Dialysis apparatus for independently controlling the concentration of at least two ionic substances inside a patient's body

A dialysis apparatus includes a dialysis liquid circuit for circulating sodium chloride and sodium bicarbonate through a haemodialyser and a circuit for infusing a patient with at least one solution containing at least one ionic substance "A" absent from the dialysis liquid. The substance "A" has a determined concentration (A)sol in the infusion solution. A dialysance detector determines the actual dialysance "D" of the haemodialyser for sodium, and a flow rate detector determines the flow rate Qinf of infusion solution such that the concentration of the substance "A" inside the patient's body tends towards a desired concentration (A)des, as a function of the dialysance "D", the concentration (A)sol of the substance "A" in the infusion solution and the desired concentration (A)des, A regulator regulates the flow rate of infusion solution, and a controller drives the regulator to control the flow rate of the infusion solution such that this flow rate is substantially equal to the determined flow rate Qinf.

BACKGROUND OF THE INVENTION
 Field of the Invention
 The present invention relates to a dialysis apparatus for independently
 controlling the concentration of at least two ionic substances inside a
 patient's body.
 The kidneys fulfill many functions, including the removal of water, the
 excretion of catabolites (or waste from the metabolism, for example urea
 and creatinine), the regulation of the concentration of the electrolytes
 in the blood (sodium, potassium, magnesium, calcium, bicarbonates,
 phosphates, chlorides) and the regulation of the acid/base equilibrium
 within the body, which is obtained in particular by the removal of weak
 acids (phosphates, monosodium acids) and by the production of ammonium
 salts.
 In individuals who have lost the use of their kidneys, since these
 excretion and regulation mechanisms no longer work, the body accumulates
 water and waste from the metabolism and exhibits an excess of electrolytes
 (in particular sodium), as well as, in general, acidosis, the pH of the
 blood plasma shifting towards 7 (the blood pH normally varies within
 narrow limits of between 7.35 and 7.45).
 In order to overcome renal dysfunction, resort is conventionally made to a
 blood treatment involving extracorporeal circulation through an exchanger
 having a semipermeable membrane (haemodialyser) in which the patient's
 blood is circulated on one side of the membrane and a dialysis liquid,
 comprising the main electrolytes of the blood in concentrations close to
 those in the blood of a healthy subject, is circulated on the other side.
 Through the effect of the physical phenomenon referred to as dialysis, the
 molecules migrate from the liquid where their concentration is higher to
 the liquid where their concentration is lower.
 In conventional dialysis apparatus, the dialysis liquid is prepared by
 metered mixing of water and two concentrated solutions, the first
 concentrated solution containing sodium chloride and sodium bicarbonate,
 and the second concentrated solution containing calcium chloride,
 potassium chloride and magnesium chloride as well as acetic acid. The
 function of the acetic acid is to limit the formation of calcium carbonate
 and magnesium carbonate precipitates which form undesirable deposits in
 the hydraulic circuit of the dialysis apparatus.
 This conventional way of preparing a dialysis liquid has several drawbacks:
 the respective concentrations of the various ionic substances involved in
 the composition of the dialysis liquid cannot be regulated independently
 of one another, even though this would be desirable at least for sodium,
 potassium and bicarbonate;
 for physiological reasons, the acetic acid concentration in the dialysis
 liquid is necessarily limited, so that carbonated deposits are formed in
 the hydraulic circuit of the dialysis apparatus. Dialysis apparatus
 therefore need to be regularly descaled, which places a further burden on
 their maintenance;
 the presence of acetic acid in the second solution causes corrosion of the
 connection and pumping means used for transferring the concentrated
 solution from the reservoir where it is contained to the mixing zone of
 the apparatus, where it is diluted in the solution obtained by metered
 mixing of water and the first solution;
 use of a dialysis liquid having a pH less than that of blood at the time
 when certain types of haemodialysers are started up, that is to say at the
 time when the dialysis liquid compartment of these haemodialysers is
 filled with dialysis liquid and the blood compartment is filled with
 diluted blood, seems to be one of the cofactors of certain
 hypersensitivity reactions.
 Document EP 0 192 588 describes a dialysis apparatus comprising:
 means for circulating a dialysis liquid which contains sodium bicarbonate
 and is free of calcium and magnesium,
 means for infusing the patient with an infusion solution containing at
 least calcium and magnesium.
