Patent Publication Number: US-2020275883-A1

Title: System and Method for Measuring a Substance Concentration in the Exhaled Breath of a Patient

Description:
The invention relates to a system for measuring a substance concentration in the exhaled breath of a patient according to the preamble of claim  1  and to a method for measuring a substance concentration in the exhaled breath of a patient. 
     A system of this kind comprises a measurement apparatus for performing an ion mobility spectrometry measurement of a gas probe of the exhaled breath of the patient to obtain a recorded data set indicative of a drift spectrum relating to the gas probe, and a processor for processing the recorded data set to determine at least one characteristic value relating to the drift spectrum and to output a concentration estimate indicative of the substance concentration in the gas probe. 
     Mechanical ventilation is used in conventional general anesthesia procedures for example in an operating center or during long-term sedation procedures for critically ill patients in an intensive care unit of a hospital. In the context of such general anesthesia procedures patients are intubated with endo-tracheal catheters to on the one hand provide for a ventilation and on the other hand administer gaseous anesthetic agents. 
     As an alternative to inhalational anesthesia procedures using gaseous anesthetic agents, in the context of intravenous anesthesia an anesthetic agent such as Propofol is administered intravenously into a patient, for example in the context of a so-called total intravenous anesthesia (TIVA) procedure. Such intravenous anesthesia may also be preferable for example for a long-term sedation procedure in an intensive care unit. 
     In particular in the context of intravenous anesthesia for example using Propofol as an anesthetic agent, it is of substantial interest to be able to monitor the concentration of the anesthetic agent in the patient&#39;s body and its related effects in particular with regard to the anesthetic impact. Generally, conclusions with regard to the drug concentration in the patient&#39;s body can be drawn by monitoring the presence and concentration of an anesthetic agent and related substances in the exhaled air of a patient. Using a suitable modeling, for example a pharmacokinetic/pharmacodynamic model, the drug concentration in the patient&#39;s body can be predicted from the drug concentration in the exhaled air. Such predictions however require on the one hand precise models and on the other hand a precise measuring of substance concentration in the exhaled air. 
     It recently has been proposed to use ion mobility spectrometry (IMS) for the detection of a substance, in particular when using an anesthetic agent such as Propofol, in the exhaled breath of a patient for monitoring during an anesthesia procedure. Within ion mobility spectrometry, components of a gas probe are ionized and are injected into a drift chamber. By applying a substantial voltage, for example several hundred volts per centimeter, to the drift chamber, the ionized components are driven towards a detector which is constituted to generate a measurement signal upon arrival of the ionized components. The ionized components encounter an opposing force whilst travelling through the drift chamber, which originates from a drift gas that flows through the same drift chamber, but in an opposing direction, thus effectively presenting an obstacle for the ionized components depending on for example their shape and cross section. Generally, the drift time of the ionized components through the drift chamber depends on the applied voltage, but is also influenced by the temperature and pressure in the drift chamber, the mass of the ionized components, their shape and charge and the like, such that different components exhibit different velocities and hence will travel through the drift chamber in different drift times. Different ionized components hence will arrive at the detector at different times, such that signals separated in time and relating to the different ionized components can be detected at the detector, giving rise to a so-called drift spectrum (signal intensity over drift time) containing peaks relating to different components of the gas probe. 
     To determine the concentration of a substance of interest in the patient&#39;s breath, for example an anesthetic agent, in particular Propofol, a peak in a drift spectrum relating to the substance of interest may be identified, and from the peak&#39;s height conclusions may be drawn with respect to the concentration of the substance of interest in the gas probe taken from the exhaled breath of the patient. Generally, using a suitable calibration the peak&#39;s height may be related to the concentration of the substance of interest, such that from the peak&#39;s height the concentration may be directly inferred. 
     However, a drift spectrum is subject to noise. In addition, concentration values typically lie in the parts-per-billion (ppb) range such that the detected signals are weak. There hence is a desire to be able to accurately determine characteristics of a drift spectrum in order to reliably draw conclusions with respect to a substance concentration in a gas probe. 
     It is an object of the instant invention to provide a system and method for measuring a substance concentration in the exhaled breath of a patient which allow for accurate measurements using the ion mobility spectrometry (IMS). 
     This object is achieved by means of a system comprising the features of claim  1 . 
     Accordingly, the processor is constituted to fit an autoregressive model to at least a portion of the recorded data set to obtain a fitted data set, wherein the processor is further constituted to determine said at least one characteristic value from the fitted data set. 
     Hence, an autoregressive model is implemented and fitted to the recorded data set. Generally, an autoregressive model (in short: AR) is capable of predicting a value at a specific instance of time from preceding instances of time, such that by applying the autoregressive model a curve may be fitted to the recorded data set obtained from the ion mobility spectrometry measurement. The fitted curve essentially does not comprise noise, such that the fitting of the autoregressive model to the recorded data set may be used to cancel out noise from the recorded data set. Characteristic values, from which the concentration of a substance of interest in the gas probe of the exhaled breath of the patient reliably may be estimated, may then accurately be determined from the fitted data set, such characteristic values being hardly affected by noise. 
     An autoregressive model which in principle is suitable for application in the instant context is for example described in U.S. Pat. No. 5,673,210 and a research article by W. Zhao et al. entitled “Gas Chromatography Data Classification Based on Complex Coefficients of an Autoregressive Model”, Journal of Sensors, 2008, Article ID 262501. 
     For example, from the fitted data set one or multiple characteristic values such as a value indicative of the height of a peak of interest of the drift spectrum, a value indicative of an area under a curve of the fitted data set (for example under a drift spectrum (along the drift-time axis) or a curve taken along the so-called retention time axis) in the region of a peak of interest, a value indicative of the full width at half maximum of a peak of interest, a value indicative of a skewness of a peak of interest, a value indicative of an inflection point of a peak of interest, and a value indicative of a slope at a peak of interest may be determined. From such characteristic values conclusions with respect to the concentration of the substance of interest in the gas probe may be drawn, wherein for example in a calibration phase prior to the actual, regular operation of the system a range of heights of a peak of interest of the drift spectrum may be related to a corresponding range of concentration values such that from the height of a peak of interest (indicating the signal intensity of a peak relating to a specific substance) the concentration of the specific substance in the gas probe may be determined. 
     Conclusions with respect to the actual substance concentration in the gas probe may be drawn by evaluating the characteristic values separately. It however is also conceivable that multiple characteristic values are combined for example using a suitable model, such as a generalized linear model, in particular a quadratic model, or the like, to determine an actual concentration value from a multiplicity of characteristic values in combination. 
     The autoregressive model may for example, in one embodiment, be defined by the equation 
     
