DE10254988A1 - Monitoring pressure in vessel entrance during extracorporal blood treatment involves computing change in pressure in vessel entrance from measured arterial or venous pressure, comparing with threshold - Google Patents

Abstract

The method involves setting upper and lower values for blood flow in extracorporal circuit (2) during extracorporal blood treatment for given period at defined levels above and below a working point, measuring arterial or venous pressure in an arterial or venous branch of the extracorporal circuit for lower and upper flow levels, computing pressure change in vessel entrance from measured pressure and comparing with threshold. AN Independent claim is also included for the following: (a) an arrangement for monitoring blood flow in a vessel entrance during extracorporal blood treatment.

Description

The invention relates to a method for surveillance the pressure in a vascular access while an extracorporeal blood treatment in which blood is arterial branch of an extracorporeal circuit connected to one arterial port connected to vascular access into a Funded blood treatment unit will and about a venous one Branch of the extracorporeal circuit that has a venous connector the vascular access connected, is returned. About that the invention also relates to a device for monitoring the pressure in a vascular access while extracorporeal blood treatment.

With extracorporeal blood treatment, for example hemodialysis, hemofiltration or hemodiafiltration, flows Blood of a patient in an extracorporeal circuit, the one Blood treatment unit, for example a dialyzer or filter includes. As access to the blood vessel system becomes common surgically an arterial-venous Fistula created, which is generally punctured with an arterial and venous cannula. Likewise, the use of a vascular implant (shunt) possible. Below is a "vascular access" understood any access to the patient's blood vessel system, but especially the connection between an artery and one Patient's vein.

Monitoring the arterial static pressure (fistula pressure) in the vascular access during extracorporeal blood treatment is of crucial importance since the fistula pressure provides information about the State of vascular access gives. For example, a decrease in fistula pressure indicates an elevated Thromboserisi ko there. Furthermore is the knowledge of the fistula pressure also for the determination of other sizes, for example the relative blood volume RBV or the hematocrit HKT required, the for blood treatment is just as important.

In everyday clinical practice, the static Pressure in the vascular access determined by the pressure in the arterial or venous branch of the extracorporeal circulation after an interruption of blood flow is measured. The blood flow is interrupted in the arterial Branch blood pump stopped. But if the blood pump stopped, there is a risk of coagulation in the blood tube system.

A method for determining the static pressure in the vascular access without switching off the blood pump is also known. The fistula pressure is determined by measuring the pressure in the arterial branch in the case of two different blood flows, which are relatively far apart, and extrapolating the fistula pressure from the measured values. A linear relationship between blood flow and pressure in the vascular access is assumed (Jacobs / Kjellestrand / Koch / Winchester "Replacment of renal function by dialysis" Fourth revised Edition, page 373, 39 ). It is disadvantageous that this extrapolation is associated with a relatively low measurement accuracy. This is because the relationship between blood flow and pressure is not linear. The reason for the non-linear relationship is that the flow in the cannula is not laminar, but turbulent.

The invention is based on the object to specify a procedure for monitoring the pressure in a vascular access while extracorporeal blood treatment without interrupting blood flow allowed in the extracorporeal bloodstream. Another job of Invention is to provide a device for monitoring the pressure in the vascular access without interrupting the blood flow.

The above tasks are solved with those in the claims 1 to 13 specified characteristics. Advantageous embodiments the invention are the subject of the dependent claims.

The method according to the invention is based on that the arterial or venous Pressure measured at an upper and a lower value for blood flow and from the measured pressures the change the pressure in the vascular access is calculated and compared with a threshold value. If the change If the control is above the threshold value, it can be the blood treatment device. critical is that the lower and upper blood flow value is only one relatively small amount below or above an operating point lies. Because blood flow is only a relatively small amount around the Increase working point or is lower, the pressure change can also only with one insufficient extrapolation determined with relatively high accuracy become.

To calculate the change in pressure in the vascular access, the arterial or venous pressure in the arterial or venous branch of the extracorporeal blood circuit can be measured, while the blood flow from a predetermined amount within a predetermined time period, which is very short in relation to the duration of treatment predetermined blood flow is decreased or increased. The predetermined blood flow, which is increased or decreased by the predetermined amount, can be the blood flow at which blood treatment is to take place. Then the working point corresponds to the current blood flow. However, the working point can also correspond to the upper or lower value for the blood flow, ie the first measurement takes place at the current blood flow and the second measurement after a decrease or increase in the blood flow.

In order to achieve a higher measuring accuracy, it may be beneficial if the blood flow at the time of measurement in an area below or above the blood flow who is being treated. In practice there is a high level of measurement accuracy dependent on from the blood pump used, for example at a given one Blood flow of preferably 100-300 ml / min, in particular 200 ml / min achieved.

In the method according to the invention can be surveillance the pressure in the vascular access without interrupting the blood flow in the extracorporeal circulation done by making the change the pressure in the vascular access is calculated continuously, the calculated pressure changes in the vascular access can be compared with predetermined threshold values. Just in case that the pressure change exceeds the threshold, the blood flow in the extracorporeal circulation is interrupted in order to the pressure in the vascular access Measurement of arterial or venous To be able to determine pressure.

The method according to the invention has the decisive one Advantage of having a continuous interruption in blood flow the aforementioned risks is not required.

To calculate the pressure change it is necessary to determine a coefficient. This coefficient is preferably calculated at the start of blood treatment, where but this requires the interruption of blood flow. It is believed that the coefficient does not change during blood treatment, which is why a continuous interruption of blood flow to calculate the Coefficient is not necessary. The measuring accuracy can, however thereby increased that the coefficient is then recalculated, if the calculated pressure change in the vascular access larger than is the predetermined threshold.

Preferably the blood flow is around increased the same value or humiliated. This has the advantage of being the change blood flow has no influence on blood volume. Basically is but also an increase or lowering the blood flow by different values possible. There the amount by which the blood flow is changed is relatively small, can be assumed to have an impact on blood volume is low.

One that is generally not negligible The determination of the change in pressure has an influence on the blood volume in the vascular system then if before the measurement to increase the measurement accuracy Blood flow is set that is much smaller or larger than is the blood flow at which blood treatment is to take place. A Compensation for the change in blood volume caused by the measurement preferably takes place in that for the same predetermined period of time, in which the blood flow increases or is lowered, blood flow is adjusted to be the difference the double blood flow at which the treatment is given, and the Half of Corresponds to the blood flow at which the measurement is made.

It is possible to determine the change in pressure in the vascular access both the pressure in the arterial and in the venous branch of the extracorporeal To measure blood circulation. Numerous experiments have shown that a higher Achieve measurement accuracy when measuring arterial pressure can because the venous Pressure signal proven to be very susceptible to interference Has.

The monitoring device according to the invention the pressure in the vascular access has a control unit to change the funding rate the blood pump, with which the blood flow in the extracorporeal circulation lets set a Measuring unit for measuring the pressure in the arterial or venous branch of the extracorporeal blood circuit and one with the control unit and the measuring unit interacting computing unit.

The following are with reference on the drawings two different embodiments of the invention explained in more detail.

Show it:

1 a first alignment example of a device for monitoring the pressure in a vascular access during an extracorporeal blood treatment in a highly simplified schematic representation, wherein the pressure in the arterial branch of the extracorporeal circuit is measured and

2 a device for monitoring the pressure in the vascular access, the pressure in the venous branch being measured.

The surveillance device the pressure in the vascular access (Fistula pressure) can form a separate assembly. But it can also be part of the blood treatment device, especially some their components in the known blood treatment devices are present anyway. Below is the monitoring device the fistula pressure together with the essential components of the blood treatment device described. The blood treatment device was in the present embodiment a conventional hemodialysis machine.

The hemodialysis machine comprises a dialysis fluid circuit 1 and an extracorporeal blood circulation 2 between which a dialyzer 3 is arranged by a semipermeable membrane 4 in a dialysis fluid chamber 5 and a blood chamber 6 is separated. From a dialysis fluid source 7 leads a dialysis fluid supply line 8th , to the inlet of the dialysis fluid chamber, from the outlet of which a dialysis fluid discharge line 9 leads to a drain 10. In the dialysis fluid discharge line there is a dialysis fluid pump for conveying the dialysis fluid 11 connected.

The patient's fistula F is punctured with an arterial and venous cannula (double-needle dialysis). From the arterial port 12 leads a blood supply line 14 , which forms the arterial branch of the extracorporeal circuit, to the conduit 14 , which forms the arterial branch of the extracorporeal circuit, to the inlet of the blood chamber 6 while a blood drain line 15 affecting the venous branch of the extracorporeal bloodstream 2 from the outlet of the blood chamber to the venous port 13 leads. In the blood supply line 19 is a blood flow in the extracorporeal circulation 2 default blood pump 16 switched that via a control line 17 with a control unit 18 connected is. With the control unit 18 the delivery rate of the blood pump can be changed within a predetermined range. In addition, the blood pump can be stopped to interrupt the blood flow in the extracorporeal circuit.

For measuring the pressure in the arterial branch 19 of the extracorporeal circulation 2 is a pressure measuring unit 25 upstream of the blood pump 16 provided an arterial pressure sensor 20 having.

In addition, the device has a computing unit 27 one hand with a data line 26 with the measuring unit 25 and on the other hand with a data line 30 with the control unit 18 connected is. The control unit 18 works with the computing and measuring unit 27 . 25 in such a way that the individual process steps for determining the pressure in the vessel access are carried out. The data exchange takes place via the data lines 30 . 26 , In addition, the control unit 18 an alarm device 18a for an acoustic and / or visual alarm.

The dialysis machine can also have more Components, for example a balance chamber, ultrafiltration pump, drip chamber etc., but the better clarity are not shown for the sake of it.

Below are the individual Process steps for determining the pressure in the vascular access (fistula pressure) described in detail.

The treatment parameters are entered at the start of the dialysis treatment. The treatment parameters are stored in the control unit 18. The control unit specifies a blood flow for the blood pump 16, which can be between 50-600 ml / min. For example, the control unit sets a blood flow Q _b0 at which the treatment is to take place of 300 ml / min.

After starting blood treatment follows first a measurement for calibration, for example five minutes after the start of treatment.

The control unit 18 initiates the measurement process by setting the blood flow to a smaller, lower value Q _bl for a predetermined period of time T ₁ and then setting it to a larger, upper value Q _{b 2} , the lower value being smaller than the upper value. For this purpose, the blood flow can be increased or decreased by a relatively small value ΔQ _b for a period of time that is very short to the treatment period, starting from a current value that corresponds to the working point Q _bm .

For a blood flow Qb = 100-300 ml / min, an increase or decrease in the blood flow by ΔQ _b 30 30 ml / min, preferably 5-30 ml / min, has proven to be advantageous.

Numerous tests have shown that with a working point Q _{b m} of 200 ml / min and a value of 20 ml / min for ΔQ _b measurement results can be achieved with particularly high measurement accuracy. When the blood flow Q _bm is increased and decreased by ΔQ _b , a blood flow which the control unit adjusts results in Q b1 = Q bm - ΔQ b and Q b 2 = Q bm + ΔQ b ,

The measuring unit measures for the two blood flows Q _b1 and Q _{b 2} 25 with the arterial sensor 20 the pressure P _a1 (0) or P _a2 (0) in the arterial branch 19 of the extra corporeal circulation 2 , The control unit then stops 18 the blood pump 16 to interrupt blood flow. Now the measuring unit measures 25 arterial pressure again. When the blood pump is switched off, this pressure corresponds to the fistula pressure P _f (0). This ends the measurement for calibration. The computing unit 27 calculates the coefficient K from the variables measured above according to the following equation:

After the calibration measurement, the control unit switches 18 the blood pump again 16 so that blood treatment continues. The change in pressure in the fistula (fistula pressure) P _f (t) is then continuously monitored without interrupting the blood flow during the blood treatment. Given after expiry At intervals of time, for example every 30 minutes, the change in the fistula pressure is recalculated.

For this purpose, the control unit 18, starting from the blood flow Q _b0 at which the blood treatment is to take place, sets the blood pump 16 to a blood flow Q _bm at which the measurement is to take place which is lower or higher than the blood flow Q _b0 . The blood flow Q _bm for the measurement should, for example, be between 100 and 300 ml / min, preferably 200 ml / min. After the blood pump 16 runs at a corresponding delivery rate, the control unit lowers the blood flow for a predetermined time period T, for example 1 minute, by a predetermined value ΔQ _b to a value Q _b1 and then increases the blood flow for the same predetermined time period T by the same Value ΔQb to the value Q _b2 . For example, ΔQb is 20 ml / min. This results in a value for Q _b1 of 180 ml / min and Q _b2 of 220 ml / min. The total measuring time is 2 min (2T).

The measuring unit 25 measures with the sensor 20 the arterial pressure with increased and decreased blood flow P _a1 (t) and P _a2 (t). The computing unit then calculates 27 the fistula pressure P _f (t) from the above measured variables according to the following equation:

wobei S einen Schwellwert darstellt.The change in fistula pressure P _f (t) is assessed as follows:

where S represents a threshold.

For example, 10% is assumed as the threshold value S. If the currently calculated fistula pressure P _f (t) deviates from the fistula pressure P _f (0) determined at the beginning of the treatment by more than 10%, the alarm generator gives the alarm 18a an acoustic and / or visual alarm. In addition, a conventional measurement of the fistula pressure is initiated. For this, the control unit stops 18 the pump 16 where the measurement unit 25 measures the arterial pressure with the sensor 20, which corresponds to the fistula pressure P _f (t). However, if the change in fistula pressure is less than 10%, it is assumed that the fistula pressure P _f (t) can be calculated with sufficient accuracy according to equation (2) without interrupting the blood treatment.

If the fistula pressure P _f (t) has been measured after the treatment has been interrupted, the computing unit calculates 27 for the new fistula pressure P _f (1) a corrected value for the coefficient K according to equation (1). The change in the fistula pressure P _f (t) is now calculated with the corrected value K, the computing unit 27 the fistula pressure P _f (t) is now monitored according to the following criterion:

So in the course of treatment if exceeded of the threshold value S the coefficient K in each case taking into account of the measured fistula pressure recalculated, the criterion for Evaluation of the change in fistula pressure is adapted in each case.

The control unit provides compensation for the change in blood volume caused by the measurement 18 a blood flow rate Q _b , which is calculated from the blood flow Q _b0 at which the treatment is to be carried out and the previously set blood flow Q _bm for the measurement according to the following equation: Q b = 2 * Q b0 - 0.5 * Q bm

The control unit 18 then sets the blood flow rate back to the current rate Q _b0 at which the treatment takes place. The entire measurement can also be carried out if the blood flow during the measurement (working point) corresponds to the blood flow during the treatment (Q _bm = Q _b0 ).

wobei P_f1(t) – Fisteldruck bei Q_b1
P_{a 1}(t) – arterieller Druck bei Q_b1
η₁ – Blutviskosität bei Q_b1 Equation (2) is derived as follows:
The fistula pressure can be measured indirectly via two measurements of the arterial pressure in two different blood flows during the treatment. The following applies to blood flow Q _b1

where P _f1 (t) - fistula pressure at Q _b1
P _{a 1} (t) - arterial pressure at Q _b1
η ₁ - blood viscosity at Q _b1

wobei P_f2(t) – Fisteldruck bei Q_b2
P_a2(t) – arterieller Druck bei Q_b2
η₂ – Blutviskosität bei Q_b2 The following applies to blood flow Q _b2 :

where P _f2 (t) - fistula pressure at Q _b2
P _a2 (t) - arterial pressure at Q _b2
η ₂ - blood viscosity at Q _b2

Assuming that there is no clear change in fistula pressure within a short time for the change from Qb to Qb ₂ , P _f1 (t) = P _f2 (t) = P _f (t) and assuming that the blood viscosity is not changed ηi = η2 = η.

wobei K ein Kompensationsparameter ist, welcher eingeführt wird zur Berücksichtigung, dass die Blutströmung in der Kanüle und dem Schlauchsegment nicht ideal laminar ist. Der Parameter wird nach dem Behandlungsstart durch eine Kalibriermessung ermittelt.This results in:

where K is a compensation parameter, which is introduced to take into account that the blood flow in the cannula and the tube segment is not ideally laminar. The parameter is determined after the start of treatment by a calibration measurement.

2 shows an alternative embodiment of the invention. This embodiment differs from that with reference to FIG 1 described embodiment only in that instead of the arterial pressure, the pressure in the venous branch 21 of the extracorporeal circulation 2 is measured. Hence the sensor 22 the measuring unit 25 in the blood return line 15 arranged. The corresponding parts of the two devices are provided with the same reference numerals. With regard to the functioning, reference is made to the embodiment of 1 directed. In the venous pressure measurement, the ultrafiltration is preferably switched off to increase the accuracy and the dialyzer valves which are generally present in a dialysis machine are closed. Only the connection between venous fistula access and venous pressure sensor must be free during the measurement.

Claims (26)

Method for monitoring the pressure in a vascular access during extracorporeal blood treatment, in which blood is conveyed into a blood treatment unit via an arterial branch of an extracorporeal circuit, which is connected to the vascular access at an arterial connection, and via a venous branch of the extracorporeal circuit, the is connected via a venous connection to the vascular access, is characterized by the following steps: setting a lower and an upper value for the blood flow Q _b1 , Q _b2 in the extracorporeal circuit during extracorporeal blood treatment, each for a predetermined period of time T, the lower Value is a predetermined amount ΔQb below and the upper value is a predetermined amount ΔQ _b above an operating point Q _bm , measuring the arterial or venous pressure P _{a / v} (t) in the arterial or venous branch of the extracorporeal circuit in the lower and upper ren value for blood flow Q _b1 , Q _b2 , and calculating the change in pressure P _f (t) in the vascular access from the measured arterial or venous pressure P _{a / v} (t) and comparing the calculated pressure change in vascular access with a predetermined threshold value. A method according to claim 1, characterized in that the blood flow in the extracorporeal circuit Exceeding the threshold value is interrupted. Method according to Claim 2, characterized in that after the blood flow has been interrupted, the arterial or venous pressure P _{a / v} (t) is measured to determine the pressure P _f (t) in the vascular access. Method according to one of claims 1 to 3, characterized in that the pressure P _f (t) in the vascular access is calculated from the arterial or venous pressure according to the following equation:

where Q _{b1 is} the lower value for blood flow, Q _{b2 is} the upper value for blood flow, P _{a / v1} (t) the arterial or venous pressure at Q _b1 , P _{a / v 2} (t) the arterial or venous pressure at Q _b2 and K is a coefficient. A method according to claim 4, characterized in that to determine the coefficient K, the blood flow Q _b in the extracorporeal circuit is interrupted and the arterial or venous pressure P _{a / v} (t) is measured to determine the pressure P _f (0) in the vascular access, and that the arterial or venous pressure P _{a / v1 / 2} (0) with two different blood flows Q _b1, Q _b2 in the extracorporeal circuit is measured without interrupting the blood flow in the extracorporeal circuit, the coefficient K being calculated according to the following equation:

A method according to claim 5, characterized in that an initial value for the coefficient K is calculated at the beginning of the blood treatment. Method according to Claim 5 or 6, characterized in that a corrected value for the coefficient K is calculated during the blood treatment if the calculated change in the pressure P _f (t) in the vascular access is greater than the threshold value S, the corrected value for the Coefficient K is used for the calculation of blood flow in the vascular access. Method according to one of claims 1 to 7, characterized in that the working point Q _bn , is a predetermined blood flow that is greater or less than the predetermined blood flow Q _b0 at which the blood treatment takes place. Method according to one of claims 1 to 8, characterized in that the blood flow Q _b compared to the working point Q _{b m is} decreased and increased by the same value ΔQ for a predetermined period of time. A method according to claim 9, characterized in that after the increase or decrease in the blood flow Q _b for a predetermined period of time 2T, a blood flow for the same predetermined period of time 2T is set, which is the difference of the double blood flow Q _b0 at which the treatment takes place, and the Half of the blood flow Q _bm at which the measurement takes place corresponds. Method according to one of claims 1 to 10, characterized in that the arterial pressure P _a1 (t) is measured. Method according to one of claims 1 to 11, characterized in that that the blood with a blood pump located in the arterial branch is conveyed into the blood treatment unit, whereby to interrupt blood flow in the extracorporeal circulation stopped the blood pump becomes. Method according to one of claims 1 to 12, characterized in that that when exceeded the threshold value is given an acoustic and / or visual alarm becomes. Device for monitoring the blood flow in a vascular access (F) during an extracorporeal blood treatment in which blood is passed through an arterial branch ( 19 ) of an extracorporeal circulation ( 2 ) connected to an arterial port ( 12 ) connected to the vascular access, into a blood treatment unit ( 3 ) ge is promoted and via a venous branch ( 21 ) of the extracorporeal circuit, which is connected via a venous 13 ) connected to the vascular access, is returned, with a blood pump in the extracorporeal circuit ( 16 ) is switched with a control unit ( 18 ) to change the flow rate of the blood pump ( 16 ) a measuring unit ( 25 ) to measure the pressure in the arterial or venous branch ( 19 . 21 ) of the extracorporeal circuit, and one with the control unit ( 18 ) and the measuring unit ( 25 ) interacting computing unit ( 27 ), characterized in that the control unit ( 18 ) with the computing unit ( 27 ) and the measuring unit ( 25 ) interacts in such a way that a lower and an upper value for the blood flow Q _b1 , Q _b2 in the extracorporeal circuit during extracorporeal blood treatment is set for a predetermined period of time T, the lower value being a predetermined amount ΔQ _b below and the upper value one predetermined amount ΔQ _b above a working point Q _bm , the arterial or venous pressure P _{a / v} (t) in the arterial or venous branch of the extracorporeal circuit is measured at the lower and upper values for the blood flow Q _b1 , Q _b2 , and the change in pressure P _f (t) in the vascular access is calculated from the measured arterial or venous pressure P _{a / v} (t) and the calculated pressure change in the vascular access is compared with a predetermined threshold value S. Device according to claim 14, characterized in that the control unit ( 18 ) with the computing unit ( 27 ) and the measuring unit ( 25 ) interacts in such a way that the blood pump ( 16 ) to interrupt the blood flow in the extracorporeal circulation ( 2 ) is stopped when the threshold value S is exceeded. Apparatus according to claim 14 or 15, characterized in that the measuring unit (25) and computing unit ( 27 ) are designed such that after the blood flow is interrupted, the arterial or venous pressure P _{a / v} (t) is measured to determine the pressure P _f (t) in the vascular access. Device according to one of claims 14 to 16, characterized in that the computing unit (27) is designed such that the pressure P _f (t) in the vascular access is calculated from the arterial or venous pressure P _{a / v} (t) according to the following equation :

where Q _{b1 is} the lower value for blood flow, Q _{b2 is} the upper value for blood flow, P _{a / v1} (t) the arterial or venous pressure. Q _b1 , P _{a / v2} (t) the arterial or venous pressure at Q _b2 and K is a coefficient. Device according to claim 17, characterized in that the control unit ( 18 ) with the computing unit ( 27 ) and the measuring unit ( 25 ) interacts in such a way that the blood pump ( 16 ) is stopped and the arterial or venous pressure P _{a / v} (t) is measured to determine the pressure P _f (0) in the vascular access, and that the arterial or venous pressure P _{a / v1 / 2} (0) with two different blood flows Q _b1 , Q _b2 , is measured in the extracorporeal circuit, the computing unit ( 27 ) is designed such that the coefficient K is calculated according to the following equation:

Device according to claim 18, characterized in that the computing unit ( 27 ) is designed such that an initial value for the coefficient K is calculated at the beginning of the blood treatment. Device according to claim 18 or 19, characterized in that the computing unit ( 27 ) is designed such that a corrected value for the coefficient K is calculated during the blood treatment if the calculated change in the pressure P _f (t) in the vascular access is greater than the threshold value S, the corrected value for the coefficient K for the calculation of the pressure in the vascular access is used. Device according to one of claims 14 to 20, characterized in that the control unit ( 18 ) is designed such that the working point Q _{bm is} a predetermined blood flow that is greater or less than that predefined blood flow Q _b0 at which the blood treatment takes place. Device according to one of claims 14 to 21, characterized in that the control unit ( 18 ) is designed such that the blood flow Q _b compared to the working point Q _{bm is} decreased and increased by the same value ΔQ for a predetermined period of time. Device according to one of claims 22, characterized in that the control unit ( 18 ) with the computing unit ( 27 ) and the measuring unit ( 25 ) interacts in such a way that after the increase or decrease in the blood flow Q _b for a predetermined period of time 2T, a blood flow for the same predetermined period of time 2T is set, which is the difference between the double blood flow Q _b0 at which the treatment takes place and half of the blood flow Q _bm at which the measurement takes place. Device according to one of claims 14 to 23, characterized in that the measuring unit ( 25 ) a sensor ( 20 ) for measuring arterial pressure P _a1 (t). Device according to one of claims 14 to 24, characterized in that the blood pump ( 16 ) in the arterial branch ( 19 ) of extracorporeal circular walking ( 2 ) is arranged. Device according to one of claims 14 to 25, characterized in that an alarm transmitter ( 18a ) is provided for an acoustic and / or visual alarm when the threshold value is exceeded.