 With this apparatus, since the ionic substances (bicarbonate, calcium,
 magnesium) which can form precipitates when they are combined, are
 separated, it is not necessary to involve acetic acid in the composition
 of the dialysis liquid. As described, however, this apparatus cannot be
 used because it does not comprise means for regulating the infusion flow
 rate which, in particular, would take into account the diffusive transport
 of calcium and magnesium through the membrane of the dialyser, from the
 patient's blood to the dialysis liquid. Now, for safety reasons, it is not
 envisageable to infuse a patient with a concentrated calcium solution
 without being able to regulate the infusion flow rate accurately so that
 there is neither an excess or deficit of this ionic substance within the
 patient's body.
 SUMMARY OF THE INVENTION
 The object of the invention is to provide a dialysis apparatus which makes
 it possible to regulate accurately the flow rate of an infusion liquid
 containing an ionic substance with a view to making the concentration of
 this substance inside the patient's body tend towards a desired
 concentration. More broadly, the object of the invention is to provide a
 dialysis apparatus which makes it possible to control separately the
 concentration of at least two ionic substances inside a patient's body by
 means of a dialysis liquid and an infusion liquid.
 This object is achieved by means of dialysis apparatus comprising:
 means for circulating a dialysis liquid containing sodium chloride and
 sodium bicarbonate through a haemodialyser;
 means for infusing a patient with at least one solution containing at least
 one ionic substance A (calcium, magnesium, potassium) absent from the
 dialysis liquid, the substance A having a determined concentration [A]sol
 in the infusion solution;
 means for determining the actual dialysance D of the haemodialyser for
 sodium;
 means for determining a flow rate Qinf of infusion solution such that the
 concentration of the substance A inside the patient's body tends towards a
 desired concentration [A]des, as a function of the dialysance D, of the
 concentration [A]sol of the substance A in the infusion solution, and of
 the desired concentration [A]des;
 regulating means for regulating the flow rate of infusion solution;
 control means for driving the means for regulating the flow rate of the
 infusion solution such that this flow rate is substantially equal to the
 determined flow rate Qinf.
 In the dialysis apparatus according to the invention, since the dialysis
 liquid is free of calcium, magnesium and, optionally, potassium, these
 substances which are present in the blood migrate by diffusion from the
 blood to the dialysis liquid in the course of the dialysis session. It is
 in order to compensate for these diffusive losses that provision is made
 to infuse the patient with a solution containing these substances. The
 difficulty resides in the fact that these diffusive losses can vary in the
 course of a dialysis session lasting several hours, and in the fact that
 an excess or deficit of potassium, calcium and magnesium in the patient's
 blood can lead to serious disorders, in particular cardiac disorders. By
 virtue of the invention, this difficulty is overcome since the flow rate
 of the infusion is slaved to the actual dialysance of the treatment
 system.
 Moreover, further to remedying the drawbacks of conventional dialysis
 apparatus mentioned above, the apparatus according to the invention also
 has the following advantage: provided that the infusion liquid is injected
 directly into the patient or, downstream of the haemodialyser, into the
 extracorporeal blood circulation circuit connecting the patient to the
 haemodialyser, the ionic calcium in the blood migrates by diffusion into
 the dialysis liquid since the latter is free of calcium. Now, ionic
 calcium is involved in the sequence of reactions constituting the
 coagulation process of blood. The extensive calcium depletion of the blood
 in the haemodialyser therefore partially inhibits the coagulation process,
 which makes it possible to reduce the amount of anticoagulant usually
 injected into the extracorporeal blood circulation circuit to prevent
 blood from coagulating in it.
 According to one characteristic of the invention, the means for determining
 the infusion flow rate Qinf comprise calculation means for calculating the
 infusion flow rate Qinf according to the formula:
EQU Qinf=Cl.times.[A]des/[A]sol-[A]des
 where Cl is the clearance of the haemodialyser for the substance A,
 extrapolated on the basis of the dialysance D for sodium.
 According to another characteristic of the invention, the infusion solution
 contains sodium at a determined concentration [Na+]sol. In this case, the
 apparatus furthermore includes:
 means for preparing the dialysis liquid, comprising means for regulating
 the sodium concentration of the dialysis liquid;
 means for determining the sodium concentration [Na+]dial of the dialysis
 liquid such that the concentration inside the patient's body tends towards
 a desired sodium concentration [Na+]des, as a function of the dialysance
 D, of the flow rate of the infusion liquid Qinf, of the sodium
 concentration [Na+]sol of the infusion solution, and of the desired sodium
 concentration [Na+]des;
 control means for driving the means for regulating the sodium concentration
 of the dialysis liquid such that this concentration is equal to the
 determined concentration [Na+]dial.
