Abstract:
A capnometer includes an airway adaptor for introducing a respiratory gas into the analyzer, an infrared radiation source emitting infrared radiation passed through the airway adaptor, a beam splitter for reflecting and transmitting infrared radiation that impinges on the beam splitter, first detecting means for detecting the infrared radiation reflected by said beam splitter and transmitting through said beam splitter, second detecting means for detecting the infrared radiation reflected by said beam splitter and transmitting through said beam splitter; a gas cell filled with CO 2  gas, said gas cell being located between one of said first and second detecting means and said beam splitter and processing means for processing a concentration of carbon dioxide gas by using output signals of said first and second detecting means.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an instrument for measuring a concentration of carbon dioxide contained in a respiratory gas by non-dispersive infrared method. 
     2. Related art 
     This type of instrument is called a capnometer. A typical example of the known capnometer is the non-dispersive infrared radiation analyzer. In the capnometer, to measure a concentration of CO 2  gas contained in the respiratory gas, infrared radiation is emitted from infrared radiation source, and passed through the respiratory gas. The concentration of CO 2  gas can be measured by passing a beam of infrared radiation through the gas, and ascertaining the attenuation of the intensity of infrared radiation in a narrow wavelength band which is remarkably absorbed by CO 2  gas. A wavelength of approximately 4.3 μm is used for this purpose as a measuring wavelength, and a wavelength of approximately 3.7 μm which is not absorbed by the carbon dioxide is used as a reference wavelength. As known, a relation between the concentration of CO 2  gas and an intensity of light is shown by the Lambert-Beer relation, and is given by 
     
       
           Iout=Iin  exp (− kcl ) 
       
     
     where 
     Iin: intensity of light going into the sample. 
     Iout: intensity of light coming out of the sample. 
     k, c, l: absorption coefficient, concentration of CO 2  gas, and optical length respectively. 
     The equation shows that a concentration c of CO 2  gas can be measured if the Iin, Iout, k and l are known. 
     The capnometer based on above principle is disclosed in U.S. Pat. No. 5,153,436. A schematic illustration of the analyzer is shown in FIG.  4 . In the figure, reference numeral  30  is a housing of a measuring section, and  31  is an airway adaptor used for introducing respiratory gases of a patient into the analyzer. The airway adaptor  31  is inserted directly in the flow path between the ventilator and the endotracheal tube (not shown), which is extended in the directions vertical to the paper surface of the drawing. Windows  32  and  33  are respectively formed in both sides of the airway adaptor  31 . These windows are made of sapphire having a good transparency to the infrared radiation. The airway adaptor  31  is firmly held in a receptacle portion  34  of the housing  30  in a detachable fashion. The airway adaptor  31  may be the reusable type or the disposal type. 
     An infrared radiation source  35  is disposed in the left hand of the receptacle portion  34 . A light beam is emitted from the infrared radiation source  35 , passes through a sapphire window  34   a  disposed in proximity to the left hand of the receptacle portion  34 , and the windows  32  and  33  of the airway adaptor  31  and a sapphire window  34   b  disposed in proximity to the right hand of the receptacle portion  34 , and reaches a beam splitter  36 . The beam splitter  36  may be a dichroic mirror which reflects the infrared radiation having a wavelength longer than about 4 μm but allows the infrared radiation having a wavelength shorter than about 4 μm to transmit therethrough. The beam splitter  36  is slanted approximately 45° with respect to the optical axis of the infrared radiation source  35 . The infrared radiation is impinging on the beam splitter  36 . Infrared radiation having a wavelength longer than 4 μm is reflected and directed to the lead selenide (PbSe) detector  38  through a bandpass filter  37  which transmits wavelength in the range of about 4.3 μm. Infrared radiation having a wavelength shorter than 4 μm is, instead, transmitted through the beam splitter  36  and impinging on the lead selenide detector  40  through a bandpass filter  39  which transmits wavelength in the range of about 3.7 μm. 
