Method of determining the pressure in an extracorporeal circuit

Abstract

The present invention relates to a method of determining the pressure or of a parameter correlated with the pressure in an extracorporeal circuit of a blood treatment apparatus, in particular of a dialyzer, wherein at least one blood pump which is driven by at least one motor is located in the blood circuit, wherein the motor current of the named motor and the blood flow or a parameter correlated therewith is measured for determining the pressure or the parameter correlated therewith and wherein the pressure p or the parameter correlated therewith is calculated from the measured values.

Claims

1. A method for regulating or controlling the pressure (p) or a parameter correlated with the pressure (p) in an extracorporeal blood circuit of a blood treatment device, wherein the pump driven by the motor (M) is located in the blood circuit, the method comprising determining the pressure (p) or a parameter correlated therewith by a method comprising measuring the motor current (I.sub.M) of the motor (M) and the blood flow (Q.sub.b) or a parameter correlated therewith, wherein the pressure (p) or the parameter correlated therewith is calculated from the measured values, and determining the pressure difference (p) between upstream and downstream of the pump in accordance with the formula I.sub.M=a+b*Q.sub.b+c*Q.sub.b*.sub.p, wherein a=P.sub.0/U.sub.0, with P.sub.0 being constant power consumption of the motor (M) and U.sub.0 being voltage output by voltage supply for the motor (M), b=(.Math.F.sub.N)/U.sub.0.Math.(2.Math.R)/V.sub.S, with (.Math.F.sub.N) being the power losses of the pump, R being radius of the blood pump motor (M), and V.sub.S being stroke volume of the blood pump per revolution, and
c=1/U.sub.0, and changing the blood flow (Q.sub.b) conveyed by the blood pump for the purpose of regulating or controlling.

2. A method in accordance with claim 1, characterized in that the method comprises the restriction of the pressure (p) or of a parameter correlated with, the pressure (p) such that a limit value is not exceeded.

3. A method in accordance with claim 1, characterized in that the method comprises the regulation of the pressure (p) or of a parameter correlated with the pressure (p) to a desired value or to a desired value range.

4. A method in accordance with claim 3, characterized in that a maximum permitted change of the pressure (p) or of a parameter correlated with the pressure (p) is predefined; and in that the regulation is only carried out when this maximum permitted change is reached or exceeded.

5. A method in accordance with claim 1, characterized in that the regulation is carried out such that the relationship (I.sub.M(t)a)/Q.sub.b(t)=(I.sub.M,0a)/Q.sub.b,0 remains constant, where t is time, I.sub.M,0 and Q.sub.b,0 are start values of the motor current (I.sub.M) and of the blood flow (Q.sub.b) at a point in time before a fault occurs, by restricting the pressure (p) or of a parameter correlated with the pressure (p) such that a limit value is not exceeded.

6. A blood treatment apparatus comprising at least one extracorporeal blood circuit containing at least one blood pump driven by at least one motor (M), at least one means for measuring the motor current (I.sub.M), at least one means for measuring the blood flow (Q.sub.b) or parameter correlated therewith, at least one of a calculating unit, control unit, and regulation unit configured to carry out a method on the basis of the measured values provided by the means, the method comprising measuring the motor current (I.sub.M) of the motor (M) and the blood flow (Q.sub.b) or a parameter correlated therewith, wherein the pressure (p) or the parameter correlated therewith is calculated from the measured values, and determining the pressure difference (p) between upstream and downstream of the pump in accordance with the formula I.sub.M=a+b*Q.sub.b+c*Q.sub.b*.sub.p, wherein a=P.sub.0/U.sub.0, with P.sub.0 being constant power consumption of the motor (M) and U.sub.0 being voltage output by voltage supply for the motor (M), b=(.Math.F.sub.N)/U.sub.0.Math.(2.Math.R)/V.sub.S, with (.Math.F.sub.N) being the power losses of the pump, R being radius of the blood pump motor (M), and V.sub.S being stroke volume of the blood pump per revolution, and
c=1/U.sub.0.

7. A blood treatment apparatus in accordance with claim 6, characterized in that at least one of the blood pump is a peristaltic pump and the motor (M) is a DC motor operated with DC current.

8. A blood treatment apparatus in accordance with claim 6 further comprising at least one dialyzer having at least one chamber flowed through by blood in operation provided in the extracorporeal circuit.

9. A blood treatment apparatus in accordance with claim 8, characterized in that the dialyzer has at least one chamber flowed through by dialysis fluid in operation.

10. A blood treatment apparatus in accordance with claim 6 further comprising at least one substituate pump for supplying a substitution fluid to the blood in the extracorporeal circuit via at least one of a line upstream of the dialyzer and a line downstream of the dialyzer.