 According to one characteristic of the invention, the means for determining
 the sodium concentration [Na+]dial of the dialysis liquid comprise
 calculation means for calculating this concentration according to the
 formula:
 ##EQU1##
 Other characteristics and advantages of the invention will become more
 clearly apparent on reading the following description. Reference will be
 made to the single appended FIGURE, which schematically represents a
 haemodialysis system according to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT
 An infusion device, comprising a pump 10 and a balance 11, is provided for
 injecting the contents of an infusion liquid bag 9 into the bubble trap 8.
 The bag 9 is suspended from the balance 11 and is connected to the bubble
 trap 8 by a line 12 in which the infusion pump 10 is arranged. The balance
 11 is used to drive the pump 10 such that the flow rate of the infusion
 liquid is equal to a target flow rate.
 The second compartment 3 of the haemodialyser 1 has an inlet connected to a
 feed line 12 for fresh dialysis liquid, and an outlet connected to a
 discharge line 13 for spent liquid (dialysis liquid and ultrafiltrate).
 The feed line 12 connects the haemodialyser 1 to a device 14 for preparing
 dialysis liquid, comprising a main line 15 to which two secondary branch
 lines 16, 17 are connected in series. The upstream end of the main line 15
 is intended to be connected to a source of running water. Each secondary
 line 16, 17 comprises connection means for fitting a cartridge 18, 19
 containing a salt in granular form. A pump 20, 21 is arranged in each
 secondary line 16, 17 downstream of the corresponding cartridge 18, 19 in
 order to circulate the liquid from the main line through it. Each pump 20,
 21 is driven on the basis of comparison between 1) a target conductivity
 value for the mixture of liquids formed where the main line 15 joins the
 downstream end of the secondary line 16, 17 and 2) the value of the
 conductivity of this mixture measured by means of a conductivity probe 22,
 23 arranged in the main line 15 immediately downstream of the junction
 between the main line 15 and the downstream end of the secondary line 16,
 17.
 The feed line 12 forms an extension of the main line 15 of the device 14
 for preparing dialysis liquid. Arranged in this feed line, in the
 direction in which the liquid circulates, there are a first flow meter 24
 and a first circulation pump 25.
 The downstream end of the discharge line 13 for spent liquid is intended to
 be connected to the drain. Arranged in this line, in the direction in
 which the liquid circulates, there are a second circulation pump 26 and a
 second flow meter 27. An extraction pump 28 is connected to the discharge
 line 13, upstream of the second circulation pump 26. The extraction pump
 28 is driven in such a way that its delivery rate is equal to a target
 value for the ultrafiltration rate in the haemodialyser 1.
 The feed line 12 and the discharge line 13 are connected by first and
 second branch lines 29, 30, which are connected together by a junction
 line 31 in which a conductivity probe 32 is arranged. These branch 29, 30
 and junction 31 lines and the conductivity probe 32 form a device for
 measuring the conductivity of the fresh and spent dialysis liquid which is
 used, as will be explained below, to determine the actual dialysance (or
 clearance) of the system for sodium or another substance of similar
 molecular weight. The first branch line 29 is connected to the feed line
 12, downstream of the first circulation pump 25, via a first three-way
 valve 33, and is connected to the discharge line 13, upstream of the
 second circulation pump 26, via a second three-way valve 34. The second
 branch line 30 and the junction line 31 are connected via a third
 three-way valve 35.
 The haemodialysis system represented in FIG. 1 also comprises a calculation
 and control unit 36. This unit is connected to a user interface
 (alphanumeric keyboard) 37 through which it receives instructions, such as
 various target values. It furthermore receives the information output by
 the measuring instruments of the system, for example the flow meters 24,
 27, the conductivity probes 22, 23, 32 and the balance 11. On the basis of
 the instructions received and the operating modes and algorithms which
 have been programmed, it drives the active components of the system, such
 as the pumps 6, 10, 20, 21, 25, 26, 28 and the valves 33, 34, 35.
 In the embodiment of the invention which is represented in the FIGURE,
 the cartridge 18 contains only sodium bicarbonate;
 the cartridge 19 contains only sodium chloride; and
 the infusion liquid bag 9 contains a solution of calcium, magnesium and
 potassium chloride. The bag 9 optionally also contains sodium.
 The haemodialysis apparatus which has just been described operates as
 follows.
