U.S. patent application number 10/507033 was filed with the patent office on 2005-09-15 for method and device for determining the hematocrit and/or blood volume.
Invention is credited to Brauer, Helge, Muller, Carsten, Spickermann, Reiner, Zhang, Wei.
Application Number | 20050202397 10/507033 |
Document ID | / |
Family ID | 27771078 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050202397 |
Kind Code |
A1 |
Zhang, Wei ; et al. |
September 15, 2005 |
Method and device for determining the hematocrit and/or blood
volume
Abstract
A method for determining the hemocrit and/or blood volume during
an extracorporeal blood treatment with an extracorporeal blood
circuit, in which blood is taken with a blood pump via an arterial
cannula and an arterial flexible-tube line and blood is fed back
via a venous flexible-tube line and a venous cannula. Pressure is
measured in the extracorporeal blood circuit and a change in the
hematocrit is determined from a change in the pressure. The
respective relationship between hematocrit HKT or blood volume RBV
and pressure P in the extracorporeal circuit is stored for various
cannula diameters and various blood-flow values. The respective
relationship for a given cannula diameter and blood flow is
selected. The hematocrit and/or blood volume is determined taking
account of the selected relationship.
Inventors: |
Zhang, Wei; (Schweinfurt,
DE) ; Brauer, Helge; (Gochsheim, DE) ;
Spickermann, Reiner; (Burghausen, DE) ; Muller,
Carsten; (Euerbach, DE) |
Correspondence
Address: |
KENYON & KENYON
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
27771078 |
Appl. No.: |
10/507033 |
Filed: |
April 26, 2005 |
PCT Filed: |
January 9, 2003 |
PCT NO: |
PCT/EP03/00126 |
Current U.S.
Class: |
435/4 ;
435/287.1; 436/70 |
Current CPC
Class: |
A61M 1/3639 20130101;
A61M 2230/207 20130101; A61M 1/1613 20140204; A61M 1/3659 20140204;
A61M 1/16 20130101 |
Class at
Publication: |
435/004 ;
436/070; 435/287.1 |
International
Class: |
C12Q 001/00; G01N
033/86; C12M 001/34 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 7, 2002 |
DE |
102 10 009.8 |
Claims
1. A method for determining the haematocrit and/or blood volume
during an extracorporeal blood treatment with an extracorporeal
blood circuit, in which blood is taken with a blood pump via an
arterial cannula and an arterial flexible-tube line and blood is
fed back via a venous flexible-tube line and a venous cannula,
whereby the pressure is measured in the extracorporeal blood
circuit and a change in the haematocrit is deduced from a change in
the pressure, characterized in that the respective relationship
between haematocrit HKT or blood volume RBV and pressure P in the
extracorporeal circuit is stored for various cannula diameters and
various blood-flow values, and the respective relationship is
selected for the given cannula diameter and blood flow and
haematocrit and/or blood volume is determined taking account of the
selected relationship.
2. The method according to claim 1, characterized in that the
pressure P.sub.art in the arterial flexible-tube line is measured
upstream of the blood pump.
3. The method according to claim 1 or 2, characterized in that, in
order to determine the cannula diameter, the change in the pressure
resulting from a change in the blood flow is determined and the
cannula diameter is deduced from the change in the pressure.
4. The method according to claim 3, characterized in that the
pressures P.sub.art1 and P.sub.art2 are measured at at least two
different values of the blood flow in each case and the difference
.DELTA.P.sub.art=P.sub.- art1-P.sub.art2 calculated from the
pressures P.sub.art1 and P.sub.art2, whereby the difference
.DELTA.P.sub.art is compared with predetermined value ranges
representative of the individual cannula diameters in order to
determine the cannula diameter.
5. The method according to any one of claims 1 to 4, characterized
in that the relationship between haematocrit or blood volume and
pressure for various diameters of the cannula and various values of
the blood flow is described by a non-linear function.
6. The method according to any one of claims 1 to 5, characterized
in that the pumping rate BPR of the blood pump is determined in
order to determine the blood flow.
7. The method according to any one of claims 1 to 6, characterized
in that the blood volume RBV is determined from the haematocrit
HKT.
8. The method according to claim 7, characterized in that the blood
volume RBV is calculated at a specified time t of the blood
treatment from the product HKT (t.sub.0) RBV (t.sub.0) of the
haematocrit HKT (t.sub.0) at a preceding time to and the blood
volume RBV (t.sub.0) at a preceding time t.sub.0, divided by the
haematocrit HKT(t.sub.0) at the specified time t.
