a bite block is a device commonly used during upper gastro - intestinal endoscopic procedures to facilitate passage of an esophago - gastro - duodenoscopy ( egd ) endoscope . the purpose of the bite block is to allow the physician to perform the procedure without the subject interfering by biting and damaging the endoscope tubing inserted via his mouth , whether voluntarily or involuntarily . the upper gastro - intestinal endoscopic procedure itself , together with the use of a bite block , is often highly uncomfortable for the subject and therefore it is very common for the subject to be sedated during the procedure . despite this , it is common for the subject to show opposition to the procedure . during upper gastro - intestinal endoscopy , and especially during long duration procedures performed under sedation , co2 monitoring is often performed using a separate nasal or oral / nasal cannula in conjunction with a bite block . concomitant use of bite blocks and cannulae may noticeably affect capnographic performance for a number of reasons , including inter alia misalignment between the cannula and the bite block and inefficient oral sampling due to the space taken up by the endoscope . the present invention provides a solution that generally does not affect the capnographic performance . reference is now made to fig1 a and 1b , which are simplified pictorial illustrations of an oral nasal sampling cannula forming part of an endoscopic bite block assembly , constructed and operative in accordance with a preferred embodiment of the present invention , in retracted and extended orientations respectively . fig1 a and 1b show an oral nasal sampling cannula 10 , which is adapted for collection of gases , such as carbon dioxide , exhaled by a subject , and for supplying oxygen to the subject . the oral nasal sampling cannula 10 comprises a main body portion 12 , having formed therein an exhaled breath collection bore 14 and an oxygen delivery bore 16 . a pair of hollow nasal prongs 18 , which are adapted for insertion into the nostrils of the subject , is integrally formed with the main body portion 12 . a hollow oral prong 22 , which is formed with a limiting rib 23 and a cut - away tip 24 , is mounted onto a bottom surface of main body portion 12 . an oral breath directing element 26 , which is preferably in the shape of a cut - away tube , is slidably mounted onto oral prong 22 by a mounting portion 28 , and positioning of the oral breath directing element 26 is limited by the limiting rib 23 of oral prong 22 . a channel formed in oral prong 22 is in fluid flow connection with channels formed in nasal prongs 18 , thereby forming a single junction 32 . single junction 32 is in fluid flow communication with exhaled breath collection bore 14 , which in turn is in fluid flow communication with an exhaled breath collection tube 34 , which is adapted to be connected to a breath test analyzer or a capnograph ( not shown ), such as microcap ® which is commercially available from oridion medical ltd . of jerusalem , israel . main body portion 12 is formed with oxygen delivery openings 36 , which are in fluid flow communication with oxygen delivery bore 16 , which in turn is in fluid flow communication with an oxygen delivery tube 38 . alternatively , at least one nasal oxygen delivery prong , adapted for insertion into the subject &# 39 ; s nostril , may be used instead of oxygen delivery openings 36 . oxygen delivery tube 38 is adapted to be connected to a source of oxygen ( not shown ). oxygen delivery tube 38 and exhaled breath collection tube 34 may optionally be placed around the ears of the subject , thereby stabilizing the oral nasal sampling cannula 10 on the subject &# 39 ; s face , such that any movement of the subject will have a negligible effect on the placement of the oral nasal sampling cannula 10 . it is appreciated that oral breath directing element 26 may be in a retracted orientation as shown in fig1 a , or in an extended orientation as shown in fig1 b , thereby allowing the oral nasal sampling cannula 10 to be suited to the facial dimensions of the subject , resulting in more efficient collection of exhaled breath . reference is now made to fig2 a and 2b , which are front - view and rear - view simplified pictorial illustrations of an endoscopic bite block forming part of an endoscopic bite block assembly constructed and operative in accordance with a preferred embodiment of the present invention and to fig3 , which is a simplified sectional pictorial illustration thereof . fig2 a , 2 b and 3 show an endoscopic bite block 50 , which is adapted to be inserted into the mouth of a subject while the subject is sedated , to ensure that the mouth of the subject is maintained open during the endoscopy process and that the subject does not interfere with the process by biting on the medical instruments used . the endoscopic bite block 50 includes a main body portion 52 , having formed therein a central opening 54 . a hollow tubular portion 56 extends distally from main body portion 52 , such that the opening of tubular portion 56 is an extension of central opening 54 . central opening 54 is of a first height , indicated by h 1 in fig3 , which is typically 16 to 20 mm in bite blocks for adult use , which is the height required by medical personnel for performing an endoscopy . in order to ensure that during breath sampling , oral prong 22 of oral nasal sampling cannula 10 ( fig1 a and 1b ) does not interfere with the space required by medical personnel for performing the endoscopy procedure , the height of tubular portion 56 is greater than the height h 1 of the central opening 54 as indicated by h 2 in fig3 , and is typically 2 to 4 mm more than height h 1 ( 18 to 24 mm ). an outer surface 58 of tubular portion 56 is formed with top and bottom teeth engagement surfaces 60 and 62 , such that top teeth engagement surface 60 is relatively forward of bottom teeth engagement surface 62 . this structure facilitates easy and accurate biting of the bite block 50 by a subject , as it is suited to the jaw morphology of a closed