11. A blood treatment apparatus in accordance with claim 6 further comprising no pressure sensor except a venous pressure sensor provided downstream of the blood pump.

Description

(1) Further details and advantages of the invention will be explained in more detail with reference to an embodiment shown in the drawings There are shown:

(2) FIG. 1; a schematic view of the peristaltic blood pump having a motor and a power supply;

(3) FIG. 2; the dependence of the motor current on the blood flow and the pressure difference over the pump;

(4) FIG. 3: an example for a power supply with a shunt resistance;

(5) FIG. 4 an example for a high-side current measurement with 12-bit ADC; and

(6) FIG. 5: a schematic representation of an extracorporeal circuit of a dialyzer in accordance with the prior art.

(7) As already stated above, in accordance with the invention, a pressure or a parameter correlated therewith, such as the pressure difference before and after the blood pump, can be calculated on the basis of the measurement of the motor current and of the blood flow or of parameters correlated therewith.

(8) This calculated value can be used to carry out a pressure regulation. It can be prevented in this manner that pressure peaks or pressure values arise which may result in damage to the blood.

(9) In a preferred embodiment, the relationship between the motor current, the blood flow and the pressure increase over the pump is reproduced by the following relationship;
I.sub.M=a+b.Math.Q.sub.b+c.Math.Q.sub.b.Math.p.

(10) If this equation is resolved for the pressure difference p it can be seen that the pressure difference can be determined on the basis of the measurement parameters I.sub.M and Q.sub.b. In this manner, the pressure downstream of the blood pump can also be calculated on a measurement of the arterial pressure.

(11) Reference is made in this respect to FIG. 5 with regard to the arrangement of the corresponding pressure sensors. FIG. 5 or such an arrangement of an extracorporeal blood circuit is also suitable for the present invention and is covered by it, with provision preferably being made that in accordance with the invention, the pressure sensor 11, i.e. the pressure sensor downstream of the blood pump 4, is dispensed with.

(12) The representation in accordance with FIG. 1 illustrates that the aforesaid relationship of motor current, blood flow and pressure difference over the pump can be determined from the values shown there P.sub.el, P.sub.R and P.sub.hyc or from the relationship P.sub.el=P.sub.0+P.sub.R+P.sub.hyc.

(13) In FIG. 1, the motor current is likewise given by the reference symbol I.sub.M.

(14) FIG. 2 shows the dependence of the motor current as a function of the blood flow and of the pressure or of the pressure difference over the pump based on the above-given relationship I.sub.M=f(Q.sub.b; p).

(15) As can be seen from FIG. 2, the motor current increases with the pressure difference in accordance with this relationship with a constant blood flow.

(16) The point A in FIG. 2 is characterized by a pressure increase p=0. No blood flow is present here, either, so that the value a results for the motor current.

(17) The path A-B is characterized by a pressure increase over the pump of approximately 0, with the blood flow varying from 100 to 700 ml/mm. In this case, the change in the motor current I.sub.M along the path A-B is I.sub.M=b.Math.Q.sub.b.

(18) The paths C shown are each characterized by constant values of the blood flow (100; 200; 300; 400; 500; and 600 ml/mm). The relationship I.sub.M=c.Math.Q.sub.b.Math.p hereby results.

(19) If not only a pressure measurement is to be carried out, but also a constant regulation of the pressure, it can be selected which pressure, for example p, p.sub.pp, is to be determined and is to be regulated to a constant value or a desired value range. The regulation preferably takes place by setting the blood pump rate or the blood speed Q.sub.b.

(20) The above-named relationship forms the starting basis:
I.sub.M=a+b.Math.Q.sub.b+c.Math.Q.sub.b.Math.P.

(21) The total differential of this equation produces dI.sub.M=b.Math.dQ.sub.b+c.Math.dQ.sub.b.Math.p+c.Math.Q.sub.b.Math.dp. The partial derivation at a constant pressure p produces:
(I.sub.M/Q.sub.b).sub.p=b+c.Math.p=const.

(22) After the partial derivation at a constant pressure p, there results for p=const on a comparison with equation (1):
(I.sub.Ma)/Q.sub.b=b+c.Math.p=const.

(23) The sensitivity E.sub.p of the motor current with respect to the pressure change of p is obtained from the partial derivation at a constant blood flow Q.sub.b:
E.sub.p=I.sub.M/(p)=(I.sub.M/p).sub.Qb=c.Math.Qb

(24) A possible procedure for operating the system or for regulating the pressure comprises the measured values I.sub.M and Q.sub.b being noted or stored at regular intervals as start values I.sub.M,0 and Q.sub.b,0 in the non-faulty state, that is in the normal operation of the pumps and in particular also of the substituate pump.