9. An apparatus for extracorporeal blood treatment with an
extracorporeal blood circuit, which has a blood pump (6) and an
arterial cannula (5a) and an arterial flexible-tube line (5) for
taking blood and a venous cannula (7a) and venous flexible-tube
line (7) for feeding back blood, with a device for determining the
haematocrit and/or blood volume, which has a pressure sensor (13,
14) for measuring the pressure in the extracorporeal circuit and a
memory and evaluation unit (15) which is designed in such a way
that a change in the haematocrit or blood volume is deduced from a
change in the pressure, characterized in that the respective
relationship between haematocrit HKT or blood volume RBV and
pressure P in the extracorporeal circuit is stored for various
cannula diameters and various blood-flow values in the memory and
evaluation unit (15) and that the memory and evaluation unit (15)
is designed in such a way that the appropriate relationship is
selected for the respective cannula diameter and blood flow and
haematocrit and/or blood volume is determined taking account of the
selected relationship.
10. The apparatus according to claim 9, characterized in that the
pressure sensor (13) is arranged in the arterial blood line (5)
upstream of the blood pump (6).
11. The apparatus according to claim 9 or 10, characterized in that
the memory and evaluation unit (15) is designed in such a way that,
in order to determine the cannula diameter, the change in the
arterial pressure resulting from a change in the blood flow is
determined and the cannula diameter is deduced from the change in
the arterial pressure.
12. The apparatus according to claim 11, characterized in that the
memory and evaluation unit (15) is designed in such a way that the
pressures P.sub.art1 and P.sub.art2 are measured at at least two
different values of the blood flow in each case and the difference
.DELTA.P.sub.art=P.sub.- art1-P.sub.art2 calculated from the
pressures P.sub.art1 and P.sub.art2, whereby the difference
.DELTA.P.sub.art is compared with predetermined value ranges
representative of the individual cannula diameters in order to
determine the cannula diameter.
Description
[0001] The invention relates to a method for determining the
haematocrit and/or blood volume during an extracorporeal blood
treatment with an extracorporeal blood circuit and an apparatus for
extracorporeal blood treatment with an extracorporeal blood circuit
and a device for determining the haematocrit and/or blood
volume.
[0002] For the purpose of removing substances usually eliminated
with urine and for the purpose of withdrawing fluid, use is made of
various methods for machine-aided blood cleaning or blood treatment
in acute or chronic kidney failure. Diffusive substance transport
predominates in the case of haemodialysis (HD), whilst convective
substance transport via the membrane takes place in the case of
haemofiltration (HF). Haemodiafiltration (HDF) is a combination of
the two methods.
[0003] An excessively high or rapid withdrawal of fluid during
haemodialysis can give rise to a possibly rapid increase in blood
volume, which often leads to an acute drop in blood pressure
(hypotony) in the patient.
[0004] Hypotony represents one of the main complications in the
treatment of blood. There are various solutions to this problem. On
the one hand, blood pressure monitors are known which continuously
monitor a change in blood pressure and regulate the ultrafiltration
depending on the change in blood pressure. On the other hand, blood
volume monitors are known which measure the relative blood volume
during the dialysis treatment and perform a regulation of the
ultrafiltration depending on the relative blood volume.
[0005] DE-C-197 46 377 describes a device for the measurement of
blood pressure, which is based on the detection of the propagation
rate of the pulse waves being propagated via the arterial vessel
system of the patient, said pulse waves being generated by the
patient's heart contractions. The device permits a continuous,
non-invasive measurement of blood pressure, but there is the
drawback that the pulse-wave running time is dependent on the
haematocrit (HKT).
[0006] DE-A-40 24 434 describes a device for the regulation of
ultrafiltration, in which the pressure in the extracorporeal
circuit is measured in order to determine the relative blood
volume. The measured pressure values are stored in chronological
sequence and the change in the blood volume is deduced from the
change in the pressure value compared with the value at the start
of the treatment. The venous return-flow or arterial
suction-pressure sensor can be used as a pressure sensor. It is
pointed out in the publication that the drop in pressure on the
arterial cannula is a function of the blood flow and the viscosity
of the blood as well as a function both of the diameter and length
of the cannula. It is further assumed that the relationship between
the blood volume and the change in pressure is linear to a good
approximation.
