CARBON DIOXIDE DETECTOR

Abstract: A carbon dioxide detector which comprises an indicating member which comprises an intimate mixture of a polymer vehicle and a carbon dioxide sensing medium. The sensing medium comprises an anionic fluorimetric dye and a lipophilic organic quaternary cation.

Description: Carbon Dioxide Detector The present invention is concerned with devices which provide a detectabl indication that a volume of gas has an increased propoπion of carbon dioxide compared t that present in normal ambient air. Such devices have a number of uses, one major us being to provide an indication of whether a tube has been correctly located in the airwa (trachea) of a patient. There are many clinical situations where it is necessary to place a tube in th trachea of a patient. This is done in order to supply air or a mixture of oxygen and othe gases to the lungs of the patient. The correct placement of the tube in the trachea is very important. Accidents have been reported where the tube has been inadvertently placed in the oesophagus. It has been observed that the air in the trachea contains about 6% carbon dioxide, whereas :he oesophagus contains air in which the concentration of carbon dioxide is normal (about 0.03%). Several indicator devices which can detect the presence of an elevated proportion of carbon dioxide compared to that in air are known. Our prior PCT application WO91/05252 discloses a carbon dioxide monitor comprising a substrate provided with a colorimetric indicating member. The indicating ' member is an intimate mixture of a transparent polymer vehicle and a salt of an indicator anion and a lipophilic cation. We have now devised a carbon dioxide detector provided with a tluorimetric sensing medium which is responsive to a carbon dioxide concentration in the proximal atmosphere. The inclusion of such fluorimetric media in a detector according to the present invention substantially enhances the sensitivity of such media in the detection of carbon dioxide. According to the present invention therefore there is provided a carbon dioxide detector which comprises an indicating member comprising an intimate mixture of a polymer vehicle and a carbon dioxide sensing medium comprising an anionic fluorimetric dye and a lipophilic organic quaternary cation. The response of the sensing medium to the level or carbon dioxide in the atmosphere involves the variation in the intensity of fluorescence emitted by the fluorimetric dye. The intensity of the fluorescence relies upon the fraction of the fluorimetric dye which is protonated which is in mm directly proportional to the carbon dioxide pressure. Protonation of the dye occurs via a reaction which involves both the lipophilic cation and the carbon dioxide as follows: > [Q~D\xH,O] + C0 2 [Q + HCO,-.(x-l)H O.HD] < Fluorescent Non Fluorescent where Q~ is the lipophilic cation, and D " is the fluorimetric dye (excitation) =430 nm: (emission) =538 nm As can be seen from the above reaction an increase in carbon dioxide pressure will result in a decrease in fluorescence. Where a qualitative indication of carbon dioxide concentration is required the change in fluorescence can be observed visually. However, where a quantative indication of carbon dioxide concentration is required the change in fluorescence can be monitored fluorimetrically. The above reaction illustrates how the cation/fluorimetric dye complex generates the protonated form of the dye on reaction with carbon dioxide. The fluorimetric dye. being in anionic form in the complex, readily abstracts a proton form the water molecule in the presence of carbon dioxide. Such a phase transfer mechanism is advantageous as it promotes the sensitivity of the dye in the detection of carbon dioxide. Preferred fluorimetric dyes include l-hydroxypyrene-3.6.8-trisulphonate. 1.3- dihydroxypyrene-6.8-disulphonate. fluorescein, umbelliferone. 4-methylumbelliferone. 3- benzothiazoylbelliferone. 7-hydroxycoumarin-3-carbox\iic acid. l-πaphthol-2-sulphonate. 1- naphthol-4-sulphonate. 2-naphthol-6-sulphonate. 7-hydroxyflavone. 7-hydroxyisoflavone. 3- hydroxyxanthone.3.6-dihydroxyxanthone.7-hydroxy-4-methylchromon. 7-hydroxylepidone. 