Abstract:
An intergrated airway adapter capable of monitoring CO 2  concentration in real time, breath by breath, using infrared absorption techniques and monitoring respiratory flow with differential pressure flowmeters under diverse inlet conditions through improved sensor configurations which minimize phase tag and dead space within the airway adapter.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 08/680,492, filed Jul. 15, 1996, now U.S. Pat. No. 5,789,660, issued Aug. 4, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an airway adapter which monitors both airway CO 2  concentration and respiratory flow. More specifically, the present invention relates to an integrated airway adapter which is capable of monitoring CO 2  concentration in real time, breath by breath, using infrared absorption techniques in combination with monitoring respiratory flow with differential pressure flowmeters under diverse inlet conditions through improved sensor configurations. 
     2. State of the Art 
     U.S. Pat. Nos. 4,859,858 (“the &#39;858 patent”) and 4,859,859 (“the &#39;859 patent”), issued Aug., 22, 1989 to Knodle et al.; and U.S. Pat. No. 5,153,436 (“the &#39;436 patent”), issued Oct. 6, 1992 to Apperson et al., each disclose apparatus including analyzers for outputting a signal indicative of the concentration of a designated gas in a sample being monitored by each apparatus. 
     The gas analyzers disclosed in the &#39;858, &#39;859, and &#39;436 patents are of the non-dispersive type. They operate on the principle that the concentration of a designated gas can be measured by passing a beam of infrared radiation through the gas and ascertaining the attenuated level of the energy in a narrow band absorbable by the designated gas. This process is accomplished using a detector capable of generating a concentration proportional electrical output signal. 
     One important application of gas analyzers is monitoring the level of carbon dioxide in the breath of a medical patient. This is typically done during a surgical procedure as an indication to the anesthesiologist of the patient&#39;s condition. Of course, such gas analyzers can also be used by doctors in numerous medical procedures, such as heart stress tests with a patient on a treadmill, and the like. 
     In a typical instrument using non-dispersive infrared radiation to measure gas concentration, infrared radiation is emitted from a source and focused into a beam by a mirror. The beam is transmitted through a sample of the gases being analyzed. After passing through the gases, the infrared radiation beam passes through a filter. The filter reflects all of the radiation except for the radiation in a narrow band which corresponds to a frequency absorbed by the gas of interest. This narrow band of radiation is transmitted to a detector which produces an electrical output signal proportional in magnitude to the magnitude of the infrared radiation impinging upon it. Thus, the radiation in the band passed by the filter is attenuated to an extent which is proportional to the concentration of the gas of interest. The strength of the signal generated by the detector is consequently inversely proportional to the concentration of the gas of interest. 
     In typical medical gas analyzers, a cuvette is used to sample a patient&#39;s gas exchange via a nasal cannula or by being placed between an endotracheal tube and the mechanical ventilator. The cuvette channels respirated gases to a specific flow path and provides an optical path between an infrared radiation emitter and an infrared radiation detector, both of which can be detachably coupled to the cuvette. 
     A typical cuvette is molded from a polymer or other appropriate material and has a passage defining the flow path for the gases being monitored. The optical path crosses the flow path of the gases through windows in the sidewalls of the cuvette aligned along opposite sides of the flow passage allowing the beam of infrared radiation to pass through the cuvette. 
     The windows are generally formed from sapphire because of sapphire&#39;s favorable optical properties. However, sapphire is a relatively expensive material. Consequently, these cuvettes are almost invariably cleaned, sterilized, and reused. The cleaning and sterilization of a cuvette is time-consuming and inconvenient, and the reuse of a cuvette may pose a significant risk of contamination, especially if the cuvette was previously used in monitoring a patient suffering from a contagious and/or infectious disease. 
     Efforts have been made to reduce the cost of cuvettes by replacing the sapphire windows with windows fabricated from a variety of polymers. One of the major problems encountered in replacing sapphire cuvette windows with polymer windows is establishing and maintaining a precise optical path through the sample being analyzed. This is attributable to such factors as a lack of dimensional stability in the polymeric material, the inability to eliminate wrinkles in the windows, and the lack of a system for retaining the windows at precise locations along the optical path. 