[0007] The problem underlying the invention is to provide a method
that permits the haematocrit and/or blood volume to be determined
with a particularly high degree of accuracy, but with a relatively
low technical outlay. Moreover, it is a problem of the invention to
provide an apparatus for extracorporeal blood treatment with a
device for determining the haematocrit and/or blood volume, which
has a relatively simple construction, but a high degree of
accuracy.
[0008] The solution to this problem tales place according to the
invention with the features of claims 1 and 9. Advantageous forms
of embodiment are the subject-matter of the sub-claims.
[0009] For reasons of safety, the known dialysis devices measure
and monitor the arterial pressure P.sub.art(t) and the venous
pressure P.sub.ven(t) in the extracorporeal blood circuit.
Moreover, the rate BPR(t) of the blood pump is also measured during
the blood treatment, i e. it is known as the control value. The
method according to the invention and the apparatus according to
the invention make use of the pressure measurement that is already
available, so that the outlay on equipment is relatively low.
[0010] The basic idea of monitoring the haematocrit and blood
volume through the measurement of pressure is based on the
following. If the relative blood volume diminishes during the blood
treatment as a result of ultrafiltration, the haematocrit in the
blood necessarily increases, since the dialysis membrane is not
permeable for the blood cells, namely erythrozytes (7.5 .PHI.m),
leucozytes (1.5-20 .PHI.m) and thromborytes (2.5 .PHI.m).
Furthermore, the viscosity increases over-proportionately with
increasing haematocrit. Since the flow resistance increases in a
markedly linear manner with viscosity, each increase in the
haematocrit caused by the reduction in blood volume signifies an
increased load on the blood pump, which leads to the fall in the
arterial pressure (negative) and the increase in the venous
pressure (positive), insofar as the blood pump is operated at the
same rate.
[0011] It has however been shown that the relationship between
blood volume or haematocrit and pressure in the extracorporeal
blood circuit is dependent not only on the blood flow, but also on
the cannula dimensions, whereby the cannula is to be regarded as
the component of the extracorporeal system determining the drop in
pressure. The inventors recognized that tie length of the cannula
does not have any significant influence on the pressure in the
extracorporeal circuit. They recognized that the diameter of the
cannula is alone decisive.
[0012] In order to increase the accuracy, the respective
relationship between haematocrit or blood volume and pressure is
stored for different diameters of the cannula and different values
of the blood flow in the case of the method and the apparatus
according to the invention. The respective data are thus already
available before the dialysis treatment. Depending on the
respective diameter of the cannula and the value of the blood flow,
the respective relationship between haematocrit or blood volume and
pressure is then selected and haematocrit and/or blood volume is
determined taking account of the selected relationship. The data
can for example be stored in the form of groups of curves, which
can be described in particular by discrete measurement values.
[0013] The increased accuracy results from the fact that account is
taken not only of the blood flow during the treatment, but also of
the cannula used.
[0014] When speaking of haematocrit and blood volume, both absolute
values as well as relative values are to be understood, which
indicate a relative change in the blood volume in respect of a
predetermined initial value, for example the start of the blood
treatment.
[0015] An evaluation of the clinical data has shown that in
practice the arterial pressure, which is measured in the arterial
blood line upstream of the blood pump, correlates with the relative
blood volume much better than the venous pressure in the venous
blood line. This can be traced back to the fact that the venous
pressure is very much more susceptible to interference than the
arterial pressure. In the case of dialysis machines which make use
of balancing chambers, the venous pressure sensor detects pressure
fluctuations which are caused not only by the ultrafiltration, but
also by switching balancing chambers. The air volume, or more
precisely the revel in the venous drip chamber, also has a strong
influence on the characteristic of the venous pressure signal. In
contrast, the arterial pressure is free from such pressure
fluctuations. It is true that the arterial pressure signal is
influenced by the blood pumping rate, but here it concerns an
unequivocal source of interference whose influence on the arterial
pressure can be compensated for.
[0016] It has been shown that the cannula diameter can be
determined unequivocally by evaluating the pressure changes in the
extracorporeal blood circuit. In order to determine the cannula
diameter, the change in pressure resulting from a change in the
blood flow is determined and the cannula diameter is deduce from
the change in pressure. For this purpose, the pressures are
preferably measured at least two different values of the blood flow
in each case, and the difference between the pressures is
calculated. In order to determine the cannula diameter, the
difference in the pressures is compared with predetermined stored
value ranges representative of the individual cannula diameters.