3-hydroxyxacridone. har ol methoiodide. salicylaldehyde semicarbazone and 2- hydroxycinnamic acid. A particularly preferred dye comprises l-hydroxypyrene-3.6.8-trisulphonate. The preference for l-hydroxypyrene-3.6,8-trisulphonate is because of its low toxicity. a pk, value in the range of 7.3-8- 1 which is advantageous for making physiological measurements- a large stoke' s shift (absorption maxima of the protonated dye and non-protonated dye bein 403 and 455nm respectively, emission maximum = 512nm. pH 2-14). high photostabilit and high water solubility. The detector according to the invention may. in some embodiments, b supplied in a sealed, gastight package, such as a pouch or sachet. Such a sachet may b formed from metallised polymer film, which is opaque in addition to being gas-impermeable When the detector according to the invention is to be used to determin whether a patient has been correctly intubated. it is preferably provided in sterilised form i such a sealed package. In this embodiment of the invention, the detector preferably consist of gamma- ray sterilisable materials. The transparent film-forming polymer vehicle should be compatible with th cation/fluorimetric dye complex such that the latter does not exude or otherwise underg phase separation over a prolonged period; it should in addition be hydrolytically stable i order to avoid unwanted changes in the pH in the absence of carbon dioxide. The polyme should furthermore be permeable to carbon dioxide. The hydrolytically stable polymer may be water-soluble or organic solvent- soluble (the latter being preferred). Examples of suitable organic solvent soluble polymers include polyvinyl butyral. polyvinyl methyl ether, polymethyl methacrylate, ethyl cellulose and polystyrene. Examples of water-soluble polymers with good resistance to hydrolysis includ hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulose, polyethylene glycol. polyvinyl alcohol (100% hydrolysed) and polypropylene glycol. According to a first aspect of the present invention, the polymer vehicle may preferably contain a plasticiser which enhances the permeability of the polymer to carbon dioxide. Preferably the plasticiser is hydrolytically stable and may be water-soluble or organic solvent-soluble (depending on the polymer vehicle). Examples of suitable organic solvent-soluble plasticisers include alkyl triesters of phosphoric acid (including tributyl phosphate, triethyl phosphate and tris butoxyethyl phosphate), branched chain esters of carboxylic acids, especially those with secondary or tertiary alcohols (including diisopropyl phthalate and diethy hexyl sebacate), suiphamides (including p-toluene sulphonamide). Examples of water-soluble plasticisers with a good resistance to hydrolysis include alcohols (including glycerine and trimethylolpropane) and ethers (including low molecular weight polyethylene glycol). According to a second aspect of the present invention, the polymer vehicle is substantially plasticiser free. The omission of plasticiser from the polymer vehicle can be seen to optimise the fluorimetric properties of the detector when used in aqueous or high humidity conditions. This is because "fogging" (general clouding or opacity) of plasticised polymer vehicles can occur in such conditions, making qualitative measurements difficult. Detectors which include non-plasticised polymer vehicles do not however exhibit such efficient response and recovery times as detectors which include plasticised polymer vehicles. This is illustrated by typical response and recovery times of a plasticised detector respectively being about 2.6 seconds and 31 seconds, and for a non-plasticised detector these times are respectively 5 seconds and 28 seconds. The decision to include a plasticiser within the polymer vehicle should therefore be balanced on the required optimal properties of the detector according to the present invention. Examples of suitable quaternary cations include ammonium cations (including benzyltrimethyl ammonium, trioctylmethylammonium, tricaprylmethyl ammonium, tetrabutyl ammonium, tetrahexyl ammonium and tetraoctyl ammonium) and phosphonium cations (including tetraphenyl phosphonium, trioctyl phosphonium and hexadecyl tributyl phosphonium). A preferred quaternary cation is the tetraoctyl ammonium ion. The salts of quaternary cations (ammonium and phosphonium) become insoluble in water when the cation is above a certain molecular weight (approximately C24 in the case of symmetrical aliphatic quaternary ammoniums). This phenomenon may be exploited to confer enhanced water resistance on non-aqueous sensor films by rendering the dye and buffer components immune to dissolution and "wash out" by liquid water. Preferably the indicating member of the detector is provided, typically in the form of a film of thickness of about 20 microns, on a substrate. The substrate should be such that it is free of mobile components capable of migrating into the indicating film: equally it should resist migration of components from the latter film. Fuπhermore. the substrate should be chemically inert relative to the film. Suitable inorganic materials for use as the substrate include glasses, ceramics, and crystalline materials: suitable organic materials include paper, polyolefins (such a polypropylene or polyethylene) and fluorocarbons (such as PTFE). When polymers are used they should be free of reactive or migratory plasticisers, lubricants, antioxidants or the like The substrate may be in the form of