IMPROVED CAPNOMETER

Abstract

We describe a capnometer for detecting a concentration of a component in a gas, wherein said gas is inhaled and/or exhaled by a patient, said capnometer comprising: an air flow region through which said gas passes to and/or from said patient's lung; a mid-IR semiconductor emitter configured to provide IR light at a wavelength in the range 3-5 m; a mid-IR semiconductor detector to detect said IR light; a reflector to reflect said IR light emitted by said emitter; wherein said emitter, said detector and said reflector are arranged such that said IR light emitted by said emitter passes through said air flow region via said reflector to said detector. The reflector is selected from a Fresnel reflector and a reflective diffractive optical element, such as.a Fresnel zone plate.

Claims

1. A capnometer for detecting a concentration of a component in a gas, wherein said gas is inhaled and/or exhaled by a patient, said capnometer comprising: an air flow region through which said gas passes to and/or from said patient's lung; a mid-IR semiconductor emitter configured to provide IR light at a wavelength in the range 3-5 m; a mid-IR semiconductor detector to detect said IR light and a reflector to reflect said IR light emitted by said emitter; wherein said emitter, said detector and said reflector are arranged such that said IR light emitted by said emitter passes through said air flow region via said reflector to said detector wherein the reflector is selected from the group consisting of a Fresnel reflector and a reflective diffractive optical element.

2. A capnometer as claimed in claim 1, wherein said emitter is located at a first location on a first side of said air flow region, wherein said detector is located at a second location on a second side of said air flow region, wherein said first side is adjacent said second side, wherein said reflector is located at a third location on a third side of said air flow region, and wherein said third side is opposite said first side or opposite said second side.

3. A capnometer as claimed in claim 1, further comprising a breath tube, wherein said breath tube defines a channel between said emitter/detector and said reflector for said air flow region.

4. A capnometer as claimed in claim 3, wherein said breath tube is removable from said capnometer.

5. A capnometer as claimed in claim 3, wherein at least one of said emitter, said detector and said reflector are mounted in said breath tube.

6. A capnometer as claimed in claim 3, wherein said emitter and said detector are located external to said breath tube, and wherein said breath tube comprises a mid-IR transmissive portion, said mid-IR transmissive portion being aligned with said emitter and said detector to allow mid-IR light to pass therethrough into and out of said breath tube.

7. (canceled)

8. A capnometer as claimed in claim 3, wherein said breath tube comprises one or more alignment features for enabling said arrangement of said reflector with said emitter and said detector.

9. A capnometer as claimed in claim 3, wherein said emitter and said detector are located external to said breath tube, and wherein said breath tube comprises a mid-IR transmissive portion, said mid-IR transmissive portion being aligned with said emitter and said detector to allow mid-IR light to pass therethrough into and out of said breath tube and said reflector is located external to said breath tube, and wherein said breath tube comprises a second mid-IR transmissive portion, said second mid-IR transmissive portion being aligned with said reflector to allow mid-IR light to pass therethrough into and out of said breath tube.

10. A capnometer as claimed in claim 3, wherein said reflector is mounted in the breath tube.

11. A capnometer as claimed in claim 3 wherein said reflector is mounted in the breath tube and the breath tube is provided with pre-formed recesses for receipt of the reflector.

12. A capnometer as claimed in claim 3 wherein said reflector is mounted in the breath tube and a flat side of the reflector on an external side of the breath tube is in contact with a heater.

13. A capnometer as claimed in claim 1 wherein the reflector is a Fresnel reflector including at least one cut in a concentric ring, preferably multiple cuts in multiple concentric rings.

14. A capnometer as claimed in claim 1 wherein the reflector is a reflective diffractive optical element in the form of a Fresnel zone plate, reflective grating or reflective hologram, the reflective diffractive optical element including at least one step, preferably multiple steps.

15. A capnometer as claimed in claim 14 wherein the reflector is a Fresnel zone plate of multiple phase levels, the plate having a series of steps of more than one step height, wherein the step heights introduce </2 phase change with no step height introducing greater than 2 of phase change.