 Via the user interface 37, an operator communicates target values
 corresponding to the various parameters of the treatment (prescription) to
 the control unit 36, namely the flow rate of blood Qb, the flow rate of
 the dialysis liquid Qd, the total weight loss WL (amount of plasma fluid
 to be withdrawn from the patient by ultrafiltration), the total duration T
 of the session, the bicarbonate concentration [HCO3.sup.- ]dial of the
 dialysis liquid (which should make it possible for the bicarbonate
 concentration inside the patient's body to tend towards a desired
 concentration [HCO3.sup.- ]des), the sodium concentration [Na.sup.+ ]dial
 of the dialysis liquid (which should make it possible for the sodium
 concentration within the patient's body to tend towards a desired
 concentration [Na+]des), the potassium concentration [K.sup.+ ]sol of the
 infusion solution, and the potassium concentration [K.sup.+ ]des to which
 the concentration inside the patient's body should tend. After a sodium
 bicarbonate cartridge 18 and a sodium chloride cartridge 19 have been
 connected to the corresponding lines 16, 17 of the device 14 for preparing
 dialysis liquid, the dialysis liquid circuit is filled with dialysis
 liquid. In order to do this, the main line 15 is connected to a source of
 running water and the pumps 20, 21, 25, 26 are turned on. The pumps 20 and
 21 are regulated by the control unit 36 such that the bicarbonate
 concentration and the sodium concentration of the dialysis liquid are
 equal to the corresponding target values [HCO3.sup.- ]dial and [Na.sup.+
 ]dial. The pumps 25, 26 for circulating dialysis liquid are regulated by
 the control unit 36 such that the delivery rate of the pump 25 located
 upstream of the haemodialyser 1 is equal to the target flow rate Qd (for
 example 500 ml/min) and the delivery rate of the pump 26 located
 downstream of the haemodialyser 1 is such that the flow rates measured by
 the flow meters 24, 27 are equal. The three-way valves 33, 34, 35 of the
 device for measuring the conductivity of the dialysis liquid are arranged
 such that the conductivity probe 32 is normally exposed to fresh dialysis
 liquid (the path of the dialysis liquid successively following the lines
 12, 29, 31, 30, 12 and 13). In order to rinse and initially fill all the
 lines for fresh dialysis liquid the valves 33, 34 and 35 are turned at
 least once so that the conductivity probe 32 is exposed to the liquid
 leaving the haemodialyser (the path of the dialysis liquid successively
 following the lines 12, 13, 30, 31, 29 and 13).
 At the same time as the dialysis liquid circuit is filled with the dialysis
 liquid according to the prescription, the extracorporeal blood circulation
 circuit is rinsed and filled with sterile saline solution.
 When the dialysis liquid circuit and the blood circuit have been primed,
 the blood circuit is connected to the patient and the treatment proper can
 begin: the pumps 20, 21 of the device 14 for preparing dialysis liquid, as
 well as the pumps 25, 26 for circulating the dialysis liquid continue to
 operate as when the circuit is being primed, while the blood pump 6, the
 extraction pump 28 and the infusion pump 10 are turned on. The blood pump
 6 is regulated to the target flow rate Qb (for example 200 ml/min) and the
 extraction pump 28 is regulated to a flow rate QUF calculated by the
 control unit 36, on the basis of the target values for the total weight
 loss WL and the total duration of the treatment T.
 According to the invention, the infusion pump 10 is regulated to a flow
 rate Qinf calculated by the control unit 36 on the basis of the following
 formula:
 ##EQU2##
 where Cl is the clearance of the haemodialyser 1 for potassium. The
 infusion pump 10 is regulated accurately by the control unit 6 on the
 basis of the information supplied by the balance 11. The actual clearance
 Cl for potassium is obtained by extrapolation from the actual dialysance
 for sodium D, which is determined by implementing the following process,
 in which the successive steps are controlled by the control unit 36. With
 the three-way valves 33, 34, 35 arranged in such a way that the fresh
 dialysis liquid irrigates the conductivity probe 32, the conductivity
 Cd1in of the fresh dialysis liquid corresponding to the prescription is
 measured and stored. The three valves 33, 34, 35 are then turned so that
 the conductivity probe 32 is irrigated by the spent liquid, and the
 conductivity Cd1out of this liquid is measured and stored. The delivery
 rate of the pump 21 for concentrated sodium chloride solution is then
 modified (increased or decreased) so that the conductivity of the dialysis
 liquid circulated is slightly different from the conductivity of the
 dialysis liquid of the prescription. For example, the conductivity of the
 second dialysis liquid is regulated so as to be 1 mS/cm greater or less
 than the conductivity of the first dialysis liquid (which is generally of
 the order of 14 mS/cm). As before, the conductivity Cd2in of the second
 dialysis liquid upstream of the haemodialyser is measured and stored,
 after which the three-way valves 33, 34, 35 are again turned so that the
 conductivity probe 32 is irrigated by the spent liquid, and the
 conductivity Cd2out of the spent liquid is measured and stored.
