IL145821A - Oral-nasal cannula - Google Patents

Description

145821/2 145821 p'Ji I 453527 τηκ rpQiD no'vyj «ITO>N!? πα ORAL BREATH COLLECTION DEVICE ORIDION MEDICAL LTD.

C: 43181 FIELD OF THE INVENTION The present invention relates to the field of oral-nasal cannulae for the sampling of breath for analysis, and especially the design of the oral prong to ensure efficient oral breath sampling.

BACKGROUND OF THE INVENTION A number of different types of oral-nasal cannulae exist, for the purpose of sampling the breath exhaled from a subject, for transfer to a capnograph or a gas analyzer to determine the breath composition. In European Patent No. EP 0 933 094 B i , for "Oral/nasal Cannula" by the present applicants, and hereby incorporated by reference in its entirety, there are described a number of prior art oral/nasal cannulae. In that patent there is also described a novel oral nasal cannula, which is the invention subject of that patent . That novel oral/nasal cannula overcomes many of the disadvantages of previous cannula, by virtue of its constructional features, whereby the nasal prongs, the oral prong, and the collection tube are connected at a single junction such that the exhaled gases flow freely from the nasal prongs and the oral prong to the collection tube. Without such free flow, the waveform of the exhaled breath would be distorted during passage of the exhaled breath from the subject to the capnographic gas analyzer, and inaccurate capnographic analysis would result.

The structure of the oral/nasal cannula of EP 0 933 094 is designed to eliminate a significant problem with certain prior art cannulae, namely the susceptibility of the cannula functionality to the intake of ambient air through that part of the cannula which is not receiving exhaled air. For example, if the subject exhales through the nasal cavity, ambient air can be sucked into prior art cannulae through the opening provided for the oral cavity, and vice versa. According to the disclosure of EP 0 933 094, a combination of the presence of a single small junction, and the bending of the distal end of the oral prong prevents ambient air from entering that portion of the cannula which is not receiving exhaled air from the subject.

According to EP 0 933 094, the distal end of the oral prong is bent, preferably at an approximately 90° angle from the remainder of the oral prong, so as to be substantially parallel to the direction of flow of orally exhaled gases from the subject. It is suggested that the oral and nasal prong diameters should be such as to promote rapid and easy removal of condensation from the interior of the cannula, yet not so large as to increase the response time. An optimal diameter for both nasal prongs and oral prong is proposed in the range from about 1.6 mm to about 2.0 mm.

The nasal prongs of the novel oral/nasal cannula of EP 0 933 094 fit into the subject's nostrils, and their diameter is only slightly smaller than the nostril openings, such that good breath collection is obtained via the nasal prongs. However, the proposed diameter of the oral probe is substantially smaller than the typical oral opening, and, unlike the nasal prongs, the end of the oral prong is not inserted into the oral opening. Consequently, despite all of the design aspects built into the cannula of EP 0 933 094, which are said to improve its performance over earlier prior art cannulae, it still may not adequately prevent mixing of ambient gases with the exhaled gases being sampled. This may be compounded because of the inability of the flow of the exhaled gases to overcome the effect of the vacuum used by the gas analyzer for sucking in samples from the collection tube.

There is therefore an important need for an improved oral/nasal cannula which will provide better sampling conditions than the prior art cannula, both that described in EP 0 933 094, and all of the earlier prior art cannulae described therein.

The disclosures of all publications mentioned in this section and in the other sections of the specification, and the disclosures of all documents cited in the above publications, are hereby incorporated by reference.

SUMMARY OF THE INVENTION The present invention seeks to provide a new oral/nasal cannula, having improved construction, such that the subject's breath is more reliably analyzed capnographically than with prior art cannulae. This novel oral/nasal cannula is different from any described in the prior art in that the bent end of the oral prong is enlarged to the shape of a funnel or a horn, such that the cross section of the oral prong at its distal end is a better match to the cross section to the oral cavity, and hence collects a more representative flow of the oral breath sample than the oral prong of prior art cannulae.