The individual value ranges can be assigned unequivocally to the
different cannula diameters. The assignment between cannula
diameter and value range can in principle be verified again by
several measurements.
[0017] Furthermore, it has been shown that the relationship between
haematocrit or blood volume and pressure for different diameters of
the cannula and different values of the blood flow can be described
to a sufficient approximation by a non-linear function, for example
a second-order polynomial. Since the blood flow correlates with the
rate of the blood pump, the pumping rate, which is preset by the
control of the blood treatment device, is preferably used to
determine the blood flow.
[0018] When the haematocrit is determined, the blood volume can be
calculated. The blood volume is calculated at a specified time in
the blood treatment from the product of the haematocrit at a
preceding time and the blood volume at a preceding time divided by
the haematocrit at the specified time.
[0019] The device for determining the haematocrit and/or blood
volume of the apparatus for extracorporeal blood treatment
according to the invention has a memory and evaluation unit, in
which the respective relationships between haematocrit and blood
volume for the different cannula diameters and blood flows are
stored. Such a memory and evaluation unit can be part of a computer
control, which is already present in the known blood treatment
apparatuses. The measurement of the pressure preferably takes place
with a pressure sensor which is also already present.
[0020] The determination of the cannula diameter on the basis of a
pressure measurement is of inherent inventive significance. The
knowledge of the influence of the cannula can be used in an
advantageous way with the method for blood pressure measurement
known from DE-C-197 46 377, in that the influence of the blood
density on the pulse-wave running time is compensated for or
corrected so that the blood pressure measurement takes place with a
higher degree of accuracy.
[0021] An example of embodiment of an extracorporeal blood
treatment apparatus with a device for determining the haematocrit
and/or blood volume as well as an example of embodiment of the
method according to the invention are explained below in greater
detail with the aid of the figures.
[0022] The figures show the following:
[0023] FIG. 1 the haematocrit (HKT(%)) as a function of the
arterial pressure (P.sub.art(mmHg)) for various cannulas of
differing diameter and differing length,
[0024] FIG. 2 the haematocrit (HKT(%)) as a function of the
arterial pressure (P.sub.art(mmHg)) for various cannulas,
[0025] FIG. 3 the haematocrit (HKT(%)) as a function of the
arterial pressure (P.sub.art(mmHg)) for various values of the blood
flow with a first cannula,
[0026] FIG. 4 the haematocrit (HKT(%)) as a function of the
arterial pressure (P.sub.art(mmHg)) for various values of the blood
flow with a second cannula,
[0027] FIG. 5 the haematocrit (HKT(%)) as a function of the
arterial pressure (P.sub.art(mmHg)) for various values of the blood
flow with a third cannula,
[0028] FIG. 6 an example of embodiment of an extracorporeal blood
treatment apparatus with a device for determining the haematocrit
and/or blood volume in a simplified diagrammatic
representation.
[0029] FIG. 1 shows the relationship between the haematocrit
(HKT(%)) of the blood and the pressure in the arterial blood lime
of the extracorporeal circuit with a constant blood pumping rate
BPR of 250 ml/min. for seven different dialysis cannulas, which
differ from one another in diameter and length. For example, the
cannula with the designation V-711 has a diameter of 1.5 mm and a
length of 15 mm. The other cannulas are correspondingly designated
in FIG. 1. It can be seen in FIG. 1 that the relationship between
haematocrit and arterial pressure is not linear. It can however be
described to a good approximation by a second-order polynomial.
Furthermore, it can be seen that the relationship between
haematocrit and pressure depends markedly on the diameter of the
cannulas. The influence of the length of the cannulas, on the other
hand, is relatively small. This can therefore be neglected to a
good approximation. For this reason, the relationship is grouped
unequivocally according to the diameter of the cannulas, i.e. 1.5,
1.6 and 1.8 mm. Due to the marked dependence of the relationship on
the diameter of cannulas, the measurement of the pressure for the
determination of the haematocrit or blood volume without a
knowledge of the cannula diameter leads to inaccurate results.
[0030] FIG. 2 shows the relationship of haematocrit and arterial
pressure of a second measurement series with a blood flow rate BPR
of 250 ml/min. Here too, the grouping according to the cannula
diameters is distinctly marked.
