Patent Publication Number: US-8114623-B2

Title: Method for determining glucose concentration in tissue fluid

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention concerns a method for determining the glucose concentration in tissue fluid in which test values for glucose and for an endogenous reference substance are detected in a sample liquid obtained from a body tissue by microdialysis, microperfusion or ultrafiltration and the test value for glucose is corrected in accordance with the test value for the reference substance. The invention also concerns a corresponding device. 
     2. Description of the Prior Art 
     Body tissue consisting of cells in a liquid environment in which metabolic products are transported between the cells and the blood vessels. In order to monitor glucose of diabetic patients, for example, it is possible to insert a probe into tissue for long periods in order to continuously obtain components from the tissue fluid by means of diffusion processes and to determine the glucose content in the tissue from the effusate. This can correlate closely with the blood glucose content without requiring an invasive access to the blood circulation. The measurement can take place outside of the body in which case the sample liquid is applied to a sensor. In this connection an ionic reference technique is known in which an ionic reference value and in particular Na +  is detected simultaneously with glucose by an ion-selective electrode in order to calibrate the recovery of the glucose in the dialysate. In this case, it is assumed that the known concentration of the reference substance in the body fluid is substantially invariant. A prerequisite of the known evaluation methods is that there is a strict linear relationship between the recovery of the endogenous calibrator and glucose in the sample liquid. However, empirical comparative measurements make this seem doubtful. 
     SUMMARY OF THE INVENTION 
     The invention considers local effects in the tissue as well as transport resistances which occur due to the special probes that are used for microdialysis, microperfusion or ultrafiltration. Accordingly, in embodiments of the invention, the concentration of lactate and/or pyruvate as a reference substance is determined in the sample liquid. This enables the specific metabolic paths of glucose degradation to be utilized in order to draw conclusions about effects on the measurement correction that are due to local tissue reactions. 
     A concentration ratio of lactate to pyruvate in the sample liquid is advantageously determined to correct the test value for glucose. In embodiments of the invention, at a concentration ratio of lactate to pyruvate in a selected middle range of between about 10:1 and about 20:1, a linear correction dependent on the concentration ratio is carried out, and in a higher range above the selected range and above a concentration ratio of 20:1 the test value for glucose is corrected by a constant. An analogous correction is also conceivable when using lactate alone as the reference substance. 
     In embodiments of the invention, the recovery rate (R GLU ) for glucose is determined from a non-linear relationship with the recovery rate (R REF ) for the ionic reference substance, and the test value for glucose is corrected therewith. In this manner it is possible to carry out a type of technical correction in a specifically adapted form for effects that are caused mainly by the specific probe technology, for example, due to the membrane processes. This allows for an accurate correction. This may be due to the fact that the recovery of the ion reference is higher than that of glucose due to the small particle size and/or due to the charge, as may be the case with rapid perfusion. Hence the non-linear compensation curve runs in a curve shape below the bisecting line. 
     According to embodiments of the invention, the recovery rate (R GLU ) for glucose is determined according to the relationship
 
1- R   REF =(1- R   GLU ) k  
 
in which k is a predetermined value. In this connection, the value k can be determined empirically as the ratio of resistances for the transfer of glucose and the reference substance between tissue fluid and sample liquid. Furthermore, it is proposed that the recovery rate (R REF ) for the reference substance is determined as the ratio of the test value for the reference substance in the sample liquid and a constant concentration value in the tissue fluid. Sodium is advantageously used as an endogenous ionic reference substance.
 