 The dialysance for sodium can then be calculated by applying the following
 formula:
 ##EQU3##
 where Qd is the flow rate of the dialysis liquid.
 Another process for calculating the dialysance D on the basis of
 measurements taken with two dialysis liquids having different
 conductivities is described in Patent Application EP 0 658 352.
 In the particular case of treatment by continuous haemodialysis, in which
 the flow rate of the dialysis liquid is very much less than the blood flow
 rate (of the order of three times), the flow rate Qinf of the infusion
 solution can be calculated at any time by applying the formula:
 ##EQU4##
 where Qout is the flow rate of spent liquid leaving the haemodialyser,
 which is equal to the sum of the flow rate of fresh dialysis liquid, as
 determined by the circulation pumps 25, 26, and the flow rate of the
 extraction pump 28.
 EXAMPLE 1
 The target to be achieved, in terms of potassium concentration [K.sup.+
 ]des inside the body is 2 mEq/l.
 A blood-isotonic infusion liquid is chosen, in which the relative
 proportions of the ions are equal to the desired relative proportions for
 the same ions inside the patient's body. The infusion liquid may, for
 example, have the following composition:
 [K.sup.+ ]=42 mEq/l
EQU [Mg.sup.++ ]=31.3 mEq/l
EQU [Ca.sup.++ ]=73.5 mEq/l
EQU [Cl.sup.- ]=147 mEq/l
 If the actual dialysance D is, for example, 150 ml/min, the infusion flow
 rate which the control unit calculates by means of formula (1) and imposes
 on the infusion pump 10 is 0.45 l/h.
 According to the invention, the intention is to be able to use the dialysis
 apparatus that has just been described to carry out a haemodiafiltration
 treatment with a high flow rate of infusion liquid (more than 1 l/h). In
 this case, it is not possible to use a blood-isotonic infusion liquid
 containing only potassium, calcium and magnesium chloride (like the one
 described in Example 1) because this would lead to the patient becoming
 overloaded with these ionic substances.
 It is therefore necessary to envisage the use of a substitute liquid in
 which the potassium, calcium and magnesium concentrations are lower and
 which furthermore contains sodium so as to remain isotonic with blood. The
 problem then encountered is of regulating the sodium concentration of the
 dialysis liquid while taking account of the variable-rate infusion of an
 infusion liquid containing sodium, so that the patient's body tends
 towards a desired sodium concentration [Na+]des.
 According to the invention, the delivery rate of the pump 21 for
 concentrated sodium chloride solution is adjusted continuously so that the
 sodium concentration [Na+]dial of the dialysis liquid, as measured by the
 conductivity probe 23, can make the sodium concentration inside the
 patient's body tend towards a desired value [Na+]des. The sodium
 concentration [Na+]dial of the dialysis liquid is calculated by the
 calculation unit 36 on the basis of the dialysance D of the haemodialyser
 for sodium, the infusion flow rate Qinf, the sodium concentration [Na+]sol
 of the infusion solution, and the desired sodium concentration [Na+]des
 towards which the patient's body should tend. It can be calculated by
 applying the following formula:
 ##EQU5##
 EXAMPLE 2
 The target to be achieved, in terms of the potassium concentration [K.sup.+
 ]des inside the body is 2 mEq/l.
 The target to be achieved, in terms of the sodium concentration [Na.sup.+
 ]des inside the body is 140 mEq/l.
 The desired infusion flow rate is about 1 l/h.
 A blood-isotonic infusion liquid is chosen, in which the relative
 proportions of the ions are equal to the desired relative proportions for
 the same ions inside the patient's body. The infusion liquid may, for
 example, have the following composition:
EQU [K.sup.+ ]=20 mEq/l
EQU [Mg.sup.++ ]15 mEq/l
EQU [Ca.sup.++ ]=35 mEq/l
EQU [Na.sup.+ ]=77 mEq/l
EQU [Cl.sup.- ]=147 mEq/l
 If the actual dialysance D is, for example, 150 ml/min, the infusion flow
 rate which the control unit 36 calculates by means of formula (1) and
 imposes on the infusion pump 10 is 1 l/h. Furthermore, the delivery rate
 which the control unit 36 imposes on the pump 21 for concentrated sodium
 chloride solution is such that the sodium concentration [Na+]dial of the
 dialysis liquid is equal to 154.8 mEq/l (concentration calculated by means
 of formula (4)).