       
         
           
             
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     where M indicates the order of the autoregressive model, y t  indicates the amplitude value of the fitted data set at time t, care regression coefficients, and is a noise term. 
     When fitting the autoregressive model to the recorded data set, the regression coefficients are determined for example in an iterative fashion until at least a local or possibly a global minimum is reached. The difference between the fitted data set hence obtained and the recorded data set can be assumed to be predominantly due to noise, such that it can be assumed that noise in the fitted data set is substantially cancelled out. By iteratively fitting the autoregressive model to the recorded data set a substantially noise-free version of the drift spectrum may thus be obtained, allowing for an analysis of the drift spectrum with regard to its characteristic features with an increased accuracy, in particular due to a substantially increased signal-to-noise ratio. 
     The order of the autoregressive model may for example lie in the range between 10 and 100, for example in between 30 and 70, for example between 40 and 60. 
     Generally, the autoregressive model may be fitted to the entire recorded data set, i.e., the entire drift spectrum recorded during an ion mobility spectrum measurement. It however is also possible that an autoregressive model is fitted only to a portion of the recorded data set, in particular a subset of the recorded data set relating to a peak of interest of the drift spectrum. 
     In one embodiment, a one-dimensional autoregressive model is used, which predicts values of a drift spectrum at a specific time from preceding values. In a further embodiment, a two-dimensional autoregressive model is used which, as an additional dimension, takes into account the time variation between different drift spectra. 
     Generally, multiple drift spectra relating to a gas probe may be determined using ion mobility spectrometry, the multiple drift spectra indicating the signal variation over time. Hence, multiple recorded data sets may be obtained, each recorded data set relating to a drift spectrum at a specific instance of time, the multiple recorded data sets hence indicating the variation of the drift spectra over time, the time between two consecutively acquired drift spectra also being referred to as retention time. 
     The second dimension of the autoregressive model may be the retention time axis. The fit of the autoregressive model to the recorded data sets hence does not only take place along the drift time axis, but also along the retention time axis, which may additionally improve accuracy and may give rise to additional characteristic values relating in particular to the time variation of peaks in recorded drift spectra. 
     By means of the system, characteristic values relating to one or multiple drift spectra may be determined, which may then be used, using a suitable calibration, to output a concentration estimate indicative of substance concentrations in the gas probe. The substance(s) of interest may in particular be at least one anesthetic agent, for example Propofol, such that the system may in particular be suitable for providing a monitoring during an anesthesia or sedation procedure, for example an anesthesia procedure in which an anesthetic agent is intravenously administered to a patient and the concentration of the anesthetic agent in the patient&#39;s body is monitored using ion mobility spectrometry on a gas probe of the exhaled breath of the patient. 
     For this, gas probes may be taken continuously or periodically in order to monitor the concentration(s) of at least one anesthetic agent, in particular Propofol, in the exhaled breath of a patient. 
     The object is also achieved by a method for measuring a substance concentration in the exhaled breath of a patient, the method comprising: performing an ion mobility spectrometry measurement of a gas probe of the exhaled breath of the patient, using a measurement apparatus, to obtain a recorded data set indicative of a drift spectrum relating to the gas probe, and processing the recorded data set, using a processor, to determine at least one characteristic value relating to the drift spectrum and to output a concentration estimate indicative of the substance concentration in the gas probe. Herein, said processing includes fitting an autoregressive model to the recorded data set to obtain a fitted data set, and determining said at least one characteristic value from the fitted data set. 
     The advantages and advantageous embodiments described above for the system equally apply also to the method, such that it shall be referred to the above. 
    
