Method for calculating or approximating a value representing the relative blood volume and devices

Abstract

The present invention relates to a method for calculating or approximating a value representing the relative blood volume (RBV) at a certain point of time, or a value representing the refilling volume of a patient that may be observed or found during or due to a blood treatment of the patient, the method involving considering one or more calculated or measured value(s) reflecting an overhydration level of the patient or an approximation thereof. It relates further to an apparatus and a device for carrying out the present invention, a blood treatment device, digital storage means, a computer program product, and a computer program.

Claims

1. A blood treatment device for treating a patient by dialysis, the blood treatment device comprising a controller configured to control the blood treatment device based on: (i) a computer-implemented method for calculating a value representing a refilling volume (V.sub._refill) of the patient that may be observed or found during or due to a blood treatment session of the patient and (ii) relative blood volume (RBV) measurements of the patient that are measured and communicated to the controller during the blood treatment session of the patient, wherein a target range of a relative blood volume (RBV.sub._end) of the patient to be achieved by the blood treatment session is set by the controller based on the refilling volume (V.sub._refill) of the patient, wherein the refilling volume (V.sub._refill) of the patient is determined by the controller using an equation:
V.sub._refill=a*UFV+b*(UFR/Hb.sub.start)+c*OH+d
or
V.sub._refill=a*UFV/Hb+b*OH+c wherein: a, b, c, and d are parameters; Hb.sub._start is a haemoglobin concentration of the patient at a beginning of the blood treatment session; Hb is a haemoglobin concentration of the patient; UFV is an ultrafiltration volume; UFR is an ultrafiltration rate; and OH is the patient's overhydration when beginning the blood treatment session, wherein the controller is configured to adjust the ultrafiltration rate (UFR) during the blood treatment session such that, once the relative blood volume (RBV) measurements of the patient meet the target range of the relative blood volume (RBV.sub._end) that was set by the controller based on the refilling volume (V.sub._refill) of the patient determined by the controller, the relative blood volume (RBV) measurements of the patient do not drop below the target range of the relative blood volume (RBV.sub._end).

2. The blood treatment device according to claim 1, wherein the controller is configured to determine a relative blood volume (RBV) or the refilling volume (V.sub._refill) using an absolute start blood volume (BV.sub._start) upon or before beginning of the blood treatment session.

3. The blood treatment device according to claim 2, wherein the controller is configured to determine the absolute start blood volume (BV.sub._start) using at least one value reflecting a lean mass (LTM) of the patient's body and at least one value reflecting a fat mass (ATM) of the patient's body, or approximations thereof.

4. The blood treatment device according to claim 2, wherein the controller is configured to predict an end value of the relative blood volume (RBV) to be arrived at by an end of the blood treatment session without having caused intradialytic morbid events.

5. The blood treatment device according to claim 2, wherein a time a certain future blood treatment session lasts is calculated or optimized by taking the relative blood volume (RBV) or an end value of the relative blood volume (RBV) into account.

6. The blood treatment device according to claim 2, wherein the controller is configured to adjust the determined relative blood volume (RBV) based on the patient's overhydration (OH) level to be a normalized or normohydrated relative blood volume (RBV.sub._normohyd, or BV(t)/BV.sub._0).

7. The blood treatment device according to claim 1, wherein the controller is configured to determine a normalized or normohydrated relative blood volume (RBV.sub._normohyd) using an equation: ${\mathrm{RBV}}_{\_\mathrm{normohyd}}\left(t\right)=\frac{{\mathrm{BV}}_{\mathrm{\_absolut}\_\mathrm{startDialysis}}}{{\mathrm{BV}}_{\_0}}*{\mathrm{RBV}}_{\_\mathrm{prior}}\left(t\right)=\frac{{\mathrm{BV}}_0+\frac{\mathrm{OH}}{K_{\_\mathrm{Guyton}}}}{{\mathrm{BV}}_{\_0}}*{\mathrm{RBV}}_{\_\mathrm{prior}}\left(t\right)$ wherein: BV.sub._absolute_startDialysis is an absolute blood volume at the beginning of the blood treatment session; K.sub._Guyton indicates what portion of overhydration is comprised within a vessel system of the patient; OH is an overhydration of the patient at the beginning of the blood treatment session; RBV.sub._prior(t) is a relative blood volume of the patient as $\frac{\mathrm{BV}\left(t\right)}{{\mathrm{BV}}_{\mathrm{absolute\_startDialysis}}};$ with BV(t) being a blood volume of the patient at time point (t); and BV.sub._0 is a normohydrated blood volume of the patient at the beginning of the blood treatment session, with BV.sub._0 being BV.sub._absolute_startDialysis minus the overhydration (OH) of the patient comprised within the vessel system of the patient.

