Patent Publication Number: US-11027113-B2

Title: Implantable mechanical circulatory support devices

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 15/385456, filed Dec. 20, 2016, now U.S. Pat. No. 10,413,647, issued Sep. 17, 2019, and is related to and claims priority to U.S. Provisional Patent Application Ser. No. 62/270156, filed Dec. 21, 2015, entitled IMPLANTABLE MECHANICAL CIRCULATORY SUPPORT DEVICES, the entirety of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     n/a 
     TECHNICAL FIELD 
     The present invention relates to mechanical circulatory support devices or “MCSDs.” MCSDs are used to assist the pumping action of the heart. Certain MCSDs are used to assist the pumping action of a ventricle of the heart and, therefore, are referred to as “ventricular assist devices” or “VADs.” 
     BACKGROUND 
     For example, as shown in U.S. Pat. Nos. 7,972,122; 8,007,254; and 8,419,609, the disclosures of which are hereby incorporated by reference herein and copies of which are attached hereto, one form of MCSD incorporates a generally cylindrical inner casing defining a flow path and a rotor mounted within the flow path for rotation about the axis of the flow path. The rotor is arranged to impel blood along the axis of the flow path. Electrical coils are mounted around the inner casing, and an outer casing surrounds the electrical coils. The coils provide a rotating magnetic field within the flow path. The rotor has a permanent magnetization that interacts with the rotating field so that the rotating field impels the rotor in rotation about the axis. The MCSD may include a volute that serves to redirect the flow from the axial direction to a direction transverse to the axis. The volute has an outlet connection that serves as the outlet connection of the MCSD. 
     Such a pump can be implanted within the thoracic cavity of a human patient as, for example, within the pericardial sack. The inlet end of the housing may be connected directly to the ventricle or connected to the ventricle by a short inlet cannula. The outlet connection may be connected, for example, to the aorta by an outlet cannula. Merely by way of example, a typical MCSD of this type has a capacity to pump about 7-10 liters per minute against a pressure difference or head of about 75 mm Hg, and thus can bear a substantial proportion or almost all of the pumping load typically carried by the left ventricle. Merely by way of example, the outer casing of such a pump may be about 21 mm in diameter, and the volute may have somewhat larger dimensions in a plane perpendicular to the axis. 
     Other MCSDs, such as those shown in U.S. Pat. Nos. 7,905,823 and 8,768,487 and in U.S. Patent Publication No. 2014/0275723, the disclosures of which are hereby also incorporated by reference herein include generally similar elements but are of smaller size. These MCSDs typically implanted outside of the thoracic cavity as, for example, under the skin within the soft tissues of the pectoral area. These devices typically are connected to the heart as, for example, to the left atrium by an inlet cannula extending from the location of the pump to the atrium. The outlet of the pump typically is connected to an artery as, for example, the subclavian artery. Because the pump is implanted outside of the thoracic cavity, remote from the heart, the implantation procedure is considerably less invasive. Typically, the cannula can be inserted into a chamber of the heart by a laparoscopic or catheter-based procedure and threaded through the tissues of the body to the location of the pump. The procedure for inserting the outlet cannula is also performed outside of the thoracic cavity. Moreover, because the pump is located outside of the thoracic cavity, the pump can be accessed readily if it becomes necessary to repair or replace it. 
     MCSDs intended for extra-thoracic placement typically have been configured to provide lower pumping capacity than MCSDs intended for intra-thoracic implantation. For example, a typical MCSD intended to extra-thoracic implantation may provide a blood flow of about 1-4 liters per minute at a 75 mm Hg head. These MCSDs thus carry a smaller proportion of the pumping load of the heart. Such pumps typically have smaller dimensions than pumps intended for intra-thoracic implantation. 