     A particularly effective and accurate compensation can be achieved by using sodium as a reference substance for a correction of deviations due to the measurement technology and lactate and/or pyruvate as a reference substance for a correction of locally-related deviations of the test value for glucose in the sample liquid from the actual glucose concentration in the tissue of the organism. 
     The compensation described above can be used particularly advantageously in procedures in which a probe located in the tissue is used to obtain sample liquid by perfusion with rinsing liquid or by applying a negative pressure. 
     An additional functionality is achieved by means of the fact that a malfunction is signalized when an upper and/or lower predetermined threshold of the test value for the reference substance is exceeded. 
     The above-mentioned process may be achieved utilizing a corresponding device for carrying out the process. Accordingly a sensor unit is provided for a device where said sensor unit is designed to determine the concentration of lactate and/or pyruvate as a reference substance in the sample liquid. 
     In embodiments of the invention, the evaluation unit has an evaluation program which determines the recovery rate (R GLU ) for glucose from a non-linear relationship with the recovery rate (R REF ) for the ionic reference substance, and the test value for glucose is corrected therewith. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features of this invention and the manner of obtaining them will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the present invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a block diagram of a measuring arrangement for determining glucose; 
         FIG. 2  shows a schematic representation of a microdialysis system as a measuring arrangement; 
         FIG. 3  shows a probe of a microdialysis system according to  FIG. 2  in a partially enlarged view; 
         FIG. 4  shows a greatly simplified reaction scheme of glucose processing in body cells; 
         FIG. 5  and  FIG. 6  show empirical measurement data of microdialysis examinations using lactate/pyruvate compensation curves; 
         FIG. 7  shows a measurement diagram corresponding to  FIG. 5  for an examination by means of ultrafiltration; and 
         FIG. 8  shows a measurement diagram of the glucose and sodium recovery by means of microdialysis using a calculated compensation curve. 
     
    
    
