Abstract:
A method and a device for determining the effective delivery rate of a peristaltic pump with which a liquid is delivered inside an elastic hose pipe or for adjusting the speed of a peristaltic pump in order to match the effective delivery rate of the pump to the desired delivery rate may be characterized in that the effective delivery rate is calculated based on the nominal speed of the pump and the pressure inside the hose pipe upstream of the pump depending on the running time of the pump. The stroke volume of the pump may be multiplied by the nominal speed of the pump and the product from the stroke volume and the speed of the pump may be corrected by a correction function, thereby determining the effective delivery rate of the pump.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and a device for determining the effective delivery rate of a peristaltic pump, with which liquid is delivered in an elastic hose pipe. Furthermore, the present invention relates to a method and a device for adjusting the speed of a peristaltic pump, with which liquid is delivered in an elastic hose pipe. 
       BACKGROUND OF THE INVENTION 
       [0002]    In medical technology, peristaltic or occluding pumps may be used for reasons of sterility. Various designs of peristaltic pump are known, one of which is the roller pump. All peristaltic pumps have in common the fact that an elastic hose pipe is inserted into the pump, in which the liquid to be delivered flows. 
         [0003]    The known extracorporeal blood treatment apparatuses are a particular area of application of peristaltic pumps in medical technology, said blood treatment apparatuses including for example hemodialysis apparatuses, hemofiltration apparatuses and hemodiafiltration apparatuses. 
         [0004]    Great demands are made on the delivery accuracy of peristaltic pumps in medical technology, for example with extracorporeal blood treatment apparatuses. It is a drawback that the effective delivery rate of a peristaltic pump, which is typically adjusted at a preset nominal speed of the pump, depends on a large number of factors. From the nominal speed of the pump, therefore, it is not readily possible to draw conclusions about its effective delivery rate. 
         [0005]    The properties of the hose pipe represent one of the main factors from which the delivery rate of a peristaltic pump depends. It has been shown in practice that a deformation of the elastic hose leads to a change in the delivery rate of the pump. 
         [0006]    German patent document DE 197 47 254 C2 describes a method for the non-invasive internal pressure measurement in elastic hose pipes. The document points out that the properties of the hose pipe change with time. 
         [0007]    There is known from U.S. Pat. No. 6,691,047 a method for calibrating a peristaltic pump for an extracorporeal blood treatment apparatus, whereby the pressure in the hose pipe is measured upstream of the pump before the start of the blood treatment, in order to be able to predict the pressure upstream of the pump in the course of the treatment. The pump is calibrated at a pressure which corresponds to the average value of the previously measured pressure. 
         [0008]    U.S. Pat. No. 4,715,786 describes a method for calibrating a peristaltic pump, but without taking account of a dependence of the delivery rate on time. 
         [0009]    PCT publication WO 99/23386 describes a method for controlling the speed of peristaltic pumps as a function of the pressure in the hose pipe upstream of the pump. The control takes place on the basis of the physical properties of the hose pipe and the pump, but once again without taking account of the dependence on time. 
         [0010]    There is known from U.S. Pat. No. 5,733,257 a calibration method for peristaltic pumps, wherein the dependence of the delivery rate on time is negated, in that the calibration does not take place until after the lapse of a preset duration. It is assumed that the delivery rate after the lapse of this duration no longer changes with time. 
         [0011]    The method described in European patent document EP 0 513 421 A1 for determining the blood flow during an extracorporeal blood treatment likewise does not take account of the time-related change in the delivery rate with the running time of the pump. 
       SUMMARY OF THE INVENTION 
       [0012]    An aspect of the invention is to make available a method and a device for determining the effective delivery rate of a peristaltic pump with a high degree of accuracy. Moreover, an aspect of the invention is to specify a method and a device for adjusting the speed of a peristaltic pump with a high degree of accuracy, in order to match the effective delivery rate to the desired delivery rate. 
         [0013]    The example methods according to the present invention and the device according to the present invention for determining the effective delivery rate of a peristaltic pump are based on the fact that, in order to achieve a particularly good accuracy, the effective delivery rate takes place not only on the basis of the nominal speed of the pump and the pressure in the hose pipe upstream of the pump, but also in dependence on the running time of the pump. 