     U.S. application Ser. No. 08/300,146, hereby incorporated herein by reference, discloses a cuvette and a method of manufacturing same which eliminates the problems encountered in previous attempts to use polymers in the place of sapphire windows. The application discloses fashioning a window from a malleable homopolymer such as biaxially oriented polypropylene in the thickness range of 0.001 to 0.005 inches. The use of this inexpensive polypropylene material allows for the fabrication of single use, disposable cuvettes. 
     Respiratory flow measurement during the administration of anesthesia in intensive care environments and in monitoring the physical condition of athletes and other individuals prior to and during the course of training programs and medical tests provides valuable information for assessment of pulmonary function and breathing circuit integrity. Many different technologies have been applied to create a flowmeter that meets the requirements of the critical care environment. Among the flow measurement approaches which have been used are: 
     1) Differential Pressure—measuring the pressure drop or differential across a resistance to flow. 
     2) Spinning Vane—counting the revolutions of a vane placed in the flow path. 
     3) Hot Wire Anemometer—measuring the cooling of a heated wire due to airflow passing around the wire. 
     4) Ultrasonic Doppler—measuring the frequency shift of an ultrasonic beam as it passes through the flowing gas. 
     5) Vortex Shedding—counting the number of vortices that are shed as the gas flows past a strut placed in the flow stream. 
     6) Time of Flight—measuring the arrival time of an impulse of sound or heat created upstream to a sensor placed downstream. 
     Each of the foregoing approaches has various advantages and disadvantages, and an excellent discussion of most of these aforementioned devices may be found in W. J. Sullivan; G. M. Peters; P. L. Enright, M. D. “Pneumotachographs: Theory and Clinical Application,” Respiratory Care, Jul. 1984, Vol. 29-7, pp. 736-49, and in C. Rader, Pneumotachography, a report for the Perkin-Elmer Corporation presented at the California Society of Cardiopulmonary Technologists Conference, October 1982. 
     At the present time, the most commonly used device for respiratory flow measurement is the differential pressure flowmeter. The relationship between flow and the pressure drop across a restriction or other resistance to flow is dependent upon the design of the resistance; thus many different resistance configurations have been proposed. The goal of many of these configurations is to achieve a linear relationship between flow and pressure differential. 
     In some prior art differential pressure flowmeters (commonly termed “pneumotachs”), the flow restriction has been designed to create a linear relationship between flow and differential pressure. Such designs include the Fleisch pneumotach in which the restriction is comprised of many small tubes or a fine screen, ensuring laminar flow and a linear response to flow. Another physical configuration is a flow restriction having an orifice variable in relation to the flow. This arrangement has the effect of creating a high resistance at low flows and a low resistance at high flows. Among other disadvantages, the Fleisch pneumotach is susceptible to performance impairment from moisture and mucous, and the variable orifice flowmeter is subject to material fatigue and manufacturing variabilities. 
     Most all known prior art differential pressure flow sensors suffer deficiencies when exposed to less than ideal gas flow inlet conditions, and further possess inherent design problems with respect to their ability to sense differential pressure in a meaningful, accurate, repeatable manner over a substantial dynamic flow range, particularly, when it is required for the flow sensor to reliably and accurately measure low flow rates, such as the respiratory flow rates of infants. 
     U.S. Pat. No. 5,379,650, issued Jan. 10, 1995 to Kofoed et al., hereby incorporated herein by reference, has overcome the vast majority of the problems with differential pressure flow sensors with a unique sensor including a tubular housing containing a diametrically-oriented, longitudinally extending strut containing first and second lumens having longitudinally-spaced pressure ports opening into respective axially-located notches at each end of the strut. 
     Developments in patient monitoring over the past several decades have shown that concurrent measurements of exhaled gas flow rate and CO 2  concentration provides information that is useful in therapy decision making. By combining these two measurement, one can calculate CO 2  production ({dot over (V)} CO2 ) which is related to the patient&#39;s metabolic status. Also, these measurements can provide a graphical representation of the expired CO 2  concentration versus expired volume which provides information about gas exchange in different compartments of the lungs. 
     Presently, the apparatus necessary to acquire the combination of these two signals requires two discrete components: a flow sensor (pneumotach) and a CO 2  sensor. This configuration is cumbersome and adds undesirable volume (dead space) and resistance to the patient&#39;s breathing circuit. 
     It would be highly desirable to have an airway adapter which combines both a CO 2  concentration monitoring sensor and a respiratory flow monitoring sensor in a configuration which is convenient to use and which minimnizes phase lag and internal dead space of the combination. 
