Cross-Reference to Related Applications

This is a continuation-in-part of Patent Application Serial No. 136,600 filed December 22, 1987 which is a contin¬ uation of Serial No. 896,360 filed August 13, 1986, now Patent No. 4,728,499. Field of the invention

This invention relates to a detector for the quanti¬ tative determination of carbon dioxide in a gas. The invention is also concerned with a method for determining the concentra¬ tion of carbon dioxide in a gas, preferably by visual means, which provides a quantitative measurement of said concentra¬ tion. Background of the invention

In Patent No. 4,728,499 and the continuation thereof, the disclosure of which is incorporated herein by reference, there is disclosed a combination rapid response device for the detection of carbon dioxide in a concentration of more than 2% in a gas mixture which includes an indicating element, comprising a chrimogenic pH-sensitive indicator and other sub¬ stances, which indicating element provides a visual color changes within 5 to 20 second when the concentration of carbon dioxide in the gas under investigation is 2% or more. In the preferred embodiment of the device said indicating element is impregnated on to a carrier to form an indicator component which is mounted within an enclosure having an inlet, an out¬ let and a transparent window through which the component and any color change thereof may be viewed.

Thus, according to Patent No. 4,728,499, the nature and concentration of the substances in the indicating element give a rapid, positive color change when the concentration of carbon dioxide in the surrounding gas reaches or exceeds a minimum concentration of 2%, thereby providing a device which enables the positioning of an endotrachael tube in a patient to be determined accurately and rapidly.

However, there are some instances wherein it is nec¬ essary to determine more quantitatively the concentration of carbon dioxide in a gas, not merely a given minimum concen¬ tration. Also, it is advantageous to have a device and method which provides on-line, real time, repetitive measurement so that temporal changes in carbon dioxide concentrations may be detected.

It has now been found that these further objectives may be achieved by extending the visual detection concept ut- ilized in the device of Patent No. 4,728,499 to (i) a compon¬ ent capable of giving progressively different response signals or (ii) a sequential array of components, each of which is pre-calibrated to give a rapid response signal, for example a visual response (color change) ; each of said response signals in either embodiment corresponding to a different pre-deter- mined concentration of carbon dioxide in the gas under inves¬ tigation, said embodiment bieng capable of responding repeti¬ tively to changing concentrations of carbon dioxide. Summary of the invention

According to the present invention there is provided a detector for the quantitative determination of carbon dioxide

in a gas which comprises a plurality of componets in which each component comprises a carrier with a surface to be exposed to the gas, said surface including an indicating element of a predetermined sensitivity which gives a rapid response signal when exposed to a given concentration of carbon dioxide, said components being arranged in a sequence wherein said predet¬ ermined sensitivity of indicating element in each component in the sequence differs progressively by an increment which gives a rapid response signal different from that of the indi¬ cating element in each preceding component, each of said dif¬ fering response signals corresponding to a particular concen¬ tration of carbon dioxide, said sequence of components thereby providing a progressive quantitative determination of carbon dioxide in the gas, and being capable of responding repetiti¬ vely, with respect to time, to changing concentrations of car¬ bon dioxide.

The active ingredient of the indicating element, here¬ in referred to as "indicator", which provides the required rapid response signal when exposed to a given concentration of carbon dioxide may be any state of the art indicator which is sensitive to the presence of carbon dioxide and is capable of being cali¬ brated as herein described to give a response signal corres¬ ponding to a given predetermined concentration of carbon dio¬ xide. The signal may be visual, e.g. a change in color, or electrical or electronic. Indicators which provide a color change in the presence of carbon dioxide include chromogenic pH-sensitive indicators and oxidation/reduction indicators.

The preferred indicators used in the detector of the present invention are chromogenic pH-sensitive indicators of

the type disclosed in Patent No. 4,728,499 and the invention will be further described hereinafter with reference to these preferred indicators.

Accordingly, the present invention also provides a detector for the quantitative determination of carbon dioxide in a gas which comprises a plurality of components in which each component comprises a carrier with a surface to be ex¬ posed to the gas, said surface being impregnated with a chrom¬ ogenic indicating element of a predetermined sensitivity which gives a rapid visual color change when exposed to a given con¬ centration of carbon dioxide, said components being arranged in a sequence wherein said predetermined sensitivity of in¬ dicating element in each component in the sequence differs progressively by an increment which gives a color change dif¬ ferent from that of the indicating element in each preceding component, each of said differing color changes corresponding to a particular concentration of carbon dioxide, said sequence of components thereby providing a progressive quantitative determination of carbon dioxide in the gas and being capable of responding repetitively, with respect to time, to changing concentrations of carbon dioxide.

