Patent Publication Number: US-2021170081-A1

Title: Percutaneous Blood Pump Systems and Related Methods

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The present application is a non-provisional application claiming the benefit of priority under 35 U.S.C. § 119(e) from commonly owned and co-pending U.S. Provisional Application Ser. No. 62/794,988 filed on Jan. 21, 2019 and entitled “Percutaneous Blood Pump System and Related Methods,” the entire contents of which are hereby incorporated by reference into this disclosure as if set forth fully herein. 
    
    
     FIELD 
     The present invention relates generally to blood pumps and, more particularly, to an improved intra-vascular blood pump system and related methods. 
     BACKGROUND 
     Blood pumps provide augmented blood flow rate for a damaged or diseased heart. 
     Flow of blood pumps are limited by blood trauma (hemolysis) resulting from shear stress and transit time. Shear stress is affected by the diameter and rotational speed of the blood pump impeller. Percutaneous blood pumps are sized to be inserted through peripheral blood vessels. The diameter of a percutaneous blood pump is limited by the anatomy of the peripheral blood vessels. Prior art percutaneous blood pumps attempt to increase flow with expandable impellers which are technically difficult to implement reliably and safely. 
     Percutaneous trans-valvular blood pumps position their inlet cannula tip in a chamber of the heart. During high flow rates or when the blood volume adjacent to the inlet tip is low due to patient hemodynamic conditions for position of the inlet tip in the heart chamber, high negative pressure within the inlet cannula may result causing hemolysis through flow disturbances through the impeller or tissue damage due to high suction forces at the tip orifices. 
     Percutaneous blood pumps are inserted in peripheral vessels. The diameter of the blood pump is maximized to provide maximum flow while minimizing blood trauma. The blood pumps are used for many hours, even days. The introducer used to insert the blood pump in the vessel and establish hemostasis blocks the native flow through the vessel to the distal extremity. Prolonged blockage can lead to amputation. Blockage of flow reduces distal extremity pressure making vascular access difficult. The blood pumps are introduced into the body under emergency situations where time is critical, preventing adjunct procedures designed to ensure distal extremity perfusion prior to initiating circulatory support. 
     To access chambers of the heart, guide wire and catheters are used. For placement, the prior art utilizes the blood flow lumen from the tip though the non-rotating impeller and exits the impeller shroud blood port for placement of a guide wire and/or catheter. A guide wire is placed through this passage prior to insertion into the body then the blood pump is tracked over the guide wire for placement in the heart. However, if the inlet cannula of the blood pump becomes dislodged from the heart during treatment, the blood pump must be removed from the body to access the mentioned lumen for back-loading onto the guide wire used to safely access the heart. Prior art also utilizes a pig-tail catheter segment attached to the inlet cannula to aid in re-accessing the heart chamber in the event the inlet cannula becomes dislodged during use after the original guidewire is removed. The drawback is that the pig-tail catheter segment limits position location of the inlet cannula tip in the heart chamber and poses risks for cannula tip dislodgement or interference with valve function. 
     Other prior art utilizes a blood pump removably attached to the inlet cannula so access to the inlet tip may be accomplished without removal of the cannula from the body. This “over the wire” configuration does not require additional diameter beyond the diameter of the impeller to house the lumen for the guidewire/catheter access. However, removal of the blood pump is required increasing risk of contamination, bleeding, and infection. 
     The present invention is directed at overcoming, or at least improving upon, the disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the limitations of prior art blood pumps by providing an improved intra-vascular blood pump having multiple impellers configured to increase flow rate. Impellers are arranged and rotationally driven in series with a trans-valvular cannula arranged for parallel trans-valvular flow through each impeller. Parallel flow results in summation of flow through each impeller for increased hemodynamic support for the patient with smaller insertion diameter for the physician. Optionally, the cannula is expandable to minimize pressure drop while being inserted in collapsed configuration similar to the size of an impeller. Rotational speed of all impellers is the same. Diameter of the impellers may be the same or progressively smaller allowing radial space for the expandable cannula in its collapsed configuration for insertion. 
     The present invention overcomes the limitations of prior art inlet cannula tips by providing an expandable structure configured to suspend the cannula inlet tip orifices away from the heart chamber tissue during use while being collapsed for insertion and removal. 
     The present invention overcomes the limitations of prior art introducers by providing a multi-lumen percutaneous blood pump introducer with access site bypass circuit configured to perfuse or drain the distal extremity after placement of the blood pump in the heart. The circuit allows blood flow through the annulus formed by the outer diameter of the blood pump drive sheath and the inner diameter of the introducer to a side-port of the introducer hemostasis valve through a connected catheter inserted percutaneously in the contralateral vessel which passes through introducer central lumen to side lumen having exit port in wall near introducer vessel access location to the distal vessel segment. Blood flow direction is dependent on anatomical placement. When placed in artery, blood flows into circuit through introducer tip under systemic pressure and exits circuit through catheter tip. When placed in vein, blood flows into circuit through catheter tip under systemic pressure and exits circuit through introducer tip. 