human mouth . surface 58 is additionally formed with jaw engagement recesses 64 , which are formed forwardly of teeth engagement surfaces 60 and 62 , respectively . a top inner surface 70 of main body portion 52 is formed with a longitudinal groove 72 having a transverse surface 73 , which is adapted to accommodate oral prong 22 and oral breath directing element 26 of the oral nasal sampling cannula 10 ( fig1 a and 1b ), as described with more detail herein below with reference to fig4 . a flexible barrier 76 , preferably comprised of several flaps 78 , is disposed within central opening 54 , thereby substantially closing off the central opening and preventing dilution of exhaled breath by ambient air during sampling . an opening 80 is preferably maintained within flexible barrier 76 , thereby ensuring a small part of central opening 54 remains open in order to enable the subject to inhale external air . the flexible barrier 76 ensures that a majority of the subject &# 39 ; s orally exhaled breath will be directed toward oral prong 22 ( fig1 a and 1b ) thereby ensuring accurate sampling of the subject &# 39 ; s breath . opening 80 is preferably placed at a top part of central opening 54 near the cut - away tip 24 of oral prong 22 ( fig1 a and 1b ), thereby directing exhaled breath toward the oral prong 22 as it is the only substantial exit . the flaps 78 are preferably formed of a plastic material selected to be of suitable thickness to maintain their position when undisturbed , yet bend readily when pushed by an endoscope probe , and thus do not limit the actions of the medical personnel performing the endoscopy . however , the flaps 78 preferably close back around the endoscope probe , thus maintaining a substantially closed oral cavity volume and allowing most of the exchange of gases to occur close to the opening 80 of the flexible barrier 76 which is close to the cut - away tip 24 of oral prong 22 ( fig1 a and 1b ) from which capnographic sampling can be performed accurately . additionally , the flaps 78 are preferably transparent , thus enabling medical personnel to see into the oral cavity during the endoscopy procedure . two attachment surfaces 82 , each formed with a slit 84 , extend horizontally outwardly from main body portion 52 . slits 84 are adapted to connect to a band which is placed around the subject &# 39 ; s head and is used to maintain the endoscopic bite block 50 firmly in position during the endoscopy procedure . preferably , slits 84 are located above a horizontal centerline of main body portion 52 , such that the connected band will tend to exert a stronger pull to the top of the main body portion 52 , thus assisting in overcoming the subject &# 39 ; s tendency to tilt the bite block 50 outward during the endoscopy procedure and in maintaining the bite block 50 upright in the subject &# 39 ; s mouth . reference is now made to fig4 , which is a simplified schematic illustration of the connection between the oral nasal sampling cannula of fig1 a and 1b and the endoscopic bite block of fig2 a - 3 . as seen in fig4 , oral prong 22 of oral nasal sampling cannula 10 is accommodated within groove 72 of bite block 50 , such that a bottom surface of oral breath directing element 26 engages transverse surface 73 of the groove 72 . it is appreciated that transverse surface 73 is located below an inner surface of tubular portion 56 in order to ensure that air exhaled by the subject into tubular portion 56 will be directed toward groove 72 and oral prong 22 . reference is now made to fig5 a , 5 b , 5 c , 5 d , 5 e , 5 f and 5 g , which are pictorial illustrations of various stages of typical use of the endoscopic bite block assembly of fig1 a - 4 . as seen in fig5 a , the nasal prongs 18 of the oral nasal sampling cannula 10 are placed in the subjects nostrils , preferably before the subject is sedated . preferably , the exhaled breath collection tube 34 and the oxygen delivery tube 38 are placed around the subject &# 39 ; s ears , in order to ensure the stability of the oral nasal sampling cannula 10 on the subject &# 39 ; s face . as seen in the enlarged portion of fig5 a , at this stage the oral breath - directing element 26 is in its retracted orientation , indicated by the length h 3 . turning to fig5 b , it is seen that the oral breath directing element 26 is extended to accommodate the facial dimensions of the subject , revealing part of oral prong 22 . preferably , the oral breath - directing element is moved down to a point in which a bottom end thereof is at the height of the top of the bottom lip of the subject , its new length being indicated by h 4 . this action is preferably preformed by medical personnel , but may alternatively be performed by the subject himself , a family member , or any other person . fig5 c illustrates the insertion of bite block 50 into the mouth of the subject , such that main body portion 52 engages the outer surface of the subject &# 39 ; s lips and the tubular portion 56 is inside the subject &# 39 ; s mouth . a strap , indicated by reference numeral 90 , is attached to slits 84 of attachment surfaces 82 and is placed around the subject &# 39 ; s head , thereby securing the bite block 50 in place . this stage is preferably performed when the subject is sedated , but may alternatively be performed prior thereto . as seen in the enlarged portion of fig5 c , the oral breath directing element 26 and the oral prong 22 are accommodated in groove 72 , such that a bottom surface of the oral breath directing element 26 engages transverse surface 73 of groove 72 . additionally , if oral breath directing element 26 has been extended more than necessary for the facial features of the subject , the transverse surface 73 pushes the oral breath - directing element 26 back , until it is optimally positioned . the lips of the subject , indicated by reference numeral 92 preferably engage jaw engagement recesses 64 , and the top and bottom teeth of the subject , indicated by reference numerals 94 and 96 engage top and bottom teeth engagement surfaces 60 and 62 , respectively . turning