     SUMMARY OF THE INVENTION 
     The present invention comprises an integrated airway adapter for monitoring CO 2  concentration in real time, breath by breath, using infrared absorption techniques and monitoring respiratory flow with differential pressure flowmeters under diverse inlet conditions through improved sensor configurations. 
     The airway adapter of the present invention has been devoloped to address the issues of adapter size and phase lag performance. The present invention combines the functions of the CO 2  concentration monitoring adapter with that of the pneumotach into a single component. Two pressure ports (on the airway adapter) provide access such that a differential pressure is generated across the orifice of the pneumotach. 
     The airway adapter of the present invention is preferably manufactured by an injection molding process. The consistency of product obtainable from the injection molding process provides a high degree of interchangeability, thereby eliminating the need for a calibration procedure to be performed during setup or with disposable adapter replacement. 
     The airway adapter comprises a CO 2  concentration monitoring portion and respiratory flow sensor portion. The CO 2  concentration monitoring portion includes a chamber having a pair of opposing, axially aligned windows flanking the flow path, wherein the windows preferably have a high transmittance for radiation in the intermediate infrared portion of the electromagnetic spectrum. The axial alignment of the windows allows an infrared radiation beam to travel from an infrared radiation emitter transversely through the chamber and the gas(es) flowing through the chamber to an infrared radiation detector to determine the concentration of CO 2  in the gas. 
     The airway adapter can be either reusable or disposable. However, if the airway adapter is designed to be disposable, the windows should be made of an inexpensive material rather than sapphire as presently used. It is essential to the accuracy of the CO 2  monitoring sensor that the material used for the window transmit a usable part of the infrared radiation impinging upon it. Thus, the window material must have appropriate optical properties. A preferred window material is biaxially oriented polypropylene. 
     The respiratory flow sensor portion of the present invention has the capability of accommodating a wide variety of gas flow inlet conditions without adding significant system volume or excessive resistance to flow. The design of the respiratory flow sensor of the present invention also substantially inhibits the entrance of liquids in the monitoring system into the pressure ports of the sensor. 
     In a first embodiment, the respiratory flow sensor portion comprises a substantially tubular housing with a diametrically-onrented, longitudinally-extending strut having pressure ports located adjacent the axis of the housing and proximate each end of the strut. The pressure ports are each associated with a lumen contained within the strut, the lumens extending to the exterior of the sensor for communication via suitable tubing with a differential pressure transducer. Depending upon the flow direction being measured, one port serves as a high pressure tap and the other as a low pressure tap. The pressure ports are oriented substantially perpendicular to the axis of the tubular housing and communicate with the interior volume of the housing via axially-placed notches in the leading and trailing edges of the strut. It is preferred that the notches extend over the entire width of the strut in the area of the pressure ports and through the side faces of the strut so that the pressure ports have reduced response to the velocity of mass flow through the sensor. 
     In this preferred embodiment, the respiratory sensor portion is positioned between the mechanical ventilator and the CO 2  concentration monitoring portion. 
     In a second embodiment, the CO 2  concentration monitoring portion is designed to be detachable from the respiratory flow sensor portion. This embodiment is particularly useful for the cleaning and sterilization of a non-disposable airway adapter. 
     In a third embodiment, the exterior of a tubular portion of the CO 2  concentration monitoring portion is provided with external circumferential ribs, preferably defining a 22 mm diameter. The ribs reduce the adapter weight while providing uniform wall dimensions that assist molding of the part. 
     In a fourth and a fifth embodiment, the pressure ports are located on opposite sides of the CO 2  monitoring portion of the sensor. A first port is placed on the proximal (nearer to the patient) side, and the second port is placed on the distal (farther from the patient) side of the CO 2  monitoring portion of the sensor. This embodiment is particularly advantageous for use in situations where the respiratory tidal volumes are extremely small. This embodiment reduces the volume of expired gases which are rebreathed by the patient. Also, this embodiment uses the pressure perturbation and pressure loss of the CO 2  portion of the sensor as part of the flow signal. This reduces the overall pressure loss of the combined adapter. Though this embodiment is particularly advantageous for use with newborn infants, it also has equal utility in adult respiratory monitoring. 
