Abstract:
In a sensor for detecting a carbon dioxide gas in an expiration gas of a living body, an airway case is adapted to be disposed below nostrils of the living body, and formed with an airway passage extending across an optical axis of a light beam emitted from a light emitter of the sensor. A mouth guide is adapted to be disposed in front of a mouth of the living body so as to define a space communicated with the airway passage. The mouth guide is pivotably supported on the airway case. A retainer is adapted to retain an oxygen supply tube on the airway adapter body in such an attitude that an oxygen gas supplied from prongs of the oxygen supply tube is not directly injected into the nostrils.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a carbon dioxide sensor for measuring concentration, partial pressure, or presence/absence of carbon dioxide in a gas expired through nostrils or a mouth of a living body.  
         [0002]     In general, when the concentration of carbon dioxide contained in an expiration gas expired from a living body is optically measured, the gas is led through a cylindrical airway adapter. An infrared ray is radiated onto the expired gas from a light-emitting element. The amount of light that remains after some of the light has been absorbed by the carbon dioxide contained in the expiration gas is detected by a light-receiving element, thus measuring the concentration of carbon dioxide.  
         [0003]      FIG. 16  shows such an apparatus for measuring the concentration of carbon dioxide. In this apparatus, one end  101   a  of an airway adapter  101 , which is formed into a substantially cylindrical shape and through which a respiration gas passes, is to be connected to a tube inserted into a trachea of a patient. Another end  101   b  is to be connected to a Y piece of a respiratory circuit, such as a respirator. An intermediate portion of the airway adapter  101  has a rectangular cross-sectional shape. Circular windows  101   c ,  101   d  are formed in respective, opposing surfaces of the intermediate portion such that the windows are concentrically aligned with each other.  
         [0004]     A sensor body  102  is formed into a substantially-rectangular shape, and a notch is formed in an intermediate portion of the sensor body  102 . The intermediate portion of the airway adapter  101  is to be detachably fitted with the notch. Two opposing surfaces defining the notch are in contact with the windows  101   c ,  101   d  of the airway adapter  101 . A light-emitting element  103  is disposed on one side with reference to the notch.  
         [0005]     An optical filter  104  for absorbing only light having a wavelength to be absorbed by carbon dioxide and a light-receiving element  105  are disposed on the side opposite the light-emitting element  103  with reference to the notch. The light-emitting element  103  and the light-receiving element  105  are connected to a monitor  107  via a lead wire  106 .  
         [0006]     In the apparatus having the foregoing configuration, the light emitted from the light-emitting element  103  enters the light-receiving element  105  by way of the window  101   c , the respiration gas in the airway adapter  101 , the window  101   d , and the filter  104 . The light-receiving element  105  detects the amount of light after some amount of the light has been reduced in accordance with the concentration of carbon dioxide. A signal output from the light-receiving element  105  is input to the monitor  107 , where the concentration of carbon dioxide is displayed.  
         [0007]     Another known apparatus has a structure in which a sampling tube is connected to a sensor body disposed in a monitor.  
         [0008]     In such an apparatus, one end of the sampling tube which introduces a portion of a respiration gas is connected to an airway adapter through which the respiration gas passes. The other end of the sampling tube is connected to the monitor. A pump is disposed in the monitor to lead the introduced respiration gas to the sensor body disposed in the monitor.  
         [0009]     Moreover, as shown in  FIG. 17 , an apparatus capable of measuring the concentration of carbon dioxide in an oral expiration gas as well as the concentration of carbon dioxide in a nasal expiration gas is known (see, e.g., U.S. Pat. No. 5,046,491).  
         [0010]     This apparatus is provided with a respiration gas collector  110  having: a nasal cannula  111  for collecting a nasal respiration gas; an outwardly-convex mouth guide  113  for collecting an oral respiration gas; an oral respiration gas collector  114  which is disposed in the mouth guide  113  and collects an oral respiration gas; and a joint stem  112  which is connected at one end thereof to an external upper portion of the mouth guide  113  and at the other end thereof to the nasal cannula  111 .  
         [0011]     However, the respiration gas collector  110  involves a large number of components, because the joint stem  112  is constituted of separate members. Further, the joint stem  112  must be attached to two points; that is, the mouth guide  113  and the nasal cannula  111 . This entails consumption of man-hours and, by extension, costs.  
         [0012]     Further, in the respiration gas collector  110 , the oral respiration gas collector  114  is disposed in the mouth guide  113  in order to cause a respiration gas to flow through an airway passage provided in the upper portion of the mouth guide  113 . Hence, the oral respiration gas collector  114  exerts gas flow resistance, which inhibits efficient flow of the oral expiration gas through the airway passage.  