 EXAMPLE 3
 The target to be achieved, in terms of the potassium concentration [K.sup.+
 ]des inside the body is 2 mEq/l.
 The target to be achieved, in terms of the sodium concentration [Na.sup.+
 ]des inside the body is 140 mEq/l.
 The desired infusion flow rate is about 2 l/h.
 A blood-isotonic infusion liquid is chosen, in which the relative
 proportions of the ions are equal to the desired relative proportions for
 the same ions inside the patient's body. The infusion liquid may, for
 example, have the following composition:
 [K.sup.+ ]=10 mEq/l
EQU [Mg.sup.++ ]=7.5 mEq/l
EQU [Ca.sup.++ ]=17.5 mEq/l
EQU [Na.sup.+ ]=112 mEq/l
EQU [Cl.sup.- ]=147 mEq/l
 If the actual dialysance D is, for example, 150 ml/min, the infusion flow
 rate which the control unit 36 calculates by means of formula (1) and
 imposes on the infusion pump 10 is 2.25 l/h. Furthermore, the delivery
 rate which the control unit 36 imposes on the pump 21 for concentrated
 sodium chloride solution is such that the sodium concentration [Na+]dial
 of the dialysis liquid is equal to 147 mEq/l (concentration calculated by
 means of formula (4)).
 EXAMPLE 4
 The target to be achieved, in terms of the potassium concentration [K.sup.+
 ]des inside the body is 2 mEq/l.
 The target to be achieved, in terms of the sodium concentration [Na.sup.+
 ]des inside the body is 140 mEq/l.
 The desired infusion flow rate is about 4.5 l/h.
 A blood-isotonic infusion liquid is chosen, in which the relative
 proportions of the ions are equal to the desired relative proportions for
 the same ions inside the patient's body. The infusion liquid may, for
 example, have the following composition:
EQU [K.sup.+ ]=6 mEq/l
EQU [Mg.sup.++ ]=4.5 mEq/l
EQU [Ca.sup.++ ]=10.5 mEq/l
EQU [Na.sup.+ ]=126 mEq/l
EQU [Cl.sup.- ]=147 mEq/l
 If the actual dialysance D is, for example, 150 ml/min, the infusion flow
 rate which the control unit 36 imposes on the infusion pump 10 is 4.5 l/h.
 Furthermore, the delivery rate which the control unit 36 imposes on the
 pump 21 for concentrated sodium chloride solution is such that the sodium
 concentration [Na+]dial of the dialysis liquid is equal to 147 mEq/l
 (concentration calculated by means of formula (4)).
 In the above examples, the target to be achieved was defined in terms of
 the potassium concentration [K.sup.+ ]des inside the body. It is obvious
 that, when calcium or magnesium are considered as the critical substance
 for a certain patient, the infusion pump 10 will be regulated on the basis
 of a target defined in terms of the desired calcium [Ca.sup.++ ]des or
 magnesium [Mg.sup.++ ]des concentration inside the patient's body.
 Variants may be made to the invention which has just been described.
 In the embodiment which was described above, only one of the ionic
 substances contained in the infusion liquid can be metered accurately
 inside the body. This is due to the fact that only one infusion solution
 is used, containing all the ionic substances to be infused. If it is
 desired to meter several ionic substances accurately, it is sufficient to
 use several infusion liquid bags, each containing a single ionic substance
 to be metered, and to provide a corresponding number of infusion means
 (balance and pump). It is also possible to use only a single balance, from
 which the various bags are suspended, and only a single pump which is
 associated with distribution means for in turn connecting each bag and the
 extracorporeal blood circulation circuit according to a determined time
 sequence.
 Instead of a single conductivity probe, to which the fresh dialysis liquid
 and the spent liquid are successively applied, the dialysis liquid circuit
 may be equipped with two conductivity probes which are arranged in the
 dialysis liquid circuit, respectively upstream and downstream of the
 haemodialyser. The dialysis liquid circuit may also include only one
 conductivity probe, arranged downstream of the haemodialyser, in which
 case the two target conductivity values used for driving the concentrate
 pump 21 in order to prepare the first and second dialysis liquids are
 substituted in the formula indicated above for the conductivity values
 measured upstream of the haemodialyser.