Abstract:
A nasal/oral cannula for the collection of exhaled gases from the nostrils of a patient, made up of two nasal prongs for insertion into the patient&#39;s nostrils and a collection tube for the collection of the exhaled gases, the nasal prongs and the collection tube being connected at a single junction, such that the exhaled gases flow freely from the nasal prongs to the collection tube. An oral prong can also be provided, whose end is placed near the oral cavity of the patient, the oral prong too being connected at the single junction of the nasal prongs and the collection tube.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a nasal cannula and to an oral/nasal cannula, and, more particularly, to a nasal cannula and an oral/nasal cannula which permits both delivery of oxygen and accurate sampling of carbon dioxide. 
     For purposes of description, the discussion herein is focused on cannulas for use with human patients, it being understood that the present invention is not limited in scope only to use with patients and can beneficially be used in various other contexts. 
     Different types of oral/nasal cannulas are used to deliver oxygen to hospital patients who require assistance to breathe properly, to collect carbon dioxide samples from patients to monitor respiration, or to perform both functions. Such cannulas are used when direct ventilation is not provided. The term “oral/nasal” refers to the adaptable configuration of such cannulas which can be in close proximity to the oral cavity or inserted into the nasal cavity of the patient. In either arrangement, a sidestream of the patient&#39;s exhaled breath flows through the cannula to a gas analyzer to be analyzed. The results of this non-invasive analysis provide an indication of the patient&#39;s condition, such as the state of the patient&#39;s pulmonary perfusion, respiratory system and metabolism. 
     The accuracy of this non-invasive analysis of exhaled gases depends on the ability of a sampling system to move a gas sample from the patient to the gas analyzer while maintaining a smooth, laminar flow of gases, such that there are as few alterations to the waveform and response time of the concentration of the gases as possible. The waveform of the concentration of the gas is critical for accurate analysis. As the gas mixtures travels from the patient to the gas analyzer, the concentration of the gases can be affected by mixing of the component gases, which reduces the accuracy of the analysis of the sample by the gas analyzer, and reduces the amount of information obtained from that analysis. 
     Prior art nasal or oral/nasal cannulas unfortunately have caused significant alterations to these important features of the internal structure of the stream of exhaled gases. Such alterations have especially arisen as the result of attempts to combine the delivery of oxygen with the sampling of the exhaled breath of the patient. For example, the simplest nasal cannula design, consisting of a tube with two double hollow prongs for insertion into the nostrils, allows significant mixing of the oxygen which is delivered from the end of one tube, and the exhaled breath which is collected from the end of the second tube. Such mixing occurs when oxygen is delivered in a stream with strong force, so that the oxygen stream penetrates deeply into the nasal cavity even during expiration, thereby artifactually altering the composition of the exhaled gases. 
     However, attempts to prevent mixing between delivered oxygen and exhaled gases have resulted in other alterations to the exhaled gases. For example, one type of prior art nasal cannula (Salter Labs, Arvin, Calif. USA) consists of a tube with two openings at either end, and two hollow prongs projecting perpendicularly from the center of the tube with a partition between them. Oxygen enters the tube from one end and exhaled breath leaves the tube from the other end. The two hollow prongs are inserted into the nasal cavity of a patient, one prong in each nostril, so that oxygen could be delivered to, and exhaled breath collected from, the patient. Unfortunately, the reliance of this cannula on a single nasal prong for collection of exhaled gases does not prevent the strong flow of delivered oxygen from the other nostril mixing with exhaled gases deep in the nasal cavity, above the nasal septum. Such mixing of delivered oxygen with exhaled gases reduces the accuracy of gas analysis. 
     In addition, this type of cannula usually has significant “void volume”, or space in which mixing of gases and concurrent alteration of the gas waveform, can occur. Such space is often referred to as “void volume” because it is not part of the pathway for the flow of gases and hence is unproductive. For example, void volume arises in this cannula between the septum dividing the main tube and the junction of each prong with that tube. The presence of such void volume is a significant hindrance to the accurate analysis of exhaled gases. Thus, this prior art nasal cannula has a reduced efficiency for the collection of exhaled gases for analysis. 
