Non-invasive estimation of arterial blood gases
FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for assessing the ventilatory status of a patient. More particularly, the present invention provides a system for implementing a non-invasive procedure for estimating the amount of dissolved carbon dioxide within the arterial portion of the vasculature. The arterial carbon dioxide content, expressed as a partial pressure, i.e., pCO 2 , is an important measure of ventilatory status which ultimately reflects pulmonary health. 
     BACKGROUND OF THE INVENTION 
     Brief Description of the Prior Art 
     Physicians and other health care providers often use elevated arterial pCO 2  (PaCO 2 ) as an indicator of incipient respiratory failure. In this regard, the determination of PaCO 2  is useful in optimizing the settings on ventilators and detecting life-threatening changes in an anesthetized patient undergoing surgery. The traditional method for obtaining arterial blood gas values is to extract a sample of arterial blood and measure the partial pressure of carbon dioxide using a blood gas analyzer (PaCO 2  ABG). Arterial puncture has inherent limitations: 1) arterial puncture carries a degree of patient discomfort and risk, 2) handling of the blood is a potential health hazard to health care providers, 3) significant delays are often encountered before results are obtained and, 4) measurements can only be made intermittently. 
     Continuous invasive monitoring requires in-dwelling arterial lines which entail inherent problems. These include sepsis, slow response times, and signal decay. The nature of this monitoring system excludes its use under routine care and is generally restricted to intensive care units within a hospital facility. 
     There have been attempts to assess PaCO 2  levels indirectly, including a technique known as Capnography. The approach utilized in Capnography involves tracking patient exhale and measuring expiratory gas CO 2  concentration against time during one or more respiratory cycles. The resulting relationship is plotted to create a graph depicting three distinct phases in breath CO 2  gas concentration during the patient exhale cycle. (See, FIG. 1.) Typically, the three phases reflect the clearing of the conducting airways which do not normally participate in gas exchange (i.e., dead space) (phase I) followed by the exhalation of air from conducting airways dynamically mixed with lung gases from the active (alveoli) membrane surfaces within the lung that have undergone gas exchange with arterial blood (Phase II). The final phase (phase III) reflects the exhalation of unmixed gas from regions of the lung which normally are in active exchange with the alveoli tissue and thus closely resembles (in healthy patients) gas properties associated with arterial blood in contact with the lung for gas exchange (CO 2  release and O 2  absorption). In normal lungs, Phase III is substantially level (slope ≈0) since ventilated and perfused alveolar regions are closely matched. In a diseased lung, Phase III may not appear level due to a mismatch in ventilation and perfusion of this lung region. See, Table I below: 
     
                       TABLE I                                                     
______________________________________                                    
Phase I Represents CO.sub.2 -free gas expired from the airway             
        conduction structures where gas exchange does not                 
        occur,                                                            
Phase II                                                                  
        The S-shaped upswing represents the transition                    
        from airway to alveolar gas, and                                  
Phase III                                                                 
        The alveolar plateau representing CO.sub.2 rich gas               
        from the alveoli.                                                 
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     In the past, capnography has utilized the peak or end-tidal (PetCO 2 ) values as an estimate of PaCO 2 . PetCO 2  is a measure of the mean alveolar partial pressure of carbon dioxide from all functional gas exchange units. PetCO 2  obtained from capnography is a measure of mean alveolar pCO 2  which approximates PaCO 2  in normal lungs. Because CO 2  readily diffuses across the alveolar-capillary membrane, the PetCO 2  closely approximates the PaCO 2  with normal ventilation-perfusion. The difference between PetCO 2  and PaCO 2  is primarily a function of the proportion of the lung where gas exchange does not occur (Fletcher, R., Johnson, G., and Brew, J., &#34;The Concept of Deadspace with Special Reference to Single Breath Test for Carbon Dioxide,&#34; Br. J. Anaesth., 53, 77, 1981). In lung disease there often exists a proportional increase in the region of the lungs where gas exchange does not occur, resulting in a significant difference between peak CO 2  obtained from capnography and PaCO 2 . 