to fig5 d , it is seen that air exhaled orally by the subject , indicated by arrows , passes through the bore of tubular portion 56 , and is directed toward oral breath directing element 26 and oral prong 22 by the flaps 78 of flexible barrier 76 . air that is exhaled nasally by the subject passes through nasal prongs 18 . fig5 e illustrates the sedated subject , having the nasal prongs 18 of the oral nasal sampling cannula 10 in his nostrils and the endoscopic bite block 50 placed in his mouth and strapped to his head . preferably , once the subject is sedated , oxygen is supplied to the nose of the subject via oxygen delivery openings 36 of oral nasal sampling cannula 10 , as indicated by arrows in the enlarged portion of fig5 e . the oxygen is supplied to oxygen delivery openings 36 via oxygen delivery bore 16 ( fig1 a and 1b ) and oxygen delivery tube 38 . turning to fig5 f , it is seen that when the subject is sedated , he tends to move or slump his head , thereby moving oral nasal sampling cannula 10 relative to bite block 50 , as indicated by angle α in the enlarged portion of fig5 f . the feature of the present invention which provides oral nasal sampling cannula 10 which is physically separated from bite block 50 and the placement of oral breath directing element 26 and oral prong 22 within groove 72 , ensure that even when the subject moves or slumps his head , the oral prong 22 and nasal prongs 18 will be maintained in their respective places , and accurate sampling will continue . additionally , the placement of oral prong 22 within groove 72 provides a counter force to force applied by the subject &# 39 ; s tongue to push at least the top portion of the bite block 50 out of the subject &# 39 ; s mouth , thus ensuring accurate placement of the bite block . as seen in fig5 g , an endoscope probe 98 is inserted into the bore of tubular portion 56 of bite block 50 , for performing an endoscopy procedure . during the insertion of endoscope probe 98 and its presence in the subject &# 39 ; s mouth and pharynx , flaps 78 of flexible barrier 76 bend slightly inward to allow the passage of the endoscope probe 98 , as seen with particular clarity in the enlarged portion of fig5 g . however , the central opening 54 of bite block 50 remains substantially closed by flaps 78 , thereby separating the exhaled breath of the subject which is in the bore of tubular portion 56 from the ambient air . additionally , the sampling may continue during the presence of the endoscope probe 98 in the pharynx of the subject , as the tubular portion 56 is of a slightly larger diameter than the central opening 54 , thereby ensuring that medical personnel have the space required for the endoscopy procedure and sampling can take place from the space defined by the difference between heights h 2 and h 1 ( fig3 ), as indicated by arrows in the enlarged portion of fig5 g . it is appreciated that following the endoscopy , the bite block 50 may be removed from the subject &# 39 ; s mouth , preferably by medical personnel . however , the sampling of exhaled breath through nasal prongs 18 which remain in the subject &# 39 ; s nostrils and through oral prong 22 which remains near the subject &# 39 ; s mouth , preferably continues until the subject has awaken from the sedation . this is necessary because the subject &# 39 ; s breath must be monitored as long as the subject is sedated . reference is now made to fig6 a and 6b , which are simplified pictorial illustrations of an oral nasal sampling cannula forming part of an endoscopic bite block assembly , constructed and operative in accordance with another preferred embodiment of the present invention , in retracted and extended orientations respectively . fig6 a and 6b show an oral nasal sampling cannula 110 , which is adapted for collection of gases , such as carbon dioxide , exhaled by a subject , and for supplying oxygen to the subject . the oral nasal sampling cannula 110 comprises a main body portion 112 , having formed therein an exhaled breath collection bore 114 and an oxygen delivery bore 116 . a pair of hollow nasal prongs 118 , which are adapted for insertion into the nostrils of the subject , is integrally formed with the main body portion 112 . a hollow oral prong 122 , which is formed with a limiting rib 123 and a cut - away tip 124 , is mounted onto a bottom surface of main body portion 112 . an oral breath directing element 126 , which is preferably in the shape of a cut - away tube , is slidably mounted onto oral prong 122 by a mounting portion 128 , and positioning of the oral breath directing element 126 is limited by the limiting rib 123 of oral prong 122 . a channel formed in oral prong 122 is in fluid flow connection with channels formed in nasal prongs 118 , thereby forming a single junction 132 . single junction 132 is in fluid flow communication with exhaled breath collection bore 114 , which in turn is in fluid flow communication with an exhaled breath collection tube 134 , which is adapted to be connected to a breath test analyzer or a capnograph ( not shown ), such as microcap ® which is commercially available from oridion medical ltd . of jerusalem , israel . main body portion 112 is formed with oxygen delivery openings 136 , which are in fluid flow communication with oxygen delivery bore 116 , which in turn is in fluid flow communication with an oxygen delivery tube 138 . alternatively , at least one nasal oxygen delivery prong , which is adapted to be inserted into the nostril of the subject , may be used instead of oxygen delivery openings 136 . oxygen delivery tube 138 is preferably formed with a t - element 140 , connecting the oxygen delivery tube 138 to an oral oxygen delivery tube 142 . oxygen delivery tube 138 is adapted to be connected to a source of oxygen ( not shown ). oral oxygen delivery tube 142 is preferably normally closed by a valve element 144 . typically , the valve is a luer type valve . oxygen delivery tube 138 and exhaled breath collection tube 134 may optionally be placed around the ears of the subject , thereby stabilizing the oral nasal sampling