         [0014]    In an example embodiment, the product of a preset stroke volume of the pump and the nominal speed of the pump is corrected with a correction function in order to determine the effective delivery rate, said correction function describing the dependence of the stroke volume of the pump on the running time and the pressure in the hose pipe upstream of the pump. The preset stroke volume of the pump operated pressureless is determined by the mechanical dimensions of the pump, for example its radius, its length etc. and the dimensions of the hose pipe. 
         [0015]    As a correction function, it may be beneficial for a polynomial with one or more parameters to be set up to describe the relative decrease in the nominal delivery rate with the running time of the pump and for a polynomial with one or more parameters to be set up to describe the relative decrease in the nominal delivery rate with the pressure in the hose pipe upstream of the pump. The polynomial degrees may be increased by adding further powers or reduced by equating parameters to zero. The independence of the individual variables may also be removed, the parameters of the one variable then being made dependent on at least another variable. 
         [0016]    The correction function with the parameters is generally a property of the pump segment. The stroke volume and the parameters may thus be ascertained in tests and be preselected for the user of the pump. The same applies to the preset stroke volume. 
         [0017]    The example device according to the present invention for determining the effective delivery rate has means for measuring the pressure in the hose pipe upstream of the pump, means for determining the nominal speed of the pump and means for calculating the effective delivery rate of the pump on the basis of the nominal speed of the pump and the pressure in the hose pipe upstream of the pump in dependence on the running time of the pump. 
         [0018]    In an example embodiment, the device for calculating the effective delivery rate comprise a means for multiplying the preset stroke volume by the nominal speed of the pump and means for correcting the product of the stroke volume and the nominal speed. The means for correction may be configured as a computing unit. For example, the required calculations may take place with a computer. 
         [0019]    According to the example methods according to the present invention and the example devices according to the present invention for adjusting the speed of a peristaltic pump, with which liquid is delivered in an elastic hose pipe, the matching of the effective delivery rate of the pump to the desired delivery rate may take place not only on the basis of the nominal speed of the pump and the pressure in the hose pipe upstream of the pump, but also in dependence on the running time of the pump. 
         [0020]    In principle, it may be possible with the example methods and devices according to the present invention to determine the effective delivery rate to be expected at a nominal speed of the pump, whereby the effective delivery rate may be compared with the desired delivery rate. Since the effective delivery rate may be lower than the desired delivery rate, the speed of the pump may be increased until the effective delivery rate corresponds to the desired delivery rate. A comparison between the setpoint value and the actual value may be possible with the example methods and devices according to the present invention in order to determine the effective delivery rate without the effective delivery rate being measured. 
         [0021]    In an example embodiment of the present invention, the matching of the effective delivery rate of the pump to the desired delivery rate first takes place in an initial compensation step. It is assumed, according to this example, that the effective delivery rate for the most part corresponds to the desired delivery rate after the performance of this compensation step. After performance of the initial compensation step, the remaining deviation of the delivery rate of the pump may then be eliminated by control. The regulation of the pump may take place in continuous iterative compensation steps. 
         [0022]    A new speed with which the pump is operated in order to match the effective delivery rate to the desired delivery rate may be calculated in the initial compensation step by multiplication of the nominal speed of the pump adjusted before the compensation step by a correction factor. 
         [0023]    In order to determine the correction factor, the pump may be operated at a preset speed, whereby the pressure that is established at the preset speed is measured in the hose pipe upstream of the pump. The preset speed, with which the pump is operated in order to determine the pressure in the hose pipe, may simply be calculated according to an equation. 
         [0024]    The correction factor may be calculated from the measured pressure which is established upstream of the pump in the hose pipe at the preset speed, according to an equation into which, apart from the pressure in the hose pipe upstream of the pump, one or more parameters enter that describe the relative decrease in the delivery rate with the running time of the pump and one or more parameters enter that describe the relative decrease in the delivery rate with the underpressure in the hose pipe upstream of the pump. 