There are certain indicators, which may be chromogenic electrical or electronic, which are capable of giving changing signals dependent upon different concentrations of carbon dio¬ xide in the surrounding gas. For example, a chromogenic chem¬ ical indicator may progressively change color through different visually distinct colors or shades of color as the concentra¬ tion of carbon dioxide increases and if the gradation of color

or color shades is sufficiently distinct to yield meaningful rapid response signals when compared with an appropriate color comparison chart then a modified detector according to the in¬ vention may comprise a single, multi-response component, rather than a plurality of components each of which gives a distinct different response signal. When using a chromogenic chemical indicator, not only the single multi-response component embo¬ diment but also the multi-component embodiment normally will be used with an appropriate color comparison chart.

Thus the present inventin further provides a modifi¬ cation of the detector as described above for the quantitative determination of carbon dioxide in a gas, which comprises a component comprising a carrier with a surface to be exposed to the gas, said surface including an indicating element hav¬ ing the capability of providing a plurality of distinguishable rapid response signals upon exposure to progressively changing concentrations of carbon dioxide, each of said signals cor¬ responding to a particular concentration of carbon dioxide, and pre-calibrated comparison means for interpreting said sig¬ nals.

In a preferred embodiment of the modified detector according to the invention the component comprises a carrier with a surface to be exposed to the gas, said surface being impregnated with a chromogenic indicating element which gives a plurality of rapid visual color or color shade changes upon exposure to progressively changing concentrations of carbon dioxide, each of said colors or color shades corresponding to a particular concentration of carbon dioxide. A precalibrated

T TU E SHEET

color comparison chart is provided to interpret the differing colors or color shades.

The invention still further provides a method for the quantitative determination of the concentration of carbon dioxide in a gas which comprises bringing said gas into con¬ tact with a surface of one or more components in a detector comprising said component or components wherein the component or each component comprises a carrier with said surface to be exposed to said gas, said surface being impregnated with a chromogenic indicating element of a predetermined sensitivity which, in the case of a single component, gives a series of rapid visual color changes corresponding to varying concentra¬ tions of carbon dioxide, or in the case of a plurality of com¬ ponents, gives a rapid visual color change when exposed to a given concentration of carbon dioxide, said plurality of com¬ ponents being arranged in a sequence wherein said predetermined sensitivity of indicating elements in each component in the sequence differs progressively by an increment which gives a color change different from that of the indicating element in each preceding component, and comparing said series of rapid visual color changes or said different color changes with the colors in a precalibra ed color comparison chart, thereby ob¬ taining a progressive quantitative determination of the con¬ centration or change in concentration of carbon dioxide in the gas. Detailed description of the invention

The preferred embodiment of the detector of the pre¬ sent invention utilizes the same concept for measurement of

end-tidal carbon dioxide as that disclosed in Patent No. 4,728,499 and retains the rapid response characteristics vital for medical applications but, additionally, by modifying the indicating entity to comprise a component or a plurality of components, each sensitive to different concentrations of car¬ bon dioxide in the surrounding gas, the present detector or modified detector allows the detection of temporal changes in the carbon dioxide concentration and quantitative measurement thereof.

Preferably the indicating element used in the detec¬ tor of the present invention comprises an aqueous solution of a colorless compound which provides an alkaline solution, i.e. a base, a chromogenic pH-sensitive indicator and a hygroscopic, colorless, transparent, water-miscible liquid, and the sensit¬ ivity of the indicating element in each of said components is established by varying the concentration or nature of the base, the concentration or nature of the chromogenic pH-sensitive indicator or any combination of these parameters.

As used herein the term "indicating element" is in¬ tended to mean that entity within each component of the detec¬ tor which provides a rapid response signal, preferably a color change, dependent upon a given concentration of carbon dioxide. The preferred "indicating element" includes at least one "in¬ dicator" which is defined herein to include a chemical compound or mixture of compounds, or a suitable electrical or electronic device.