8. The blood treatment device according to claim 1, further comprising an output device for outputting results of the computer-implemented method.

9. The blood treatment device according to claim 7, wherein the controller is configured to control the blood treatment device in relation to a value or target range representing the relative blood volume (RBV) of the patient or the normalized or the normohydrated blood volume (RBV.sub._normohyd) of the patient.

10. The blood treatment device according to claim 1, the controller being configured to control the blood treatment device in relation to a value or target range representing a target relative blood volume (RBV), wherein the target range is defined by a target overhydration status.

11. The blood treatment device according to claim 1, the controller being configured to control the blood treatment device such that the blood treatment session is terminated or interrupted once an end value of the relative blood volume (RBV) is measured or calculated that has been predicted as an end value or target range of the relative blood volume (RBV) at which the patient has not suffered intradialytic morbid events.

12. The blood treatment device according to claim 1, the controller being configured to control the blood treatment device such that the blood treatment session is terminated or interrupted once a threshold or a predetermined value of the patient's absolute blood volume has been detected or calculated.

13. The blood treatment device according to claim 1, the controller being configured to control the blood treatment device such that the blood treatment session is terminated or interrupted once a threshold or a predetermined value of the absolute blood volume of the patient has been detected or calculated, wherein the absolute blood volume of the patient is determined during the blood treatment session by taking the relative blood volume (RBV) of the patient determined during the blood treatment session into account.

14. The blood treatment device according to claim 1, wherein the blood treatment device is configured for treating the patient by haemofiltration, ultrafiltration, and/or haemodialysis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other aspects, features, and advantages will be apparent from the description, figures, and claims.

(2) FIG. 1 shows the relative blood volume drop per liter ultrafiltration volume during dialysis over an initial overhydration (in liters);

(3) FIG. 2 shows data including those already used for the plot of FIG. 1 in another, classified presentation;

(4) FIG. 3 shows a relation between the refilling volume as predicted over a refilling volume reference;

(5) FIG. 4 shows the concordance between the predicted relative blood volume and the relative blood volume actually measured at the end of the treatment;

(6) FIG. 5 shows the relative blood volume as measured in the course of one single treatment session;

(7) FIG. 6 shows another way of how a blood treatment machine may be controlled according to certain embodiments of the present invention;

(8) FIG. 7 shows another way of how a blood treatment machine may be controlled according to certain embodiments of the present invention;

(9) FIG. 8 shows another way of how a blood treatment machine may be controlled according to certain embodiments of the present invention;

(10) FIG. 9 shows a normohydrated relative blood volume over the overhydration;

(11) FIG. 10 shows a first apparatus comprising a controller for carrying out the method according to the present invention; and

(12) FIG. 11 shows a second apparatus comprising a controller for carrying out the method according to the present invention.

DETAILED DESCRIPTION

(13) FIG. 1 shows a correlation between the relative blood volume drop per liter ultrafiltration fluid in percent (short: % ARBV/UFV, UFV measured in liters [L]) during dialysis and an initial overhydration (in liters). The data of FIG. 1 have been gathered from a number of patients treated by ultrafiltration.

(14) As can be seen from FIG. 1, the relative blood volume drop per liter ultrafiltration fluid is lower the higher the overhydration is before dialysis (“pre-Dx” overhydration).

(15) FIG. 2 shows data including those already used for the plot of FIG. 1 plus data of additional measurements. Another difference between FIG. 1 and FIG. 2 is the different way of representing the available data. In FIG. 2, the patients have been classified to have started dialysis with either an overhydration OH below one liter or above one liter.