         [0013]     In a case where oxygen is also supplied in conjunction with collection of the respiration gas, an oxygen supply tube is also attached to the patient. In such a case, prongs are inserted into nostrils. Alternatively, even in a case where an oxygen supply tube which does not entail insertion of the prongs into the nostrils, the prongs are oriented so that oxygen supplied by way of the prongs is injected directly toward the nostrils, which induces a problem of abrupt drying of the nostrils, causing the patient discomfort.  
       SUMMARY OF THE INVENTION  
       [0014]     It is therefore an object of the invention to provide an airway adapter for a carbon dioxide sensor in which an oral expiration gas can be efficiently supplied to an airway passage located at an upper portion of a mouth guide.  
         [0015]     It is also an object of the invention to provide an airway adapter for a carbon dioxide sensor in which the position of the mouth guide can be adjusted in accordance with the contour or size of a patient&#39;s face.  
         [0016]     It is also an object of the invention to provide an airway adapter for a carbon dioxide sensor in which the number of element and the manufacturing cost can be reduced.  
         [0017]     It is also an object of the invention to provide an airway adapter for a carbon dioxide sensor in which oxygen supplied from prongs can be prevented from directly injecting into patient&#39;s nostrils, in order to avoid abrupt drying of the nostrils.  
         [0018]     In order to achieve the above objects, according to the invention, there is provided an airway adapter of a sensor for detecting a carbon dioxide gas in an expiration gas of a living body, comprising:  
         [0019]     an airway case, adapted to be disposed below nostrils of the living body, and formed with an airway passage extending across an optical axis of a light beam emitted from a light emitter of the sensor; and  
         [0020]     a mouth guide, adapted to be disposed in front of a mouth of the living body so as to define a space communicated with the airway passage, the mouth guide being pivotably supported on the airway case.  
         [0021]     In such a configuration, the mouth guide can be disposed in the vicinity of the mouth in accordance with the contour or size of a face of the living body.  
         [0022]     Further, since the space defined by the mouth guide is communicated with the airway passage, an oral expiration gas can be smoothly guided to the airway passage to be subjected to the carbon dioxide detection.  
         [0023]     Preferably, a shaft member is integrally molded with the mouth guide, and fitted into a hole formed in the airway case, so that the mouth guide is pivoted about the hole. In such a configuration, the number of parts can be reduced.  
         [0024]     Here, it is preferable that the shaft member is extending in a first direction substantially parallel with a face of the living body, and the mouth guide is pivotable about the shaft member in a second direction perpendicular to the first direction.  
         [0025]     Here, it is also preferable that the shaft member is formed with a flexible material so as to have a size which is no less than a size of the hole. In such a configuration, appropriate resistance is afforded when the mouth guide is pivoted.  
         [0026]     It is also preferable that at least one of the airway case and the mouth guide is formed with an elastic material, so as to generate an elastic force directed in an extending direction of the shaft member. In such a configuration, appropriate resistance is afforded when the mouth guide is pivoted.  
         [0027]     According to the invention, there is also provided an airway adapter of a sensor for detecting a carbon dioxide gas in an expiration gas of a living body, comprising:  
         [0028]     an airway case, adapted to be disposed below nostrils of the living body, and formed with an airway passage extending across an optical axis of a light beam emitted from a light emitter of the sensor; and  
         [0029]     a retainer, adapted to retain an oxygen supply tube on the airway case in such an attitude that an oxygen gas supplied from prongs of the oxygen supply tube is not directly injected into the nostrils.  
         [0030]     In such a configuration, an oxygen gas can be supplied during the detecting operation for the carbon dioxide gas. Further, since the supplied oxygen gas is not directly injected into the nostrils, abrupt drying of the nostrils can be avoided.  
         [0031]     Preferably, the oxygen supply tube is retained at such a position that a gap is defined between the prongs and the nostrils.  
         [0032]     Preferably, the airway adapter further comprises a mouth guide, adapted to be disposed in front of a mouth of the living body so as to define a space communicated with the airway passage, the mouth guide being pivotably supported on the airway case.  
         [0033]     In such a configuration, the mouth guide can be disposed in the vicinity of the mouth in accordance with the contour or size of a face of the living body.  
