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.

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 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 art1  and P art2  are measured at at least two different values of the blood flow in each case and the difference ΔP art =P art1 −P art2  calculated from the pressures P art1  and P art2 , whereby the difference ΔP 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 0 ) RBV (t 0 ) of the haematocrit HKT (t 0 ) at a preceding time to and the blood volume RBV (t 0 ) at a preceding time t 0 , divided by the haematocrit HKT(t 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 ( 5   a ) and an arterial flexible-tube line ( 5 ) for taking blood and a venous cannula ( 7   a ) 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 art1  and P art2  are measured at at least two different values of the blood flow in each case and the difference ΔP art =P art1 −P art2  calculated from the pressures P art1  and P art2 , whereby the difference ΔP art  is compared with predetermined value ranges representative of the individual cannula diameters in order to determine the cannula diameter.