cannula 110 on the subject &# 39 ; s face , such that any movement of the subject will have negligible effect on the placement of the oral nasal sampling cannula 110 . it is appreciated that oral breath directing element 126 may be in a retracted orientation as shown in fig6 a , or in an extended orientation as shown in fig6 b , thereby allowing the oral nasal sampling cannula 110 to be suited to the facial dimensions of the subject , resulting in more efficient collection of exhaled breath . reference is now made to fig7 a and 7b , which are front - view and rear - view simplified pictorial illustrations of an endoscopic bite block forming part of an endoscopic bite block assembly constructed and operative in accordance with a preferred embodiment of the present invention and to fig8 , which is a simplified sectional pictorial illustration thereof . fig7 a , 7 b and 8 show an endoscopic bite block 150 , which is adapted to be inserted into the mouth of a subject while the subject is sedated , to ensure that the mouth of the subject is maintained open during the endoscopy process and that the subject does not interfere with the process by biting on the medical instruments used . the endoscopic bite block 150 includes a main body portion 152 , having formed therein a central opening 154 . a hollow tubular portion 156 extends distally from main body portion 152 , such that the opening of tubular portion 156 is an extension of central opening 154 . central opening 154 is of a first height , indicated by h 1 in fig8 , which is typically 16 to 20 mm in bite blocks for adult use , which is the height required by medical personnel for performing an endoscopy . in order to ensure that during breath sampling , oral prong 122 of oral nasal sampling cannula 110 ( fig6 a and 6b ) does not interfere with the space required by medical personnel for performing the endoscopy procedure , the height of tubular portion 156 is greater than the height h 1 of central opening 154 as indicated by h 2 in fig8 , and is typically 2 to 4 mm more than height h 1 ( 18 to 24 mm ). an outer surface 158 of tubular portion 156 is formed with top and bottom teeth engagement surfaces 160 and 162 , such that top teeth engagement surface 160 is relatively forward of bottom teeth engagement surface 162 . this structure facilitates easy and accurate biting of the bite block 150 by a subject , as it is suited to the jaw morphology of a closed human mouth . surface 158 is additionally formed with jaw engagement recesses 164 , which are formed forwardly of teeth engagement surfaces 160 and 162 , respectively . a top inner surface 170 of main body portion 152 is formed with a longitudinal groove 172 having a transverse surface 173 , which is adapted to accommodate oral prong 122 and oral breath directing element 126 of the oral nasal sampling cannula 110 ( fig6 a and 6b ), as described with more detail hereinbelow with reference to fig9 . a tubular portion 174 is formed on a side of outer surface 158 of tubular portion 156 . tubular portion 174 is adapted to threadably engage oral oxygen delivery tube 142 ( fig6 a and 6b ), thereby opening valve 144 to the passage of gases and thus supplying oxygen directly to the oral cavity of the subject . preferably , tubular portion 174 includes a luer portion corresponding to luer valve element 144 . it is appreciated that tubular portion 174 is formed on outer surface 158 of tubular portion 156 , in order to ensure that the oral oxygen delivery does not interfere with the procedure performed by the medical personnel and so that the oxygen flow does not directly interfere with the co2 sampling . a flexible barrier 176 , preferably comprised of several flaps 178 , is disposed within central opening 154 , thereby substantially closing off the central opening and preventing dilution of exhaled breath by ambient air during sampling . an opening 180 is preferably maintained within flexible barrier 176 , thereby ensuring a small part of central opening 154 to remain open in order to enable the subject to inhale external air . the flexible barrier 176 ensures that a majority of the subject &# 39 ; s orally exhaled breath will be directed toward oral prong 122 ( fig6 a and 6b ) thereby ensuring accurate sampling of the subject &# 39 ; s breath . opening 180 is preferably placed at a top part of central opening 154 near the cut - away tip 124 of oral prong 122 ( fig6 a and 6b ), thereby directing exhaled breath toward the oral prong 122 as it is the only substantial exit . the flaps 178 are preferably formed of a plastic material selected to be of suitable thickness to maintain their position when undisturbed , yet bend readily when pushed by an endoscope probe , and thus do not limit the actions of the medical personnel performing the endoscopy . however , the flaps 178 preferably close back around the endoscope probe , thus maintaining a substantially closed oral cavity volume , and allowing most of the exchange of gases to occur close to the opening 180 of the flexible barrier 176 , which opening is close to the cut - away tip 124 of oral prong 122 from which capnographic sampling can be performed accurately . additionally , the flaps 178 are preferably transparent , thus enabling medical personnel to see into the oral cavity during the endoscopy procedure . two attachment surfaces 182 , each formed with a slit 184 , extend horizontally outwardly from main body portion 152 . slits 184 are adapted to connect to a band which is place around the subject &# 39 ; s head and is used to maintain the endoscopic bite block 150 firmly in position during the endoscopy procedure . preferably , slits 184 are located above a horizontal centerline of main body portion 152 , such that the connected band will tend to exert a stronger pull to the top of the main body portion 152 , thus assisting in overcoming the subject &# 39 ; s tendency to tilt the bite block 150 outward during the endoscopy procedure and in maintaining the bite block 150 upright in the subject &# 39 ; s mouth . reference is now made to fig9 , which is a simplified schematic