(25) If a fault occurs during the treatment, a check can first be made whether a maximum permitted pressure change has been reached. This maximum pressure change can be represented by (p).sub.GW. The change of the motor current is monitored for this purpose. It is conceivable that if the motor current changes by more than
I.sub.M=c.Math.Q.sub.b.Math.(p).sub.GW,
the active regulation phase is started in which regulation takes place to a pressure value or to a pressure value range.

(26) When taking account of the recorded or noted start values for the motor current and the blood flow, the blood flow can now be changed by the regulation or by a regulating unit such that the relationship (I.sub.M(t)a)/Q.sub.b(t)=(I.sub.M,0a)/Q.sub.b,0)=const remains constant.

(27) It is furthermore conceivable that the procedure is ended after a timeout or after a certain conveyed blood volume. The blood flow is then set back to the original value Q.sub.b,0.

(28) As can be seen from the above-named relationship for the keeping constant of the pressure, the parameter b is not needed and the parameter c is only needed at a trigger point in time, namely when it is to be determined whether the permitted pressure change has been exceeded or not. This value c does not have to be very precise. The accuracy of the parameter a only plays a role with relatively large changes.

(29) Only one system parameter, namely a, has to be determined from the initially named parameters a, b and c. It is sufficient to measure the motor current I.sub.M at a low flow e.g. Q.sub.b<100 ml/minute and at a low pressure p, for example within the framework of the preparation or on the filling of the hose system.

(30) FIG. 3 shows an example for a current supply with a shunt resistance. In this case, a shunt resistance R.sub.Shunt=0.1 is inserted into the 24 volt supply line of the motor. The voltage drop at the shunt is converted by a high-side current filler into an output current or output voltage with a ground (GND) reference and is digitized via an ADC. The further signal processing takes place in an microcontroller.

(31) This results from the representation in accordance with FIG. 4.

(32) The fit parameters a, b, and c can be determined as follows: The fit parameters are selected so that the sum of the error square S becomes minimal. The sum of the error square is S=(I.sub.M,iab.Math.Q.sub.b,ic.Math.Q.sub.b,i.Math.p.sub.i).sup.2, where the sum is respectively formed over i in this relationship and in the following equations.

(33) The extreme value of S is determined for this purpose:
S/a=0.fwdarw.(I.sub.M,iab.Math.Q.sub.b,ic.Math.Q.sub.b,i.Math.p.sub.i)=0
.fwdarw.I.sub.M,i=N.Math.a+b.Math.Q.sub.b,i+c.Math.(Q.sub.b,i.Math.p.sub.i)
.fwdarw.y.sub.1=a.Math.s.sub.1,1+b.Math.s.sub.2,1+c.Math.s.sub.3,1
S/b=0.fwdarw.(I.sub.M,iab.Math.Q.sub.b,ic.Math.Q.sub.b,i.Math.p.sub.i).Math.Q.sub.b,i=0
.fwdarw.I.sub.M,i.Math.Q.sub.b,i=a.Math.Q.sub.b,i+b.Math.Q.sub.b,i.sup.2+c.Math.(Q.sub.b,i.sup.2.Math.p.sub.i)
.fwdarw.y.sub.2=a.Math.s.sub.1,2+b.Math.s.sub.2,2+c.Math.s.sub.3,2
S/c=0.fwdarw.(I.sub.M,iab.Math.Q.sub.b,ic.Math.Q.sub.b,i.Math.p.sub.i).Math.Q.sub.b,i.Math.p.sub.i=0
.fwdarw.(I.sub.M,i.Math.Q.sub.b,i.Math.p.sub.i)=a.Math.(Q.sub.b,i.Math.p.sub.i)+b.Math.(Q.sub.b,i.sup.2.Math.p.sub.i)+c.Math.(Q.sub.b,i.Math.p.sub.i).sup.2
.fwdarw.y.sub.3=a.Math.s.sub.1,3+b.Math.s.sub.2,3+c.Math.s.sub.3,3

(34) The three equations for determining y.sub.1, y.sub.2 and y.sub.3 can be combined to one vector equation: y=a.Math.s.sub.1+b.Math.s.sub.2+c.Math.s.sub.3

(35) With the aid of the determinant det ( ), the solutions are obtained: a=det(y; s.sub.2; s.sub.3)/det(s.sub.1; s.sub.2; s.sub.3) b=det(s.sub.1; y; s.sub.3)/det(s.sub.1; s.sub.2; s.sub.3) c=det(s.sub.1; s.sub.2; y)/det(s.sub.1; s.sub.2; s.sub.3)