     Extra-thoracically implanted MCSDs typically are housed in a pocket within the soft tissues outside of the thoracic cavity. Such pockets normally are created by surgical procedures as, for example, separating skin or subcutaneous fat from the underlying muscular tissue or separating layers of muscular tissue from one another. In some instances, the tissues forming the wall of a pocket surrounding an extra-thoracic MCSD can erode. Such erosion arises from mechanical action of the MCSD against the surrounding tissues. Mechanical action of the MCSD can lead to inflammation and necrosis of the tissues surrounding the pocket, and can cause the pocket to become enlarged. This difficulty can be particularly pronounced where the pocket closely overlies bones such as ribs. Enlargement of the pocket may allow movement of the MCSD, which creates an uncomfortable sensation for the patient. In severe cases, these conditions may require correction by additional surgical procedures. 
     Certain aspects of the present invention provide MCSDs and implantation methods that can address these concerns. Moreover, the improved MCSDs and implantation methods may allow implantation of larger MCSDs in extra-thoracic locations. For example, MCSDs of the type typically used heretofore for intra-thoracic implantation can be implanted extra-thoracically. 
     SUMMARY 
     The present invention advantageously provides for a mechanical circulatory support device. The mechanical circulatory support device includes an inner casing defining a fluid flow path, the fluid flow path defines a longitudinal axis. A rotor is mounted within the fluid flow path and configured to rotate about the longitudinal axis. A housing is included, the inner casing and the rotor being substantially disposed within the housing. The housing having a cross-sectional shape in a plane transverse to the longitudinal axis which decreases in thickness extending from a medial position to opposite lateral positions. 
     In another aspect of this embodiment, the housing defines first surface disposed about the longitudinal axis and a second surface opposite the first surface, the first surface having a medial portion, the first surface being sloped on opposite sides of the medial portion toward the second surface. 
     In another aspect of this embodiment, the housing defines an opening to an interior of housing proximate the second surface. 
     In another aspect of this embodiment, the second surface is substantially planar. 
     In another aspect of this embodiment, the second surface is curved. 
     In another aspect of this embodiment, at least a portion of the second surface is sloped in a direction toward the longitudinal axis. 
     In another aspect of this embodiment, the medial portion is convex. 
     In another aspect of this embodiment, the first surface includes a pairs of end regions disposed on opposite ends of the first surface, and wherein the end regions are disposed substantially orthogonal to the longitudinal axis. 
     In another aspect of this embodiment, electrical coils are disposed within the inner cases and disposed about the longitudinal axis, the electrical coils being configured drive the rotor in rotation about the longitudinal axis. 
     In another aspect of this embodiment, an outer casing surrounding the electrical coils is included, the outer casing being substantially disposed within the housing, and wherein the outer casing is releasably attached to the housing. 
     In another aspect of this embodiment, electrical coils are configured to drive the rotor in rotation about the longitudinal axis, and wherein one or more electronic components are disposed within the housing, the one or more electronic components including power semiconductors connected to the electrical coils, the power semiconductors being configure to selectively conduct and selectively block current flow to the coils. 
     In another aspect of this embodiment, an outlet port extending through the housing is included, the outlet port being disposed transverse to the longitudinal axis and in communication with the flow path. 
     In another aspect of this embodiment, the rotor is an axial flow rotor configured to impel blood in a downstream direction along the flow path, and wherein the device further includes a volute disposed between the flow path and the outlet port. 
     In another embodiment, a method of implanting an MCSD includes forming a pocket within the body of the patient outside of the thoracic cavity. A housing is implanted within the pocket, the housing having an inner casing defining a fluid flow path and a longitudinal axis along the fluid flow path and a rotor mounted within the fluid flow path. The housing has a cross-sectional shape in a plane transverse to the longitudinal axis which decreases in thickness extending from a medial position to opposite lateral positions the medial position of the housing facing outwardly when the housing is implanted within the pocket. The fluid flow path is fluidly coupled to the circulatory system of the patient. 
     In another aspect of this embodiment, the pocket is formed in the pectoral region of the patient&#39;s body. 