     Infrared spectrum of carbon dioxide gas is shown in FIG.  5 . As seen from the spectrum diagram, the least transmittance of the carbon dioxide gas appears at its wavelengths near to 4.3 μm, and the transmittance is approximately 100% at 3.7 μm. In other words, most of infrared radiation having a wavelength of 4.3 μm is absorbed by the carbon dioxide gas, while infrared radiation having a wavelength of 3.7 μm is not absorbed. From this fact, it is seen that a concentration of the CO 2  gas can be obtained by calculating a ratio of electrical signals, which are derived from the two detectors  38  and  40 , propotional to the intensity of the infrared radiation impinging on them. 
     A heater h and a thermistor s are attached to a portion (of the receptacle portion  34 ) of the housing  30  where the housing comes in contact with the airway adaptor  31 . The thermistor s senses temperature of the heater h. The heater h heats the airway adaptor  31  in order to avoid the condensation of water vapor on the inner surfaces of the windows  32  and  33  by highly humidified respiratory gases. 
     In the conventional art, as seen from the foregoing description, where the inner surfaces of the windows  32  and  33  are soiled with secretion, e.g., sputum, whose absorption amounts of the infrared radiation at 4.3 μm and 3.7 μm are different from each other, the absorption amount difference causes a false calculation of the carbon dioxide concentration. 
     In the conventional art, a heat source, a lamp, or the like is used for the infrared radiation source. If such an infrared radiation source suffers from degradation, drift or the like, its temperature varies. As a result, not only the intensity of the emitted light varies at 4.3 μm and 3.7 μm, but also the ratio of the intensity of the infrared radiation impinging on the two detectors  38  and  40  varies as shown by Planck&#39;s law of radiation. 
     As described above, the prior airway adaptor is high in cost to manufacture because expensive sapphire is used for the windows of the airway adaptor. 
     To avoid the codensation of water on surfaces of the windows of the airway adaptor, the airway adaptor is heated by the heater. The use of the heater causes an increase of power consumption, requires a long warm-up time. In other words, a quick measurement of the CO 2  gas concentration from cold start is impossible in the prior analyzer. 
     The infrared radiation of two wavelengths, 4.3 μm and 3.7 μm, are used for measuring the carbon dioxide gas concentration. Therefore, the CO 2  gas concentration measurement may be inaccuate by the soils of the windows of the airway adaptor, the degradation and drift of the infrared radiation source, and is instable. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a capnometer which is free from the adverse influence by soils of windows, and the degradation and drift of the infrared radiation source, and further consumes less electric power. 
     According to an aspect of the present invention, there is provided a capnometer comprising: an airway adaptor for introducing a respiratory gas into the analyzer; an infrared radiation source emitting infrared radiation passed through the airway adaptor; a beam splitter for reflecting the infrared radiation impinging thereon and allowing the infrared radiation to transmit therethrough; first detecting means for detecting the infrared radiation reflected by said beam splitter or transmitting through said beam splitter; second detecting means for detecting the infrared radiation reflected by said beam splitter or transmitting through said beam splitter; a gas cell filled with CO 2  gas, said gas cell being located between one of said first and second detecting means and said beam splitter; and processing means for processing a concentration of CO 2  gas by using output signals of said first and second detecting means. 
     As seen from the foregoing description, in the capnometer of the present invention, the detectors detect the each infrared radiation having an equal wavelength. Therefore, the analyzer can exactly measure the concentration of carbon dioxide independently of soils of the windows, and the degradation and drift of the infrared radiation source. 
     In the embodiment of the invention, there is no need for the heater and thermistor, which are indispensable for preventing the windows of the airway adaptor from being fogged in the conventional capnometer. This feature contributes to reduction of power consumption by the analyzer and simplification of the analyzer construction. 
     Further, there is no need for expensive material, such as sapphire, for the windows of the airway adaptor. Besides, such a simple and inexpensive beam splitter as to be able to reflect the infrared radiation and allow the same to transmit therethrough is available for the capnometer of the embodiment of the invention, while an expensive dichroic mirror capable of splitting two infrared radiation of different wavelengths is used for the conventional analyzer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view showing a key portion of a capnometer which is an embodiment of the present invention. 