     Other techniques have been utilized for assessing patient blood gas levels with mixed results. Transcutaneous sensors measure tissue pCO 2  diffused through the heated skin but have practical and theoretical limitations. Oximetry is a widely used, non-invasive method for estimating the arterial oxygen carried on hemoglobin. For example, U.S. Pat. Nos. 4,759,369, 4,869,254 and 5,190,038 describe pulse oximeters which measure the percentage of hemoglobin which is oxygenated. However, neither measure the amount of dissolved oxygen present, nor the amount of oxygen carried when hemoglobin levels are reduced. Low hemoglobin levels are found when there is a significant blood loss or when there is insufficient red blood cell formation. Additionally, oximeter readings are specific to the point of attachment, which is typically the finger tip or ear lobe, and may not reflect the oxygen level of vital organs during conditions such as shock or hypothermia. 
     There remains a significant need in the art for an accurate, non-invasive, sensitive method for accurately determining the levels of arterial blood gases. As will be seen hereinafter, the instant invention sets forth a non-invasive system to overcome the problems of the prior art. 
     OBJECTS AND SUMMARY OF THE PRESENT INVENTION 
     It is a primary object of the present invention to provide a system to rapidly and accurately derive a patient&#39;s arterial carbon dioxide concentration. 
     It is a further object of the present invention to provide a non-invasive method for monitoring arterial partial pressure of carbon dioxide in a patient as an indicator of ventilatory status. 
     It is another object of the present invention to provide a system for detecting expiratory CO 2  concentration and volumetric rate data and accurately deriving actual arterial pCO 2  based thereon. 
     It is yet another object of the present invention to provide a system for non-invasive, substantially real time determination of blood gas concentrations as derived from current expiratory data as correlated with processed data collected from past expiratory measurements. 
     It is still another object of the present invention to provide a system for normalizing expiratory data by converting the data from a time domain to a volume domain. 
     It is yet another object of the present invention to provide a system to ascertain arterial CO 2  partial pressures of a patient by selectively analyzing the slope and intercept values associated with phase II and phase III expiratory data converted to the volume domain. 
     It is still another object of the present invention to provide for the accurate determination of arterial CO 2  partial pressure by measuring expiratory gas data and statistically filtering this data to ascertain readings having the highest correlation to actual pulmonary performance with a statistically significant level of confidence. 
     The above and other objects of the present invention are realized in a specifically delineated gas analysis and data processing system operated in accordance with select data qualifying and enhancing procedures. In particular, the inventive system provides for the collection of concise expiratory data from a patient undergoing treatment. This data includes details on CO 2  gas partial pressure, concentration, and total gas volume sampled as a function of time during the breath exhaust cycle. Multiple readings are made to enhance accuracy. The expiratory data is normalized by converting to a volume domain from the time domain. The normalized expiratory data is then charted to establish three distinct phases within the expiratory cycle. The associated linear details of these three phases are extracted and used to project current arterial CO 2  partial pressure, PaCO 2 . This value is then utilized to quantify pulmonary performance and/or determine the existence of lung failure or distress. 
     In accordance with the varying aspects of the present invention, the system includes a sophisticated artificial intelligence engine that iteratively analyzes many separate and distinct measurements, including actual blood levels of CO 2 , and the calculated values associated with the expiratory data. Based on these permutations, the engine quantifies those measured and derived values having the highest correlation to the actual arterial CO 2  partial pressures, resulting in a fixed relationship including specifically weighted variables for projecting arterial CO 2  partial pressures. This relationship, expressed as a vector, is implemented according to system parameters in actual patient monitoring during surgical procedures and other periods of time associated with potential pulmonary failure. 