illustration of the connection between the oral nasal sampling cannula of fig6 a and 6b and the endoscopic bite block of fig7 a - 8 . as seen in fig9 , oral prong 122 of oral nasal sampling cannula 110 is accommodated within groove 172 of bite block 150 , such that a bottom surface of oral breath directing element 126 engages transverse surface 173 of the groove 172 . it is appreciated that transverse surface 173 is located below an inner surface of tubular portion 156 in order to ensure that air exhaled by the subject into tubular portion 156 will be directed toward groove 172 and oral prong 122 . additionally , valve 144 ( fig6 a and 6b ) of oral oxygen delivery tube 142 is accommodated in tubular portion 174 of endoscopic bite block 150 , thereby opening the valve element and forming a fluid flow engagement between oxygen delivery tube 138 and tubular portion 174 of endoscopic bite block 150 , which is in fluid flow engagement with the oral cavity of the subject . reference is now made to fig1 a , 10 b , 10 c , 10 d , 10 e , 10 f and 10 g , which are pictorial illustrations of various stages of typical use of the endoscopic bite block assembly of fig6 a - 9 . as seen in fig1 a , the nasal prongs 118 of the oral nasal sampling cannula 110 are placed in the subjects nostrils , preferably before the subject is sedated . preferably , the exhaled breath collection tube 134 and the oxygen delivery tube 138 are placed around the subject &# 39 ; s ears , in order to ensure the stability of the oral nasal sampling cannula 110 on the subject &# 39 ; s face . as seen in the enlarged portion of fig1 a , at this stage the oral breath - directing element 126 is in its retracted orientation , indicated by the length h 3 . at this stage , oral oxygen delivery tube 142 is not connected to the bite block 150 ( fig7 a - 8 ). turning to fig1 b , it is seen that the oral breath directing element 126 is extended to accommodate the facial dimensions of the subject , revealing part of oral prong 122 . preferably , the oral breath - directing element is moved down to a point in which a bottom end thereof is at the height of the top of the bottom lip of the subject , its new length being indicated by h 4 . this action is preferably preformed by medical personnel , but may alternatively be performed by the subject himself , a family member , or any other person . fig1 c illustrates the insertion of bite block 150 into the mouth of the subject , such that main body portion 152 engages the outer surface of the subject &# 39 ; s lips and the tubular portion 156 ( fig7 a - 8 ) is inside the subject &# 39 ; s mouth . additionally , valve 144 of oral oxygen delivery tube 142 is inserted , preferably by medical personnel , into tubular portion 174 of endoscopic bite block 150 , as indicated by an arrow in the enlarged portion of fig1 c , thereby opening the valve and allowing passage of fluids from the oral oxygen delivery tube 142 into the oral cavity or the subject . a strap , indicated by reference numeral 190 , is attached to slits 184 of attachment surfaces 182 and is placed around the subject &# 39 ; s head , thereby securing the bite block 150 in place . this stage is preferably performed when the subject is sedated , but may alternatively be performed prior thereto . turning to fig1 d , it is seen that air exhaled orally by the subject , indicated by arrows , passes through the bore of tubular portion 156 , and is directed toward oral breath directing element 126 and oral prong 122 by the flaps 178 of flexible barrier 176 . air that is exhaled nasally by the subject passes through nasal prongs 118 . fig1 d illustrates the oral breath directing element 126 and the oral prong 122 being accommodated in groove 172 , such that a bottom surface of the oral breath directing element 126 engages transverse surface 173 of groove 172 . additionally , if oral breath directing element 126 has been extended more than necessary for the facial features of the subject , the transverse surface 173 pushes the oral breath - directing element 126 back until it is optimally positioned . the lips of the subject , indicated by reference numeral 192 preferably engage jaw engagement recesses 164 , and the top and bottom teeth of the subject , indicated by reference numerals 194 and 196 engage top and bottom teeth engagement surfaces 160 and 162 , respectively . fig1 e illustrates the sedated subject , having the nasal prongs 118 of the oral nasal sampling cannula 110 in his nostrils and the endoscopic bite block 150 placed in his mouth and strapped to his head . preferably , once the subject is sedated , oxygen is supplied to the nose of the subject via oxygen delivery openings 136 of oral nasal sampling cannula 110 , and to the mouth of the subject via oral oxygen delivery tube 142 and tubular portion 174 , as indicated by arrows . the oxygen is supplied to the oxygen delivery openings 136 via oxygen delivery bore 116 ( fig6 a and 6b ) and to oral oxygen delivery tube 142 via oxygen delivery tube 138 and t - element 140 . turning to fig1 f , it is seen that when the subject is sedated , he tends to move or slump his head , thereby moving oral nasal sampling cannula 110 relative to bite block 150 , as indicated by angle α in the enlarged portion of fig1 f . the feature of the present invention which provides oral nasal sampling cannula 110 which is physically separated from bite block 150 and the placement of oral breath directing element 126 and oral prong 122 within groove 172 , ensure that even when the subject moves or slumps his head , the oral prong 122 and nasal prongs 118 will be maintained in their respective places , and accurate sampling will continue . additionally , the placement of oral prong 122 within groove 172 provides a counter force to force applied by the subject &# 39 ; s tongue to push at least the top portion of the bite block 150 out of the subject &# 39 ; s mouth , thus ensuring accurate placement of the bite block . as seen in fig1 g , an endoscope probe 198 is inserted into the bore of tubular portion 156 of bite block 150 , for performing the