     Another design for a nasal cannula (Hospitak, Lindenhurst, N.Y., USA) has two parallel overlapping tubes, one for delivering oxygen and one for receiving exhaled gases. The tube which receives exhaled gases has two nasal prongs, while the tube which delivers oxygen has two holes parallel to these prongs. Both tubes have two holes, such that the gases can flow freely from the prongs to the holes. This configuration allows delivered oxygen to easily mix with expired gases, even at the end of the expiration period, thereby reducing the accuracy of the gas analysis. 
     U.S. Pat. No. 5,046,491 discloses another type of nasal cannula which also includes a first tube with two double nasal prongs and a septum placed between the prongs. One prong delivers oxygen and the second prong collects exhaled gases. A second tube is attached to the first tube and has two holes which are placed in or near the oral cavity of the patient for collecting exhaled breath. One problem with this cannula is that the exhaled gases are collected through two outputs, which are then connected to two separate tubes. These separate tubes then join together before delivering the gases to the capnograph. If gases are not flowing at exactly the same rate through both tubes, for example due to condensation, then the waveform of the gas concentration is altered and the results of the analysis are affected. In addition, this cannula has significant void volume because of the large dimension of the tubes and because there are two outputs for collecting the exhaled gases. The large void volume also causes mixing of the gases. Thus, the cannula of U.S. Pat. No. 5,046,491 does not solve the prior art problems for accurate gas analysis by nasal cannulas. 
     Furthermore, none of these prior art cannulas is a true oral/nasal cannula, which can be placed in either the oral or nasal cavities of the patient interchangeably. Such prior art oral/nasal cannulas, which are described below in the “Description of the Preferred Embodiments”, also have significant problems regarding the collection of gases for accurate analysis, but offer the desirable feature of flexibility concerning the respiratory cavity from which exhaled gases are collected. Patients often alternately exhale through the nasal cavity and the oral cavity. The advantage of the oral/nasal cannula is that exhaled gases can be automatically collected from either cavity. The disadvantage is that many prior art oral/nasal cannulas are susceptible to the intake of ambient air through that portion of the cannula which is not receiving exhaled air. For example, if the patient exhales through the oral cavity, ambient air can be sucked into the cannula through the opening provided for the nasal cavity. Such ambient air can dilute the concentration of gas in the exhaled breath of the patient, thus giving misleading results for the gas analysis. 
     Hereinafter, the term “respiratory cavity” refers to the oral cavity, the nasal cavity, or both cavities, of a patient. 
     In addition, the effectiveness of oxygen delivery by a cannula is determined by two principles, neither of which is completely fulfilled by prior art cannulas. The first principle is that the distribution of the delivered oxygen stream should be equal between the two nostrils of the patient. In most prior art cannulas, one nostril receives 1.2-2.0 times as much oxygen as the other. However, an equal distribution of oxygen is preferably for the following reasons. First, if one of the nostrils is blocked, the second will continue to deliver oxygen. Second, even flow rates for both nostrils will not cause the patient to feel excess pressure in one nostril, even at high flow rates for the delivered oxygen. Third, producing even flow rates through the presence of oxygen “clouds” near the nostrils of the patient will cause such “clouds” to be the same size at both nostrils, and will permit the more effective use of ambient oxygen present near the nostrils before the inspiration phase. 
     The second principle is that the oxygen stream should be delivered at a relatively slow rate, rather than being forced into the nostrils at a high rate, for the following reasons. First, an oxygen stream which is delivered at a slow rate will not penetrate deeply into the nostrils of the patient and so will not be collected during the exhalation phase, thereby preventing distortion of the carbon dioxide measurements because of dilution of the exhaled gases. Second, the patient will feel more comfortable since the oxygen stream will not be so forceful. 
     If both principles are fulfilled, then oxygen delivery and analysis of exhaled gases will be optimized. Unfortunately, many prior art cannulas fail to implement these principles and are thus lacking in this respect. 
     There is thus a widely recognized need for, and it would be highly advantageous to have, a cannula which does not alter the gas waveform, which does not easily become blocked or clogged, which has minimal added void volume, and which can deliver oxygen without disturbing the waveform of exhaled gases, yet which has the flexibility and adaptability of an oral/nasal cannula. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a nasal cannula for collection of exhaled gases from a patient having nostrils, comprising: (a) two nasal prongs for insertion into the nostrils of the patient; and (b) a collection tube for the collection of the exhaled gases from the patient, the nasal prongs and the collection tube being connected at a single junction, such that the exhaled gases flow freely from the nasal prongs to the collection tube. Preferably, the collection tube is a single collection tube. Also preferably, the nasal prongs are joined in an are substantially before being connected to the junction. Preferably, the collection tube delivers the exhaled gases to a capnograph for gas analysis. 
