Patent Publication Number: US-2023137197-A1

Title: Blood pump

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to an intravascular blood pump to support or replace the function of the heart by creating an extra blood flow in a patient&#39;s blood vessel. 
     BACKGROUND OF THE INVENTION 
     Blood pumps of different types are known such, as axial blood pumps, centrifugal blood pumps and mixed-type or diagonal blood pumps, where the blood flow is caused by both axial and radial forces. Intravascular blood pumps are usually inserted percutaneously, such as through the femoral artery into the left ventricle so as to bridge the aortic valve or through the femoral vein into the right ventricle. 
     A rotary blood pump has an axis of rotation. In this patent application, the terms “radial” and “axial” refer to the axis of rotation and mean “in radial direction in relation to the axis of rotation” and “along the axis of rotation”, respectively. The term “inner” means radially toward the axis of rotation, and the term “outer” means radially away from the axis of rotation. 
     An intravascular blood pump typically comprises a pumping device as a main component. The pumping device has a pump section including a primary impeller for pumping the blood from a blood flow inlet to a blood flow outlet and a drive section including a motor for driving the primary impeller. The pump section may include a flexibly bendable cannula between the blood flow inlet and outlet. 
     The pumping device comprises a pump section end which is arranged at a pump side of the pumping device. The pumping device further comprises a drive section end which is arranged at a drive side of the pumping device. The blood pump may further comprise a catheter connected to the pumping device in order to supply the pumping device e.g. with energy, and/or a purge fluid. The catheter may be connected to the pump section end but is mostly connected to the drive section end of the pumping device. It is also conceivable to rotate the impeller in a forward and in a reverse direction. Then, the blood flow inlet and the blood flow outlet of the pump section may interchange. 
     Usually, the impeller is supported within the pumping device by means of at least one impeller bearing. Different rotor bearing types are known, such as sliding bearings, in particular hydrodynamic sliding bearings, pivot bearings, hydrostatic bearings, ball bearings etc., and combinations thereof. In particular, contact-type bearings may be realized as “blood-immersed bearings”, where the bearing surfaces have blood contact. Problems during operation may be friction and heat. In case of a blood-immersed bearing, a further problem may be blood clotting due to heat or not enough rinse. 
     An example for blood-purged radial sliding rotor bearings is disclosed in WO 2017/021465 which describes an intravascular blood pump comprising a generally cylindrical primary impeller and a generally cylindrical secondary impeller which rotate together. The secondary impeller is arranged at the radial center of the primary impeller. Vanes of the secondary impeller extend toward an axis of rotation of both impellers. The tips of the vanes of the secondary impeller form an outer bearing surface of a sliding bearing. A cylindrical outer surface of a pin which is arranged at the center of the secondary impeller forms the inner bearing surface of the sliding bearing. In another embodiment, blood from the center of an arriving blood stream can enter the blood pump through a central axial passage in the impeller. In all embodiments the primary and the secondary impeller are mounted on a non-rotating central pin. This increases the hydraulic resistance of the secondary blood flow. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a blood pump with reduced hydraulic resistance for the pumped blood. 
     This is achieved according to the present invention by a blood pump having the features of independent claim  1 . Preferred embodiments and further developments of the invention are specified in the claims dependent thereon. 
     According to a first aspect of the invention, an intravascular blood pump comprises a pumping device with a pump casing having a primary blood flow inlet and a primary blood flow outlet which are hydraulically connected by a primary blood flow passage, wherein a primary impeller has an upstream end and a downstream end and is configured to convey a primary blood flow from the primary blood flow inlet to the primary blood flow outlet along the primary blood flow passage. The pumping device further comprises a drive unit configured to rotate the primary impeller about an axis of rotation. An impeller bearing supports the upstream end of the primary impeller, wherein a central opening axially extends through the impeller bearing. The pumping device further comprises at least one secondary blood flow passage in the primary impeller, the at least one secondary blood flow passage having a secondary blood flow inlet in axial alignment with the central opening of the impeller bearing. Each secondary blood flow passage has a secondary blood flow outlet and is configured to convey a secondary blood flow from the secondary blood flow inlet to the secondary blood flow outlet. The secondary blood flow outlet connects the at least one secondary blood flow passage to the primary blood flow passage at a location axially between the upstream and downstream ends of the primary impeller. 
