Abstract:
Apparatus or systems which employ luminescence-quenching to produce a signal indicative of oxygen concentration. Components of such systems include: (1) an airway adapter, sampling cell, or the like having a casing and a sensor which is excited into luminescence with the luminescence decaying in a manner reflecting the concentration of oxygen in gases flowing through the airway adapter or other flow device and is in intimate contact with a window in the casing; (2) a transducer which has a light source for exciting a luminescable composition in the sensor into luminescence, a light sensitive detector for converting energy emitted from the luminescing composition as that the composition is quenched into an electrical signal indicative of oxygen concentration in the gases being monitored, and a casing which locates the light source and detector in close physical proximity to the window but on the side thereof opposite the sensor; and (3) subsystems for maintaining the sensor temperature constant and the temperature of the window above condensation temperature and for processing the signal generated by the light sensitive detector. Airway adapters, sampling cells, and transducers for such systems are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the monitoring of oxygen concentration and, more particularly, to novel, improved methods and apparatus for monitoring the concentration of oxygen in respiratory and other gases. 
     2. State of the Art 
     The most common cause of anesthetic and ventilator related mortality and morbidity is inadequate delivery of oxygen to a patient&#39;s tissues. Therefore, the monitoring of static inspired oxygen concentration has long been a safety standard of practice to ensure detection of hypoxic gas delivery to patients undergoing surgery and to those on mechanical ventilators and receiving supplemental oxygen therapy. However, monitoring the static inspired fraction of inhaled oxygen does not always guarantee adequate oxygen delivery to the tissues because it is the alveolar oxygen concentration that eventually enriches the blood delivered to the cells. 
     It is this alveolar gas phase that is interfaced with pulmonary perfusion which, in turn, is principally responsible for controlling arterial blood gas levels. It is very important that the clinician know the blood gas levels (partial pressure) of oxygen (pO 2 ) and carbon dioxide (pCO 2 ) as well as the blood pH. Blood gas levels are used as an indication of incipient respiratory failure and in optimizing the settings on ventilators. In addition, blood gas levels can detect life-threatening changes in an anesthetized patient undergoing surgery. 
     The traditional method for obtaining arterial blood gas values is highly invasive. A sample of arterial blood is carefully extracted and the partial pressure of the gases is measured using a blood gas analyzer. Unfortunately, arterial puncture has significant limitations: (1) arterial puncture requires a skilled health care provider and it carries a significant degree of patient discomfort and risk, (2) handling the blood is a potential health hazard to the health care provider, (3) significant delays are often encountered before results are obtained, and (4) measurements can only be made intermittently. 
     Noninvasive methods for estimating blood gas levels are available. Such methods include the use of capnography (CO 2  analysis). These methods employ fast gas analyzers at the patient&#39;s airway and give a graphic portrayal of breath-by-breath gas concentrations and, therefore, can measure the peak exhaled (end tidal) concentrations of the respective respired gases. Although gradients can occur between the actual arterial blood gas levels and the end tidal values, this type of monitoring is often used as a first order approximation of the arterial blood gas values. 
     Other techniques have been utilized for assessing patient blood gas levels with mixed results. Transcutaneous sensors measure tissue pO 2  and pCO 2  diffused through the heated skin surface. This type of sensor has a number of practical limitations including a slow speed of response and difficulty of use. 
     Pulse oximetry is widely used to measure the percentage of hemoglobin that is saturated with oxygen. Unfortunately, it does not measure the amount of dissolved oxygen present nor the amount of oxygen carried by the blood when the hemoglobin levels are reduced. This is important because low hemoglobin levels are found when there is a significant blood loss or when there is insufficient red blood cell information. In addition, pulse oximeter readings are specific to the point of contact, which is typically the finger or ear lobe, and may not reflect the oxygen level of vital organs during conditions such as shock or hypothermia. 
     Oxygraphy measures the approximate concentration of oxygen in the vital organs on a breath-by-breath basis and can quickly detect imminent hypoxemia due to decreasing alveolar oxygen concentration. For example, during hypoventilation, end tidal oxygen concentration changes more rapidly than does end tidal carbon dioxide. During the same conditions, pulse oximetry takes considerably longer to respond. Fast oxygen analysis (oxygraphy) can also readily detect inadvertent administration of hypoxic gas mixtures. 
     Oxygraphy reflects the balance of alveolar O 2  available during inspiration minus the O 2  uptake secondary to pulmonary perfusion. An increasing difference between inspiratory and end tidal oxygen values is a rapid indicator of a supply/demand imbalance which could be a result of changes in ventilation, diffusion, perfusion and/or metabolism of the patient. This imbalance must be quickly corrected because failure to meet oxygen demand is the most common cause of organ failure, cardiac arrest, and brain damage. Oxygraphy provides the earliest warning of the development of an impending hypoxic episode. 
     Oxygraphy has also been shown to be effective in diagnosing hypovolemic or septic shock, air embolism, hyperthermia, excessive positive-end expiratory pressure (PEEP), cardiopulmonary resuscitation (CPR) efficacy, and even cardiac arrest. During anesthesia, oxygraphy is useful in providing a routine monitor of preoxygenation (denitrogenation). It especially contributes to patient safety by detecting human errors, equipment failures, disconnections, misconnections, anesthesia overdoses, and esophageal intubations. 
     Combining the breath-by-breath analysis of oxygen with the measurement of airway flow/volume as outlined in U.S. Pat. Nos. 5,347,843 and 5,379,650 gives another dimension to the clinical utility of oxygraphy. This combination parameter, known as oxygen consumption (VO 2 ), provides an excellent overall patient status indicator. Adequate cardiac output, oxygen delivery, and metabolic activity are all confirmed by oxygen consumption because all of these physiological processes are required for oxygen consumption to take place. Oxygen consumption is also useful in predicting ventilator weaning success. 
     A metabolic measurement (calorimetry) includes determination of a patient&#39;s energy requirements (in calories per day) and respiratory quotient (RQ). Interest in the measurement of caloric requirements has closely paralleled the development of nutritional support. For example, the ability to intravenously provide all the necessary nutrition to critically ill patients has only been accomplished within the last 25 years. Along with the realization that we need to feed patients has come the need to know how much to feed them, what kind of nutrients (carbohydrates, lipids, protein) to feed them, and in what ratio the nutrients need to be supplied. The only true way to measure the caloric requirements of patients and to provide a noninvasive quality assessment of their response to nutrition is with indirect calorimetry. Airway O 2  consumption and CO 2  production can be measured noninvasively and provide a basis for the computations needed for a measurement of indirect calorimetry, a direct measurement of the metabolic status of the patient, and the patients&#39; respiratory quotients. 
     With the above clinical need in mind, it is important to ensure that clinicians have the proper equipment to monitor breath-by-breath oxygen. While there are adequate devices for measuring static levels of oxygen, the measurement of breath-by-breath (fast) airway oxygen concentration requires more sophisticated instruments. Very few of these devices can be directly attached to the patient airway. Instead, most require the use of sampling lines to acquire the gas and send it to a remote site for analysis. Fast airway oxygen monitors are typically large, heavy, fragile instruments that consume considerable power. They must sample airway gases via a small bore plastic tube (sidestream) and remotely detect the oxygen gas as it passes from the airway to the sensor. The problems associated with this type of gas sampling are well known. Gas physics dictates painstaking, careful measurements because water vapor concentration pressure and temperature can vary within the patient&#39;s airway and gas sample line. The presence of water and mucous create problems for long term patency of the sample tube. Also, the sample line acts like a low pass filter and affects the fidelity of the measurement. Finally, the pressure variable delay introduced by the sample line creates difficulty in accurately synchronizing the airway flow and oxygen concentration signals required to calculate oxygen consumption. 
     On-airway (mainstream) monitoring of oxygen has the potential to solve all of the above problems, especially when breath-by-breath oxygen consumption measurements are made. However, most of the available fast oxygen sensors are simply too big, too heavy, too fragile, and/or otherwise not suited to be placed in line with a patient&#39;s breathing tube. 
     There are various other technologies which have been employed in monitoring airway oxygen concentration. Some of the most widely used are electrochemical sensors. These fall into two basic categories: polarographic cells and galvanic cells. These cells produce an electric current proportional to the number of oxygen molecules which diffuse across a membrane. The advantages of these types of sensors are simplicity and low cost. The disadvantages of these types of sensors include limited lifetime (chemistry depletes) and slow response (not breath-by-breath). In some cases, these cells have demonstrated sensitivity to certain anesthetic agents, which introduces inaccuracies into the oxygen concentration measurement. Generally, this type of sensor is too large to attach to the patient airway. 
     There have been a few reported developments where electrochemical cell membranes were improved to enable faster response. There are also silicon micromachined cells using the principle of “Back Cell” electrochemical technology. Their time response approaches 150 ms but they appear to be subject to the typical problems of this type of cell (i.e., stability and calibration). 