When the indicator is a chemical compound it is usu¬ ally a chromogenic pH-sensitive indicator which will provide a

color change upon exposure to a given concentration of carbon dioxide in the presence of other ingredients of the element which provide the appropriate chemical conditions to induce the required color change. When the indicator is an electrical or electronic device it may be an electrode or transistor which is adapted to detect and measure changes in the ambient chemi¬ cal parameters induced by the presence of critical amounts of carbon dioxide.

When the response signal provided by the indicating element is a color change, the term "indicator" is intended to mean the actual chemical compound which provides a rapid color change relative to the concentration of carbon dioxide. In this embodiment, the preferred indicator is a chromogenic pH- sensitive indicator i.e. a compound, or mixture of compounds, which rapidly changes color when there is a change in pH in the surrounding medium. For such an indicator to be capable of giving a determination of carbon dioxide it has to be used in combination with a suitable base which provides an alkaline solution. The hydroxyl ions or amine residues present in said alkaline solution react chemically with carbon dioxide to pro¬ duce a carbonate, bicarbonate and/or carbamate moiety. As des¬ cribed in Patent No. 4,728,499, 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 indi¬ cating element. The lowering of the pH causes a color change in the indicator.

Since carbon dioxide will not react with the base without water, the presence of a certain minimum amount of water

is necessary for the indicating element to provide the required color change. Accordingly, the indicating element also should include a substance which ensures that the required minimum amount of water is present when the detector is used. The preferred substance is a hygroscopic, colorless, transparent, water-miscible liquid.

Thus, a particularly preferred embodiment of the in¬ vention is a detector as described above in which said indi¬ cating element comprises an aqueous solution of a base, a chro¬ mogenic pH-sensitive indicator and a hygroscopic, colorless, transparent, water-miscible liquid, and the sensitivity of the indicating element in each of said components is established by varying the concentration or nature of the base, the concen¬ tration or nature of the chromogenic pH-sensitive indicator or any combination os these parameters.

Preferred indicators which may be used in the detec¬ tor of the present invention include the chromogenic pH-sen- sitive indicators disclosed in Patent No. 4,728,499, namely: 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, α-naphthol- phthalein and orange I. Examples of other indicators which may be used in the detector of the present invention include brom- cresol purple, bromphenol red, p-nitrophenol, m-nitrophenol, curcumin, quinoline blue, thymolphthalein and mixtures thereof.

Particularly preferred indicators are metacresol purple, cresol red, bromthymol blue and a mixture of cresol red and bromthymol blue.

The other ingredients of the indicating element in the preferred embodiments of the invention are a base, which is a colorless compound which provides an alkaline solution with water, and a hygroscopic, high-boiling, transparent, col¬ orless, water-miscible liquid.

The base should be a compound whose reaction product with carbon dioxide is water-soluble and which will attain rea¬ sonable equilibrium or steady state conditions with carbon dio¬ xide. Suitable bases include sodium carbonate, lithium hydro¬ xide, sodium hydroxide, potassium hydroxide, potassium carbon¬ ate, sodium barbitol, tribasic sodium phosphate, dibasic sodium phosphate, potassium acetate, monoethanolamine, diethanolamine and piperidine. A particularly preferred base is sodium car¬ bonate.

The third ingredient in the preferred embodiment of the invention is a hygroscopic, high-boiling, transparent, col¬ orless, water-miscible liquid. The purpose of this ingredient is to entrap sufficient water in the indicating element of each component to enable the exposed surface of the element to act as a reaction zone with the surrounding gas.

An important criterion of each component of the de¬ tector according to the invention is that the indicating ele¬ ment be immobilized in or on the carrier. This requires active drying or blotting of the impregnated carrier to achieve min¬ imal moisture retention so as to prevent migration or flow of material while in use. However, since carbon dioxide will not react with the base without water, the presence of a certain minimum amount of water is necessary for the component to work.

The hygroscopic liquid ensures that the required minimum amount of water is present in the indicating element when exposed to the gas under investigation.

Examples of suitable hygroscopic liquids for use in the detector of the invention are glycerol, propylene glycol, monethylene glycol, diethylene glycol, polyethylene glycol and various aliphatic alcohols. Because they are non-toxic and have antiseptic properties which inhibit bacterial and fungal growth, glycerol and propylene glycol or mixtures thereof are particularly preferred.