16. A capnometer as claimed in claim 14 wherein the reflector is a Fresnel zone plate of multiple phase levels, the plate having a series of steps of more than one step height, wherein the step heights are <0.5 m and the total step heights are <5 m.

17. A capnometer as claimed in claim 14, wherein the reflector is a Fresnel zone plate, reflective grating or reflective hologram of a single phase level comprising a series of steps of a single height, wherein the height of each step is around one quarter the wavelength of light.

18. A capnometer as claimed in claim 14 wherein the reflector is provided as a thin film single level phase FZP focussing element.

19. A capnometer as claimed in claim 14 wherein the reflector is provided as a thin film single level phase FZP focussing element and further comprises a backside conducting element provided on the thin film.

20. (canceled)

21. (canceled)

22. A capnometer as claimed in claim 1 further comprising a diversion device for blocking one of a first and a second air-flow path in the capnometer at a time, wherein said first air-flow path connects an inhaling/exhaling portion of the capnometer at which a user of the capnometer inhales/exhales air and an intake portion of said capnometer through which air enters said capnometer, and wherein said second air-flow path connects said inhaling/exhaling portion and an exit portion of said capnometer at which air exits said capnometer.

23. An inhaler comprising a capnometer according to claim 1 wherein said capnometer is configured to monitor a CO.sub.2 level in air inhaled through or exhaled through said inhaler.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] These and other aspects of the invention will now be further described by way of example only, with reference to the accompanying figures, wherein like numerals refer to like parts throughout, and in which:

[0052] FIG. 1 shows a schematic illustration of components of a capnometer according to the prior art;

[0053] FIG. 2 shows a schematic of a capnometer comprising a silicon optic window according to the prior art;

[0054] FIG. 3 shows a schematic illustration of a capnometer comprising a Fresnel reflector and a breath tube incorporating two IR-transmissive windows according to an embodiment of the present invention;

[0055] FIGS. 4A and 4B illustrate path lengths traversed by light rays from emitter to detector via a spherical reflector and a Fresnel lens reflector respectively;

[0056] FIG. 5 shows a schematic illustration of a capnometer incorporating a Fresnel reflector within a disposable breath tube according to another embodiment of the present invention;

[0057] FIG. 6A shows a schematic illustration of a capnometer incorporating a Fresnel zone plate according to yet another embodiment of the present invention, with FIG. 6B being an expanded partial view of the Fresnel zone plate;

[0058] FIG. 7A shows a schematic illustration of a capnometer incorporating a Fresnel zone plate according to yet a further embodiment of the present invention, with FIG. 7B being an expanded partial view of the Fresnel zone plate;

[0059] FIG. 8 shows a schematic illustration of a capnometer incorporating a thin film Fresnel zone plate with integral heater according to yet another embodiment of the present invention; and

[0060] FIG. 9 is a cross-sectional view of a capnometer according to yet a further embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0061] FIG. 1 shows a schematic illustration of components of a capnometer according to the prior art, such as that described in the Applicant's earlier co-pending Patent Application Publication No. WO 2016/092308. The device may be exploited to measure the concentration of a gas component, such as carbon dioxide in a sample gas (for example, exhaled air) flowing across an air flow region interposed between an emitter (2)/detector (3) pair and a reflector (4). In FIG. 1, an emitter (2)/detector (3) pair and a curved reflector (4) are incorporated into a breath tube (1) and are connected to a suitable electronic drive and detecting circuitry. The value of the detector current may be proportional to the emitter current and the amount of gas to be sampled in the sampling area.

[0062] The emitter (2) and/or the detector (3) of the capnometer may be composed of III-V semiconductors. It will be appreciated that materials suitable for emitter (2) and detector (3) will be known to those skilled in the art. Ideally, an emitter diode is designed to emit IR radiation centred around 4.26 m and the detector (3) is a photo-diode which has a peak sensitivity centred around 4.26 m.