    
     
       The idea underlying the invention shall subsequently be described in more detail with reference to the embodiments shown in the drawings. Herein: 
         FIG. 1  shows a schematic view of a system comprising an endo-tracheal catheter and a ventilation system for providing ventilation to a patient; 
         FIG. 2  shows a graphical representation of a drift spectrum recorded by using ion mobility spectrometry on a gas probe taken from the exhaled breath of a patient; 
         FIG. 3  shows a portion of a drift spectrum in the range of a peak relating to Propofol (top graph) and the difference between recorded data and fitted data (bottom graph); and 
         FIG. 4  shows a flow chart of a workflow for determining a concentration estimate from a recorded data set obtained using ion mobility spectrometry. 
     
    
    
       FIG. 1  shows an embodiment of a system as it generally may be used for example in the context of an anesthesia procedure. 
     For example in an intravenous anesthesia procedure, an anesthetic agent such as Propofol is intravenously administered to a patient  4  and hence enters into the patient&#39;s bloodstream. In order to monitor the concentration of the anesthetic substance within the patient&#39;s body, a gas detector  2  connected in a sidestream arrangement to a connection piece  11  of an endo-tracheal catheter  1  continuously or periodically measures a drug concentration in a gaseous flow taken from the patient&#39;s lungs  41  via a catheter tube  10  of the endo-tracheal catheter  1  inserted into the trachea  40  of the patient  4 . By means of such concentration measurements, hence, a monitoring of the substance concentration in the exhaled air of the patient  4  may be conducted, allowing for conclusions with respect to the concentration of the anesthetic substance within the patient&#39;s body, for example using a suitable pharmacokinetic/pharmacodynamic model or the like. 
     In an embodiment, the gas detector  2  comprises a processor  20  and a measurement apparatus  21 , the measurement apparatus  21  being designed to conduct ion mobility spectrometry measurements on gas probes taken via a side-stream line  12  connected at a port  110  to the connection piece  11  of the endo-tracheal catheter  1 . 
     By means of the side-stream line  12  gas probes may be taken in a continuous or periodic fashion from a gas flow streaming through the connection piece  11 . In order to measure the concentration of a substance of interest in a gas probe taken from the exhaled breath of the patient  4 , ion mobility spectrometry is used in which components of the gas probe are ionized and injected into a drift chamber, through which the components are driven by a substantial voltage, for example larger than 100 volt or even a few hundred volt per centimeter of drift, towards a detector. By means of the detector a measurement signal is obtained, generated by the ionized components arriving at the detector and causing a low voltage signal at the detector. Because different components exhibit different drift velocities through the drift chamber—for example dependent on the temperature and pressure in the drift chamber, the component&#39;s mass, and the component&#39;s shape and charge—different components will arrive at the detector at different drift times, causing a drift spectrum in which peaks occur relating to the different drift times of the different components. From the signal intensity at a peak relating to a specific component relating to a substance of interest, hence, conclusions can be drawn with regard to the concentration of the substance of interest in the gas probe taken from the gas flow. 
     In order to draw conclusions with regard to the substance concentration in the gas probe, characteristic values in relation to a drift spectrum are determined, such characteristic values allowing to infer a concentration estimate (by for example calibrating, in an initial calibration phase, a range of a characteristic value to a range of concentration values). Recorded data obtained from ion mobility spectrometry however generally is subject to noise, such that it may not be possible, without further ado, to accurately determine characteristic values such as the height of a peak relating to a substance of interest in a drift spectrum with sufficient accuracy. 
     For this it is proposed to use an autoregressive model defined by the equation 
     