     The present invention overcomes the limitations of prior art access cannula systems by providing a lumen in the drive sheath of the blood pump configured to pass a removable guide sheath through a side-port proximal to the impeller crossing over the outer diameter of the impeller housing to access a lumen in the inlet cannula via a separate side-port distal to the impeller thereby bypassing the impeller region without adding additional diameter to the system beyond the size of the impeller housing. A guidewire may be passed through guide sheath to access the inlet cannula tip without removal of the blood pump from the body. The present invention provides for an “over the wire” type guide mechanism for selectively positioning and repositioning the intravascular blood pump and cannula at a predetermined location within the circulatory system of a patient without requiring removal of the blood pump from the patient. 
     In summary, the percutaneous blood pump system of the present invention boasts a variety of advantageous features, including but not limited to: An improved intra-vascular blood pump with multiple impellers and expandable cannula which provides the ability to produce increased flow rate at safe levels of blood trauma without increasing the diameter of the intravascular segments of the system compared to a single impeller blood pump; An expandable inlet cannula tip which provides the ability to prevent tip inflow occlusion when the tip is placed within anatomy that could block the tip inlet orifices; An introducer and distal extremity infusion catheter system to which provides the ability to bypass the insertion site obstruction and perfuse the distal extremity which the blood pump was introduced into the body; and a transvalvular percutaneous blood pump having one or more lumens which provide the ability to access the inlet cannula tip with guidewire or catheter for insertion and re-insertion into chambers of the heart without having to remove the blood pump from the body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
         FIG. 1  is a side plan view of one example of a percutaneous blood pump system according to one embodiment of the disclosure; 
         FIG. 2  is a top plan view of an example of a distal end portion of the blood pump system of  FIG. 1 , comprising an inlet cannula with multiple impeller blood pumps according to one example embodiment; 
         FIG. 3  is a bottom plan view of the inlet cannula of  FIG. 2 ; 
         FIG. 4  is a side plan view of the inlet cannula of  FIG. 2 ; 
         FIG. 5  is a side sectional view of the inlet cannula of  FIG. 2 , taken along line  1 - 1  of  FIG. 2 ; 
         FIG. 6  is a sectional view of the inlet cannula of  FIG. 2 , taken along line  2 - 2  of  FIG. 4 ; 
         FIG. 7  is a sectional view of the inlet cannula of  FIG. 2 , taken along line  3 - 3  of  FIG. 4 ; 
         FIG. 8  is a perspective view of an example of a first pump assembly forming part of the blood pump system of  FIG. 1 ; 
         FIG. 9  is an exploded perspective view of the first pump assembly of  FIG. 8 ; 
         FIG. 10  is a top plan view of the first pump assembly of  FIG. 8 ; 
         FIG. 11  is a side plan view of the first pump assembly of  FIG. 8 ; 
         FIG. 12  is a plan view of the distal end of the first pump assembly of  FIG. 8 ; 
         FIG. 13  is a plan view of the proximal end of the first pump assembly of  FIG. 8 ; 
         FIG. 14  is a bottom plan view of the first pump assembly of  FIG. 8 ; 
         FIG. 15  is a sectional view of the first pump assembly of  FIG. 8 , taken along line  4 - 4  of  FIG. 14 ; 
         FIG. 16  is a perspective view of an example of a second pump assembly forming part of the blood pump system of  FIG. 1 ; 
         FIG. 17  is an exploded perspective view of the second pump assembly of  FIG. 16 ; 
         FIG. 18  is a top plan view of the second pump assembly of  FIG. 16 ; 
         FIG. 19  is a side plan view of the second pump assembly of  FIG. 16 ; 
         FIG. 20  is a plan view of the distal end of the second pump assembly of  FIG. 16 ; 
         FIG. 21  is a plan view of the proximal end of the second pump assembly of  FIG. 16 ; 
         FIG. 22  is a bottom plan view of the second pump assembly of  FIG. 16 ; 
         FIG. 23  is a sectional view of the second pump assembly of  FIG. 16 , taken along line  5 - 5  of  FIG. 22 ; 
         FIG. 24  is a side plan view of an example of a proximal end portion of the blood pump system of  FIG. 1 , comprising a proximal impeller drive sheath, hub, and motor assembly according to one example embodiment; 
         FIG. 25  is a side plan view of an example of an introducer forming part of the blood pump system of  FIG. 1 ; 