endoscopy procedure . during the insertion of endoscope probe 198 and its presence in the subject &# 39 ; s mouth and pharynx , flaps 178 of flexible barrier 176 bend slightly inward to allow the passage of the endoscope probe 198 , as seen with particular clarity in the enlarged portion of fig1 g . however , the central opening 154 of bite block 150 remains substantially closed by flaps 178 , thereby separating the exhaled breath of the subject which is in bore of tubular portion 156 from the ambient air . additionally , the sampling may continue during the presence of the endoscope probe 198 in the pharynx of the subject , as the tubular portion 156 is of a slightly larger diameter than the central opening 154 , thereby ensuring that medical personnel have the space defined by the difference between heights h 2 and h 1 ( fig8 ), as indicated by arrows in the enlarged portion of fig1 g . it is appreciated that following the endoscopy , the bite block 150 may be removed from the subject &# 39 ; s mouth , preferably by medical personnel . prior to this stage , the valve 144 of oral oxygen delivery tube 142 is removed from tubular portion 174 thereby closing the valve and thus fully decoupling the oral nasal sampling cannula 110 from the endoscopic bite block 150 . however , the sampling of exhaled breath through nasal prongs 118 which remain in the subject &# 39 ; s nostrils and through oral prong 122 which remains near the subject &# 39 ; s mouth , preferably continues until the subject has awaken from the sedation . this is necessary because the subject &# 39 ; s breath must be monitored as long as the subject is sedated . reference is now made to fig1 a and 11b , which are simplified pictorial illustrations of an oral nasal sampling cannula forming part of an endoscopic bite block assembly , constructed and operative in accordance with yet another preferred embodiment of the present invention , in retracted and extended orientations respectively . fig1 a and 11b show an oral nasal sampling cannula 210 , which is adapted for collection of gases , such as carbon dioxide , exhaled by a subject , and for supplying oxygen to the subject . the oral nasal sampling cannula 210 comprises a main body portion 212 , having formed therein an exhaled breath collection bore 214 and an oxygen delivery bore 216 . a pair of hollow nasal prongs 218 , which are adapted for insertion into the nostrils of the subject , is integrally formed with the main body portion 212 . a hollow oral prong 222 , which is formed with a limiting rib 223 and a cut - away tip 224 , is mounted onto a bottom surface of main body portion 212 . an oral breath directing element 226 , which is preferably in the shape of a cut - away tube , is slidably mounted onto oral prong 222 by a mounting portion 228 , and positioning of the oral breath directing element 226 is limited by the limiting rib 223 of oral prong 222 . a channel formed in oral prong 222 is in fluid flow connection with channels formed in nasal prongs 218 , thereby forming a single junction 232 . single junction 232 is in fluid flow communication with exhaled breath collection bore 214 , which in turn is in fluid flow communication with an exhaled breath collection tube 234 , which is adapted to be connected to a breath test analyzer or a capnograph ( not shown ), such as microcap ® which is commercially available from oridion medical ltd . of jerusalem , israel . main body portion 212 is formed with oxygen delivery openings 236 , which are in fluid flow communication with oxygen delivery bore 216 , which in turn is in fluid flow communication with an oxygen delivery tube 238 . alternatively , at least one nasal oxygen delivery prong , adapted for insertion into the subject &# 39 ; s nostril , may be used instead of oxygen delivery openings 236 . oxygen delivery tube 238 is preferably formed with a t - element 240 , preferably terminating at an end thereof in a normally closed valve element 244 , which is preferably a luer valve . oxygen delivery tube 238 is adapted to be connected to a source of oxygen ( not shown ). oxygen delivery tube 238 and exhaled breath collection tube 234 may optionally be placed around the ears of the subject , thereby stabilizing the oral nasal sampling cannula 210 on the subject &# 39 ; s face , such that any movement of the subject will have negligible effect on the placement of the oral nasal sampling cannula 210 . it is appreciated that oral breath directing element 226 may be in a retracted orientation as shown in fig1 a , or in an extended orientation as shown in fig1 b , thereby allowing the oral nasal sampling cannula 210 to be suited to the facial dimensions of the subject , resulting in more efficient collection of exhaled breath . reference is now made to fig1 a and 12b , which are front - view and rear - view simplified pictorial illustrations of an endoscopic bite block forming part of an endoscopic bite block assembly constructed and operative in accordance with yet another preferred embodiment of the present invention and to fig1 , which is a simplified sectional pictorial illustration thereof . fig1 a , 12 b and 13 show an endoscopic bite block 250 , which is adapted to be inserted into the mouth of a subject while the subject is sedated , to ensure that the mouth of the subject is maintained open during the endoscopy process , and that the subject does not interfere with the process by biting on medical instruments used . the endoscopic bite block 250 includes a main body portion 252 , having formed therein a central opening 254 . a hollow tubular portion 256 extends distally from main body portion 252 , such that the opening of tubular portion 256 is an extension of central opening 254 . central opening 254 is of a first height , indicated by h 1 in fig1 , which is typically 16 to 20 mm in bite blocks for adult use , which is the height required by medical personnel for performing an endoscopy . in order to ensure that during breath sampling , oral prong 222 of oral nasal sampling cannula 210 ( fig1 a and 11b ) does not interfere with the space required by