Abstract:
A liquid separator removing a liquid from a sample of a breathing gas flowing through an airway adapter having a channel surrounded by a wall is disclosed herein. The separator includes a chamber receiving the sample, and a membrane having an outer surface exposed to the gas flow, the membrane at least partially surrounding the chamber, which membrane separates the liquid received by the chamber. The separator also includes a supporting structure for supporting the membrane, and a connector operationally attached to the supporting structure, the connector being connectable to the adapter. The connector comprises a cavity providing a flow path for the sample from the chamber through an opening of the cavity to a sample tube. The membrane branches from a central part of the channel into at least two different branches extending to different directions.

Description:
BACKGROUND OF THE INVENTION 
     This disclosure relates generally to a liquid separator for removing a liquid from a sample of a breathing gas and an airway adapter. 
     When a patient is mechanically ventilated with a conventional ventilator, an endotracheal tube is placed into a trachea so that it goes through oral or nasal cavity and larynx. The other end of the endotracheal tube is connected to a breathing circuit Y-piece through a luer type connector. If the patient is gas monitored with a sidestream gas analyzer, an airway adapter used for sampling the breathing gas that is analyzed by the gas analyzer is normally connected between connectors of the endotracheal tube and the breathing circuit Y-piece. During an inspiration the fresh breathing gas including higher oxygen (O 2 ) concentration flows into the patients lungs through an inspiratory limb of the breathing circuit Y-piece, the airway adapter, the endotracheal tube and their connectors, then to a trachea, a bronchus, a bronchi, bronchioles and finally reaching an alveoli deep in the lungs, where all the gas exchange actually occurs. Carbon dioxide (CO 2 ) molecules in hemoglobin of a blood flowing in tiny blood vessels around the alveoli are replaced with O 2  molecules in the fresh breathing gas through the thin walls of the alveoli. O 2  molecules take their place in the hemoglobin, whereas CO 2  molecules flow out from the patient within the used expired breathing gas, through the same path as the fresh gas came in during the inspiration. Thus a gas concentration of the breathing gas measured by the gas analyzer is somewhat proportional to the gas concentration in the blood. If anesthetic agents are used they flow in to the patient during inspiration and the content not adsorbed by the patient flows out from the patient during expiration, which can be monitored with a gas analyzer as well. 
     The conventional patient side part of the breathing circuit, which is also shown in  FIG. 1 , usually consists of an endotracheal tube  2  connected to a patient  1  and to a sidestream type airway adapter  3  used for sampling the breathing gas for the gas analyzing purposes and a Y-piece  4  that connects the patient side part of the breathing circuit to the ventilator  5  through breathing circuit tubing for inspiratory gas  6  and expiratory gas  7 . The gas analyzer  8  is placed further away from the patient close to or into the host device such as the ventilator  5 . The breathing gas sample withdrawn from the patient&#39;s breathing is sucked by the gas analyzer  8  from the airway adapter  3 , through a sampling port  9 , which in connection with the breathing gases, through a sampling tube  10  and through a water separation unit  11  into the gas analyzer  8  to be analyzed. The length of the sampling tube  10  may vary from 2 to 6 meters and the inner diameter of the tube may vary from 1.2 to 2 mm. The breathing gas includes close to 100% humidity, which condensates into water in the sampling tube  10 . The breathing gas may also include other liquid substances such as blood, mucus etc. that may enter the sampling tube  10 . The water separation unit  11  usually comprises a porous membrane or a similar structure that separates the water or liquid substance from the gas flowing in the tube  10 , preventing it to enter the sensitive parts inside the analyzer. 
     The inner diameter and the length of the sampling tube together with the sampling gas flow speed mostly determine the total system response time and the total system rise time of the gas analyzer. The length of the tubing is normally determined by the use environment in the hospital and is between 2 to 6 meters. It would be beneficial to have high sampling gas flow speed to decrease the total system response and rise times, but the tendency is to have the sampling gas flow speed below 200 ml/min or advantageously approximately at 50 ml/min to enable the gas monitoring of small patients whose tidal volumes may be as low as 2 ml. The advantageous choice to decrease the total system response and rise times is to decrease the diameter of the sampling tube. However as the diameter is decreased the condensed water and other liquid substances block the sampling tube easily deteriorating the measurement system rise time and increasing the measurement system response time or even preventing the whole gas analyzing as the sample gas is not allowed to enter the gas analyzer. 