[0063] The path length traversed by the IR light emitted by the emitter (2) and detected by the detector (3) via the reflector (4) is chosen such that the attenuation of the signal by absorption by CO.sub.2 molecules is such that a change in intensity can be detected by the detector (3) for the concentration range that is appropriate for the gas stream. For human breath, the range of CO.sub.2 concentration varies between the background CO.sub.2 level which is inhaled by the human and over 10 kPa which is exhaled by the hypercapnic patients. For the mid-range of 5 kPa the distance traversed by the IR light may be approximately 20 mm.

[0064] FIG. 2 shows another capnometer according to the prior art. The capnometer incorporates a disposable breath tube (1) with a curved reflector (4) and an optical layer (5) which may comprise silicon. The optical layer (5) forms a seal with the breath tube (1). The purpose of the optical layer (5) comprising silicon is both to protect the emitter (2)/detector (3) pair from breath, e.g. to avoid a contamination of the emitter (2)/detector (3) pair, and to increase the collection efficiency of the system by directing more light emitted from the emitter (2) to impinge on the detector (3). Other materials suitable for the optical layer (5) will be known to those skilled in the art, and include, but are not limited to ZnS, ZnSe, Ge, chalocogenide glasses and certain polymers. The capnometer includes a seal (9) to prevent gas from the surrounding area from entering the sampling area. In the illustrated example, the seal (9) is arranged at two locations where the breath tube (1) and the optical layer (5) connect with each other. It will be appreciated that the seal (9) may be arranged at one or more locations. The seal (9) may be arranged on a side facing the air flow region where the breath tube (1) and the optical layer (5) connect, and/or on a side facing away from the air flow region (as shown in FIG. 2).

[0065] The emitter (2)/detector (3) pair are mounted on a printed circuit board (6) which may incorporate high precision locating lugs for locating pins (8) that are incorporated into the injection moulded breath tube (1). This allows for a high precision alignment of the emitter (2)/detector (3) pair to the curved reflector (4) so that high collection efficiency may be achieved. The printed circuit board (6) allows connection between the emitter (2)/detector (3) pair and a driving circuit. An electrical connector (7) is connected to the printed circuit board (6) in order to drive the emitter (2)/detector (3) pair.

[0066] The system may be designed such that the whole assembly may be removed from the driving electronics and body of the capnometer, and may be replaced. Therefore, the breath tube assembly comprising curved reflector (4), optical layer (5) and emitter (2)/detector (3) printed on the circuit board (6) may be disposed after use of the capnometer.

[0067] While the aforementioned capnometers are suitable for purpose and enable high temporal resolution with high accuracy over a specific concentration range, the use of spherical/elliptical curved mirrors lead to a number of difficulties, including uneven heating leading to problems with condensation, the curvature of the mirror causing gas to be trapped in the apex reducing the temporal resolution of the system and the single path length meaning that the sensor is suited to only a narrow range of measurement. Furthermore, the curvature of the lens does not lend itself to easy introduction into a replaceable breath tube.

[0068] FIG. 3 of the accompanying drawings illustrates a capnometer according to one embodiment of the present invention. The capnometer incorporates a Fresnel reflecting lens (12) in place of the curved reflector (4). The capnometer comprises a breath tube (1) which constrains the air flow, the tube having incorporated into it one narrow and one relatively wide IR-transmissive window (10a, 10b) on opposing sides of the tube. The emitter (2) and detector (3) are provided adjacent the narrow window (10a) and are mounted on a printed circuit board (6). The Fresnel reflector (12) is provided behind the wider window (10b). The emitter/detector pair (1) may be composed of III-V semiconductors and suitable materials will be known to those skilled in the art. In this example the emitter is designed to emit IR radiation centred around 4.26 m and the detector has a sensitivity peak centred around 4.26 m. The emitter/detector pair and Fresnel lens will be aligned so that light emitted from emitter (2) is focused by the lens (12) on to the detector (3). The gas flows through the air region interposed between the emitter/detector pair (2,3) and the Fresnel reflector (12). The air flow region is isolated from the emitter/detector and the reflector by the silicon IR-transmissive windows (10a, 10b) which are provided with anti-reflections coatings and a heater (not shown).