     In another aspect of this embodiment, fluidly coupling the fluid flow path to the circulatory system includes connecting an inlet cannula between a ventricle of the heart of the patient and the housing and connecting an outlet cannula from the housing to an artery of the patient. 
     In another aspect of this embodiment, the rotor is an axial flow rotor configured to impel blood in a downstream direction along the flow path. 
     In another aspect of this embodiment, the housing includes an outlet port disposed transverse to the longitudinal axis and in communication with the flow path. 
     In another aspect of this embodiment, the housing is coupled to a volute, and wherein the volute is in fluid communication with the fluid flow path. 
     In yet another embodiment, the MCSD includes an inner casing defining a fluid flow path, the fluid flow path defining a longitudinal axis. An axial flow rotor mounted within the fluid flow path and configured to rotate about the longitudinal axis and to impel blood in a downstream direction along the flow path is included. A housing is included, the inner casing and the rotor being substantially disposed within the housing. The housing has a cross-sectional shape in a plane transverse to the longitudinal axis which decreases in thickness extending from a medial position to opposite lateral positions. The housing defines first surface disposed about the longitudinal axis and a second surface opposite the first surface. The first surface has a medial portion. The first surface is sloped on opposite sides of the medial portion toward the second surface and the second surface is substantially planar. An outlet port extends through the housing, the outlet port is disposed transverse to the longitudinal axis and in communication with the flow path. A volute disposed between the flow path and the outlet port is included. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a diagrammatic sectional view depicting a pump used in one embodiment of the invention; 
         FIG. 2  is a plan view of the pump depicted in  FIG. 2 ; 
         FIG. 3  is a perspective view of an MCSD incorporating the pump of  FIGS. 1 and 2 ; 
         FIG. 4  is an exploded view of the MCSD shown in  FIG. 3 ; 
         FIG. 5  is a diagrammatic sectional view taken along line  5 - 5  in  FIG. 4 ; 
         FIG. 6  is a diagrammatic view depicting the MCSD of  FIGS. 3-5  implanted in a human patient; 
         FIG. 7  is the MCSD shown in  FIG. 5  in an implanted condition; 
         FIG. 8  is a view similar to  FIG. 7  depicting the pump of  FIGS. 1 and 2  implanted in the patient; 
         FIGS. 9  is diagrammatic sectional view depicting an MCSD according to another embodiment of the invention; 
         FIGS. 10  is diagrammatic sectional view depicting an MCSD according to another embodiment of the invention; 
         FIGS. 11  is diagrammatic sectional view depicting an MCSD according to another embodiment of the invention; 
         FIG. 12  is a diagrammatic plan view of the MCSD of  FIG. 11 ; 
         FIG. 13  is diagrammatic sectional view depicting an MCSD according to another embodiment of the invention; and 
         FIG. 14  is diagrammatic sectional view depicting an MCSD according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     Now referring to the drawings in which like reference designators refer to like elements, there is shown in  FIGS. 1-2  a blood pump for mechanical circulatory support device according to one embodiment of the invention and designated generally as “20”. The blood pump  20  may be generally as disclosed in the aforementioned U.S. Pat. Nos. 7,972,122; 8,007,254; and 8,419,609. The blood pump  20  includes an inner housing  22  defining a generally cylindrical flow path  24  having an axis  26 , an inlet end  28 , and an outlet end  30 . A hollow inlet fitting  32  projects axially from the inlet end  28  of the flow path  24  and is in fluid communication with the flow path  24 . A volute  34  is provided at the outlet end  30  of the flow path. The volute  34  includes a center post  36  extending generally in the axial direction towards the flow path  24 . The volute  34  defines a chamber  38  extending around the center post  36  and around axis  26 . Chamber  38  has a radius R v  measured from axis  26 . The radius of the volute increases progressively in a circumferential direction around axis  26 . 