     The foregoing features of the present invention are more fully and readily understood from the following detailed description of a specific illustrative embodiment thereof, presented hereinbelow in conjunction with the accompanying drawings of which: 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 graphically illustrates three phases of the CO 2  /volume curve in a healthy person; 
     FIG. 2 represents the normal expiration of CO 2  plotted against the expired volume and the represented deadspaces; 
     FIG. 3 is a schematic illustration of a system in accordance with a preferred embodiment of the present invention; 
     FIG. 4 is a flow chart of a preferred method of the present invention; 
     FIG. 5 is a capnograph for a healthy individual; and 
     FIG. 6 is a capnograph of a person suffering from lung distress. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First briefly in overview, the present invention is a non-invasive system and procedure for deriving the gas content of arterial blood through analysis of the expiratory gas concentrations and volumes. The expiratory gas concentrations and volumes are collected in a time domain typically using a capnometer and pneumotachometer. The raw data for both pCO 2  and volume are digitized at a frequency high enough to avoid aliasing. In adults with normal ventilatory frequencies, the data is typically collected at 100 Hz, producing a data point every 0.01 seconds. The measurements are converted from the time domain (typically 1 point per 0.01 seconds) to a volume domain (typically 1 point per 1 ml of expired volume) utilizing a polynomial fit. 
     Once converted to the volume domain, the slope of Phase II segment is calculated. The initial point of Phase II is where the curve transcends a threshold value (typically 0.5% for adults). The final point of Phase II is defined, in this application, as the point at which the line deviates by a specified amount (typically 5% for adults). The threshold and deviation values are based on lung size and respiration rate. The slope of the Phase III segment is calculated, in an analogous manner, by calculating from the last data point of expiration toward Phase II until the line deviates by a fixed amount (typically 5% for adults). The slopes of Phase II and Phase III are used as initial input variables, which are combined into a vector with other parameters to produce an estimate of content of the gas in the blood, i.e., the partial pressure of CO 2 . 
     The following abbreviations and definitions are salient to invention description: 
     
         ______________________________________                                    
ABBREVIATIONS AND DEFINITIONS                                             
______________________________________                                    
CO.sub.2  Carbon dioxide                                                  
pCO.sub.2 Partial pressure of carbon dioxide                              
PetCO.sub.2                                                               
          End tidal CO.sub.2                                              
PECO.sub.2                                                                
          Expiratory CO.sub.2                                             
PaCO.sub.2                                                                
          Arterial partial pressure of CO.sub.2                           
PaCO.sub.2 ABG                                                            
          Arterial partial pressure of CO.sub.2                           
          measured by an arterial blood gas                               
          analyzer                                                        
% CO.sub.2                                                                
          Carbon dioxide value expressed as a percent-                    
          age of the total gas content                                    
COPD      Chronic obstruction pulmonary disease                           
ml        Milliliters                                                     
fds       Fowler dead space (Airway dead space)                           
I2        Intercept of phase II on CO.sub.2 axis                          
I3        Intercept of phase III on CO.sub.2 axis                         
m2        Slope of phase II                                               
m3        Slope of phase III                                              
ph2i      Phase II deviation index                                        
ph3i      Phase III deviation index                                       
Tb        Phase transition bend                                           
Tbi       Phase transition bend index                                     
TV        Tidal volume of air exhaled                                     
ang       Angle between phases II and III                                 
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     The expired CO 2  pattern obtained from capnography is graphed as a three-phase curve. In a normal person (FIG. 5), the CO 2  curve forms a plateau at the start of Phase III and reaches a value approximately equal to the PaCO 2  at the end of Phase III (PetCO 2 ). In a patient with increased volume of the lung where gas exchange does not occur (physiologic dead space, FIG. 6), as typically occurs in COPD, the curve does not plateau and the final values are a less reliable indication of the actual PaCO 2 . The instant invention determines an estimated value which closely approximates the actual PaCO 2  under these and other conditions. 
     The inventive system is illustrated in FIG. 3 and comprises a differential pressure flowmeter or pneumotachometer (item 15), a gas sensor (item 16), a processor (item 12) and a data display (item 11). This system can be used with or without mechanical ventilation. 
