Abstract:
The present invention relates to a method of controlling a dialysis apparatus, which can be used for e.g. hemodialysis, hemodiafiltration or hemofiltration. Numerous alarms occur during treatments creating additional workloads for the operator. The present method of controlling the apparatus allows an automatic switch form a first control mode to another, thus allowing the treatment to continue without operator interference.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a method of controlling a dialysis apparatus, which can be used for e.g. hemodialysis, hemodiafiltration or hemofiltration.  
       BACKGROUND ART  
       [0002]     U.S. Pat. No. 5,366,630 discloses a method of controlling a dialysis apparatus for hemofiltration treatment by establishing the positive transmembrane pressure (TMP) between first and second compartments of a dialyser having two compartments separated by a semi permeable membrane, the first compartment being connected to a blood circuit for conveying blood outside of a human body and the second compartment having an inlet connectable to dialysate liquid circuit. The value of the TMP is measured and compared with a threshold value. When the measured value is greater than the threshold value treatment liquid is supplied to the inlet of the second compartment the exchanger and effectively the treatment mode is switched from hemofiltration to hemodiafiltration.  
         [0003]     The Gambro AK 200 ULTRA™ online dialysis machine can be controlled in a number of different modes of operation of which the two most relevant modes will be discussed here. Before the treatment is started, the desired weight loss of the patient is determined, and is divided by the treatment time in order to get a desired weight loss rate. In a first mode of operation, usually referred to as “volume control”, the desired total replacement volume is divided by a treatment time to get the infusion rate. The pump for the replacement fluid is set to deliver fluid at this desired rate. The fluid balancing system will compensate for the amount of replacement fluid. A certain TMP will result, depending on the characteristics of the membrane and the blood composition. Thus, a predetermined amount of fluid is ultrafiltered to the dialyser and the correct weight loss of the patient is reached at the end of the treatment time. In another mode of operation, usually referred to as “pressure control”, a desired TMP is set in order to achieve a high ultrafiltration rate higher than the desired weight loss rate. Often this TMP set point is chosen close to the maximum TMP allowed. The chosen TMP will resolve in a certain ultrafiltration rate, which will depend on the blood composition and characteristics, i.e. permeability of the chosen dialyser. The resulting ultrafiltration will determine the infusion flow rate that has to be produced by the apparatus. The treatment is stopped when the accumulated ultrafiltration liquid volume reaches the predetermined value.  
         [0004]     These two methods for control can each under certain circumstances produce alarms because some threshold values are exceeded. In order to shorten the length of the treatment in volume control, it is desirable to set the infusion rate as high as possible. It is however difficult to know exactly the limits set by the filtering capacity of the dialyser. In addition, as the treatment progresses and the blood becomes thicker due to the accumulating weight loss, this filtering capacity will gradually decrease. The result is frequently that the infusion rate is set at a value which may work a the start, but as the treatment progresses there is an increasing number of TMP alarms due to insufficient filtering capacity.  
         [0005]     There is a similar situation in pressure control. If the TMP is set too close to the maximum allowable TMP, the resulting ultrafiltration rate may become so high that the necessary replacement fluid rate cannot be achieved by the pump. This will create an alarm situation.  
       DISCLOSURE OF THE INVENTION  
       [0006]     On this background, it is the object of the present invention to provide a method of controlling a hemodialysis, a hemodiafiltration or hemofiltration treatment of the kind referred to initially, which overcomes the above-mentioned problem. This object is achieved in accordance with claim  1  by switching to controlling another process parameter.  
         [0007]     According to an embodiment of the invention the controlled parameter is controlled so as to maintain a predetermined value or follow a predetermined profile  
         [0008]     In a further embodiment, the dialyses apparatus is suitable for hemodialysis, and/or hemodiafiltration and/or hemofiltration. The control parameters preferably comprise TMP and/or ultrafiltrate liquid flow rate and/or replacement fluid flow rate.  
         [0009]     According to yet another embodiment the method comprises the step of selecting an initial parameter to be controlled, if a switch from controlling said initial parameter to controlling another parameter has taken place, returning to said initial control parameter after a predetermined time span since said switch.  
         [0010]     Further objects, features, advantages and properties of the method of controlling a hemodialysis, a hemodiafiltration or hemofiltration treatment according to the invention will become apparent from the detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     In the following detailed portion of the present description, the invention will be explained in more detail with reference to the exemplary embodiments shown in the drawings, in which  
         [0012]      FIG. 1  is a diagrammatic view of a first preferred embodiment of the invention, and  
         [0013]      FIG. 2  is a diagrammatic view of another preferred embodiment of the invention 
     
    
     DETAILED DESCRIPTION  
       [0014]     In  FIG. 1 , a dialysis apparatus according to a first preferred embodiment of the invention includes a dialyser  50  having two compartments  51  and  52  that are separated by a semi permeable membrane  54 . The compartment  51  is connected to a circuit for convening a flow of blood outside the body of a patient comprising an upstream duct  80 , having a peristaltic pump  84  disposed therein, and a downstream duct  70 . Duct  70  is provided with a bubble trap  74  and the free ends of the ducts are fitted respective needles or catheter connections to enable them to be connected to the vascular circuit of a patient.  
         [0015]     A system for preparing dialysate from dialysate concentrate and powder comprises a tank  10  having an inlet  12  for water from for example a reversed osmosis unit, a dry powder cartridge  20  containing bicarbonate is disposed in duct  21 . A part of the heating reservoir  10  is flown through the cartridge  20  by a precisely controlled pump  22 . The dialysate preparation system further comprises a main duct  5  to which the so-called “A-concentrate” is added by withdrawing it from a canister  26  by means of a pump  28 . The main duct includes a pump  8  and directs the dialysate to an inlet of compartment  52  of the dialyser  50 . An outlet of the compartment  52  is connected to a downstream duct  61  having an extraction pump  64  disposed therein for establishing variable suction inside the compartment  52 . The duct  61  leads to a waste liquid (ultrafiltrate and/or waste dialysis liquid) container  67 . Duct  5  leading to compartment  52  and duct  61  leading away from compartment  52  both pass a flow rate cell  40 . The flow rate cell  40  generates a signal indicative of the difference in flow rate between the incoming dialysate and the outgoing ultrafiltrate and waste dialysis liquid. The signal is therefore indicative of the ultrafiltration flow rate. A control unit  45  receives the signal from the flow rate cell  40  and operates in a manner of explained below to control the flow rate of pumps  8 ,  64  and  84 . In this manner the control unit  45  can adjust the ultrafiltration flow rate and the TMP. Before describing the operation on this dialysis apparatus a brief comment on the hemodialysis treatment follows. The flow rate of ultrafiltrate through a dialyser membrane is a function of the pressure difference (the transmembrane pressure) that exist between the two compartments of the dialyser. Before a treatment the desired weight loss of the patient is determined, and divided by the treatment time in order to get a desired weight loss rate.  
         [0016]     In the light of the above, the dialysis apparatus of the invention operates on the following principles. Before the beginning of a treatment session, an operator stores in the memory of the control unit  45  both a desired reference blood flow rate QB and a desired weight loss rate QWL as prescribed by the medical doctor (or as derived from a total desired weight loss prescribed by the medical doctor) (were QWL is equal by definition to the ultrafiltration flow rate). In accordance with a correspondence relationship previously stored in its memory, the control unit automatically associates the blood flow rate QB with an upper threshold value TMP 1 and a lower threshold value TMP 2 for the transmembrane pressure specific to the dialyser  50  being used. After initial rinsing and filling of the ducts and after the circuit for convening a flow of blood outside the body has been connected to the vascular circuit of the patient, the pumps  8 ,  64  and  84  are adjusted to a constant flow rate. Two different modes of control can now be selected for the initial control. If the ultrafiltration rate is selected as the control parameter, the flow rate of pump  64  is controlled to a value which corresponds to the flow rate value of pump  8  added with the desired ultrafiltration flow rate. In order to perform this control the dialysis apparatus disposes over a control loop formed by the flow rate cell  40  that generates a signal to the control unit  45 . The control unit  45  generates a control signal to both pumps  64  and  8  to adjust their performance to achieve exactly the desired ultrafiltration flow rate. The desired ultrafiltration flow rate is usually constant and stored in the control unit  45 , but alternatively the ultrafiltration flow rate may follow a profile stored in the control unit. The resulting TMP depends on the characteristics of the selected membrane and the blood composition. The resulting TMP is monitored by using the pressure of the dialysate downstream of the dialyser  50  indicated by sensor  65  which generates a signal that is sent to the control unit  45  and the pressure sensor  72  which produces a signal indicative of the blood pressure downstream of the dialyser  50  and sends a corresponding signal to the control unit  45 . If the measured value of TMP is outside the allowable range, i.e. either above the upper threshold or below the lower threshold, the control unit will switch to controlling the TMP by controlling the pumps  8 ,  64  and  84  in response to the measured TMP. The TMP will be controlled so as to maintain a predetermined value stored in the control unit  45  or in accordance with a profile stored in a control unit  45 . The resulting ultrafiltration flow rate depends on the characteristics of the membrane and the blood composition and will be monitored by the flow rate cell  40 .  
         [0017]     The treatment may also start in the TMP control mode. If the ultrafiltration flow rate exceeds its threshold, the control unit  45  will switch to controlling the ultrafiltration flow rate. According to a preferred embodiment the control is switched back to the initial in the control parameter after a time stored in control unit  45 .  
         [0018]      FIG. 2  shows a second embodiment of the invention, which is particularly suitable for carrying out a hemofiltration treatment. It has in common with first embodiment most of its circuits for convening blood outside the body and it circuits for dialysate and waste liquids. It differs there from in that it comprises means for providing sterile replacement fluid for the patient. Therefore it comprises in addition to the system of the first embodiment a first ultrafilter  14  which is placed in the inlet duct  12 . A second ultrafilter  44  is placed in the main duct  5  after the flow rate cell  40 . Downstream of the second ultrafilter  44  the main duct splits in a branch leading to the waste liquid duct  61  and another branch continuing towards the extracorporeal blood flow for delivering replacement fluid to the patient. A third ultrafilter  46  is placed in the branch delivering the replacement fluid. The three ultrafilters in series guarantee a sufficient sterile quality of the replacement fluid. Fewer ultrafilters may be used, however with an increased risk of insufficient sterility of the replacement fluid.  
         [0019]     Further a replacement fluid pump  48  is placed in the branch delivering the replacement fluid to the patient. The replacement fluid is mixed with the patient blood at a mixing point  58  in the downstream duct  70 .  
         [0020]     The flow rate of the replacement fluid pump  48  determines the amount of replacement fluid delivered to the patient. The flow rate cell  40  generates a signal indicative of the difference in flow rate between the incoming replacement fluid and the outgoing ultrafiltrate and waste replacement fluid. The signal is therefore indicative of the weight loss rate of the patient. The ultrafiltration flow rate, which is in hemofiltration unlike in hemodialysis, not equal to the weight loss rate, but determined by adding the replacement fluid rate to the weight loss rate.  
         [0021]     A control unit  45  receives the signal from the flow rate cell  40  and operates in a manner of explained below to control the flow rate of pumps  8 ,  64  and  84 . The control unit  45  also steers the flow rate of the replacement fluid pump  48 . In this manner the control unit  45  can adjust the ultrafiltration flow rate and the TMP. Before describing the operation on this dialysis apparatus a brief comment on the hemofiltration treatment follows. The flow rate of ultrafiltrate through a dialyser membrane is a function of the pressure difference (the transmembrane pressure) that exists between the two compartments of the dialyser. Before a treatment the desired weight loss of the patient is determined, and divided by the treatment time in order to get a desired weight loss rate. The total treatment is determined by the total ultrafiltrate volume and the weight loss of the patient.  
         [0022]     In the light of the above, the dialysis apparatus according to the second embodiment of the invention operates on the following principles. Before the beginning of a treatment session, an operator stores in the memory of the control unit  45  both a desired reference blood flow rate QB, desired treatment time and a derived desired weight loss rate QWL, and the ultrafiltration flow rate and the replacement fluid rate as prescribed by the medical doctor (were QWL is equal by definition to difference at any instant between the ultrafiltration flow rate and the replacement fluid flow rate). In accordance with a correspondence relationship previously stored in its memory, the control unit  45  automatically associates the blood flow rate QB with a number thresholds value TMP 1  and a lower threshold value TMP 2  for the transmembrane pressure specific to the dialyser  50  being used. After initial rinsing and filling of the ducts and after the circuit for convening a flow of blood outside the body has been connected to the vascular circuit of the patient, the pumps  8 ,  64  and  84  are adjusted to a constant flow rate.  
         [0023]     Two different modes of control can now be selected for the initial control, replacement fluid flow or TMP. If the replacement fluid rate is selected as the control parameter, the flow rate of pump  48  is continuously adjusted to the desired value. The flow rate of pump  64  is continuously adjusted to a value which corresponds to the flow rate value of pump  8  added with the desired weight loss rate. In order to perform this control the dialysis apparatus disposes over a control loop including the ultrafiltration flow rate cell  40  that generates a signal to the control unit  45 . The control unit  45  generates a control signal to both pumps  64  and  8  to adjust their performance to achieve exactly the desired weight loss rate. The desired ultrafiltration flow rate/and or replacement fluid flow rate is usually constant and stored in the control unit  45 , but alternatively the ultrafiltration flow rate may follow a profile stored in the control unit. The resulting TMP depends on the characteristics of the selected membrane and the blood composition. The resulting TMP is monitored by using the pressure of the dialysate downstream of the dialyser  50  indicated by sensor  65  which generates a signal that is sent to the control unit  45  and the pressure sensor  72  which produces a signal indicative of the blood pressure downstream of the dialyser  50  and sends a corresponding signal to the control unit  45 . From these signals the TMP is derived. If the measured value of TMP is outside the allowable range, i.e. either above the upper threshold or below the lower threshold, the control unit will switch to controlling the TMP by controlling the pumps  8 ,  64  and  84  in response to the measured TMP. The TMP will be controlled so as to maintain a predetermined value stored in the control unit  45  or in accordance with a profile stored in a control unit  45 . The resulting ultrafiltration flow rate depends on the characteristics of the membrane and the blood composition and will be monitored by the ultrafiltration flow rate cell  40 . The replacement fluid rate will then be set in accordance with the resulting ultrafiltration flow rate to a value corresponding to the resulting ultrafiltration flow rate and subtracting the desired weight loss rate.  
         [0024]     The treatment may also start in the TMP control mode. The control unit  45  regulates the pumps  8 , 64  and  84  so as to maintain the set TMP. This setting results in a certain ultrafiltration flow rate. The control unit derives from the ultrafiltration flow rate and the desired weight loss rate a required replacement fluid rate. If this required amount of replacement fluid is exceeds its threshold, the control unit  45  will switch to controlling the replacement fluid flow rate instead.  
         [0025]     According to a preferred embodiment the control unit  45  switches back to controlling the initial in the control parameter after a switch has taken place. The return to controlling the initial control parameter is performed after a predetermined time set or stored in control unit  45  or when the initial control parameter has returned below its threshold or a return-threshold.  
         [0026]     Generally in all embodiments, a return-threshold is set for determining if the initial control parameter has returned to an acceptable value. This return-threshold is different from the normal threshold in order to take hystereses errors in the measuring equipment.  
         [0027]     The general term dialysis as used here includes hemodialysis, hemofiltration, hemodiafiltration and therapeutic plasma exchange (TPE), among other similar treatment procedures. The general term “dialyser” as used here includes hemofilters, ultrafilters, and hemodiafilters.  
         [0028]     The general term “means for measuring transmembrane pressure” as used here includes any conventional pressure transducers or sensors, which in general measure the pressures from the venous line of the extracorporealblood circuit, and the dialysis fluid downstream from the dialyser. From these measurements an estimate of the transmembrane pressure is derived. It is also possible to measure the pressure both before and after the dialyser in the dialysate flow path and in the blood flow path and derive an estimate of the transmembrane pressure from these four measurements.  
         [0029]     The general term “means for measuring the volume or ultrafiltration flow rate” as used here includes flow meters of coreolis (mass flow), turbine, orifice nozzle, venturi flow or electromagnetic induction (Faradays law) type. The ultrafiltration flow rate can be determined by comparing signals from two meters, one for incoming dialysate and one for waste liquid, or the flow meters may be of the differential type that gives one signal representative of the ultrafiltration flown rate. If the dialysis apparatus uses the balance chamber system for the transport of dialysate, it is not always necessary to use differential flow meters. Instead the flow can be derived from the speed of the ultrafiltrate pump used in the balance chamber system, or by simply collecting the waste liquid and measuring it by weight or volume.  
         [0030]     The invention is not limited to the two exemplary embodiments described above, and variants may be provided. The preparation of the fluids does not have to be as described online. Instead ready mixed fluids from e.g. pre-packaged bags of sterile replacement fluid, which will also do away with the need for ultrafilters, may be used.  
         [0031]     In general, the threshold values for the purpose of switching the control parameter may be different from, values at which the dialyses machine will activate an alarm to warn the operator. Therefore the present invention will normally reduce the number for alarms that occur regularly in certain circumstances, but not do altogether away with them.  
         [0032]     Although the dialyses apparatus has been described with a sensor measuring the flow rate in the main duct, measuring flow rate in the waste liquid duct the, and determining the ultrafiltrate flow rate by comparing the two measure flow rates, the dialyses apparatus can be modified to the so called “double chamber principle” which is well known in the art. The The apparatus can also perform hemodiafiltration treatment, in which the branch of the main duct leading to the waist duct is in a well-known manner connected to the inlet of the dialyser. Several parameters have been described as control parameters, other parameters of the apparatus could also be used e.g. the TMP and the treatment time could be fixed whereas the total ultrafiltrate volume will depend on the resulting accumulated flow rate ultrafiltrate flow rate. According to another embodiment the dialysate flow rate could be controlled instead of the replacement fluid rate when the incoming (cold) water flow exceeds the heating capacity of the monitor. The maximum heating capacity of the heating reservoir  10  can in particular under circumstances with relatively cold incoming water be exceeded. In a conventional dialysis apparatus first an alarm with be set and eventually the machine will be stopped. According to this embodiment the machine will switch to controlling the dialysate flow rate and limit the flow rate to the heating capacity of the heating element. Thus the apparatus can continue the treatment.  
         [0033]     In another embodiment the blood pressure in the extracorporeal circuit and the blood flow rate in the extracorporeal circuit are used as parameters to switch between. Normally desired blood flow rate is set at the start of the treatment. The control unit  45  regulates the peristaltic pump  84  in accordance with the desired blood flow rate. The chosen blood flow rate results under certain circumstances in unacceptable blood pressures e.g. too high blood pressure before the dialyser or too low blood pressure before the pump. These occurrences give rise to alarms in conventional dialysis machines. In this embodiment of the invention, the control unit will switch form controlling the blood flow rate controlling the blood pressure control when a blood pressure threshold is exceeded. The control unit will thus regulate the peristaltic pump in accordance with a preset value of the blood pressure stored in the control unit  45 . When the blood pressure regains a normal value, the apparatus may switch back after a predetermined time to the blood flow rate control.  
         [0034]     The above-described different embodiments can be combined in a single apparatus. Thus the apparatus could both have automatic switching between transmembrane pressure/replacement fluid rate control and between blood pressure/blood flow control.  
       LIST OF REFERENCE NUMERALS  
       [0035]    
       
           5  Main duct  
           8  Pump  
           10  Heating reservoir  
           12  Inlet  
           14  1 st  Ultrafilter  
           20  Bicarbonate cartridge  
           21  Duct  
           22  Pump  
           26  A-concentrate container  
           28  Pump  
           40  Flow rate cell  
           44  2 nd  Ultrafilter  
           46  3 rd  Ultrafilter  
           45  Control unit  
           48  Replacement fluid pump  
           50  Dialyser  
           51  First compartment  
           52  Second compartment  
           54  Membrane  
           61  Waste liquid duct  
           64  Extraction pump  
           65  Pressure sensor  
           67  Waste liquid container  
           70  Downstream duct  
           72  Pressure sensor  
           74  Bubble trap  
           80  Upstream duct  
           84  Peristaltic pump