     Another popular medical oxygen sensor is the paramagnetic type. This sensor uses the strong magnetic property of oxygen as a sensing mechanism. There are two basic types of paramagnetic cells: static and dynamic. The static type is a dumbbell assembly suspended between the poles of a permanent magnet. The magnetic forces of the surrounding oxygen molecules cause a torsional rotation of the dumbbell which can be sensed optically and employed as a measure of oxygen concentration. The dynamic type (see U.S. Pat. No. 4,633,705) uses a magneto-acoustic approach. This requires a gas sample and a reference gas that are mixed within an electromagnetic field. When the field is switched on and off, a pressure signal proportional to the oxygen content is generated. The signal can be detected by a differential microphone. The advantages of the paramagnetic sensor are good linearity and stability. The dynamic type has an inherently faster response than the static type. Both types are subject to mechanical vibration, and the dynamic type has the disadvantage of requiring a reference gas. Neither type is suitable for on-airway applications. 
     Zirconium oxide cells are frequently used in the automotive industry to measure oxygen concentration. The cell is constructed from a solid electrolyte tube covered by platinum electrodes. When heated to approximately 800 degrees C., a voltage proportional to the logarithm of the ratio between a sample gas and a reference gas is generated. The advantages of this sensor are wide dynamic range, very fast response, and simplicity. The high cell temperature is clearly a disadvantage as is power consumption. Also, the cell is degraded in the presence of anesthetic agents. Clearly, this type of cell cannot be used on a patient airway. 
     Ultraviolet absorption uses the principle that oxygen exhibits absorption properties in the ultraviolet part of the electromagnetic spectrum (about 147 nm). This technique has been used in several medical applications but has never been reduced to commercial viability. There are numerous technical difficulties which make this a difficult technique for on-airway applications. 
     Mass spectrometers spread ionized gas molecules into a detectable spectrum according to their mass-to-charge ratios and can accordingly be used to measure oxygen concentration. These instruments are generally large assemblies with ionizing magnets and high vacuum pumps. The advantages of mass spectrometers include high accuracy, multi-gas analysis capability, and rapid response. The disadvantages include high cost, high power consumption, and large size. Mass spectrometers are not suitable for on-airway applications. 
     Raman scattering spectrometers (as described in U.S. Pat. No. 4,784,486) can also be used to measure oxygen concentration. These devices respond to photons emitted by the collision of a photon with an oxygen molecule. A photon from a high-power laser loses energy to the oxygen molecule and is re-emitted at a lower energy and frequency. The number of photons re-emitted at the oxygen scattering wavelength is proportional to the number of oxygen molecules present. Like mass spectrometers, Raman spectrometers have multi-gas analysis capability and rapid response time. Disadvantages include large size and power consumption. Therefore, Raman scattering photometers are not suitable for on-airway applications. 
     Visible light absorption spectrometers (as described in U.S. Pat. Nos. 5,625,189 and 5,570,697) utilize semiconductor lasers that emit light at near 760 nm, an area of the spectrum comprised of weak absorption lines for oxygen. With sophisticated circuitry, the laser can be thermally and/or electronically tuned to the appropriate absorption bands. The amount of energy absorbed is proportional to the number of oxygen molecules present. The advantages of this system are precision, fast response, and no consumable or moving parts. The disadvantages include somewhat fragile optical components, sensitivity to ambient temperature shifts, and a long gas sample path length. While there have been attempts to utilize this technology in an on-airway configuration, no commercially viable instruments have so far been available. 
     Luminescence-quenching has also been proposed as a technique for measuring oxygen concentration. In this approach, a sensor contacted by the gases being monitored is excited into luminescence. This luminescence is quenched by the oxygen in the monitored gases. The rate of quenching is related to the partial pressure of oxygen in the monitored gases, and that parameter can accordingly be used to provide an indication of the oxygen in the monitored gases. However, the prior art does not disclose an oxygen concentration monitor employing luminescence-quenching which addresses the problems associated with this type of measurement device in any practical application. These problems include: photo-degradation-associated and other instabilities of the sensor, low signal level, noise leading to difficulties in assessing the decay of sensor luminescence, acceptably fast response times, thermal drift of the sensor, reproducibility of the sensors, inaccuracies attributable to stray light reaching the data photodetector, and the need for light weight, ruggedness, and low power consumption. Disclosed in copending applications Ser. Nos. 09/128,918 and 09/128,897, both filed Aug. 4, 1998, are devices for monitoring oxygen concentration in gaseous mixtures which differ from the majority of the oxygen monitors described above in that they are compact, lightweight, and otherwise suited for on-airway mainstream monitoring of the oxygen concentration in a person&#39;s respiratory gases. These monitoring devices utilize the fast (or breath-by-breath) approach to oxygen concentration monitoring with the quenching of a luminescent dye being used in determining the concentration of oxygen in the gases being monitored. 
     Fast (breath-by-breath) monitoring of end tidal oxygen is an important diagnostic tool because, as examples only: 
     1. It is a sensitive indicator of hypoventilation. 
     2. It aids in rapid diagnosis of anesthetic/ventilation mishaps such as (a) inappropriate gas concentration, (b) apnea, and (c) breathing apparatus disconnects. 
     3. End tidal oxygen analysis reflects arterial oxygen concentration. 
     4. Inspired-expired oxygen concentration differences reflect adequacy of alveolar ventilation. This is useful for patients undergoing ECMO (Extracorporeal Membrane Oxygenation) or nitric oxide therapies. 
     5. When combined with a volume flow device (e.g. a pneumotach), VO 2  (oxygen consumption) can be determined. Oxygen consumption is a very useful parameter in determining (a) oxygen uptake during ventilation or exercise, (b) respiratory exchange ratio or RQ (respiratory quotient) and (c) general patient metabolic status. 
     The novel sensor devices disclosed in the copending applications locate a luminescent chemical in the patient airway. Modulated visible light excites the chemical and causes it to luminesce. The lifetime of the luminescence is proportional to the amount of oxygen present. A transducer containing a photodetector and associated electronic circuitry measures decay time and relates the measured parameter to the ambient oxygen partial pressure. 
     The transducer device is small (&lt;1 cubic inch), lightweight (less than 1 ounce), and does not contain moving parts. It utilizes visible light optoelectronics and consumes minimal power (system power less than 2 watts). The unit warms up in less than 30 seconds, which is advantageous in on-airway applications because of the need to take prompt remedial action if a change occurs in a patient&#39;s condition reflected in a change in respiratory oxygen concentration. The assembly does not require any significant optical alignment and is very rugged (capable of being dropped from 6 feet without affecting optical alignment or otherwise damaging the device). 
     The principles of the inventions disclosed in the copending applications can be employed to advantage in sidestream (sampling) type systems as well as in mainstream systems. This is important because some gas analysis systems, such as anesthetic analyzers, employ sidestream techniques to acquire their gas sample. 
     A typical transducer unit is easy to calibrate, is stable (±2 torr over 8 hours at a 21 percent oxygen concentration), and has a high resolution (0.1 torr) and a wide measurement range (oxygen concentrations of 0 to 100 percent). Response to changing oxygen concentrations is fast (&lt;100 ms for oxygen concentrations of 10-90 percent at flow rates ≈1|/min). The transducer is not susceptible to interference from anesthetic agents, water vapor, nitrous oxide, carbon dioxide, or other gases and vapors apt to be present in the environment in which the system is used. 
     The sensor comprises a polymeric membrane in which a luminescable composition such as a porphyrin dye is dispersed. The sensor membrane is the mediator that brings about dye-oxygen interaction in a controlled fashion. In a functional sensor, the dye is dispersed in the polymeric membrane, and oxygen diffuses through the polymer. The characteristics of the sensor are dependent upon the dye-polymer interaction and permeability and the solubility of oxygen in the polymer. Such characteristics include the sensitivity of response of the sensor to oxygen, the response time of the sensor to a change in oxygen concentration, and the measured values of phosphorescence intensity and decay time. Thus the composition and molecular weight of the polymer determines the sensor characteristics. Also, if the sensor is prepared by evaporation of a solution as described in the copending applications, the film characteristics depend on the solvent that is used and conditions during casting or evaporation. If the dye is separately doped into the film from another solution, the solvent and conditions in the doping medium also affect the sensor characteristics. When the polymer film is prepared by polymerization of a monomer or mixture, the sensor characteristics depend on the conditions of polymerization and such resultant polymer characteristics as degree of crosslinking and molecular weight. 
     The luminescent chemical sensor is not toxic to the patient and is a part of a consumable (i.e., disposable) airway adapter weighing less than 0.5 ounce. The sensor shelf life is greater than one year and the operational life exceeds 100 hours. The cost of the consumable airway adapter is minimal. 
     It is also important that the oxygen monitoring systems disclosed in the copending applications have sufficient accuracy (1.0%), precision(0.01%), and response time (&lt;100 ms) to monitor breath-by-breath oxygen concentrations. The sensor is not sensitive to other gases found in the airway, including anesthetic agents, and is accordingly not excited into luminescence by those gases. The sensitivity of the sensor to temperature, flow rate, pressure and humidity change is well understood, and algorithms which provide compensation for any errors due to these changes are incorporated in the signal processing circuits of the device. 