     The respiratory flow sensor has a flow resistance element (whether the strut or the CO 2  concentration monitoring portion) which results in a differential pressure signal which is non-linear. To obtain adequate precision at extremely high and low flow rates, a very high resolution 18-bit or 20-bit analog to digital conversion device may be used. The use of such a converter allows a digital processor to compute flow from the measured differential pressure by using a sensor characterizing look-up table. This technique eliminates the need for variable or multiple gain amplifiers and variable offset circuits that might otherwise be required with use of a 12-bit resolution or lower resolution converter. 
     The airway adapter preferably incorporates a specific instrument connection scheme and is color or optically coded to prevent incorrect assembly. The airway adapter is to be manufactured in both permanent and disposable configurations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded prospective view of a first preferred embodiment of the airway adapter of the present invention in combination with a transducer housing for containing electronics for CO 2  determination; 
     FIG. 2 is a side elevation view of a first preferred embodiment of the airway adapter of the present invention; 
     FIG. 3 is an end elevation view of the airway adapter of FIG. 2, looking from plane  3 — 3 ; 
     FIG. 4 is a side sectional elevation view of the airway adapter of FIG. 2; 
     FIG. 5 is a secional view of the airway adapter of FIG. 4, looking upward from plane  5 — 5  extending laterally across the axis of the present invention; 
     FIG. 6 is a side elevation view of a second preferred embodiment of the airway adapter of the present invention; 
     FIG. 7 is a side elevation view of a third preferred embodiment of the airway adapter of the present invention; 
     FIG. 8 is a side sectional elevation of the airway adapter of FIG. 7; 
     FIG. 9 is a bottom view of the airway adapter of FIG. 7; 
     FIG. 10 is a side elevation view of a fourth preferred embodiment of the airway adapter of the present invention; 
     FIG. 11 is a side sectional elevation of the airway adapter of FIG. 10; 
     FIG. 12 is an end elevation view of the airway adapter along lines  12 — 12  of FIG. 10; 
     FIG. 13 is an end elevation view of the airway adapter along lines  13 — 13  of FIG. 10; 
     FIG. 14 is a sectional view of the airway adapter of FIG. 10, looking from plane  14 — 14 ; 
     FIG. 15 is a sectional view of the airway adapter of FIG. 10, looking from plane  15 — 15 ; 
     FIG. 16 is a sectional view of the airway adapter of FIG. 10, looking from plane  16 — 16 ; and 
     FIG. 17 is a sectional view of the airway adapter of FIG. 10, looking from plane  17   
       13   17 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1-4 illustrate an airway adapter  20 . The preferred airway adapter  20  is a unitary, injection-molded plastic element, so as to afford low manufacturing cost and permit disposal of the sensor after a single use, with a separate transducer housing  22  for housing an infrared emitter and an infrared absorption device which monitors CO 2  concentration. However, this configuration is not a requirement and the materials and method of fabrication are not critical to the invention. Suitable plastics include polycarbonates such as Lexan®, manufactured by General Electric, or Makrolon®, manufactured by Miles Chemicals. 
     The preferred airway adapter  20  is designed for connection between a patient ventilation device, such as an endotracheal tube inserted in a patient&#39;s trachea, attached to a first tubular portion  24  and the tubing of a mechanical ventilator attached at a second tubular portion  26 . The first and second tubular portions  24  and  26  have bores of varying diameters and substantially circular cross-sections, with a CO 2  concentration monitoring portion  28  disposed therebetween. The second tubular portion  26  houses a respiratory flow monitoring device  30  therein. 
     FIG. 1 illustrates the transducer housing  22  for electronics designed to output a signal proportional in magnitude to the concentration of carbon dioxide flowing through airway adapter  20 , and a reference signal. These signals can be ratioed to provide a third signal accurately and dynamically representing the concentration of the carbon dioxide flowing through the airway adapter. The transducer housing  22  also includes an infrared radiation emitter (not shown). The internal configuration and design of the infrared absorption device housed in transducer housing  22  which monitors CO 2  concentration in real time is thoroughly discussed in U.S. application Ser. No. 08/300,383, previously incorporated by reference. It is understood that infrared CO 2  monitor devices such as those disclosed in the &#39;858, &#39;859, and &#39;436 patents, as well as other CO 2  detection devices could be used in the transducer housing  22 . 
     Referring more specifically to FIGS. 1-3, airway adapter  20  embodies the principles of the present invention and is typically molded from a polycarbonate or a comparable rigid, dimensionally stable polymer. The airway adapter  20  has a generally parallelepipedal center section  32  between and axially aligned with the first and second tubular portions  24  and  26  with a flow passage  34  extending from end-to-end through the airway adapter. 