     Many devices for measuring the volume of a person&#39;s expiratory breath actually integrate a measured flow and can be used in this invention. Typically, flow-measuring devices use one of the following methods to determine flow: 
     1. measure pressure drop or differential pressure across a fixed resistance (differential pressure flowmeter or pneumotachometer), 
     2. measure the temperature change of a heated wire cooled by the airflow (hot wire anemometer), 
     3. measure frequency shift of an ultrasonic beam passed through the airstream (ultrasonic Doppler), 
     4. count the number of vortices shed as air flows past a strut (vortex shedding), or 
     5. measure transmission time of a sound or heat impulse created upstream to a downstream sensor (time of flight device). 
     Alternately, volume may be measured directly by counting revolutions of a vane placed in the flow path (spinning vane). A discussion of the aforementioned devices and associated technology can be found in Sullivan, et al., Pneumotachographs: Theory and Clinical Application, Respiratory Care, Vol. 29-7, pages 736-749 (1984), which is incorporated by reference herein. Examples of known differential pressure flowmeters include those described in U.S. Pat. Nos. 4,047,521, 4,403,514, 5,038,773 and 5,088,332, the teachings of which are incorporated by reference herein. 
     The exemplary device for respiratory flow measurement is the differential pressure flowmeter or &#34;pneumotachometer&#34; (FIG. 3, item 15) which provides electrical data representing the relationship between flow and pressure differential. The flowmeter (item 15) is manufactured and sold by Novametrix Medical Systems, Inc., Wallingford, Conn. However, any of the aforementioned type of devices may be utilized in the inventive system. 
     Sensors capable of measuring carbon dioxide content in a person&#39;s expiratory breath are well known. The embodied device for measuring carbon dioxide content is a gas analyzer of the type employing non-dispersive infrared radiation which presents data representing the %CO 2  (or pCO 2 ) of a sample of exhaled breath. Examples of known infrared gas analyzers include those described in U.S. Pat. Nos. 4,859,858, 4,859,859, 4,914,720, 4,958,075, 5,146,092, 5,153,436, 5,206,511 and 5,251,121, the teachings of which patents are incorporated by reference herein. Other technologies used to measure the concentration of carbon dioxide such as Raman spectroscopy and mass spectroscopy can also be used in the present invention. 
     The exemplary gas sensor capable of measuring carbon dioxide content in a patient&#39;s exhaled breath is available from Novametrix Medical Systems, Inc., Wallingford, Conn., under the trade name CAPNOSTAT. Other methods of measuring carbon dioxide content both at the airway (mainstream) or by removing a sample (sidestream) may be used in the present invention. 
     Such gas analyzers employ non-dispersive infrared radiation to measure the concentration of a selected gas in a mixture of gases. The infrared radiation can be emitted from a thick film source and focused by a mirror on the mixture of gases being analyzed. After passing through the gas mixture, the infrared beam is passed through a filter which reflects all of the radiation except for that in the narrow bands centered on a wavelength which is absorbed by the gas of concern (such as CO 2 ) in the mixture being analyzed (such as the air flow from a person&#39;s expired breath). This narrow band of radiation, which typically extends approximately 190 angstroms to each side of the wavelength on which the radiation is centered, is allowed to reach a detector which is capable of producing an electrical output signal proportional to the magnitude of the infrared radiation impinging upon it. Thus, the radiation in that band is attenuated to an extent which is proportional to the concentration of the designated gas in the mixture of gases being analyzed. The strength of the signal generated by the detector is consequently inversely proportional to the concentration of the designated gas and can be inverted to provide a signal indicative of that concentration. The processor can be either a personal computer with a suitable digital interface for receiving the digital signals from the gas sensor (FIG. 3, item 16) and pneumotachometer (FIG. 3, item 15) or any specially designed processor capable of calculating the vectors as disclosed further herein. 