     The visible light oxygen measurement transducers disclosed in the copending applications employ a sensor heater arrangement and a proportional-integrated-differential (PID) heater control system for keeping the oxygen concentration sensor of the transducer precisely at a selected operating temperature. This is particularly significant because those oxygen measurement transducers employ a sensor which involves the use of the diffusion of oxygen into a luminescable layer in measuring oxygen concentration. The rate of diffusion is temperature dependent. As a consequence, the measurement of oxygen concentration becomes inaccurate unless the sensor temperature is kept constant. Also, if the window through which the excitation energy passes is not kept warm, it may fog over. This also affects the accuracy of the oxygen concentration measurement. 
     The location of the oxygen concentration sensor in a replaceable, simple component is a feature of the systems disclosed in the copending applications. This makes it possible to readily and inexpensively ensure that the system is sterile with respect to each patient being monitored by replacing the airway adapter between patients, avoiding the non-desirability (and perhaps the inability) to sterilize that system component. 
     The provision of an airway adapter sensor and a separate signal-producing transducer also has the practical advantage that a measurement of oxygen concentration can be made without interrupting either the ventilation of a patient or any other procedure involving the use of the airway circuit. This is effected by installing the airway adapter in the airway circuit. When the time comes to make oxygen measurements, all that is required is that the transducer be coupled to the airway adapter already in place. 
     Another important feature of the invention ensures that the airway adapter and transducer are assembled in the correct orientation and that the airway adapter and transducer are securely assembled until deliberately separated by the system user. 
     The signals generated by the oxygen-measurement transducers of the previously disclosed system are processed to remove noise and extract the luminescence decay time, which is the oxygen-sensitive parameter of interest. A lock-in amplifier is preferably employed for this purpose. The lock-in amplifier outputs a signal which has a phase angle corresponding to the decay time of the excited, luminescent composition in the oxygen concentration sensor. The lock-in detection circuitry rejects noise and those components of the photodetector-generated signal which are not indicative of oxygen concentration. This noise reduction also allows a higher level of signal gain which, in turn, makes possible enhanced measurement precision while decreasing the level of the visible excitation. This reduces instability from photoaging of the sensor, increasing accuracy and useable life. All of this processing, which can be done with digital, analog, or hybrid method, is fast enough for even the most demanding applications such as those requiring the breath-by-breath monitoring of a human patient. Various pathological conditions result in a change of oxygen demand by the body. If a decrease of oxygen utilization by the body, for example, can be detected on a breath-by-breath basis, timely and effective remedial steps can be taken to assist the patient. 
     In the novel oxygen measurement transducers of the present invention, the concentration of oxygen in the gases being monitored is reflected in the quenching of an excited luminescent composition in the oxygen concentration sensor by oxygen diffusing into the sensor matrix. A source consisting of a light-emitting diode (LED) produces visible exciting light which strikes the surface of the sensor film. Some of the light is absorbed by the luminescent chemical dye in the film whereupon it produces luminescent light at a second, shifted wavelength. This light is captured by a photodetector which thereupon generates a signal reflecting the intensity and decay pattern of the intercepted light. All light directed toward the photodetector can potentially result in a signal. A suitable optical filter placed over the surface of the photodetector discriminates against all but the luminescent light, thereby ensuring that the photodetector is producing a signal related to oxygen concentration only. 
     SUMMARY OF THE INVENTION 
     There have now been invented and disclosed herein new and novel oxygen concentration measuring devices which differ from those disclosed in the copending applications in that the light-sensitive, oxygen concentration sensor is located on the same side of the gas sampling device (typically an airway adapter or a sampling cell) as the light source and detector of an associated transducer. 
     This “single-sided” arrangement of the light source, oxygen sensor, and photodetector has a number of significant advantages. Specifically, in the systems disclosed in the copending applications, intimate contact between heater element components of the transducer and the sampling device is required, and this can prove difficult to achieve. This problem is eliminated in the single-sided systems disclosed herein by supporting the sensor from a near side optical window and by heating that window which thereupon transfers thermal energy to the associated sensor. 
     Another important advantage of the single-sided arrangements disclosed herein is that the energy of excitation indicative of oxygen concentration does not have to traverse the gases flowing through the sampling component. Consequently, the degradation in signal attributable to interactions between the gas being sampled and the energy of excitation is eliminated, making a significantly less-degraded signal available to the photodetector. 
     One of the two apertures present in the sampling component of the previously disclosed systems is eliminated, along with a sensor film heating component installed in that aperture. This leads directly to a less complex, less expensive sampling component. This is important because the sensor film has a finite, relatively short life, and the sampling unit must accordingly be periodically replaced. In fact, in an important application of the present invention—on-airway use in a hospital—it is highly desirable that the cost of the sampling unit be low enough to make it feasible to discard this unit after a single use. 
     The location of the sensor film on the opposite side of a flow passage from an optical window in the previously disclosed systems leaves the optical window essentially unheated, making it particularly prone to fogging. Contamination of this window may also be a problem, creating obstructions in the optical path between the sensor and the window. 
     The single-sided arrangement also makes feasible systems embodying the principles of the present invention where it is desirable to have a unit such as a freestanding film reader in close proximity to the sensor film as can be done with fiber-optics, for example. Such arrangements can be beneficially used in sensor film quality control and in transcutaneous oxygen monitoring, for example. Such arrangements are made practical by employing the principles of the present invention because the sensor film is associated with the optical window and not isolated from the exterior of the sampling component by a thermal component as disclosed in the copending applications. 
     Systems with the advantages just described differ physically from those disclosed in the copending applications in that the optical window in the airway adapter or sampling cell is employed as a mount or support for the sensor film and is also employed to transfer to the film the heat needed to keep it at a constant temperature. As will be apparent, this also results in the window being heated to a high enough temperature to eliminate fogging. Various schemes for heating the transparent window might be employed. One suitable approach is to surround the transparent window of the gas sampling device with a heater in a ring configuration. Of importance in systems employing the principles of the present invention is a secure application of the film-type sensor to the optical window of the sampling device. An adhesive layer may be employed to bond the sensor film to the window, or it may be solvent bonded to the window. Another approach is to employ a retaining ring to stretch the film over and secure it to the window. A related approach is to employ a retaining ring bounded on one side with a fine mesh to retain the film and press it against the window. The last-mentioned approach has the advantage that the film is physically retained without an adhesive and will not loosen. In addition, the mesh, with its location on the gas side of the sensor, enhances heat conduction over that side of the sensor, producing exceptional thermal stability. 
     In monitoring apparatus embodying the principles of the present invention, light not indicative of the concentration of oxygen in the gas being monitored is preferably kept from the detector of that apparatus by locating a blue dichroic filter and an infrared-blocking filter in line with and on the output side of the light source and by summarily locating a red dichroic filter and a red glass filter in front of the detector apparatus. Because this arrangement eliminates essentially all of the light which is not part of the signal indicative of oxygen concentration, the light collection efficiency is increased to the extent that the intensity of the exciting light from the LED or other source can be reduced. This is important because reducing the intensity of the light from the source significantly increases the service life of the sensor. This is particularly significant in sidestream applications of the present invention where the sensor is not apt to be replaced each time it is used. 
     Other objects, advantages, and features of the present invention will be apparent to the reader from the foregoing and the appended claims, and as the ensuing detailed description and discussion is read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE OF THE DRAWINGS 
     In the drawings, like reference numerals refer to like parts throughout the various views: 
     FIG. 1 is a block diagram of several main elements of a luminescence-quenching gas detection apparatus of the present invention showing general interrelationships between the main elements. 
     FIG. 2 is a graph that depicts characteristic emission curves of an excited sensing film of the present invention showing qualitative emission intensity vs. time. 
     FIG. 3 is a perspective view of an airway adapter and a complementary transducer that shows a particular physical embodiment of a portion of the block diagram of FIG.  1 . 
     FIG. 4 is a generally pictorial view of an inline system for monitoring the oxygen concentration in a patient&#39;s breath. 
     FIG. 5 is a perspective view of an alternative airway adapter and a complementary transducer that shows the relationship of an optical block assembly to the airway adapter. 
     FIG. 6 is a diagram showing the optical alignment of key optical components of a transducer and sampling cuvette of the present invention in a “single-sided” arrangement. 
     FIGS. 7 and 8 are diagrams showing the relationship of the optical components in “straight-through” and “two-sided” arrangements respectively, as disclosed in a prior co-pending application. 
     FIG. 9 is a cross-sectional view of the airway adapter and transducer assembly of FIG. 5 showing the spatial relationship of key optical components in the optical block assembly. 
     FIG. 10 shows a perspective view of a sidestream embodiment of the present invention. 
     FIG. 11 illustrates a nasal canula component for sampling a patient&#39;s respiratory gases for subsequent monitoring by a sidestream monitor such as that shown in FIG.  10 . 
     FIG. 12 depicts an exploded view of the sidestream embodiment shown in perspective in FIG. 10 showing pertinent details of device assembly. 
     FIG. 13 is a cross-sectional view of the sidestream gas measurement system of FIGS. 10 and 12 showing especially details of optical alignment and heater-to-sensing film relationship. 