[0069] The Fresnel reflector (12) is made from a high density polymer which may have been injection moulded. In the embodiment shown in FIG. 3 the Fresnel reflector contains five concentric elements, however there may be more or less elements. The reflector surface may be coated by evaporation, sputtering or other physical vapour deposition or via an electrochemical method with a thin metal film, such as silver, gold or aluminium. It will be appreciated that a variety of deposition techniques known to a skilled person may be exploited. In this example the reflector is coated with a gold film. However, it will be understood that a variety of other materials known to those skilled in the art may be used for this purpose.

[0070] The breath tube (1) is made from high-density polymer which may have been injection moulded and incorporate recesses for the silicon windows (10a, 10b) which may be bonded using a suitable adhesive. The silicon windows may have anti-reflection coatings to minimise the signal loss at the air/silicon interfaces. The silicon windows will also incorporate heaters to maintain a temperature of the window in contact with breath to minimise the effect of breath condensation on the optical path. Other materials for the optical windows transparent to infra-red radiation will be known to those skilled in the art and include but not limited to ZnS, ZnSe, Ge, chalcogenide glasses and certain polymers. The silicon window adjacent to the Fresnel lens (12) is placed in close proximity to the lens to limit the non-sampled gas volume.

[0071] It should be appreciated that compared to a spherical reflecting geometry where the distance traversed by a light ray from emitter to detector via the reflector is substantially the same for all rays emitted at solid angles illuminating the reflector, the Fresnel lens (12) has a larger path length for rays emitted at larger angles (relative to the emitter surface normal direction), as illustrated in FIGS. 4A and 4B wherein FIG. 4A shows the path length for a spherical reflector and FIG. 4B shows the multiple path lengths for a Fresnel reflector. The sensitivity of the sensor is dependent on this path length. Measurements made at low gas concentrations require long path lengths to allow enough absorption of the ray to be detected and conversely, at high gas concentrations only small path lengths are required to prevent too small a signal being received by the detector. Therefore, there is an optimum path length required for a particular gas concentration. The range of carbon dioxide concentration in breath is large, therefore it is an advantage to have multiple path lengths as provided by the Fresnel lens incorporated into the capnometer of the present invention. Those skilled in the art can determine the optimum path length range in the gas being sampled depending on the power output of the emitter, the sensitivity of the detector and the losses in the optical path.

[0072] The use of the Fresnel lens reflector provides a number of advantages over the prior art. The flatter geometry allows the breath tube to be inserted with a smaller non-sampled dead space between the silicon window and the reflector thereby reducing the errors associated with the absorption of signal by gas in the dead space volume. The flat geometry also gives rise to a range of path lengths as detailed above, the consequence of which is to increase the range of measurement of the capnometer.

[0073] FIG. 5 illustrates a similar capnometer to that of FIG. 3 but having the Fresnel lens (12) provided as part of a replaceable breath tube (1). The emitter/detector pair (2,3) are again mounted on a PCB (6), the emitter (2) illuminating a gas flow region defined by the breath tube (1) incorporating a heated silicon window (10a) on the emitter/detector side and a Fresnel reflector (12) on the opposite side. The Fresnel reflector contains a heater (14) to prevent condensation affecting the signal focused by the Fresnel lens onto the detector. This arrangement not only provides for a higher signal level due to removal of reflection loss associated with the second silicon window (10b) but lowers the materials cost of the device. The heater enables the surfaces that are in contact with the sample gas to be heated so that the surface temperature is high enough to prevent condensation on the reflector (12) and/or the transmissive layer (10a). The flatter Fresnel reflector geometry enables the reflector to be heated more uniformly because a flat heater can be used, allowing it to be in more uniform contact with the surface to be heated. This enables the heater to use less energy, reducing the power consumption of the capnometer. Furthermore, the low heat energy requirement reduces the thermal expansion of the optical system, allowing it to maintain its optimum efficient geometry.

