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 xe2x80x9coral/nasalxe2x80x9d 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""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""s condition, such as the state of the patient""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 xe2x80x9cvoid volumexe2x80x9d, or space in which mixing of gases and concurrent alteration of the gas waveform, can occur. Such space is often referred to as xe2x80x9cvoid volumexe2x80x9d 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 xe2x80x9cDescription of the Preferred Embodimentsxe2x80x9d, 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 xe2x80x9crespiratory cavityxe2x80x9d 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 xe2x80x9ccloudsxe2x80x9d near the nostrils of the patient will cause such xe2x80x9ccloudsxe2x80x9d 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.
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 xe2x80x9cattachedxe2x80x9d is defined as connected to, or integrally formed with. Hereinafter, the term xe2x80x9cconnected xe2x80x9d is defined as communicating with. Hereinafter, the term xe2x80x9cprongxe2x80x9d refers to a hollow tube with two openings, one at each end of the tube.