medical personnel for performing the endoscopy procedure , the height of tubular portion 256 is greater than the height h 1 of central opening 254 as indicated by h 2 in fig1 , and is typically 2 to 4 mm more than height h 1 ( 18 to 24 mm ). an outer surface 258 of tubular portion 256 is formed with top and bottom teeth engagement surfaces 260 and 262 , such that top teeth engagement surface 260 is relatively forward of bottom teeth engagement surface 262 . this structure facilitates easy and accurate biting of the bite block 250 by a subject , as it is suited to the jaw morphology of a closed human mouth . surface 258 is additionally formed with jaw engagement recesses 264 , which are formed forwardly of teeth engagement surfaces 260 and 262 , respectively . a top inner surface 270 of main body portion 252 is formed with a longitudinal groove 272 having a transverse surface 273 , which is adapted to accommodate oral prong 222 and oral breath directing element 226 of the oral nasal sampling cannula 210 ( fig1 a and 12b ), as described with more detail hereinbelow with reference to fig1 . a tubular portion 274 is formed on a side of outer surface 258 of tubular portion 256 . extending out of tubular portion 274 is an oral oxygen delivery tube 275 including a tip 276 , which is adapted to engage valve 244 ( fig1 a and 11b ), thereby supplying oxygen directly to the oral cavity of the subject . preferably , tip 276 comprises a luer corresponding to luer valve 244 . it is appreciated that tubular portion 274 is formed on outer surface 258 of tubular portion 256 , in order to ensure that the oral oxygen delivery does not interfere with the procedure performed by the medical personnel . a flexible barrier 277 , preferably comprised of several flaps 278 , is disposed within central opening 254 , thereby substantially closing off the central opening and preventing dilution of exhaled breath by ambient air during sampling . an opening 280 is preferably maintained within flexible barrier 277 , thereby ensuring a small part of central opening 254 remains open in order to enable the subject to inhale external air . the flexible barrier 277 ensures that a majority of the subject &# 39 ; s orally exhaled breath will be directed toward oral prong 222 ( fig1 a and 11b ) thereby ensuring accurate sampling of the subject &# 39 ; s breath . opening 280 is preferably placed at a top part of central opening 254 near the cut - away tip 224 of oral prong 222 ( fig1 a and 11b ), thereby directing and amplifying exhaled breath toward the oral prong 222 as it is the only substantial exit . the flaps 278 are preferably formed of a plastic material selected to be of suitable thickness to maintain their position when undisturbed , yet bend readily when pushed by an endoscope probe , and thus do not limit the actions of the medical personnel performing the endoscopy . however , the flaps 278 preferably close back around the endoscope probe , thus maintaining a substantially closed oral cavity volume , and allowing most of the exchange of gases to occur close to the opening 280 of flexible barrier 277 , which opening is close to the cut - away tip 224 of oral prong 222 ( fig1 a and 11b ) from which capnographic sampling can be performed accurately . additionally , the flaps 278 are preferably transparent , thus enabling medical personnel to see into the oral cavity during the endoscopy procedure . two attachment surfaces 282 , each formed with a slit 284 , extend horizontally outwardly from main body portion 252 . slits 284 are adapted to connect to a band which is placed around the subject &# 39 ; s head and is used to maintain the endoscopic bite block 250 firmly in position during the endoscopy procedure . preferably , slits 284 are located above a horizontal centerline of main body portion 252 , such that the connected band will tend to exert a stronger pull to the top of the main body portion 252 , thus assisting in overcoming the subject &# 39 ; s tendency to tilt the bite block 250 outward during the endoscopy procedure and in maintaining the bite block 250 upright in the subject &# 39 ; s mouth . reference is now made to fig1 , which is a simplified schematic illustration of the connection between the oral nasal sampling cannula of fig1 a and 11b and the endoscopic bite block of fig1 a - 13 . as seen in fig1 , oral prong 222 of oral nasal sampling cannula 210 is accommodated within groove 272 of bite block 250 , such that a bottom surface of oral breath directing element 226 engages transverse surface 273 of the groove 272 . it is appreciated that transverse surface 273 is located below an inner surface of tubular portion 256 in order to ensure that air exhaled by the subject into tubular portion 256 will be directed toward groove 272 and oral prong 222 . additionally , tip 276 of oral oxygen delivery tube 275 engages valve 244 ( fig1 a and 11b ) of t - element 240 of oral nasal sampling cannula 210 , thereby opening the valve 244 and forming a fluid flow engagement between oxygen delivery tube 238 and tubular portion 274 of endoscopic bite block 250 , which is in fluid flow engagement with the oral cavity of the subject . reference is now made to fig1 a , 15 b , 15 c , 15 d , 15 e , 15 f and 15 g , which are pictorial illustrations of various stages of typical use of the endoscopic bite block assembly of fig1 a - 14 . as seen in fig1 a , the nasal prongs 218 of the oral nasal sampling cannula 210 are placed in the subjects nostrils , preferably before the subject is sedated . preferably , the exhaled breath collection tube 234 and the oxygen delivery tube 238 are placed around the subject &# 39 ; s ears , in order to ensure the stability of the oral nasal sampling cannula 210 on the subject &# 39 ; s face . as seen in the enlarged portion of fig1 a , at this stage the oral breath - directing element 226 is in its retracted orientation , indicated by the length h 3 . at this stage , oral oxygen delivery tube 275 ( fig1 a - 13 ) is not connected to the t - element 240 of oral nasal sampling cannula 210 . however , even if oxygen is supplied to