BACKGROUND OF INVENTION 
       [0001]    Embodiments of this invention relate generally to, but are not limited to, the provision and use of medical devices and, more particularly, to the provision and use of miniature, implantable blood pumps for assisting or supplementing cardiac function. 
       SUMMARY OF INVENTION 
       [0002]    At least some aspects and embodiments of the invention are directed to systems and methods for assisting the functioning of the human heart. In particular, one embodiment is directed to a system and method for pumping blood that utilizes a miniature blood pump with a tubular housing including a drive unit carrying an impeller, where the drive unit is maintained free of clots or other blood deposits without the use of a purge fluid. 
         [0003]    Existing miniature blood pumps, such as that of U.S. Pat. No. 6,942,611, employ a purge fluid to maintain the motor section free of obstructions that could impair performance or, in the long term, cause seizure of the pump. While the purge fluid arrangement is relatively effective, the additional structure it requires may increase the risk of pump failure when the device is used for longer durations, such as those in excess of thirty days. 
         [0004]    One embodiment in accord with the present invention includes a blood pump that may comprise a tubular housing in which an electromotive drive unit is located. The drive unit may be arranged coaxially within the housing, creating a primary blood flow channel in the annular space between the drive unit and the interior surface of the housing. In this configuration, blood is driven in a generally helical motion by an impeller arranged on the upstream side of the drive unit, circulates along the flow channel, and exits the housing through a laterally branching outlet tube. 
         [0005]    The housing and the outlet tube may have an L-shaped configuration, whereby the pump may be inserted into a port created in the cardiac wall, while the outlet tube may be connected to a target vessel. In this configuration, the blood pump may serve to support the heart temporarily without being totally inserted into the heart like larger, intracardiac blood pumps. 
         [0006]    One aspect of the invention is directed to a miniature blood pump that is maintained free of clots or other blood deposits or accumulations without the use of a purge fluid. The blood pump may include an annular, secondary flow channel constructed between the rotor and the stator, through which blood flows to continually flush the areas between the relatively moving parts of the rotor and stator. The secondary blood flow may be in a direction generally opposite the primary blood flow. 
         [0007]    Another aspect of the invention is directed to a miniature axial blood pump in which a rotor carrying an impeller does not require mechanical support in the axial direction. The blood pump may employ a magnetic bearing that supports the rotor and impeller in the axial direction. In some embodiments, the axial magnetic bearing may be at least partially active. 
         [0008]    Another aspect of the invention provides a catheter-based pump that employs the purge fluid free secondary blood flow path arrangement described above. In this device, the primary blood flow is directed outside and around the housing immediately behind the impeller, and additional ports positioned behind the rotor allow for the intake of blood to form the secondary blood flow. 
         [0009]    Another aspect of the invention may provide for a catheter-based pump that employs a magnetic bearing to control the axial position of a rotor carrying an impeller. The magnetic bearing may be active, or passive, or both. 
         [0010]    Another aspect of the invention is directed to method for providing cardiac assistance by the use of a miniature blood pump that includes a housing, a drive unit positioned within the housing and having a rotor and a stator, an impeller positioned at one end of the drive unit and connected to the rotor, and a cannula extending from the end of the housing nearest the impeller. The pump may be operated to create a primary blood flow around the drive unit and a secondary blood flow in an annular space formed between the rotor and the stator. The secondary blood flow may be generally in a direction opposite the primary blood flow. Axial support may be provided by mechanical bearings and/or by active or passive magnetic bearings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
           [0012]      FIG. 1  is a schematic cross-section of one embodiment of a blood pump according to the present invention; and 
           [0013]      FIG. 2  is a schematic cross-section of one embodiment of a catheter-based blood pump according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Various embodiments and aspects thereof will now be described in more detail with reference to the accompanying figures. It is to be appreciated that this invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, elements and features discussed in connection with any one or more embodiments are not intended to be excluded from a similar role in any other embodiments. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
         [0015]    A blood pump according to one embodiment of the invention is shown in schematic cross section in  FIG. 1 . 
         [0016]    The blood pump  100  of this illustrative embodiment has an elongated, substantially cylindrical housing  200  in which a drive unit  210  is generally co-axially positioned. The drive unit  210  includes an electric motor comprising a stator portion  300  and a rotor portion  400 . Electrical conductors  220  passing through a catheter  230  provide power and control signals to the electric motor. 
         [0017]    The annular space between the drive unit  210  and the interior of the housing forms the primary blood flow path  240 . In one embodiment, the inner and outer diameters of the primary blood flow path may be on the order of 7.0 and 10.0 mm, respectively, providing for a cross-sectional area in the primary blood flow path of approximately 160 mm 2 . 
         [0018]    The stator portion  300  comprises a pair of permanent stator bearing magnets  310   a,    310   b.  One stator bearing magnet  310   a  is positioned near the upstream end of the stator, while the other stator bearing magnet  310   b  is fixed near the downstream end. 