         [0025]    The equation describing the relationship between the pressure in the hose pipe upstream of the pump and the correction factor may, in principle, be solved in real time. It may be beneficial, however, that the individual pairs of values of pressure and correction factor are stored in a memory, so that access to the data is possible in real time, but without the equation having to be solved. The hardware and software expenditure for the determination of the correction factor may thus be reduced. 
         [0026]    The initial compensation step may take place after the starting of the pump or the adjusting of a new setpoint delivery rate. In further compensation steps, deviations of the effective delivery rate of the pump from the desired delivery rate may be continuously compensated for. The correction is achieved in the initial compensation step. Only smaller deviations are generally eliminated in the following control. 
         [0027]    A maximum speed or delivery rate, for example relative to an initial start value, may be taken into account as an upper threshold value in the regulation of the delivery rate of the pump. An upper threshold value for the amount of the pressure upstream of the pump may also be provided. If the individual magnitudes reach the upper threshold values, this may be used as an indication of the fact that the effective delivery rate can no longer be matched to the desired delivery rate. In this case, it is possible to emit an optical and/or acoustic alarm which draws the user&#39;s attention to the deviation in delivery rate. 
         [0028]    In principle, the regulation only has to be carried out when the amount of the deviation in the delivery rate lies above a preset lower threshold value. For example, further matching of the effective delivery rate to the desired delivery rate is not in general necessary in the case of a deviation of the delivery rate of less than one percent. 
         [0029]    Some embodiments make provision such that the preset stoke volume of the pump and the individual parameters for determining the correction factor for the various hose systems are made available, so that the appropriate stroke volume and the respective parameters may be preset by selecting the hose system. 
         [0030]    Moreover, some embodiments of the present invention relate to a blood treatment apparatus with a device for determining the effective delivery rate of a peristaltic pump and/or for adjusting the speed of the peristaltic pump, in order to be able to deliver liquid in an elastic hose pipe exactly at a desired delivery rate. 
         [0031]    Various example embodiment of the invention are explained in greater detail below by reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  shows a general schematic representation of an extracorporeal blood treatment apparatus together with a device for determining the effective delivery rate of the peristaltic pump of the blood treatment apparatus and a device for adjusting the speed of the pump, in order to deliver the liquid at a desired delivery rate, 
           [0033]      FIG. 2  shows the effective delivery rate of the pump as a function of the pressure upstream of the pump for various delivery rates and 
           [0034]      FIG. 3  shows the dependence of the effective delivery rate of the pump on the pressure upstream of the pump for various speeds of the pump. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]      FIG. 1  shows, in a general schematic representation, the main components of an extracorporeal blood treatment apparatus, for example a hemodialysis apparatus, which includes an extracorporeal blood circuit  1  and a dialysing fluid circuit  2 . Dialysing fluid flows from a dialysing fluid source  3  through a dialysing fluid supply line  4  into a dialysing fluid chamber  5  of a dialyser  8  divided by a semipermeable membrane  6  into dialysing fluid chamber  5  and a blood chamber  7 , whilst dialysing fluid flows out of dialysing fluid chamber  5  of dialyser  8  via a dialysing fluid discharge line  9  into a drain  10 . A dialysing fluid pump  11  is disposed in dialysing fluid discharge line  9 . 
         [0036]    The patient&#39;s blood flows via a blood supply line  12  into blood chamber  7  and out of chamber  7  of dialyser  8  via blood discharge line  13  back to the patient. The blood pump  14  is disposed in blood supply line  12 . Both dialysing fluid pump  11  and blood pump  14  are peristaltic pumps, in particular roller pumps. Blood supply and discharge lines  12 ,  13  and dialysing fluid supply and discharge lines  4 ,  9  may be elastic hose pipes made of plastic, which are made available as disposables for single use especially on the blood side and are inserted into the pumps. It is, however, also possible for the hoses to be part of a cassette-like module, from which the hose-side pump segment projects in the form of a loop. 
         [0037]    The blood treatment apparatus includes a control unit  15 , which is connected via control lines  16 ,  17  to blood pump  14  and dialysing fluid pump  11 . The dialysis apparatus further includes computing unit  18 , which communicates via a data line  19  with control unit  15 . 