       
         
           
             
               
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     where M indicates the order of the autoregressive model, y t  indicates the amplitude value of the fitted data set at time t, E are regression coefficients, and is a noise term. The autoregressive model may iteratively be fitted to the recorded data obtained from ion mobility spectrometry, such that iteratively the regression coefficients of the autoregressive model are determined until a reasonable fit of the autoregressive model to the recorded data is reached, for example when a local minimum in the iterative fitting procedure is reached. By fitting the autoregressive model to the recorded data, fitted data is obtained, wherein the difference between the fitted data and the recorded data can be assumed to substantially be due to noise, the fitted data hence essentially representing a noise-reduced version of the drift spectrum of the recorded data. From the fitted data, hence, characteristic values of the drift spectrum may be determined with increased accuracy, because the fitted data generally exhibits a large signal-to-noise ratio. 
     This makes it possible to determine a variety of different characteristic values with sufficient accuracy such that conclusions with respect to the concentration of the substance of interest in the gas probe can be drawn. For example, the height of one or multiple peaks of interest relating to one or multiple substances of interest, an area under the curve of the drift spectrum at a peak of interest, the full width at half maximum at a peak of interest, the peak skewness, a peak&#39;s inflection points, slope values or the like may be determined. In addition, for example a derivative of a drift spectrum may be determined and processed. Furthermore, conclusions may be drawn from the regression coefficients as such. 
     Such characteristic values may be used separately to determine a concentration estimate. For example, from a peak&#39;s height, which directly relates to the signal intensity of a substance of interest, a concentration estimate may be inferred. 
     In addition or alternatively, it is possible to combine several characteristic values for example in a suitable model in order to determine a concentration estimate from a combination of different characteristic values. 
       FIG. 2  shows an example of a recorded drift spectrum S obtained by an ion mobility spectrometry measurement using the measurement apparatus  21  of the gas detector  2 . As it is visible, the drift spectrum S contains various peaks, the different peaks relating to different components of the gas probe. 
       FIG. 3  relates to a portion of a drift spectrum S, showing—in the top graph—a recorded data set R (represented by the dots in the top graph) in the range of a peak of interest and a fitted data set F (represented by the continuous line in the top graph) which is obtained by fitting the autoregressive model to the recorded data set R and—in the bottom graph—the difference D between the recorded data set R and the fitted data set F. (The increased discrepancies between the recorded data R and the fitted data F in the first portion of the drift time window are due to the nature of the autoregressive model and the chosen data excerpt and can be significantly reduced by selecting a broader window including more data points.) The autoregressive model indicated by the above equation is one-dimensional in that a value at a specific time depends on previous values along the drift time axis. In addition, a second dimension may be introduced taking into account the so-called retention time axis. 
     Specifically, multiple drift spectra S may be recorded over a period of time in order to take into account the variation of the drift spectra over time. Multiple recorded data sets R relating to multiple drift spectra S hence are obtained, to which the autoregressive model may be fitted using the retention time axis as an additional dimension for the model (the retention time denotes the time between instances of consecutively recorded drift spectra). 
       FIG. 4  illustrates a workflow for processing recorded data sets obtained using ion mobility spectrometry for analyzing gas probes of a gas flow for example during an anesthesia procedure. For the processing of data, in a first step  51  data is acquired to obtain one or multiple recorded data sets R. To the recorded data sets R an autoregressive model is fitted in a step S 2  to obtain one or multiple fitted data sets F, and in a step S 3  one or multiple drift spectra S obtained according to the fitted data sets F are evaluated to determine one or multiple characteristic values of the drift spectra S. According to a calibration the one or the multiple characteristic values are converted into a concentration estimate in a step S 4 , which in a step S 5  is then the output. 
     The idea underlying the invention is not limited to the embodiments described above, but may be implemented in an entirely different fashion. 
     In particular, the approach as described herein may not only be used within the context of an anesthesia procedure, but may generally be used to monitor a substance of interest in the exhaled breath of a patient using ion mobility spectrometry. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Endo-tracheal catheter 
           10  Catheter tube 
           11  Connection piece 
           110  Port 
           12  Side-stream line 
           2  Detection device 
           20  Processor 
           21  Measurement apparatus 
           3  Ventilation system 
           30  Ventilator 
           31  Filter 
           32  Ventilation lines 
           4  Patient 
           40  Trachea 
           41  lungs 
         D Difference 
         F Fitted data set 
         R Recorded data set 
         S Drift spectrum 
         S 1 -S 5  Steps