         [0019]    The stator bearing magnets  310   a,    310   b  cooperate with permanent rotor bearing magnets  410   a,    410   b  to form a pair of radial magnetic bearings. The radial bearings allow the rotor to rotate relative to the stator without significant radial contact and create an annular space to allow for a secondary blood flow path  255 . The stator bearing magnets  310   a,    310   b  and rotor bearing magnets  410   a,    410   b  may be comprised of stacked thin disc magnets, tubular magnets with an axial magnetization, or any other appropriate magnetic arrangement. 
         [0020]    In the illustrative embodiment, the inner and outer diameters of the secondary blood flow path  255  may be on the order of 3.2 and 3.8 mm, respectively, providing a cross-sectional area of approximately 3.3 mm 2 . 
         [0021]    Located on the stator portion  300  between the stator bearing magnets  310   a,    310   b  is a stator motor magnet  320 . The stator motor magnet  320  combines with a rotor motor magnet  420  to form an electromagnetic motor that may be driven by conventional means to cause rotation of the rotor  400 . 
         [0022]    In this embodiment, an impeller  500  is attached to the rotor  400 . The impeller  500  includes a hub  510  and a plurality of vanes  520  that project from the hub  510 . The vanes may take any appropriate shape and be of any appropriate number. The impeller  500  rotates in a cylindrical pump ring  530 , the diameter of which is about the same as that of the envelope of the impeller  500 . The upstream end of the pump ring  530  has an axial inlet  540  that includes an input bevel  545 . The downstream end of the pump ring  530  is followed by an impeller-free transition region  550 , within which the inside diameter continuously enlarges from that of the pump ring  530  to that of the inner diameter of the housing  200  surrounding the drive unit  210 . 
         [0023]    The operation of the rotor  400  creates a force that draws the rotor forward, i.e., in the direction opposite the blood flow. To prevent the rotor  400  from being drawn so far forward that it contacts the housing, an axial hydraulic bearing  260  may be positioned at the end of the rotor  400  that includes the impeller  500 . The axial hydraulic bearing  260  serves to arrest the forward translation of the rotor  400 . In this embodiment, the axial hydraulic bearing  260  is provided with a plurality of openings that allow blood to pass though the bearing. In some embodiments, the axial bearing may also be configured as a contact bearing, so as to physically arrest forward movement of the rotor  400 , particularly where external forces are operating on the device or when the rotor is starting up or winding down. 
         [0024]    In some embodiments, the axial position of the rotor  400  may be controlled by an axial magnetic bearing positioned at either end of the rotor, with or without the additional use of a mechanical contact bearing. In one embodiment, for example, the axial magnetic bearing may comprise a permanent axial housing magnet  270 , positioned in the housing  200  near the end of the rotor  400  opposite the impeller  500 , and cooperating with a permanent axial rotor magnet  410 , positioned in the end of the rotor opposite the impeller  500 . In another embodiment, the axial bearing may include an active magnetic bearing that operates alone or in conjunction with the passive magnetic bearing and/or mechanical contact bearing. In one embodiment, the axial magnetic bearing comprises a cylindrical passive magnet designed to counteract the axial forces encountered when the rotor  400  is up to speed, surrounded by an active magnet, designed to compensate for additional axial loads, such as those present during pre-load or after-load of the impeller. In some cases, the axial position of the impeller may be determined by measuring the back emf within the system, thus eliminating the need for an additional position sensor. In other cases, a separate position sensor may be employed to provide feedback concerning the position of the rotor and facilitate control by an active magnetic bearing. 
         [0025]    Towards the downstream end of the flow channel  240 , an outlet tube  600  laterally branches from the pump housing  200 . This outlet tube  600  extends in a direction generally perpendicular to the flow channel  240 . In some embodiments, the inner diameter of the outlet tube  600  may enlarge in the direction of flow. The enlargement may be on the order or 5-10 degrees or, in one particular embodiment, approximately 8 degrees. 
         [0026]    In certain embodiments, there may be a pressure detection opening in the inner wall of the housing that communicates with the primary flow channel  240 . From the pressure detection opening, a pressure channel may be in fluid communication with the lumen of a hose extending through the catheter  230 . A pressure sensor may be connected at the proximal end of the catheter to detect the pressure at the place of the pressure detection opening within the primary flow channel  240 . As an alternative, a local pressure sensor can be installed within the blood pump  100 . 
         [0027]    In some embodiments, a tubular cannula  700  may be mounted on the pump ring  530  and extend from the housing  200 . The cannula  700  may have longitudinally extending slots  710  arranged about its periphery, and/or an axial opening  720  at the front end. The length of the cannula  700  may, in some cases, not exceed the length of the housing  200 , including the pump ring  530 . 
         [0028]    In the illustrative embodiment, the outer diameter of the housing  200  is approximately 11 mm. The inner diameter of the housing in the region of the flow channel  240  is approximately 10 mm. The drive unit  210  has an outside diameter of approximately 7.0 mm. The inner diameter of the pump ring  530  is approximately 6 mm, the outer diameter of the cannula  700  is approximately 10 mm, and the inner diameter of the cannula  700  is approximately to 8 mm. 
         [0029]    In this embodiment, the entire housing  200 , including the pump ring  530 , has a length of about 50 mm, and the portion of the cannula  700  projecting beyond the pump ring  530  has a length of about 35 mm. 