(16) As can be seen both from FIG. 1 and FIG. 2, the relative blood volume drop per liter ultrafiltration fluid (short: % ARBV/UFV [L]) is lower with patients (24 patients in number) who started dialysis with a higher overhydration, and vice versa. Further, as in FIG. 1, in FIG. 2 only data are shown that have been observed with patients from who more than 1.3 liters of ultrafiltration fluid have been withdrawn (UFV>1.3 liters).

(17) As can be also seen from FIG. 1 and FIG. 2, for determining a target RBV value to be reached at the end of the treatment session, or for determining an optimized or critical RBV value (RBV_critical or RBV_min_tolerated), an overhydration level obviously is an important information and should hence be considered in setting an individual and optimal RBV_critical or RBV_min_tolerated.

(18) The representations of FIG. 1 and FIG. 2 may help understand why certain patients suffered from relevant or severe blood pressure drops (hypotensive episode or crisis) during dialysis at different relative blood volume values in the past whereas others did not. One reason therefor may be the enhanced refilling (due to the higher amount of water comprised within the interstices) in overhydrated patients; another reason may be the enhanced absolute blood volume.

(19) Also, FIGS. 1 and 2 help to understand, why a particular patient may collapse when a certain relative blood volume level or value is reached during a first dialysis session whereas he or she do not collapse when the same relative blood volume is reached during another, second dialysis session.

(20) In the following, by way of example an approach reflecting said relevance of the overhydration before dialysis and its derivation are explained (with ABV being the drop, or change in general, of the absolute blood volume during dialysis):

(21) $\begin{array}{cc}\Delta \mathrm{BV}={\mathrm{BV}}_{\mathrm{\_end}}-{\mathrm{BV}}_{\mathrm{\_start}}=-\mathrm{UFV}+V_{\mathrm{\_refill}}& \left(1\right)\\ V_{\mathrm{\_refill}}={\mathrm{BV}}_{\mathrm{\_end}}-{\mathrm{BV}}_{\mathrm{\_start}}+\mathrm{UFV}& \left(2\right)\\ V_{\mathrm{\_refill}}=\mathrm{UFV}-{\mathrm{BV}}_{\mathrm{\_start}}*\left(1-\frac{{\mathrm{RBV}}_{\mathrm{\_end}}}{100}\right)& \left(3\right)\end{array}$

(22) BV_start equals the normohydrated BV_0 plus the part of the excess fluid present in the blood compartment:

(23) $\begin{array}{cc}{V}_{\mathrm{\_refill}}=\mathrm{UFV}-({\mathrm{BV}}_{\_0}+\frac{\mathrm{OH}}{K_{\mathrm{\_Guyton}}}*\left(1-\frac{{\mathrm{RBV}}_{\mathrm{\_end}}}{100}\right)& \left(4\right)\end{array}$

(24) Assuming that BV_0 equals 0.1*LTM+0.01*ATM, with LTM being the muscle mass and ATM the fat mass of the patient in question, the refilling volume may be expressed as follows:

(25) $\begin{array}{cc}{V}_{\_\mathrm{refill}}=\mathrm{UFV}-\left(\left(0.1+\mathrm{LTM}+0.01*\mathrm{ATM}\right)+\frac{\mathrm{OH}}{K_{\mathrm{\_Guyton}}}\right)*\left(1-\frac{{\mathrm{RBV}}_{\mathrm{\_end}}}{100}\right)& \left(5\right)\end{array}$

(26) Solving (5) for RBV_end:

(27) $\begin{array}{cc}{\mathrm{RBV}}_{\mathrm{\_end}}=\frac{100*\left(V_{\mathrm{\_refill}}-\mathrm{UFV}\right)}{\left(\left(0.1*\mathrm{LTM}+0.01*\mathrm{ATM}\right)+\frac{\mathrm{OH}}{K_{\mathrm{\_Guyton}}}\right)}+100& \left(6\right)\end{array}$

(28) In any equation presented within the present description, UFV stands for ultrafiltration volume, OH stands for the overhydration before starting the dialysis.