[0031] FIG. 3 shows the relationship between haematocrit (HKT(%))
and arterial pressure (P.sub.art(mmHg)) in the case of a needle
with a diameter of 1.8 mm and a length of 20 min for a large number
of blood flows BPR between 100 ml/min. and 550 mm/min. Here too,
the relationship is not linear. It can however again be described
to a good approximation by a second-order polynomial. In a range of
blood flow from 160 to 400 ml/min., the curves for different blood
flows exhibit a similar gradient. Since the dependence of the blood
flow, i.e. the blood pumping rate, is essentially expressed by the
fact that the curves are displaced parallel to the x-axis and that
the displacement is dependent on the diameter of the needle, the
needle diameter can be determined unequivocally. On the assumption
that the haematocrit of a dialysis patient lies in the range from
30% to 40%, the diameter of the cannula can be detected without
knowledge of the haematocrit. The detection takes place via
measurement of the pressure difference with two different blood
flows, i.e. blood pumping rates, whereby typical values lie between
130 ml/min. and 310 ml/min.
[0032] FIGS. 4 and 5 show the groups of curves of a needle with a
diameter of 1.6 mm and a length of 20 mm and respectively a needle
with a diameter of 1.5 mm and length of 15 mm.
[0033] The determination of the cannula diameter with the aid of
the groups of curves in FIGS. 3-5 is explained in greater detail
below. Arterial pressures P.sub.art1 and P.sub.art2 are measured
for this purpose at at least two predetermined blood pumping rates
BPR1 and BPR2. The difference
.DELTA.P.sub.art=.DELTA.P.sub.art1-P.sub.art2 is then calculated,
which is represented in FIGS. 3-5 as a horizontal bar. Values for
.DELTA.P.sub.art that can unequivocally be assigned to the
individual cannula diameters arise for an HKT range of
approximately 30-40%. These value ranges are previously determined
and stored, whereby an appropriate assignment is carried out after
measurement of the change in pressure.
[0034] The following table shows the pressure difference
.DELTA.P.sub.art(mmHg) for the three cannulas of differing diameter
(1.8, 1.6 and 1.5 min) with a haematocrit HKT of 30 and 40%. The
measurement magnitudes can be grouped into the value ranges 70-90
mmHg for a cannula diameter of 1.8 mm, 100 to 120 mmHg for a
cannula diameter of 1.6 min and 130 to 150 mmHg for a cannula
diameter of 1.5 mm. After measurement of pressure difference
.DELTA.P.sub.art, it can thus be unequivocally decided what
diameter the cannula has. It emerges that the haematocrit does not
have any influence on the unambiguousness of the detection of the
needle diameter when it lies in the physiological range between 30
and 40%.
1 .DELTA. P.sub.art(mmHg) HKT (%) V-501 ( 1.8 mm) V-601 ( 1.6 mm)
V-701 ( 1.5 mm) 30 72 102 130 40 89 118 148
[0035] FIG. 6 shows the essential components of an extracorporeal
blood treatment apparatus together with a device for determining
the haematocrit and/or blood volume in a simplified diagrammatic
representation.
[0036] As a blood treatment device, the dialysis apparatus has a
dialyser 1, which is divided by a semipermeable membrane 2 into a
blood chamber 3 and a dialysis-fluid chamber 4. An arterial blood
line 5 leads to the inlet of blood chamber 3, a peristaltic blood
pump 6 being connected into said arterial blood line. A venous
blood line 7 leads off from blood chamber 3, a drip chamber 8 being
connected into said venous blood line. To the ends of the arterial
and venous blood line 5, 7 there are connected cannulas 5a, 7a,
which are jabbed into the patient. The arterial and venous blood
line are a component of a flexible-tube line system designed to be
disposable.
[0037] Fresh dialysis fluid is prepared in a dialysis-fluid source
9. A dialysis-fluid supply line 10 leads from dialysis-fluid source
9 to an inlet of dialysis-fluid chamber 4 of the dialyser, whilst a
dialysis-fluid discharge line 11 leads from the outlet of the
dialysis-fluid chamber to a drain 12. The dialysis apparatus also
has further components, e.g. a balancing device and an
ultrafiltration device etc., which however are not represented for
the sake of better clarity. Moreover, the central control unit,
which is a component of the dialysis apparatus, is not
represented.