     As discussed above, Capnography systems in the prior art have attempted to estimate the PaCO 2  by using the peak or end-tidal value for a single breath (PetCO 2 ). Such systems are effective at tracking gas changes in normal lungs but have been shown to be unreliable in diseased lungs such as COPD or when a significant pulmonary shunt exists. The inadequacy of PetCO 2  to measured PaCO 2  is in part attributed to regions within the lung with high ventilation to perfusion ratios. Exhaled gases from regions where gas exchange does not occur due to inadequate perfusion reduce the obtained PetCO 2 . For example, in the diseased lung, the increased slope of Phase III makes the PetCO 2  a less reliable indicator of PaCO 2 . 
     In the instant invention, the digitized (time domain) expired CO 2  is converted to the volume domain to account for variations between different people and thereby improve accuracy. This can be described by showing the curve as a series of units, each defined by a pair of points. In the time domain, the progression from unit to unit is based on fractions of a second, regardless of the quantity of air expelled during that time period. The rate of sampling depends on the patient&#39;s rate of respiration. In the volume domain, each unit to unit movement is based on a unit of volume of air expelled, regardless of the expiratory time. By transferring the data from a time domain, to a volume domain a more physiologic relationship is expressed. Toward the end of the breath, percentage increments in expiratory time and volume vary greatly in magnitude. By using the volume domain, the latter portion of the breath receives a weighting more indicative of its physiologic importance. For example, in a mechanically ventilated, chemically paralyzed patient with COPD experiencing bronchospasm, the exhalation period is prolonged due to the narrowing of the airways (FIG. 6). With pharmaceutical intervention, expiratory resistance can be reduced, thereby reducing time required to complete exhalation. The exhalation time greatly varies between the non-medicated and medicated patient, yet the exhaled CO 2  to volume exhaled ratio is relatively constant. In the volume domain, the capnograph of the non-medicated and medicated would be the same. Conversely, in the prior art systems (time domain), measurements taken before and after medication would vary greatly. 
     As shown in FIG. 3, as a person exhales, the CO 2  sensor (item 16) measures the pCO 2  in the person&#39;s expiratory breath. Virtually simultaneously, the pneumotachometer (item 15) measures the flow of the person&#39;s expiratory breath using differential pressures across a fixed resistance as elaborated herein. The analog output signals from the pneumotachometer (item 15) and the CO 2  sensor (item 16) are digitized. The digital signals are processed, as further disclosed herein, in the processor (item 12) and displayed (item 11). The PaCO 2  is displayed after several breaths. The actual time is dependent on the consistency of the data with a minimum of six breaths initially and every third breath thereafter. This data flow is illustrated, as part of the complete conversion, in FIG. 4. As stated heretofore, the sensor (item 16) and associated circuitry (item 13) measures the CO 2  content of the patient&#39;s breath. The output from sensor (item 15) is the flow of the expiratory breath and is integrated in processor (item 14) into volume. The pneumotachometer (item 15) outputs are input (digitized) as time dependent signals representing the flow rate of the patient&#39;s breath. These signals are sent to the processor (item 12). 
     As shown in the flow chart in FIG. 4, the flow data and CO 2  data are isolated breath-by-breath and converted to the volume domain in order to express a more physiologic relationship. The volume domain progression from unit to unit is based on a unit of volume expired without regard to time. As a preliminary programming step, a variety of variables are computed which depict features of Phase II, the transition period, and Phase III. The computed variables are selected to include all features of the capnograph. The derived variables are of two types: 1) those with established physiologic importance, and 2) those which mathematically represent features of the capnograph whose physiologic significance is less clear. These derived variables have utility in normalization of the data to allow for diverse lung sizes and respiratory patterns. Artificial intelligence is then used to evaluate each variable and assign a mathematical weight. In particular, a neural network is employed to insure the variables are evaluated without imposing an initial bias. Such software systems are readily available in the marketplace and are exemplified by the Neural Network presently offered by NeuralWare, Inc. located at Penn Center West, Building IV, Suite 227, Pittsburgh, Pa. 15276. These variables are computed as described hereafter. 