     FIG. 14 is a block diagram of a DSP-based controller that is especially well adapted for a mainstream embodiment of the invention. 
     FIG. 15 is a block diagram that describes more specifically the methodology for determining oxygen concentration from the luminescence characteristics of a sensing film. 
     FIG. 16 is a block diagram of a DSP-based controller adapted for a sidestream embodiment of the invention, showing especially functionality of the transducer-cuvette assembly. 
     FIG. 17 is a block diagram of a controller for a sidestream gas measurement system, showing especially functionality of the DSP controller, with correction for pressure and various output interfaces 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The descriptions contained herein adhere to a numbering convention intended to facilitate understanding and make for easy cross-referencing of described features between figures. In this convention, the first digit (for features indicated by a three-digit reference number) or the first two digits (for features indicated by a four-digit reference number) correspond(s) to the figure number in which the feature is first described. Like features are thus identified by the same reference number throughout the detailed description. In some instances, features described by the same reference number may have a different physical appearance in two or more figures. In this case, the use of a like reference number is especially useful in drawing the attention of the reader to various physical embodiments that a given feature of the invention may have. Features first introduced within the same figure are numbered more-or-less consecutively in a manner corresponding to the order in which they are described. 
     In each instance, physical forms depicted herein are intended to be illustrative of particular embodiments of the invention. They are given such particular physical form to facilitate understanding. In no case is the choice of a particular physical form intended to be limiting unless specifically so stated. A reader skilled in the art will readily recognize many alternative but equivalent physical embodiments, each of which is intended to fall within the scope of the invention taught herein. 
     Referring now to the figures, and in particular to FIG. 1, there is illustrated a block diagram showing the main components and relationships therebetween of a luminescence-quenching oxygen concentration monitoring apparatus in accordance with the principles of the present invention. A cuvette or airway adapter  101  contains a volume  102  that serves as a gas sampling cell. For applications requiring sidestream sampling of respiration or other gases, inlet/outlet ports  103   a  and  103   b  provide means for introduction of the gas to the sampling volume  102  (also referred herein as “sensing volume  102 ”) and venting of gas from the sampling volume, respectively. For mainstream applications and other applications requiring bidirectional transmission of the gas through the sampling volume  102 , the role of inlet/outlet ports  103   a  and  103   b  alternates with respect to the instantaneous direction of gas passage therethrough. A sensing film  104  held in intimate contact with gas in the sampling cell provides a medium for a luminescence-quenching reaction that forms the basis of the measurement technique of the present invention. 
     A transducer  105  is closely coupled to the cuvette  101  so as to allow a light source  106  to illuminate the sensing film  104  with electromagnetic radiation. The light or excitation energy emitted from light source  106  is illustrated as a wavy line  107 . For many applications, it is desirable for the sampling volume  102  to be isolated from the transducer  105 . In these cases, an aperture  108  may take the form of a window set into the housing of airway adapter  101  or may be formed integrally therein. 
     According to the reaction used for gas measurement, light or excitation energy  107  causes the sensing film  104  to emit a luminescence, indicated by wavy lines  109 , in a substantially omnidirectional manner at a wavelength different from that of the excitation energy  107 . The emitted luminescence or luminescent energy  109  falls on a photodetector  110  for measurement. The intensity and persistence of this luminescence rises and falls according to the concentration of one or more gas components contained within the sampling volume  102 . In a preferred embodiment of the present invention, oxygen causes a modification of the intensity and persistence of the luminescent energy by quenching the luminescence reaction as its concentration increases. Thus the luminescence-quenching reaction is used to measure the amount of oxygen available to reaction sites within the sensing film  104 . The quantity of oxygen available to the reaction sites may, in turn, be related to its partial pressure or concentration within the measured gas. 
     According to a preferred embodiment of the present invention, light source  106 , which may be in the form of a blue or green light-emitting diode (LED), is pulsed so as to provide to the sensing film  104  excitation energy  107  that varies in time. Accordingly, luminescent energy  109  emitted from the film varies in time at a substantially red wavelength. The photodetector  110 , in turn, senses a cyclical variation in emitted energy, the persistence and intensity of which is proportional to the oxygen concentration of the gas introduced into the sampling volume  102  of the airway adapter  101 . The inventors have discovered that for many applications, the persistence of the emitted luminescent energy  109  forms a more reliable and repeatable basis for measurement of oxygen concentration than does the intensity or amplitude of the emitted energy. 
     Transducer  105  is connected to control and measurement circuitry  112  by means of electrical connections indicated by the line  111 . Control and measurement circuitry  112  may, in turn, be connected to an external computer, communication, display or other device by means of connections  113 . 
     A temperature regulation apparatus  114 , which, in a preferred embodiment, is a heater held in intimate contact with the sensing film  104 , is maintained in a relationship to the sensing film to provide adequate control of film temperature while not interfering with the light transmission paths of excitation energy  107  and luminescence energy  109 . As will be appreciated by the following discussion, control of sensing film temperature is important to the luminescence-quenching rate as a function of oxygen concentration. 
     Taken together, the components of the block diagram illustrated in FIG. 1 form an oxygen concentration monitoring apparatus  115 . 
     Turning our attention now to FIG. 2, there is illustrated a qualitative graph showing the relationship of the intensity and persistence of luminescence in the sensing film as they may vary with oxygen concentration. The vertical axis is an arbitrary indication of intensity or brightness of the luminescence, while the horizontal axis is an arbitrary indication of time. While no units are given in the illustration, the total time scale of the horizontal axis is generally well under 1 second. For purposes of understanding FIG. 2, one may assume that excitation energy begins to illuminate the sensing film at to and ceases at t 1 . Curve  201  indicates the natural luminescence of the sensing film in the absence of oxygen. Higher concentrations of oxygen progressively decrease both the peak luminescence and the luminescence decay time. Curve  202  illustrates the effect of luminescence-quenching in the presence of a moderate oxygen concentration of, for example, 21% at 1 atmosphere pressure. Curve  203  shows a higher degree of luminescence-quenching caused by a higher oxygen concentration of, for example, 50% at 1 atmosphere pressure. 
     By inspection of FIG. 2, one can see that both the peak luminance and the decay time decrease as oxygen concentration increases. By measuring the decay time over a series of excitation pulses, real-time measurement of oxygen concentration is effected. 
     It is of particular note that characteristic luminescence response of the sensing film  104  as a function of oxygen concentration is a strong function of film temperature. This is due to the fact that it is the presence of oxygen within the sensing film at the site of each luminescence reaction that determines whether or not that particular luminescence reaction will be quenched. In this manner, it is the statistical proximity of oxygen molecules to the population of luminescence reaction sites within the sensing film that determines the overall macroscopic luminescence-quenching effect illustrated by curves  201 ,  202 , and  203 . The presence and concentration of oxygen within the sensing film  104  is a function of the rate of diffusion of oxygen within the film. As with most or all diffusion rate-limited reactions, oxygen luminescence-quenching is thus a strong function of temperature. Accordingly, embodiments of temperature regulation apparatus play a significant role in the enablement of the present invention. 
     Referring now to FIG. 3, there illustrated is a perspective view of an embodiment of certain parts of the present invention wherein the sampling cell is in the form of a mainstream airway adapter. The airway adapter  101  includes inlet/outlet ports  103   a  and  103   b  respectively. Aperture  108  is indicated by dashed lines and lies on an unseen side of the airway adapter. A transducer  105  is formed to securely attach to the airway adapter  101  by a snap fit, for instance. By forming the sampling cell and transducer in separate couplable bodies, the airway adapter  101  may readily be made replaceable or even disposable without incurring the extra cost of replacing all the optical and signal conditioning components every time an airway adapter is discarded. It is particularly advantageous to form the sampling cell as a disposable unit for mainstream applications so that each patient can be provided with his or her personal airway adapter without fear of contamination by another individual. Making the airway adapter replaceable also serves to make connection of oxygen monitoring apparatus quick and easy and allows the more expensive transducers to be easily shared among multiple patients without causing an interruption in airway flow while removing or inserting a measuring apparatus. Finally, making the mainstream airway adapter disposable also ensures that fresh sensing films are provided to each patient. This is important due to a tendency for the sensing film to gradually undergo photo-degradation. 
     The mainstream airway adapter body  101  may be comprised of any of a number of suitable materials. In one embodiment, airway adaptor  101  is a one-piece unit typically molded from Valox polycarbonate or a comparable polymer that is rugged and can be molded to close tolerances. An opaque material is employed to keep ambient light from reaching the sensing film  104  through the walls of the airway adapter. Such extraneous light would adversely affect the accuracy of the oxygen concentration reading that the system is designed to provide, or at least degrade the signal-to-noise ratio of the characteristic signal, thus requiring more sophisticated and expensive control and detection means. 
     Airway adapter  101  has a generally parallelepipedal center section  301  and hollow, cylindrical inlet/outlet ports  103   a  and  103   b  at opposite ends of center section  301 . Axially aligned passages  302   a ,  102 , and  302   b  found in airway adapter elements  103   a ,  301 , and  103   b , respectively, define a flow passage extending from end-to-end of airway adapter  101 . Port section  103   a  may be configured as a female connector and port  103   b  may be configured as a male connector, thus allowing the airway adapter to be connected to conventional anesthetic and respiratory circuits. 