[0038] For safety reasons, the arterial pressure in arterial blood
line 5 is monitored upstream of blood pump 6 and the venous
pressure in the venous blood line is monitored downstream of drip
chamber 8 in the dialysis apparatus. For this purpose, an arterial
pressure sensor 13 is provided in arterial blood line 5 and a
venous pressure sensor 14 is provided in venous blood line 7. The
device for determining the haematocrit and/or blood volume has
arterial pressure sensor 13 already available in the dialysis
apparatus as well as a memory and evaluation unit 15. Memory and
evaluation unit 15 receives the pressure signal of arterial
pressure sensor 13 via a data line 16. Alternatively, the memory
and evaluation unit can receive the pressure signal of a venous
pressure sensor 14 via a data line 17. Data line 17 is shown by a
dashed line in FIG. 6. Furthermore, memory and evaluation unit 15
is connected to blood pump 6 via a data line 18. A blood pump
signal proportional to the blood pumping rate is transmitted via
data line 18. The curve groups represented in FIGS. 3-5, which
describe the relationship between haematocrit and arterial
pressure, are stored in the memory and evaluation unit. The memory
and evaluation unit operates as follows.
[0039] The cannula diameter, in which the blood pumping rate is
varied, is first determined in an initial measurement during the
dialysis treatment, whereby the arterial pressures P.sub.art1 and
P.sub.art2 are measured at two predetermined blood pumping rates
BPR of, for example, 310 and 130 mm (FIG. 3). The memory and
evaluation unit calculates from the measured values the amount of
the pressure difference .DELTA.P.sub.A=P.sub.art1-P.- sub.art2,
which in the present example amounts to 89 mmHg with a haematocrit
of 40%. In principle, however, measurements can also be carried out
for other haematocrit values, insofar as the haematocrit lies in
the physiological range of the patient and thus between 30 and 40%.
Apart from the groups of curves, there are stored in the memory and
evaluation unit the value ranges from 70 to 90, 100 to 120 and 130
to 150 mmHg characteristic of the cannula diameter, which are
described above. The memory and evaluation unit performs an
assignment between the measured pressure difference
.DELTA.P.sub.art and the stored value ranges. Since the measured
pressure difference .DELTA.P.sub.art lies here in the value range
between 70-90 mmHg, the memory and evaluation unit assumes that the
cannula has a diameter of 1.8 mm (FIG. 3).
[0040] After the cannula diameter has been ascertained in the
initial measurement, the memory and evaluation unit carries out a
selection between the different curve groups (FIGS. 3-5), which
respectively describe the relationship of haematocrit and arterial
pressure for the respective needle diameter. The memory and
evaluation unit selects here the group of curves according to FIG.
3, which are representative of the present needle diameter of 1.8
mm.
[0041] After the selection of the appropriate curve group, the
memory and evaluation unit determines from the appropriate curve
group, with a high degree of accuracy, the appropriate haematocrit
in dependence on the blood pumping rate BPR(t) taking account of
the diameter of the employed cannula, without the diameter of the
used cannula needing to be inputted manually. If, for example, an
arterial pressure of 100 mmHg is measured with the arterial
pressure sensor, a haematocrit of approx. 33% results with a blood
pumping rate of 310 mm (FIG. 3). With decreasing blood pumping
rate, the haematocrit increases according to the curve group.
[0042] The determination of the blood volume takes place after the
haematocrit has been ascertained. The blood volume at a specified
time in the blood treatment RBV(t) is calculated from the
haematocrit HKT according to the following equation: 1 RBV ( t ) =
HKT ( t 0 ) RBV ( t 0 ) HKT ( t )
[0043] whereby RBV(t) is the blood volume at time t,
[0044] HKT(t) is the haematocrit at time t and RBV (t.sub.o) and
HKT (t.sub.0) are respectively the blood volume and the haematocrit
at an arbitrary time t.sub.o, which lies before time t.
[0045] Since RBV (t.sub.o)=1 at the start of the dialysis
treatment, the memory and evaluation unit can determine RBV(t)
relative to this time. On the other hand, the above equation can
also be used for two arbitrary times t.sub.0 and t.sub.0 if to does
not coincide with the start of the treatment and RBV (t.sub.o) is
thus not necessarily 1. If RBV (t.sub.o) is not known, the memory
and evaluation unit can however determine relative changes in RBV
according to the above equation compared with a value of RBV
(t.sub.o) of 1.
* * * * *