     A tubular outlet port  40  extends in a plane transverse to axis  26  and is in fluid communication with the chamber  38  of the volute  34 . In one configuration, the maximum radius R v  of the volute  34  is greater than the radius of the cylindrical flow passage  24 . One particularly preferred form of volute is shown and described in the copending, commonly assigned U.S. provisional patent application filed of even date herewith and entitled “AXIAL FLOW IMPLANTABLE MECHANICAL CIRCULATORY SUPPORT DEVICES WITH OUTLET VOLUTE,” the disclosure of which is also incorporated by reference herein and a copy of which is annexed hereto. 
     A rotor  42  is disposed inside the flow path  24  defined by inner casing  22 . In one configuration, the rotor is symmetrical about an axis coincident with the central axis  26  of the flow path  24 . In one configuration the rotor  42  includes plurality of vanes spaced circumferentially around the axis  26  and a plurality of channels between adjacent vanes. The vanes are configured to impel blood in the downstream direction along the axis upon rotation of the rotor about its axis in a predetermined direction. An exemplary rotor  42  is disclosed in U.S. Patent Application Publication No. 2015/0051438 (“the &#39;438 Publication”), the disclosure of which is also incorporated by reference herein. As also explained in the U.S. Pat. No. 7,972,122; 8,007,254; and 8,419,609 patents and in the &#39;438 Publication, the rotor  42  may be formed from a magnetic material such as a platinum-cobalt alloy and has a permanent magnetization in a direction transverse to the axis  26 . The rotor  42  may further include hydrodynamic bearing surfaces on the tips of the vanes, remote from axis  26 . The blood pump  20  further includes electrical windings  44  carried on ferromagnetic metal cores  46 . The windings  44  typically are composed of numerous turns of wire encircling the cores. Windings  44  are disposed in an array around the outside of inner casing  22 . As best appreciated with reference to  FIG. 5 , the windings are disposed in pairs, with the windings of each pair being diametrically opposed to one another on opposite sides of the axis  26 . 
     As discussed further below, the windings  44  may be energized in alternating sequence using a three-phase excitation system so as to create a rotating magnetic field within the flow path  24 . Magnetic coupling between this rotating field and the permanent magnetization of rotor  42  drives the rotor  42  in rotation about the axis  26  of the flow path. As described in the aforementioned patents, the hydrodynamic bearing surfaces on the rotor will maintain the rotor out of contact with the wall of inner casing  22  and with a thin film of blood disposed between the tip surface of each vane and the wall of the casing, and thus maintain the rotor with its axis aligned with the axis  26  of the flow path. Magnetic interaction between the magnetic field of the rotor and the ferromagnetic cores  46  of the stator prevents the rotor  22  from moving axially. Rotation of the rotor  42  drives blood in a downstream direction, from the inlet end  26  of the flow path to the outlet end  30  and out through the volute  34  and the outlet port  40 . 
     An outer casing  48  surrounds the inner casing  22 , the windings  44  and cores  46 . In one configuration, the outer casing  48  is cylindrical and coaxial with the inner casing  22  and axis  26 . The outer casing  48  forms a sealed enclosure around the windings  44 . Electrical connections to the windings  44  may be made through an opening  50  in the outer casing  48 , which may further be sealed. 