     The center section  301  of the airway adapter  101  is formed so as to fit snugly into a correspondingly shaped section  303  of transducer  105 . When airway adapter  101  is properly snapped into transducer  105 , aperture  108  in the airway adapter is held in an orientation relative to a corresponding aperture  304  so as to allow passage of light therebetween. As described and shown in FIG. 1, excitation energy  107  (see FIG. 1) comprised of blue or green light is transmitted from transducer  105 , through apertures  304  and  108 , and into contact with a sensing film  104  (see FIG. 1) held in intimate contact with the gas contained within sensing volume  102 . In response, and with a signal strength and duration characteristic of the oxygen concentration of the gas in sensing volume  102 , the sensing film  104  emits electromagnetic radiation back through apertures  108  and  304  onto a photodetector  110  (see FIG. 1) held inside transducer  105  with a field of view comprising at least a portion of the sensing film  104  (through apertures  304  and  108 ). In a preferred embodiment, apertures  108  and  304  contain windows which permit the transmission of both excitation and luminescence radiation therethrough. 
     Incorrect assembly of the airway adapter  101  into transducer  105  is precluded by the inclusion of location features such as stops  305  and  306  on the airway adapter  101  and complementary stops  307  and  308 , respectively, on the transducer  105 . 
     FIG. 4 depicts an oxygen concentration monitoring apparatus or system  115  as it may be used in operation. A mainstream airway adapter  101  and transducer  105 , as illustrated in FIG. 3, make up the major components of inline assembly or system  401 . The monitoring system  115  illustrated in FIG. 4 also includes a hand-held control and measurement circuitry display unit  112  that is connected to transducer  105  by a conventional electrical connection  111 . 
     In the particular application of the present invention illustrated in FIG. 4, system  115  is employed to monitor the concentration of oxygen in a patient&#39;s respiratory gases. To this end, airway adapter  101  is connected in line between an endotracheal tube  402  inserted in the patient&#39;s trachea and the breathing circuit  403  of a mechanical ventilator (not shown). 
     Airway adapter  101  and transducer  105  cooperate to produce an electrical signal indicative of the oxygen concentration in the gases flowing from endotracheal tube  402  through airway adapter  101  to breathing circuit  403 . This signal is transmitted to unit  112  through electrical connection  111  and converted to a numerical designation that appears on the display array  404  of unit  112 . 
     The two-component system  401  just described meets the requirement that monitoring be accomplished without interrupting the flow of gases through breathing circuit  403  or other patient-connected flow circuit. Transducer  105  can be removed—for example, to facilitate or enable the movement of a patient—leaving airway adapter  101  in place to continue the vital flow of gases. 
     System  115  has, in this regard, the advantage that there are no electrical components in the airway adapter. Hence, there are no potentially dangerous electrical connections to the airway adapter which might expose the patient to an electrical shock. 
     FIG. 5 illustrates another embodiment of two-piece assembly  401 . Airway adapter  101  includes the three sections  103   a ,  301 , and  103   b  that together form an inline gas flow passage  302   a ,  102 , and  302   b . Center section  301  of inline airway adapter  101  is formed to fit snugly into corresponding section  303  of transducer  105 . Stops  305  and  306  on airway adapter are formed so as to create a snug fit with corresponding stops  307  and  308 , respectively, when inline airway adapter  101  is coupled to transducer  105 . Aperture  108 , formed in a side of airway adapter center section  301 , contains a window  501 . Window  501  supports sensing film  104  (not shown) within sensing volume  102  and provides a thermal energy transmission path from a temperature regulation apparatus  114  (see FIG. 1) housed within transducer  105 . 
     Transducer  105  contains an optical block assembly  502 . Optical block assembly  502  contains the light source  106  and photodetector  110  (see FIG. 1) in proper alignment. Optical block assembly  502  also houses a heater assembly  114  (not shown) for maintaining a constant temperature within sensing film  104  (not shown). The use of an optical block assembly  502  as a subassembly aids in the manufacturability of the transducer  105 . By containing all critical alignments and tolerances associated with transducer  105  within optical block assembly  502 , the manufacturing tolerances of the outer housing of transducer  105  may be loosened somewhat, thus reducing cost. Furthermore, service related to failure of one or more components within the optical block assembly  502  may be treated as a subassembly level repair, rather than forcing a replacement of the entire transducer assembly  105 . 
     FIG. 6 is a conceptual diagram of the main optical components of an embodiment of the present invention. Light emitting diode (LED)  106  emits blue or green light in response to an energization signal transmitted via leads  601 . The blue or green light passes through dichroic filter  602  and infrared-blocking filter  603 . In the embodiment illustrated in FIG. 6, the light energy then passes through an aperture in heater  114 , through window  501 , and falls upon sensing film  104 . Sensing film  104  is held in intimate contact with window  501  by any of a number of methods, such as adhesive or solvent bonding, or via a retaining ring or mesh covering. This allows the sensing film  104  to freely contact the gas within sensing volume  102 . 
     LEDs are known to generally emit a relatively broad range of light wavelengths extending to some degree even into the infrared. The dichroic filter  602  and infrared-blocking filter  603  cooperate to significantly reduce wavelengths other than the narrow range of wavelengths passed by the dichroic filter. The particular wavelength chosen for passage by the dichroic filter  602  may be selected to correspond to the peak output of LED  106  and to a suitable energization wavelength for the sensing film  104 . In a preferred embodiment, this wavelength is chosen to be in the blue range of the visible electromagnetic spectrum. 
     Energization light incident upon sensing film  104  causes the film to begin to emit light of a different wavelength. The sensing film may be comprised, for instance, of a microporous polycarbonate film having a platinum-porphyrin dye contained therein as in a guest-host system. The microporosity of the film represents a novel approach in the preparation of films designed for the monitoring of gaseous oxygen concentrations. The preparation of the polymeric membrane is well known in the art of manufacturing microporous screens and will not be described in detail herein. Suffice it to say that the process involves two steps wherein the polymer film is exposed to collimated, charged particles in a nuclear reactor which pass through the polymer, leaving behind sensitized tracks which are then etched into uniform cylindrical pores. The incorporation of the luminescent sensing material into the film is more fully described in co-pending U.S. patent application entitled “Oxygen Monitoring Methods and Apparatus” having Ser. No. 09/128,897, hereby incorporated herein in its entirety by this reference. 
     In one embodiment of the present invention, the emission wavelength of the sensing film  104  corresponds to light in the red portion of the visible electromagnetic spectrum. An LED  106  is repeatedly pulsed at a frequency of 20 kilohertz with its output excitation energy  107  rising and falling as a sinusoidal wave. This causes a rise and fall in luminescence energy emitted from the sensing film  104  that is a function of oxygen concentration in sensing volume  102 . The effect of a single pulse is qualitatively illustrated in FIG.  2 . 
     Luminescence emitted by sensing film  104  passes through window  501 , through an aperture in heater  114 , through red dichroic filter  604 , through red filter  605 , and impinges upon photodetector  110 . Red filter  605  may be comprised of a conventional glass or gel filter. Red dichroic filter  604  and red filter  605  cooperate to virtually eliminate any light emitted by LED  106  through dichroic filter  602  and infrared-blocking filter  603  from reaching photodetector  110 . The geometric relationship of emitter and detector field-of-views further serves to reduce the amount of excitation energy reaching photodetector  110  arising, for instance, from specular reflection off a surface of window  501 . 
     Heater  114  is maintained in intimate contact with window  501  so as to maximize the effectiveness of the energy conduction path from heater  114  through window  501  into sensing film  104 . Maintaining a constant temperature within sensing film  104  is advantageous for keeping the relationship between oxygen concentration within sensing volume  102  and the amount of luminescence-quenching sensed by photodetector  110  constant. Window  501  is preferably comprised of a material having relatively high thermal conductivity and high transparency such as sapphire, glass, quartz, polycarbonate, or other material apparent to those skilled in the art. Window  501  should be constructed so as to maximize transmission of excitation energy and especially to maximize transmission of luminescence energy. The materials listed above also accomplish this aim. Furthermore, it is advantageous to maintain the temperature of the sensing film  104  and window  501  somewhat above the temperature of the gas in sensing volume  102 . This serves to avoid condensation of vapors on the window, which may otherwise obscure the window and reduce the effectiveness of the sensing apparatus. 
     The arrangement of emitter, detector, filters, and sensing film described by FIG. 6 is particularly effective at maximizing the signal-to-noise ratio of the detection apparatus of the present invention. The arrangement of electrical components shown in FIG. 6 on one side of sensing volume  102  serves to reduce cost and improve reliability compared to other arrangements wherein electrical components are arrayed on opposing sides of sensing volume  102 . FIGS. 7 and 8 illustrate configurations of the optical components representative of such arrangements and of those disclosed in co-pending application Ser. No. 09/128,918. 