         [0038]    The hemodialysis apparatus also has other components, which are generally known to the person skilled in the art and, for the sake of clarity, are not represented. 
         [0039]    The device according to the present invention and the method for determining the effective delivery rate of blood pump  14  and for adjusting the speed of the blood pump are described in detail below. Corresponding devices may also be provided for dialysing fluid pump  11 . 
         [0040]    The present invention is based on the properties of blood pump  14  with respective hose pipe  12 , which is inserted into the blood pump, described as follows. 
         [0041]    Effective blood flow Q b,ist  of blood pump  14  is calculated according to the following equation: 
         [0000]        Q   b,ist   =n*V   S   equation (1) 
         [0000]    where n is the rotor speed of the blood pump [l/min], and Vs is the stroke volume with a revolution of the blood pump [ml]. 
         [0042]    It is assumed that stroke volume Vs of blood pump  14  is a function of the mechanical dimensions r [mm] of the blood pump and the hose, running time t [h] of the blood pump, and pressure P art  [mmHg] in blood supply line  12  upstream of the blood pump: 
         [0000]        V   S   =V   S ( r,t,P   art )  equation (2) 
         [0000]    where r represents the mechanical dimensions and tolerances of the blood pump [mm], t is the running time of the blood pump [h], and P art  is underpressure at the entrance of the blood pump [mmHg]. 
         [0043]    Apart from the running time of the pump, its speed or cycle number in particular is of interest in practice, which is directly proportional to the loading of the pump segment and is responsible for the plastic behaviour of the hose. With a constant delivery rate, however, this difference may be less relevant. If, however, the delivery rate is changed at different times, this may have an effect. Variable t may therefore not only be the running time, but also a parameter in an unequivocal relationship therewith, for example the accumulated speed of the pump. Instead of the running time of the pump, the number of revolutions of the pump determined, for example, with a Hall sensor may also be taken into account. 
         [0044]    The stroke volume of the blood pump as a function of pressure P art  upstream of the pump in hose pipe  12  and running time t of the pump is described by the following equation: 
         [0000]        V   S   =V   S,0 ( r )*(1 −a   1   *t )*(1 −b   1   *P   art   −b   2   *P   2   art )  equation (3) 
         [0000]    where V S,0 (r) is stroke volume [ml] after a preset run-up time to with zero pressure at the entrance of the blood pump, a 1  is a parameter [%/h] which describes the relative decrease in the delivery rate with the running time, and b 1  and b 2  are parameters [%/mm Hg 2 ] which describe the relative decrease in the delivery rate with the arterial underpressure. 
         [0045]    Preset stroke volume V S,0 (r) [ml] after a preset run-up time to of the blood pump of, for example, 5 min with an underpressure at the entrance of the pump of 0 is determined by the mechanical dimensions of the pump and of the hose. 
         [0046]    Since many types of hose exhibit a deviation from the linear time-related behavior according to equation (3), which after a few minutes running time may be neglected, it is a tried and tested practice to determine preset stroke volume V S,0 (r) for this time. On account of the short run-up time, the deviation of the actual pump rate for this period is also negligible. In principle, however, it is also possible to specify preset stroke volume V S,0 (r) without run-up effects, if this is not necessary due to the employed functional time-related relationship of the correction factor. 
         [0047]    Parameter a 1  describes the relative decrease in the delivery rate of the pump with running time t, while parameters b 1  and b 2  describe the relative decrease in the delivery rate with the underpressure. The preset stroke volume and the individual parameters are magnitudes which are characteristic of the blood pump used together with the hose pipe, said magnitudes being ascertained in tests and made available to the user. 
         [0048]    The nominal delivery rate (blood flow) Q b,0  [ml/min] after the preset running time of, for example, 5 min at a zero pressure at the entrance of the pump, is obtained according to the following equation: 
         [0000]        Q   b0   =n   alt   *V   s,0 ( r )  equation (4) 
         [0049]    Effective delivery rate Q b,ist  (blood flow) of the blood pump that is to be expected when the pump is operated at speed n is obtained according to the following equation: 
         [0000]        Q   b,ist   =n*V   S,0 ( r )*(1 −a   1   *t )*(1 −b   1   *P   art   −b   2   *P   2   art )  equation (5) 
         [0050]      FIG. 2  shows the dependence of effective delivery rate Q b,ist  on the pressure upstream of the blood pump for different delivery rates Q b,t . It is clear that the delivery rate decreases with increasing arterial underpressure. The higher the delivery rate (blood flow), the greater the absolute decrease. 