     In other words, the secondary blood flow passage or passages extend diagonally through the primary impeller, starting centrally at the distal tip of the impeller and terminating in a lateral surface of the impeller so that the secondary blood flow is taken from the center of the arriving blood stream, where the blood stream is fastest and has the most kinetic energy, and meets and supports the primary blood flow in the primary blood flow passage. 
     Due to their diagonal extension, the secondary blood flow passages also generate pressure in the secondary blood flow by centrifugal forces. Thus, blood can be pumped through the intravascular blood pump with greater ease. 
     Thus, the secondary blood flow outlet is not arranged at the downstream end of the primary impeller. In such a case, the secondary blood flow would mix with the primary blood flow downstream beyond the primary impeller. This would require a comparably long secondary blood flow passage with increased hydraulic resistance. 
     A further advantage of the construction is that no stationary parts are present along the secondary blood flow passage. This reduces the hydraulic resistance of the blood pump. 
     The velocity of the blood can be utilized as it is not decelerated by a stationary pin bearing as according to the state of the art. 
     The one or more secondary blood flow passages can be considered to constitute a secondary impeller within the primary impeller. The primary and secondary impeller rotate together. The secondary impeller may be realized completely or partly as an inlay part in the tip end of the primary impeller. 
     It is preferred that at least a part of the secondary impeller is arranged inside the central opening. Then, arriving blood can immediately be conveyed by the secondary impeller. The impeller bearing can be arranged at an outer circumference of the secondary impeller. 
     The central opening may define the outer impeller bearing surface of the impeller bearing. Inside the central opening, a part of the primary or the secondary impeller may be arranged and form a corresponding inner impeller bearing surface. Preferably, at least one secondary blood flow passage extends into the central opening. For example, the inner impeller bearing surface may be formed by the outer circumference of one or more secondary impeller vanes defined by the secondary blood flow passages. 
     It is, however, also possible that the inner impeller bearing surface is arranged at the outer circumference of a part of the primary impeller which is not the secondary impeller. This part can be arranged inside the central opening. 
     Preferably, the primary blood flow inlet is separated from the secondary blood flow inlet by an inflow separator. The inflow separator preferably has the form of a ring. Thus, blood that arrives at the blood pump is divided and flows into the primary or into the secondary blood flow inlet. Preferably, the inflow separator is stationary. Then, the inflow separator can form the outer impeller bearing surface of the impeller bearing. Alternatively, an outer surface of the inflow separator may form an inner impeller bearing surface of the impeller bearing. The impeller bearing radially supports the primary impeller. It is possible that the primary impeller is mounted on the impeller bearing by the secondary impeller or a part thereof. 
     The inflow separator may comprise an additional impeller bearing ring as a separate component. The impeller bearing ring may form the outer or inner impeller bearing surface of the impeller bearing, but it is preferably arranged on the inside of the inflow separator to form an inner impeller bearing surface. The impeller bearing ring may be made of a material that is different from the material of the inflow separator. Particularly, the impeller bearing ring may be made of a ceramic material, especially of silicon carbide. 
     The inflow separator may be supported by at least one strut that connects the inflow separator with the pump casing. Preferably, a minimum of three struts is provided. The struts may extend across the primary blood flow inlet. It is preferred that the struts are configured with a low hydraulic resistance. 