An important factor in the prevention of the ingress of ambient air into the sampled breath is thought to be the level of increased internal pressure generated by the breath intake in the cannula tubes, and particularly at the single junction. For this single junction to operate optimally, the positive pressure generated by the exhaled breaths from at least one of the oral or nasal prongs must be higher than the vacuum generated by the gas analyzer. A good level of positive pressure from either one of the oral or the pair of nasal prongs thus generates a positive pressure at the junction, effectively preventing significant ambient air leakage into the other of the two sets of prongs. The prior art oral prong, even when bent to effectively face the stream of exhaled breath, does not have a sufficient cross-section or diameter to provide a positive internal pressure large enough to overcome the sampling vacuum, and as a result, ambient air from the nasal region may be sucked in. This is particularly problematic when the subject is not breathing through his nose, or if one or both of the nares are blocked, such that the nasal prongs are situated in a blind cavity, and would readily ingest ambient air from around the nostril if there were not sufficient positive pressure at the junction to provide a net outflow from the nasal prongs.

When breathing nasally, the overpressure from the exhaled nasal breaths is generally sufficient to force exhaled air into the collection tube, and any excess out of the oral prong. The funnel-shaped end of the oral prong, according to the various embodiments of the present invention, is believed to achieve the desired equivalent effect by concentrating the collection of exhaled breath from an area more comparable with the oral opening than that of prior art oral prongs, and by forcing this breath into a smaller diameter tube, thus increasing the pressure of the gas. The internal pressure thus generated should be sufficient to ensure that the oral breath sample flows into the collection tube and any excess into the nasal prong, so preventing inflow of ambient air from the nasal region. In this way, the oral/nasal cannula according to the various preferred embodiments of the present invention, increases the reliability and accuracy of the breath collected for analysis by a capnographic gas analyzer.

There is thus provided in accordance with a preferred embodiment of the present invention, an oral/nasal cannula for collection of exhaled gases from a subject having nostrils and an oral cavity, comprising two nasal prongs for insertion into the nostrils of the subject, an oral prong for placement near the oral cavity of the subject, and a collection tube for the collection of the exhaled gases from the subject, the nasal prongs, the oral prong and the collection tube being connected at a single junction configured to be located substantially near the nostrils of the subject, wherein the oral prong features a distal end having a cross section substantially larger than the cross section of the oral prong at its proximal end.

In the above described oral/nasal cannula, the oral prong may preferably be smoothly curved at an angle such that its distal end is directed towards the oral cavity of the subject. Furthermore, the distal end cross section may be constructed to be substantially larger than the cross section of the oral prong at its proximal end by means of a taper in the oral prong. 5 145821/2 There is further provided in accordance with yet another preferred embodiment of the present invention, an oral prong for collecting samples of breath exhaled from the oral cavity of a subject, comprising an input aperture end adapted to be positioned opposite the oral cavity of the subject, and a junction at another end for connection to a pair of nasal prongs and a collection tube, wherein the cross section of the input aperture end is substantially larger than the cross section of the other end. The input aperture end of the oral prong may be runnel shaped.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: Figs. 1A and IB schematically illustrate a prior art oral/nasal cannula, such as that described in of EP 0 933 094. Fig. 1 A is an isometric view of the complete prior art oral/nasal cannula, while Fig. IB is a side view of the nasal and oral prongs of this prior art cannula; Fig. 2 is a schematic drawing of an oral/nasal cannula, constructed and operative according to a preferred embodiment of the present invention; and Figs. 3A, 3B and 3C are schematic cross-sectional shapes of the distal end of the oral prong of oral/nasal cannulae, according to a typical embodiment used in prior art oral/nasal cannulae (Fig. 3 A), and according to two different preferred embodiments of the present invention (Figs. 3B and 3C).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is now made to Figs. 1A and IB, which illustrate schematically a prior art oral/nasal cannula, such as that described in of EP 0 933 094. Fig. 1A is an isometric view of the complete prior art oral/nasal cannula, showing the single junction 10, the collection tube 12 leading to the gas analyzer, the nasal prongs 14, and the oral prong 16. Fig. IB is a side view of the nasal and oral prongs of this prior art cannula, showing the 90° bend 22 at the distal end of the oral prong 16 opposite the subject's oral cavity 20. From Fig. IB, it is apparent that the efficiency of collection of oral exhaled breath is limited compared to that of nasal exhaled breath, because of the large disparity in size between the subject's oral cavity 20, and the diameter of the oral prong 16. Furthermore, the 90° bend 22 at the distal end of the oral prong 16 is likely to impede the free flow of the exhaled oral breath down the oral prong, and to reduce the pressure generated.

Reference is now made to Fig. 2, which is a schematic drawing of the nasal prongs 32' and the oral prong 34 of an oral/nasal cannula 30, constructed and operative according to a preferred embodiment of the present invention. For the sake of clarity, the position of the collection tube 36 is shown only in dotted outline form. All of these tubes are connected together at a single junction 38. Towards its distal end, the oral prong widens to a horn shaped ending 40, whose cross section is cut such that it faces the subject's oral cavity. In addition, the oral prong preferably has a smooth curve, operative to position the cross section at the correct angle opposite the exhaled breath from the subject's oral cavity, yet without introducing any sharp bends in the oral prong. Though the distal end of the oral prong in the embodiment of Fig. 2 is shown as an angled cut in its gradually widening section, it is understood that the invention will be equally operable using an oral prong with a funnel-shaped end, a cone-shaped end, a horn-shaped end, a trumpet-shaped end, or any similar such type of end, positioned with its wide end opposite the oral cavity of the subject.

Reference is now made to Figs. 3 A, 3B and 3C, which are schematic cross-sectional shapes of the distal end of the oral prong of an oral/nasal cannula, according to a typical embodiment used in prior art oral/nasal cannulae, and according to preferred embodiments of the present invention. Calculations are made of the sampling efficiency of the shapes shown, in order to compare the performance of the oral/nasal cannula of the present invention with prior art oral/nasal cannulae. The diameter of all the oral prongs tested, at a position remote from their distal ends is 2.5 mm.

Fig. 3A shows a typical prior art oral prong ending, such as that shown in Fig. IB. The diameter is 2.5 mm for its entire length.

Fig. 3B shows an oral prong ending according to a preferred embodiment of the present invention, having an input aperture widened to an elliptical shape of dimensions 3.5mm x 5mm.

Fig. 3C shows an oral prong ending according to another preferred embodiment of the present invention, having an input aperture widened to an elliptical shape of dimensions 5mm x 8mm.

Reference is now made to table I below, which shows the results of breath sampling tests performed on the above three embodiments of oral prongs. The oral prongs were parts of oral/nasal cannulae such as that shown in Fig. 2. The tests were performed with a healthy adult subject. Since the efficiency of the oral prong was tested, the subject was constrained to breathing only through his mouth, by means of a nasal clamp. The nasal prongs were however open, in order that the oral/nasal cannula operate in its normal mode. The measurements were performed with three different openings of the lips, 5mm, 10mm, and 15mm, in order to compare the efficiency of the oral prongs under different types of oral breathing. The oral nasal cannula used for these tests had a distance from the nostrils to the center line of the mouth of 19 mm.

The tests were performed by measuring the difference between the carbon dioxide concentration obtained using each prong, and the true carbon dioxide in the exhaled breath, as obtained from a separate reference probe tube inserted well into the oral cavity, such that there can be no dilution from the ambient air. As is seen from table 1, with the prior art oral prong of Fig. 3 A, significant differences were found to exist, even of up to 30% with a fully opened mouth, except when the mouth was almost closed, with a lip opening of 5 mm. The oral prong, according to the preferred embodiment of the present invention, shown in Fig. 3B, showed better results, with an error of only 8% in the measured carbon dioxide level for a mouth with a 15 mm. lip opening. The oral prong, according to the preferred embodiment of the present invention, shown in Fig. 3C, showed results almost identical to those of the reference measurements, within the accuracy of the experimental methods used. The maximum error measured was only 3.6%, and this was for a mouth with a 10 mm. lip opening.

Table I 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. 145821/2 9

Claims (5)

CLAIMS We claim:

1. · An oral/nasal cannula for collection of exhaled gases from a subject having nostrils and an oral cavity, comprising: two nasal prongs for insertion into the nostrils of the subject; an oral prong for placement near the oral cavity of the subject; and a collection tube for the collection of the exhaled gases from the subject, said nasal prongs, said oral prong and said collection tube being connected at a single junction configured to be located substantially near the nostrils of the subject; wherein said oral prong features a distal end having a cross section substantially larger than the cross section of said oral prong at its proximal end.

2. An oral/nasal cannula according to claim 1 and wherein said oral prong is smoothly curved at an angle such that its distal end is directed towards the oral cavity of the subject.

3. An oral/nasal cannula according to claim 1 and wherein said distal end cross section is made to be substantially larger than the cross section of said oral prong at its proximal end by means of a taper in said oral prong.

4. An oral prong for collecting samples of breath exhaled from the oral cavity of a subject, comprising an input aperture end adapted to be positioned opposite the oral cavity of said subject, and a junction at another end for connection to a pair of nasal prongs and a collection tube, wherein the cross section of said input aperture end is substantially larger than the cross section of said other end. 145821/2 10

5. An oral prong according to claim 4, and wherein said input aperture end is funnel shaped. Patent Attorneys