a simple tube or plate: in one preferred embodimen the substrate is in the form of a connector or junction piece for a catheter. In another embodiment of the invention, the substrate may be in the form of a tube which is open a both ends; this may be used to monitor the carbon dioxide content of expired breath in method in which the patient breaths through the tube and the fluorescence change in th indicator film monitored. This method may be used for determination of the end-tidal carbo dioxide (that is, the carbon dioxide present in the final expiration of a deep breath from th lungs). The carbon dioxide sensor according to the invention may be used for lon term continuous monitoring of carbon dioxide concentration if acidifying and oxidising gase are removed from the air prior to contact with the film. This may be achieved using "guard" or "scrubbing" device incorporating (i) the salt of an involatile acid with a pK intermediate between carbonic acid and the polluting volatile acid (e.g. sulphurous acid) an (ii) an antioxidant or antiozonant. The sensor typically has a shelf life of over six months, this shelf life can however be depleted by prolonged exposure of the detector polymer vehicle to a dry environment or substantial heating. There is further provided by the present invention a method of preparing a carbon dioxide detector as hereinbefore described, which method comprises mixing an anionic fluorimetric dye, a lipophilic quaternary cation and a polymer to produce a carbon dioxide indicating member, and substantially curing said indicating member. Typically the indicating member is cast as a film on a substrate, and curing of the film results in a decrease in film thickness generally from about 100 microns (as applied) to about 20 microns (when cured). Typical reagents used in the method are substantially as hereinbefore described with reference to the detector. There is further provided by the present invention a carbon dioxide detector obtained bv a method as described above. The present invention will now be further illustrated by the following example, in which is described a method of preparation of a carbon dioxide detector according to the present invention, and spectrophotometric analysis of the polymer film of the detector. Reference will now be made to the accompanying drawings, in which: Figure 1 is a side elevation showing one form of a detector according to the invention; Figure 2 is a simplified anatomical diagram showing how correct intubation of the trachea is effected; Figure 3 shows the absorption spectra of a sensor according to the present invention when exposed (i) to CO 2 (maximum absorption 394 nm as shown in broken line) and (ii) to nitrogen (maximum absorption 467 nm); and Figure 4a shows the variation of fluorescence intensity of the film with % CO ; and time (excitation wavelength 430nm, emission wavelength 517nm), and Figure 4b the subsequent plot of the data in the form of fluorescence intensity versus %CO 2 ; and Figure 5a shows the output of a commercially available capnostat when exposed to an atmosphere of varying CO 2 concentration, and Figure 5b the output from a sensor according to the present invention when exposed to the same atmosphere. Referring to Figure 1, there is shown a device comprising a relatively gas- impermeable tube 1 , the internal surface thereof being provided with a coating of fluorimetric sensing material. Referring to Figure 2, a portion of a head and neck is shown; passing along the mouth 21, through the epiglottis 22 to the trachea 23 is a dotted line 24 showing the path of correct intubation. If the tracheal tube failed to pass through the epiglottis, it would pass down the oesophagus 25 in error. If a device such as that illustrated in Figure 1 were to be passed along the tracheal tube, when the leading edge thereof reaches the tracheal zone, the device would respond to the higher carbon dioxide content by undergoing a change of intensity of fluorescence emitted by the indicator layer. On the contrary, if the leading end of the device were to be located erroneously in the oesophagus, there would be no resulting change in fluorescence intensity. This therefore provides a simple and effective test for whether or not a tracheal tube has been correctlv inserted in the trachea. The present invention has been described primarily with reference to a devic for use in determining whether a patient has been correctly intubated; other uses of a monit according to the invention are envisaged, such as for use as a personnel monitor for use i potentially hazardous environments, such as ships' holds, tanks or the like. Example A film solution was prepared by mixing together 2.07 x 10 "3 mol dm '3 1 hydroxypyrene-3,6,8-trisulphonate (HPTS) in methanol (2 cm 3 ), -0.2 mol dm tetraoctylammonium hydroxide in methanol, lOg