[0074] The breath tube (1) may be made from a high-density polymer which may have been injection moulded to form the required shape. The Fresnel lens is provided with an additional coating of material to achieve a reflecting surface as previously described in relation to FIG. 3. The Fresnel lens may be incorporated into the breath tube using a suitable adhesive or it may be fabricated in the same injection moulding tooling as the breath tube in two halves so that the side containing the Fresnel lens can be coated, the two halves subsequently attached using suitable adhesive or by interference fit. It can be appreciated that alignment features (not shown) are required to ensure that the breath tube (1) now incorporating the focussing element (12) can be aligned accurately to the emitter/detector pair (2,3).

[0075] The breath tube (1) may in embodiments contain channels which allow a sample gas to be diverted from the air flow region where the sample gas is examined, so that a conductance may be achieved which is high enough for the flow to be sampled.

[0076] Additionally, the surfaces of the reflector (12) and/or transmissive window (10a, 10b) which are in contact with the sample gas may be coated with materials which modify the surface energy. Hence, when water condensate from breath condenses on these surfaces, in the case of high surface energy modifiers, the water condensate forms a thin film rather than a droplet or droplets which potentially scatter and therefore reduce the IR radiation impinging on the detector (3). The surface energy modifying materials may be, but are not limited to, hydroxyl and carboxyl containing hydrocarbon thiols. Hydroxyl and carboxyl containing hydrocarbon thiols may be particularly suitable for coating a gold surface to form a self-assembled mono-layer. Poly-ethylene oxide or an amine of cyano containing polymers may be deposited on the surface of the reflector (12). In the case of low surface energy modifiers, the water condensate forms droplets that bead up and fall from the window, or in the case of the reflector (12) coat less of the reflector (12) and so have less effect on the signal. Low surface energy materials may be, but are not limited to fluoro-carbon containing molecules. Fluoro-carbon thiols may be particularly suitable for coating a gold surface using a self-assembled mono-layer.

[0077] FIGS. 6A and 6B show a schematic of a capnometer according to another embodiment of the present invention. In this embodiment, the reflector comprises a reflective diffractive optical element in the form of a Fresnel Zone Plate FZP (12) of the multi-level type (see FIG. 6B) which is incorporated into the body of the removable breath tube (1). In this type of zone plate a series of steps (12s) that approximate to the continuous phase change introduced by a curved reflector can be made. The plate is again provided with a heater (14) and an IR transmissive window (10a) is provided opposite the plate in line with which are emitter (2) and detector (3) mounted on PCB (6). Typically, the series of steps (12s) introduce </2 phase change with no step height introducing greater than 2 of phase change therefore limiting the total height of the step and therefore limiting the total thickness of the mirror to this height plus any supporting thickness required to ensure mechanical integrity. For mid-infrared wavelengths the step heights are of the order of 0.5 m and the total step heights are <5 m.

[0078] The FZP may be fabricated using high-density polymer and injection moulding. It will be known by those skilled in the art that the injection moulding tooling for the small step heights required for multi-level phase FZPs suitable for infra-red wavelengths can be fabricated using silicon microfabrication techniques including but not limited to photolithography and etching of silicon dioxide and silicon nitride and silicon oxynitride layers grown by chemical vapour deposition (CVD) or plasma enhanced chemical vapour deposition (PECVD) on silicon. The master zone plate patterns may be used in the injection moulding tooling directly or be used to make an electroformed mould insert, typically using electroplated nickel. As for the Fresnel lens, the FZP requires coating with a reflective material.

[0079] FIGS. 7A and 7B show another embodiment of a capnometer according to the present invention incorporating a disposable breath tube (1) provided with a Fresnel Zone Plate FZP (12) with a single phase level (see FIG. 7B). This is one of the simplest diffracting elements with just one step height. In this embodiment, a series of concentric steps are patterned and coated with a suitable reflector such that the interference of wave fronts scattered from the surface of the zone plate (12) result in maximising of the flux of radiation on the detector (3). In the case where the FZP is separate to the breath tube the planar geometry also allows a breath tube to be inserted with minimum dead space, thereby minimising dead space error.