         [0030]    The foregoing approximate dimensions are for the illustrative embodiment only, and it is to be understood that the dimensions may vary, proportionally or otherwise, in other embodiments of the invention. 
         [0031]    The dimensions of this particular embodiment are expected to result in blood flow rates of from 1.5 to 3.0 m/s in the region of the pump ring  530 , from 1.0 to 1.5 m/s in the region of the flow channel  240 , and of 0.5 m/s in the region of the outlet tube  600 . The drive unit  210  is configured to run at a relatively high rotational speed of from 10,000 to 33,000 rpm. At those speeds, the impeller  500  would move in the range of 4 to 6 l (liters) of blood per minute under physiological pressure conditions. 
         [0032]    It is anticipated that exemplary device of  FIG. 1  would, with the above dimensions, result in a flow rate in the secondary blood flow path  255  greater than approximately 20 ml/min, with a shear rate of less than 150 N/m 2 , a transition time of less than approximately 200 ms, and that it would maintain the inner surfaces of the motor at or below approximately 44° C. 
         [0033]    In order to maintain sufficient flow through the secondary flow path to prevent the accumulation of clots or other deposits in a miniature pump of this approximate size and configuration, it has been determined that the pressure differential between the secondary flow path and the primary flow path should be no less than approximately 60 mmHg. 
         [0034]    In the illustrative embodiment, the desired pressure differential is achieved by use of the laterally branching outlet tube  600 , described above, which creates a secondary pressure rise. In addition, as shown in  FIG. 1 , the secondary flow path  255  can be connected to the primary flow channel by means of a small gap  560  positioned behind the impeller  500  and running in a direction approximately perpendicular to the direction of flow through the primary flow channel. This gap  560  can help create a “water pump” effect that reduces the pressure within the secondary flow path  255  and helps to draw blood through the secondary flow path  255  in a direction opposite the flow through the primary flow path  240 . 
         [0035]    The exemplar paracardiac blood pump of  FIG. 1  may be inserted though a puncture in the cardiac wall and introduced into the heart in such a manner that the housing  200  sealingly closes the puncture hole, while the cannula  700  is in the interior of the heart and the outlet tube  600  outside the heart. The puncture hole in the cardiac wall may be made without removing cardiac wall tissue. This facilitates the closing of the hole in the cardiac wall after the future withdrawal of the pump. For a better axial fixing of the pump on the cardiac wall, a peripheral enlargement  290  may be provided on the housing  200 . 
         [0036]    With the blood pump arranged as described above, an essential portion of the length of the housing  200  and the cannula  700  is located in the interior of the heart, while a relatively short portion of the housing  200  projects from the heart, and the outlet tube  600  together with a hose connected to the outlet tube  600  lies close against the outside of the heart. Therefore, the blood pump does not occupy substantial room within the chest cavity. 
         [0037]    The blood pump may be implanted into the open heart to provide heart support for the duration of an operation or another intervention or to provide longer term support following an operation. An advantage of a miniature pump is that no heavy-weight and voluminous pumps need to be borne on the thoracic region of the patient. In addition, the pump is so small and light that even the fragile right or left atrium is not substantially deformed by applying and introducing the pump. In all cases, the positioning of the pump may be effected in a space-saving manner, and disturbances and impairments of the access to the heart being kept as low as possible. 
         [0038]    The systems described above may also be employed in an intravascular, catheter-based device, as shown in  FIG. 2 . In this embodiment, a catheter-based pump includes a housing  200  that carries a stator motor magnet  320  and stator bearing magnets  310   a,    310   b.  A rotor  400  carries a rotor motor magnet  420 , rotor bearing magnets  410   a,    410   b,  and an impeller  500 . The rotor and stator bearing magnets cooperate to form forward and rear radial magnetic bearings. 
         [0039]    In this embodiment, the rotating impeller creates a primary blood flow that passes outside the housing through primary blood flow openings  245 . Secondary blood flow openings  285 , located downstream of the primary blood flow openings  245  allow for a secondary blood flow path  255  between the rotor  400  and the housing  200  including the stator magnets. As in the embodiment of  FIG. 1 , this secondary blood flow path  255  is generally in a direction opposite the primary blood flow path. The flow of blood through the secondary blood flow path  255  serves to prevent the accumulation of clots or other blood deposits within the motor. 
         [0040]    In this embodiment, an axial magnetic bearing, which may be passive, active, or both, may be located at the rear of the rotor and counteracts the forces drawing the rotor forward, as described above. In other embodiments, the axial forces may be addressed hydraulic or contact bearings. 
         [0041]    Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.
Methods and systems are provided for the circulation of blood using a purge-free miniature pump. In one embodiment, a pump is provided that may comprise a housing including a rotor and a stator within a drive unit. In this embodiment, the pump may establish a primary blood flow through the space between the drive unit and the housing and a secondary blood flow between the rotor and stator. In another embodiment, a pump establishes a primary blood flow outside the housing and a secondary blood flow between the rotor and stator. In yet another embodiment, a method is provided for introducing the pump into the body and circulating blood using the pump.