     The CO 2  concentration monitoring portion  28  of airway adapter  20  provides a seat for transducer housing  22 . An integral, U-shaped casing element  36  positively locates transducer housing  22  across the airway adapter  20  and, also, in the transverse direction indicated by arrow  38  in FIG.  1 . The arrow  38  also shows the direction in which transducer housing  22  is displaced to detachably assemble it to the airway adapter  20 . In a preferred embodiment, the airway adapter  20  snaps into place (see the above-cited &#39;858 and &#39;859 patents); no tools are needed to assemble or remove the adapter. 
     The center section  32  includes first and second axially aligned windows  40  and  42 , respectively (window  42  is shown only in FIG.  4 ). The windows  40  and  42  preferably have a high transmittance for radiation in the intermediate infrared portion of the electromagnetic spectrum. The axial alignment of first window  40  and second window  42  allows an infrared radiation beam to travel from the infrared radiation emitter in one leg of transducer housing  22  transversely through airway adapter  20  and the gas(es) flowing through airway adapter flow passage  34  to an infrared radiation detector (not shown) in the opposing, parallel leg of transducer housing  22 . 
     Cuvette windows have typically been fabricated from sapphire because of sapphire&#39;s favorable optical properties; stability; and resistance to breakage, scratching, and other forms of damage. However, sapphire windows are expensive, and this makes it impractical to discard the cuvette after it is used to monitor a single patient. Instead, the cuvette must be cleaned, sterilized, and reused, which is inconvenient and poses a risk of infection to the patient. 
     In a preferred embodiment, the cost of the cuvette can be reduced to the point of making it practical to dispose of the cuvette after a single use by fabricating the cuvette windows from an appropriate polymer rather than the many times more expensive sapphire. It is essential to the accuracy of the CO 2  concentration monitor that the polymer transmit a usable part of the infrared radiation impinging upon it. Thus, the window material must have the appropriate optical properties. Such appropriate preperties may include a polymer that is malleable, of which a preferred window material is biaxially oriented polypropylene. 
     As discussed above, airway adapter  20  includes the respratory flow monitoring device  30  within the first tubular portion  24  (most clearly seen in FIGS.  4  and  5 ). The respiratory flow monitoring device  30  includes a diametrically-oriented, longitudinally-extending strut  44  of axial length L and height H 1 . The strut  44 , which provides a position for pressure ports apertures  62  and  66  and conditions the velocity profile of the flowing gas, offset from the inner wall  48  of the tubular housing  46 , is secured at both ends to the inner wall  48  of the tubular housing  46 , has first and second end faces  50  and  52 , respectively, and has first and second side faces  54  and  56 , respectively. The cross-sectional area of the strut  44  transverse to a bore axis A should be minimized. The minimization of this dimension is constrained by the diameter of the pressure ports  62  and  66 . Typically this may be five percent (5%) of the cross-sectional bore area of the tubular housing  46  at the strut location. 
     It should be noted that the bore diameter of the tubular housing  46  depicted in FIGS. 4-5 is different between first portion  24  and second portion  26 . This is intentional in the preferred embodiment to accommodate a male connecting tube element shown in broken lines and designated as M on the left-hand side or first portion  24  of the airway adapter  20 , and a female connecting tube element F on the right-hand side or second portion  26  of the airway adapter  20 . Also, the internal bores may be tapered to allow for mold release from a plastic injection molding machine. 
     The strut  44  further includes notch structures comprising substantially symmetrical first notch  58  and a second notch  60  located substantially on axis A of the tubular housing  46 , notches  58  and  60  extending axially inwardly from first and second end faces  50  and  52 , respectively, and laterally through first and second side faces  54  and  56 , respectively. A first pressure port  62  of a first lumen  64  opens into the first notch  58 , and a second pressure port  66  of a second lumen  68  opens into the second notch  60 , first and second lumens  64  and  68  comprising passages internal to the strut  44  which extend into and through a first and second male stem  70  and  72 , respectively, on the exterior surface  74  of the tubular housing  46 . The respiratory flow monitoring device  30  is proportional to the square root the differential pressure as measured at the pressure ports  62  and  66 . 