[0080] The pattern of steps is a set of concentric circles or ellipses optimised so that the phase of wavelets arriving at the detector are in substantially in-phase and those off the detector or out of phase. Various optimisations are known to those skilled in the art, where a particular pattern may also be referred to as a photon sieve or hologram. Typically, the step height is one quarter of the wavelength of light, therefore this structure provides the thinnest optical element. Like the previous embodiments of FIGS. 3 and 5A to 6B, the structure requires a coating of reflective metal and can be fabricated by injection moulding of a high-density polymer using a suitable injection mould.

[0081] It is to be appreciated that in any of the aforementioned embodiments the capnometer may include a body (not shown), for example, of injection moulded high density polymer, in relation to which the breath tube is inserted. The body may contain locating pins which align to lugs in a printed circuit board. The Fresnel reflector (12) may optionally be integral to the body. The emitter (2)/detector (3) pair may be arranged on top of the printed circuit board and an electrical connector may be connected to the printed circuit board in order to drive the emitter (2)/detector (3) pair. The breath tube (1) may be interspersed between the moulded body and the printed circuit board.

[0082] In embodiments described herein, there is a problem of contamination of the reflective surface and/or any IR-transmissive windows/components. Contamination may reduce the signal received by the detector (3) and it may appear as an increase in CO.sub.2 level even if no change has actually occurred. One method to mitigate this is to monitor the inspiration phase of the breath cycle where fresh air with low CO.sub.2 levels is passed through the sampling area. Typically, the CO.sub.2 concentration is approximately 450 ppm. It will be appreciated that the threshold may vary and may preferably be adjusted depending on conditions of the area surrounding the device. If the measured CO.sub.2 level is higher than the threshold, the system may indicate that the measurement is void. A correction to the measured CO.sub.2 level may be made.

[0083] FIG. 8 shows another capnometer according to the present invention, the capnometer incorporating a breath tube (1) with a thin film single-level phase FZP focussing element (12) attached to the breath tube using a pre-formed recess and push-fit clip (16). The advantage of a film FZP is that it may be fabricated on a sheet using a hot embossing method. A foil which may be either of thin metal or polymer such as PET or PEN is coated with a thermosetting polymer capable of being moulded using a hot embossing roller. The foil can then be coated with a reflecting coating and the FZP cut from the foil. The FZP foil can incorporate a backside conducting element that may be attached to a current source using conductors. Not only is this method of FZP formation low cost it also has low thermal capacitance and so will use less power to heat the element to prevent condensation affecting the signal level received by the detector.

[0084] FIG. 9 of the accompanying drawings, illustrates yet another embodiment of the present invention having a Fresnel reflector 12 provided within the breath tube (1) as shown in FIG. 5 but the breath pathway area is reduced by the provision of a restriction 18 between the emitter/detector (2,3) and the Fresnel lens (12). This serves to increase the velocity of the gas flowing through the sampling area, increasing a time-resolution of the determination of the concentration of the gas component to be determined. This increase in breath velocity ensures that the response time of the capnometer is not dependent on the transit time of breath in the sampling area.

[0085] In embodiments described herein, it may be necessary to add an additional mid-IR window between the emitter/detector pair and the breath tube to provide protection to these devices during replacement of the breath tube.

[0086] It is also necessary to achieve accurate calibration of the system. This is particularly the case where a replaceable breath tube (1) is employed since there is a possibility of small variations in alignment. The device may be calibrated assuming the background CO.sub.2 level is known. This might be the case if the air is ambient air. Alternatively, the CO.sub.2 level may be obtained from a different information source.

[0087] Alternatively, a breath tube (1) incorporating a gas of known CO.sub.2 level may be used for calibrating the device. The breath tube (1) may comprise end caps which may have removable elements such that after calibration the removable elements may be peeled off and the device is ready to use.

[0088] No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art and lying within the spirit and scope of the claims appended hereto.