     Referring now to  FIGS. 3-5 , a housing  52  of the MCSD surrounds the outer casing  48  and the volute  34  such that the inner casing is at least substantially disposed within the housing  52 . The housing  52  includes first surface  54  having a medial portion  56  defining a convex top surface  66  facing in an upward direction, transverse to the central axis  26  of the flow path  24  when the MCSD is implanted within the patient. The upward direction as used in this disclosure refers to a direction transverse to axis  26 , indicated by arrow U in  FIG. 5 , whereas the downward direction as used in this disclosure refers to the opposite direction, indicated by arrow D. The upward direction is the direction that will face toward the outside or skin of the subject when the device is implanted. The housing  52  also includes a second surface or bottom wall  58  defining a downwardly facing bottom surface. In the particular embodiment depicted in  FIGS. 4 and 5 , bottom wall  58  includes a central element  62  and a pair of side elements  60 . As best seen in  FIG. 4 , the side elements  60  are connected to one another by a tab  64  projecting generally in the axial direction from the side elements  60 . In the embodiment of  FIGS. 4 and 5 , bottom surface  58  is a flat, downwardly facing surface. The medial portion  56  slopes downwardly, in lateral directions indicated by arrows L and R in  FIG. 5 , from the top surface  66  of the medial portion  56  disposed above the central axis  26  to edge regions  68  remote from the axis in the lateral direction. The first surface  54  slopes downwardly to a level below the axis  26 . In one configuration, the medial portion  56  of housing  52  is aligned with the flow path  22  and outer casing  48 , housing  52  has a cross-sectional shape that tapers gradually from a relatively large thickness adjacent the axis  26  to relatively small thicknesses at edge regions  68  remote from the axis  26  and offset from the axis  26  in opposite lateral directions transverse to the axis. In one configuration, the housing  52  is axially aligned with the inner casing  22 , outer casing  48 , and rotor  42 , and the cross-sectional dimensions of the housing  52  are substantially uniform. Thus, the top portion  66  of the medial portion  56  lies at a constant elevation above the bottom surface  58 , so that the housing  52  has substantially uniform thickness at the medial portion  56 . 
     The housing  52  has an outlet end region  70  axially aligned with the volute  34  and with the outlet port  40 . As best seen in  FIGS. 4 and 5 , the housing  52  has an indentation in the outlet end region  70  adjacent one edge region  68  so as to form a substantially vertical wall  72  extending upwardly from bottom wall  58  and intersecting the medial portion  56  between the top surface  66  of the medial portion  56  and edge region  68 . A portion of the outlet port  40  of the volute  34  projects from within the housing  52  to outside of the housing  52 . A portion of the outlet port  40  extends through the medial portion  56 , whereas another portion of the outlet port  40  extends through the vertical wall  72 . A further vertical wall  74  is also provided on the outlet end region  70  of the housing  52 , on the side opposite from vertical wall  72 . This vertical wall  72  also extends upwardly from the bottom wall  58  and faces in a lateral direction opposite to the lateral-facing direction of vertical wall  72 . A further vertical wall  76  faces generally axially so that walls  74  and  76  cooperatively define a notch in the housing  52 . An electrical cable  78  extends into the housing  52  through this notch and through wall  76 . A shroud  81 , shown in  FIG. 4  as transparent, covers the cut-away portion. The shroud  81  defines a smooth surface continuous with the first surface  54  of the housing  52 . The top surface  66  of the medial portion  56  curves downwardly within outlet end portion  70  in an axial direction toward the end of the housing  52 , so that the housing  52  has a rounded profile if seen in an elevational view transverse to the axis. The housing  52  has an inlet end portion  80  adjacent the inlet fitting  32 . Here again, the medial portion  56  curves downwardly within the inlet end portion  80 . Within the inlet end portion  80  and within the outlet end portion  70 , the bottom edge of the housing  52  curves inwardly toward the axis  26 , so that the bottom surface  58  of the housing has rounded edges. 