     According to another embodiment of the present invention, there is provided a cannula for collection of exhaled gases from a patient having nostrils and an oral cavity, including: (a) two nasal prongs for insertion into the nostrils of the patient; (b) an oral prong for being located proximately to the oral cavity of the patient; and (c) a collection tube for the collection of the exhaled gases from the patient, the nasal prongs, the oral prong and the collection tube being connected at a single junction located substantially near the nostrils of the patient, such that the exhaled gases flow freely from the nasal prongs and the oral prong to the collection tube. Preferably, the collection tube is a single collection tube. Also preferably, the oral prong features a distal portion, the distal portion being bent at an angle. More preferably, the angle is about 90 degrees, such that the distal portion is located proximately to the oral cavity of the patient. Most preferably, the distal portion features a cap, the cap being attached to the distal portion, and the cap being made of a substantially hydrophilic material, such that the cap absorbs condensation from the distal portion. Also preferably, the nasal prongs are joined in an arc substantially before being connected to the junction. Preferably, the collection tube delivers the exhaled gases to a capnograph for gas analysis. 
     According to preferred embodiments of the present invention, the cannula further includes (d) an oxygen tube for delivery of oxygen, the oxygen tube being located near the nostrils of the patient; and (e) two oxygen inlets connected to the oxygen tube and being disposed such that the oxygen flows from the oxygen tube into the nostrils of the patient. 
     Preferably, the oxygen tube is located either above or below the nostrils of the patient. Also preferably, the oxygen tube includes a centrally located input for receiving oxygen being placed substantially equidistant from both oxygen inlets. Preferably, the oxygen inlets are holes. More preferably, the holes have an first diameter at an inner surface of the oxygen tube and the holes have a second diameter at an outer surface of the oxygen tube, the first diameter being smaller than the second diameter. Most preferably, the oxygen tube features a screen, the screen being placed within the oxygen tube such that the oxygen flows from the oxygen tube through the screen. Preferably, the screen is constructed of a material selected from the group consisting of a hydrophobic porous material, a wide mesh and a netting. 
     Alternatively and preferably, the inlets are oxygen prongs for being inserted into the nostrils of the patient. More preferably, the oxygen prongs are substantially shorter in length than the nasal prongs, such that the nasal prongs extend farther into the nostrils than the oxygen prongs. Also more preferably, the oxygen prongs are formed of a substantially porous material, such that the oxygen prongs are permeable to gases. Most preferably, the oxygen prongs are formed from an inner cylinder and an outer cylinder, both cylinders being made from the substantially hydrophobic porous material, and the inner cylinder being substantially shorter in length than the outer cylinder. 
     According to other preferred embodiments of the present invention, at least a portion of the oxygen tube is formed from a substantially porous material such that the at least a portion of the oxygen tube is permeable to gases. More preferably, the at least a portion of the oxygen tube is located substantially between the oxygen prongs. 
     According to another embodiment of the present invention, there is provided a method of using the cannula of claim  1  for collecting the exhaled gases from the patient, including: (a) inserting the nasal prongs into the nostrils of the patient; (b) attaching the collection tube to a conduit for conducting gas; (c) connecting the conduit to a gas analyzer; and (d) applying a force at the gas analyzer, such that the exhaled gases flowing through the cannula moves from the collection tube to the gas analyzer. 
     According to yet another embodiment of the present invention, there is provided a cannula for collection of exhaled gases from a patient and for delivery of oxygen to a patient, the patient having nostrils and an oral cavity, including: (a) two nasal prongs for insertion into the nostrils of the patient; (b) an oral prong for being located proximately to the oral cavity of the patient; (c) a collection tube for the collection of the exhaled gases from the patient, the nasal prongs, the oral prong and the collection tube being connected at a single junction, such that the exhaled gases flow freely from the nasal prongs and the oral prong to the collection tube; (d) an oxygen tube for delivery of oxygen, the oxygen tube being located near the nostrils of the patient; and (e) two oxygen inlets connected to the oxygen tube and being disposed such that the oxygen flows from said oxygen tube into the nostrils of the patient. 
     Hereinafter, the term “attached” is defined as connected to, or integrally formed with. Hereinafter, the term “connected ” is defined as communicating with. Hereinafter, the term “prong” refers to a hollow tube with two openings, one at each end of the tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1 is an illustrative prior art oral/nasal carbon dioxide cannula: 
     FIG. 2 is an illustrative prior art double nasal oxygen/carbon dioxide cannula for oxygen delivery and collection of exhaled gases; 
     FIG. 3 is a second illustrative prior art divided nasal oxygen/carbon dioxide cannula for oxygen delivery and collection of exhaled gases; 
     FIG. 4 is an illustrative oral/nasal cannula for the collection of exhaled gases according to the present invention; 
     FIGS. 5A-5C are cross-sectional views of the cannula of FIG. 4 according to the present invention; 
     FIGS. 6A and 6B show cross-sectional views of a second illustrative embodiment of an oral/nasal cannula according to the present invention; 
     FIGS. 7A and 7B show portions of the oral/nasal cannula of FIGS. 6A and 6B in more detail, with the preferred addition of a porous screen to the oxygen tube according to the present invention; 
     FIGS. 8A and 8B show detailed cross-sectional views of portions of a third embodiment of an oral/nasal cannula with porous oxygen delivery tubes according to the present invention; and 
     FIG. 9A shows a prior art cannula for oxygen delivery, and FIGS. 9B-9C show a cannula with equal oxygen delivery to each nostril according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is of a cannula which can effectively be used to collect samples of gas without reducing the accuracy of the analysis of the collected gas, and which is less likely to become blocked by condensed moisture, or by liquid or solid material, or their mixtures thereof, such as mucous or saliva. Specifically, the present invention has two prongs for insertion into the nostrils of a patient. These two prongs are joined outside the nasal cavity to a single output tube for collection of the exhaled gases. According to preferred embodiments of the present invention, a second tube is attached to the two prongs, which is parallel to the nasal prongs, for placement of the distal end of the tube near the oral cavity of the patient, thereby providing an oral/nasal cannula. According to other preferred embodiments of the present invention, an additional tube is provided for the delivery of oxygen, the additional tube having two additional prongs for insertion into the nostrils of the patient, and the additional tube being perpendicular to the additional nasal prongs. 
     The principles and operation of an airway adapter according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Referring now to the drawings, FIG. 1 shows a prior art oral/nasal carbon dioxide cannula. A cannula  10  has two nasal prongs  12  for insertion into the nostrils of a patient (not shown). Nasal prongs  12  are connected to a first side  14  of a hollow tube  16 . Hollow tube  16  is substantially perpendicular to nasal prongs  12 . Two oral prongs  18  are also connected to a second side  20  of tube  16  in a substantially perpendicular orientation, such that gas flow from nasal prongs  12  to oral prongs  18  through tube  16  is substantially free and unimpeded. Tube  16  also has two holes  22 , one at each end of tube  16 , for connection to one of a plurality of connectors  24 . Each connector  24  is attached to a gas line (not shown) which is then connected to a Y-connector  26 . Y-connector  26  is attached to a line which leads to a capnograph (not shown). Thus, cannula  10  is suitable only for collection of exhaled gases for analysis. 
     Prior art cannula  10  unfortunately has a significant void volume  28  (also designated as V o ) between nasal prongs  12 , within which gases do not properly circulate. Two smaller void volumes  30  (also designated as V 1  and V 2 ) are also present parallel to nasal prongs  12  and oral prongs  18 . Such void volumes  28  and  30 . and especially the larger void volume  28 , permit the mixing of exhaled gases from a currently exhaled breath with previously exhaled breaths, thereby increasing the response time, altering the waveform and introducing an artifact into the gas analysis. Furthermore, the separation of the exhaled gases into two streams from nasal prongs  12  by tube  16 , the later reintegration of the two streams at Y-connector  26  and the subsequent great distance of about 0.5 m between tube  16  and Y-connector  26 , also increases the response time if there is even a slight difference in the flow rate of the gases between tubes  16 . Such separation potentially also results in two stream having different flow properties. For example, if one tube  16  accumulated more condensed water than the other, the corresponding stream of exhaled gases would have a lower flow rate, thereby altering the waveform of the gas concentrations and increasing the response time for gases in that tube  16 . Thus, prior art cannula  10  cannot provide completely accurate collection of gases for analysis. 
     FIG. 2 shows an exemplary prior art double oxygen/carbon dioxide nasal cannula for the collection of exhaled gases and the delivery of oxygen. A prior art nasal cannula  32  again has a first pair of nasal prongs  34  for insertion into nostrils  36  of a patient. First nasal prongs  34  are again connected to a first hollow tube  38 . First hollow tube  38  is again substantially perpendicular to first nasal prongs  34 . In addition, nasal cannula  32  has a second pair of nasal prongs  40  for insertion into nostrils  36 . Second nasal prongs  40  are attached to a second hollow tube  42  in a substantially perpendicular orientation. First nasal prongs  34  and first hollow tube  38  are intended for the collection of exhaled gases from the patient, in a substantially similar configuration as that shown in FIG.  1 . Second nasal prongs  40  and second hollow tube  42  are intended to deliver oxygen to the patient, so that nasal cannula  32  is capable of simultaneous oxygen delivery and gas collection. 
     Unfortunately, prior art nasal cannula  32  also permits the mixing of delivered oxygen and exhaled gases between first nasal prongs  34  and second nasal prongs  40  in nostrils  36 , thereby diluting the true concentration of expired carbon dioxide. Thus, prior art nasal cannula  32  also introduces artifacts into the analysis of expired gases. 
     Also, the efficiency of oxygen delivery by prior art nasal cannula  32  is not sufficient because the oxygen flow rate varies between nasal prongs  40 . Specifically, nasal prong  40  which is closer to the input of hollow tube  42  will have a higher flow rate than the other nasal prong  40 . In addition, the strong oxygen stream into the nostrils creates discomfort for the patient, the alleviation of which is especially important for long term oxygen delivery. 
     FIG. 3 shows a second exemplary prior art divided oxygen/carbon dioxide nasal cannula for the simultaneous delivery of oxygen and collection of exhaled gases. A prior art nasal cannula  44  has a single tube  46  for both delivery of oxygen and collection of gases. Tube  46  has two nasal prongs  48  and  50  for insertion into nostrils  52  of a patient. Oxygen is delivered through nasal prong  48  and exhaled gases are collected from nasal prong  50 . A septum  54  is present inside tube  46  between nasal prong  48  and nasal prong  50  to separate the delivered oxygen from the exhaled gases. However, particularly forceful streams of delivered oxygen can pass from nasal prong  48 , penetrate deeply into nostrils  52 , entering nasal prong  50  and dilute the true concentration of exhaled carbon dioxide. Furthermore, a significant void volume  56  is present between septum  54  and nasal prong  50 , both increasing the response time and mixing the exhaled gases, which also reduce the accuracy of the analysis of the exhaled gases. Thus, prior art nasal cannula  44  is still not able to collect gases for a completely accurate analysis. In addition, the strong oxygen stream into the nostrils creates discomfort for the patient, the alleviation of which is especially important for long term oxygen delivery. 
     FIG. 4 shows a schematic illustration of an exemplary novel oral/nasal carbon dioxide cannula for collection of exhaled gases according to the present invention. An oral/nasal cannula  58  also have a pair of nasal prongs  60  for insertion into the nostrils  62  of a patient. Cannula  58  preferably features an oral prong  64  for placement near the oral cavity of the patient (not shown) to form an oral/nasal cannula. If oral prong  64  is absent, then cannula  58  is a nasal cannula according to the present invention. Cannula  58  also has a collection tube  66  for collection of the exhaled gases for analysis by a capnograph (not shown). Nasal prongs  60 , oral prong  64  and collection tube  66  meet at a single junction  68 , which is preferably minimized to reduce void volume. Hereinafter, the term “single junction” refers to the joining of nasal prongs  60 , oral prong  64  and collection tube  66  at least in close proximity, and preferably at exactly one junction. 
     At the very least, having the single junction  68  between all portions of oral/nasal cannula  58  significantly reduces the void volume, thereby reducing mixing of the gases and maintaining the response time. In addition, having the single collection tube  66 , rather than two such tubes as in prior art cannulas, eliminates the division of the stream of exhaled gases as well as reducing the amount of void volume created. 
     A cross-sectional view of the oral/nasal cannula of FIG. 4 is shown in FIGS. 5A-5C, clearly illustrating the small void volume created within the cannula. FIG. 5A shows a front cross-sectional view of oral/nasal cannula  58 . As clearly shown in the illustration, nasal prongs  60 , oral prong  64  and collection tube  66  all meet at a single small junction  68  with a minimum void volume. In practice, the void volume can be almost completely eliminated through this configuration, because there are no poorly ventilated areas within oral/nasal cannula  58 . As shown in the illustration, a portion  70  of collection tube  66  does extend past nasal prongs  60  opposite to the collection point. However, portion  70  is blocked and is only intended to permit the attachment of a symmetrical loop which extends around the head of the patient (not shown). 
     FIG. 5B shows a side cross-sectional view of the connection between one nasal prong  60  and oral prong  64 . Preferably, a distal end  72  of oral prong  64  is bent, more preferably at approximately a 90 degree angle from the remainder of oral prong  64 , so as to be substantially parallel to the direction of flow of orally exhaled gases from the patient. Such an orientation both provides optimal response time for gas analysis and promotes self-clearing of condensation from oral/nasal cannula  58 . Furthermore, preferably nasal prongs  60  are joined in an arc, so that condensation tends to move into oral prong  64  under dynamic pressure of the nasal exhalation of gases by the patient. 
     The structure of oral/nasal cannula  58  is designed to eliminate one significant problem with certain prior art oral/nasal cannulas, which is the susceptibility of these prior art cannulas to the intake of ambient air through that portion of the cannula which is not receiving exhaled air. For example, if the patient exhales through the nasal cavity, ambient air can be sucked into the prior art cannula through the opening provided for the nasal cavity. Such ambient air can dilute the concentration of gas in the exhaled breath of the patient, thus giving misleading results for the gas analysis. The structure of oral/nasal cannula  58  reduces or eliminates this problem with the presence of single small junction  68 , and the bending of distal end  72  of oral prong  64 . The resultant structure substantially prevents ambient air from entering the portion of cannula  58  which is not directly receiving exhaled air from the patient. 
     Also preferably, nasal prongs  60  and oral prong  64  have an optimal diameter, sufficiently large to promote rapid and easy removal of condensation from the interior of nasal cannula  58 , yet not so large as to increase the response time. For this configuration, an optimal diameter for both nasal prongs  60  and oral prong  64  is in a range of from about 1.6 mm to about 2.0 mm. 
     Most preferably, distal end  72  of oral prong  64  features a porous, hydrophilic cap  74 , as shown in cross-section in FIG.  5 C. Porous hydrophilic cap  74  covers distal end  72  and absorbs water droplets formed from condensation which collects in nasal cannula  58 . The particular advantage of cap  74  is that the material of cap  74  preferably attracts water away from oral prong  64 , and then provides a relatively large surface area for evaporation of that water. Additionally, cap  74  relieves potential patient discomfort from water dripping from cannula  58  into the mouth of the patient. 
     FIGS. 6A and 6B show cross-sectional views of a second preferred embodiment of the oral/nasal cannula for oxygen delivery and gas collection of the present invention. Detailed illustrations of portions of the cannula of FIGS. 6A and 6B are shown in FIGS. 7A and 7B. FIGS. 7A and 7B also show the preferred addition of a porous screen to the oxygen tube. 
     In this preferred embodiment, as shown in FIG. 6A, an oral/nasal cannula  76  again has a pair of nasal prongs  78  for insertion into the nostrils of a patient (not shown). Cannula  76  again preferably features an oral prong  80  for placement near the oral cavity of the patient (not shown) to form an oral/nasal cannula. Cannula  76  also has a collection tube  82  for collection of the exhaled gases for analysis by a capnograph (not shown). Nasal prongs  78 , oral prong  80  and collection tube  82  again meet at a single junction  84 , which is preferably minimized to reduce void volume. 
     Although cannula  76  also features an oxygen tube  86  for lying near the nostrils of the patient (not shown) and more preferably above or below the nostrils of the patient, substantially parallel with the upper lip of the patient (not shown), oxygen is not delivered through a second set of nasal prongs. Instead, oxygen tube  86  has two holes  88 , through which oxygen is delivered to the patient. Holes  88  are placed near the nostrils of the patient yet do not enter the nostrils, thereby preventing the delivered oxygen from entering as a forceful stream of gases which dilutes the exhaled gases and reduces the accuracy of gas analysis. 
     FIG. 6B shows a side cross-sectional view of junction  84  between one nasal prong  78  and oral prong  80 , as well as a portion of oxygen tube  86 . Oxygen is shown being dispersed from oxygen tube  86  through hole  88 . 
     FIG. 7A shows holes  88  in more detail. Holes  88  preferably have a relatively large diameter. Most preferably the diameter of holes  88  increases from the inner surface of oxygen tube  86  to the outer surface of oxygen tube  86 , in order to reduce the force of the delivered oxygen stream. Holes  88  have a first smaller diameter  90  at the inner surface of oxygen tube  86 , and a second larger diameter  92  at the outer surface of oxygen tube  86 , with the diameter of holes  88  preferably gradually increasing from the inner to the outer surface of oxygen tube  86 . 
     In addition, as shown in FIG. 7A, oxygen tube  86  preferably features a screen  94  made from a substantially porous material which is permeable to oxygen, such as a wide mesh, a hydrophobic porous screen, netting or cotton wool, for example. The advantages of screen  94  are that the force of the delivered oxygen stream is reduced and an oxygen “cloud” is created near the nostrils of the patient. The combination of the dispersion of oxygen through screen  94  and hole  88  is shown in a side, cross-sectional view in FIG. 7B, which also shows junction  84 . 
     FIGS. 8A and 8B provide a detailed illustration of a portion of a third embodiment of an oral/nasal cannula according to the present invention. FIG. 8A shows a portion of an oral/nasal cannula  96 , showing a section of a pair of nasal prongs  98  for receiving exhaled carbon dioxide, an oxygen tube  100  and a pair of second nasal prongs  102 . As clearly illustrated, oxygen is delivered through oxygen tube  100  and is then dispersed through second nasal prongs  102 . 
     Preferably, second nasal prongs  102  are constructed from two cylinders, in order to ensure that oxygen is delivered to the nostrils of the patient efficiently, yet is quickly dispersed within the nasal cavity. The first cylinder is an inner cylinder  104 , preferably made from a substantially porous hydrophobic material. The material is preferably hydrophobic to prevent absorption of moisture. Inner cylinder  104  is surrounded by an outer cylinder  106 , also preferably made from a substantially porous hydrophobic material, such that oxygen is dispersed throughout the nostrils of the patient, rather than entering the nasal cavity as a highly pressurized stream of gas. 
     FIG. 8B shows a side, cross-sectional view of the portion of the cannula illustrated in FIG. 8A. A junction  108  between one nasal prong  98  and an oral prong  110  is shown, as is one second nasal prong  102  with inner cylinder  104  and outer cylinder  106 . The advantage of constructing second nasal prong  102  from a porous material is that such material would be permeable to oxygen, thereby allowing oxygen to disperse evenly from second nasal prong  102 . Such dispersion reduces the force of the delivered oxygen stream. 
     FIGS. 9A-9C show a comparison between a prior art oral/nasal cannula in which oxygen is delivered unequally to the nostrils of the patient (FIG.  9 A), and a oral/nasal cannula according to the present invention in which oxygen is delivered at equal flow rates (FIGS.  9 B and  9 C). FIG. 9A shows a cross-sectional view of the oxygen-delivery portion of a typical prior art oral/nasal cannula  112 . Prior art cannula  112  has an oxygen delivery tube  114  for delivery oxygen to two outputs  116  and  118 . Outputs  116  and  118  could be holes or nasal prongs as shown previously. The problem with this configuration is that oxygen is not distributed evenly between both outputs  116  and  118 . Output  116 , which is closest to the start of oxygen delivery tube  114 , has a greater flow of oxygen than output  118 , as indicated by the arrows. Such a situation arises because of the resistance of outputs  116  and  118  to the flow of oxygen is much lower than the resistance of the connecting portion of oxygen delivery tube  114 . 
     FIG. 9B shows a cross-sectional view of the oxygen-delivery portion of a first exemplary oral/nasal cannula  120  according to the present invention. First cannula  120  has an oxygen delivery tube  122  for delivery oxygen to two sets of outputs  124  and  126 . Each set of outputs  124  and  126  includes at least two outputs, although three are shown here for illustrative purposes, without any intention of being limiting. Again, the outputs could be holes, with a porous screen, or nasal prongs as shown previously. The advantage of this configuration is that oxygen is distributed more evenly between both sets of outputs  124  and  126 . Such a situation arises because the resistance of both sets of outputs  124  and  126  to the flow of oxygen is much greater than the resistance of the connecting portion of oxygen delivery tube  122 . 
     FIG. 9C shows a cross-sectional view of the oxygen-delivery portion of a second exemplary oral/nasal cannula  128  according to the present invention. Second cannula  128  has an oxygen delivery tube  130  for delivery oxygen to two sets of outputs  132  and  134 . Each set of outputs  132  and  134  includes at least one output, although only one is shown here for illustrative purposes, without any intention of being limiting. Again, the outputs could be holes, holes with a porous screen, or nasal prongs as shown previously. Additionally, oxygen delivery tube  130  features a centrally located input  136  for the delivery of oxygen. Preferably, centrally located input  136  is located substantially equidistantly to outputs  132  and  134 . The advantage of this configuration is that oxygen is distributed more evenly between both sets of outputs  132  and  134  even for their relatively lower resistance to air flow in comparison to the resistance of oxygen delivery tube  130 . Such a situation arises because the resistance of each output  132  and  134  to the flow of oxygen is equal. 
     TESTING OF THE ORAL/NASAL CANNULA 
     The features and embodiments illustrated herein may be better understood with reference to the experiments described below. These experiments were conducted on oral/nasal cannulas according to the present invention, as well as on examples of prior art cannulas. 
     Experimental Methods 
     The first test performed was the self-cleaning test. Self-cleaning is important for preventing the accumulation of condensed water, which can disturb the sampling of carbon dioxide. The term “V ex ” is defined as the minimal volume of expired breath required for self-cleaning of water from the cannula. 
     The second test was the response time test, performed in accordance with Regulation prEN 864:1992 (European Union standard) for capnography. All measurements were conducted on a capnograph with low flow rate of 47 ml/min. Response times (in mSec) were tested for nasal cannula blanks only, nasal cannula systems which also included the set of sample lines, and the entire capnograph set which included the nasal cannula system with a typical capnograph flow system. 
     The third test determined the accuracy of measurements of expired carbon dioxide (EtCO 2 ). Expired carbon dioxide was measured both with and without oxygen delivery. In the absence of oxygen delivery, the alteration to the true EtCO 2  caused by the influence of the response time was calculated as: 
     Δ (EtCO 2 )=EtCO 2  (True value)−EtCO 2  (with Entire Capnograph Set) 
     In the presence of oxygen delivery, the alteration to the true EtCO 2  was calculated as 
     Δ (EtCO 2 )=EtCO 2  (Q=0)−EtCO 2 (Q≠ 0)    
     The fourth test measured the effectiveness of the delivery of oxygen according to the flow distribution between the two nasal cannula oxygen delivery outputs. Oxygen was delivered at the rate of 8 L/min. The flow of oxygen from each output, given as Q 1  and Q 2 , was measured. The efficiency (K eff ) was determined according to the ratio of Q 1  to Q 2 . 
     These four tests were performed on several different types of cannulas. Three types of cannulas were obtained and tested from Salter Labs (Arvin, Calif., USA): a nasal cannula (catalog number 4000); a dual oral/nasal cannula (catalog number 4001); and divided oxygen/carbon dioxide nasal cannula (catalog number 4707). Two types of cannulas were obtained and tested from Hudson (Temecula, Calif., USA): a nasal cannula (catalog number 1103); and an oxygen/carbon dioxide nasal cannula (catalog number 1843). An oral/nasal cannula according to the present invention was also tested, in the embodiment of an oxygen/carbon dioxide oral/nasal cannula with inserts of braid or cotton wool for oxygen dispersion as shown in FIG.  7 B. Results for all tests are shown in Table 1. 
     Essentially, the cannula of the present invention performed at least as well as, any in many respects better than, the prior art cannulas. In particular, the cannulas of the present invention had a much lower response time than any of the other tested prior art cannulas. For example, without any additional connections, the cannulas of the present invention had a response time of 14, while those of the cannulas of Hudson were 97 and 47, and those of the cannulas of Salter Labs were 167, 143 and 239. Thus, clearly the cannula of the present invention had a far better response time than these tested cannulas. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.