         [0051]    The device according to the invention for determining the effective delivery rate of blood pump  14  includes means for measuring the pressure in hose pipe  12  upstream of blood pump  14  in the form of a pressure sensor  20 , which may case present in the known blood treatment apparatuses. Blood sensor  20  is connected via a data line  21  to control unit  15 . Moreover, means are provided for determining the nominal speed of blood pump  14 , which are a component of control unit  15  of the dialysis apparatus inasmuch as control unit  15  presets a specific speed for blood pump  14 . The same applies to dialysing fluid pump  11 . 
         [0052]    When control unit  15  for blood pump  14  presets a specific speed n, the blood pump delivers the blood at an effective delivery rate Q b,ist  (blood flow). The measured value of the arterial underpressure from pressure sensor  20  and speed n of blood pump  14  from control unit  15  are available at computing unit  18 . Furthermore, parameters a 1 , b 1  and b 2 , as well as stroke volume V S,0 (r), are available at the computing unit. These empirically determined magnitudes are stored in a memory  22 , which is connected via a data line  23  to computing unit  18 . 
         [0053]    According to equation (5), computing unit  18  calculates effective delivery rate Q b,ist  (blood flow) which is established at preset speed n of blood pump  14 . Since it is to be expected that the effective delivery rate will be smaller than the desired delivery rate, control unit  15  increases speed n of blood pump  14  until the effective delivery rate corresponds to desired delivery rate Q b,soll . 
         [0054]    The device and the method for matching the effective delivery rate of the blood pump to the desired delivery rate by adjusting the speed of the pump are described in detail below. 
         [0055]    The control of the speed of the blood pump begins with an initial compensation step, which may be carried out immediately after starting the pump. A further compensation then follows, which may take place continuously or iteratively. If the setpoint delivery rate is to be changed, the initial compensation step takes place again, but parameter t is not reset. In this way, the time-related influence on the delivery rate may also be taken into account with a change in the delivery rate. 
         [0056]    Control unit  15  first sets blood pump  14  at a preset speed, which is calculated in the computing unit according to the following equation 
         [0000]    
       
         
           
             
               
                 
                   
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         [0057]    At speed n alt , preset by the control unit, arterial underpressure P art,alt  is established, which is measured by pressure sensor  20 . 
         [0058]      FIG. 3  shows delivery rate (blood flow) Q b,ist  of blood pump  14  as a function of arterial underpressure P art . Effective delivery rate Q b,ist,alt  to be expected is obtained at measured underpressure P art,alt  according to equation (5). In the initial compensation step, control unit  15  increases speed n in order to compensate for the delivery deviation. 
         [0059]    On account of new speed n neu , the arterial pressure of P art,alt  changes to P art,neu . Pressure change ΔP art  is fixed proportional to speed change Δn. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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         [0000]    where x is a correction factor. 
         [0060]    With new arterial underpressure P art,neu , new stroke volume V S,neu  is obtained: 
         [0000]        V   S,neu   =V   S,0 ( r )*(1 −a   1   *t )*(1 −b   1   *P   art,neu   −b   2   *P   2   art,neu )  equation (8) 
         [0061]    With new stroke volume V S,neu , delivery rate Q b,ist,zw  would result at previous speed n alt : 
         [0000]        Q   b,ist,zw   =n   alt   *V   S,neu   equation (9) 
         [0062]    The new expected value of the blood flow Q b,ist,neu  results from new speed n neu  and current stroke volume V S,neu  with: 
         [0000]        Q   b,ist,zw   =Q   b,soll   =n   neu   *V   S,neu   equation (10) 
         [0000]    where the new expected value of the blood flow is set equal to setpoint value Q b,soll . Hence: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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         [0063]    If equations (7), (8), (9) are put into equation (11), the following equation is obtained: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0064]    According to equation (6), the left-hand side of equation (12) yields the value 1 independently of setpoint value Q b,soll . The defining equation for correction factor x follows as a function of arterial underpressure P art : 
         [0000]        b   2   *P   2   art   *x   3   +b   1   *P   art   *x   2   −x+ 1=0  equation (13) 
         [0065]    Computing unit  18  calculates correction factor x according to equation (13) from arterial underpressure P art  ascertained at preset speed n alt . After the determination of correction factor x, computing unit  18  calculates speed n neu  according to equation (11) by multiplying speed n alt , preset by control unit  15 , by correction factor x, said speed n neu  being set by control unit  15  in order to match effective delivery rate Q b,ist  (effective blood flow) to desired delivery rate Q b,soll  (blood flow). 
         [0066]    Since the solving of equation (13) during the running time is very expensive, an alternative embodiment of the invention makes provision to store the relationship between arterial underpressure P art  and correction factor x in a value table, which is compiled in advance and stored in memory  22 . In this embodiment, computing unit  18  takes correction factor x belonging to ascertained arterial underpressure P art  directly from memory  22 , without solving equation (13) in real time. 
         [0067]      FIG. 3  shows that, upon selection of new speed n neu , a new arterial underpressure P art,neu  results, at which the effective delivery rate of the blood pump Q b,ist,neu  (blood flow) is equal to desired delivery rate Q b,soll  (blood flow). 
         [0068]    At running time t, the setpoint value will diverge from the actual value of the blood pump without further compensation. The device according to example embodiments the present invention therefore provides a continuous control of the speed of pump  14  by means of further compensation steps. The theoretical principles of the continuous control are next described: 
         [0069]    Whereas the initial compensation step may be carried out only after starting the blood pump without compensation, equation (6) is no longer satisfied after the initial compensation step, and correction factor x is dependent on the ratio of desired delivery rate Q b,soll  (blood flow) to actual speed n art . 
         [0070]    Equation (12) is reduced to equation (13), whereby the following is defined for the left-hand side of equation (12): 
         [0000]    
       
         
           
             
               
                 
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         [0071]    If equation (12) is divided by equation (14), the following equation, which is formally identical to equation (13), is obtained: 
         [0000]        b   2   *P   2   art,r   *x   3   r   +b   1   *P   art,r   *x   2   r   −x   r +1=0  equation (15) 
         [0000]    where P art,r =q*P art  equation (15a) and x r =x/q equation (15b). 
         [0072]    In order to be able to use the table stored in memory  22 , which in each case assigns a correction factor x r  to an arterial underpressure P art  according to equation (13), a reduced correction factor x r  is determined for a reduced arterial underpressure P art,r . For this purpose, computing unit  18  first calculates ratio q between reduced correction factor x r  and correction factor x according to equation (14). Speed n alt  is the speed instantaneously preset by control unit  15  after the initial compensation step. By multiplying arterial underpressure P art  measured by pressure sensor  20  by factor q, the computing unit calculates reduced arterial pressure P art,r  according to equation (15a). The computing unit then takes, from the table stored in memory  22 , the value of reduced correction factor x r  that is assigned to reduced arterial underpressure P art,r . After reduced correction factor x r  and factor q are determined, computing unit  18  calculates speed n neu  to be set by control unit  15  from: 
         [0000]        n   neu   =x   r   *n   alt   equation (16) 
         [0073]    Control unit  15  sets new speed n neu , so that the actual value of the delivery rate is again matched to the setpoint value. The next iterative compensation step then follows, whereby factor q is first calculated again at speed n alt  now set by control unit  15 , which speed n alt  corresponds to new speed n neu  determined in the preceding compensation step. 
         [0074]    The essential correction is achieved in the initial compensation step. Consequently, it would in principle also be possible to dispense with the following control. Only smaller deviations are as a rule eliminated in the continuous control, whereby the amount of the maximum change per iteration is limited to 2% for an arterial underpressure ≦150 mmHg and to 4% for an arterial underpressure ≧150 mmHg.