Each component of the preferred detector according to the invention comprises a carrier with a surface to be ex¬ posed to the gas under investigation and said surface is im¬ pregnated with an indicating element as described above. The carrier may be made of any material capable of being impreg¬ nated with the indicating element and of exhibiting the requi¬ red sensitivity to carbon dioxide when so impregnated. Prefer¬ ably the carrier is a bibulous material, such as filter paper or fibrous synthetic material. However, the carrier may be any solid material having no inherent acid or basic properties and to which said indicating element may be impregnated, for exam¬ ple certain porous plastics, in the form of thin films or beads, and inorganic crystals. The carrier is usually impregnated with the indicating element by applying the latter in the form of an aqueous solution and drying or blotting to remove excess moi¬ sture, as described in Patent No. 4,728,499.

In the multi-component embodiment of the detector according to the invention the components are arranged sequen-

tially according to carbon dioxide sensitivity so that when the detector is exposed to the gas to be tested a quantita¬ tive determination of the concentration of carbon dioxide in the gas (if any) , is given by a change in color of that com¬ ponent which is calibrated to change color when the concen¬ tration is a predetermined amount greater than that indicated by components which have already changed color but less than that which would be indicated by components which have not changed color. The increments of calibration may be chosen according to the accuracy desired. Normally increments of 1.0 to 2.0% are sufficient for most purposes. Also the detector normally is calibrated so that the last one or two components do not change color under the expected test conditions.

The actual configuration of the sequence of compon¬ ents is not critical, provided the response can be easily vi¬ ewed and interpreted. Preferably, for ease of operation, the components are arranged sequentially in a strip form. If the carrier is a bibulous material, such as filter paper, the com¬ ponents may be formed by applying the indicating element in strips to a single sheet of the carrier material wherein each strip is separated from each adjacent strip by a thread of hydrophobic material to prevent migration of the indicating element from one strip to another. Alternatively, each com¬ ponent may be a separate strip of carrier material impregnated with an indicating element of the required sensitivity and the detector is made up by mounting each strip sequentially on a suitable support, for example a cylindrical spool such as that used in the device of Patent No. 4,728,499.

Each of the components in the detector is calibrated by selecting one or more of three parameters of the indicating element, namely:

1. the nature of the base;

2. the concentration of the base in the solution used to impregnate the carrier. This parameter provides the initial pH of the solution in equi¬ librium or steady state with the baseline con¬ centration of carbon dioxide; and

3. the pK of the chromogenic pH-sensitive indicator. The critical factors for calibration are: a) the difference between the initial pH and the pK of the indicator and b) the new equilibrium or steady state pH of the par¬ ticular indicating element at the specific carbon dioxide concentration of interest for the parti¬ cular base used in the indicating element.

By utilizing a plurality of components each with an indicating element of different initial pH to pK spread cali¬ brated to a different concentration of carbon dioxide, i.e. different equilibrium or steady state pH, it is possible to precisely quantitate the concentration of carbon dioxide pre¬ sent in the test gas at any point in time and to detect changes of carbon dioxide concentration with time. Thus, the detector is capable of responding repetitively to changing concentra¬ tions of carbon dioxide.

Precision will depend upon the number of components and the magnitude of color variation exhibited by each indica-

tor around its pK when subjected to small variations in pH which will translate to the incremental color change in the indicating element of each adjacent component. Precision also depends upon the choice of detector to exhibit a suitable re¬ sponse according to the range of concentration of carbon dio¬ xide in the test gas.

The indicating element in each component will res¬ pond to a specific concentration range of carbon dioxide since the color change in the transition pH range of the indicator will be continuous with small changes in pH.

Color comparison charts for a variety of.known car¬ bon dioxide concentrations will provide rapid quantitation of unknown carbon dioxide concentrations in a gas provided the gas is known not to contain other acidic gases, such as sulfur dioxide or hydrogen sulfide, which would interfere with the measurement.

When the components of the detector are in the form of a sequence of strips as described above, said components may be mounted on a spool supported within a transparent enclosure in a device such as that described and illustrated in Patent No. 4,728,499.