     Electrical components  82  are mounted within the housing  52 . As best seen in  FIGS. 4 and 5 , the electrical components  82  are disposed on a circuit board  84  and are disposed adjacent the edges regions  68  of the housing  52 . Stated another way, some or all of the electrical components  82  lie adjacent the edge regions  68  of the housing  52 , laterally remote from the axis  26 . Within these edge regions  68 , the housing  52  provides space above the circuit board  84  so that components  82  of substantial size can be mounted. The electrical components  82  are directly or indirectly connected to the coils  46  of the pump  20 . For example, the electrical components  82  may include switching elements such as field effect transistors that are used to make and break electrical connections between the various coils and power and ground voltages, so as to produce the rotating magnetic field to drive the rotor. In a typical circuit of this type, a pump with three pairs of opposed coils may require six separate switching elements and six connections between switching elements and coils. These connections carry the full power used to drive the coils. Where the switching elements are disposed inside the housing  52 , these connections need extend only within the housing  52 . Thus, in one configuration, only two power leads—a ground connection and a supply voltage connection—extend out of the housing, along with any desired control signal leads. By contrast, where the switching elements are disposed in a controller separate from an MCSD housing  52  containing the pump  20 , six power leads extend to the MCSD. Reducing the number of power leads allows the use of a substantially smaller cable for feeding power to the MCSD and thus simplifies installation of the cable. Alternatively or additionally, other electrical components  82  may be disposed within the housing  52 . For example, the other electrical components  82  may be used to perform some or all of the functions of a pump controller. Typically, the pump controller monitors operation of the pump, actuates the switching elements to provide the desired rotor speed and flow characteristics. The controller may also perform additional functions such as providing alarm signals denoting abnormal conditions, recording abnormal events during operation and the like. Pump controllers are known in the art; certain controllers are described, for example, in U.S. Pat. Nos. 8,628,460 and 8,597,350, the disclosures of which are hereby incorporated by reference herein. Some or all of the components  82  that perform the controller functions may be disposed within the housing. Even where the switching elements are disposed external to the housing  52 , some or all of the controller components may be mounted within the housing  52 . Mounting some of the controller components within the housing  52  reduces the bulk of a separate controller housing. Where all of the controller components are mounted within the housing of the MCSD itself, there is no need for a separate controller housing. 
     In a method according to a further aspect of the invention, the MCSD discussed above is implanted within the body of a human or non-human mammalian patient, at a location outside of the thoracic cavity of the patient as depicted in  FIG. 6 . The implantation procedure includes forming a pocket within the body of the patient at the location where the MCSD is to be implanted. Typically, the pocket is formed by forming a cut in tissues of the subject and separating adjacent layers of tissue. The MCSD is then inserted between the separated layers. In one configuration, the pump  20  is positioned in the pocket with the top surface  66  of the housing  52  facing upwardly, toward the skin of the subject, and with the bottom surface  58  facing downwardly, toward the interior of the patient&#39;s body. Before or after insertion into the pocket, the inlet fitting  32  ( FIG. 3 ) is connected to a source of blood by a flexible elongated inlet cannula  86 . The outlet port  40  is connected to an artery by an outlet cannula  88 . Most typically, where the MCSD is used to assist the pumping action of the left side of the heart, supporting the general circulation, the inlet cannula  86  is connected to the left atrium or left ventricle of the heart, and the outlet cannula  88  is connected to an artery of the general circulation as, for example, the subclavian artery. 
     The drive line  78  is connected to a source of power and control signals  90 . Such a source may be an implanted controller or a connection such as a skin-penetrating connection to an external controller worn by the subject. In other embodiments, the source  90  of power and control signals may incorporate a transcutaneous energy transfer or “TET” arrangement for transferring electrical energy through the skin without a physical skin-penetrating connection. For example, a TET may incorporate an induction coil disposed within the patient&#39;s body and another induction coil disposed outside of the patient&#39;s body. The procedures used for installing the inlet cannula  86 , outlet cannula  88 , and drive line  78  may be substantially conventional. For example, each of these elements may be “tunneled” through the patient&#39;s body. Where an implanted controller is used, power may be supplied to the controller through a skin-penetrating connection or through a TET arrangement. A battery (not shown) may be included in the controller or may be disposed in a separate implanted battery housing (not shown) so as to provide temporary power when external power is disconnected. 
     As schematically illustrated in  FIG. 7 , the housing  52  provides a shape that allows conformance of the tissue layers with the housing  52 , and which tends to minimize severe stresses on the tissues surrounding the pocket. The medial portion  56  allows the overlying tissues  90  to conform to the housing  52  so that the tissue contacts the housing  52  over a relatively large area and thus distributes stresses on the tissue caused by implantation of the MCSD. Likewise, the bottom surface  58  spreads the load over the underlying tissue layer  92 . 