     Turning our attention now to FIG. 9, a cross-sectional view of two-component assembly is illustrated generally at  401  showing especially the means for optical alignment of key components. The arrangement of components correlates most closely to the embodiment depicted in FIG. 6 in accordance with the principles of the present invention. The center section  301  of inline airway adapter  101  is held in place within transducer housing  105 . Center section  301  of the inline airway adapter  101  is held in correct optical alignment with optical block assembly  502  by means of the close fit between stop features  306  and  308  (not shown) and between the outer walls of airway adapter  101  and the inner walls of the transducer body  105  as illustrated by FIGS. 3 and 5. 
     Optical block assembly  502  is comprised of an optical block casing or body  901  that holds key optical components in boresight alignment by means of two bores created therein, light source bore  902  and detector bore  903 , each of which is aligned to hold their respective components so as to create substantially coincident fields of view of sensing film  104 . LED  106  and filters  602  and  603  are held in LED mounting tube  904 . LED mounting tube  904  may be constructed of brass tubing or other appropriate material. LED mounting tube  904  is coupled to light source bore  902  and holds the LED and filters for illuminating the sensing film  104 . LED  106  receives a signal via leads  601  from optical block circuit board  905 . In another embodiment, LED  106  receives a signal through leads  601  from optical block circuit board  905 . Optical block circuit board  905  further provides means for mounting photodetector  110  and holding it aligned with detector bore  903 . Light emitted from sensing film  104  thus passes through window  501 , traverses detector bore  903 , passes through red dichroic filter  604  and red filter  605 , and impinges upon photodetector  110 . In a preferred embodiment, photodetector  110  is comprised of a photodiode. 
     Heater  114  is shown in cross-section with its aperture therethrough allowing passage of both excitation energy and luminescent emission. Parts of heater  114  peripheral to the aperture are held in intimate contact with window  501 . Sensing film  104  is maintained in intimate contact with window  501  by optional porous member  906  or by other means as described previously. Porous member  906  may be comprised of any material that allows free passage of the gas in sensing volume  102  to sensing film  104  and has appropriate tensile strength and heat-resistance properties. In practice, it has been found that it is especially advantageous for porous member  906  to be comprised of a stainless steel screen. In this embodiment, heat conduction along the wires of stainless steel screen  906  aids in the control and maintenance of the temperature of sensing film  104 . 
     FIG. 10 shows a perspective view of a sidestream embodiment of the present invention. Circuit board  1001  supports an optical block assembly  502 . A sampling cuvette  101  containing a sampling volume  102  and inlet/outlet ports  103   a  and  103   b  is affixed to the optical block with machine screws (not shown) or by other means known in the art. Optical block  502  also includes a light source bore  902  which contains LED  106 . LED  106  is, in turn, connected to circuit board  1001  and the circuit thereon by means of leads  601 . 
     The cuvette  101  may be made from machined and anodized aluminum with ports  103   a  and  103   b  press-fit therein. Optical block casing  901  may be similarly constructed from machined and anodized aluminum. 
     Circuit board  1001  may contain all or part of control and measurement circuitry in addition to providing a mounting point for optical block assembly  502 . In some embodiments, circuit board  1001  may be mounted inside diagnostic equipment such as an anesthesia monitor and provide connections  113  (not shown) to such equipment. 
     FIG. 11 illustrates a nasal canula component which may be employed to sample a patient&#39;s respiratory gases for subsequent monitoring by a sidestream monitor such as that shown in FIG.  10 . The nasal canula of FIG. 11 is of the conventional type typically found in hospitals or other health care facilities. It includes tubing  1101  that fits over the head of a patient  1102 . An insert  1103  in the tubing features a pair of protruding tube-shaped members  1104  that fit into the patient&#39;s nostrils. The nasal canula is connected as by tubular fitting  1105  to a flexible Nafine drying tube  1106 . The drying tube removes moisture from gases exhaled by patient  1102 , thereby eliminating errors that moisture might cause. At the far end of the Nafine drying tube  1106  is the female component  1107  of a conventional Leur fitting. A male Leur fitting (not shown) may be connected to a gas sampling tube (not shown) and transmitted to a sidestream oxygen sensing device such as that of FIG. 10 by means of a pump (not shown) such as a peristaltic pump. 
     FIG. 12 shows an exploded view of the sidestream gas measurement device illustrated in FIG.  10 . Photodetector  110 , in the form of a photodiode, is mounted through holes in photodiode mounting block  1201  to circuit board  1001  and thus connected into the circuit thereon. Photodiode mounting block  1201  is itself glued to the surface of circuit board  1001  in order to hold photodetector  110  at the correct height in detector bore  903 , which is formed in optical block body  901 . Filters  604  and  605  are mounted into the detector bore  903  of optical block body  901  in the manner indicated. Optical block body  901  is affixed to circuit board  1001  using optical block mounting screws  1202   a  and  1202   b  which extend through holes in circuit board  1001  into tapped holes  1203  (only one hole,  1203   a , is indicated for clarity) formed diagonally across detector bore  903  in optical block body  901 . Optical block locating stops  1204   a  and  1204   b  (not shown) are located on the opposite diagonal of detector bore  903  to optical block mounting screws  1202   a  and  1202   b  and extend into holes formed in circuit board  1001  for aiding the proper location of optical block body  901 . 
     LED mounting tube  904  extends into light source bore  902  in optical block body  901  and is held therein via a press fit, trapping dichroic filter  602  and infrared blocking-filter  603  against a shoulder formed within the light source bore. An optional diffuser may be inserted between dichroic filter  602  and LED  106  for reducing hot spots in the LED emission pattern. LED  106  is held inside LED mounting tube  904  using a press fit, adhesive mounting, or any suitable alternative mounting method. LED leads  601  extend through an aperture  1205  formed in circuit board  1001  and are soldered to traces on the bottom of the circuit board  1001 . 
     Cuvette  101  is coupled to optical block body  901  with gas sensing volume  102  registered on axis to detector bore  903  using two screws  1206   a  and  1206   b  extending through corresponding holes in cuvette  101  formed diagonally to gas measurement volume  102 . Screws  1206   a  and  1206   b  couple into corresponding tapped holes  1207   a  and  1207   b , respectively, formed in optical block body  901 . Ports  103   a  and  103   b  are inserted into cuvette  101  and may be attached via screws, press fitting, or adhesive, or may be formed integrally into the cuvette body, or may be held in place using other means apparent to one skilled in the art. Stops  1208   a  and  1208   b  formed in optical block body  901  extend into corresponding holes  1209   a  and  1209   b  formed in cuvette  101  at an opposite diagonal to screws  1206   a  and  1206   b  relative to detector bore  903  and sensing volume  102 . Stops  1208   a  and  1208   b  and their corresponding holes  1209   a  and  1209   b  aid in locating the cuvette relative to the optical block body  901  and are especially useful during assembly. The cuvette body may be constructed of machined aluminum, machined stainless steel, die cast metal, molded plastic, or other suitable material. 
     Porous member  906 , sensing film  104 , and window  501  are captivated on a shoulder formed circumferentially to gas sensing volume  102  in cuvette  101 . These may be affixed by press fit or may be affixed in place using silicone adhesive or other alternative means apparent to those skilled in the art. Window  501  may be comprised of sapphire, glass, quartz, plastic or other material. Materials for window  501  may be chosen for their combination of high transparency at excitation and emission wavelengths as well as high thermal conductivity and low thermal mass. Heater  114  is urged into intimate contact with window  501  by heater springs  1210  which extend into corresponding holes  1211  formed in optical block body  901 . In one embodiment, heater  114  is a ceramic heater with integral thermister. The use of springs  1210  to hold heater  114  against window  501  helps to eliminate point loading and/or tight tolerance requirements on heater  114  and the corresponding gap between cuvette  101  and optical block body  901 . For the case where heater  114  is formed of ceramic or other brittle material, this arrangement also serves to reduce heater breakage during assembly and during service. In one embodiment, springs  1210  may be formed from silicone rubber. 
     Referring now to FIG. 13, a cross-sectional view of the sidestream gas measurement system of FIGS. 10 and 12 is shown. Detector bore  903  in optical block body  901  has two shoulders  1301  and  1302  formed circumferentially at the bottom of the bore  903 . Shoulder  1301  serves as a stop for locating of the top of red dichroic filter  604 . Shoulder  1302  serves as a stop for locating the top of photodiode mounting block  1201 . Photodetector  110  is supported on photodiode mounting block  1201  and presses up against red filter  605 . Red filter  605 , in turn, presses against the bottom of red dichroic filter  604  and urges it against shoulder  1301  in detector bore  903 . When circuit board  1001  is affixed to optical block body  901  using screws  1202   a  (not shown) and  1202   b , photodiode mounting block  1201  is urged against shoulder  1302  in detector bore  903 . Photodiode mounting block  1201  also presses the assembly comprising photodetector  110 , red filter  605 , and red dichroic filter  604  against shoulder  1301  in the detector bore  903 . In this way, when optical block body  901  is affixed to circuit board  1001 , the entire detector assembly is securely coupled to its correct location in the optical block body. 
     Light source bore  902  has one shoulder  1303  formed therein for locating the end of LED mounting tube  904 . Shoulder  1303  furthermore serves to locate the top of infrared-blocking filter  603 . When LED mounting tube  904  is pressed into emitter bore  902  of optical block body  901 , it pushes against the bottom of dichroic filter  602 , urging it up into its correct location above LED  106 . The top of dichroic filter  602 , in turn, presses against the bottom of infrared-blocking filter  603 , which itself is urged against shoulder  1303  in light source bore  902 . In this way, the proper insertion of LED mounting tube  904 , with LED  106  held therein, in light source bore  902  captures the entire light source assembly comprising the LED  106 , dichroic filter  602 , and infrared blocking filter  603  at its correct position in optical block body  901 . 
     LED mounting tube  904  and the rest of the light source assembly may be inserted into the light source bore  902  of optical block body  901  through aperture  1205  in circuit board  1001  after securely affixing the optical block body  901  to the circuit board using screws  1202   a  and  1202   b . Alternatively, the light source assembly may be inserted into the light source bore  902  prior to attaching the optical block body  901  to circuit board  1001 . In either case, LED leads  601  may be subsequently bent into position contacting their corresponding electrical traces (not shown) on circuit board  1001  and soldered thereto. Alternatively, other types of socketed connectors may be used to receive LED leads  601  or their equivalent or other types of permanent connection may be made. 
     Cuvette body  101  has a shoulder  1305  formed circumferentially to the bottom aperture of gas sensing volume  102 . Shoulder  1305  serves as a location feature for locating the sensor and window assembly comprising porous member  906 , sensing film  104 , and window  501  relative to gas sensing volume  102 . Optical block body  901  has a depressed planar area  1304  corresponding to and extending beyond shoulder  1305  formed between cuvette mounting surfaces. This serves to provide a volume for accepting heater  114  and any protruding thickness of window  501 . Four heater spring holes  1211  extend from planar area  1304  into the volume of optical block body  901 . Four heater springs  1210  are inserted into heater spring holes  1211  prior to placing heater  114  thereon with its aperture located axially along detector bore  903 . Cuvette  101  with the sensor and window assembly seated therein is placed over heater  114  and located with window  501  aligned axially to detector bore  903 . Stops  1208   a  (see FIG. 12) and  1208   b  formed in optical block body  901  extend into holes  1209   a  (see FIG. 12) and  1209   b , respectively, formed in cuvette  101 . Stops  1208   a  and  1208   b  and their corresponding holes  1209   a  and  1209   b  aid in the alignment of window  501 , sensing film  104 , porous member  906 , and gas sampling volume  102  to the detector bore  903  formed in the optical block body  901  during assembly and service. As cuvette mounting screws  1206   a  and  1206   b  are tightened, heater springs  1210  compress in their holes  1211  and urge heater  114  against the bottom of window  501 . This upward pressure on window  501  further compresses sensing film  104  and porous member  906  against shoulder  1305  in sensing volume  102  of cuvette  101 . As screws  1206   a  and  1206   b  are torqued to predetermined values, the bottom of cuvette  101  comes into close coupling with the top surface of optical block body  901 . Thus the use of heater springs  1210  to compress the assembly comprising heater  114 , window  501 , sensing film  104 , and porous member  906  against shoulder  1305  causes the entire sensor and window assembly to be brought into correct optical alignment with other components of optical block assembly  502  when cuvette  101  is properly coupled against optical block body  901 . 
     FIG. 14 is a block diagram of a controller for controlling the gas measurement apparatus of the present invention and for receiving data that may be converted to gas concentration information. The controller of FIG. 14 is particularly applicable to a mainstream gas analyzer such as that depicted by FIGS. 3 through 5. 
     The main assemblies shown in FIG. 14 include a controller corresponding to circuitry and display  112  from FIG. 1, transducer  105 , and cuvette or airway adapter  101  containing sensing film  104 . Transducer  105  contains LED  106 , photodetector  110 , and heater  114 , and additionally a thermostat  1401 , a memory  1405 , and a photodetector pre-amp  1409 . 
     Control and electrical connections  11  connect control and measurement circuitry  112  to transducer  105  and include cuvette temperature signal  1402 , heater control line or signal  1403 , data line  1406 , LED drive  1407 , and oxygen signal  1410 . Excitation light  107 , luminescence light  109 , and heat conduction path  1404  form the interface between transducer  105  and airway adapter  101 . 
     Digital Signal Processing (DSP) controller  112  may, for example, contain control and detection circuitry as well as communications circuitry and logic for communicating with a host computer and/or for displaying gas concentration measurement data to the user. One aspect of system operation controlled by DSP controller  112  is the temperature of the sensing film  104 . 
     Heater  114  may contain an integral thermostat  1401  or, alternatively, may contain a separate thermostat  1401 . In any event, heater  114  may preferably contain a circuit to cut heater drive in the event of heater control failure. Thermostat  1401  and associated heater cut-off circuit serves as a fail-safe device to avoid runaway heater drive and a resultant possibly unsafe situation or destruction of sensing film  104 . Cuvette temperature is transmitted to the DSP controller circuit by an analog signal  1402 , the voltage of which is proportional to the temperature of heater  114  and, by extension, the temperature of sensing film  104 . Analog cuvette temperature signal  1402  may, for instance, be generated by a thermistor integral to or otherwise coupled to heater  114  or, alternatively, coupled to a convenient location whose temperature varies proportionally to the temperature of heater  114 . Heater control signal  1403  is driven from DSP controller  112  as a pulse width modulated (PWM) digital control signal whose duty cycle is controlled by a fuzzy logic controller embedded within DSP controller  112 . The fuzzy logic portion of the DSP controller is programmed in a manner similar to a proportional integral-differential (PID) controller. Fuzzy logic embedded in the DSP controller  112  monitors the analog cuvette temperature signal  1402  via an analog-to-digital (A/D) converter and controls the duty cycle of PWM heater control signal  1403  in response. The duty cycle of heater control signal  1403  is controlled to be higher when the cuvette temperature is cooler and controlled to be lower when the cuvette temperature is warmer. In practice, this control methodology may be used to maintain a constant temperature in sensing film  104 . Heater control signal  1403  drives a transistor (not shown) that may, for instance, be integral to heater  114 . The transistor driven by PWM heater control signal  1403  acts as a relay that switches drive current to heater  114  on or off. Heat flows from heater  114  to sensing film  104  via a heat conduction path  1404 . By setting the temperature of sensing film  104  above that of the flowing gas to be sensed, heat always flows from the heater  114  to the sensing film. The amount of heat modulated by heater control signal  1402  thus may always act as a positive control signal, heat never needing to be removed from the system. 
     Memory element  1405 , which may, for instance, be embodied as electrically erasable programmable read-only memory (EEPROM) or flash memory, is associated with a transducer  105 . Memory  1405  contains a transducer serial number and calibration information indicating oxygen concentration vs. phase shift. At boot-up, controller  112  reads the transducer serial number from memory  1405  to determine if proper calibration information has been loaded. If the transducer  105  is the same unit that had been connected to DSP controller  112  during its previous operational session, no further data is read from memory  1405  and boot-up continues. If the serial number encoded within memory  1405  indicates that transducer  105  is a new pairing with DSP controller  112 , calibration data and the serial number is read from memory  1405  and written in non-volatile form into memory (not shown) contained within DSP controller  112 . Upon subsequent boot-ups with the same transducer  105 , this previously stored calibration data is used directly. 
     During operation, controller  112  drives LED  106  with a phase angle modulated signal via LED drive  1407 . Light energy  107  emitted from LED  106  is pulsed onto sensing film  104  with phase angle modulation corresponding to the LED drive signal  1407 . In a preferred embodiment, excitation energy  107  emitted from LED  106  has a spectral distribution predominantly in the blue portion of the electromagnetic spectrum and serves to excite sensing film  104  into luminescence. Photodetector  110  transforms luminescence into a current- or voltage-modulated electrical signal  1408  which, in turn, is amplified to a usable oxygen signal  1410  by pre-amplifier  1409 . Pre-amplifier  1409  may be, for instance, a low noise operational amplifier. Oxygen signal  1410  is transmitted to DSP controller  112  via a conventional conductive wire where it is used to determine oxygen concentration within airway adapter  101 . 
     The oxygen signal  1410  may be a function of several factors in addition to oxygen concentration including pre-amp  1409  characteristics, photodetector  110  characteristics, and other detector optical idiosyncrasies. Luminescent energy  109  emitted from sensing film  104  has a temporal intensity curve (similar to curves shown in FIG. 2) related to excitation energy  107  received from LED  106 , sensing film temperature, oxygen concentration within airway adapter  101 , and possibly the amount of previous photo-degradation of sensing film  104 . The particular amount and quality of excitation energy  107  emitted by LED  106  varies according to LED output efficiency and spatial distribution, variations in alignment and transmissivity of the particular components of the transducer emitter assembly as well as the phase angle modulated signal input via LED drive  1407 . 
     The effects of factors other than oxygen concentration and LED drive signal may, to a great extent, be eliminated, thus simplifying the problem of determining concentration. Transducer-specific factors such as pre-amp characteristics, detector assembly characteristics, variations in heater calibration, variations in overall LED output efficiency, and other alignment variations may be eliminated from consideration by use of the transducer-specific calibration data contained within memory  1405  according to the method described above. Variations in sensing film oxygen diffusivity (as a function of temperature) may be eliminated by keeping the sensing film  104  at a constant temperature using methods described above. Deleterious effects due to sensing film photo-degradation may be largely eliminated by packaging the sensing film  104  as a part of a disposable airway adapter  101 , thus ensuring that the sensing film is always fresh. Thus, the problem of determining oxygen concentration is simplified to comparing the oxygen signal  1410  to the phase angle modulated LED drive signal  1407 . 
     FIG. 15 is a block diagram that describes more specifically the process of comparing the LED drive signal  1407  to the oxygen signal  1410  to determine oxygen concentration. A portion of the DSP controller  112  is shown with connections to the transducer  105  comprising an LED drive  1407  and an oxygen signal  1410 . The memory heater and thermostat, as well as their corresponding connections are omitted from FIG. 15 for the sake of clarity. DSP integrated circuit  1520  forms the heart of processing functionality and CODEC  1521  provides analog/digital interfaces on DSP controller  112 . Current voltage converter  1409  corresponds to pre-amp  1409  in FIG.  14  and is indicative of one embodiment. As described in conjunction with FIG. 14, LED drive  1407  pulses LED  106  which emits a corresponding excitation energy  107  to excite luminescence in fluorescent sample  104 . Upon receiving a pulse of excitation energy  107 , sensing film  104  emits luminescence energy  109  with an intensity and duration inversely proportional to oxygen concentration in the sampling volume  102  (not shown) of the airway adapter  101 , as shown by FIG.  2 . Photodetector  110  converts variations in luminescence  109  to corresponding variations in electrical signal  1408  that current voltage converter  1409 , in turn, amplifies and converts to variations in voltage prior to transmitting the resultant oxygen signal  1410  back to the DSP controller  112 . Signals  109 ,  1408 , and  1410  thus are effectively phase-retarded output signals with the amount of phase retardation determined by oxygen concentration. 
     For the purposes of the signal processing to be done, transducer  105  may be considered a trans-impedance amplifier. LED drive  1407  and reference channel  1501  are driven as pure sine waves. Due to perturbations introduced by sensing film  104 , oxygen signal  1410  is modified somewhat from the pure sine wave of LED drive  1407 . The perturbations introduced by sensing film  104  are, of course, the very signal from which oxygen concentration may be derived. 
     Oxygen signal  1410  is passed to DSP controller  112  and sent through anti-aliasing filter  1502  to remove phase delays relative to LED drive  1407  introduced by propagation delays along the signal path length, thus producing anti-aliased oxygen signal  1503 . Reference channel  1501 , nominally driven in quadrature to LED drive  1407 , is similarly passed through anti-aliasing filter  1504  to produce anti-aliased reference signal  1505 . 
     Anti-aliased oxygen signal  1503  and anti-aliased reference signal  1505  are converted to digital signals by passing through analog-to-digital (A/D) converter channels  1506  and  1507 , respectively. Digital oxygen signal  1508  and digital reference signal  1509 , which result from the A/D conversion, are then mixed by mixer  1510  to create AC coupled error signal  1511 . Digital mixer  1510  multiplies signals  1508  and  1509  point-by-point to produce error signal  1511 . AC coupled error signal  1511  is subsequently processed by digital low pass filter  1512  to remove the AC coupling and produce DC error signal  1513 . DC error signal  1513  has a voltage proportional to the signal perturbations (phase delay) introduced by the luminescence-quenching oxygen measurement sensing film  104  in converting LED drive signal  1407  to oxygen signal  1410 . Less phase delay in the signal channel relative to the reference channel, corresponding to higher oxygen concentrations, results in a lower DC error signal  1513 . Conversely, greater phase delay in the signal channel relative to the reference channel corresponds to lower oxygen concentration and a higher DC error signal  1513 . 
     Dual output variable phase drive  1514  outputs digital waveforms along channels  1515  and  1516  which are converted by digital-to-analog (D/A) converter channels  1517  and  1518 , respectively, to create LED drive  1407  and reference channel  1501 , respectively. Frequency is held constant by drive  1514  while the phase of the two channels  1517  and  1518  is varied relative to one another. Specifically, drive  1514  advances the phase of digital reference channel  1516  in response to DC error signal  1513  to minimize the magnitude of DC error signal  1513 . The amount of phase advance, indicated as N 0 , required to minimize the magnitude of DC error signal  1513  is thus proportional to oxygen concentration. The value of N 0  is output via digital output line  1519  for further processing and interpretation, either by embedded processes or by a host computer. 
     FIG. 16 is a block diagram of controller components for a sidestream gas measurement transducer and cuvette such as the system shown in FIGS. 10,  12 , and  13  focusing especially on functionality incorporated in transducer/cuvette assembly  401 . FIG. 16 also corresponds relatively closely to FIG. 14, which is an implementation specific to a mainstream gas measurement system. 
     The main difference between the block diagram of FIG.  16  and the block diagram of FIG. 14, aside from the physical implementation, is the addition of a pressure-sensing transducer  1601  and corresponding data line  1602  in the block diagram of FIG.  16 . Because gases delivered to sidestream gas analysis systems are pumped to the sampling cuvette  101 , there is a possibility of an overpressure situation in which the gas pressure within cuvette  101  is above atmospheric pressure. As was described in conjunction with FIG. 2, a higher sample gas pressure could lead to mistaken calculation of a higher-than-actual oxygen concentration. 
     The addition of pressure-sensing transducer  1601  yields two advantages. First, oxygen concentration calculated using an atmospheric pressure assumption may be corrected according to measured pressure to yield actual oxygen concentration. Secondly, feedback control may be used to control the pump (not shown) to reduce actual sample volume pressure to atmospheric pressure. 
     Other functionality of the block diagram of FIG. 16 is similar to corresponding features shown and described in FIG.  14 . 
     FIG. 17 is a block diagram of a sidestream gas measurement controller showing especially functionality incorporated in the DSP controller  112 . Signals from transducer/cuvette assembly  401  are as shown and described in FIG.  16 . 
     Analog-to-digital (A/D) converter  1701  is configured as a multichannel device, receiving analog input from various sensors and providing digital representations of said analog signals to the integrated circuit  1520  via digital signal path or line  1704 . Cuvette temperature signal  1402  is provided as a DC voltage and converted by A/D converter  1701  into a digital form for processing by DSP chip  1520  which, in response, modulates PWM heater control line  1403 . An ambient pressure transducer  1702  is connected to A/D converter  1701  by analog line  1703  and the cuvette pressure-sensing transducer  1601  (not shown) is connected to A/D converter  1701  by analog data line  1602 . These analog signals are converted to corresponding digital signals and transmitted to DSP chip  1520  via digital line  1704 . Digital line  1704  may, for instance, be configured as a multichannel parallel interface. By comparing the ambient pressure to cuvette pressure differential, DSP chip  1520  may, for instance, provide feedback to process computer  1705  to enable pump control. By measuring cuvette pressure, DSP chip  1520  may correct for errors in measured oxygen concentration due to absolute pressure variations. 
     DSP controller  112  may communicate with process computer  1705  via a serial data communications line or interface  1706 . Serial communications interface  1706  may use, for instance, an RS-232 protocol. Communications interface  1706  may utilize fixed commands by the process computer  1705  to control and calibrate DSP controller  112 . In one embodiment, oxygen concentration data is sent from DSP controller  112  to process computer  1705  as a response to command by the process computer. In this way, the process computer only receives data when such data is needed and it is ready to receive data. 
     CODEC  1521  receives an oxygen signal  1410  from the sidestream assembly  401 , converts it into digital signal  1508 , and transmits digital signal  1508  to DSP chip  1520  as shown and described in FIG.  15 . CODEC  1521  provides an interface between the digital input and output (I/O) of DSP chip  1520  and various analog lines, only two of which are shown in FIG. 17 for clarity. Digital interface  1707  is actually a composite of several digital channels including  1508 ,  1509 ,  1515 , and  1516 . CODEC  1521  converts a digital LED drive signal or wave form transmitted along channel  1515  into a corresponding LED analog signal  1708 . LED analog signal  1708  is then amplified by LED driver  1709  and sent to sidestream assembly  401  via LED drive  1407  to drive LED  106  (not shown). 
     EEPROM data line  1406  operates as shown and described in FIGS. 14 and 16. 
     Digital output line  1519  is converted to an analog signal or line  1711  by digital-to-analog converter (DAC) (elsewhere referred to as “D/A converter”)  1710 . Analog line  1711  may be used, for instance, to drive analog gauges or other devices for displaying oxygen concentration data to a user. 
     While the invention is described and illustrated here in the context of a limited number of preferred embodiments, the invention may be embodied in many forms without departing from the spirit of the essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the forgoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.