     Both pressure ports  62  and  66  face substantially perpendicular to axis A of tubular housing  46 , and the notches  58  and  60  extend axially inwardly to a depth D at least past the pressure ports  62  and  66 , and may so extend a distance equal to the height H 2  of the notches  58  and  60 , which in turn should be less than or equal to four-tenths ({fraction (4/10)}) of the height H 1  of the strut  44 . 
     The back walls  78  and  80  of the notches  58  and  60 , respectively, together with the restrictions (ridges or lands)  90  comprise an obstruction  76  and/or perturbation to the gas flow which generates the differential pressure signal which is measured at first and second pressure ports  62  and  66 . The measured differential pressure signal is from either pressure loss or from vena contracta. Vena contracta is the contraction of the flowing gas velocity profile caused by the flow obstruction. The differential pressure generated from the vena contracta can be modeled by standard fluid mechanics equations such as Euler&#39;s or Bernoulli&#39;s equation. The differential pressure signal generated from vena contracta is considered “lossless”, meaning that the pressure is restored as the velocity profile is returned to that incident to the sensor. 
     The flow obstruction  76  may be varied in a number of ways to yield a different magnitude of measured differential pressure for a given flow rate. First, the cross-sectional area of the restrictions (ridges or lands)  90  may be increased or decreased in the plane perpendicular to axis A. Also, the distance from the center of the first pressure port  62  to the back wall  78  of notch  58 , and likewise the center of the second pressure port  66  to the back wall  80  of notch  60 , may be varied to change the flow response characteristics. The magnitude of the differential pressure signal for a given flow rate can be further increased by reducing the cross-sectional bore area by necking down the inner wall of tubular housing  46 . 
     The length and width of the strut may be altered as desired to change flow characteristics. These flow characteristics include flow conditioning, signal strength, and signal stability. Ideally the incident velocity profile to the obstruction should be the same regardless of the velocity profile incident to the airway adapter  20 . Signal stability may be compromised when unstable multi-dimensional vortex formations are generated by the obstruction  76 . The strut  44  with notch means provides flow conditioning that yields some immunity to inlet velocity profile and yields a stable differential pressure signal in response to the gas flow. 
     It is contemplated that the end faces  50  and  52  may be substantially perpendicular to axis A as shown in FIGS. 4 and 5, chamfered and rounded as shown, so long as the end face configuration is symmetrical when viewed from above. The major characteristic of the end faces  50  and  52 , aside from symmetry, is that they do not incline toward notches  58  and  60  or otherwise collect or direct flow through the respiratory flow monitoring device  30  toward the notches  58  and  60  and the pressure ports  62  and  66 . The end faces  50  and  52  are to be aerodynamically designed to minimize resistance to the gas flow. 
     Side faces  54  and  56  of strut  44  are flat as shown in FIGS. 4 and 5, again the major requirement as with the end faces  50  and  52  being one of symmetry between the sides of the strut  44 . 
     Back walls  78  and  80  of the notches  58  and  60 , respectively, are arcuate or radiused as shown in FIG. 5, or otherwise symmetrically shaped, as with the end faces  50  and  52 . The back walls  78  and  80  may also be flat. 
     Floors  82  and  84  and ceilings  86  and  88  of the notches  58  and  60 , respectively, are preferably flat as shown in FIGS. 4 and 5, or may be otherwise symmetrically shaped. Likewise, the transition edges or lines between the end faces  50  and  52  and the notches  58  and  60  are preferably radiused, although alternatively chamfered or bevelled. 
     The foregoing modifications of the respiratory flow monitoring device  30  of FIGS. 4 and 5 may be selectively employed to adapt to the conditions under which the sensor is to operate. In particular, the modification of the cross-sectional flow area in the vicinity of the strut  44  may be employed to adjust the dynamic range of the respiratory flow monitoring device  30 , as may modifications of the configuration of the end faces, the back walls of the notches, and to the lines of transition between the notches and the end faces and side faces. It is preferred to use laterally extending, transversely oriented center (strut  44 ) restrictions (ridges or lands)  90  and a gradual inner wall transition in the strut area axial length to add symmetry to the flow pattern, normalize the flow, provide immunity to moisture, and provide better repeatability of readings. The notch height H 2  or the length of the structure may be increased or decreased to accommodate a wider range of inlet conditions, such as might result from employment of the respiratory flow monitoring device  30  with a variety of endotracheal tubes. 
     The airway adapter  20  is preferably oriented with the first and second male stems  70  and  72  directed upward, such that the water condensation and mucous does not clog or otherwise impair the pressure ports  62  and  66 . 