     Computation Of Intermediate Variables 
     A threshold level of CO 2  is detected by finding the point were the curve transcends from a value below 0.5% to a value above 0.5%. In the instant invention, this point is used as the initial point of Phase II. From the threshold point, subsequent CO 2  data points are adjoined and tested for linearity to the data point where the signal deviates from the linear points. The amount of allowed deviation is typically 5% in adults. The point where the deviation occurs marks the termination of Phase II and the slope of the segment is derived. As described further herein, the Phase II slope is used later. The slope of Phase III is determined in an analogous manner by starting at the last data point of expiration and progressing toward the termination point of Phase II. Once the Phase slopes are computed the remaining input variables are derived as the following: 
     
                       TABLE II                                                    
______________________________________                                    
Fds     The Fowler dead space volume is determined by                     
        examining the volume that gives equal area                        
        between the start of the Phase II data and the                    
        state of the Phase III data. This computation is                  
        done by fractional difference. (Fowler W. S. Lung                 
        Function studies II. The Respiratory Dead Space.                  
        Am. J. Physiol. 154: 405, 1948 - the teachings of                 
        which are hereby incorporated by reference).                      
i2      The CO.sub.2 axis intercept of Phase II. The effec-               
        tive Phase II slope (m0) is used at the Fds                       
        volume. The equation is: i2 = modelCO.sub.2 (Fds) -               
        m0 * Fds where modelCO.sub.2 (Fds) is the model                   
        evaluated at Fds.                                                 
m3      The slope of Phase III.                                           
i3      The CO.sub.2 axis intercept of Phase III. The Phase               
        III slope is used at the end tidal volume. The                    
        equation is: i2 = modelCO.sub.2 (PetCO.sub.2) - m3 * PetCO.sub.2  
        where modelCO.sub.2 (PetCO.sub.2) is the model evaluated at       
        PetCO.sub.2.                                                      
ang     The angle in degrees between the Phase II and                     
        Phase III. The effective slope of Phase II (m0)                   
        is used. The angle between the lines is:                          
        (tan(m3) - tan(m0) + π) * (180/π).                          
PetCO.sub.2                                                               
        The end tidal value of pCO.sub.2. The last element in             
        the CO.sub.2 (vol) data array is used for PetCO.sub.2.            
TV      The total volume of air exhaled expressed in ml.                  
ph2i    The Phase II deviation index. This is smallest                    
        volume at which the Phase II line deviates from                   
        the smoothed CO.sub.2 data. The comparison is done by             
        fractional difference.                                            
ph3i    The Phase III deviation index. This is the                        
        smallest volume at which the Phase III line                       
        deviates from the smoothed CO.sub.2 data by the phase             
        transition width (Tw). The negative of the                        
        second derivative of the model curve exhibits a                   
        peak in the region between Phase II and Phase                     
        III. The width of this peak is Tw and is                          
        determined as the full width at half the maximum                  
        of the peak. All second derivative curves have                    
        peaks that fall below half the maximum of                         
        increasing volume. However, the peaks in some                     
        derivative curves do not fall to 1/2 the maximum                  
        for decreasing volume. Thus, Tw is found from                     
        the right side half-width of the peak at half-                    
        maximum and multiplying by 2 to construct the                     
        full-width.                                                       
Tb      The phase transition bend. This value is the                      
        maximum of the negative of the second derivative                  
        of the model curve. It reflects the measure of                    
        the sharpness of the transition.                                  
Tbi     The phase transition bend index. This value is                    
        the index (volume) at which the phase transition                  
        bend (Tb) occurs.                                                 
ModAng  A modified version of the angle in degrees                        
        between the Phase II and Phase III lines. The                     
        modification scales the actual angle to enhance                   
        differences in the model fit curves. The scaling                  
        is heuristic. The modified angle is computed as:                  
        (tan(400 * m3) - tan(400 * m0) + π) (180/π).                