oral nasal sampling cannula 210 via oxygen delivery tube 238 , there is no oxygen leakage , as the t - element 240 is sealed by valve 244 . turning to fig1 b it is seen that the oral breath directing element 226 is extended to accommodate the facial dimensions of the subject , revealing part of oral prong 222 . preferably , the oral breath - directing element 226 is moved down to a point in which a bottom end thereof is at the height of the top of the bottom lip of the subject , its new length being indicated by h 4 . this action is preferably preformed by medical personnel , but may alternatively be performed by the subject himself , a family member , or any other person . fig1 c illustrates the insertion of bite block 250 into the mouth of the subject , such that main body portion 252 engages the outer surface of the subject &# 39 ; s lips and the tubular portion 256 is inside the subject &# 39 ; s mouth . additionally , tip 276 of oral oxygen delivery tube 275 is inserted , preferably by medical personnel , into valve 244 of t - element 240 of oral nasal sampling cannula 210 , as indicated by an arrow in the enlarged portion of fig1 c , thereby opening the valve 244 . a strap , indicated by reference numeral 290 , is attached to slits 284 of attachment surfaces 282 and is placed around the subject &# 39 ; s head , thereby securing the bite block 250 in place . this stage is preferably performed when the subject is sedated , but may alternatively be performed prior thereto . turning to fig1 d , it is seen that air exhaled orally by the subject , indicated by arrows , passes through the bore of tubular portion 256 , and is directed toward oral breath directing element 226 and oral prong 222 by the flaps 278 of flexible barrier 277 . air that is exhaled nasally by the subject passes through nasal prongs 218 . fig1 d illustrates the oral breath directing element 226 and the oral prong 222 being accommodated in groove 272 , such that a bottom surface of the oral breath directing element 226 engages transverse surface 273 of groove 272 . additionally , if oral breath directing element 226 has been extended more than necessary for the facial features of the subject , the transverse surface 273 pushes the oral breath - directing element 226 back until it is optimally positioned . the lips of the subject , indicated by reference numeral 292 preferably engage jaw engagement recesses 264 , and the top and bottom teeth of the subject , indicated by reference numerals 294 and 296 engage top and bottom teeth engagement surfaces 260 and 262 , respectively . fig1 e illustrates the sedated subject , having the nasal prongs 218 of the oral nasal sampling cannula 210 in his nostrils and the endoscopic bite block 250 placed in his mouth and strapped to his head . preferably , once the subject is sedated , oxygen is supplied to the nose of the subject via oxygen delivery openings 236 of oral nasal sampling cannula 210 , and to the mouth of the subject via oral oxygen delivery tube 275 and tubular portion 274 , as indicated by arrows . the oxygen is supplied to the oxygen delivery openings 236 via oxygen delivery bore 216 ( fig1 a and 11b ) and to oral oxygen delivery tube 275 via oxygen delivery tube 238 and t - element 240 . turning to fig1 f , it is seen that when the subject is sedated , he tends to move or slump his head , thereby moving oral nasal sampling cannula 210 relative to bite block 250 , as indicated by angle α in the enlarged portion of fig1 f . the feature of the present invention which provides oral nasal sampling cannula 210 which is physically separated from bite block 250 and the placement of oral breath directing element 226 and oral prong 222 within groove 272 , ensure that even when the subject moves or slumps his head , the oral prong 222 and nasal prongs 218 will be maintained in their respective places , and accurate sampling will continue . additionally , the placement of oral prong 222 within groove 272 provides a counter force to force applied by the subject &# 39 ; s tongue to push at least the top portion of the bite block 250 out of the subject &# 39 ; s mouth , thus ensuring accurate placement of the bite block . as seen in fig1 g , an endoscope probe 298 is inserted into the bore of tubular portion 256 of bite block 250 , for performing the endoscopy procedure . during the insertion of endoscope probe 298 and its presence in the subject &# 39 ; s mouth and pharynx , flaps 278 of flexible barrier 277 bend slightly inward to allow the passage of the endoscope probe 298 , as seen with particular clarity in the enlarged portion of fig1 . however , the central opening 254 of bite block 250 remains substantially closed by flaps 278 , thereby separating the exhaled breath of the subject which is in the bore of tubular portion 256 from the ambient air . additionally , the sampling may continue during the presence of the endoscope probe 298 in the pharynx of the subject , as the tubular portion 256 is of a slightly larger diameter than the central opening 254 , thereby ensuring that medical personnel have the space defined by the difference between heights h 2 and h 1 ( fig1 ), as indicated by arrows in the enlarged portion of fig1 g . it is appreciated that following the endoscopy the bite block 250 may be removed from the subject &# 39 ; s mouth , preferably by medical personnel . prior to this stage , the tip 276 of oral oxygen delivery tube 275 is removed from valve 244 ( fig1 a and 11b ) of t - element 240 , thereby closing the valve and fully decoupling the oral nasal sampling cannula 210 from the endoscopic bite block 250 . however , the sampling of exhaled breath through nasal prongs 218 which remain in the subject &# 39 ; s nostrils and through oral prong 222 which remains near the subject &# 39 ; s mouth , preferably continues until the subject has awaken from the sedation . this is necessary because the subject &# 39 ; s breath must be monitored as long as the subject is sedated . it is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove . rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art .