     The inflow separator preferably comprises at least one cut-out at a downstream end of the inflow separator. Preferably, the cut-out has a circumferential width that is comparable to a circumferential width of a secondary blood flow passage. This way, the rotational position of the primary impeller may be defined by at least one secondary blood flow passage extending into and matching the circumferential position of the cut-out. Then, blood clots that begin to build up on the inner (or outer) impeller bearing surface can be removed by an edge of the cut-out when the inner (or outer) impeller bearing surface rotates over it. In case that the corresponding outer (or inner) impeller bearing surface is discontinuous, such as the tip ends of the vanes defined by the second blood flow passages of the secondary impeller, the outer (or inner) impeller bearing surface at the inflow separator can be cleaned by an edge of such vanes in a similar way. A further advantage of the cut-outs is that the cleaning edges of the cut-outs can be flushed by blood flowing through the cut-outs such that blood clots or debris do not accumulate. An inner or outer impeller bearing surface formed by the primary or secondary impeller preferably overlaps the cut-outs axially such that the whole impeller bearing surface of the primary or secondary impeller will be cleaned. Preferably, the outer impeller bearing surface of the inflow separator preferably overlaps axially with the end surfaces of the vanes of the secondary impeller, which form the inner impeller bearing surface, such that the whole outer impeller bearing surface will be cleaned. It is preferred that the tips of the vanes of the secondary impeller have a greater axial length than the circumferential length of the cut-outs. Preferably, at least one cut-out extends between two struts. 
     The cut-out is preferably not only arranged in the inflow separator, but extends through the aforementioned impeller bearing ring which may be present optionally. Then, the cut-out is completely open on both sides such that the cut-out can be washed effectively. 
     The impeller bearing may be a sliding bearing. Preferably, the impeller bearing is a blood-purged sliding bearing. This has the advantages that the blood from the blood stream can be used to purge the bearing and, thereby, also cool the bearing. 
     Preferably, a center of area in a mathematical sense, meaning the middle of a certain region, of the primary blood flow inlet is arranged in the secondary blood flow inlet. The primary blood flow inlet is arranged around the secondary blood flow inlet such that the middle of a blood stream that arrives at the blood pump enters the blood pump through the secondary blood flow inlet. The middle of a laminar blood stream has the highest velocity and is, thus, supplied to the secondary blood flow. 
     After the blood has passed the primary and secondary blood flow inlet, it enters the primary impeller and secondary impeller, respectively, at respective primary and secondary channel intakes into the primary and secondary blood flow passages. Preferably, at least one secondary channel intake is arranged at a position of the axis of rotation. Usually, the tip end of an impeller does not have a rational velocity so that blood may clot by adhesion and accumulation may arise on a stationary solid tip end. However, with the proposed arrangement, the middle part of the bloodstream flows into the intakes of the secondary blood flow passages. Thus, blood clotting cannot occur. 
     The secondary channel intakes are preferably arranged upstream of the primary channel intakes. An end of the secondary impeller may then be arranged inside the impeller bearing. 
     The primary impeller comprises at least one blade having a primary pitch at the upstream end of the primary impeller and a secondary blood flow passage having a secondary pitch, the secondary pitch being approximately or exactly the same as the primary pitch. The pitches may deviate from each other as much as necessary to prevent undesirable flow conditions, such as turbulences. 
     Preferably, at least two of the at least one secondary blood flow passages are arranged asymmetrically in regard to the axis of rotation. This renders it possible to arrange a secondary channel intake at the axis of rotation. 
     Preferably, two secondary blood flow passages are arranged opposite to each other in regard to the axis of rotation. This enables a compact design of the secondary impeller. 
     Preferably, as already mentioned, a secondary blood flow passage defines an edge moving over the outer impeller bearing surface upon rotation of the secondary impeller so as to clean the outer impeller bearing surface. The edge acts as a wiper for the outer impeller bearing surface, and blood clots or debris can be removed in this way. 