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     The product of these intermediate variables and their assigned mathematical weights is used in arguments in a hyperbolic scaling function to compute the final PaCO 2 . The mathematical weights for each variable were initially defined from a set of 100 derived variables used as inputs to a fully connected, back-propagation, neural network and the actual arterial carbon dioxide used as the desired output. Once trained for 75,000 iterations, the resultant weights were extracted and used for subsequent derivations where the actual PaCO 2  was to be determined. 
     The system operation based on the stored vector arguments are shown in the flow chart in FIG. 4. System inputs the flow data and CO 2  data as isolated breath-by-breath and converted to the volume domain in order to express a more physiologic relationship. The volume domain progression from unit to unit is based on a unit of volume expired without regard to time. Referring to FIG. 4, logic begins at start block 100, and begins a processing loop defined by block 110, defining a sequence of 10 iterations (exemplary). At block 120, the digitized respiratory data is loaded as variable RESP(I), converted to the volume domain at block 130, and then Mapped, setting forth the three phases of the capnograph, block 140, VRESP(I). 
     At block 150, phase II threshold is detected, THRESH(I), followed by block 160 and the detection of the phase II deviation to phase III, DEVIA(I). Based thereon, the variables for the capnograph are determined, VAR(I), at block 170. The stored arguments based on the assessed weighting for the vector are recalled at block 180, and the final vector generated, VEC(I). Based thereon, the PaCO 2  is derived and stored for the Ith iteration, block 190. At block 200, this process is repeated for the next sequence of respiratory data from the patient for 10 iterations (exemplary). 
     Starting at block 210, the data is refined by discarding four of the ten readings (two high and two low), XPaCO 2  (I), the resulting six iteratives are statistically assessed and accepted if meeting a minimum confidential level (95 or 99%). If accepted, the mean value of the remaining iterations is displayed, MPaCO 2  (I), block 220. The process continues giving pseudo real time data, block 230. 
     The results of system operation are depicted for the following patients: 
     
                       TABLE III                                                   
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Derived PaCO.sub.2      PaCO.sub.2                                        
Patient No.  PaCO.sub.2 ABG                                               
                        mean     std  err                                 
______________________________________                                    
 1.          33.00      34.30    2.40 1.30                                
 2.          33.00      31.33    1.84 1.67                                
 3.          33.00      32.20    2.33 0.80                                
 4.          33.50      32.00    1.77 1.50                                
 5.          38.30      39.67    1.36 1.39                                
 6.          40.00      38.37    0.86 1.63                                
 7.          40.00      41.38    0.62 1.38                                
 8.          42.00      42.63    0.15 0.63                                
 9.          42.00      44.01    1.60 2.01                                
10.          44.00      45.47    0.18 1.47                                
11.          45.00      45.66    2.35 0.66                                
12.          46.00      46.40    1.31 0.40                                
13.          47.00      47.42    0.96 0.42                                
14.          49.00      51.03    0.12 2.03                                
15.          50.50      50.73    0.50 0.23                                
16.          51.00      49.77    0.31 1.23                                
17.          51.00      52.07    1.29 1.07                                
18.          51.00      52.22    0.69 1.22                                
19.          53.00      53.20    0.72 0.20                                
20.          57.00      56.60    1.22 1.00                                
21.          63.00      63.34    0.23 0.34                                
22.          65.00      62.20    0.77 2.80                                
23.          67.00      64.98    0.62 2.02                                
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     The exemplary data are computed for each breath contained in, a 2-5 minute collection period. The calculated PaCO 2  values and standard deviations included in Table I were determined by analysis of the median six (6) values of ten (10) consecutive breaths. The standard deviation for the six (6) median values was computed for assurance of reproducibility of the data. The computed values were then compared to the PaCO 2  ABG values obtained simultaneously from an arterial blood gas sample. Using this method the accuracy for determination of PaCO 2  by the method of the instant invention is typically about ±2 mmHg. However, the actual data range for a 95 or 99% confidence levels may be graphically displayed, on the data display (FIG. 3, item 11). 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for the purposes of disclosure and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.