     Some prior art systems may comprise a cylindrical water separation unit located partially inside the airway adapter breathing flow path and the volume inside the sample gas tube, which is in connection with the airway adapter. Such systems generate high and unwanted flow resistance to the gas flow disturbing the gas exchange in the lungs. Such systems are also position sensitive and easily get blocked by the condensed water and other liquid substances in certain positions. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
     In an embodiment, a liquid separator for removing a liquid from a sample of a breathing gas flowing through an airway adapter having a channel surrounded by a wall, the channel being configured to locate between a patient and a ventilator, includes a chamber configured to receive the gas sample of the breathing gas, and a membrane having an outer surface exposed to the breathing gas flow, the membrane at least partially surrounding the chamber and which membrane is configured to separate the liquid from the gas sample received by the chamber. The liquid separator also includes a supporting structure for supporting the membrane, and a connector operationally attached to the supporting structure and which connector is connectable to the airway adapter, the connector comprising a cavity providing a flow path for the sample gas from the chamber through an opening of the cavity to a sample tube. The membrane is configured to branch from a central part of the channel into at least two different branches, each of the at least two branches extending to different directions to obtain the sample. 
     In another embodiment, an airway adapter includes a channel surrounded by a wall for a breathing gas flow, the channel being configured to locate between a patient and a ventilator. The airway adapter also includes a first port for delivering breathing gas, and a second port for delivering breathing gas, the second port being in flow communication with the first port through the channel. The airway adapter also includes a liquid separator extending into the channel for removing a liquid from a sample of the breathing gas flowing through the channel, the liquid separator comprising a chamber configured to receive the sample of the breathing gas, and a membrane having an outer surface exposed to the breathing gas flow, the membrane at least partially surrounding the chamber and which membrane is configured to separate the liquid from the gas sample received by the chamber. The liquid separator also includes a supporting structure for supporting the membrane, and a connector operationally attached to the supporting structure, the connector comprising a cavity providing a flow path for the sample gas from the chamber through an opening of the cavity to a sample tube, the connector being operationally connected to the wall. The membrane is configured to branch from a central part of the channel into at least two different branches, each of the at least two branches extending to different directions towards the wall to obtain the sample. 
     In yet another embodiment, an airway adapter includes a channel surrounded by a wall for a breathing gas flow, the channel being configured to locate between a patient and a ventilator. The airway adapter also includes a first port for delivering breathing gas, and a second port for delivering breathing gas, the second port being in flow communication with the first port through the channel. The airway adapter also includes a liquid separator extending into the channel for removing a liquid from a sample of the breathing gas flowing through the channel, the liquid separator comprising a chamber configured to receive the sample of the breathing gas, and a membrane having an outer surface exposed to the breathing gas flow, the membrane at least partially surrounding the chamber and which membrane is configured to separate the liquid from the gas sample received by the chamber. The liquid separator also includes a supporting structure for supporting the membrane, and a connector operationally attached to the supporting structure, the connector comprising a cavity providing a flow path for the sample gas from the chamber through an opening of the cavity to a sample tube, the connector being operationally connected to the wall. The membrane is configured to branch from a central part of the channel into at least two different branches, each of the at least two branches extending to different directions towards the wall to obtain the sample, and that the branches are configured to restrict the breathing gas flow through the channel providing a signal indicative of the breathing gas flow. 
     Various other features, objects, and advantages of the invention will be made apparent to those skilled in art from the accompanying drawings and detailed description thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic view of a prior art breathing circuit; 
         FIG. 2  shows an exploded partial cross-sectional view of an airway adapter with a liquid separator in accordance with an embodiment; 
         FIG. 3  shows a cross-sectional schematic end view of the airway adapter connected with the liquid separator of  FIG. 2 ; 
         FIG. 4  shows a cross-sectional schematic view of the airway adapter connected with the liquid separator of  FIG. 3  taken along lines A-A; and 
         FIG. 5  shows a perspective cross-sectional view of an airway adapter with a liquid separator in accordance with another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Specific embodiments are explained in the following detailed description making a reference to accompanying drawings. These detailed embodiments can naturally be modified and should not limit the scope of the invention as set forth in the claims. 
