 4
showing the distal end of the catheter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of this invention consists of a sensor for in vivo
measurement of the concentration of carbon dioxide in a body fluid, such
as the blood, wherein the sensor includes a single optical fiber waveguide
having attached at one end a cylindrical body of polymeric material of
fixed length defining a gas-absorbing region. Both the fiber and body are
substantially transparent to the energy absorption peak for carbon
dioxide. The body is permeable to carbon dioxide while the fiber is
substantially impermeable to carbon dioxide. The distal end of the body is
provided with a reflective coating. An incident energy signal is directed
into the proximal end of the fiber and is transmitted along the fiber to
the distal end and then into the body. The incident signal makes a first
pass along the fixed length of the body, is reflected off the reflective
coating, and returns to make a second pass along the fixed length of the
body and is then transmitted down the fiber back to the proximal end of
the fiber and directed to a detector. The sensor thus provides a known and
fixed absorption path length equal to twice the fixed length of the body.
Knowing this path length, the absorption coefficient of the body for
carbon dioxide, and measuring the intensity of the emitted energy signal
in relation to the incident signal, the partial pressure of carbon dioxide
can be calculated.
Although the preferred embodiment is directed to an in vivo fiber optic
CO.sub.2 sensor, the invention is not limited to in vivo sensors, nor to
carbon dioxide sensors, nor to fiber optic waveguide means. Nonfiber
waveguide means may be used such as, for example, a hollow or
liquid-filled waveguide. The principal requirement is that the waveguide
means be impermeable to the gas being measured. If fiber optics are used,
a plurality of fibers in a bundle may be used. Alternatively, the gas
permeable segment may be interposed between the abutting ends of two glass
fibers, with one fiber leading to the incident energy source and the
second fiber to the detector. Thus, the gas permeable segment may be
disposed either at the distal end of the nonpermeable waveguide means or
at any other point along its length, and rather than a reflective coating,
a second optical fiber can be used for returning the partially absorbed
signal which has passed through the segment to the detector. In addition,
other gases having spectrally isolated absorption peaks can be measured
with the sensor of this invention.
The preferred fiber optic sensor of this invention and associated apparatus
is shown in FIG. 1. The sensor includes an optical fiber 10 having a
proximal end 12 and a distal end 14. The sensor further includes a solid
body 16 of a selected length defined by first and second ends 18, 20. The
first end 18 is disposed adjacent the distal end of the fiber and the
second end 20 has an inwardly reflective surface 22.
The apparatus to the left of the sensor fiber in FIG. 1 consists of a
radiation source 30, a detector 32, and various optical elements
Preferably, an incoherent energy source 30, such as a black body radiator,
produces broadband radiation which is partially collected and collimated
by a lens 34. The collimated signal from the lens is received by a
narrowband filter 36 which rejects nearly all the radiation from the
source except for a narrow segment which includes part or all of the
predetermined wavelength range for which the gaseous component to be
measured is selectively absorptive. This narrowband signal is imaged onto
the face at the proximal end of the fiber by a second lens 38. The face at
proximal end 12 thus defines an inlet aperture for receiving the incident
energy signal. The radiation travels to the distal end of the fiber where
it passes directly into a gas-absorbing region defined by the body 16. The
radiation then encounters a mirrored retro-reflector 22 and retraces its
path back through the fiber 10 and exits the proximal end 12 of the fiber.
The proximal end thus defines an emission aperture from which the emitted
energy signal exits, as well as the inlet aperture. The emitted signal
then passes through lens 38 to a partially reflecting mirror 40 and third
lens 42 for focusing it onto the active area of a detector 32.
The return signal is diminished in intensity over the predetermined
wavelength range in proportion to the concentration of the gaseous
component in the gas-absorbing region The detector measures the intensity
of the emitted signal. By comparing the output of the detector with a
calibration curve for the system, the concentration of the gaseous
component can be determined.
The calibration curve is determined by making at least two measurements
with the system for known concentrations of the gaseous component, and
constructing a line determined by those points. The output of the detector
in the absence of the gaseous component constitutes the intensity of the
incident signal minus any system losses. After this initial one-time
calibration, a reference energy signal outside of the predetermined
wavelength range may be sent along with the absorption signal and
separately detected in order to continuously monitor the optical alignment
of the system and system losses during operation.
A preferred sensor body 16 is shown in FIG. 2. A gas-absorbing region 60 is
composed of a solid cylindrical polymeric body which is directly bonded at
its proximal end 62 to the exit face at the distal end of the optical
fiber 10. The region 60 has substantially the same diameter as the fiber.
The gas-permeable polymer material has a high value of solubility for the
gaseous component of interest and is optically clear at the predetermined
wavelength range defined by the absorption peak of the gaseous component.
The region 60 is covered at its distal end 64 with a plane metal
reflective coating 66, and preferably, except for proximal end 62, the
entire outer surface 68 of the body is partially covered with a very thin
reflective coating 68 in order to preserve the light-guiding properties of
the body. Such a partial and very thin (e.g., 2-5 micron) coating of
metal, e.g., aluminum, gold, silver or nickel, can be formed by
electrodeposition or vacuum deposition, to be both reflective and still
gas permeable.
In an alternative embodiment, not shown, the sensor body 16 is disposed
between two segments of an optic fiber. In this alternative embodiment the
inlet and emission apertures are not the same, rather the incident energy
enters one end (inlet aperture) of a first optic fiber segment, the signal
travels through the first optic fiber segment, the sensor body, and the
second optic fiber segment in succession, and then exits from the opposing
end (emission aperture) of the second optic fiber segment.
The fiber optic sensor of this invention can be inserted into a blood
vessel or body cavity via a flexible catheter as shown in FIGS. 4-6. The
catheter is formed from an elongate flexible body 102 and, for example,
may be extruded from an appropriate plastic material such as polyurethane
or polyvinyl chloride. The body 102 has a first lumen 104 in which the
optical fiber 110 of the sensor is enclosed and a second lumen 106 for
fluid infusion. Both of lumens 102 and 106 are open at their distal ends.
The proximal end of the catheter includes a molded fitting 120 which is
secured to the catheter body 102. Projecting from the proximal end of the
fitting 120 are a pair of flexible tubes 122, 124. The tube 122 is adapted
to receive the optical fiber 110, which extends through the fitting 120.
The proximal end of the tube 122 is provided with a connector 126 which is
connected to the proximal end of the optical fiber 110. Connector 126 is
adapted to be mounted with respect to a source of radiant energy, such as
a laser or black body radiator (illustrated diagramatically at 127) so
that the proximal end of the optical fiber 110 may | | |
