Abstract:
A pump includes: (a) an elongated pump housing having first and second ends; (b) a primary impeller mounted in the housing for rotation about an axis, the impeller comprising a plurality of vanes whose outer tips define an impeller plane; (c) an inlet disposed in fluid communication with the primary impeller; and (d) an annular volute housing communicating with the primary impeller and with an outlet, where the volute housing is axially offset from the impeller plane.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to pumps, and more particularly to centrifugal pumps used in medical applications. 
         [0002]    It is known to use centrifugal pumps as cardiac assist devices, also known as left or right ventricular assist devices (“LVAD” or “RVAD”). In such applications, the pump is implanted in the patient along with a power source and a control system. Alternatively, the power source and control system may be located externally. 
         [0003]    A centrifugal pump includes a rotating impeller contained in a housing which defines an inlet, and an annular chamber which surrounds the impeller, which is commonly referred to as a “volute”. Fluid flow enters the impeller near its center and exits from the periphery of the impeller. The flow exiting the impeller is collected in the volute and channeled to an outlet. Conventional centrifugal pump design places the volute section in axial alignment with the outside diameter of the impeller. This results in a very short fluid residence time in the impeller and volute, and a greater residence time of recirculating fluid in the more remote sections of the pump. 
         [0004]    When used as a blood pump for a ventricular assist system, extended residence time of blood within a pump can cause thrombus (clot) formation, and hemolysis (damage of red blood cells), both of which are undesirable. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    These and other shortcomings of the prior art are addressed by the present invention, which provides a centrifugal pump that minimizes residence time of fluids therein. 
         [0006]    According to one aspect of the invention, a pump includes: (a) an elongated pump housing having first and second ends; (b) a primary impeller mounted in the housing for rotation about an axis, the impeller comprising a plurality of vanes whose outer tips define an impeller plane; (c) an inlet disposed in fluid communication with the primary impeller; and (d) an annular volute housing communicating with the primary impeller and with an outlet, where the volute housing is axially offset from the impeller plane. 
         [0007]    According to another aspect of the invention, a cardiac assist device includes: (a) an elongated housing having first and second ends; (b) a primary impeller mounted in the housing for rotation about an axis, the impeller defining an impeller plane; (c) an inlet disposed in fluid communication with the primary impeller; and (d) an annular volute housing communicating with the primary impeller and with an outlet, where the volute housing is axially spaced away the impeller plane. The housing, the primary impeller, the inlet, and the volute housing are constructed from biologically compatible materials. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0009]      FIG. 1  is a side view of a centrifugal pump constructed according to an aspect of the present invention; 
           [0010]      FIG. 2  is a cross-sectional view of the centrifugal pump of  FIG. 1 ; and 
           [0011]      FIG. 3  is a cross-sectional view of a prior art centrifugal pump. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS. 1 and 2  depict a centrifugal pump  10  of the type used to pump blood or similar products. The pump  10  includes a pump housing  12  with opposed first and second ends  14  and  16 , and a central axis “A”. 
         [0013]    In the illustrated example the pump housing  12  is split into a body  18  and a separate cover plate  20 . The cover plate  20  closes off the second end  16  and may be secured to the body  18  by one or more fasteners, for example. 
         [0014]    A central portion of the pump housing  12  is generally cylindrical. The first end  14  of the pump housing  12  defines a centrally-located, axially-aligned inlet  22  of a conventional profile with a throat  24  and a generally conical portion  26 . 
         [0015]    A stator housing  28 , which may be integral with the cover plate  20 , extends from the cover plate  20  into the center of the pump housing  12 . The distal end of the stator housing  28  terminates in a conical surface  30 . An electrical stator  32  comprising a plurality of coil windings is contained in the interior of the stator housing  28 . A cable  34  which penetrates the cover plate  20  provides electrical connections for power, control, and sensing functions to the stator  32 . 