(29) V_refill may be estimated, by way of example, as follows:

(30) $\begin{array}{cc}{V}_{\mathrm{\_refill}}=a*\mathrm{UFV}+b*\frac{\mathrm{UFR}}{{\mathrm{hb}}_{\mathrm{\_start}}}+c*\mathrm{OH}+d& \left(7\right)\end{array}$

(31) As stated above, in certain embodiments according to the present invention, parameter a equals 0.6015, b equals 0.0097, c equals 0.0223 and d equals 0.0442. This is, however, not to be understood as limiting. Any other estimation is also contemplated.

(32) The above stated values for parameters a, b, c and d have been empirically found. They have been step wisely analyzed for significance and cross validated.

(33) Other approaches for estimating the refilling volume based on the UFV, UFR and OH are of course also contemplated. In those alternative approaches, hb_start may be comprised.

(34) This is, however, not mandatory.

(35) Also, the duration of the dialysis session T_dialysis may be calculated by dividing UFV by UFR.

(36) Further, the overhydration OH may be expressed by a function f(UFV, UFR, LTM, ATM, hb_start, K_Guyton, RBV_end). The duration of the dialysis session T_dialysis may be expressed by a function f(UFV, OH, hb_start, LTM, ATM, K_Guyton, RBV_end). The ultrafiltration rate UFR may be expressed by a function f(UFV, OH, hb_start, LTM, ATM, K_Guyton, RBV_end, T_dialysis).

(37) FIG. 3 illustrates a relation between the refilling volume (Vol_refill_ref in liter) as predicted by equation (5) over a refilling volume reference (Vol_refill_ref in liter). These reference values were calculated for the illustrated treatments by means of equation (5) and depicted along the x-axis, whereas the values found by means of equation (7) were depicted along the y-axis.

(38) In the embodiment that corresponds to FIG. 3, it is believed that the expansion of the blood volume is proportional to the overhydraton OH because of a constant value for K_Guyton, and that LTM, ATM, etc. have been determined sufficiently correct.

(39) As can be seen from FIG. 3, the predicted refilling volume (Vol_refill_ref in liter) corresponds quite well to the actually observed refilling volume reference (Vol_refill_ref in liter).

(40) Similarly, what has been demonstrated above with reference to FIG. 3 may also be observed in FIG. 4. The data plotted in FIG. 4 show that the predicted relative blood volume RBV_pred [in %] corresponds very well to the relative blood volume (RBV_end_reached [in %] reached once the ultrafiltration volume that was set by the physician before starting the treatment was withdrawn from the blood.

(41) The standard deviation (SD) of the values shown is +/−2.2%. Each point shown in FIG. 4 represents one complete treatment session (in total, 109 measurements).

(42) It is noted that the data shown in the figures discussed here were achieved during or by treatments that were carried out with a constant ultrafiltration rate applied. It is, however, contemplated that the idea of the present invention may also be embodied with ultrafiltration rates that are not kept constant during treatment.

(43) FIG. 5 represents the relative blood volume as measured (RBV_meas in [%]) in the course of one single treatment session. The relative blood volume (RBV) is illustrated over the duration of the session (time t in minutes).

(44) Reference sign RBV_meas depicts the actual, measured course of the relative blood volume over time t. Reference sign RBV_pred shows the predicted relative blood volume. As can be seen, there is hardly any deviation (the deviation is represented by the arrow of FIG. 5) between the predicted and the matter-of-fact end relative blood volume. In any case, controlling the dialysis apparatus based on the in advance calculated or predicted predicted relative blood volume RBV_pred would not have caused any hypotensive crisis with regard to the particular treatment session reflected in FIG. 5.

