WO2002025277A1

WO2002025277A1 - Heart-lung machine provided with a device for electrical impedance measurement, and method therefore

Info

Publication number: WO2002025277A1
Application number: PCT/NL2001/000701
Authority: WO
Inventors: Gheorghe Aurel Marie Pop
Original assignee: Martil Instruments B.V.
Priority date: 2000-09-22
Filing date: 2001-09-24
Publication date: 2002-03-28
Also published as: NL1016247C2; AU1438602A

Abstract

The invention relates to a blood transporting machine, at least comprising transport means for transporting blood, wherein the blood transporting machine is further provided with a device for electrical impedance measurement of the blood, which comprises means for generating an electrical current in the blood, means for measuring the electrical impedance and means for recording the electrical impedance and/or impedance change in the blood. The blood transporting machine also comprises processing means for processing recorded impedance to a blood viscosity value.

Description

HEART-LUNG MACHINE PROVIDED WITH A DEVICE FOR ELECTRICAL IMPEDANCE MEASUREMENT ,

AND METHOD THEREFORE

The present invention relates to a blood transporting machine as according to the heading of claim 1.

A blood transporting machine is used for extracorpo- real circulation of blood. Examples hereof are a heart- lung machine and an artificial kidney machine.

An open-heart operation is usually performed with a heart-lung machine (HLM) . Cerebral dysfunction and to a lesser extent kidney function disorders are frequently occurring problems for patients as a result of this operation. These cerebral and renal problems not only cause a greater or lesser degree of permanent invalidity for the patient, but also result in hospital accommodation costs in the short term and costs of care in the longer term.

One of the causes of renal and cerebral disorders is formed by a rise in the viscosity of the blood, which is caused by hypothermia and by the "acute phase" reaction generated in the body by the operation. Increased viscosity is linked to reduced cerebral microcirculation. Hyperviscosity resulting from hypothermia affects the renal function. In addition, the moments of canulating, manipulation of the heart and declamping of the aorta are significant surgical sources of the occurrence of micro- embolisms. Despite the use of different filters in conjunction with the HLM, a significant part of the micro- embolisms is still found to occur as a result of perfusi- on problems associated with the HLM.

The present invention has for its object to monitor the viscosity of blood when it is transported extracorpo- really by a blood transporting machine.

This objective is achieved with the present invention by providing a blood transporting machine of the above stated type, further comprising means for processing the impedance value to a viscosity value. The blood viscosity of a patient being treated can be measured with the blood transporting machine according to the invention.

It is favourable to embody the blood transporting machine as according to claim 2. It is hereby possible to monitor the blood viscosity value during open-heart operations. When the recorded blood viscosity value changes in adverse manner, external intervention can take place, for instance with medication.

Claim 3 is preferably applied. The electrical impe- dance measurement in the blood hereby takes place as soon as it is transported out of the blood transporting machine. A viscosity value for the blood is hereby obtained in a short time.

Claim 4 is preferably applied. Measuring of the impedance of the blood hereby becomes possible in efficient manner.

According to claim 5 the blood transporting machine is preferably embodied with a set of measuring electrodes . The impedance of the blood can hereby be measured between the measuring electrodes.

In the preferred embodiment the electrodes are circular. Such an embodiment allows the electrodes to enclose the perfusion tube.

The blood transporting machine is preferably embo- died with platinum electrodes. The use of platinum electrodes has a favourable effect on the accuracy of the value determination of the viscosity of the blood.

The electrodes are preferably arranged at a regular distance from each other in the longitudinal direction of the perfusion tube. A homogeneous electrical field in the blood is hereby achieved.

It is favourable to embody the heart-lung machine as according to claim 9. Such a ratio is effective for the homogeneity of the electrical field to be applied and measured.

The measure as according to claim 10 has a favourable effect on the accuracy of the measurement of the impedance value, and therefore on the accuracy of the blood viscosity to be determined.

In a preferred embodiment according to the invention the measure of claim 11 is applied in the blood transpor- ting machine. Particular medications administered to a patient when blood is circulated outside the body influence the sodium concentration in the blood of the patient. The sodium concentration influences the impedance of the blood. Inclusion of the sodium concentration in the algorithm for processing the impedance signal produces a more accurate determination of the viscosity value of the blood.