         [0016]    A rotor  36  is disposed in the pump housing  12 , surrounding the stator housing  28 . The rotor  36  is generally cylindrical with first and second ends  38  and  40  corresponding to the first and second ends  14  and  16  of the pump housing  12 . The rotor  36  includes a primary impeller  41  at its first end  38  which comprises an annular array of vanes located between the inlet  22  and the conical surface  30 . The outer tips of the vanes of the primary impeller  41  lie generally within an impeller plane, which is shown schematically at “P” in  FIG. 2 . One or more permanent magnets  42  are disposed in an annular array within the walls of the rotor  36 . A secondary impeller  44  comprising an annular array of vanes is located at the second end  40  of the rotor  36 . The rotor  36  and the stator  32  operate as a brushless DC motor through the application of varying electrical currents to the stator  32  through the cable  34 , in a known manner. 
         [0017]    All of the portions of the pump  10  which will come into contact with blood or tissue, including the pump housing  12  and the rotor  36 , are constructed from known biologically compatible materials such as titanium, medical grade polymers, and the like. 
         [0018]    Together, the stator housing  28  and the rotor  36  are configured so as to operate as a hydrodynamic bearing for the rotor  36  in operation. Specifically, the secondary impeller  44  causes a small portion of the blood flowing through the primary impeller  41  to flow axially to the cover plate  20 , radially inward through the secondary impeller  44 , and axially towards the primary impeller  41  between the rotor  36  and the stator housing  28 . This bearing and recirculation function is explained in more detail in U.S. Pat. No. 7,189,260 to Horvath, et al. 
         [0019]    The pump housing  12  includes an annular passage which collects the flow exiting the primary impeller  41  and channels it to a single outlet  46  (see  FIG. 1 ). This passage is referred to as a “volute” or volute housing  48 . As shown in  FIG. 2 , the axial position of the volute housing  48  is substantially offset away from the plane P of the primary impeller  41  and towards the cover plate  20 . The actual offset distance between the impeller plane P and the midplane “V” of the volute housing  48 , denoted “D”, is not a critical dimension, however generally the volute housing  48  is offset as much as possible towards the cover plate  20  within the physical constraints of the pump housing  12  and the walls of the volute housing  48 . In the illustrated example, the midplane V of the volute housing  48  is located approximately halfway between the impeller plane P and the second end  16  of the pump housing  12 . 
         [0020]    This positioning of the volute housing  48  is in substantial contrast to a conventional centrifugal pump design. For Example,  FIG. 3  illustrates a prior art centrifugal pump  110  having a pump housing  112 , a primary impeller  141 , and a volute housing  148 . It can be seen that the volute housing  148  and the outer vane tips of the primary impeller  141  line substantially in a single plane, denoted “P′”. 
         [0021]    Surprisingly, it has been found that the offset position of the volute housing  48  greatly decreases fluid residence time during operation of the pump  10 . By “residence time” it is meant the duration that a specific, identifiable volume of fluid remains within the pump  10 , from the time it enters the inlet  22  until it finally exits the outlet  46 . Residence time is not necessarily related to the average mass or volume flow rate. For example, it has been found that the prior art pump  110  may exhibit a relatively long residence time. Flow visualization has revealed that peak residence time in the pump  10  is approximately cut in half as compared to the prior art pump  110 . 
         [0022]    Despite the unconventional placement, overall pump performance is maintained across its operating range. Mechanical efficiency of the pump  10  is also virtually unchanged by moving the volute housing  48 . 
         [0023]    The reduction in residence time is especially advantageous when using the pump  10  as an implantable blood pump for a ventricular assist system, e.g. an LVAD or RVAD, in which it is desired to minimize residence time of blood to avoid thrombus (clot) formation, and hemolysis (damage of red blood cells). However, the concepts described herein are also useful for other fluid pumping applications where the working fluid is sensitive to shear and mechanical damage, such as whole blood, plasma, serum, or other therapeutic fluids containing complex molecules. 
         [0024]    The foregoing has described a centrifugal pump. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.