     FIG. 6 illustrates a second embodiment of the airway adapter  20  which includes a plurality of ribs  92  around the outside diameter of the first tubular portion  24  of the airway adapter  20 . The ribs preferably define a 22 mm diameter and reduce the adapter weight while providing uniform wall dimensions that assist molding of the part. 
     FIGS. 7-9 illustrate a third embodiment of an airway adapter  100  which is particularly suitable for use in situations where the respiratory tidal volumes are extremely small, such as with newborn infants, although it has equal utility in adult respiratory monitoring. 
     The preferred airway adapter  100  is also designed for connection between a patient ventilation device, such as an endotracheal tube inserted in a patient&#39;s trachea, attached to a first tubular portion  104  and the tubing of a mechanical ventilator attached at a second tubular portion  106 . The first and second tubular portions  104  and  106  have bores of varying diameter and of substantially circular cross-section, with a CO 2  concentration monitoring portion  108  disposed therebetween. 
     The CO 2  concentration monitoring portion  108  of the airway adapter  100  provides a seat for a ransducer housing (not shown), simila to the transducer housing  22  shown in FIG.  1 . An integral, U-shaped casing element  112  positively locates the transducer housing into position. In a preferred embodiment, the airway adapter  100  snaps into place without the need for tools to assemble or remove the adapter. 
     The CO 2  monitoring portion  108  also includes a first axially aligned window  116  and a second axially aligned window  118  (shown in FIG. 8 only) to allow an infrared radiation beam to travel from the infrared radiation emitter in the transducer housing transversely through a sampling chamber  114  in adapter  100  for monitoring CO 2  gas as discussed above. 
     The airway adapter  100  includes a respiratory flow monitoring device  110  which partially resides in the first tubular portion  104 , partially resides in the second tubular portion  106 , and partially resides in the intermediate CO 2  monitoring portion  108 . The respiratory flow monitoring device  110  is most clearly seen in FIG.  8 . The respiratory flow monitoring device  110  comprises a first pressure port  120  of a first lumen  122  which opens into a first tubular chamber  124  of the first tubular portion  104 , and a second pressure port  126  of a lumen  128  which opens into the second tubular chamber  130 , lumens  122  and  128  extending to a first recess  132  and a second recess  134 , respectively. The recesses  132  and  134  are configured to minimize dead space and accommodate male connecting tubes shown in broken lines and designated as T 1  and T 2 . Tubes T 1  and T 2  are connected to a flow monitor (now shown) which determines flow rate through a pressure differential detected between the pressure ports  120  and  126 . This pressure differential is produced through the use of necked-down ports  136  and  138  at the longitudinal ends of CO 2  sampling chamber  114 . 
     In this embodiment, an annular recess  142  is formed in the first tubular portion  104  to accommodate a male connecting tube element shown in broken lines and designated as M 1  on the left-hand side or first tubular portion  104  of the airway adapter  100 . The second tubular portion  106  accommodates a second male connection be element M 2 , as shown in broken lines. The element M 2  includes a bore of like diameter to the bore of second tubular chamber  130 , and snaps into the second portion  106  by engaging a stepped slot  140 . 
     It has been found that this embodiment has many advantages such as minimization of dead space and moldability in one piece. The sampling chamber  114  with ports  136  and  138  serves a dual function by adding a differential pressure flow signal to CO 2  measurement. The heat from the CO 2  transducer housing placed over the airway adapter  100  should help to reduce the tendency of breath moisture to condense in the airway adapter  100 . The effects of water condensation are of particular concern in this embodiment due to its small volume and intended neonatal use; therefore the airway adapter  100  should be positioned such that recesses  132  and  134  are directed upward to prevent clogging. 
     FIGS. 10-17 illustrate a fourth preferred embodiment of an airway adapter  200  which is similar to the airway adapter  100  of FIGS. 7-9; therefore, components common to FIGS. 7-9 and FIGS. 10-17 retain the same numeric designation. The airway adapter  200  is also particularly suitable for use in situations where the respiratory tidal volumes are extremely small, such as with newborn infants, although it has equal utility in adult respiratory monitoring. 
     The preferred airway adapter  200  is also designed for connection between a patient ventilation device, such as an endotracheal tube inserted in a patient&#39;s trachea, attached to the first tubular portion  104  and the tubing of a mechanical ventilator attached at second tubular portion  106 . The first and second tubular portions  104  and  106  have bores of varying diameter and of substantially circular cross-section, with the CO 2  concentration monitoring portion  108  disposed therebetween. 
     The CO 2  concentration monitoring portion  108  of the airway adapter  200  provides a seat for a transducer housing (not shown), similar to the transducer housing  22  shown in FIG.  1 . An integral, U-shaped casing element  112  positively locates the transducer housing into position. In a preferred embodiment, the airway adapter  200  snaps into place without the need for tools to assemble or remove the adapter. 
     The CO 2  monitoring portion  108  also includes a first axially aligned window  116  and a second axially aligned window  118  to allow an infrared radiation beam to travel from the infrared radiation emitter in the transducer housing transversely through the sampling chamber  114  in the adapter  200  for monitoring CO 2  gas as discussed above. 
     The airway adapter  200  includes a respiratory flow monitoring device  110  which partially resides in the first tubular portion  104 , partially resides in the second tubular portion  106 , and partially resides in the intermediate CO 2  monitoring portion  108 . The respiratory flow monitoring device  110  comprises the first pressure port  120  of a first lumen  122  which extends through a first strut  202  and opens into the first tubular chamber  124  of the first tubular portion  104 . The first strut  202  has a tapered portion  204  directed toward the first tubular portion  104  to minimize potential flow disturbances. The respiratory flow monitoring device  110  further comprises the second pressure port  126  of a lumen  128  which extends through a second strut  206  and opens into the second tubular chamber  130 . The second strut  206  has a tapered portion  208  directed toward the second tubular portion  106  to minimize potential flow disturbances. The lumens  122  and  128  extend to the first recess  132  and the second recess  134 , respectively. 
     The recesses  132  and  134  are configured to minimize dead space and accommodate male connecting tubes shown in broken lines and designated as T 1  and T 2 . The recesses  132  and  134  may have internal ribs  210  to securely grip the tubes T 1  and T 2 . The tubes T 1  and T 2  are connected to a flow monitor (now shown) which determines flow rate through a pressure differential detected between the pressure ports  120  and  126 . This pressure differential is produced through the use of a first annular port  212  and a second annular port  214  at the longitudinal ends of CO 2  sampling chamber  114 . The first annular port  212  is formed by a first restriction member  216  extending from the first strut  202  and blocking a portion of the first tubular chamber  124  of first tubular portion  104 . The face surfaces  220 ,  222  of the first restriction member  216  are preferably substantially perpendicular to the flow of the respiratory gas within the airway adaptor  200 . The second annular port  214  is formed by a second restriction member  218  extending from the second strut  206  and blocking a portion of the second tubular chamber  130  of the second tubular portion  106 . The face surfaces  224 ,  226  of the second restriction member  218  are preferably substantially perpendicular to the flow of the respiratory gas within the airway adaptor  200 . The first restriction member  216  and the second restriction member  218  can be any shape such a circular, oval, rectangular, or the like. However, the preferred shape is a planar disk. 
     In this embodiment, as with the embodiment of FIGS. 7-9, an annular recess  142  is formed in the first tubular portion  104  to accommodate a male connecting tube element shown in broken lines and designated as Ml on the left-hand side or first tubular portion  104  of the airway adapter  200 . Second tubular portion  106  accommodates the second male connecting tube element M 2 , as shown in broken lines. The element M 2  includes a bore of like diameter to the bore of second tubular chamber  130 , and snaps into second tubular portion  106  by engaging stepped slot  140 . 
     It has been found that this embodiment has many advantages such as minimization of deadspace and moldability in one piece. The sampling chamber  114  with ports  212  and  214  serves a dual function by adding a differential pressure flow signal to CO 2  measurement. The heat from the CO 2  transducer housing placed over the airway adapter  200  should help to reduce the tendency of breath moisture to condense in adapter  200 . The effects of water condensation are of particular concern in this embodiment due to its small volume and intended neonatal use; therefore, the airway adapter  200  should be positioned such that recesses  132  and  134  are directed upward to prevent clogging. 
     While the airway adapter of the present invention has been disclosed herein in terms of a preferred and alternative embodiment and modifications thereto, those of ordinary skill in the art will appreciate that many other additions, deletions and modifications to the disclosed embodiments may be effected without departing from the scope of the invention as hereinafter claimed.