The preferred embodiment of the device disclosed in Patent No. 4,728,499 provides a convenient and comparatively simple device for obtaining a rapid and substantially fool¬ proof indication of the proper or improper placement of an endotracheal catheter in the trachea of a patient, particu¬ larly an apneic patient.

Accordingly, said preferred embodiment includes an

indicating element which responds positively and rapidly to the presence of a certain concentration of carbon dioxide, i.e. the amount of carbon dioxide which is present in the ex¬ haled breath of a human being. This concentration is normally of the order of 4.5 - 5.0%, but possibly may be as low as 2%.

However, although an extremely rapid response, of the order of 5 to 20 seconds, is crucial for the successful op¬ eration of said device, it is equally important that the indi¬ cating element should not be so sensitive that it changes color too quickly when exposed to an atmosphere containing some min¬ imal amount of carbon dioxide, for example, ambient air which normally contains about 0.03% carbon dioxide, which minimal amount is substantially less than that present in exhaled brer- ath.

Accordingly, the indicator used in the device accord¬ ing to Patent No. 4,728,499 is selected so that it does not change color instantaneously upon exposure to an atmosphere which contains a certain minimum amount of carbon dioxide, for example, ambient air and the resultant delay provides the operator with ample time to open the package in which the de¬ vice is sealed and connect the device to an endotrachael cath¬ eter after having placed the catheter in the patient's throat and having inflated the sealing cuff on the catheter.

Since the exposed indicator may eventually change color upon continued exposure to ambient air, or any atmos¬ phere containing minimal amounts of carbon dioxide, because even a slow rate of diffusion of carbon dioxide into the in¬ dicator zone will lead, in time, to a sufficient depletion of

base to cause a color change, the device is normally enclosed in atmospheric sealing means which is constructed to be opened immediately prior to use of said device.

Similar considerations apply to the detector of the present invention, particularly when it is calibrated to be used for the monitoring of carbon dioxide concentrations in conjun¬ ction with an endotracheal tube, an anesthesia mask or a resus- ciation mask.

A preferred atmospheric sealing means for storing the detector is a gas impermeable metallic foil.

If a hydrophilic filter is included at the proximal end of the device, the device may be attached to an endotrach¬ eal tube, anesthesia mask or resuscitation mask or to the ex¬ haust port of resuscitation bag for extended periods of time, thereby providing breath to breath quantitation of end-tidal carbon dioxide and detection of changes in this parameter with time under varying physiological conditions.

If the device is attached to the expiratory port of a resuscitation bag the filter may be unnecessary.

Preferably the filter comprises a porous or fibrous bibulous material impregnated with a hydrophilic liquid, such as glycerol or propylene glycol. The presence of such a filter ensures that minimal water vapor in exhaled breath will not con¬ dense within the device and the filter thus prevents leaching of the ingredients of the indicating element from its carrier.

The hydrophilic filter may include a porous hydro- phobic membrane, made from a material such as foamed polyeth¬ ylene or styrofoam, attached to the distal end of the filter

just inside the inlet of the device. The filter, without or with the membrane, should offer minimal resistance to gas flow up to 100 1/min.

The surface of the multi-component detector may be coated with a water-repellant film of a carbon dioxide-per¬ meable material to augment resistance to leaching by condensed water droplets. Preferred materials are silicone and fluoro- carbons. The specific material used should have no signifi¬ cant effect on the pH of the indicator element.

In another embodiment of the invention the compon¬ ents containing the indicator elements may be enveloped in or laminated with a polymeric film which is water resistant, trans¬ parent and is permeable to carbon dioxide. Examples of such polymeric films are ultra thin films of cellulose butyrate or linear low density polyethylene. The presence of the film ex¬ tends the response time and this embodiment would be used only when a rapid response time, i.e. a response time of 5 to 20 seconds, is not critical. However, the.embodiment is still useful for many applications wherein a longer response time, of the order of 20 to 30 seconds or more, is tolerable.

Attachment of the detector to the exhaust port of a resuscitation bag, as described hereinabove, obviates the con¬ cern regarding leaching of chemicals from the indicating ele¬ ment, since in this embodiment the entire detector is physi¬ cally separated from the patient's trachea by valves.

The following Examples illustrate the preparation of the indicating element and component used in the detector ac¬ cording to the invention.

Example 1

In this Example the indicating element for each com¬ ponent is prepared from an aqueous solution of sodium carbon¬ ate as the base, meta cresol purple as the chromogenic pH- sensitive indicator and glycerol as the hygroscopic liquid.

For each component the amount of indicator and gly¬ cerol is kept constant while the amount of base is varied to provide the desired response characteristic.

Four components are prepared from the following in¬ dicating elements

Strip Sodium carbonate Metal cresol purple Glycerol ^No ' lθ ^"2 M % w/v % by vol

1. 1.0 0.15 50

2. 1.26 0.15 50

3. 1.58 0.15 50

4. 2.0 0.15 50 The four components are made up by immersing strips of Whatman No. 1 filter paper in aqueous solutions of the above indicating elements until they are fully impregnated with the solution. The strips are blotted until no further solution can be expressed therefrom and they are then assembled in order of increasing base concentration and affixed to a support, for example of cardboard or celluloid. The support is mounted in an appropriate enclosure or chamber wherein it is exposed to gases containing varying concentrations of carbon dioxide. A progressive change in color in each strip is observed depend¬ ent upon the concentration of carbon dioxide in the gas until the concentration reaches the saturation point for these par-

ticular indicating elements. The results are set out in the following Table 1.

Table 1

Concentration of COLOR

Carbon Dioxide Strip 1 Strip 2 Strip 3 Strip 4 Remarks

.03% purple purple purple purple No color change

.50% grayish- yellowish- grey dull yellow grey fuschia

1.00% golden dirty dirty grayish- yellow yellow yellow yellow

1.50 light golden golden dirty yellow yellow yellow yellow

2.00% light golden golden golden yellow yellow yellow yellow

2.50% light light light light Saturati yellow yellow yellow yellow

By comparing the above results with a pre-calibrated standardized color chart the concentration of carbon dioxide in the surrounding gas at any given time may be determined.

Example 2 A three-component detector is prepared from aqueous solutions of the following indicating elements:

Strip Base Indicator Glycerol No. w/v % by volume

1.5 x 10 -2 M sodium 0.22% metacresol 50 carbonate purple

6. 1.5 x 10 -2 M sodium 0.1% cresol red 50 carbonate +0.18% bromthymol blue

7. 1.0 x 10 -2 M sodium 0.24 bromthymol 50 carbonate

The above indicating elements are impregnated on to porous hydrophlic polyethylene foam 0.0625 inch thick (Porex DBE polyethulene foam-fine pore) .

The elements were assembled to form a detector in a manner similar to that described in Example 1 and this detector was exposed to gases containing varying amounts of carbon dio¬ xide. The results are shown in Table 2.

Table 2

Concentration of COLOR

Carbon Dioxide Strip 5 Strip 6 Strip 7

.03% purple blue violet light blue

1.00% greyish-purple light blue NO CHANGE

2.00% greyish-yellow blue grey NO CHANGE

3.00% dirty yellow grey lighter blu

4.00% dark yellow dark green grey

5.00% golden yellow green greenish gr

6.00% lighter yellow yellow green dirty green

7.00% bright yellow greenish green yellow

8.00% NO CHANGE dark yellow lighter gre

9.00% NO CHANGE golden yellow greenish- yellow

10.00% NO CHANGE _. lighter yel¬ dark yellow low

Again, a quantitative determination of the carbon dio¬ xide concentration for each gas is obtained by comparing the above results with a pre-calibrated standardized color chart.

Example 3

This Example illustrates a modified detector accord¬ ing to the invention comprising a single component with an in¬ dicating element which provides a plurality of distinguishable color changes when exposed to varying concentrations of carbon dioxide.

Strip 4 of Example 1 is used to illustrate this em¬ bodiment. This strip comprises a strip of Whatman No. 1 filter paper impregnated with an indicating element formed from an

_2 aqueous solution of 2.0 x 10 M solium carbonate, 0.15 w/v meta cresol purple and 50% by volume glycerol.

The strip is mounted as described in Example 1 and exposed to gases containing increasing concentrations of carbon dioxide from 0.03% (normal ambient air) to 2.50%. The results are the same as given in Table 1. Comparison of the changing shades of color against a standarized color chart give similar information to that obtained in Example 1. Thus, technically, the single component detector of this Example is comparable to the multi-component detector illustrated in Example 1 and such single component detector is certainly within the scope of the invention. However, in practice, the fine gradations of color shade for differing concentrations of carbon dioxide require very careful color comparisons against the precalibrated chart and the multi-component detector of Example 1 is therefore pre¬ ferred. 3rief Description of the Drawings

A preferred embodiment of the invention is a multi- component detector in which each component is an impregnated strip of bibulous material mounted on a support spool in a

device similar to that described and illustrated in Patent No. 4,728,499. The invention will now be further described with reference to the accompanying drawings in which:

Figure 1 is a side cross-section of a simplified ver¬ sion of the device of Patent No. 4,728,499 and the modified detector of the present invention; and

Figure 2 is a similar view of the device of Figure 1 but with the single rapid response strip replaced by a multi- component detector according to the present invention. Detailed Description of the Drawings

The device illustrated in Figure 1 of the drawings comprises a cylindrical housing 1 having at its proximal end a cone-shaped coupling 2 terminating in a cylindrical connector 3 and at its distal end a cone-shaped coupling 4 terminating in a cylindrical connector 5.

The cylindrical housing 1 is made from a clear, col¬ orless, transparent plastic, for example, an acrylic polymer, such as that available under the Trademark PLEXIGLAS, polyme- thyl acrylate, polymethyl methacrylate, polycarbonate, polysty¬ rene or styrene-acrylonitrile copolymer.

The combination of housing and coupling/connector units effectively forms an enclosure having an inlet formed by prox¬ imal connector 3 and an outlet formed by distal connector 5.

The clear transparent plastic used for the cylindrical housing provides an effective window 6 for viewing the indicator component 7, which comprises a strip of filter paper impregnated with an appropriate indicating element. Fogging of said window by the humidity in exhaled breath is prevented by coating the

the inner surface of the window with a suitable anti-fogging surfactant, such as dioctyl sodium sulfosuccinate.

The indicator strip 7 is securely wrapped around a cylindrical spool which is made from a rigid plastic, such as polyethylene. The spool 8 is mounted at each end on an in¬ wardly facing flange 9 of a support 10.

The supports 10 are substantially circular in shape and have a plurality of apertures 11 which allows substantia¬ lly unrestricted flow of gas through the device when in use.

The spool supports 10 are preferably made from a cl¬ ear plastic, similar to that used for the cylindrical housing.

The device is preferably enclosed within atmospheric sealing means, for example a gas-impermeable metal foil 12, until required for use.

Figure 2 illustrates a detector according to the pre¬ sent invention mounted on a spool 8 in a device similarly to that illustrated in Figure 1.

In this embodiment the single indicator strip 7 is replaced by four strips 21, 22, 23 and 24 impregnated, respec¬ tively, with the indicating elements 1, 2, 3 and 4 of Example 1 herein.

The resulting device may be connected to an endotra- cheal tube through connector 3 and be used to obtain quanti¬ tative determinations of the carbon dioxide concentration in the exhaled breath of a patient over a given time. If desired, a hydrophilic filter 13 as described above may be included in the device.

It is a characteristic of the present invention that

the response time of each indicator element is comparable to that of the device disclosed in Patent No. 4,728,499. Thus each indicating element normally will give a rapid response signal corresponding to the concentration of car¬ bon dioxide to which it is sensitive within 5 to 20 seconds after being exposed to the gas. Possible exceptions are when the element is coated with a water-repellent film or is enveloped in a polymeric film, as described above, when the response time might be somewhat extended. However, even in these embodiments the response time, of the order of 20 to 30 seconds or more, is sufficiently fast for many applications. Of course, such an embodiment should not be used when a rapid response time is critical.

CLAIMS 1. A detector for the quantitative determination of carbon dioxide in a gas which comprises a plurality of com¬ ponents in which each component comprises a carrier with a surface to be exposed to the gas, said surface including an indicator element of a predetermined sensitivity which gives a rapid response signal when exposed to a given concentration of carbon dioxide, said components being arranged in a sequence wherein said predetermined sensitivity of indicator element in each component in the sequence differs progress¬ ively by an increment which gives a response signal different from that of the indicating element in each pre¬ ceding component, each of said differing response signals corresponding to a particular concentration of carbon dioxide, said sequence of components thereby providing a