     Although the drawings represent embodiments of various features and components according to the present invention, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate and explain the present invention. The exemplification set out herein illustrates embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings, which are described below. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. The invention includes any alterations and further modifications in the illustrated device and described methods and further applications of the principles of the invention, which would normally occur to one skilled in the art to which the invention relates. Moreover, the embodiments were selected for description to enable one of ordinary skill in the art to practice the invention. 
     The measurement arrangement according to  FIG. 1  includes a probe  10  that can be inserted into subcutaneous tissue for obtaining a sample liquid, a sensor system  12  for detecting components in the sample liquid, an evaluation and control unit  14  for processing the sensor signals and controlling the measurement process as well as optionally an output unit  16  and an interface  18  to display or transmit the measured results. The probe  10  can be loaded in a known manner with perfusion fluid (microdialysis, microperfusion) or be placed under negative pressure (ultrafiltration) in order to collect the sample liquid. 
       FIG. 2  illustrates in more detail a microdialysis system for continuous sample collection and measurement data recording. The microdialysis probe  10  has a double-lumen membrane catheter  22  which is located in the tissue  20 . The catheter  22  can be rinsed with a perfusion liquid  28  from a liquid reservoir  26  via a feed line  24 . The perfusion liquid  28  may comprise any suitable solution, such as physiological saline, for example. The return line  30  of the dialysis probe  10  runs through a peristaltic or roller dosage pump  32  to a collecting vessel  34 . A flow measuring cell  36  of the probe  10  may be arranged extracorporeally in the return line  30 . The measuring cell  36  detects the concentration of the sought-after analyte in the dialysate, such as glucose, for example, and a reference or control substance, for example, by means of an electrochemical measurement. 
     The mode of operation of the microdialysis probe  10  can be seen in more detail in  FIG. 3 . The distal end of the catheter  22  has a dialysis membrane  38  which is embedded in the tissue  20  such that the perfusate flowing in through the inlet channel  24  is loaded through the semi-permeable membrane with constituents  40  from the intercellular tissue. The porosity of the membrane  38  is dimensioned so that the metabolic products to be measured, such as glucose, lactate and pyruvate, for example, can pass into the perfusate almost without resistance, and larger molecules are held back. Suitable alternatives may be utilized, such as microperfusion, for example. In the case of microperfusion, there may be no membrane, and the inner space of the probe  10  may be directly coupled to the tissue via perforations in the wall. In both cases the collected sample liquid  42  is aspirated through the mouth of the return channel  30 , which is proximally set back from the outlet opening of the line  24 . 
     However, due to various circumstances and in particular to local changes and transport resistances and perfusion rates, the glucose concentration in the dialysate or effusate is not equal to that of the glucose content in the tissue fluid. In order to remedy this situation the sensor unit  10 , together with the evaluation unit  14 , is designed to additionally determine the concentration of an endogenous reference or control substance in order to carry out a compensation for effects which falsify the measurement. Generally, the glucose concentration in capillary blood is regarded as a “gold standard” which can be determined by spot measurements using known tests. 
     One aspect concerns the correction of the test value for glucose on the basis of the concentration ratio of lactate to pyruvate in the sample liquid. The background to this is glucose processing in the tissue cells is illustrated in a greatly simplified manner in  FIG. 4 . Glucose is degraded by glycolysis to pyruvate which can in turn be utilized in an aerobic process with a high energy yield in which the largest possible number of H atoms (in the form of NADH/H +  and FADH 2 ) are obtained which react with oxygen to form water and ATP (citrate cycle). Under anaerobic conditions pyruvate is degraded to lactate in a competing metabolic pathway. This process generates less energy so that the cells have to locally process more glucose to cover their energy requirements. The compensation method, which is elucidated in more detail in the following, is based on the idea that when lactate increases, an exceptional case is present which indicates that the general glucose concentration in the body is higher than the local concentration in the probe environment. Such local changes could, for example, result from the insertion of the probe, or from probe movements, in the tissue. 
       FIG. 5  shows a measurement diagram in conjunction with an embodiment of a proposed compensation curve. The deviation or difference of the glucose values measured in parallel for sample liquid and blood is plotted against the ratio of lactate to pyruvate in the sample liquid. The aim of the compensation is to reduce the deviation compared to the values in blood. For this purpose, a correction calculation is performed in the evaluation unit which is such that a linear correction is carried out at a concentration ratio of lactate/pyruvate between about 10:1 and about 20:1 (curve section  44 ) and that the test value for glucose is corrected by a constant above a concentration ratio of about 20:1 (curve section  46 ). This constant results directly from  FIG. 5  by the amount for the abscissa value for section  46 . 
       FIG. 6  shows a measurement diagram in conjunction with an embodiment of a proposed compensation curve when only lactate is used as a reference substance. The deviation or difference of the glucose values measured in parallel for sample liquid and blood is plotted against the lactate content in the sample liquid. In order to reduce the deviation compared to the values in blood, a linear correction is carried out at a lactate content between about 1.3 and about 2.5 millimoles per liter whereas the test value for glucose is corrected by a constant when the values of the lactate content are higher than this range. The constant results from  FIG. 6  by the amount of the abscissa value of the line of best fit in the upper range. 
     With reference again to  FIG. 1 , in ultrafiltration a negative pressure may be applied to a membrane probe  10  by means of a suction pump or suction syringe without feeding in perfusion liquid. As a result, interstitial tissue fluid enters the suction line and is transported to a sensor  12 . The semi-permeable membrane of the catheter generally only allows relatively small molecules to diffuse through together with the liquid and thus form a sample liquid which is separate from the tissue fluid. In this case, reactions due to injury can occur in the tissue which result in local changes in the glucose concentration and can be corrected by means of lactate/pyruvate measurements in the sample liquid. 
       FIG. 7  shows test values obtained by ultrafiltration in which the difference between the detected glucose value in the sample liquid and comparative measurements in blood is plotted as the ordinate against the ratio of lactate to pyruvate. The dashed correction curve  48  corresponds to the compensation elucidated above in connection with  FIG. 5 . It is also possible to only carry out a linear correction corresponding to the continuous line  50  for example in a concentration range between about 10:1 and about 30:1 lactate/pyruvate wherein the slope of the compensation curve was obtained by a fit to the test values. 
     According to an embodiment of the invention, test values for ions that are kept very constant in the tissue, such as Na +  ions, for example, are detected as an endogenous ionic reference substance, in addition to the glucose values in the sample liquid. In this case, it is assumed that Na +  in the tissue fluid remains substantially constant independently of the glucose processing and that a simultaneous measurement of glucose and sodium ions in the sample liquid can thus be used to deduce the transport or flow resistances in the sample collection. In the example, the recovery rate is defined as the concentration ratio of the respective substance in the tissue fluid and sample liquid. With reference to  FIG. 1 , the evaluation unit  14  has an evaluation routine  52  for the ionic reference correction which determines the recovery rate R GLU  for glucose from a non-linear relationship with the recovery rate R REF  for the ionic reference substance (Na + ) and thus corrects the test value for glucose. 
     In particular the evaluation program  52  determines the recovery rate R GLU  for glucose from the relationship
 
