as introduced above , a fistula is generally used in a dialysis procedure to access a patient &# 39 ; s blood stream . the general term dialysis as used here includes hemodialysis , hemofiltration , hemodiafiltration and therapeutic plasma exchange ( tpe ), among other similar treatment procedures . in dialysis generally , blood is taken out of a patient &# 39 ; s body and exposed to a treatment device to separate substances therefrom and / or to add substances thereto , and is then returned to the body . although the dialysis procedure used in the present invention will be described by way of example with respect to hemodialysis , it is understood that the invention is not so limited in scope . [ 0019 ] fig1 shows an arterio - venous fistula 60 created , for example , in the arm 18 of a dialysis patient . the surgically created connection 60 between an artery 86 and a vein 9 serves as the location of vascular access to the patient &# 39 ; s blood . blood needing to be dialyzed is withdrawn from the fistula and cleaned blood that has been dialyzed is returned to the patient through the fistula . a fistula is usually located in the arm of a patient , but may be located anywhere a fistula may be placed . [ 0020 ] fig2 shows a graph of the x - ray spectral patterns of the ions found deposited on the interior walls of a human stenotic fistula . as shown in the graph , the concentration of phosphorus ions to calcium ions are found in a 1 : 1 ratio . this corresponds to descriptions of brushite formation in the literature , which describe a 1 : 1 ratio of phosphorus to calcium . ( elliot , j . c . ; structure and chemistry of the apatites and other calcium orthophosphates , stud . in inorganic chem ., 18 , 23 - 30 ( 1994 )). in brushite formation , phosphorus exists as monohydrogen phosphate , and deposition of brushite occurs through the direct reaction between the monohydrogen phosphate ion and the calcium ion . in comparison , fig3 shows a graph of the x - ray spectral patterns of the ions deposited on the interior wall of a non - stenotic fistula . as shown in the graph of fig3 the concentration of phosphorus to calcium ions in a non - stenotic fistula is not found in a 1 : 1 ratio . this finding corresponds to the lack of brushite crystals found in a non - stenotic fistula . in order to prevent the formation of brushite in a fistula due to the 1 : 1 concentration of calcium ions to phosphorous ions such as that shown in fig2 the concentration of calcium administered to a patient during the dialysis procedure may be varied over time . as shown in fig4 the amount of calcium present in the dialysate may be varied over the time of the procedure , as well as varied in accordance with any decrease in concentration of phosphorous in the plasma . alternatively , calcium may be varied over time in a step - wise fashion ( not shown ). a sensor may also be used which detects the concentration of phosphorous in the blood plasma of the individual patient and adjusts the calcium concentration accordingly . in another alternative , a calcium profile could be set up which presumes that the phosphorous concentration in blood plasma decreases at a standard rate regardless of the patient , and so utilizes a standard profile . calcium profiling is premised on the fact that the blood level of monohydrogen phosphate decreases during the dialysis session . therefore , at some time period after the start of the dialysis procedure , when monohydrogen phosphate level is low enough that it is unlikely brushite formation will occur , the addition of calcium to the blood or to the dialysate fluid may be initiated . to further clarify , the calcium ion concentration in the fistula depends to some extent on the concentration of calcium contained in the dialysate fluid , whereas the phosphorous ion concentration comes solely from plasma phosphate . if the concentration of phosphate in blood plasma may be decreased by dialysate having a low concentration of calcium , then when the dialysis session has been going on for some period of time , for example between 15 to 30 minutes as shown in the exemplary profile of fig4 the concentration of plasma calcium may then be increased by addition of calcium to the dialysate fluid . such calcium profiling may help decrease the likelihood of brushite formation . this concept assumes that the concentration of phosphorous ions in the blood is highest at the beginning of a dialysis procedure and subsequently decreases over time as the procedure continues . furthermore , by keeping the ph of the dialysate high , calcium and phosphate ions will more easily remain in solution , and possible brushite formation in a fistula may be potentially avoided . [ 0025 ] fig4 shows one proposed profile of the ratio of calcium ions in the dialysate to phosphorus ions in the blood of a dialysis patient during a dialysis procedure in accordance with the instant invention . the concentration of calcium ions in the dialysate is graphed against the concentration of phosphorus ions in blood plasma over time . as shown in fig4 at the beginning of the dialysis procedure , at a first period of time , the concentration of phosphorus in the blood plasma is high . accordingly , the concentration of calcium administered to the patient in either the dialysate fluid or directly into the patient &# 39 ; s blood is kept low . as the dialysis procedure progresses , at a second period of time , the amount of phosphorous in the blood decreases due to filtration by the dialyzer . accordingly , the concentration of calcium in the dialysate solution is increased . by varying the concentration of calcium in response to the concentration of phosphorus in the blood in accordance with the instant invention , the formation of brushite crystals in the fistula may be avoided , thereby decreasing the probability of calcification of the fistula and subsequent stenosis due to brushite formation . in another embodiment , ( not shown ) the amount of calcium administered to the patient either in the dialysis fluid or directly into the patient &# 39 ; s blood may be increased by increasing the flow rate of the solution containing calcium over time . the profile shown in fig4 is merely exemplary , and is not meant to be limiting . it is understood that other profiles could be developed by those skilled in the art utilizing the principles described herein . the use of different profiles will be described in greater detail below . here below follows descriptions of embodiments which are currently believed to be solutions to avoid the formation of brushite in fistulas of dialysis patients . referring to the figures , in which like reference numerals refer to like portions thereof , fig5 shows by way of a schematic diagram one embodiment of an extracorporeal blood treatment system capable of performing a calcium profiling procedure according to the present invention . a first flow circuit 40 for a dialysis procedure comprises a main or primary conduit 1 which originates from a suitable source of water , such as a liquid reservoir or heating vessel 2 . the liquid reservoir 2 may include an inlet 15 for introduction of pure water thereinto , for example , from a reverse osmosis unit ( not shown ). the main conduit 1 may include a throttling mechanism 3 , a pressure gauge 4 , a pump 5 and a deaerating device 6 which may be provided with an air outlet ( not shown ). the main conduit may also contain one or more conductivity meters 14 and 26 respectively . water may enter the first flow circuit 40 from the liquid reservoir 2 via the main or primary conduit 1 or alternatively may enter the circuit through a first concentrate circuit 8 . concentrate circuit 8 may contain a powder concentrate column 10 , which may contain sodium bicarbonate powder . the first concentrate circuit 8 communicates with the main conduit 1 at a mixing point 7 . a conductivity meter 14 or other measuring device may also be provided in the main conduit 1 . the conductivity meter 14 or other measuring device is adapted to control a flow regulating device or pump 13 provided in the concentrate conduit 8 downstream of the powder concentrate column 10 . if , as described below , the flow regulating device 13 is a throttle , the main line throttle device 3 should be located upstream of the mixing point 7 as shown . according to another embodiment , the flow regulating device may be a metering dosage pump , a variable displacement pump , or a proportional valve ( not shown ). as mentioned , the flow regulating device 13 may be a simple adjustable throttling device . this is advantageous in that a single pump 5 may be employed for withdrawing water from the reservoir 2 for both the main dialysate flow through line 1 and for production of the concentrate fluid in fluid conduit 8 . if the throttling device 3 is located in the main line 1 between the source of water 2 and mixing point 7 , and if the deaerating device 6 is located in the main duct downstream of pump 5 , the same pump 5 may also be used to deaerate both the main line 1 and the prepared dialysate fluid . for the preparation of dialysate fluids , the pump 5 is preferably operative to handle flow rates up to at least 500 ml / min , and more preferably , up to approximately 1 , 000 ml / min in the main line 1 . the flow regulating means 13 on the other hand should be preferably operative to handle flow rates up to approximately 40 ml / min or at least 30 ml / min at flow rates of approximately 1 , 000 ml / min in the main line 1 . a second mixing point 23 is provided downstream of conductivity meter 14 . at mixing point 23 , a second concentrate fluid preferably containing sodium chloride , magnesium chloride , potassium chloride , small amounts of acetic acid and glucose may be introduced into the main line 1 via a second concentrate conduit or duct 24 . this second concentrate may be in a solid or a liquid form , however , in the preferred embodiment , the concentrate is in a liquid form . the second concentrate 25 corresponds substantially to the conventional “ a ” concentrate known in the art . in a preferred embodiment , the second concentrate does not contain calcium . the flow of second concentrate fluid through the second concentrate duct 24 may be regulated with the aid of a conductivity meter 26 or other measuring device which may be located downstream of mixing point 23 in the main conduit 1 . conductivity meter 26 controls a flow regulating device 27 , located in the second concentrate duct 24 . in the embodiment shown in fig5 a third mixing point 53 may be provided downstream of conductivity meter 26 . at mixing point 53 , a fluid containing concentrated calcium may be introduced into the primary conduit 1 via a third concentrate conduit or duct 54 . duct 54 communicates with a source of concentrate 55 , which in this instance , is a container containing calcium concentrate . the concentrated calcium may be in a solid or a liquid form such as a calcium solution without departing from the spirit and scope of the invention . according to one embodiment , the calcium concentration in a dialysate solution may be a solution containing calcium chloride . the calcium solution may have a variable amount of calcium of between 1 mm to 1 . 75 mm ( kracler , m ., scharfetter , h ., wimsberger , g . h ., clinical nephrology , 2000 , 54 : 35 - 44 , and argiles i ciscart , a , nephrol dial . transplant . 1995 , 10 : 451 - 454 ). the amount of calcium concentrate released through the third concentrate duct 54 may be regulated with the aid of a conductivity meter 56 or other measuring device located in the main conduit 1 . conductivity meter 56 may control a flow regulating device 57 located in concentrate duct 54 . flow regulating device 57 may be a variable output pump or may be a proportional valve . thus , as shown in fig5 it will be appreciated that if three concentrates 10 , 25 and 55 respectively are to be conducted to the main duct 1 at three separate mixing points 7 , 23 and 53 it is important that conductivity meters 14 , 26 and 56 or other similar measuring devices for accurate monitoring of the composition of the prepared solution be used . in this fashion , the dialysate solution composition may be accurately monitored both upstream as well as downstream of the second and third mixing points 23 and 53 . for ultimate monitoring of the ph of the prepared dialysate solution , an optional ph meter 28 maybe located in the main conduit 1 downstream of the third mixing point 53 , but upstream of a bypass valve 29 and a main valve 30 through which the system may be connected to a dialyzer 100 . if the measurements obtained in the main conduit 1 from any one or all of conductivity meters 14 , 26 or 56 and / or ph meter 28 are not in accord with the desired values , the main valve 30 may be closed and bypass valve 29 opened . for this purpose , conductivity meters 14 , 26 and 56 and ph meter 28 are all shown as providing input for controlling valves 29 and 30 . although the various meters for measuring the properties of the fluid being conducted through main conduit 1 preferably control the valves 29 and 30 , it will also be appreciated that it is possible instead to control one or more of the pumps 5 , 13 , 27 and 57 to stop or otherwise alter the flow of fluid into and through the various conduits . as shown in fig5 control unit 110 is preferably connected to the variable output pump 57 for controlling the concentration of calcium in the dialysate as a function of time . for this purpose the control unit 110 receives a signal from conductivity meter 56 and sends a control signal to pump 57 . thus the variable output pump 57 is controlled by a closed loop feedback system . a number of profiles of a desired calcium concentration versus time may be stored in the control unit 110 . one example of such a profile is shown in fig4 described above . because patients react very differently to low calcium concentrations , one embodiment may comprise the personal calcium concentration profiles of individual patients stored in control unit 110 . another embodiment may be to store specific profiles for certain patient types or patient groups . the control unit 110 may also comprise a user interface 115 for manual or automatic adjustment and selection of a specific calcium profile . according to another embodiment the control unit 110 communicates with other control elements ( not shown ) of the dialysis system for exchange of data in order to perform an automatic selection and adjustment of a calcium profile . in the embodiment of fig5 downstream of valve 30 a flow meter 46 may be located in the primary conduit 1 . the primary conduit 1 extends to the filtration or processing unit 100 . in dialysis , filtration unit 100 may be a dialyzer , which may also be referred to as a filter . the dialyzer or filtration unit 100 may be a hemodialfiltration unit , a hemofiltration unit , an ultrafiltration unit , or other types of filtration devices known in the art . filtration unit 100 is shown schematically divided into a primary chamber 101 separated from a secondary chamber 102 by a semi - permeable membrane 103 ( not shown in detail ). in this extracorporeal system , primary chamber 101 of the dialyzer 100 accepts fluid from the dialysate or first flow circuit 40 and secondary chamber 102 accepts blood from the blood or second flow circuit 12 . a conduit 68 extends from flow meter 47 to pump 63 , which transports the dialysate to an outlet 64 . another conduit 69 connects the outlet of valve 29 to conduit 68 . as introduced above , the system generally includes a second flow circuit 12 , which is an extracorporeal blood flow circuit , having first and second conduits 71 and 72 which are both connected to the vascular system of a patient ( see element 60 of fig1 ). blood access and return devices 76 and 77 respectively , remove and return blood to the patient . the access and return devices 76 and 77 may be cannulas , catheters , winged needles or the like as understood in the art . conduits 71 and 72 are also connected to the filtration or processing unit 100 . a peristaltic pump 80 is disposed in operative association with the first conduit 71 . in fig5 the extracorporeal blood flow circuit 12 preferably includes a conventional anticoagulant pump 85 for mixing anticoagulant such as heparin into the flow of blood at a mixing point 74 . the anticoagulant pump 85 may be a syringe filled with heparin concentrate and may contain an actuator 87 that may be controlled from a control unit ( not shown ). as understood in the art , an air bubble trapping drip chamber 66 for deaerating the blood is shown in the second conduit 72 . a bubble detector 67 is often included on or adjacent to bubble trap 66 . numerous other component devices may be used in the extracorporeal blood flow circuit 12 without departing from the spirit and scope of the invention . pressure sensors 88 , 89 and 90 may be included in the extracorporeal circuit as well as tubing clamps 61 and 62 . as shown in fig6 and as previously described above with reference to the embodiment described in fig5 the first flow circuit for a dialysis solution comprises a main or primary conduit 1 in which various concentrates may be mixed . except as described in further detail below , the embodiment of fig6 is similar to the embodiment described in fig5 wherein like numbers represent corresponding like elements . repeat description of these elements will not be further repeated here . in fig6 the calcium concentrate sub - system ( see mixing point 53 , tubing 54 , container 55 and pump 57 of fig5 ) is not included for connection into primary line 1 . in fig6 a calcium pump 95 similar in construction to conventional anticoagulant pump 85 may be used to deliver calcium to the blood flow side of extracorporeal circuit 12 . the pump 95 delivers calcium to the circuit 12 at a calcium mixing point 75 located in conduit 71 downstream of the anticoagulant mixing point 74 . some calcium added to blood circuit 12 from pump 95 may migrate across membrane 103 of the filter 100 and may enter the dialysis circuit 40 . once calcium enters the dialysis circuit 40 , some calcium may be lost via the dialysate outlet 64 . because of this , calcium must be added to the system in a higher concentration or amount than necessary for the patient , with the understanding that some amount of calcium will be lost to the dialysis circuit side 40 . an alternative embodiment ( not shown ) to prevent the loss of calcium across the membrane 103 is to connect a calcium pump similar to pump 95 shown in fig6 to the blood circuit side 12 at location 42 of tubing segment 72 . such a connection may prevent calcium from entering the dialysis circuit . the calcium would flow directly into the patient via blood return device 76 . the calcium pump 95 may be a syringe containing calcium concentrate infusion fluid and may also be connected to an actuator mechanism 97 , which may in turn be connected to control unit 110 . according to another embodiment ( not shown ) the calcium pump for delivering the calcium concentrate may be a peristaltic pump . for accurate dosing of a patient , the calcium concentrate may also be supplied from a bag that is suspended from a balance . a signal from the balance may be used by the control unit 110 to drive the pump . the addition of calcium into the extracorporeal circuit may also be added at other locations within the circuit without departing from the spirit and scope of the present invention . calcium addition can be by other well known methods and means including but not limited to a stepper motor . it has been further hypothesized that the ph of blood may play a role in the formation of brushite crystals in a fistula . at a ph less than 7 . 3 , calcium phosphate may precipitate out of the blood in such a way as to form brushite crystals . at a blood ph greater than 7 . 5 however , calcium phosphate may precipitate out of the blood as hydroxyapatite crystals , which do not contribute to the formation of stenosis in a fistula . another way to avoid brushite formation is to keep the ph of plasma sufficiently high in some way , either with or without the calcium profiling described above . this might be achieved by acetate free bio - filtration ( not shown ) or by infusing bicarbonate directly into the blood stream ( not shown ). it should be understood that various changes and modifications to the described embodiments will be apparent to those skilled in the art . these examples are not meant to be limiting , but rather are exemplary of the modifications that can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages .
the invention is directed towards a method for preventing the loss of functionality of a fistula due to the formation of calcium phosphate and other precipitates within the fistula . the method comprises profiling the amount of calcium in the dialysis fluid or blood in relation to the amount of phosphorous in the blood plasma . this invention also comprises a system for profiling calcium during a dialysis procedure .