The invention also relates to and provides a.method for the operation of the heart-lung machine according to the invention. The method according to the invention relates in particular to detecting embolisms in the blood.

The method according to the invention preferably comprises of detecting air embolisms and/or micro-embo- lisms.

In a preferred embodiment according to the invention the blood transporting machine is embodied with temperature measuring means and means for measuring the haemato- crit value of the blood. These values can hereby also be monitored. The relation between the electrical impedance of the blood and the viscosity depends on the temperature and is also determined by the haematocrit value. For processing of the impedance signal to a viscosity value a calculation of the impedance signal is necessary with an algorithm dependent on the temperature and the haematocrit value.

It is favourable to determine correlation values for the relation between impedance of the blood and viscosity in relation to the temperature and the haematocrit value. The correlation coefficients are determined by doing an experiment with a large group of people. These are incorporated in the algorithm for processing the impedance signal to a viscosity value. An apparatus and a method for detecting the presence of embolisms in a tube through which blood or other fluid flows using a blood impedance measuring system is otherwise known from US 4,014,206. The invention is further elucidated with reference to the annexed figures, in which:

Figure 1 shows a schematic view of a preferred embodiment of the device for electrical impedance measurement of the blood in a heart-lung machine; Figure 2 shows a graph of a measurement signal for the impedance with the embodiment of figure 1 ;

Figure 3 shows a graph of an example of change in the impedance in accordance with a measurement with the embodiment of figure 1; Figure 4 shows a schematic view of a device for electrical impedance measurement of the blood, wherein particles flow in the blood;

Figure 5 shows a graph of the relation between impedance and viscosity corrected according to the inven- tion.

Figure 1 shows a schematic view of a preferred embodiment of the measuring device 1 for electrical impedance measurement of the blood in a blood transporting machine. The measuring device of the blood transpor- ting machine comprises two outer current electrodes 2,3 and two inner measuring electrodes 4,5 in the perfusion tube 6, immediately after the source from the blood transporting machine. Electrodes 2-5 are preferably circular and of platinum or stainless steel with a layer of precious metal. In order to obtain a homogeneous electrical field the distances between electrodes 2-5 are chosen so as to be more than twice the diameter of the perfusion tube 6 coming out of the blood transporting machine. By generating a low alternating current (10mA - 1mA) with a frequency between 4 kHz and 5000 kHz with an alternating current source 7 via the two outer electrodes 2,3, it will be possible via measuring electrodes 4,5 to continuously measure an impedance (ZO) of the blood passing the measuring electrodes. The impedance is acquired by measuring means 8.

Also shown is a sodium concentration measuring means 9 which is arranged in perfusion tube 6. Processing means 10 processes the measured value to a blood viscosity value which can be shown on a screen 11. The processing means can also be embodied to detect the occurrence of embolisms in the blood, air and/or micro-embolisms in particular. An example of an impedance signal as measured with the embodiment of figure 1 is shown in figure 2. A continuous recording of the change in the impedance signal (delta Z) can likewise take place. An example of a measurement signal of the change in the impedance signal is shown in figure 3.

It is known that blood has electrical properties. These electrical properties differ for plasma and blood cells. The plasma and the interior of the cells consist of conducting fluids with a determined electrical resis- tance and cell membranes consist of phospholipids and proteins with di-electrical properties. The electrical impedance of blood is determined primarily by three parameters: plasma resistance, internal resistance in the cell and the capacitance of the cell membrane. The elec- trical impedance of the blood increases with an increased viscosity of the blood. Just as the viscosity, the electrical impedance of blood increases during hypothermia and this is determined to a large degree by the haematocrit . Small particles (< 100 micron) with an impedance other than blood will become manifest in a sudden change in the impedance signal (Figures 2 + 3) . These changes can be seen particularly in the deltaZ signal and the magnitude of this change depends on the difference in specific resistance of the particles and of blood. During passage of air particles a large deflection will occur in the deltaZ signal in view of the clearly higher specific resistance of air relative to blood. This deflection of

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According to the invention a relation can be formulated between the viscosity value, the electrical impedance, the haematocrit value, the. temperature and the sodium level in the blood. The latter three factors influence the impedance of the blood. A test arrangement with blood from ten volunteers was used to establish correlation coefficients for said relation. The test arrangement simulated a heart-lung machine. Gelofusine was also added to change the haematocrit value. It was found that:

(1) Ln (visco) = 0.774 + 2876 x 10^"2 x HCT - 2.104 x 10^"2 x T + 2.765 x 10^"3 X Na.

(2) Ln (imp.) = 5.466 + 2.386 x 10^"2 x HCT - 1.961 x 10^"2 x T - 5.995 x 10^"3 x Na. (3) Visco = 0.364 + 3.782 x 10^"2 x imp., wherein Ln is the natural logarithm, visco is the viscosity value, HCT the haematocrit value, T the temperature, Na the sodium level, and imp. the impedance according to the measurement. Measurement took place in the test arrangement with an alternating current with a frequency of 20 kHz and an alternating current value of 300 μA.

During the test arrangement the levels of Ht, sodium, potassium, calcium and the pH were measured. Of these variables only the sodium was found to have significant influence on the impedance/viscosity. For the processing of the impedance measurement to a viscosity value a correction is therefore necessary which depends on the measured sodium concentration.

The third formula shows a direct correlation between the viscosity value and the impedance measurement.

In figure 5 is shown the correlation between the viscosity and the impedance of a test measurement with the blood of ten people. The viscosity and impedance are shown to be correlated. The measurement of the impedance can be used to determine the viscosity value. The processing means are provided with an algorithm which calculates the relation according to formula (3) , wherein a correction for the measured sodium concentration is also carried out .

Claims

1. Blood transporting machine, at least comprising transport means for transporting blood, wherein the blood transporting machine is further provided with a device for electrical impedance measurement of the blood, which comprises means for generating an electrical current in the blood, means for measuring the electrical impedance and means for recording the electrical impedance and/or impedance change in the blood, characterized in that the blood transporting machine also comprises processing means for processing recorded impedance to a blood viscosity value.

2. Blood transporting machine as claimed in claim 1, characterized in that the machine is provided with means for functioning as heart-lung machine.

3. Blood transporting machine as claimed in claim 1, characterized in that transport means comprise at least one perfusion tube and the current generating means and the measuring means are arranged in the perfusion tube close to the source thereof out of the blood transporting machine.

4. Blood transporting machine as claimed in claim 1, 2 or 3 , characterized in that the current generating means comprise a set of current generating electrodes which are connected to a source of alternating voltage and which can generate an alternating current in the blood.

5. Blood transporting machine as claimed in any of the claims 1-4, characterized in that the measuring means comprise a set of measuring electrodes.

6. Blood transporting machine as claimed in claim 4 or 5, characterized in that the electrodes are circular.

7. Blood transporting machine as claimed in claim 4, 5 or 6, characterized in that the electrodes are platinum _.electrodes .

8. Blood transporting machine as claimed in any of the foregoing claims 1-7, characterized in that the electrodes are arranged at a regular distance from each other in the perfusion tube in the direction of the blood flow.

9. Blood transporting machine as claimed in claim 8, characterized in that the distance is at least twice the diameter of the perfusion tube.

10. Blood transporting machine as claimed in any of the foregoing claims, characterized in that in the direction of the blood flow the current generating electrodes comprise the outer electrodes and the measuring electrodes the inner electrodes .

11. Blood transporting machine as claimed in any of the foregoing claims, characterized in that means are also arranged for measuring the sodium concentration in the blood.

12. Blood transporting machine as claimed in claim 11, characterized in that the processing means can cor- rect the blood viscosity value for the measured sodium concentration .

13. Blood transporting machine as claimed in any of the foregoing claims, characterized in that means are also arranged for measuring the haematocrit value of the blood.

14. Blood transporting machine as claimed in any of the foregoing claims, characterized in that the processing means determine the viscosity value with an algorithm for the impedance measurement, wherein the algo- rithm depends on the temperature and the haematocrit value .

15. Blood transporting machine as claimed in any of the foregoing claims, characterized in that the algorithm comprises correction coefficients for the dependence of the viscosity value on temperature and haematocrit value during an impedance measurement .

16. Use of a blood transporting machine as claimed in any of the foregoing claims for detecting the occurrence of embolisms in the blood.

17. Use as claimed in claim 13, characterized in that the embolisms comprise air embolisms and/or micro- embolisms .