of a solution comprising lOg of eth cellulose dissolved in 20 cm 3 ethanol and 80 cm 3 toluene, and 2 cm 3 of tris-n-but phosphate. A film was cast from the film solution onto the surface of a gas impermeabl tube as illustrated in Figure 1. As cast, the film was approximately a 100 micron thick laye (using a brass template) and was reduced to a thickness of about 20 microns when dried Thicker films, of thickness of about 40 microns, could of course be prepared. Spectrophotometric and steady-state fluorimetric analysis of the film wer respectively carried out using a Perkin Elmer Lambda 9 UV/VIS/NIR spectrophotometer an a Perkin Elmer LS-5 Fluorimeter/Phosphorimeter. In measurement of the response of the film, as a function of time, to rapi variations in the level of the ambient CO 2 , an optical system with a resolution of ca. 12 ms was used. This system comprises a 100W quartz-iodide lamp coupled with 430 n interference filter (half band width ± 40 nm) which was used to excite the dye in the thi film (at right-angles to the excitation beam). The emission from the edge of the film passe through a high resolution monochromator (Applied Photophysics, U.K.) set at 517 nm an the intensity of the emission at the wavelength was measured using a photomultiplier. In th measurement of the response and recovery times a rapid-switching gas administration syste was used. In order to provide a spectrophotometric comparison of the absorption spectr of the protonated and non-protonated forms of the film, the film was exposed to: (i) an atmosphere of 100% N 2 , and (ii) an atmosphere of 100% CO 2 . Exposure of the film to high level of CO 2 is able to completely convert the deprotonated form of the dye into it protonated form (wavelength of maximum absorption = 394 nm). It can be readily de onstrated that this process is reversible and can be repeated many times over a period of a day with little or no evidence of degradation in film performance. Protonation of the film, and measurement of its absorption spectrum, were carried out. Figure 3 shows the absorption spectra of the film when exposed to (i) CO 2 (maximum absorption 394 nm) and (ii) to nitrogen (maximum absorption 467 nm) as described above. From the inspection of the absorption spectra in Figure 3 an excitation wavelength of 430 nm was employed to study the effect of CO, upon the intensity of film fluorescence. This is because the wavelength of 430 nm excites the deprotonated form of the dye exclusively and therefore avoids the complicating factor of exciting the protonated form of the dye. The fluorescence due to the excited form of deprotonated HPTS present in the film has a maximum at 517 nm (slightly red shifted compared to 512 nm for HPTS in aqueous solution). 517 nm was then chosen as the emission wavelength at which the fluorescence intensity of the film was measured as a function of %CO 2 . Figure 4a shows the variation of fluorescence intensity of the film with % CO, and time (excitation wavelength 430nm, emission wavelength 517nm), and Figure 4b the subsequent plot of the data in the form of fluorescence intensity versus %CO 2 . The plot of fluorescence intensity versus %CO 2 illustrated in Figure 4b is similar in shape to the profile expected for a direct sensor, i.e. one in which there is a 1:1 equilibrium reaction between the analyte and the "immobilised" reagent. This situation would arise if the overall interaction between {QT) ~ .xH 2 O} and CO 2 was as follows: K > {Q ^ D\xH 2 O} ÷ CO 2 (g) {Q + HCO 3 -.(x-7)H 2 O.HD} <- It follows that the relationship between the %CO 2 present and the concentrations of {QT-) ~ .xH 2 O} and {Q + HCO 3 ~ -fx-.-'JH 2 O.HD} will be given by the following expression: %CO 2 xK = [{Q + HCO 3 -fx-7_JH 2 O.HD}]/[{Q + D-.xH 2 O}] The film was subsequently exposed to a gas stream alternating in CO 2 :N 2 composition from 5%:95% to 0%: 100% every 20s; this was achieved using two pressu actuator, solenoid activated valves. From the resulting measured variation in the fluorescence intensity of the film the 0 to 90% response (0% CO, to 5% CO ) and recove (5% CO 2 to 0% CO,) times for the film were respectively found to be 4.3 and 7.1 s. In another set of experiments, the %CO, in the gas phase was varied random with time and the response of the film monitored, so as to estimate the potential of a detect according to the present invention for use as a continuous CO 2 sensor. At the same time t %CO, in the gas phase was also monitored using a commercial capnostat, i.e. an infra-re CO, detector ("End Tidal CO 2 monitor", model 1260, Novametric Medical Systems In U.S.A.). The output from the capnostat and the film are illustrated in figs. 5(a) an 5(b), respectively, and the reasonable degree of matching shows the suitability of a detecto according to the present invention for continuous CO, sensing.

Claims: Claims: 1. A carbon dioxide detector which comprises an indicating member comprising an intimate mixture of a polymer vehicle and a carbon dioxide sensing medium comprising an anionic fluorimetric dye and a lipophilic organic quaternary cation. 2. A detector according to claim 1, wherein said fluorimetric dye comprises any of 1.3- dihydroxypyrene-6,8-disulphonate,fluorescein,umbelliferone,4-methylumbelliferone, 3-benzothiazoylbelliferone, 7-hydroxycoumarin-3-carboxylic acid, l-naphthol-2- sulphonate, l-naphthol-4-sulphonate, 2-naphthol-6-sulphonate, 7-hydroxyflavone, 7- hydroxyisoflavone, 3-hydroxyxanthone, 3,6-dihydroxyxanthone, 7-hydroxy-4- methylchromon, 7-hydroxylepidone, 3-hydroxyxacridone, harmol methoiodide, salicylaldehyde semicarbazone and 2-hydroxycinnamic acid. 3. A detector according to claim 1 or 2, wherein said fluorimetric dye comprises 1- hydroxy-3,6,8-trisulphonate. 4. A detector according to any of claims 1 to 3, wherein said indicating member is in the form of a film provided on a substrate. 5. A detector according to claim 4, wherein said film has a thickness of about 20 microns. 6. A detector according to any of claims 1 to 5. which consists of gamma-ray sterilisable materials. 7. A detector according to any of claims 1 to 6, wherein said polymer vehicle comprises any of polyvinyl butyral, polyvinyl methyl ether, polymethyl methacrylate, ethyl cellulose and polystyrene. 8. A detector according to any of claims 1 to 6, wherein said polymer vehicle compris any of hydroxypropyl cellulose, methyl cellulose, carboxymethyl cellulos polyethylene glycol, polyvinyl alcohol and polypropylene glycol. 9. A detector according to any of claims 1 to 8, wherein said polymer vehicle contain a plasticiser. 10. A detector according to claim 9, wherein said plasticiser comprises any of tribut phosphate, triethyl phosphate, tris butoxyethyl phosphate, diisopropyl phthalat diethylhexyl sebacate, and p-toluene sulphonamide. 11. A detector according to claim 9, wherein said plasticiser comprises any of glycerine trimethylolpropane and low molecular weight polyethylene glycol. 12. A detector according to any of claims 1 to 8, wherein said polymer vehicle i substantially plasticiser free. 13. A detector according to any of claims 1 to 12, wherein said quaternary catio comprises any of benzyltrimethyl ammonium, trioctylmethyl ammonium tricaprylmethyl ammonium, tetrabutyl ammonium, tetrahexyl ammonium, tetrapheny phosphonium, trioctyl phosphonium and hexadecyl tributyl phosphonium. 14. A detector according to any of claims 1 to 13, wherein said cation comprise tetraoctyl ammonium.

Inventors: CHANG QING, MILLS ANDREW, CHANG, QING, MILLS, ANDREW
Applicants: JOHNSON & JOHNSON PROFESSIONAL, JOHNSON & JOHNSON PROFESSIONAL PRODUCTS LTD.
Application year: 1993
Main classification: G01N31
IPC codes: A61M16, G01N21, G01N31