1- R   REF =(1- R   GLU ) k    (1)
 
in which k is a predetermined value.
 
     The recovery rate R REF  for the reference substance is determined as a ratio from the test value for the reference substance in the sample liquid and a known constant concentration value in the tissue fluid. 
     The equation (1) is based on a non-linear model for recovery: 
                   recovery   =     1   -     exp   [     -     1     Q   ·     W   tot           ]               (   2   )               
in which Q denotes the flow rate of perfusion and W tot  denotes the total resistance as a function of the transport resistances of dialysate, membrane and tissue.
 
     From equation (2) it firstly follows that: 
                     1     1   ⁢     n   ⁡     (     1   -   recovery     )           =     -     QW   tot               (   3   )               
i.e. a logarithmic transformation results in a linear dependence on the flow rate.
 
     A factor k is now sought after which characterizes the ratio of the total resistances for glucose and sodium for a certain dialysis membrane:
 
 W   tot (glucose)= k W   tot (sodium).
 
     This factor k can be estimated with the aid of in vivo studies and in vitro data from equation (3) by forming the quotient. 
     The factor k is a function of the resistances i.e. in practice a function of the flow rate, membrane and interstitium properties: 
     
       
         
           
             k 
             = 
             
               
                 
                   
                     W 
                     tot 
                   
                   ⁡ 
                   
                     ( 
                     glu 
                     ) 
                   
                 
                 
                   
                     W 
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                   ⁡ 
                   
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                     Na 
                     ) 
                   
                 
               
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                     n 
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                       ) 
                     
                   
                 
                 
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     In vivo studies have shown that the influence of the membrane and the physiological environment (determined by osmotic effects) can also be modeled and enables an additional correction of the recovery. 
     If k is known, then the relationship between recovery (glucose) and recovery (sodium) can be derived from equation (2) as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           1 
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                             recovery 
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                               - 
                               
                                 1 
                                 
                                   
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                         = 
                         
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       FIG. 8  shows a diagram of the test values of the glucose recovery rate plotted against the Na +  recovery rate. In addition a proportional relationship between the recovery rates, represented by the line  54 , and the compensation curve  56  according to the first equation set forth above, is shown in the diagram. It can be clearly seen that the proportional or linear relationship according to line  54  is not confirmed by the test values whereas the compensation curve  56  runs positively curved completely below the bisecting line and thus correctly describes the empirical measurements. 
     Thus, the compensation curve  56  can compensate for a variation in the recovery by also appropriately correcting the value for glucose on the basis of a changing Na +  recovery. In order to take into account different flow rates, the compensation curve  56  could be compressed by an additional factor which thus takes into consideration the fact that a 100% recovery cannot be reached due to the high throughput. 
     Any suitable substance, such as Na + , for example, may be utilized as a reference substance to correct for metrological substance. Any suitable substance, such as lactate/pyruvate, for example, may be utilized as a reference substance to correct for locally-related deviations of the test value for glucose in the sample liquid from the actual glucose concentration in the tissue. 
     In embodiments, the signaling of a malfunction of the measuring arrangement by a signal transmitter when a threshold value of the test value for the reference substance is exceeded may be included in the probe  10 . 
     While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. The application is intended, therefore, to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains.