Abstract:
An apparatus for extracorporeal blood treatment has a dialysis fluid circuit and a blood circuit which are separated by a dialyzer. Arranged in the dialysis fluid circuit there is a sterile filter for producing a sterile dialysis fluid which flows into the dialyzer. To check the sterile filter, a chemical and/or physical property of the dialysis fluid, for example the conductivity, is changed upstream of the sterile filter, and the change in the property of the dialysis fluid is detected downstream of the sterile filter. From the time shift between the initiation of the conductivity impulse and the detection of the latter, it is possible to tell whether the blood treatment apparatus is fitted with a sterile filter. Moreover, the volume of the sterile filter can be inferred from the length of the time shift.

Description:
FIELD OF THE INVENTION 
     The invention relates to an apparatus for extracorporeal blood treatment, with a device for checking a sterile filter which is arranged in a dialysis fluid circuit of the blood treatment apparatus. The invention further relates to a method of checking a sterile filter, arranged in a dialysis fluid circuit of an apparatus for extracorporeal blood treatment, before the start of the extracorporeal blood treatment. 
     BACKGROUND 
     It is known to produce a dialysis fluid on-line from fresh water and one or more concentrates. The fresh water generally contains no microorganisms and the concentrates are normally sterile, but it is nevertheless difficult to guarantee that the dialysis fluid produced on-line always meets the very stringent sterility requirements imposed in extracorporeal blood treatments. 
     In hemodiafiltration, it is known to produce a substitution fluid on-line from a dialysis fluid. The substitution fluid in particular should satisfy the stringent requirements concerning sterility. 
     In order to enable the sterility of the dialysis fluid and substitution fluid, known blood treatment apparati contain sterile filters, which can be replaced after one use or after they have been used several times. European Patent No. EP 0 930 080 A1 describes a blood treatment apparati which has a first sterile filter for producing a sterile dialysis fluid from fresh water and from a dialysis fluid concentrate, and a second sterile filter for producing a sterile substitution fluid from the dialysis fluid. Both sterile filters are arranged in the dialysis fluid circuit upstream of the dialyzer and are flushed with the dialysis fluid. 
     In practice it is desirable that a sterile filter provided for a blood treatment apparatus is actually fitted in the apparatus. Otherwise there is a risk that the dialysis fluid and/or substitution fluid may not be sterile. Automatic detection of whether the apparatus is fitted with a filter typically requires additional equipment and costs. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an apparatus for extracorporeal blood treatment which permits checking of the sterile filter with relatively little additional equipment. A further object of the invention is to provide a method which permits checking a sterile filter of an extracorporeal blood treatment apparatus and which requires only relatively little outlay in terms of equipment. 
     According to one embodiment of the present invention, a sterile filter is checked based on determining the time interval between a change in a physical and/or chemical property of the dialysis fluid, initiated in the dialysis fluid circuit upstream of the sterile filter, and the detection of the change in the property of the dialysis fluid in the dialysis fluid circuit downstream of the sterile filter. From the length of the determined time interval, it is then concluded whether the blood treatment apparatus is fitted with a sterile filter. For this purpose, the length of the determined time interval is compared to one or more predefined reference values which are representative of one sterile filter or of various sterile filters of different configurations. In another embodiment, instead of a time interval, a parameter correlating with the time interval, for example the balance chamber cycles, can also be evaluated if the blood treatment apparatus has a balance device with balance chambers. 
     If the blood treatment apparatus is not fitted with a sterile filter, e.g., the connection terminals of the sterile filter are bridged by a tube line, an impulse-type change in the physical and/or chemical properties of the dialysis fluid initiated upstream of the sterile filter requires a shorter propagation time in order to be able to be detected downstream of the sterile filter than if the blood treatment apparatus is fitted with a sterile filter. This may be attributed to the different volume of dialysis fluid in the predefined section of the dialysis fluid circuit, without or with sterile filter. 
     One advantage of an apparatus according to one embodiment of the present invention and of the method according to one embodiment of the invention is that additional sensors on the sterile filter itself may not be necessary. Thus, the outlay in terms of equipment may be reduced. Checking of the sterile filter can be carried out before the actual dialysis treatment. The treatment thus may begin when it has been ascertained that the blood treatment apparatus is fitted with the sterile filter. For checking the sterile filter, it may be immaterial whether the dialysis fluid, whose chemical and/or physical property is changed, flows through only the first chamber or the second chamber or through both chambers. For this reason, the sterile filter can also be checked when it is connected into the circuit only for tangential flushing. 
     The physical and/or chemical property of the dialysis fluid can be any parameter which is detectable in the dialysis fluid. The physical and/or chemical property is preferably the concentration of a defined substance in the dialysis fluid. However, the physical and/or chemical property can also be, for example, the temperature, density or pressure. The measurement of the concentration of a defined substance in the dialysis fluid, for example Na, is preferably done by measuring the electrical conductivity of the dialysis fluid. In measuring the conductivity, it is advantageous that use can be made of conductivity sensors already provided in the dialysis fluid circuit of the blood treatment apparatuses. 
     To check the sterile filter, it may be sufficient for the physical and/or chemical property of the dialysis fluid to be changed, not constantly, but instead only for brief periods. The property of the dialysis fluid is preferably changed abruptly only for a short time interval. In a particularly preferred embodiment of the invention, the change in the physical and/or chemical property of the dialysis fluid is effected by changing the mixing ratio of water and concentrate(s), preferably only for a short time, during preparation of the dialysis fluid. 
     In addition to checking that the blood treatment apparatus is fitted with a sterile filter, the apparatus and method according to the invention also make the checking of whether the blood treatment apparatus is fitted with a sterile filter of the correct size. Checking the size of the sterile filter is preferably done by determining the volume of dialysis fluid which flows through the predefined section of the dialysis fluid circuit until the change in the physical and/or chemical property of the dialysis fluid can be detected. This volume is dependent on the volume of the sterile filter through which dialysis fluid flows in the predefined section of the dialysis fluid circuit. Comparison of the determined volume and predefined reference values, which are representative of the various types of sterile filters of different size, permits identification of the respective sterile filter. 
     In a blood treatment apparatus having a balance device fresh dialysis, fluid may be balanced against used dialysis fluid. The balance device may have at least one balance chamber in which, in successive balance chamber cycles, a predetermined amount of dialysis fluid is conveyed in each case. In this arrangement, the volume of dialysis fluid is advantageously determined from the number of balance chamber cycles and from the predetermined amount of dialysis fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One embodiment of the invention is explained in more detail below with reference to the drawings, in which: 
         FIG. 1  shows a very much simplified, diagrammatic representation of the main structural components of a hemodialysis apparatus with a device for checking the sterile filter, according to one embodiment of the present invention; and 
         FIG. 2  shows the conductivity of the dialysis fluid as a function of time, with and without sterile filter, according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , the main structural components of a hemodialysis apparatus according of one embodiment are shown in a simplified diagrammatic representation. The dialysis apparatus has a dialyzer  1  which is divided by a semipermeable membrane  2  into a first chamber  3 , through which dialysis fluid flows, and a second chamber  4  through which blood flows. The first chamber  3  is coupled into a dialysis fluid circuit  5 , which has a dialysis fluid admission line  6  and a dialysis fluid discharge line  7 , while the second chamber of the dialyzer  1  is coupled into a blood circuit  8 . 
     The dialysis fluid admission line  6  of the dialysis fluid circuit  5  has a first line section  9  and a second line section  10 . The first line section  9  connects a device  11 , for preparing dialysis fluid, to the inlet of a first chamber  12  of a sterile filter  15  which is divided into the first chamber  12  and a second chamber  14  by a membrane  13  that filters microorganisms. The second admission section  10  connects the outlet of the second chamber  14  of the sterile filter  15  to the inlet of the first chamber  3  of the dialyzer. The outlet of the first chamber  3  of the dialyzer  1  is connected to an outflow  16  via the dialysis fluid discharge line  7 . 
     The device  11  for preparing fresh dialysis fluid has a fresh-water source  11   a  and two dialysis fluid concentrate sources  11   b  and  11   c . The water source  11   a  is connected via a water line  11   d , and the concentrate sources  11   b ,  11   c  via concentrate lines  11   e  and  11   f , to a mixing point M from which the dialysis fluid admission line  9  issues. Proportioning pumps P 1 , P 2  and P 3  are coupled into the water and concentrate lines, the flow rates of the pumps being used to set the mixing ratio of water and concentrates for mixing the dialysis fluid. 
     To balance fresh dialysis fluid against used dialysis fluid, a balance device  18  in employed which is designed as a balance chamber and which has first and second subsidiary chambers  17 ,  19 . The first subsidiary chamber  17  is coupled into the first line section  9  of the dialysis fluid admission line  6 , while the second subsidiary chamber  19  is coupled into the dialysis fluid discharge line  7 . Upstream of the second subsidiary chamber  19 , a dialysis fluid pump  20  is coupled into the dialysis fluid discharge line. In practice, a second balance chamber operating in counter-phase may be used parallel to the first balance chamber  18  in order to permit an almost continuous flow. For reasons of clarity, however, the second balance chamber has not been shown. 
     From the outlet of the first chamber  12  of the sterile filter  15 , a first bypass line  21 , into which a first bypass valve  22  is coupled, leads to the dialysis fluid discharge line  7  upstream of the dialysis fluid pump  20 . A first shut-off member  23  is arranged upstream of the dialyzer  1  in the second line section  10  of the dialysis fluid admission line  6 , and a second shut-off member  24  is arranged downstream of the dialyzer in the dialysis fluid discharge line  7 . A second bypass line  25 , into which a second bypass valve  26  is coupled, connects the second line section  10  of the dialysis fluid admission line  6  upstream of the first shut-off member  23  to the dialysis fluid discharge line  7  downstream of the second shut-off member  24 . 
     A device  28  for measuring the conductivity of the dialysis fluid is coupled into the dialysis fluid discharge line  7  downstream of the first and second bypass lines  21 ,  25  and downstream of the dialysis fluid pump  20 . Downstream of the first bypass line  21  and upstream of the dialysis fluid pump  20 , an ultrafiltration line  46  branches off from the dialysis fluid discharge line  7  and opens into the dialysis fluid discharge line downstream of the second subsidiary chamber  19 . An ultrafiltration pump  45  is coupled into the ultrafiltration line  46 . 
     The blood circuit  8  has an admission line  29  coming from the patient and connected to the inlet of the second chamber  4  of the dialyzer  1 . The outlet of the second chamber  4  of the dialyzer  1  leads to the patient via a blood discharge line  30  to which a drip chamber  31  is connected. 
     The dialysis apparatus also has a central control unit  34  which, via control lines (not shown), controls the dialysis fluid pump and ultrafiltration pump  20 ,  45 , the proportioning pumps P 1 , P 2 , P 3 , the shut-off members  22 ,  23 ,  24  and  26 , and the balance device  18 . 
     During the dialysis treatment, the first and second shut-off members  23 ,  24  are opened and the first and second bypass valves  22 ,  26  are closed, so that dialysis fluid flows from the device  11 , for preparing dialysis fluid, via the first subsidiary chamber  17  of the balance device  18  and via the sterile filter  15  into the first subsidiary chamber  3  of the dialyzer. From the first chamber of the dialyzer, the dialysis fluid then flows to the outflow  16  via the second subsidiary chamber  19  of the balance device  18 . The sterile filter  15  provides for sterile dialysis fluid to reach the dialyzer  1 . 
     For flushing the sterile filter  15 , the first and second shut-off members  23 ,  24  are closed and the first bypass valve  22  is opened, so that the dialysis fluid runs off directly into the outflow  16 . With the second bypass valve  26  opened, the second bypass line  25  serves to circumvent the dialyzer after flushing mode with the first and second shut-off members  23 ,  24  closed. Here once again, the dialysis fluid flows directly into the outflow  16 . 
     The sterile filter  15  may be used one or more times and can be replaced while the lines are disconnected. A device  35  for checking the sterile filter determines that the dialysis apparatus is fitted with a sterile filter at the start of the actual dialysis treatment. The device  35  for checking the sterile filter  15  has a computing unit  36  which is connected via a data line  37  to the central control unit  34  and via a data line  40  to an alarm unit  38 , which emits an acoustic and/or visual alarm. The alarm unit  38  is in turn connected via a data line  39  to the control unit  34  which, in the event of an alarm, interrupts the routine of preparing for the dialysis treatment in order to fit the dialysis machine with a sterile filter. 
     During the routine of preparing for the dialysis treatment, the device  35  checks the sterile filter in the following way: 
     First, the dialysis fluid circuit  6  is flushed with fresh water. At this stage, only the proportioning pump P 1  is in operation, not the pumps P 2  and P 3 . Then one of the two pumps P 2  and P 3  is triggered briefly to generate a concentrate bolus, which can be detected as a conductivity impulse in the dialysis fluid. This conductivity impulse propagates through the dialysis fluid admission line  6  (which includes the sterile filter  15 ), the dialyzer  1  and the dialysis fluid discharge line  7  until it reaches the device  28  for measuring the conductivity of the dialysis fluid and is detected. 
     The computing unit  36  is configured to determine the time interval Δt between the first time, at which the conductivity impulse is generated by the device  11  for preparing the dialysis fluid, and the second time, at which the conductivity impulse is detected by the device  28  for measuring the conductivity. The computing unit  36  is also configured to compare the length of the time interval Δt to predefined reference values which are representative of the sterile filters  15  used. The time t, at which the measurement starts, can be defined by any desired signals associated with the development of the change in the property, for example the conductivity impulse. A further conductivity sensor can be provided for detection of the conductivity impulse at a certain point on the measurement length to the time of the start of the measurement. 
       FIG. 2  shows the conductivity of the dialysis fluid as a function of time, the time axis being given in balance chamber cycles. Since the balance device  18 , in successive balance chamber cycles, delivers in each case a predetermined amount of dialysis fluid, there may be a fixed predefined relationship between the volume of dialysis fluid delivered and the number of balance chamber cycles. In the present example, a dialysis fluid flow rate of 500 ml/min has been set, with a balance chamber cycle of 3.3 seconds duration. In the present illustrative embodiment, a balance chamber filling is 30 m/l. Thus, for example, 15 balance chamber cycles correspond to a time shift of 49.5 seconds and a volume difference of 450 ml. 
       FIG. 2  shows that the conductivity impulse initiated at the time t=0 can be detected at the time t=t 1  after a time shift Δt of 132 s, corresponding to 40 balance chamber cycles, if the dialysis apparatus is not fitted with a sterile filter  15 . If the dialysis apparatus is fitted with a sterile filter  15 , the conductivity impulse is not detected until the time t=t 1 ′ after 55 balance chamber cycles, i.e. 181.5 seconds. Here, the conductivity impulse is detected by detection of the abrupt rise in conductivity. 
     In the computing unit  36 , the number of balance chamber cycles corresponding to the time shift is compared to a predefined reference value lying between 40 and 55 balance chamber cycles, for example 50 balance chamber cycles. In the event that the determined time shift is smaller than the reference value, the computing unit  36  sends an alarm signal to the alarm unit  38 , which emits an acoustic and/or visual alarm and, via the control unit, interrupts the routine of preparing for the dialysis treatment. 
     The time shift Δt may be dependent on the volume of the dialysis fluid flowing through the predefined section of the dialysis fluid circuit that includes the sterile filter. A sterile filter with a greater volume may consequently cause a greater time shift than a sterile filter with a smaller volume. 
     To identify a sterile filter with a specified volume, the computing unit compares the time shift to predefined limit values which are each characteristic of a sterile filter with a specified volume. If the time shift is greater than the respective characteristic limit value, it may be concluded that the respective sterile filter is used. 
     In the illustrative embodiments described, the sterile filter  15  may be checked both with fluid flowing through the dialyzer  1  and also with fluid flowing through the first or second bypass line  21 ,  25 , because the device  28  for measuring the conductivity is arranged downstream of the first and second bypass lines  21 ,  25 . However, the device  28  can also be arranged in the second section of the dialysis fluid admission line  10 . Then, however, it may be difficult to check the sterile filter with fluid flowing through the first bypass line  21 . On the other hand, the predefined volume enclosed by the sterile filter in such an arrangement of the device  28  is smaller, because part of the dialysis fluid admission and discharge lines is omitted. At the same time, the reference values to be used may change. When measuring via the bypass line  21 , it may be noted in particular that dialysis fluid flows through one chamber of the sterile filter  15 .