(45) FIG. 6 to FIG. 8 are intended to explain further ways of how a blood treatment machine may be controlled according to certain embodiments of the present invention. In FIG. 6 to FIG. 8, the course of the relative blood volume RBV is illustrated over time t [min]. The data have been recorded during three different dialysis treatments of patient “46249” who had a constant, normohydrated absolute start blood volume BV_0 of 4.7 liter at the beginning of each treatment session. The patient's overhydration OH differed between 1.4 (FIG. 6) and 2.3 liter (FIG. 7). In these three sessions, an ultrafiltration volume between 1.5 (FIG. 6) and 2.3 liter [L] (FIG. 8) was removed, respectively.

(46) In each of FIGS. 6 to 8, graph RBV_prior shows the development of the relative blood volume during the time that has passed since the beginning of the treatment. With respect to the present invention, the term RBV_prior reflects the results of a blood volume monitor measurement as is carried out to date in the prior art. RBV_prior (that could, therefore, also be called RBV classical or RBV standard or the like and that is calculated as BV(t)/BV_start) reveals the relative blood volume as measured. RBV_prior is, however, not a normohydrated blood volume. As can be seen from FIGS. 6 to 8, the treatment starts with a relative blood volume RBV_prior of 100% and ends at an reduced relative blood volume between 85 and 95%. Hence, as can easily be seen from these figures, the relative blood volume measured at the end of the treatment may differ from treatment to treatment, even though the same amount of fluid was reduced at each treatment, because the relative blood volume was determined relative to different absolute values for the blood volume at the beginning of the treatment. For that reason, controlling a dialysis machine (or adequately setting the ultrafiltration rate or volume) based on a critical or end value of the relative blood volume set or determined in advance may be difficult in the prior art with certain patients—in contrast to what can be achieved by means of the present invention.

(47) According to some embodiments of the present invention, a relative blood volume RBV_normohyd, (here also called a normohydrated relative blood volume) that is “corrected” for the overhydration found in the patient at issue before the dialysis session is used for controlling the dialysis machine.

(48) The normohydrated relative blood volume RBV_normohyd can be calculated, e.g., as follows:

(49) $\begin{array}{cc}{\mathrm{RBV}}_{\mathrm{\_normohyd}}\left(t\right)=\frac{{\mathrm{BV}}_{\mathrm{\_absolut}\mathrm{\_startDialysis}}}{{\mathrm{BV}}_{\_0}}*{\mathrm{RBV}}_{\mathrm{\_prior}}\left(t\right)=\frac{{\mathrm{BV}}_0+\frac{\mathrm{OH}}{K_{\mathrm{\_Guyton}}}}{{\mathrm{BV}}_{\_0}}*{\mathrm{RBV}}_{\mathrm{\_prior}}\left(t\right)& \left(8\right)\end{array}$
wherein BV_absolute_startDialysis stands for the absolute blood volume at the beginning of the treatment session, and wherein BV_0 stands for the absolute blood volume corrected for the fluid contribution to the vessel system due to the overhydration. The Guyton factor K_Guyton indicates what portion of the overhydration is comprised within the vessel system.

(50) That way, an overhydrated patient would start his or her dialysis treatment with a normohydrated relative blood volume RBV_normohyd that is higher than 100% as can be seen from FIGS. 6 to 8. In the example of FIG. 6, the patient started treatment with an overhydration of 1.4 liters. 1.5 liters were removed. The actual blood volume BV_absolute_start Dialysis was 4.7 liters plus 1.4/3 equals 5.2 liters. The normohydrated absolute start blood volume BV_0 was 4.7 liters. The Guyton factor K_Guyton indicating what portion of the overhydration is comprised within the vessel system was assumed to be 3. Hence, the patient started with a fictive relative blood volume or normohydrated relative blood volume of 110% (5.2 liters/4.7 liters) with the dialysis session. When the dialysis comes to an end, the normohydrated relative blood volume RBV_normohyd is 100%. The absolute blood volume of 4.7 liters has been restored by then. The overhydration has been reduced to 0 liter by then. Keeping in mind that the ultrafiltration volume UFV was 1.5 liters in the present treatment, the patient's refilling volume was 4.7 liters minus (5.2 minus 1.5 liters) equals 1 liter.