     Now referring to  FIG. 8 , by contrast, an MCSD incorporating a generally similar pump  20  without housing  52  is shown in an implanted condition in  FIG. 8 . The outer casing  48  tends to engage the tissue only in areas close to the axis  26 . This implies relatively high concentrated loads applied between the casing and the tissue layers  90  and  92 . Such concentrated loads tend to promote necrosis of the tissue. Moreover, the overlying tissue layer  90  is in tension, which may tend to promote peeling or separation of the tissue layers around the pocket at the junctures of the tissue layers. The concentrated loads are particularly problematic where one of the tissue layers includes boney structures. For example, if the pump  20  without the housing is implanted as shown in  FIG. 8  over a layer that includes boney structures  94  and if the pump  20  outer casing  48  is disposed close to one of these boney structures  94 , the tissue between the outer casing  48  and the boney structure  94  can be compressed to such a degree that blood circulation to the tissue is impaired. By contrast, in an MCSD having the housing  52  discussed above with reference to  FIGS. 3-5 , the relatively wide distribution of force over the bottom surface  58  of the housing  52  ( FIG. 7 ) tends to avoid this problem. 
     The embodiments discussed above can be varied in numerous ways. For example, as shown in  FIG. 9 , the outer casing of the pump is omitted. In this arrangement, the stator, including the core  46  and coils  44 , is exposed to the environment within housing  152  so that the coils and their electrical connections are protected from the environment within the subject&#39;s body only by the housing. This permits some reduction in the thickness of the housing at its medial portion adjacent the axis. Also, in this arrangement, the bottom wall  158  is concave, so that the bottom surface slopes upwardly in the lateral directions from edge regions  159  remote from the axis  26  of the pump to a medial region adjacent the axis. This concavity can be useful where the MCSD is installed in a pocket within a naturally convex region of the subject&#39;s body. In other respects, the embodiment of  FIG. 9  is generally similar to the embodiments discussed above. 
     In the embodiment of  FIG. 10 , the bottom surface  258  is generally convex, rather than concave, so that the bottom surface slopes downwardly from edge regions  259  laterally remote from the axis of the pump to medial regions  261  adjacent the axis. Also in this embodiment, the top surface  252  is generally convex and provides a smooth, downwardly sloping surface extending from the medial region adjacent the axis to edge regions  268  laterally remote from axis  26 . In this embodiment, although the top surface  252  is generally convex, it includes some local regions  269  where it is slightly concave. Stated another way, a generally convex surface need not be entirely convex at every point. In the embodiment of  FIG. 10 , the convexity of the bottom surface  258  is less than the convexity of the top surface. Stated another way, the top wall  254  has a greater vertical extent (in the upward and downward directions) than the bottom wall  258 . The top and bottom walls meet at junctures defined by edge regions  259  and  266 . These junctures are disposed below the axis  26  of the pump components. In the embodiment of  FIG. 10 , the outlet port  40  is directed downwardly as well as laterally and exits through the bottom surface  258  of the housing. 