     As also briefly mentioned above, the primary impeller may comprise an inlay in which the at least one secondary blood flow passage is formed. Then, the secondary impeller may be made of another material as the primary impeller. For example, the secondary impeller may be made of a ceramic material. This is advantageous for the inner impeller bearing surface. The primary impeller and the secondary impeller may alternatively form one integral piece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The a foregoing summary as well as the following detailed description of preferred embodiments will be better understood when read in conjunction with the appendant drawings. The scope of the disclosure is not limited, however, to the specific embodiments disclosed in the drawings. In the drawings: 
         FIG.  1    shows a cross section of a first embodiment of a blood pump according to the invention, 
         FIG.  2    shows a part of  FIG.  1    with the pump section in an enlarged view, 
         FIG.  3    shows a part of  FIG.  1    with the drive section in an enlarged view, 
         FIG.  4    shows a perspective view toward the pump section end of the first embodiment of the blood pump, 
         FIG.  5    essentially shows the view of  FIG.  4   , but with a transparent pump casing, 
         FIG.  6    essentially shows the same view as  FIG.  5   , but of a second embodiment of the blood pump, 
         FIG.  7    shows a perspective view of an embodiment of the secondary impeller, 
         FIG.  8    shows a perspective view of a separator ring of the first embodiment of the blood pump, 
         FIG.  9    shows a perspective view of a separator ring of the second embodiment of the blood pump, 
         FIG.  10    shows a cross section of the drive section end of the blood pump in a perspective view depicting an ancillary impeller, 
         FIG.  11 A  shows a perspective view of the ancillary impeller and of a rotor bearing ring, 
         FIG.  11 B  shows a perspective view of a rotor bearing ring having cut-outs, and 
         FIG.  12    shows a perspective view of a tertiary impeller of the first or the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG.  1   , a cross sectional view of a first embodiment of an intravascular blood pump is illustrated. Rotating parts are not shown cut. The intravascular blood pump  1  comprises a pumping device  11  and a supply line in the form of a catheter  5  attached thereto. 
     The pumping device  11  comprises a pump casing  2  of substantially cylindrical form, at least in an intermediate section thereof. The pump casing  2  comprises a blood flow inlet  21  and a blood flow outlet  22 . In  FIG.  1   , the pump casing  2  seems to comprise two separate sections, but these sections are either integral or connected to form a single piece. 
     As can be better seen in the enlarged representation of the pump section shown in  FIG.  2   , together with the front perspective views depicted in  FIGS.  4  and  5   , the blood flow inlet  21  comprises a primary blood flow inlet  211  and a secondary blood flow inlet  212 . The primary blood flow inlet  211  surrounds the secondary blood flow inlet  212 . The primary blood flow inlet  211  and the secondary blood flow inlet  212  are separated by an inflow separator  26 . Inside the inflow separator  26 , the inflow separator  26  comprises an impeller bearing ring  27 , which is separately shown in  FIG.  8   . Further, the pumping device  11  comprises a primary impeller  31  which has integrated therein a secondary impeller  32 . The primary and secondary impellers  31 ,  32  are rotatable together about an axis of rotation  10 . The secondary impeller  32  may, as shown in  FIG.  7   , have the form of an inlay and may be arranged inside a secondary impeller cavity  312  of the primary impeller  31 . The secondary impeller cavity  312  is open toward a pump section end PSE of the pumping device  11 . Alternatively, the primary and secondary impellers  31 ,  32  are integrally formed. 
     A primary blood flow  1 BF flows from the primary blood flow inlet  211  to the primary impeller  31  outside of the inflow separator  26  to be conveyed further by the primary impeller  31  through a primary blood flow passage  30  to the primary blood flow outlet  22 . A secondary blood flow  2 BF flows from the secondary blood flow inlet  212  through the inflow separator  26  to the secondary impeller  32  to be conveyed further by the secondary impeller  32  through a plurality of secondary blood flow passages  321  to the primary blood flow passage  30 . 
     Thus, a blood stream arriving at the pumping device  11  at the pump section end, preferably about almost the whole cross section of the pumping device  11 , can flow into the primary and secondary blood flow inlets  211 ,  212  without significant deflection. Because of the central position of the secondary blood flow inlet  212 , also blood from the middle of the blood stream can enter the pumping device  11  without deflection. This is advantageous because usually a blood stream is a laminar flow in which the flow velocity is greatest in the center. 
     The primary impeller  31  comprises primary impeller vanes  313  which extend into the primary blood flow passage  30  and between which primary impeller channels  311  are arranged. The primary impeller channels  311  have a primary pitch at a primary channel intake  314  at each end of the primary impeller channels  311  toward the pump section end PSE. The secondary impeller  32  comprises at least one and particularly exactly two secondary blood flow passages  321  in channel form, which are therefore referred to hereinafter also as secondary impeller channels  321  (see also  FIG.  7   ). The secondary impeller channels  321  have a secondary pitch at a secondary channel intake  324  which is arranged at an upstream end of the secondary impeller channel  321 . The secondary pitch is preferably the same as the primary pitch or may vary to a certain degree as long as undesirable flow conditions, such as turbulences, are prevented. At an end of the secondary impeller  32  toward a drive section end DSE, a connecting breakthrough  315  between the secondary impeller cavity  312  and one of the primary blood flow passages  311  is arranged. An end of this breakthrough  315  in the direction of the blood flow defines the secondary blood flow outlet  213 . The secondary blood flow outlet  213  is arranged further radially outward in regard to the axis of rotation  10 . Therefore, blood is forced outward in a radial direction by centrifugal forces caused by the rotation of the secondary impeller  32 . In this way, the secondary blood flow  2 BF is conveyed through the secondary blood flow inlet  212  and further through the secondary impeller channel  321  of the secondary impeller  32  and unites with the primary blood flow  1 BF flowing through the primary impeller channel  311  of the primary impeller  31 . In this way, a pumped blood flow PBF is formed. The pumped blood flow PBF leaves the pumping device  11  at the blood flow outlet  22 . 
     The primary and secondary impellers  31 ,  32  are jointly mounted in an impeller bearing  37 . They are connected via the secondary impeller cavity  312  or integrally formed as one single piece. The inflow separator  26  comprises an impeller bearing ring  27  arranged inside the inflow separator  26 . An outer impeller bearing surface  277  of the impeller bearing  37  is arranged at the inside of the impeller bearing ring  27 . The impeller bearing  37  further comprises an inner impeller bearing surface  327  which is arranged at an outer circumference of the secondary impeller  32 . 
     The primary impeller  31  is fixedly connected to a tapered section  314  leading to the blood flow outlet  22 . The tapered section  314  directs the pumped blood flow PBF in a direction radially outward in regard to the axis of rotation  10 . The blood then reaches the blood flow outlet  22 . 
     From the tapered section  314  in a direction toward the pump section end PSE, a drive section  4  is arranged inside the pump casing  2  of the pumping device  11 , which comprises a stator  40  and a rotor  41 . Between the stator  40  and the rotor  41 , an axial gap  401  is arranged. In order to cool the stator  40  and the rotor  41 , the axial gap  401  is blood-purged. For this, an ancillary blood flow ABF enters the drive section  4  through an ancillary blood flow inlet  23  arranged at the drive section end DSE. The blood is then conveyed by an ancillary impeller  42  through an ancillary pump gap  423  which is arranged between the ancillary impeller  42  and an inner wall of the pump casing  2 . From there, the blood continues to flow into the axial gap  401 . From the axial gap  401 , the ancillary blood flow ABF enters a radial gap  241 . At the radial outer end of the radial gap  241 , an ancillary blood flow outlet  24  is arranged. The ancillary blood flow ABF flows in the axially gap  401  in a direction opposite to the pumping direction of the primary and secondary impellers  31 ,  32 . The ancillary blood flow ABF inside the drive section  4  also flows substantially in an opposite direction to a general blood flow GBF flowing around the blood pump  1 . 
     As can be better seen in reference to the enlarged representation shown in  FIG.  3   , a rotor bearing ring  43  surrounds the ancillary impeller  42 . The ancillary impeller  42  comprises ancillary impeller vanes  421 . The ancillary impeller vanes  421  protrude in a direction of the axis of rotation  10  toward the drive section end DSE of the pumping device  11 . A radial rotor bearing  47  is arranged at the drive section end DSE and comprises an outer rotor bearing surface  4211  and an inner rotor bearing surface  4311 , between which an axially extending bearing gap is arranged. The outer rotor bearing surface  4311  is arranged on the rotor bearing ring  43 . Blood conveyed by the ancillary impeller  42  flows through the bearing gap and further to the axial gap  401  between the rotor  41  and the stator  40 . From the axial gap  401 , the blood flows to a radial gap  241 . The radial gap  241  extends between the tapered section  314  of the primary impeller  31  and the stator  40 . An ancillary blood flow outlet  24  is arranged at the transition between the radial gap  241  and the surrounding of the pumping device  11 . The ancillary blood flow outlet  24  is arranged perpendicularly to the axis of rotation  10 . Here, the blood from the ancillary blood flow ABF unites with the pumped blood flow PBF from the pump section  2  and the surrounding general blood flow GBF. When the ancillary blood flow outlet  24  is, as shown, arranged close to the outer diameter of the pump casing  2  and close to the primary blood flow outlet  22 , the pumped blood flow PBF and the general blood flow GBF support the drawing of blood out of the radial gap  241  because of their flow velocity. This enhances the ancillary blood flow ABF through the axial gap  401 . 
     At a center of the ancillary impeller  42 , through which the axis of rotation  10  extends, and at a side of the ancillary impeller  42  opposite to the rotor  41 , a hump  422  is arranged. In a direction of the axis of rotation  10  toward the drive section end DSE and adjacent to the hump  422 , a bearing pin  44  is arranged. The bearing pin  44  is connected to the pump casing  2 . An axial bearing surface of the bearing pin  44  toward the ancillary impeller  42  has a convex shape. The axis of rotation  10  runs through an apex of the axial bearing surface of the bearing pin  44  and through an apex of the axial bearing surface of the hump  422 . In this way, the bearing pin  44  interacts with the hump  422  and forms a thrust bearing in order to transmit axial forces in regard to the axis of rotation between the hump  422  and the bearing pin  44 , wherein the aforementioned parts are rotatable relative to each other. Obviously, the contact surface is small, such that rotational friction is low. 
     The drive section end of the blood pump  1  comprises one or more, preferably three, ancillary inlet through-holes  231 . The ancillary inlet through-holes  231  extend from the ancillary blood flow inlet  23  to an ancillary impeller cavity  232  in which the ancillary impeller  42  is arranged. Thus, the blood flows from the ancillary blood flow inlet  23  to the ancillary impeller  42  via the ancillary inlet through-hole  231 . 
     At least one wire through-hole  25  is arranged at the drive section end DSE of the pumping device  11 . The wire through-holes  25  may extend from the catheter  5  to the stator  40 . Preferably, three wire through-holes  25  are arranged about the axis of rotation  10 . Between two ancillary inlet through-holes  231 , one wire through-hole  25  may be arranged. In a wire through-hole  25 , at least one supply line  51 ,  52  and/or  53  may extend to be connected to the stator  40 . Preferably, as shown, the supply wires  51 ,  52  and/or  53  extend through the inside of the catheter  5  to the outside of the patient&#39;s body. The supply wires  51 ,  52  and/or  53  run from the catheter  5  to the stator  40  without contact to blood. 
       FIG.  4    shows a perspective front view on the pump section end PSE of the pump section  3 . As is shown, the secondary impeller  32  is arranged inside the impeller bearing ring  27 . The impeller bearing ring  27  is arranged inside the inflow separator  26 . Alternatively to this embodiment, the additional impeller bearing ring  27  can be omitted such that the outer impeller bearing surface  277  is formed by the inflow separator  26 . Here, the inflow separator  26  is mounted between the primary blood flow  1 BF and the secondary blood flow  2 BF by three struts  28 . It is shown that the secondary blood flow  2 BF flows into the secondary impeller  32  through the secondary blood flow inlet  212  which is arranged at an inflow into the impeller bearing ring  27 . In the secondary impeller  32 , the blood flows along the secondary impeller channel  321  and through the through-opening  315  to the secondary blood flow outlet  213 . Here, the secondary blood flow  2 BF unites with the primary blood flow  1 BF to form the pumped blood flow PBF. 
       FIG.  5    shows the pump section end PSE of the pump section  3  in a perspective view, wherein the pump casing  2  is shown in transparency. It is visible that through-opening  315  and the secondary blood flow outlet  213  are arranged between two primary impeller vanes  313 . As shown, the struts  28  are connected by an outer strut connection ring  29 . The strut connection ring  29  is arranged inside an inner circumferential surface of the pump casing  2  at the pump section end PSE. The impeller bearing ring  27  is supported by the struts  28 . It is conceivable to manufacture the strut connection ring  29  and the struts  28  as one piece. Preferably, also the impeller bearing ring  27  is a part of this piece. Said piece may also be formed in one piece with the pump casing  2 . 
       FIG.  6    shows a perspective view of the pump section end PSE of the pump section  3  in which the pump casing  2  is shown in transparency. Different from the embodiment shown in  FIGS.  3  to  5   , the inflow separator  26  comprises at least one, preferably three, cut-outs  261  at a downstream end of the inflow separator  26 . The cut-out  261  is arranged between two struts  28 . The impeller bearing ring  27  is part of and fixedly connected to the inflow separator  26 , and the cut-out  261  also extends through the impeller bearing ring  27 . Due to the cut-out  261 , the secondary impeller channel  321  has an increased cross section when it aligns with the cut-out  261  during rotation of the secondary impeller. The secondary impeller  32  extends inside the impeller bearing ring  27  in a direction toward the pump section end PSE maximally up to an end of the cut-out  261 . This has the effect that, in operation, an edge of the cut-out  261  runs over the inner impeller bearing surface  327  and removes blood clots at the beginning of their formation or preferably prevents their formation, since the mating part of the rotating axial thrust bearing surface  328  is in direct blood contact at the cut-out  261 . This helps to avoid stagnant blood within the axial thrust bearing. Also the inner impeller bearing surface  327  has edges  325 , as can be seen in  FIG.  7   , which have the effect to remove blood clots from the outer impeller bearing surface  277  ( FIGS.  8  and  9   ). 
       FIG.  7    shows the secondary impeller  32  in detail in a perspective view. There, the secondary impeller  32  is configured as an inlay and has roughly the form of a cylinder. It may be made of different material as the primary impeller  31 , for instance of a ceramic material. The inlay comprises a cylindrical section  323  which is arranged inside the secondary impeller cavity  312  of the primary impeller  21 . A circumferential protrusion  329  forms an axial stop for the secondary impeller  32  in the secondary impeller cavity  312 . The inner impeller bearing surface  327  is arranged at an outer circumference of the secondary impeller  32 . Two secondary impeller channels  321  are arranged at the end of the secondary impeller  32  toward the pump section end PSE. The secondary impeller channels  321  have their largest cross section at the upstream end of the secondary impeller  32 . Thus, the channels  321  decrease in cross section away from the blood flow inlet  21 . In this way, blood is directed from a mainly axial direction to an axial-radial direction when it flows through the secondary impeller channel  321 . 
     The secondary impeller channels  321  are arranged asymmetrically in regard to the axis of rotation  10  of the secondary impeller  32 . At an end of the secondary impeller  32  directed toward the blood flow inlet  21 , the axis of rotation  10  extends through one of the secondary impeller channels  321 . In this way, the center of rotation, which is located at the axis of rotation  10 , does not coincide with a solid part of the secondary impeller  32 . This has the advantage that blood clotting at the center of rotation, where no differential velocity to neighboring blood flow is present, can be avoided. 
     At the transition between the secondary impeller channels  321  and the inner impeller bearing surface  327 , edges  325  are arranged. As mentioned above, such edges  325  serve to push away formations of blood clotting on the outer impeller bearing surface  277 . The inner impeller bearing surface  327  provides an inner surface of a radial bearing at the pump section end PSE. The secondary impeller  32  further comprises an axial impeller bearing surface  328 . It is arranged at the circumferential protrusion  329 . The axial impeller bearing surface  328  foul&#39;s a part of the above-mentioned axial stop or axial thrust bearing. The axial stop may be configured as an axial bearing which is capable of transmitting forces from the secondary impeller  32  to the bearing ring  27  during rotation of the impeller. The axial bearing is necessary to counter the axial force which stems from the purging action of the impeller. 
       FIG.  8    shows an enlarged view of the impeller bearing ring  27 . The outer impeller bearing surface  277  is arranged at the inside of the impeller bearing ring  27 . The impeller bearing ring  27  comprises an axial bearing ring surface  278 . As shown, the axial bearing ring surface  278  may be arranged at an axial end of the impeller bearing ring  27 . 
       FIG.  9    shows a perspective view of an impeller bearing ring  27  according to a further embodiment which differs from the embodiment shown in  FIG.  8    in that it comprises the cut-outs  261 , as previously mentioned, which are arranged at a downstream end of the impeller bearing ring  27 . The number of cut-outs  261  preferably matches the number of struts  28 . 
       FIG.  10    shows a perspective view of a cross section through the drive section end DSE of the drive section  4 . Rotating parts are not shown cut. As shown, the ancillary blood flow ABF enters the pump casing  2  at the ancillary blood flow inlet  23 . The ancillary impeller  42  accelerates the blood, which continues to flow into the axial gap  401 . As is shown by the arrow ABF inside the axial gap  401 , the blood does not flow directly in the direction of the axis of rotation  10 , but rather has a strong circumferential flow component so that it flows along the axial gap  401  along helices. 
       FIG.  11    shows a perspective view of an end of the rotor  41  at the drive section end DSE of the pumping device  11 . The ancillary vanes  421  of the ancillary impeller  42  are clearly recognizable, and they extend straight in a radial direction. The ancillary impeller vanes  421  provide, at their outer circumference, the inner rotor bearing surface  4211  of the radial rotor bearing  47 . Further, the ancillary impeller vanes  421  each have a chamfer  4212 . This chamfer  4212  is advantageous in order to build a tapered drive section end DSE of the pumping device  11  as shown in  FIG.  10   . Further, the ancillary impeller vanes  421  comprise radially extending end surfaces  4214  at an axial end of the secondary impeller  42 . A hump  422  is formed at the center of the axial end of the secondary impeller  42 . The hump  422  interacts with the bearing pin  44 , as shown in  FIG.  10   . 
       FIG.  11 A  further shows the rotor bearing ring  43  to be arranged around the inner rotor bearing surface  4211  of the secondary impeller  42 . The outer rotor bearing surface  4311  of the rotor bearing ring  43  forms the rotor bearing  47  together with the inner rotor bearing surface  4211  of the ancillary impeller  42 . The ancillary impeller  42  has an axial Length L and a diameter D. Alternatively, as shown in  FIG.  11 B , the rotor bearing ring  43  may have cut-outs with a form, function and arrangement similar to the cut-outs  261  of above-described impeller bearing ring  27 . 
       FIG.  12    shows in a perspective view an end of the rotor  41  connected to the tapered section  314  of the primary impeller  31 . A tertiary impeller  242  is arranged between the tapered section  314  and the pump section end of the rotor  41 , and extends radially from an outer diameter of the rotor  41  to an outer diameter of the tapered section  314  to form a shoulder. An axial plane of this shoulder forms a rotatable wall  2411  of the radial gap  24 . From the rotatable wall  2411 , the tertiary impeller vanes  2412  project toward the drive section end DSE of the pumping device  11 . Preferably, the tertiary impeller vanes  2412  extend axially along the axis of rotation  10 . Particularly, the tertiary impeller vanes  2412  are straight and extend in a radial direction relative to the axis of rotation  10 . Further in the alternative, the tertiary impeller vanes  2412  can be omitted (not shown).