(51) As is readily understood from FIG. 6 to FIG. 8, controlling the dialysis based on the normohydrated relative blood volume RBV_normohyd is quite easy as it suffices to stop the ultrafiltration once a normohydrated relative blood volume RBV_normohyd of 100% is reached in the respective treatment session, assuming no refilling takes place after dialysis. This can be achieved by running at a very low UFR at the end of the treatment, so that vascular and interstitial spaces approach more or less equilibrium (in terms of filtration pressures). As is also obvious from the above, the stop value of 100% may remain unaltered for the normohydrated relative blood volume RBV_normohyd, both in every single treatment and irrespective of what particular patient is treated. Controlling the machine as proposed here may reduce the programming effort.

(52) FIG. 7 reflects another treatment of the same patient. Due to the higher overhydration OH of 2.3 liters (when compared to the treatment of FIG. 6, it was 1.4 liters there), the dialysis of FIG. 7 starts at a normohydrated relative blood volume RBV_normohyd of 117% (compared to 112% in FIG. 6).

(53) Additionally, as is indicated in FIG. 8, it may be contemplated to set a target range TR for the normohydrated relative blood volume RBV_normohyd that is to be met at the end of the treatment.

(54) For example, the target range TR may be set 3%, 5% or more below and/or above the 100% envisaged.

(55) Further, the target range TR does not necessarily cover an end value of the normohydrated relative blood volume RBV_normohyd that is always 100%. The range may also be used to cover an area around any desired end value for the treatment in question. Hence, under certain circumstances, a target range may relate to an end value of, e. g., 90% or 95%, depending on the patient.

(56) FIG. 9 shows the normohydrated relative blood volume RBV_normohyd depicted over the overhydration OH. Further, in FIG. 9, a start point Dx_start and an end point Dx_end for the dialysis treatment at issue are illustrated. A target range TR is set that indicates an area in the plot of FIG. 9 that is acceptable for the values of both the normohydrated relative blood volume RBV_normohyd and the overhydration OH at the end of the treatment.

(57) It is noted that instead of OH as described above, a time averaged value of OH (TAFO, a mean between pre and post overhydration values) can be used for embodying the idea of the present invention, including the idea described with regard to FIG. 9 without being limited thereto.

(58) In FIG. 9, two possible way of treatment or of controlling the dialysis machine are depicted as C1 and C2.

(59) At end point C2 the patient has a normohydrated blood volume BV; however, the patient is still overhydrated. In consequence, a rebound of water from the interstices into the blood vessels has to be expected as the end point C2 will still rise in the illustration of FIG. 9 after the treatment (OH will stay at a constant level, but the blood volume BV will rise).

(60) On the other hand, at end point C1 there is no rebound because the patient is not overhydrated, and also because the distribution of water between blood volume BV and interstices has found an equilibrium.

(61) In certain embodiments, a control according to the present invention is contemplated as follows:

(62) After 10 to 30 minutes after the begin of the treatment the direction of the curve (e. g. in a representation like that of FIG. 9) is determined. It is assessed whether or not the curve will most probably meet the target range. This assessment may be done by mere observation on the monitor (naked eye) or via a more sophisticated approach such as an algorithm. If it is assumed that the curve will end within the target range, nothing has to be done. However, in case the curve declines too steeply, as is the case with C2, this may indicate that the blood volume decreases too quickly or strongly, both of which may indicate that the refilling is restricted or limited. Hence, it might be wise to limit the ultrafiltration rate UFR and to prolong the duration of the diayisis treatment. That end, the curve may eventually meet the target range in a “flat manner”.

(63) In contrast, in case the curve of FIG. 9 runs above of the target range, the treatment control may increase the ultrafiltration rate UFR until the curve heads towards the target range again and/or is believed to meet the target range or to end therein.

(64) In certain embodiments of the control method or the devices for carrying out the methods, the manipulated variables comprise in first place the ultrafiltration rate and/or the duration of the dialysis treatment. Besides, additional means such as salt boli and their administration for enhancing the refilling are contemplated.

(65) FIG. 10 shows an apparatus 61 comprising a controller 63 configured to carry out the method according to a first embodiment of the present invention. The apparatus 61 is optionally connected to an external database 65 comprising the results of measurements and the data needed for the method according to the present invention. The database 65 can also be an internal means of the apparatus 61. The apparatus 61 may optionally have means 67 for inputting data into the controller 63 or into the apparatus 61 itself. Such data may be information about the ultrafiltration rate set, the ultrafiltration volume planned to be eliminated from the body, etc., or approximations thereof. The results provided by the controller 63 and/or the apparatus 61 can be displayed on a monitor 60 or plotted by means of a—in FIG. 10 not displayed but optionally also encompassed—plotter or stored by means of the database 65 or any other storage means. The database 65 can also comprise a computer program initiating the method according to the present invention when executed.

(66) As can be seen from FIG. 11, for corresponding measurements, the apparatus 61 according to a second embodiment can be connected (by means of wires or wireless) with a bioimpedance measurement means 69 as one example of a means for measuring or calculating the overhydration, the lean mass, the fat mass or other parameters of the body or approximations thereof. Generally, the means for measuring or calculating can be provided in addition to the external database 65 comprising the results of measurements and the data needed for the method according to the present invention, or in place of the external database 65 (that is, as an substitute).

(67) The bioimpedance measurement means 69 can be capable of automatically compensating for influences on the impedance data like contact resistances.

(68) An example for such a bioimpedance measurement means 69 is a device from Xitron Technologies, distributed under the trademark Hydra™ that is further described in WO 92/19153, the disclosure of which is hereby explicitly incorporated in the present application by reference.

(69) The bioimpedance measurement means 69 may comprise various electrodes. In FIG. 7, only two electrodes 69a and 69b are shown which are attached to the bioimpedance measurement means 69. Additional electrodes are, of course, also contemplated.

(70) Each electrode implied can comprise two or more (“sub”-)electrodes in turn. Electrodes can comprise a current injection (“sub”-)electrode and a voltage measurement (“sub”-)electrode. That is, the electrodes 69a and 69b shown in FIG. 11 can comprise two injection electrodes and two voltage measurement electrodes (i.e., four electrodes in total).

(71) Generally spoken, the apparatus according to the present invention can be provided with means such as weighing means, a keyboard, a touch screen etc. for inputting the required data, sensors, interconnections or communication links with a lab, any other input means, etc.

(72) Similarly, the apparatus 61 may have further means 71 for measuring or calculating means for obtaining a value reflecting the overhydration and/or for obtaining values reflecting the mass, the volume or the concentration of Hb that can be provided in addition to the external database 65 or in place of the external database 65 (that is, as an substitute).

(73) The means 71 can be provided as a weighing means, a keyboard, touch screen etc. for inputting the required data, sensors, interconnections or communication links with a lab, an Hb concentration probe, any other input means, etc.

(74) Below, an exemplary way is described of how an apparatus according to the present invention works that is configured to control a device for treating a patient's blood such that the treatment session is terminated or interrupted once a threshold or a predetermined value of the patient's absolute blood volume has been detected or calculated: The patient's absolute blood volume at the beginning of the treatment session is known as BV_start=BV_0+OH/K_Guyton. In the following example, BV_start is 5.0 L. Now, the relative blood volume determined by, e.g., a blood volume monitor several times during the treatment is multiplied with BV_start. Once the relative blood volume (in %) has fallen to 95%, the absolute blood volume may be calculated as 5.0 L*0.95=4.75 L.

(75) It may be desired that the blood treatment has to be terminated once the absolute blood volume has fallen under a threshold of, e.g., 4.0 L. That way, the relative blood volume is taken into account during the blood treatment session. Of course, the threshold may be set for each patient and/or treatment session individually.

(76) Of course, what has been explained above with regard to the approximation or prediction of a tolerated relative blood volume during blood treatment may in certain embodiments also be true for the absolute blood volume. In other words, by means of the present invention it may be possible to approximate or predict also an absolute blood volume that is still tolerated. Such a tolerated absolute end blood volume BV_end may be obtained by multiplying BV_start with RBV_predicted.

(77) Therefore, what has been said above with regard to the present invention in the light of a relative blood volume is in many embodiments also true for an absolute blood volume.