     FIG. 2 is a sectional view showing a key portion of a capnometer which is another embodiment of the present invention; 
     FIG. 3 is a graph showing variations of the output signals of the first and second detectors of the capnometer with respect to the concentration of carbon dioxide, and a ratio of the output signals; 
     FIG. 4 is a sectional view showing a key portion of a conventional capnometer; and 
     FIG. 5 is an infared transmittance spectrum diagram of CO 2  gas and water. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of an instrument (referred to as a capnometer) for measuring a concentration of carbon dioxide contained in respiratory gases will be described with reference to the accompanying drawings. FIG. 1 is a sectional view showing a major portion of a capnometer which is an embodiment of the present invention. FIG. 3 is a graph showing variations of the intensity of infrared radiation impinging on first and second detectors with respect to the concentration of carbon dioxide, and a ratio of the intensity of infrared radiation impinging on the first and second detectors as shown in FIG.  1 . 
     An airway adaptor  12  is detachably inserted in a receptacle portion  11  of a housing  10  of an instrument (referred simply to as a capnometer) for measuring a concentration of carbon dioxide contained in a respiratory gas of a person. The airway adaptor  12 , like the conventional one, is of the disposal type. An infrared radiation source  13  with a reflecting mirror is located on the left side of the receptacle portion  11 . The airway adaptor  12  is inserted directly in the flow path between the ventilator and the endtracheal tube (not shown), which is extended in the directions vertical to the paper surface of the drawing. 
     Windows  12   a  and  12   b  are provided on both sides of the airway adaptor  12  when viewed in the direction of the optical axis of the infrared radiation source  13 . A thin plastic film is used for forming the windows  12   a  and  12   b  of the airway adaptor  12  because it has a good transparency to the infrared radiation. If the windows  12   a  and  12   b  of the airway adaptor are not heated by a heater, water droplets deposit onto the inner surface of the windows and windows become fogged due to high humidity of respiratory gas. And scattering the infrared radiation by the droplets lower the transparency of the windows. To avoid fogging of the windows without heating the windows, the inner surface of the window is anti-fogging processed. A specific example of the film is a polyester film with anti-fogging coating. Not water droplets, but thin layer of the water is formed on the film surface because the film surface is hydrophilic. Infrared radiation is not scattered by thin water layer on the surface of the window, and fogging of the window is prevented. Therefore, there is no need of heating the windows in order to avoid fogging of the windows. 
     In the conventional art, as already described, the windows  12   a  and  12   b  of the airway adaptor  12  are made of expensive sapphire. Those expensive windows may be substituted by films with anti-fogging coating. As described above, there is no need of heating the windows in order to prevent the fogging of the windows. However, a thin layer of water is inevitably formed on the film surface. Sometimes, secretion of the patient, which contains mainly water, is deposit onto the inner surfaces of the windows  12   a  and  12   b.    
     The detectors  40  and  38  generate electrical signals Is and Ir, which is propotional to the intensity of the incident infrared radiation. A ratio of those electrical signals is given by Is/Ir. As seen from an infrared transmittance spectrum of FIG. 5, the infrared transmittance of water (H 2 O) varies with wavelength of light, viz., it is a function of wavelength λ. In a case where thin water layers are formed on  15  the inner surfaces of the windows  32  and  33  of the airway adaptor  31 , the infrared transmittance is given by T(λ). The output signals of the detectors  40  and  38  depend on T(λs)·Is and T(λr)·Ir where λs is the wavelength of light absorbed by the CO 2  gas, and λr is the wavelength of light not absorbed by the same. At this time, a ratio of the output signals of the detectors  40  and  38  is T(λs)/T(λr)·Is/Ir. As seen from the infrared transmittance spectrum for water shown in FIG. 5, the infrared transmittance for water varies with the wavelength. With the transmittance variation, there is no case where the ratio of T(λs)/T(λr) is 1. For this reason, it is impossible to use of the anti-fogging film for the airway adaptor  31  without heating in the conventional capnometer. 
     Openings  11   a  and  11   b  are provided also on both sides of the receptacle portion  11  when viewed in the direction of the optical axis of the infrared radiation source  13 . Sapphire windows  14   a  and  14   b  are attached to the openings  11   a  and  11   b  of the receptacle portion, respectively. 
     A beam splitter  15  is slanted 45° with respect to the optical axis of the infrared radiation source  13 . An infrared radiation impinges on the beam splitter  15 , through the receptacle portion  11  and the airway adaptor  12 . The beam splitter  15  allows part of the received infrared radiation in equal wavelength to transmit therethrough, but reflects the rest of the infrared radiation. For this reason, the beam splitter  15  may be formed with an inexpensive silicon plate, for example, while an expensive dichroic mirror for splitting the infrared radiation of different wavelengths is used in the prior capnometer. 
     After reflected by the beam splitter  15 , the infrared radiation is impinging on a first detector  17  through a bandpass filter  16  of 4.3 μm in wavelength. The first detector  17 , like the corresponding one in the prior analyzer, is a lead selenide detector, for example. The first detector  17  produces an electric signal, propotional to the intensity of the infrared radiation impinging on it. The present invention is not limited by this embodiment. The measurement could be performed to use a bandpass filter through which the infrared radiation within a range of 4.2 to 4.4 μm transmits. 
     After transmitting through the beam splitter  15 , the infrared radiation impinges on a second detector  19  through a bandpass filter  18  of 4.3 μm in wavelength. The second detector  19  may have the same construction as of the first detector  17 . The second detector  19  produces an electric signal, propotional to the intensity of the infrared radiation impinging on it. 
     In this embodiment of the invention, a gas cell  20  is disposed between the beam splitter  15  and the second detector  19 . The gas cell  20  is filled with high concentration of CO 2  gas. Sapphire windows  20   a  and  20   b  are provided on both sides of the gas cell  20  when viewed in the direction of the optical axis of the infrared radiation impinging on the gas cell. The gas cell  20  absorbs the infrared radiation of 4.3 μm in wavelength, while allowing the infrared radiation of other wavelengths to transmit therethrough. In other words, the gas cell  20  has such a filtering function. 
     In the capnometer thus constructed, the infrared radiation is emitted from the infrared radiation source  13 , and passes through the window  14   a  of the receptacle portion  11 , the windows  12   a  and  12   b  of the airway adaptor  12 , and the window  14   b  of the receptacle portion  11 , and reaches the beam splitter  15 . Part of the infrared radiation is reflected by the beam splitter  15  and impinges on the first detector  17  through the bandpass filter  16 . The first detector  17  produces an electrical signal, propotional to the intensity of the infrared radiation impinging on it. 
     The rest of the infrared radiation transmits through the beam splitter  15 , and reaches the second detector  19  by way of the gas cell  20  and the bandpass filter  18 . The second detector  19  produces an electrical signal, propotional to the intensity of the infrared radiation impinging on it. 
     Variations of the output signals of the first and second detectors with respect to the concentration of carbon dioxide, and a ratio of the output signals of infrared radiations impinging on the first and second detectors, will be described with reference to FIG.  3 . The output signal of the first detector  17  greatly decreases with an increase of the concentration of carbon dioxide within the airway adaptor  12  (as indicated by a curve A in FIG.  3 ). The output signal of the second detector  19  through the gas cell  20  slightly varies with a variation of the amount (concentration) of carbon dioxide within the airway adaptor  12  (as indicated by a curve B in FIG.  3 ). This is because the infrared radiation is greatly absorbed by the high concentration of the carbon dioxide within the gas cell  20 . 
     A concentration of the carbon dioxide can be obtained by calculating a ratio (B/A) of the output signal A of the first detector  17  and the output signal B of the second detector  19 , without any influence of a variation of the intensity of infrared radiation that is emitted from the infrared radiation source  13  the water layer, and soils of the windows  12   a  and  12   b  of the airway adaptor  12 . Actually, a control unit (not shown) calculates the concentration of carbon dioxide by the utilization of the output signals of the first and second detectors  17  and  19 . 
     In the embodiment under discussion, the infrared radiation of equal wavelength is detected by the first and second detectors  17  and  19 . For this reason, the ratio of the intensity of the infrared radiation impinging on the first and second detectors is invariable even if the water layer is formed on the inner surfaces of the windows  12   a  and  12   b  of the airway adaptor  12 . While the calculation error arises from the difference of the absorption amounts of the infrared radiation of 4.3 μm and 3.7 μm when the windows are soiled, and the light source suffers from the degradation and drift. 
     As described above, the wavelengths of the infrared radiations incident on the first and second detectors are equal to each other. The transmittance of a medium is a function of the wavelength of light transmitting through the medium, as described above. Therefore, the transmittance T is given by T(λ) where λ is the wavelength of light. An intensity of infrared radiation impinging on the first detector  17  is denoted as Is, and an intensity of infrared radiation impinging on the second detector  19  is denoted as Ir. Actual intensities of the infrared radiation impinging on the first and second detectors  17  and  19  are given by T(λs)·Is and T(λs)·Ir respectively. The ratio of the output signals of detectors are given by 
     
       
           T (λ s )· Ir/T (λ s )· Is=Is/Ir   
       
     
     As seen from the above equation, the ratio of the output signals of the detectors is independent of T(λ). This fact implies that even if a water layer is formed on the windows  12   a  and  12   b  in the airway adaptor  12  and the windows are soiled, the concentration of carbon dioxide can be measured independently of their transmittance values. 
     Since both the first and the second detectors detect the infrared radiation of approximately 4.3 μm in wavelength, the ratio (Is/Ir) of the output signals of the detectors  17  and  19  is invariable if the light source  13  suffers from its degradation and drift. Therefore, the concentration of the carbon dioxide may be measured free from the degradation and drift of the infrared radiation source. 
     In the embodiment mentioned above, the gas cell  20  is disposed between the beam splitter  15  and the second detector  19 . If necessary, the gas cell  20  may be disposed between the beam splitter  15  and the first detector  17 . Also in this case, it is possible to measure the concentration of carbon dioxide by use of the ratio of the output signals of the detectors  17  and  19 , as a matter of course. The locations of the bandpass filters are not limited to those in the embodiment. For example, as shown in FIG. 2, a bandpass filter  16   a  may be disposed between the infrared radiation source  13  and the beam splitter  15 . If so done, use of only one bandpass filter will do. This results in reduction of cost to manufacture. Usually, nitrogen is used for the gas filling the housing of the detector ( 19 ). CO 2  gas may be used in place of the N 2  gas. In this case, the detector may also be used as the gas cell. In other words, the detector and the gas cell are constructed as a unit. This leads to the size and cost reduction. 
     It should be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings. 
     As mentioned above, in the capnometer of the present invention, the detectors detect infrared radiation having an equal wavelength. Therefore, the analyzer can exactly measure the concentration of carbon dioxide independently of the water layer formed on the inner surfaces of the windows in the airway adaptor, soils of the windows, and the degradation and drift of the infrared radiation source. 
     In the embodiment of the invention, there is no need for the heater and thermistor, which are indispensable for preventing the windows of the airway adaptor from being fogged in the conventional capnometer. This feature contributes to reduction of power consumption by the analyzer and simplification of the analyzer construction. 
     Further, there is no need for expensive material, such as sapphire, for the windows of the airway adaptor. Besides, such a simple and inexpensive beam splitter as to be able to reflect the infrared radiation and allow the same to pass therethrough is available for the capnometer of the embodiment of the invention, while an expensive dichroic mirror capable of splitting two infrared radiation of different wavelengths is used for the conventional analyzer.