Abstract:
In a centrifugal flow blood pump, usable in left ventricular assist applications, blood is pumped from an inlet ( 16 ) to an outlet ( 22 ) by a primary impeller ( 18 ). A portion of the blood that enters the pump follows a secondary channel ( 24 ) where a secondary impeller ( 70 ) routes the blood to lubricate a bearing between an impeller assembly ( 14 ) and a post formed by a component of the pump housing. The unique shape of the secondary impeller ( 70 ) prevents blood stagnation and provides for a well-washed fluid bearing.

Description:
FEDERAL RESEARCH STATEMENT 
   The U.S. Government may have certain rights in this invention pursuant to contract number N01-HV-58159 awarded by the U.S. National Heart, Lung and Blood Institute of the National Institutes of Health. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to the medical arts. It finds particular application in cardiac assist technologies using rotodynamic blood pumps, also known as left ventricular assist devices (LVAD) in assisting patients with failing hearts and will be described with particular reference thereto. It is to be appreciated that the present invention is also applicable to other types of pumps, and is not limited to the aforementioned application. 
   Rotodynamic pumps (axial flow, mixed flow, and centrifugal) have prospective applications in cardiac assist technologies. A typical cardiac assist system includes the blood pump itself, electric motor (usually a brushless DC motor integrated into the pump), drive electronics, microprocessor control unit, and an energy source, such as rechargeable batteries. These pumps can be used in fully implantable systems for chronic cardiac support where the whole system is located inside the body and there are no drive lines penetrating the skin. For more temporary support, the pump is located inside the body but some system components, including drive electronics and energy source, may be placed outside the patient body. 
   The inverted, shaftless, brushless motor design is utilized because it has a significant advantage over typical motor/drive shaft configurations. There are no openings in the housing that would allow blood into the motor, and the housing precludes air or other fluid from entering the bloodstream. A primary drive impeller of the pump encloses a drive magnet and is driven by a stator and coil assembly disposed radially inward from the motor rotor, i.e., an inverted motor. In order to avoid friction and subsequent heat buildup, the blood of the patient is used as a fluid bearing between the impeller and the stator. 
   A potential problem with this system is that the blood can become heated and/or stagnant, and partially solidify by forming a thrombus or heat coagulation of blood proteins on the stator housing surface or on the secondary impeller of the motor rotor in the inverted fluid film bearing assembly. Such a situation is undesirable and potentially life-threatening to the patient who is dependant on the proper function of such a device. Accordingly a need exists for a well-washed or continuous flow of blood that serves as the bearing between the rotor and stator components. 
   The present invention provides a new and improved method and apparatus that avoids thrombus and/or coagulated protein formation/deposition and overcomes the above referenced problems and others. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, a cardiac assist device is provided. A drive stator is received within a housing along with an impeller assembly. The impeller assembly includes primary and secondary impellers, and a drive rotor. The primary impeller provides a motive force which transports blood from an inlet port to an outlet port. The secondary impeller cycles blood to lubricate and cool a bearing between the impeller assembly and the stator housing. 
   In accordance with a more limited aspect of the present invention, the secondary impeller comprises radial vanes that exhibit symmetry relative to radii extending from a center of the impeller assembly. 
   In accordance with another aspect of the present invention, a left ventricular assist device is provided. A brushless DC motor and an impeller assembly are contained within a volute housing assembly. The impeller assembly comprises a primary impeller, an annular magnet drive rotor, and a secondary impeller. The secondary impeller comprises a plurality of radial vanes that are smooth and rounded, with an axial height at an outer radius greater than an axial height at an inner radius. 
   One advantage of the present invention is a blood pump with a single moving part with no seal between the motor and blood compartments. 
   Another advantage resides in avoiding problems associated with drive shaft interfaces. 
   Another advantage is the creation of blood flow and wash patterns that avoid the formation of blood element depositions. 
   Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
       FIG. 1  is a cross-sectional view of a blood pump in accordance with the present invention; 
       FIG. 2A  is a cross-sectional view of a volute housing assembly in accordance with the present invention; 
       FIG. 2B  is a cross-sectional view of the volute housing assembly taken generally along the lines  2 B— 2 B of  FIG. 2A ; 
       FIG. 3  is a cross-sectional view of a stator assembly in accordance with the present invention; 
       FIG. 4A  is an elevational view of an impeller assembly, particularly illustrating the primary impeller, in accordance with the present invention; 
       FIG. 4B  is a cross-sectional view of the impeller assembly taken generally along the lines  4 B— 4 B of  FIG. 4A ; 
       FIG. 5A  is an elevational view of a secondary impeller in accordance with the present invention; 
       FIG. 5B  is a cross-sectional view of the secondary impeller taken generally along the lines  5 B— 5 B of  FIG. 5A . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIG. 1 , a centrifugal flow blood pump comprises three primary subassemblies, namely a volute housing assembly  10 , a stator assembly  12 , and a rotating assembly or rotor  14 . Blood from a patient flows into an inlet port  16  of the blood pump. Arrows (unnumbered) indicate the direction of travel of the blood through the pump in the preferred embodiment. The blood specifically by primary impeller  18 . The blood proceeds around the volute housing, first entering a volute channel  20  and a primary portion exiting the pump via a discharge port  22 . 
   A small portion of the blood flows into a second channel or passage  24 , specifically a first passage portion  24   a  that extends axially from a rear face of the primary impeller and is radially interposed between the rotor and a post formed by an axial extension  10   a  of the stator housing that protrudes into a pump chamber defined in the volute housing. As is well known, the chamber is in fluid communication with the inlet and outlet, and the primary impeller pumps the blood from the axial inlet to the tangential outlet. The secondary impeller (to be described in greater detail below) is provided at an opposite end of the rotor assembly remote from the primary impeller. A second passage portion  24   b  of the second channel defined at a second end of the rotor, i.e., remote from the primary impeller, continues from the first passage portion  24   a  and flows radially inward through the secondary impeller toward a rotational axis of the rotor. The small portion of blood flow then proceeds axially along a third passage portion  24   c  between the rotor  14  and the axial extension of the housing. The secondary channel thus forms a fluid or blood bearing that is continuously renewed during operation of the pump. Blood exits the bearing near the primary impeller  18  and is replenished by new blood flowing through the secondary channel  24 . 
   As illustrated in  FIG. 1 , the cross-sectional dimension of the second channel varies from one portion to the next. Particularly, the channel has the largest dimension along the first portion  24   a  and the smallest dimension along the third portion  24   c . The channel dimensions, in conjunction with the impeller geometry, speed, number of blades, clearance, pressure gradient, and flow recirculation, provide parameters for an effective fluid film bearing free of blood element deposition. 
   A jacketed cable  26  is received through a base portion  28  of the housing for connection with the stator assembly  12  contained in the axial extension. The cable carries power and control connections to and from the pump particularly to the stator assembly  12 . 
   With reference to  FIGS. 2A and 2B , the volute housing assembly  10  in the preferred embodiment is formed by a single casting that is subsequently cut into two separate pieces before being joined again to form a one-piece housing. When separated, the individual pieces are machined and polished, removing any casting imperfections or anomalies within the volute section  20 , prior to being welded together. Alternately, the volute housing assembly can be a single cast component, with no seams to weld. Preferably, the volute housing assembly  10  is made of titanium with walls approximately 2.5 mm thick. Both the inlet and discharge ports  16 ,  22  are configured for coupling  34  to adjacent fluid lines or passages. For example, the housing is externally threaded at each port for ease of connection with a corresponding female threaded coupling to provide a secure, sealed interconnection. The ports are preferably equipped with a guide surface or fitting pilot  36  for orienting connection to a fluid conduit (not shown). 
   The inlet port  16  communicates with an inlet throat  38  that has a slightly smaller diameter than the inlet port. In this manner, blood passes through the throat and is accelerated. This reduces pre-whirl of the blood entering the impeller  18 . A conical diffuser  40  leading to the discharge port  22 , and downstream of the primary channel, decreases the velocity of the pumped blood before it enters the aorta. In the preferred embodiment, the conical diffuser  40  widens to the discharge port  22  at an included angle of approximately 7°, although other diffuser angles and configurations can be used without departing from the scope and intent of the present invention. 
   A recess  42  is included in a base of the volute housing body  30 . Inserts  44 , are circumferentially spaced about the housing and adjusted to receive fasteners after the impeller assembly  14  and the stator assembly  12  have been inserted into the housing assembly. A volute tongue  46  extends inwardly from the housing along a tangent with the rotor to separate the diffuser from the pump chamber and direct the blood into the conical diffuser  40  at the end of the primary channel. 
   With reference to  FIG. 3 , stator windings  50  are located within the axial extension of the housing. An electrical connector  52  represented at a distal end of the jacketed cable  26  connects the pump to a power supply and control circuitry (not shown). In the preferred embodiment, the stator windings  50  are connected to the power supply which is located outside the body of the patient. The stator windings  50  and electrical connections are inserted into the stator housing axial extension and multiple inserts or shims  54  disposed about the stator windings  50 , adjust position and ensure a tight and secure fit of the windings within the axial extension. The housing cover  28  is secured to the axial extension of the housing  10  with an attachment device  56 , preferably a single fastener or screw that locks into a self-locking helical receptacle  58  at an opposite end of the axial extension. Seal members such as O-rings  60 ,  62  seal any possible apertures through which body fluids might enter the housing. 
   It will also be appreciated from a close examination of  FIG. 3  that the stator assembly is offset within the axial extension. That is, the wall thickness of the axial extension differs over its circumferential extent. For example, the wall thickness along the top (as viewed in  FIG. 3 ) is less than the wall thickness along the bottom. This provides a purposeful offset for controlling motion of the rotor and controlling the fluid film bearing formed between the rotor and housing. More particular details of this offset feature are shown and described in U.S. Pat. No. 5,324,177, which is hereby incorporated by reference. 
     FIGS. 4A and 4B  illustrate three main features of the impeller assembly  14 , namely the primary impeller  18 , a secondary impeller  70 , and an annular magnet  72 . The primary impeller  18  includes multiple blades, e.g. seven blades, shaped such that together, the primary blades provide a mixed flow, i.e., combined axial and radial flow. The annular magnet  72  extends around a circumference of the impeller assembly  14  and mates with the post containing the stator windings  50  of the stator housing  12 . The annular magnet  72  is preferably magnetized in a longitudinal, circumferentially spaced pattern, commonly known as a four pole pattern. Alternately, a plurality of individual magnets can be arranged in a similar pattern. The annular magnet  72  is inserted into the impeller assembly, sealing the magnet  72  within the rotor assembly envelope formed between the primary  18  and secondary  70  impellers. The assembly is welded or otherwise bonded shut. 
   Details of the secondary impeller  70  disposed at one end or on a base of the impeller assembly  14 , are more particularly illustrated in  FIGS. 5A and 5B . The secondary impeller  70  comprises a plurality of straight, radial blades  80 , nine blades in the preferred embodiment. Each blade  80  has a rounded outboard tip  82  that is approximately twice the height of a radial inner portion  84 . The height difference is realized with a revolved scallop cut, shaping each of the blades  80  similarly. All transitions or edges of the blades are gradual, avoiding sharp corners or other crevices, wherein blood can become lodged. The preferred embodiment of the secondary impeller  70  as described establishes a rotor-balancing pressure distribution across the back of the rotating assembly while allowing a radial inflow of blood to continually wash through the secondary channel  24  between the impeller assembly  14  and the housing. The particular shape of the secondary impeller blades  80  keeps the blood moving to prevent stagnation/long residence times and the blades  80  free of thrombus formation. 
   In the preferred operation of the blood pump, the secondary impeller  70  establishes a radial pressure gradient across the base of the impeller assembly  14 , such that control of rotating assembly hydraulic thrust and a differential pressure across the bearing is achieved. The pressure gradient and circulation of blood around the vanes help to avoid thrombus formation on the impeller and within the bearing. 
   The blood flow through the secondary channel  24  supplying the bearing is very low, relative to the flow through the primary channel  20 . The design of the secondary impeller allows a balance between bearing flow, and creating too great an axial hydraulic loading. The thrust resulting from axial hydraulic loading is balanced by the axial magnetic stiffness of the motor components. The pressure at the outboard tips  82  of the blade is essentially equal to and fixed at the pressure at the primary impeller  18 . A radial pressure gradient is created inboard of the secondary impeller tips. The higher the gradient, the lower the pressure at the secondary end of the bearing. If the pressure gradient is equal to the opposing primary impeller, then both the hydraulic thrust and the net bearing pressure and flow are zero. If the pressure gradient is too low, then both the bearing flow and hydraulic thrust on the impeller assembly  14  increases. 
   The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.