         [0034]     In any of the above airway adapters, it is preferable that the airway adapter further comprises an inlet member, adapted to be inserted into at least one of the nostrils having a passage for guiding a nasal expiration gas to the airway passage, the inlet member being formed with a vent hole communicating the passage and an exterior of the inlet member.  
         [0035]     In such a configuration, the gas stayed in the airway passage can be smoothly escaped therefrom by the oral expiration gas with the aid of the vent hole. Further, even if the inlet member is clogged with a nasal mucus, the gas stayed in the airway passage is discharged to the exterior by way of the vent hole. Therefore, superior escape of the gas from the airway passage is achieved. Even when the amount of respiration is small, a sufficient amount of oral expiration gas for the detection can be introduced into the airway passage.  
         [0036]     Here, it is further preferable that the passage of the inlet member is defined by a pair of tube members adapted to be inserted into the nostrils and a junction at which the tube members are merged. The vent hole is formed at the junction.  
         [0037]     In such a configuration, when the oral expiration gas is guided to the airway passage, the gas remaining in the airway passage is discharged to the exterior by way of the pair of tube members and the vent hole. Even if the tube members are clogged with a nasal discharge, the gas in the airway passage is discharged to the exterior. Consequently, superior escape of the gas from the airway passage is achieved. Even when the amount of respiration is small, a sufficient amount of oral expiration gas for the detection can be introduced into the airway passage.  
         [0038]     It is also preferable that the vent hole is arranged such that a flow of a gas discharged from the vent hole is not substantially interfered by the living body.  
         [0039]     It is also preferable that the vent hole is arranged so as not to oppose to a face of the living body.  
         [0040]     In such configurations, when the oral expiration gas is guided to the airway passage, the gas remaining in the airway passage can be efficiently discharged to the exterior while being less likely to be impeded by the living body. Even when the amount of respiration is small, a sufficient amount of oral expiration gas for detection can be introduced into the airway passage.  
         [0041]     According to the invention, there is also provided a sensor for detecting a carbon dioxide gas in an expiration gas of a living body, comprising:  
         [0042]     a photo emitter;  
         [0043]     a photo receiver; and  
         [0044]     an airway adapter, which supports the photo emitter and the photo receiver such that a light beam emitted from the photo emitter is received by the photo receiver, the airway adapter comprising:  
         [0045]     an airway case, adapted to be disposed below nostrils of the living body, and formed with an airway passage extending across an optical axis of the light beam; and  
         [0046]     a mouth guide, adapted to be disposed in front of a mouth of the living body so as to define a space communicated with the airway passage, the mouth guide being pivotably supported on the airway case.  
         [0047]     According to the invention, there is also provided a sensor for detecting a carbon dioxide gas in an expiration gas of a living body, comprising:  
         [0048]     a photo emitter;  
         [0049]     a photo receiver; and  
         [0050]     an airway adapter, which supports the photo emitter and the photo receiver such that a light beam emitted from the photo emitter is received by the photo receiver, the airway adapter comprising: 
        an airway case, adapted to be disposed below nostrils of the living body, and formed with an airway passage extending across an optical axis of the light beam; and     a retainer, adapted to retain an oxygen supply tube on the airway case in such an attitude that an oxygen gas supplied from prongs of the oxygen supply tube is not directly injected into the nostrils.        
 
         [0053]     According to the invention, there is also provided a sensor for detecting a carbon dioxide gas in an expiration gas of a living body, comprising:  
         [0054]     a photo emitter;  
         [0055]     a photo receiver;  
         [0056]     an oxygen supply tube;  
         [0057]     an airway adapter, which supports the photo emitter and the photo receiver such that a light beam emitted from the photo emitter is received by the photo receiver, the airway adapter comprising: 
        an airway case, adapted to be disposed below nostrils of the living body, and formed with an airway passage extending across an optical axis of the light beam; and     a retainer, which retains the oxygen supply tube on the airway case in such an attitude that an oxygen gas supplied from prongs of the oxygen supply tube is not directly injected into the nostrils.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0060]     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein:  
         [0061]      FIG. 1  is a perspective view of a carbon dioxide sensor according to a first embodiment of the invention, showing a state that the sensor is attached to a patient;  
         [0062]      FIG. 2  is a cross-sectional view of an airway case in the sensor of the first embodiment;  
         [0063]      FIG. 3  is a cross-sectional view of the sensor taken along line III-III shown in  FIG. 2 ;  
         [0064]      FIG. 4  is a view of the sensor when viewed in the direction of arrow IV shown in  FIG. 3 ;  
         [0065]      FIG. 5  is a fragmentary perspective view of a carbon dioxide sensor according to a second embodiment of the invention, showing a state that the sensor is attached to a patient;  
         [0066]      FIG. 6  is a fragmentary perspective view showing a hook in the sensor of the second embodiment;  
         [0067]      FIG. 7A  is an overall perspective view showing the sensor of the second embodiment;  
         [0068]      FIG. 7B  is a fragmentary perspective view of a first modified example of the sensor of the second embodiment;  
         [0069]      FIG. 8A  is a fragmentary perspective view of a second modified example of the sensor of the second embodiment;  
         [0070]      FIG. 8B  is a fragmentary perspective view of a third modified example of the sensor of the second embodiment;  
         [0071]      FIG. 9A  is a fragmentary perspective view of a fourth modified example of the sensor of the second embodiment;  
         [0072]      FIG. 9B  is a fragmentary perspective view of a fifth modified example of the sensor of the second embodiment;  
         [0073]      FIG. 10A  is a fragmentary perspective view of a sixth modified example of the sensor of the second embodiment;  
         [0074]      FIG. 10B  is a fragmentary perspective view of a seventh modified example of the sensor of the second embodiment;  
         [0075]      FIG. 11  is a top plan view of a carbon dioxide sensor according to a third embodiment of the invention;  
         [0076]      FIG. 12  is a cross-sectional view of the sensor taken along line XII-XII shown in  FIG. 11 ;  
         [0077]      FIG. 13  is a view showing flow of an oral expiration gas when the sensor of the third embodiment is attached to the patient;  
         [0078]      FIG. 14  is a graph for explaining an advantageous effect of the sensor of the third embodiment;  
         [0079]      FIG. 15  is a cross-sectional view showing a modified example of the sensor of the third embodiment;  
         [0080]      FIG. 16  is a view showing a first related-art carbon dioxide sensor; and  
         [0081]      FIG. 17  is a view showing second related-art carbon dioxide sensor.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0082]     Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.  
         [0083]      FIG. 1  shows a carbon dioxide sensor  1  according to a first embodiment of the present invention. In order to measure the concentration, partial pressure, or presence/absence of a carbon dioxide in an expired gas of a patient (living body)  3 , the carbon dioxide sensor  1  comprises a light-emitting element  10  and a light-receiving element  11 , which are arranged so as to oppose to each other on an optical axis thereof; a lead wire  16   a  for transmitting a light emission signal from the unillustrated carbon dioxide measurement apparatus to the light-emitting element  10 ; and a lead wire  16   b  for transmitting a light detection signal from the light-receiving element  11  to the carbon dioxide measurement apparatus; and an airway adapter  2 .  
         [0084]     The airway adapter  2  comprises: an airway case  12  for supporting the light-emitting element  10  and the light-receiving element  11 ; and an airway passage  13  (see  FIG. 2 ) which enables a respiration gas of the patient  3  to pass through the optical axis when the airway case  12  is attached to an area located below nostrils  31  of the patient  3 ; a horizontal shaft  14  which is disposed on a wall portion  19  extending downward from the airway case  12  so as to be parallel to the surface of the face of the patient  3 ; a mouth guide  15  which is pivotable about the horizontal shaft  14  to approach or depart from a mouth  32  of the patient  3  with appropriate pivotal resistance; and a flexible tube (nasal tube)  21  (see  FIG. 3 ).  
         [0085]     The airway case  12  is formed from nonflexible resin. As shown in  FIG. 2 , the light-emitting element  10  and the light-receiving element  11  are hermetically sealed within the airway case  12  by anti-fogging films  17  whose mutually-opposing surfaces permit transmission of light and prevent fogging which would otherwise be caused by the respiration gas.  
         [0086]     The airway passage  13  is defined by interior walls  12   a ,  12   b  and the anti-fogging films  17 , both being provided in the airway case  12 .  
         [0087]     An optical filter (not shown) for permitting passage of only light having a wavelength to be absorbed by a carbon dioxide is disposed on the side of the light-receiving element  11 . In  FIG. 2 , reference numeral  18  designates an anti-fogging film case.  
         [0088]     The light-emitting element  10  is equipped with the lead wire  16   a , and the light-receiving element  11  is equipped with the lead wire  16   b.    
         [0089]     As shown in  FIG. 3 , the airway passage  13  is connected to the flexible tube  21 . This flexible tube  21  is formed from silicon rubber or the like or from vinyl chloride, polypropylene, polyethylene, an elastomer, or the like.  
         [0090]     The flexible tube has a pair of inlets  21   a ,  21   b . When the inlets  21   a ,  21   b , which are Y-shaped, are inserted into the nostrils  31  of the patient  3  (see  FIG. 1 ), a nasal expiration gas is guided to the airway passage  13  by way of the flexible tube  21 .  
         [0091]     In the side of the airway case  12  opposite to the side where the flexible tube  21  is attached, the mouth guide  15  is attached such that the respiration gas flows into the airway passage  13 . The mouth guide  15  is formed from a soft material and is tongue-shaped when viewed from the front side thereof so as to have an appropriate width “b”; e.g., 20 mm or less.  
         [0092]     The width “b” is preferably sufficiently narrow that a suction tube  23  (see  FIG. 1 ) can be inserted into the mouth  32  while the patient  3  is wearing the carbon dioxide sensor  1 , and sufficiently wide enough to receive the respiration gas from the mouth  32 . To this end, the width “b” of the mouth guide  15  is preferably set to approximately 5 to 20 mm.  
         [0093]     In order to minimize escape of the respiration gas, a sidewall  22  (see  FIG. 4 ) is provided on both sides of the mouth guide  15  such that a concave space facing the mouth  32  is defined.  
         [0094]     As shown in  FIG. 4 , the mouth guide  15  is configured so as to be pivotable about the horizontal shaft  14  latched by the wall portion  19  in a direction X in which the mouth guide  15  approaches and departs from the mouth  32  of the patient  3  (see  FIG. 1 ); that is, in the forward and backward directions. The mouth guide  15  is attached to the sensor  1  such that the direction designated by arrow F is directed toward the face.  
         [0095]     Material of the mouth guide  15  can be selected, as required, from soft materials such as vinyl chloride, polypropylene, polyethylene, silicon rubber, or an elastomer.  
         [0096]     As shown in  FIGS. 3 and 4 , the wall portion  19  is constituted of continuous walls  19   a ,  19   b , and  19   c  so as to define a space opened to the face of the patient  3 . Holes  20  are coaxially formed in the opposing walls  19   a  and  19   b  so as to extend horizontally and parallel to the face of the patient  3 .  
         [0097]     As shown in  FIG. 3 , the horizontal shaft  14  is formed integrally from the mouth guide  15  and made up of mushroom-shaped shafts  14   a ,  14   b  having the same dimensions. The outer diameters of small-diameter sections of the respective shafts  14   a ,  14   b  are determined so as to be slightly larger than the diameters of holes  20  of the walls  19   a ,  19   b , in a state before the shafts  14   a ,  14   b  are fitted into the holes  20 . The shafts  14   a ,  14   b  are tightly fitted into the holes  20  so that the mouth guide  15  can pivot about the holes  20  (shafts  14   a ,  14   b ) with appropriate resistance.  
         [0098]     A slit may be formed in each mushroom-shaped top of the shafts  14   a ,  14   b  to facilitate the insertion of the shafts  14   a ,  14   b  into the holes  20 .  
         [0099]     As shown in  FIG. 4 , the wall  19   c  of the wall portion  19  is configured so as to cover an end section  15   a  proximate to the horizontal shaft  14  of the mouth guide  15  even when the mouth guide  15  is situated a position close to the face of the patient  3  (i.e., a position indicated by a dashed chain line), thereby reducing a flow resistance against the expiration gas flowing from the mouth  32  into the airway passage  13 .  
         [0100]     The mouth guide  15  is pivotable about the horizontal shaft  14  back and forth with appropriate resistance. Therefore, even when the shape and size of the face of the patient  3  varies from that corresponding to the current configuration the mouth guide  15  is subjected to positional adjustment along the contour of the face and can be caused to approach the mouth  32  of the patient  3 .  
         [0101]     Therefore, the expiration gas from the mouth  32  of the patient  3  can be reliably led to the airway passage  13  of the airway case  12 . As a result, the concentration of carbon dioxide in the oral expiration gas can be measured reliably.  
         [0102]     Since the horizontal shaft  14  is formed integrally with the mouth guide  15 , the mouth guide can be manufactured inexpensively. Therefore, the number of parts and the manufacturing cost can be reduced.  
         [0103]     In the present embodiment, the shafts  14   a ,  14   b  are tightly fitted into the holes  20  to impart resistance to the pivotal movement of the mouth guide  15 . Alternatively, the dimension “c” of the mouth guide  15  shown in  FIG. 3  (i.e., a distance between proximal ends of the shafts  14   a ,  14   b ) may be made greater than the inner distance between the walls  19   a ,  19   b  before the shafts  14   a ,  14   b  are assembled into the holes  20 . In such a case, a horizontal repulsion force develops in an area where the mouth guide  15  is in contact with the walls  19   a ,  19   b , so that appropriate resistance can be imparted to the pivotal movement of the mouth guide  15 .  
         [0104]     In this case, in order to facilitate the assembling operation, the small diameters of the shafts  14   a ,  14   b  may be made smaller than the internal diameters of the holes  20 , so that the shafts  14   a ,  14   b  may be respectively loosely fitted into the holes  20 .  
         [0105]     In order to further facilitate the assembling operation, in the above case, the shafts  14   a ,  14   b  may be merely shaped cylindrical to be loosely fitted into the holes  20 .  
         [0106]     In the loose-fitting configuration, the material of the mouth guide  15  may not be the soft material. However, it is necessary to configure the marginal shape of the mouth guide  15  so as not to inflict any pain on the living body even if the mouth guide  15  comes into contact with the living body.  
         [0107]     In this embodiment, the horizontal shaft  14  is formed integrally with the mouth guide  15 , and the holes  20  are formed in the wall portion  19  of the airway case  12 . However, the horizontal shaft  14  may be formed integrally with the wall portion  19 , and the holes  20  may be formed in the mouth guide  15 .  
         [0108]     In this case, so long as the mouth guide  15  is formed from a soft material, the horizontal shaft  14  made of unsoft resin can be readily inserted into the holes  20 .  
         [0109]     In this embodiment, the wall portion  19  is constituted of the three walls  19   a ,  19   b , and  19   c  so as to have a rectangular horizontal cross section. However, the wall portion  19  may be configured so as to have a semi-circular or a semi-oval horizontal cross section, for example, only if the wall portion  19  defines a space opened to the face of the patient  3 .  
         [0110]     There will be described a second embodiment of the invention in which a hook  33  is provided in the carbon dioxide sensor  1  for retaining an oxygen supply tube (which may also be for general purpose use).  
         [0111]     As shown in  FIG. 5 , the hook  33  is provided on the back face of the airway case  12  of the carbon dioxide sensor  1  (i.e., the side opposite to the side facing the face of the patient  3  when the sensor is attached to the living body), to thereby enable attachment of an oxygen supply tube  34 .  
         [0112]     As shown in  FIG. 6 , the hook  33  has a curved portion  33   a  defining an opened section  33   c . A tube portion of the oxygen supply tube  34  located between two prongs  35  is attached to the hook  33  by way of the opened section  33   c . In order to prevent deformation of the prongs  35 , which would otherwise be caused by application of external force, the width of the curved portion  33   a  is preferably made equal to the distance between the two prongs  35 . The curved portion  33   a  can be applied to the oxygen supply tube  34  regardless of the diameter thereof, through use of an elastic material. Such a hook  33  is bonded to the back face of the airway case  12 .  
         [0113]     Alternatively, as a first modified example shown in  FIG. 7B , the airway case  12  and the hook  33  may be formed integrally.  
         [0114]      FIG. 7A  is a view showing a state in which the hook  33  is used while the oxygen supply tube  34  is attached to the hook.  
         [0115]     Reference numeral  34   a  designates an oxygen supply port of the oxygen supply tube  34 . Reference numeral  16   c  designates a connector for electrically connecting an electric current employed for driving the light-emitting element  10  and the signal detected by the light-receiving element  11  to the not shown measurement apparatus.  
         [0116]     Incidentally, the prongs  35  are made not to be inserted into the nostrils and arranged such that the oxygen supplied from the prongs  35  is not injected directly into the nostrils. As a result, abrupt drying of the nostrils can be prevented.  
         [0117]     In the embodiment shown in  FIG. 7A , in order to realize such an arrangement, the hook  33  is disposed such that the prongs  35  are arranged on a top face of the airway case  12 . In this case, the oxygen supplied from the prongs  35  is not injected directly into the nostrils. The oxygen is aspirated by the nostrils after having come into collision with the skin located below the nose and wafted.  
         [0118]      FIG. 8A  shows a second modified example of this embodiment featuring a different arrangement of the hook  33 . The hook  33  is provided such that the prongs  35  are aligned in line with the back of the airway case  12 . In this case, the oxygen supplied from the prongs  35  is directed parallel to the back face of the airway case  12  toward the flexible tube  21  and wafted and aspirated by the nostrils.  
         [0119]     As a third modified example of this embodiment shown in  FIG. 8B , the hook  33  may be formed integrally with the airway case  12 .  
         [0120]      FIG. 9A  shows a fourth modified example of this embodiment featuring a further different layout of the hook  33 . The hook  33  is provided such that the prongs  33  are aligned with a bottom face of the airway case  12 . Even in this case, as in the case shown in  FIG. 7A , the oxygen supplied by way of the prongs  35  is not injected directly into the mouth and aspirated.  
         [0121]     As a fifth modified example of this embodiment shown in  FIG. 9B , the hook  33  may be formed integrally with the airway case  12 .  
         [0122]      FIG. 10A  shows a sixth modified example of this embodiment featuring a still further different layout of the hook  33 . The hook  33  is disposed on the back face of the airway case  12  such that the extremities of the prongs  35  are directed toward the back face of the airway case  12  by rendering a handle  33   b  of the hook sufficiently long. By adoption of such a configuration, the oxygen supply tube  34  is attached to the curved portion  33   a , and the oxygen can be supplied to the face. Once having come into collision with the airway case  12  and wafted, the oxygen supplied by way of the prongs  35  can be aspirated by the nostrils and the mouth  32 . The orientation in which the extremities of the prongs  35  are directed toward the face can be adjusted by the angle at which the oxygen supply tube  34  is attached to the curved portion  33   a  of the hook  33 .  
         [0123]     As a seventh modified example of this embodiment shown in  FIG. 10B , the hook  33  may be integrally formed with the airway case  12 .  
         [0124]     The hook  33  may be attached to another side face of the airway case  12 , and the hook  33  may retain the prongs  35 .  
         [0125]     There will be described a third embodiment of the invention in which a vent hole for immediately discharging a gas remaining in the airway passage  13  during the oral expiration, in order to enhance the measurement accuracy for the concentration, the partial pressure, or the presence/absence of the carbon dioxide even in a case where the amount of a respiration gas is small.  
         [0126]     This embodiment shown in  FIGS. 11 through 14  differs from the first embodiment shown in  FIGS. 1 through 4  only in terms of ventilation. Therefore, the same elements are designated by the same reference numerals, and the repetitive explanations for those will be omitted.  
         [0127]     In an upper portion of the airway case  12 , the pair of inlets  21   a ,  21   b  of the flexible tube  21  having a relatively small cross-sectional area are merged with each other in the vicinity of the airway passage  13 , thereby defining a merge section  40  having a relatively large cross-sectional area. The merge section  40  is in close proximity to and in communication with the airway passage  13  having a much larger cross-sectional area. The nasal expiration gas inlet member  42  is constituted of the soft tube  21  and the merge section  40 . In order to immediately discharge the gas remaining in the airway passage  13  simultaneously when breath is expired from the mouth, a vent hole  41  is formed for bringing the inside of the merge section  40  into communication with the outside.  
         [0128]     The vent hole  41  is disposed downstream of the airway passage  13  with respect to the direction of flow of the oral expiration gas. The location and shape of the vent hole  41  are determined so as not to oppose to the flow of the nasal expiration gas, in order to block leakage of the nasal expiration gas to the outside by way of the vent hole  41 . Further, the location and shape of the vent hole  41  are determined such that the face does not hinder circulation of the gas passing through the vent hole  41 .  
         [0129]     As shown in  FIGS. 11 through 13 , the vent hole  41  is circular (e.g., having a diameter of 2 mm) and formed in an exterior wall of the merge section  40  so as to situate at the center of the exterior wall facing away from the face. In  FIG. 11 , the carbon dioxide sensor  51  is attached such that the direction of arrow F faces the face of the patient  3 .  
         [0130]     As indicated by the arrow in  FIG. 13 , the oral expiration gas is guided to the airway passage  13  by way of the mouth guide  15 . The gas remaining in the airway passage  13  is pushed to the merge section  40 . The thus-pushed gas flows to the outside by way of the vent hole  41  and simultaneously enters the nostrils  31  by way of the soft tube  21 , subsequently flowing outside. Since the soft tube  21  is elongated and has a relatively small cross-sectional area, the flow resistance against the gas is large. Moreover, the vent hole  41  is provided in the merge section  40  adjacent to the airway passage  13 . Consequently, the gas is likely to flow outside via the vent hole  41 .  
         [0131]     Even when the soft tube  21  has been clogged with a nasal mucus, the gas can flow outside from the vent hole  41  as a result of inflow of the oral expiration gas to the airway passage  13 .  
         [0132]     As mentioned above, since the carbon dioxide sensor has the vent hole  41 , excellent escape of the gas from the inside of the airway passage  13  is achieved. Therefore, when the oral expiration is performed, the gas remaining in the airway passage  13  is discharged to the outside, and the oral expiration gas immediately flows into the airway passage  13 . Consequently, even when the amount of respiration is small, the concentration, partial pressure, or presence/absence of carbon dioxide in the oral expiration gas can be measured accurately.  
         [0133]     Next will be described an experiment in which the concentration of carbon dioxide in the oral expiration gas is measured and evaluated through use of the carbon dioxide sensor having the vent hole and the carbon dioxide sensor not having any vent hole.  
         [0134]     The measurement was evaluated by measuring the concentration of carbon dioxide through use of the carbon dioxide sensor  51  having the vent hole  41 , the sensor being shown in  FIGS. 11 through 13 , and the carbon dioxide sensor  1  not having the vent hole  41 , and by comparing the results of measurement. Measurement was performed as described below.  
         [0135]     A model for a human face and a nostril was used. As shown in  FIG. 13 , the sensor was attached to the model. The amount of gas to be measured corresponding to weak expiration was delivered by a delivery pump for a given time period corresponding to a time period during which an ordinary person expires at a single breath, to thereby discharge the gas from the mouth. Subsequently, the amount of gas corresponding to weak respiration was sucked by a vacuum pump in place of the delivery pump for a given time period corresponding to a time period during which the ordinary person inspires at a single breath. These operations were performed alternately and consecutively. A gas, whose concentration is close to the concentration of carbon dioxide in an expired gas of the human attained by mixing carbon dioxide in air, was used as the gas to be measured.  
         [0136]     Measurement results are shown in  FIG. 14 . A solid line shows the results obtained by the sensor having the vent hole  41 , whereas a dashed line designate the results obtained by the sensor not having the vent hole  41 .  
         [0137]     As can be seen from  FIG. 14 , when the vent hole  41  is formed, the concentration of carbon dioxide has increased and become saturated immediately after initiation of ejection of the gas to be measured. This shows immediate flow of the gas to be measured into the airway passage  13 , and the effect of the vent hole  41  is exhibited well.  
         [0138]     In contrast, when the vent is not formed, the concentration of carbon dioxide gradually increases with a lag even when the gas to be measured has been discharged. The increase continues until initiation of sucking action. Subsequently, the concentration of carbon dioxide does not saturate and gradually decreases. This shows that flow of the gas to be measured into the airway passage  13  is performed gradually.  
         [0139]     As mentioned above, even when the amount of respiration is small, the concentration of carbon dioxide in the expired gas from the mouth can be measured accurately by the vent hole  41 .  
         [0140]     When the vent hole  41  is formed, the concentration of carbon dioxide has decreased rapidly after initiation of sucking operation. This shows that an external gas flows into the airway passage  13  by way of the vent hole  41  and that the gas to be measured has been immediately discharged to the outside.  
         [0141]     The nasal expiration gas inlet member  42  has been described as being constituted of a pair of tubes (i.e., the pair of inlets  21   a ,  21   b  of the soft tube  21  to be inserted into the nostrils  31 ) and the merge section  40  merged with the respective ends of the tube. However, the nasal expiration gas inlet member  42  may be constituted of a single tube. In this case, a vent hole is formed in an area of the tube, where is in close proximity to a node between the tube and the airway passage  13 .  
         [0142]     The vent hole  41  is given a circular shape having a diameter of 2 mm and is formed in the exterior wall of the merge section  40  so as to be located in the center of the portion thereof facing away from the living body. The vent hole  41  may assume any shape or location, so long as the above-described requirements are satisfied.  
         [0143]      FIG. 15  shows a modified example of this embodiment featuring a different layout of the vent hole  41 . As illustrated by phantom lines, the vent hole can be formed in any one of locations  41   a ,  41   b ,  41   c , and  41   d . In a position where the vent hole  41   c  or the like may be closed by the living body, an opening of the vent hole  41   c  may be set to an oval shape. Although the diameter of the vent hole  41  is taken as 2 mm, the diameter can be set so as to satisfy the foregoing requirements in view of the structure, such as the soft tube  21 , the merge section  40 , and the airway passage  13 .  
         [0144]     Although the present invention has been shown and described with reference to specific preferred embodiments, various changes and modifications will be apparent to those skilled in the art from the teachings herein. Such changes and modifications as are obvious are deemed to come within the spirit, scope and contemplation of the invention as defined in the appended claims.