       FIG. 2  shows an exploded schematic view of an airway adapter  30  having a first port  31  and a second port  32 . The ports may connect typically through an endotracheal tube to a patient  38  and through a breathing circuit Y-piece to a ventilator  39  respectively. The airway adapter  30  comprises a channel  36  which is surrounded by a wall  37 , the adapter  30  allowing breathing gases to flow between the first port  31  and the second port  32 . Thus the second port is in flow communication with the first port through the channel  36 . The adapter may also comprise a liquid separator  40  for removing a liquid, such as water, from a sample of breathing gas flowing through the channel. Furthermore the airway adapter may comprise a third port  33  located between the first port  31  and the second port  32  for the liquid separator  40 . The liquid separator  40  can be integrated with the wall  37  or the third port, but as well it can be detachably connected into the third port  33  through an opening  34  to be in connection with the breathing gases flowing between the ports  31  and  32 . The liquid separator typically extends into the channel  36  when it is connected to the airway adapter  30  as shown in  FIGS. 3 and 4 . 
     The liquid separator  40  comprises a porous membrane  41  having an outer surface  51  exposed to the breathing gas flow enabling the breathing gases to flow through the membrane into the chamber  50 , which may be defined by a supporting structure  42 , such as a frame, and the membrane  41 . The supporting structure is for supporting the membrane  41 . The membrane may be attached on both sides of the supporting structure  42 , but prevent the liquid, such as water substances to enter through the membrane  41  keeping them outside the chamber  50 . The supporting structure  42  may be made of plastic or similar material. The membrane  41 , which may be attached on both sides of the supporting structure  42  (only part of the top membrane shown in  FIG. 2 ) by gluing, with ultrasonic welding, with laser welding or similar attaching method to form a double layer construction with clearance between. 
     The liquid separator  40  further comprises a connector  43  having a fixed or removable connection to a sample tube  60 . The connector  43 , which may be operationally attached to the supporting structure, comprises a cavity  44  with a predetermined cross sectional area, which is aligned with a tube cavity  61  of the sample tube  60 . The cavity  44  opens into the chamber  50  through an opening  46  and the other end of the tube cavity  61  opens into the gas analyzer and is in connection with the gas pump (not shown in  FIG. 2 ). Thus the cavity  44  provides a flow path for the sample gas from the chamber  50  through the opening  46  of the cavity to the sample tube  60  and further to the gas analyzer. In  FIG. 2  the supporting structure  42  is rectangular shaped forming a rectangular shape chamber  50 , but the supporting structure  42  can be shaped multiform, circular disk or similar as well forming other than rectangular shaped chamber  50 . If desired the chamber  50  can be divided into sections. 
     The connector  43  may be fixedly or removably connected to the opening  33  of port  34  in the airway adapter  30 . When the liquid separator  40  is connected to the airway adapter  30  the chamber  50  may locate between the first port  31  and the second port  32  so that the side surfaces  47  of the supporting structure  42  face towards the ports  31  and  32  allowing the breathing gas to flow from the side of the membrane  41 . As the gas pump (not shown in Figures) in connection with the sample tube  60  sucks breathing gas through the opening  46 , the cavity  44  and the tube cavity  61  it generates an underpressure into the chamber  50 , which causes the breathing gases flowing inside the airway adapter, between the ports  31  and  32 , to flow through the membrane  41  into the chamber  50  and through the opening  46  and the cavity  44  and the tube cavity  61  towards the gas analyzer  45 . The condensed water or vapor, mucus, blood or similar liquid substances does not penetrate through the membrane  41  into the chamber  50  and the cavity  44  and the tube cavity  61 , but stay inside the airway adapter  30  in the channel  36 , and outside the chamber  50 . 
     All liquid substances such as water, mucus and blood have inertia as they move within the breathing gas flow. It is advantageous that the side surfaces  47  of the supporting structure  42  face against the first port  31  and the second port  32  whereas the outer surface  51  of membrane  41  is aligned parallel with the breathing gas flowing between the ports  31  and  32  to prevent the excessive liquid substances to collide with the membrane  41  and block the outer surface  51  of membrane  41  preventing the sample gas to flow into the chamber  50 . Thus the outer surface  51  of the membrane  41  may extend parallel with a longitudinal axis of the channel  36 . It is advantageous to shape the surfaces  47  so that they generate less turbulence into the breathing gas flowing past the surfaces  51  to cause less mixing of gases, especially the boundaries of different gas columns, but also to minimize the liquid substances to collide with the outer surfaces  51  and to resist less the breathing gas flow. 
     The breathing cycle includes an inspiration and an expiration changing by turns. The volume of inspiration or expiration is called the tidal volume and the frequency the inspiration and the expiration changing by turns is called the respiration rate. The tidal volume and the respiration rate are dependent on the size and the physiology of the patient. In general the tidal volume decreases and the respiration rate increases as the size of the patient decreases. 
     The size of the chamber  50  needs to be small enough to allow only one gas concentration volume corresponding to the inspiration or the expiration to fill up the volume of the chamber  50  at a time during the respective inspiration or expiration phase to minimize the mixing of different gas concentration columns sucked through the membrane  41  in the chamber  50 . The mixing of different concentration gas columns degrade the gas concentration signal rise time that can be seen as rounded transitions in a capnogram. The volume of the chamber  50  may be defined by the thickness of the supporting structure  42 , which may be the same as the distance between the membrane  41  on the opposite sides of the chamber  50 , and the surface area of the membrane  41 . The volume of the chamber  50  can be reduced by decreasing the thickness of the frame  42 , which also decreases the undesired flow resistance generated by the areas of the side surfaces  47  of the supporting structure  47 . However, the minimum thickness of the supporting structure  42  cannot be smaller than the inner diameter of the opening  46  and the wall thickness of the supporting structure  42  around the opening  46 . 
     To reduce the mixing of gas columns inside the chamber  50  and to minimize the sample gas transit time differences between the different surface areas of the membrane  41  and the opening  46  it is advantageous to locate the opening substantially close into the midpoint in regard to the side surfaces  47  to equalize the distances that the sampled gas needs to travel through the different areas of the membranes  41  into the opening  46 . 
     It is also advantageous to minimize the area of the outer surface  51  to reduce the sample gas transit time differences and the sample gas transit time between the different points across the surfaces of the membrane  41  and the opening  46 . On the other hand the membrane  41  must have a certain minimum outer surface area to minimize the pressure difference across the membrane  41 , which is inversely proportional to the surface area of the membrane  41  to degrease the work that the gas pump needs to do to generate enough flow to get the gas samples for analyzes. Thus the larger the area of the outer surface of the membrane  41 , the smaller the pressure difference across the membranes  41  and the less work the gas pump needs to do. Furthermore the size of the area of the outer surface  51  of the membrane  41  affect to how easy the membrane  41  get blocked due to the collisions with liquid substances. In general the smaller the area of the outer surface  51  of the membrane  41 , the easier they get blocked of liquid substances and even harder it gets for the gas pump to suck the gas samples. The height (H) of the supporting structure  42 , which may correspond to the length of the side surfaces  47 , is advantageous to extend across the inner diameter of the airway adapter  30  to maximize the area of the outer surface  51  of the membrane  41 . However it is also advantageous that the side surfaces  47  are not in connection with the inner walls of the airway adapter  30  to prevent the condensed water or other liquid substances floating on the inner walls of the airway adapter to block the membrane. The length of the side surfaces  47  along the direction parallel to the diameter, which is the direction perpendicularly against the direction of breathing gas flow or the longitudinal axis of the channel  36 , is advantageous to increase since the boundary of inspiratory and expiratory gases pass the membrane  41  across the whole outer surface area of the membrane  41  simultaneously. This means that the gas samples sucked through across the different areas of the membrane  41  in the direction of the diameter are more synchronous than that in the direction of the breathing gas flow. It is also advantageous direction in transition time difference sense since due to the laminar flow dominating in the airway adapter  30  the breathing gases travel in separate gas concentration columns proportional to inspiration and expiration with a clear boundary. 
     The fixed connection between the airway adapter  30  and the liquid separator  40  ensure that the there is no leakages in the breathing circuit, but on the other hand the manufacturability of such a complicated combination of parts, as described above, would become more difficult. It is also disadvantageous if there is need for changing the airway adapter, when entire surfaces of the liquid separator  40  has been blocked due to an extensive water or liquid substance such as mucus or blood. When the breathing circuit is opened to replace the airway adapter with a new one the positive end expiratory pressure keeping the sick lungs open is released and the lungs collapse preventing the gas exchange in the alveoli. Thus it is advantageous to have a removable connection between the airway adapter  30  and the liquid separator  40  when the blocked liquid separator is replaced so that there is no need to open the breathing circuit (separating the airway adapter from the endotracheal tube and the breathing circuit Y-piece) and thus loose the positive end expiratory pressure. 
     The third port  33  may comprise an elastic penetrable membrane  35 , which covers the opening  34  preventing the positive end expiratory pressure to escape from the breathing circuit through the opening  34 . When the liquid separator  40  is placed into the port  34  the supporting structure  42  and the membrane  41  attached to the supporting structure displace the elastic penetrable membrane  35  attached to the third port  33  covering the opening  34  enabling the supporting structure  42  and the membrane  41  attached to the supporting structure to be placed between the first port  31  and the second port  32 . When the liquid separator  40  is removed from the third port  33  the elastic penetrable membrane  35  returns to its closed state preventing the positive end expiratory pressure to escape from the breathing circuit. 
     Alternatively the connection between the liquid separator  40  and the sample tube  60  can be fixed or removable. The removable connection enables the sample tube  60  to be removed without removing the airway adapter  30  from the breathing circuit and losing the positive end expiratory pressure. However, this does not help the situation if the membrane  41  for liquid separation is blocked, but it helps if there are problems with the sample line  60 . The disadvantage would be that every additional connection, a step like change or a dead volume along the sample gas flow bath, the connection between the cavity  44  and the tube cavity  61 , generate turbulences and mixes the gas columns having different gas concentrations, thus degrading the rise time of the measurement. 
       FIGS. 3 and 4  show from different directions cross-sectional schematic views of the airway adapter, when the liquid separator  40  is connected to the airway adapter. Both the airway adapter and the liquid separator were described in connection with  FIG. 2 . In  FIG. 3  there is shown a central part  70  of the channel  36  extending perpendicular to a longitudinal axis of the channel towards the wall  37  of the channel. The membrane may branch from this central part  70  into at least two different branches  71 , each of the at least two branches extending to different directions towards the wall  37  to obtain the sample from the breathing gas. The branches typically extend crosswise in respect to the longitudinal axis of the channel. In this specific embodiment shown in  FIGS. 3 and 4  the branches  71  extend across the channel between the opposite parts of wall  37 . An angle between these branches of  FIGS. 3 and 4  is substantially 180 degrees, but the angle can vary and may be less than 180 degrees. Advantageously the angle may depend on the number of branches extending from the central part of the channel. The angle between different branches may be between 5-degrees depending on the number of branches, but technically it may be difficult to manufacture. More specifically the angle between different branches may be more than 90 degrees, but not more than 180 degrees to improve manufacturing and decrease the gas flow resistance and turbulences. Advantageously the angle is same between various branches extending from the central part  70  of the channel. 
     The opening  46  of the cavity  44 , when the liquid separator  40  is connected to the airway adapter, may locate inside the channel at a predetermined distance from the wall  37 . The predetermined distance from the wall may be at least 10% of a diameter of the channel  36 , more specifically at least 30% of the diameter of the channel  36 , or even more specifically at least 40% of the diameter of the channel  36 . The most advantageous place would be in the middle of the channel  36 . The opening  46  of the cavity is typically in flow communication with the at least two branches. 
     At least one branch may extend towards the wall and reaching the wall, but advantageously the outer surface  51  of the at least one branch may stay at a predetermined distance from the wall. The predetermined distance may be at least 2% of the diameter of the channel  36 , more specifically at least 5% of the diameter of the channel  36 , or even more specifically at least 10% of the diameter of the channel  36 . 
     The central part  70  of the channel covers a middle of the channel  36 , but may also cover a central area around the middle of the channel. The central area may extend around the middle of the channel towards the wall less than 25% of the diameter of the channel, more specifically less than 15% of the diameter of the channel, or even more specifically less than 5% of the diameter of the channel. 
     A width (W) of the branch  71  along a longitudinal axis of the channel  36  may be at least as long as the diameter of the opening  46 , but typically it is at least 2 times the diameter of the opening  46 , more specifically at least 4 times the diameter of the opening  46 , or even more specifically at least 5 times the diameter of the opening  46 . The thickness of the branch may be at least as long as the diameter of the opening  46 . Typically it may be less than 6 times the diameter of the opening  46 , more specifically less than 4 times the diameter of the opening  46 , or even more specifically less than 2 times the diameter of the opening  46 . The thickness includes a distance between the opposite outer surfaces  51  of the membrane  41  leaving therebetween the chamber  50 . 
     A perspective cross-sectional view of the airway adapter  130  with the liquid separator  140  according to another embodiment is shown in  FIG. 5 . The liquid separator in the airway adapter  130  differs from the one shown in  FIGS. 2-4 , but the liquid separation may be made following same principles as explained in connection with  FIG. 2 . Also in this case the liquid separator may be integrated with the airway adapter  130  or may be detachable. The rectangular supporting structure  42  with the membrane  41  has been replaced by differently shaped supporting structure  142 , such as a star-shaped supporting structure, comprising the membrane  141  attached to both sides of each of the three branches  171  of the star-shape structure in liquid separator  140 . One of the branches  171  in connection with the tube cavity  161  inside the sample tube  160  comprises opening  146 , which opens into the middle of the star-shaped chamber  150  divided into sections, which are defined by the star-shaped supporting structure  142  and the membrane  141 . The tube cavity  161  inside the sample tube  160  further connects to a sample gas pump (not shown in Figures), which sucks sample gas through the tube cavity  161 , the opening  146  and the membrane  141  from the channel  136  inside the airway adapter  130  into the gas analyzer  39 . The volume of the chamber  150  should be minimized, whereas the surface area of the membrane  141  is maximized as described and shown in the embodiment of  FIG. 2 . The star-shaped liquid separator inside the channel  136  of airway adapter  130  is position insensitive and insensible to condensate water or other liquid substances accumulating on the bottom of the channel  136  surrounded by the wall  137  since at least two of the branches  171  are always independent. 
     The star-shaped liquid separator  140  inside the channel  136  of the airway adapter  130  increases the flow resistance of the breathing gas flow between the first port  131  and the second port  132 , but this turns into an advantage when it is combined with a breathing gas flow measurement based on pressure difference over the flow barrier, which is in this case the star-shaped construction. Thus one of the branches  171  comprises at least two pressure cavities  162  and  163  inside the sample tube  160  in connection with pressure openings  164  and  165  respectively, which further connect to pressure sensors (not shown in figure) to measure the pressure and/or the pressure difference over the flow resistance, which is in this case the star-shaped construction inside the channel  136 . In  FIG. 3  the cavities  161 ,  162  and  163  are located inside one branch  171  and one sample tube  160 , which tube cavity  161  further connects to the sample gas pump and which pressure cavities  162 ,  163  further connects to the pressure sensors respectively located further away (not shown in Figures), but advantageously inside the gas analyzer. However it is clear that the cavities  161 ,  162  and  163  can be located inside the different branches  171  and the separate sample tubes if desired. 
     The advantage of the embodiments shown in  FIGS. 2-4 and 5  is that the liquid substances are separated from the breathing gas as close to the patient as possible inside the airway adapter  30  where the liquid substances are built up and the pure gas sample continues through the sample tube  60 ,  160  towards the gas analyzer  45 . The diameter of the cavity  44 , and the tube cavity  61 ,  161  of the sample tube  60 ,  160  as well as all the cavities inside the gas analyzer (not shown in Figures) can be reduced considerably to increase the gas analyzer performance since the liquid substances cannot block the tiny cavity as they are separated already at their origin. The reduction of diameters of cavities between the liquid separator  40 ,  140  and the gas analyzer  45  shortens the response time, the time delay between the gas sample is taken from the breathing gas and it is analyzed since the flow speed of sample gas becomes higher in the sample tube  60 ,  160  and all the cavities in connection with it. Also the rise time is shortened and the capnogram accuracy increased as the mixing between different gas concentration columns and the diffusion is decreased. It is also possible to reduce the sample gas flow, but to maintain the measurement performance, which is advantageous especially with smaller children with very low tidal volumes and high respiration rates. 
     The written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.