     The embodiment of  FIG. 11  includes a pump having an external casing  348  surrounding the windings  346 . Thus, in this case, the windings and other electrical components of the pump are protected by the external casing. The housing  352  is not sealed. The housing includes resilient elements  302  adapted to releasably engage the outer casing of the pump. Also, in this embodiment, the bottom surface  358  has an opening in the medial region, adjacent the axis  26  of the pump. In this embodiment as well, housing  352  has a cross-sectional shape, when viewed in a direction parallel to the axis of the pump, which tapers from a relatively large thickness at medial regions close to the axis to a relatively small thickness laterally remote from the axis. As best seen in  FIG. 12 , a top view of the assembled pump and housing, housing  352  encompasses only a portion of outer casing  348 . The volute  334  projects axially beyond housing  352  at one end of the housing, whereas casing  348  of the pump projects axially beyond housing  352  at the opposite end. A housing of this nature can be readily installed on a pump that was originally manufactured without the housing  352 , simply by forcing the pump casing into the housing until it is engaged between resilient elements  302 . In this arrangement, the electronic elements associated with the pump are either housed within the casing  348  of the pump itself or within a separate controller housing. In a variant of this approach, the bottom surface  358  may be continuous across the entire lateral extent of the housing so that opening  304  is omitted. In this arrangement, the casing of the pump may be inserted into the housing by moving it into the housing axially. In a further variant, the wall defining the bottom surface of the housing may be movable between a closed position, in which it extends laterally across the bottom of the housing, and an open position. 
     In the embodiment of  FIG. 13 , the housing includes two separate housing elements  552   a  and  552   b,  which are attached to laterally opposite sides of the pump casing  548 . Portions of the pump housing  548  are exposed at medial regions of the combined structure. The combined shape defined by the pump housing  548  and by housing elements  552   a  and  552   b  again includes a relatively large thickness dimension (in the upward and downward directions) at locations laterally adjacent axis  26  and again tapers to a relatively small dimension at edges remote from the axis in opposite lateral directions. The housing still serves to distribute the loads applied between the tissue layers and the MCSD. 
     An MCSD according to yet another embodiment of the invention ( FIG. 14 ) includes a housing  652  which includes a main section  601  housing the pump. Main section  601  may be generally similar to any of the housings discussed above. The housing  652  further includes an additional section  602  extending laterally from the main section  601 . Section  602  includes a portion  603  containing electronics and a further portion  604  containing a battery. Preferably, the only implantable battery used to power the pump is disposed in housing  652 , and the implantable controller is entirely contained within the housing. This arrangement provides a simpler implantation procedure, as well as reduced electrical losses because there is no need for long cables connecting implanted components to one another. Typically, a driveline is connected between housing  652  and a component such as a percutaneous connector used to supply power and signal connections to outside of the body. In a further variant, the internal coil of a transcutaneous energy transfer system and telemetry devices may be disposed within the housing, so that only the housing and cannulas need be implanted. In yet another variant, the internal coil of a TET coil may be mounted in any of the housings discussed above. The features of the various embodiments discussed above can be combined with one another. For example, the separate housing elements shown in  FIG. 13  can be hollow and can house components such as electrical components. 
     In another variant, where the housing  352  is open to the environment within the subject as, for example, in the embodiment of  FIGS. 11 and 12 , the bottom wall may be entirely omitted. Also, in an embodiment where the bottom wall  358  is entirely omitted, or where the bottom wall has an opening, the pump casing may project slightly below the remainder of the housing. 
     The housings may be formed of various materials. Where the housing is intended to provide a sealed arrangement, the housing should have relatively low permeability to moisture. The housing may be formed of a metal or from a composite including a metal or other impervious material. The housing may be rigid or flexible and may have varying degrees of rigidity or flexibility. Also, the exterior surfaces of the housing, such as the top and bottom surfaces discussed above, may be provided with a porous material or other material that promotes ingrowth of tissue so as to securely bond the MCSD in place within the pocket. 
     In the embodiments above, the pump has been described as an axial flow pump. However, the present invention also can be applied to radial flow pumps. A radial flow pump likewise has a rotor rotatable about an axis within a flow path. In a radial flow pump, however, the fluid flow within the rotor is primarily in the radial direction, away from the axis. Likewise, the present invention can be applied in connection with mixed-flow pumps, in which the fluid flow within the rotor has substantial axial and radial components. 
     As these and other variations and combinations of the features described above can be employed, the foregoing description should be taken by way of illustration rather than by limitation of the present invention. Additional features of the present invention are disclosed in the numbered paragraphs set forth below: