Abstract:
A rotary blood pump comprises a housing and a rotor. The housing includes a blood inlet, a blood outlet, a blood flow conduit disposed between the blood inlet and the blood outlet, and a rotary bearing assembly disposed within and fixed to the housing. The rotor is rotatably disposed within the housing and includes one or more impellor blades disposed within the blood flow conduit for pumping blood through the conduit, and a shaft affixed to and rotating with the rotor. The shaft rotatably engages the bearing assembly to define an intersection between the rotor and the housing and to provide relative rotation between the rotor and the housing. In addition, the shaft further defines an axis of rotation for the rotor. In the rotary pump of the invention, at least one of the rotor and the housing defines a swirl region proximate to the intersection between the rotor and the housing. The swirl region includes a curved surface adapted to cause blood being pumped through the blood flow conduit to swirl about an axis that is transverse to and spaced apart from the axis of rotation of the rotor. This swirl washes the intersection between the housing and the rotor, and further provides blood flow in a region that would otherwise have low flow.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to rotary blood pumps and more particularly to rotary blood pumps configured to minimize thrombus formation. As is known in the art, blood pumps are used to assist in bumping blood through the body. Blood pumps are often configured as rotary blood bumps of which there are two general categories: centrifugal and axial. In each of these categories of blood pumps, an impellor element rotates with respect to a stationary housing in order to impart motive power to the blood.  
           [0002]    An exemplary rotary blood pump of the centrifugal category is disclosed in U.S. Pat. No. 5,017,103(which is hereby incorporated by reference). This pump, known as the St. Jude pump after its developing institution, has a broad, relatively flat impellor situated within a housing that has inlet and outlet tube connector ports. The impellor has a generally disc-shaped lower body portion with vanes on its upper surface and an inlet at the center, so that blood entering at the inlet along a central rotation axis is urged radially outward by the vanes to exit at higher pressure along an outflow path at the disc periphery. A shaft extending through the bottom of the disc on the opposite side from the vaned top surface centers the assembly, with the shaft and corresponding bearings being located out of the blood flow path and shielded therefrom by seals. Multiple circumferentially-spaced ferromagnetic plates are embedded in the disc body portion, and the pump assembly is driven by a separate driver unit that fastens to the housing and rotates a similarly-poled magnetic disc positioned directly below and closely parallel to the impellor so that the driver disc magnetically engages the plates on the rotor.  
           [0003]    In general, rotary pumps have low flow in regions proximate to the center of rotation of their impellor component. For the exemplary pump of U.S. Pat. No. 5,017,103, this region is the region closest to the rotating shaft and bearings that support the rotating centrifugal impellor, however, such regions can also be present in axial flow pumps and in bearingless rotary pumps that do not employ rotating shafts. Regions of low flow in blood conducting elements give rise to the possibility of detrimental blood stagnation and thrombus formation. Accordingly, it is one goal of the present invention to ensure that such low flow regions in a rotary blood pump become washed by blood during the pumping process to reduce the possibility of these detrimental effects.  
         SUMMARY OF THE INVENTION  
         [0004]    In accordance with the present invention, a rotary blood pump is provided having a housing and a rotor. The housing includes a blood inlet, a blood outlet, a blood flow conduit disposed between the blood inlet and the blood outlet, and a rotary bearing assembly disposed within the housing. The rotor is rotatably disposed within the housing and includes one or more impellor blades located within the blood flow conduit for pumping blood through the conduit, and a shaft affixed to and rotating with the rotor. The shaft rotatably engages the bearing assembly to define an intersection between the rotor and the housing and to provide relative rotation between the rotor and the housing. In addition, the shaft farther defines an axis of rotation for the rotor. In the rotary pump of the invention, at least one of the rotor and the housing defines a swirl region proximate to the intersection between the rotor and the housing. The swirl region includes a curved surface adapted to cause blood being pumped through the blood flow conduit to swirl about an axis that is transverse to and spaced apart from the axis of rotation of the rotor. This swirl washes the intersection between the housing and the rotor, and further provides blood flow in a region that would otherwise have low flow.  
           [0005]    The rotor can also include a thick central region extending along the axis of rotation of the rotor in the direction of the shaft. The thick central region can have a predetermined thickness extending outward from the axis of rotation to fill a low flow region along the axis of rotation which can resemble the “eye” of a storm. The thick central region can also define at least a portion of the swirl region.  
           [0006]    In further embodiments, the impellor can include a central hub located on the axis of rotation of the rotor and a plurality of impellor blades extending outwardly from the central hub, the impellor blades being separated by channels that allow blood to flow through the rotor in a direction along the axis of rotation of the rotor. The impellor blades in this configuration can have a cross-sectional shape proximate to the central hub designed to encourage blood to flow in a direction parallel to the axis of rotation of the rotor— down into the swirl region. Still further, the blood inlet can be disposed proximate to the axis of rotation of the rotor and angled so as to direct blood flowing into the rotary pump in a direction parallel to the axis of rotation of the rotor. The cross-sectional shape of the impellor blades proximate to the central hub can be tilted to match the velocity of the blood flowing from the blood inlet into the rotor, thus further encouraging blood to flow over the hub and into the swirl zone.  
           [0007]    A further flow enhancing feature that can be included in a rotary pump of the invention is to provide a cross-sectional flow area through an intermediate portion of the blood flow conduit that is smaller than a cross-sectional flow area of the blood inlet. This encourages blood flowing through the pump to speed up regardless of the effects of the impellor blades. In one embodiment, the cross-sectional flow areas through the intermediate portion of the blood flow conduit and blood inlet are sized so that for a nominal blood flow of approximately 5 liters per minute through the rotary blood pump, flow velocity of blood through the intermediate portion of the blood flow conduit is approximately 2 meters per second.  
           [0008]    These and other features of the invention may be combined or used singly to enhance blood flow through a rotary blood pump and to reduce low flow regions that can lead to problematic thrombosis. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIGS. 1 and 1A illustrate cross-sectional views of a rotary pump according to the invention;  
         [0010]    [0010]FIGS. 2 and 2A illustrate side and top views respectively of a rotor used in the rotary pump of FIG. 1;  
         [0011]    [0011]FIGS. 3 and 3A are cross-sectional views of intersection configurations between a rotor and housing of a rotary pump of the invention; p FIG. 3B is a cross-sectional view of a blood flow region of a rotary pump of the invention;  
         [0012]    [0012]FIG. 3C is a cross-sectional view of the rotor of FIG. 3B taken along line C-C;  
         [0013]    [0013]FIG. 4 is a cross-sectional view of a shaft and bearing assembly of a rotary pump of the invention;  
         [0014]    [0014]FIG. 4A is a side view of an axial thrust bearing shown in FIG. 4;  
         [0015]    [0015]FIGS. 4B and 4C are side views of further axial thrust bearings useful with a rotary pump of the invention;  
         [0016]    [0016]FIG. 5A is a diagrammatic representation of a stator coil configuration of the rotary pump of FIG. 1; and  
         [0017]    [0017]FIG. 5B is a diagrammatic representation of a further stator coil configuration useful with a rotary pump of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The present invention provides structures and techniques for improving the flow of fluid within a rotary pump so as to reduce or eliminate low flow regions, particularly in areas where rotating portions (such as a rotating impellor) of the rotary pump are proximate to a stationary portion of the rotary pump (such as a housing containing the impellor). These structures and techniques can be particularly important in a blood pump as regions of low flow can give rise to thrombus formation. While the primary embodiment disclosed below is a centrifugal pump having an impellor supported by a shaft and bearing arrangement with a housing, the principles of the invention can readily be applied to axial flow pumps and to rotary pumps not having supporting shafts as well.  
         [0019]    [0019]FIGS. 1 and 1A illustrate a rotary pump  10  of the invention. Rotary pump  10  is a centrifugal blood pump having a stationary housing  12  and a rotating impellor or rotor  26 . Housing  12  includes a blood inlet conduit  14 , a blood outlet conduit  16 , a blood flow conduit  18  disposed between the blood inlet conduit  14  and the blood outlet conduit  16 , and a bearing receiving bore  20  having a bearing assembly  22  disposed therein.  
         [0020]    Exemplary rotor  26  (further illustrated in FIGS. 2 and 2A) is configured to be disposed within blood flow conduit  18  of housing  12  and to draw blood in through housing inlet  14  and expel blood out through housing outlet  16  upon rotation of rotor  26  (in a counter-clockwise direction in the embodiment illustrated in FIG. 2A). Rotor  26  includes a plurality of impellor blades  28  connected at a central hub  60  and through connecting elements  62 . Impellor blades  28  are large enough to carry magnets  52  and are spaced apart by channels that are made small enough to maintain a nearly constant cross-sectional area as measured normal to the flow of blood through pump  10 . Gaps between impellor blades  28  and housing  12  may generally be maintained in the range of between about 0.020 and 0.050 inches. Such gap distances are small enough to maintain reasonable efficiency and to keep shear rates large enough at part surfaces to reduce or prevent thrombosis. On the other hand, the distance across such gaps should be large enough so that shear rates do not lead to problematic hemolysis. In addition, the geometric tolerances should not be so tight that a small amount of wear leads to failure. A person of ordinary skill in the art will recognize that the number of impellor blades  28 , the shape of impellor blades  28 , as well as the connecting features between impellor blades  28  can be varied within the spirit of the present invention.  
         [0021]    Rotor  26  is also connected to and rotates with shaft  30 , which interacts with bearing assembly  22  to allow for relative movement between rotor  26  and housing  12 . Illustrated bearing assembly  22  comprises toroidal bearing elements  32  and an axial thrust element  34 . Bearing assembly  22  is further described and alternative bearing configurations are provided below by reference to FIGS.  4  to  4 C, however, a person of ordinary skill in the art will recognize that a variety of bearing assemblies, including those compatible with magnetic and hydrodynamic bearing support, can be used to allow relative motion between rotor  26  and housing  12  within the spirit of the invention.  
         [0022]    An exemplary motor is provided integrally with the housing  12  and rotor  26  to drive the rotation of rotor  26 . In the illustrated embodiment, permanent magnets  52  are provided on each of impellor blades  28  while stator windings  54  may be provided integrally with housing  12 . As further illustrated in FIGS. 5A and 5B, the motor stator may be a split stator including windings  54  on either side of magnets  52  (illustrated in FIGS. 1 and 5A with stator windings  54  on the inflow  14  and bearing assembly  22  sides of rotor  26 ). Alternatively, the motor stator may include a flux return path  94  on the inflow  14  side of rotor  26  (FIG. 5B) with stator windings  54  provided on the bearing assembly  22  side. By providing ferromagnetic material on both sides of the rotor  26 , the total axial thrust can be minimized, thereby reducing the wear rate on any axial bearings. Heat generated in stator windings  54  can be dissipated across a large area of housing  12  into regions of very high blood flow and high surface shear. In this situation several watts of heat may be dissipated with no measurable heating effect on the blood.  
         [0023]    These motor arrangements are provided for illustrative purposes only as a person of ordinary skill in the art will be able to vary the motor configuration in a variety of ways consistent with the spirit of the invention, including for example the motor configurations provided with the centrifugal pump of Hart et al. as described in U.S. Pat. No. 6,071,093 which is hereby incorporated by reference. Where pump  10  is used as an implanted blood pump, the motor preferably includes stator windings  54  integral with housing  12 . Where pump  10  is used as an external blood pump, an external rotating magnet drive (such as that illustrated in Hart et al.) may preferably be employed to drive rotor  26 .  
         [0024]    Pump  10  of the invention includes several features configured to address the problem of low flow regions within known pumps. For example, in known rotary pumps, low flow is typically encountered in regions along an axis of rotation  36  of rotor  26 , particularly where a portion of rotor  26  along axis  36  is proximate to a stationary portion of the pump such as housing  12 . In this region, fluid in the pump is swirling in the same direction in which rotor  26  is spinning and, like the linear velocity of any point on the rotor, the velocity of a typical fluid “particle” will correspond at least in part to the angular velocity of the swirl multiplied by the distance of the particle from the center or axis  36 . Approaching axis  36 , this component of the velocity of the fluid approaches zero and an area of flow stagnation exists. This phenomenon bears some similarity with “the eye of a storm,” where winds in a storm may swirl at high velocity at a distance apart from a central axis of the storm, but may be quite calm proximate to the axis.  
         [0025]    One such feature illustrated in pump  10  (most clearly shown in FIGS. 1A, 3 and  3 A) is a thick central region  38  formed along central axis  36  on a side of rotor  26  proximate to stationary housing  12 . Central thick region  38  “fills” the “eye” region present along the central axis, helping to ensure that only the “swirl” portion of the flow remains between rotor  26  and housing  12 . The thickness of central thick region  38  can be determined experimentally for a given pump configuration by emperically determining the diameter of the “eye,” and providing the central thick region  38  with a diameter at least as large as that determined for the eye.  
         [0026]    Another feature designed to reduce low flow regions is the definition of a swirl zone  40  proximate to central axis  36 . Where a central thick region  38  is present, swirl zone  40  may preferably be formed by providing a surface  42  on central thick region  38  that encourages a flow of pumped fluid that swirls about an axis  43  (axis  43  extends out of the page in FIG. 3) that is transverse to and spaced apart from central axis  36  as indicated by flow arrows  44  (FIG. 3). Viewed in its entirety around the circumference of surface  42 , swirl region  40  takes a toroidal shape centered on axis  36 . Swirl region  40  is bounded on one side (a superior side) by rotating rotor  26  and on an opposed side (an inferior side) by stationary housing  12 . As a result, pumped fluid will be forced outward (away from central axis  36 ) along a region proximate to the rotor  26 , causing the swirl indicated by arrows  44 .  
         [0027]    Still another such feature is the configuration of cross-sectional flow areas through blood flow conduit  18  to encourage higher velocity fluid flow. In general, reducing the cross-sectional area through which a fluid flows results in a higher fluid velocity as the same volume throughput (assuming an incompressible or nearly incompressible fluid) of fluid must pass through a smaller area. Referring to FIG. 3B, the cross-sectional flow area through cross-section A is configured (with consideration given to the cross-sectional area taken up by rotor  26 ) to be smaller than the cross-sectional flow area through the pump inlet  14 . This reduction in cross-sectional area results in greater fluid velocity through cross-section A, which in turn promotes a higher velocity swirl in swirl region  40  as well as higher velocity flow in other regions of potentially low flow, such as regions surrounding the junction between impellor blades  28  and rotor hub  62 . Preferably, where the pump is used as a blood pump providing physiologically appropriate levels of blood flow, the cross-sectional flow areas are sized so that, at a nominal blood flow of 5 liters per minute, flow velocity through cross-section A is approximately 2 meters per second. This limitation of cross-sectional flow area through cross-section A also provides a down-stream constriction that encourages the formation and continuation of fluid swirl  44  in swirl zone  40 .  
         [0028]    Another feature for reducing low flow regions is illustrated in FIG. 3C which shows a cross-section taken along line C-C of FIG. 3B. FIG. 3C illustrates, in particular, the cross-sectional shape of an impellor blade  28  in a region proximate to hub  62 . Impellor blade  28  is tilted as it emerges from hub  60  so that it matches the velocity  64  (having lateral  66  and vertical  68  components) of the blood or other fluid as it approaches the impellor vanes  28 . In this way, the flow of fluid over the impellor blades  28  near their junction with hub is encouraged, as well as the downward flow of fluid into swirl region  40 . This configuration also tends to avoid dead spots or areas of potential stagnation on the back side of impellor blades  28 .  
         [0029]    Another characteristic of rotary pumps addressed by the present invention is the location of, and fluid flow across, any gaps that might exist at junctions between rotating and stationary parts. While some rotary pumps will not have such gaps (for example those supported solely by hydrodynamic or magnetic bearings), pumps having shaft  30  and bearing assembly  22  arrangements will have a gap  46  between rotating parts (such as rotor  26  and shaft  30 ) and stationary parts (such as housing  12  supporting bearing assembly  22 ). In the embodiment illustrated in FIGS. 3 and 3A, housing  12  provides one surface  48  defining one side of gap  46  while rotor  26  and/or shaft  30  provide an opposed surface  50 . Gap  46  terminates in a mouth  70  that is exposed to blood flow in the vicinity of swirl region  40 . FIGS. 3 and 3A illustrate two different configurations for exposing mouth  70  to swirl region  40 .  
         [0030]    The distance across gap  46  (denominated as A in FIG. 3, and B in FIG. 3A) should be small enough to maintain reasonable efficiency and to keep shear rates large enough at surfaces  48 ,  50  to prevent thrombosis. The size of gap  46 , along with the amount of washing provided over mouth  70 , will determine the size of any clots that may be released into the blood flow as a result of any blood trapped in a volume around rotor  26 . By providing and enhancing swirl region  40  and exposing mouth  70  to swirl region, the size of any released clots can be minimized for practical gap  46  sizes. Distance A across gap  46  of FIG. 3 may be made as small as the tolerance on radial bearings  32  will allow, typically on the order of 25 microns. An axially configured gap distance (distance B across gap  46  of FIG. 3A) can be as small as the wear expected on axial bearing  34  over the expected life of pump  10 . Distance B can be effectively reduced (and under some circumstances made effectively zero) by either allowing the materials lining gap B to wear at the same rate as axial bearing  34 , or by forming an axial thrust hydrodynamic bearing across gap B by machining wedges into the surface of one or the other of the surfaces opposed across the gap.  
         [0031]    In the illustrated embodiments, no seal is provided to isolate bearing  32  surfaces from blood, although a person of ordinary skill in the art will recognize that a seal could be put in place if needed. The spaces around bearings  32  may be made small enough so that there is no significant volume for infection. Further, blood products that deposit around the shaft  30  likely will not add to bearing  30  friction or wear.  
         [0032]    Exemplary bearing assembly  22 , illustrated in place within a blood pump of the invention in FIGS. 1 and 1A, is illustrated in greater detail in FIG. 4. Bearing assembly  22  supports shaft  30  of rotor  26 , and accordingly it supports rotor impellor  28  and provides forces to constrain the position of the impellor axially and radially. In the exemplary bearing assembly, a set of radial bearings  32  having a toroidal shape is held in place by spacers  72  along shaft  30 , as shown in FIG. 4. An axial thrust bearing  34  located beneath shaft  30  takes up the thrust generated by the electro-magnetic driver provided by rotor  26  mounted permanent magnets  54  (FIG. 1) and electric coils  52  mounted in housing  12 .  
         [0033]    Three possible configurations for axial bearing  72  are shown in FIGS.  4 A- 4 C. One configuration (the “sliding” embodiment of FIG. 4A; also the configuration illustrated in FIG. 4) includes a rotating contact area  80  disposed on the end of shaft  30  which slides on a stationary (i.e., housing affixed) pedestal  82 . In the illustrated embodiment, the diameter of shaft  30  is 0.05 inches, however, this diameter may be increased in order to provide a larger contact area  80  and thus improve axial wear rates. In one exemplary pairing of bearing materials, at least contact area  80  of shaft  30  can be formed of zirconia, a ceramic, to slide on a pedestal  82  formed of sapphire. In another potential combination, a zirconia contact area  80  can slide against a pedestal  82  of ultra-high molecular weight polyethylene (UHMWP). This combination of materials is known to have a very low wear rate.  
         [0034]    A second illustrated axial bearing  34  configuration (the “rolling” configuration of FIG. 4B) is one in which a rolling ball  84  is moveably disposed between the shaft  30  and a housing affixed pedestal  86 . For typical sizes, speeds and loads as described herein, linear velocities between moving parts in this configuration may be as low as 50 mm/s and axial loads near 1 Newton.  
         [0035]    A third potential axial bearing  34  configuration (the “magnetic” configuration of FIG. 4C) provides an axial bearing thrust bearing  34  using a pair of small permanent magnets  88 ,  90 . In this embodiment, magnet  88  is affixed to the end of shaft  30  and magnet  90  is affixed to the housing. It is an important consideration for this axial bearing configuration that radial bearings  32  hold shaft  30  precisely on center so that magnets  88 ,  90  provide the desired axial forces.  
         [0036]    Each of the three illustrated axial bearing  34  configurations should provide for absorption of axial shocks. In the sliding (FIG. 4A) and rolling (FIG. 4B) embodiments, axial shocks can be absorbed by providing a relatively soft material to support pedestals  82 ,  86 . In the case of the magnetic axial bearing (FIG. 4C), damping of axial shocks occurs as fluid is pushed in and out of a gap  92  between magnets  88 ,  90 . It is also important to dissipate heat that may be generated by radial  32  and/or axial  34  bearings having contact surfaces that may generate frictional heat by directing that heat to an area of the pump that is well washed by moving blood. This can be accomplished by making sure that the external surfaces of metal parts in thermal contact with such radial bearings reach locations within blood flow conduit  18  (FIG. 1A) having areas of high surface shear with respect to blood flowing over the surface.  
         [0037]    Referring again to FIG. 1, pump  10  can be assembled by inserting rotor  26  into bearing assembly  22  which can be located in a bottom portion  96  of housing  12 . A top portion  94  of housing  12  can then be disposed on bottom portion  96  and fastened there by bolts  98 . In addition to gap  46 , this construction of housing  12  further results in the creation of seam  100 . Preferably, this seam is located within 0.5 mm of a surface of impellor blades  28  so that seam  100  is continuously washed at the point where it contacts blood.  
         [0038]    A person of ordinary skill in the art will appreciate further features and advantages of the invention based on the above-described embodiments. Accordingly, the invention is not to be limited by what has been particularly shown and described, except as indicated by the appended claims. All publication and references cited herein are expressly incorporated herein by reference in their entity.