         FIG. 26  is a bottom plan view of the introducer of  FIG. 25 ; 
         FIG. 27  is a detail view of a portion of the introducer of  FIG. 25 ; 
         FIG. 28  is a cross-sectional view of the introducer of  FIG. 25 , taken along line  6 - 6  of  FIG. 26 ; 
         FIG. 29  is a detail sectional view of a portion of the introducer of  FIG. 25 ; 
         FIG. 30  is a side sectional view of an example of a bypass circuit using the introducer of  FIG. 25  with the blood pump of  FIG. 1 ; 
         FIG. 31  is a bottom plan view of an example of a distal tip of an inlet cannula of  FIG. 2  having a tapered shape according to one embodiment; 
         FIG. 32  is a plan view of the distal tip of the inlet cannula of  FIG. 31 ; 
         FIG. 33  is a side plan view of an example of a distal tip of an inlet cannula of  FIG. 2  having a duckbill shape according to one embodiment; 
         FIG. 34  is a plan view of the distal tip of the inlet cannula of  FIG. 33 ; 
         FIG. 35  is a side plan view of an example of a distal tip of an inlet cannula of  FIG. 2  having an expandable balloon cage tip according to one embodiment; 
         FIG. 36  is a plan view of the distal tip of the inlet cannula of  FIG. 35 ; 
         FIG. 37  is a side plan view of an example of a distal tip of an inlet cannula of  FIG. 2  having an expandable mesh cage tip according to one embodiment; 
         FIG. 38  is a plan view of the distal tip of the inlet cannula of  FIG. 37 ; 
         FIG. 39  is a side plan view of an example of a distal end portion of the blood pump system of  FIG. 1 , comprising an inlet cannula with multiple impeller blood pumps according to another example embodiment; 
         FIG. 40  is a section view of the inlet cannula of  FIG. 39 , taken along line  7 - 7  of  FIG. 39 ; 
         FIG. 41  is a perspective view of the inlet cannula of  FIG. 39  with a guide wire/catheter placed therethrough; 
         FIG. 42  is a side plan view of a portion of the inlet cannula of  FIG. 39 ; 
         FIG. 43  is a side plan view of another example of a percutaneous blood pump system according to one embodiment of the disclosure; 
         FIG. 44  is a broken plan view of the blood pump system of  FIG. 43 ; 
         FIG. 45  is an exploded plan view of the blood pump system of  FIG. 43 ; 
         FIG. 46  is a plan view of an example of an introducer sheath forming part of the blood pump system of  FIG. 43 ; 
         FIG. 47  is an exploded plan view of the introducer sheath of  FIG. 46 ; 
         FIG. 48  is a sectional view of the introducer sheath of  FIG. 46 , taken along line A-A of  FIG. 46 ; 
         FIG. 49  is an exploded plan view of an example of a catheter forming part of the blood pump system of  FIG. 43 ; 
         FIG. 50  is a top plan view of the catheter of  FIG. 49 ; 
         FIG. 51  is a sectional view of the catheter of  FIG. 49 , taken along line B-B of  FIG. 50 ; 
         FIG. 52  is a top plan view of an example of an expandable cannula forming part of the catheter of  FIG. 49 ; 
         FIG. 53  is a plan view of the distal end of the expandable cannula of  FIG. 52 ; 
         FIG. 54  is a side plan view of the expandable cannula of  FIG. 52 ; 
         FIG. 55  is a plan view of the proximal end of the expandable cannula of  FIG. 52 ; 
         FIG. 56  is a bottom plan view of the expandable cannula of  FIG. 52 ; 
         FIG. 57  is a side sectional view of the expandable cannula of  FIG. 52 , taken along line C-C of  FIG. 52 ; 
         FIG. 58  is an exploded plan view of an example of a pump system forming part of the blood pump system of  FIG. 43 ; 
         FIG. 59  is an exploded plan view of an example of a first pump assembly forming part of the pump system of  FIG. 58 ; 
         FIG. 60  is an exploded plan view of an example of a second pump assembly forming part of the pump system of  FIG. 58 ; 
         FIG. 61  is an exploded plan view of an example of a third pump assembly forming part of the pump system of  FIG. 58 ; 
         FIG. 62  is a side plan view of the distal region of the pump system of  FIG. 58 , showing in particular the first, second, and third pump assemblies arranged in tandem; 
         FIG. 63  is a bottom plan view of the distal region of  FIG. 62 ; 
         FIG. 64  is a section view of the distal region of  FIG. 62 , taken along ling H-H of  FIG. 63 ; 
         FIG. 65  is a section view of the first pump assembly of  FIG. 59 , taken along line H-H of  FIG. 63 ; 
         FIG. 66  is a section view of the third pump assembly of  FIG. 61 , taken along line H-H of  FIG. 63 ; 
         FIG. 67  is a section view of the third pump assembly of  FIG. 61 , taken along line G-G of  FIG. 62 ; 
         FIG. 68  is top plan view of a distal region of the blood pump assembly of  FIG. 43 ; 
         FIG. 69  is a side sectional view of the distal region of  FIG. 68 , taken along line L-L of  FIG. 68 ; 
         FIG. 70  is a side sectional view of a portion of the distal region of  FIG. 68 ; 
         FIG. 71  is a side plan view of an obturator assembly forming part of the percutaneous blood pump system of  FIG. 1 ; 
         FIG. 72  is a side plan view of the percutaneous blood pump system of  FIG. 1  assembled in an insertion configuration according to one example embodiment; 
         FIG. 73  is a side section view of the assembled blood pump system of  FIG. 72 , taken along line P-P of  FIG. 72 ; 
         FIG. 74  is a section view of the assembled blood pump system of  FIG. 72 , taken along lines N-N of  FIG. 72 ; 
         FIG. 75  is a perspective view of another example of a pump subsystem according to one embodiment of the disclosure; 
         FIG. 76  is a perspective view of the distal end of the pump subsystem of  FIG. 75 , illustrating in particular first and second pump assemblies arranged in tandem; 
         FIG. 77  is a perspective view of a first pump assembly forming part of the pump subsystem of  FIG. 75 ; 
         FIG. 78  is a partially exploded perspective view of the first pump assembly of  FIG. 77 , illustrating in particular the impeller assembly in exploded form; 
         FIG. 79  is another partially exploded perspective view of the first pump assembly of  FIG. 77 , illustrating in particular the bearing assembly in exploded form; 
         FIG. 80  is a partially exploded perspective view of the distal end of the pump subsystem of  FIG. 76 ; 
         FIG. 81  is a top plan view of the distal end of the pump subsystem of  FIG. 76 ; 
         FIG. 82  is a side sectional view of the distal end of the pump subsystem of  FIG. 76 , taken along line Q-Q in  FIG. 81 ; 
         FIG. 83A  is a detail view of highlight area R in  FIG. 82 , illustrating in particular a sectional view of the bearing assembly of  FIG. 79 ; 
         FIG. 83B  is a detail view of highlight area S in  FIG. 82 , illustrating in particular a sectional view of the impeller assembly of  FIG. 78 ; 
         FIG. 83C  is a sectional view of the bearing assembly of  FIG. 79 , taken along line  6 - 6  of  FIG. 81 ; 
         FIG. 84  is a side plan view of the first pump assembly of  FIG. 77 ; 
         FIG. 85  is an axial sectional view of the first pump assembly of  FIG. 77 , taken along line T-T in  FIG. 84 ; 
         FIG. 86  is an axial sectional view of the first pump assembly of  FIG. 77 , taken along line U-U in  FIG. 84 ; 
         FIG. 87  is an axial sectional view of the first pump assembly of  FIG. 77 , taken along line V-V in  FIG. 84 ; 
         FIG. 88  is a perspective view of a second pump assembly forming part of the pump subsystem of  FIG. 75 ; 
         FIG. 89  is an exploded perspective view of the second pump assembly of  FIG. 88 ; 
         FIG. 90  is a top plan view of the second pump assembly of  FIG. 88 ; 
         FIG. 91  is a side plan view of the second pump assembly of  FIG. 88 ; 
         FIG. 92  is a side sectional view of the second pump assembly of  FIG. 88 , taken along line W-W in  FIG. 90 ; 
         FIG. 93  is an axial view of the proximal end of the second pump assembly of  FIG. 88 ; 
         FIG. 94  is an axial view of the distal end of the second pump assembly of  FIG. 88 ; 
         FIG. 95  is an axial section view of the second pump assembly of  FIG. 88 , taken along line X-X of  FIG. 90 ; 
         FIG. 96  is a side plan view of another example of an expandable cannula according to one embodiment; 
         FIG. 97  is a top plan view of the expandable cannula of  FIG. 96 ; 
         FIG. 98  is a side sectional view of the expandable cannula of  FIG. 96 , taken along line Y-Y of  FIG. 97 ; 
         FIG. 99  is a bottom plan view of the expandable cannula of  FIG. 97 ; 
         FIG. 100  is a plan view of the distal end of the expandable cannula of  FIG. 97 ; 
         FIG. 101  is a plan view of the proximal end of the expandable cannula of  FIG. 97 ; 
         FIG. 102  is an axial sectional view of the expandable cannula of  FIG. 97 , taken along lines Z-Z of  FIG. 99 ; 
         FIGS. 103-114  are plan views of various assembly configurations of the percutaneous blood pump system of  FIG. 1 , shown in order of method steps on using the system; and 
         FIG. 115  is a sectional view of a heart illustrating the percutaneous blood pump system of  FIG. 1  in use. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The percutaneous blood pump systems and related methods disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination. 
       FIG. 1  illustrates an example of a percutaneous blood pump system  10  according to one embodiment of the disclosure. By way of example only, the blood pump system  10  of the present example includes a blood pump  12  configured to pump a volume of blood from one portion of a target site to another portion of a target site and an introducer  14  configured to facilitate the placement of the blood pump  12  within the target site. The blood pump  12  includes a distal end portion  16 , a proximal end portion  18 , and a middle portion  20  extending between the distal and proximal end portions. The distal end portion  16  comprises an expandable cannula  22  having a first or proximal pump assembly  24  and a second or distal pump assembly  26  arranged in series or tandem. The proximal end portion  18  comprises a drive hub  28  and drive motor assembly  30 . The middle portion  20  comprises a flexible drive cable  32  contained within a drive cable sheath  34 . The drive cable  32  connects the drive motor assembly  30  to the first and second pump assemblies  24 ,  26  and transfers rotational energy from the drive motor assembly  30  to the first and second pump assemblies  24 ,  26  to activate the pumps. The cannula  22  includes an inlet tip  36  that feeds into a plurality of separate and distinct internal lumens (by way of example only) providing inlet blood flow to each of the pump assemblies. In the instant example embodiment, a first lumen  38  provides a dedicated inlet flow into the first pump assembly  24  and a second lumen  40  provides a dedicated inlet flow into the second pump assembly  26  (See  FIG. 5 ). In use, the blood pump  16  is a trans-valvular blood pump in which the inlet tip  36  of the multi-lumen cannula  22  is placed within a chamber of the heart (e.g. left ventricle) while outlet ports of the first pump assembly  24  and second pump assembly  26  are positioned in a trans-valvular manner (e.g. outlet ports of the proximal pump assembly  24  and distal pump assembly  26  are in the aorta on the opposite side of the aortic valve from the inlet tip  36 ) for providing left ventricular hemodynamic support. 
     By way of example only, two pump assemblies are shown. However, depending on the flow augmentation and insertion diameter required, additional pump assemblies may be configured. For example, as the desired insertion diameter of the system decreases, the number of pump assemblies may increase to achieve the same total flow augmentation amount and hemolysis index. Each pump assembly must have impeller blade design to produce a minimum amount of positive flow augmentation (e.g. &gt;0.2 LPM average over the cardiac cycle) against physiological pressure differential between the cannula inlet and pump outlet (e.g. 60 mmHg average of the cardiac cycle). 
       FIGS. 2-7  illustrate an example of the inlet cannula  22  in greater detail. By way of example only, the inlet cannula  22  has a distal portion  42 , a proximal portion  44 , and a middle portion  46  extending between the distal and proximal portions. The distal portion  42  includes the inlet tip  36  comprising an axial aperture  48  at the distal tip of the cannula  22  and a plurality of large side apertures  50  and small side apertures  51  spaced about the distal portion  42 . The axial aperture  48  and side apertures  50 ,  51  are configured to minimize pressure drop losses through the inlet tip and be tolerant of partial blockage by anatomical structures to maximize blood flow through first and second pump assemblies  24 ,  26 . The proximal portion  44  connects to the distal end of the drive cable sheath  34  and includes a first or proximal set of egress apertures  52  configured for enabling flow of blood pumped through the first pump assembly  24  and a second or distal set of egress apertures  54  configured for enabling flow of blood pumped through the second pump assembly  26 . Due to the structural configuration of linear or tandem pumps, the distal set of egress apertures  54  may be provided on only one side of the cannula  22 , for example the bottom side. The proximal set of egress apertures  52  may be formed on any (or all) sides of the cannula  22 . Egress apertures  52 ,  54  may include curved boundary surfaces that create a flow straightener or diffuser to recover rotational kinetic energy in the fluid exiting the impeller into pressure head energy thereby improving the efficiency of the pump and allowing for lower speeds and lower hemolysis for the same flow rate. The curved boundary surface may extend beyond the diameter of the pump housing and elongate into rib/blade features when released from the introducer. A guide wire aperture  56  positioned between the first and second pump assemblies  24 ,  26  (by way of example only) provides access to the guide wire lumen  58  extending distally within the cannula  22 . A similar guide wire aperture  60  near the distal end of the drive cable sheath  34  allows the guide wire or a catheter (not shown) to bypass the proximal pump assembly  24  to access the inlet tip  36  of the cannula  22 . Proximal access to the guide wire lumen  58  is via the hemostasis valve  136  on the drive hub  28 , described below. The cannula middle portion  46  extends between the distal and proximal portions  42 ,  44  and includes a pair of distinct parallel lumens  38 ,  40  that fluidly connect the inlet tip  36  to the first and second pump assemblies  24 ,  26 , respectively. The cannula middle portion  46  may be shaped straight or curved/angled for anatomical fit. In some embodiments, the cannula  22  may include only a single lumen that fluidly connects the inlet tip  36  to the first and second pump assemblies  24 ,  26 . 
     As shown by way of example in  FIG. 5 , the proximal pump assembly  24  and distal pump assembly  26  are arranged in series or tandem and are driven by a drive cable  32  contained within a drive cable sheath  34 . The blood flow into each pump assembly  24 ,  25  is accomplished within separate lumens  38 ,  40  of the multi-lumen cannula  22 . Blood flows from the left ventricle of the heart (for example) into the cannula  22  through the inlet tip  36 , passes through the second lumen  40  and into the distal pump assembly  26 , and then exits the cannula  22  into the aorta (for example) through distal egress apertures  54  formed within the distal pump housing  104 . Concurrently, blood also flows from the inlet tip  36  through the first lumen  38  and into the proximal pump assembly  24 , and then exits the cannula  22  into the aorta (for example) through proximal egress apertures  52  formed within the proximal pump housing  66 . Any single blood cell will only pass through one of the pump assemblies. Thus, the first and second pump assemblies  24 ,  26  are arranged in series (or tandem) but operate in parallel, enabling the blood pump  12  of the present example to pump twice the amount of blood compared to a single pump of the same size while exhibiting the same amount of hemolysis per volume pumped compared to a single pump of the same size. 
     The proximal pump assembly  24  and distal pump assembly  26  are connected to one another by way of a pump coupler  62 . By way of example only, the pump coupler  62  is a flexible tube extending between the proximal and distal pump assemblies  24 ,  26  that contains the drive cable as it passes between the proximal and distal pumps, contains pressurized purge fluid for the distal pump(s) hydrodynamic bearings, and also allows the percutaneous blood pump  12  to be inserted through anatomy having a curved path, for example through a vein or artery. The cannula  22  may be constructed of flexible material (e.g. polyurethane, silicone) with resiliently elastic support material (e.g. Nitinol, nylon) or resiliently foldable frame material (e.g. laser cut stainless steel tubing) embedded in the wall or connected to the wall, which expands to operation configuration after being released from the confines of introducer sheath  14  then re-collapses to the confines of the introducer sheath  14  for removal from the patient. 
       FIG. 6  illustrates a section view of the cannula  22  taken along lines  2 - 2  of  FIG. 4 , looking axially into the cannula  22  proximally toward the distal pump assembly  26 . The first lumen  38  and second lumen  40  are separated by a septum  64  to enable the parallel flow arrangement. Each lumen cross-sectional area is sized to minimize and optimize pressure drop losses from the cannula inlet tip  36  to the pump housing  104  inlet. 
       FIG. 7  illustrates a section view of the cannula  22  taken along lines  3 - 3  of  FIG. 4 , looking axially into the cannula  22  proximally toward the proximal pump assembly  24 . By way of example, the first lumen  38  at this point merges coaxial with the impeller  68  of the proximal pump assembly  24 . As described below, the proximal pump assembly  24  includes a tip bearing  72  having radial support struts  100  to locate the tip bearing  72  inside the proximal pump housing  66  while allowing blood flow to pass between the radial support struts  100 . The distal end of the impeller  68  couples with the drive cable  32  housed within the pump coupler  62  (see  FIG. 5 ). The drive cable  32  transmits rotational energy from the drive motor assembly  30  to the proximal pump impeller  68  and the distal pump impeller  106 . The pump coupler  62 , made of flexible metal or plastic tubing, houses the rotating interpump drive cable  33  (see  FIG. 5 ), connects the proximal pump tip bearing  72  to the distal pump shaft bearing  108  (see below), and transmits pressurized purge fluid for the hydrodynamic bearings of the distal pump(s). 
       FIGS. 8-15  illustrate an example of the first or proximal pump assembly  24  in greater detail according to one embodiment. By way of example, the proximal pump assembly  24  includes a housing  66 , an impeller  68 , a shaft bearing  70 , and a tip bearing  72 . The housing comprises a generally cylindrical tube configured to contain the impeller  68 , shaft bearing  70 , and tip bearing  72  therein. The housing  66  has a plurality of egress apertures  74  formed therein in the proximity of the impeller  68  to enable blood flow out of the proximal pump assembly  24 . The impeller  68  has a generally frustoconical shape including a base  76 , a fulcrum  78 , and a plurality of blades  80  (e.g. straight or curved) extending along the hub  82  from the base  76  to the fulcrum  78 . The impeller  68  further includes a generally cylindrical shaft  84  extending proximally from the base  76  and a generally cylindrical post  86  extending distally from the fulcrum  78 . The proximal shaft  84  is sized and configured to pass through the central aperture  96  of the shaft bearing  70  and engage the drive cable  32  directly as shown through inner lumen  88  or indirectly with a cylindrical coupler (not shown), thereby coupling the drive cable  32  to the impeller  68  so that the drive cable  32  may transfer rotational energy from the drive motor assembly  30  to the proximal pump impeller  68  to draw blood flow through the proximal pump assembly  24 . The distal post  86  is sized and configured to pass through the central aperture  102  of the tip bearing  72  and engage the interpump drive cable  33  in the same manner, which couples with the proximal shaft  120  of the distal pump impeller  106 , so that the drive cable  32  may transfer rotational energy from the motor drive motor assembly  30  to the distal pump impeller  106  by way of proximal pump impeller  68  and interpump drive cable  33  to draw blood flow through the distal pump assembly  26  at the same time (e.g. in parallel) as blood flow is being drawn through the proximal pump assembly  24 . In addition, the proximal shaft  84 , hub  82 , and cylindrical post  86  may have an internal passage configured for transporting purge fluid to the distal pump(s). The frustoconical shape of the impeller  68  forces the blood to flow out of the egress apertures  74 . This is known as a “mixed-flow” impeller design. Alternatively, the impeller  68  may have a hub  82  that is generally cylindrical in shape to create an “axial-flow” impeller, omitting the base  76  from the impeller  68 . This would have a second tip bearing  72  on the proximal end of the impeller in place of shaft bearing  70 . For the “axial-flow” design, the radial support struts  100  of the proximal tip bearing  72  may be configured with a curved shape to create a flow straightener or diffuser to recover rotational kinetic energy in the fluid exiting the impeller into pressure head energy thereby improving the efficiency of the pump allowing for lower speeds and lower hemolysis for the same flow rate. Alternatively, the pumps may have impellers of multi-stage design where blood passes through the multiple impellers in series, increasing pressure head performance allowing for further diameter reduction (e.g. &lt;6 Fr). 
     The shaft bearing  70  is generally circular in shape and has a planar distal surface  90 , a planar proximal surface  92 , a curved radial outer surface  94 , and a central aperture  96 . The shaft bearing  70  is sized to fit snugly within the housing  66 . The central aperture  96  is sized and configured to receive the proximal shaft  84  of the impeller  68  and allow the proximal shaft  84  and therefore the impeller  68  to rotate at high speed while maintaining axial alignment of the impeller  68  to ensure efficient rotation. The central aperture  96  may include one or more axial grooves to allow passage of pressurized purge fluid from the sheath  34  to the interface between the impeller base  76  and the bearing planar distal surface  90  to create a hydrodynamic bearing. The shaft bearing  70  may be comprised of two components a distal component and a proximal component with a compression spring element between them. The distal bearing outer surface  94  is sized for press-fit or adhesive bonding to the impeller housing preventing rotation while the proximal bearing is slip-fit on its outer surface  94  to allow axial translation with minimal radial run-out from the compression spring. The proximal surface  92  of the proximal shaft bearing is constrained from proximal axial movement by a shaft collar fixed to the rotating proximal shaft  84  and/or drive cable  32 . The spring compression force is transmitted from the distal bearing through the spring to the “floating” but non-rotating proximal bearing to the rotating shaft collar and proximal shaft  84  to the impeller proximal surface  92  which is suspended on a thin-film of purge fluid (e.g. saline, dextrose solution) that is pressurized by the spring force reaction to the distal surface  90  of the distal bearing. This arrangement, or others providing the same functional effect as described below, reduces frictional heat between the rotating impeller and shaft bearing while minimizing the radial runout of the impeller at high speeds. Excessive heat from rotational friction is known to activate the clotting cascade which poses risk of vascular embolism to the patient. Excessive impeller runout can cause flow disturbances within the impeller flow region reducing pump efficiency, cause blood damage, or activate platelets. Instead of a separate compression spring element, the proximal shaft  84  may be hollow with lateral slits to form a rotating tension spring. This configuration would involve only one shaft bearing  70  and the shaft collar and the bearing load path would be through the shaft instead of the compression spring. 
     The tip bearing  72  has a base  98 , a plurality of radial struts  100 , and a central aperture  102  extending axially through the base. The radial struts  100  extend radially outward from the base  98  and are sized to span the distance between the base  98  and the housing  66  so that the tip bearing  72  may be sized and configured to fit snugly within the housing  66 . The radial struts  100  may be straight or curved to form an inducer to precondition the fluid flow path to minimize hydraulic instability (e.g. flow separation, cavitation, vortices) within the impeller blade region. The central aperture  102  is sized and configured to receive the distal post  86  of the impeller  68  and allow the distal post  86  and therefore the impeller  68  to rotate at high speed while maintaining axial alignment of the impeller  68  to ensure coaxial rotation. Although shown in  FIG. 9  by way of example only as having three radial struts  100 , the tip bearing  72  may have any number of radial struts  100  without departing from the scope of the disclosure. In a similar manner to the hydrodynamic bearing arrangement described above for the proximal end of the impeller, the tip bearing may be fitted with a hydrodynamic bearing. In this instance, the shaft collar would be attached to the drive cable or impeller distal shaft and react the spring force on the distal face of the tip bearing while the fulcrum  78  would react the spring force on the proximal face of the tip bearing. In addition, the impeller housing would include anti-rotation and axial sliding feature to permit the tip bearing to self-align in the axial direction. Alternatively, proximal bearing and tip bearing may be comprised of blood immersed hydrodynamic bearings constructed from passive magnets for magnetic levitation of the pump shaft or of low friction materials (e.g. ruby, ceramic). 
       FIGS. 16-23  illustrate an example of the second or distal pump assembly  26  in greater detail according to one embodiment. By way of example, the distal pump assembly  26  includes a housing  104 , an impeller  106 , and a shaft bearing  108 . The housing  104  comprises a generally cylindrical tube configured to contain the impeller  106  and shaft bearing  108  therein. The housing  104  has a plurality of egress apertures  110  formed therein in the proximity of the impeller  106  to enable blood flow out of the distal pump assembly  26 . The impeller  106  has a generally frustoconical shape including a base  112 , a fulcrum  114 , and a plurality of blades  116  (e.g. straight or curved) extending along the sidewall  118  from the base  112  to the fulcrum  114 . The impeller  106  further includes a generally cylindrical shaft  120  extending proximally from the base  112 . The proximal shaft  120  is sized and configured to pass through the central aperture  128  of the shaft bearing  108  and engage an inner lumen  88  of the drive cable  32 , thereby coupling the drive cable  32  to the impeller  106  so that the drive cable  32  may transfer rotational energy from the drive motor assembly  30  to the distal pump impeller  106  to draw blood flow through the distal pump assembly  26 . The blades  116  are sized and configured to create a turbulence or current that draws the blood into the distal pump assembly  26 . The frustoconical shape of the impeller  106  forces the blood to flow out of the egress apertures  110 . 
     The shaft bearing  108  is generally cylindrical in shape and has a planar distal surface  122 , a sloped proximal surface  124 , a curved radial outer surface  126 , and a central aperture  128 . The shaft bearing  108  is sized to fit snugly within the housing  104 . The central aperture  128  is sized and configured to receive the proximal shaft  120  of the impeller  106  and allow the proximal shaft  120  and therefore the impeller  106  to rotate at high speed while maintaining axial alignment of the impeller  106  to ensure efficient rotation. The sloped proximal surface  124  is configured to gently urge blood flow toward the proximal pump assembly  24 . The proximal surface  124  may further include a generally cylindrical coupler recess  130  axially aligned with the central aperture  128  and configured to receive therein at least a portion of the pump coupler  62 . 
     In some embodiments, the shaft bearing  108  (and/or any other bearing disclosed herein) may be a hydrodynamic bearing and blood seal. In such a case, the bearing may have axial slots inside the central aperture  128  to allow passage of purge fluid so the impeller  106  “hydroplanes” on bearing cooling interface to prevent thrombus formation and hemolysis. The impeller  106  may be spring loaded against the bearing  108  to create a rotating check valve for pressure and creating thin film for fluidic suspension of the impeller  106  on the bearing  108 . See, e.g.  FIGS. 77-95  below. 
     By way of example, the proximal and distal impellers  68 ,  106  are shown herein as fixed diameter components but can also be self-expanding flexible blades that are delivered in a folded or collapsed state inside a folded or collapsed pump housing inside the introducer sheath. Moreover, while the proximal and distal pump assemblies  24 ,  26  are shown herein as located at the proximal end  44  of the cannula  22 , the proximal and distal pump assemblies  24 ,  26  may alternatively be located on the distal end  42  of the cannula  22 , in which case the cannula  22  may not require resiliently strong support material in the wall because the pump outlet pressure may be sufficient to support the cannula wall from collapse. 
       FIG. 24  illustrates an example of the proximal end portion  18  in greater detail according to one embodiment. By way of example, the proximal end portion  18  comprises a drive hub  28  and drive motor assembly  30 . The drive hub includes a first port  132  providing access to a first lumen within the drive cable sheath  34 , a second port  134  providing access to a second lumen within the drive cable sheath  34 , and a hemostasis valve  136  for insertion of the guide wire. The drive motor assembly includes the drive motor (not shown) including a drive motor rotor located within the rotor housing  138 . 
     By way of example, the drive cable sheath  34  comprises a flexible tubing of adequate length to locate the proximal pump assembly  24  in the desired anatomical position while the drive hub  28  is located outside the body. For example, in a typical transvalvular heart pump scenario, the blood pump  12  is advanced through the femoral artery accessed near a patient&#39;s groin such that one or more pumps (in this case proximal pump assembly  24  and distal pump assembly  26 ) is positioned in the aorta proximate the aortic valve. The drive cable sheath  34  houses the drive cable  32 , which is made from multiple wires (filars) and layers for torque transmission and flexibility suitable for the anatomical route required. The drive cable  32  connects to a motor rotor (magnet) supported by bearings inside of the rotor housing  138 . The first port  132  is fluidically connected to the central lumen of the drive cable sheath  34 . The second port  134  is fluidly connected to a side lumen of the drive cable sheath  34 . An infusion pump (not shown) may be fluidically connected to the second port  134 . The infusion pump supplies the pump bearings (e.g. proximal pump shaft bearing  70 , proximal pump tip bearing  72 , and distal pump shaft bearing  108 ) with fluid (e.g. 30% dextrose intravenous solution) via the side lumen to de-air the system prior to insertion into a patient and to lubricate and flush the pump bearings during rotational operation. Return flow from the infusion pump travels along the central lumen and exits the through the first port  132  into a waste bag (not shown) while flushing wear particulate from the rotating drive cable  32  and drive cable sheath  34  interaction. The hemostasis valve  136  fluidically connects to another guide wire lumen  60  provided in the drive cable sheath  34  for passage of a guide wire sheath and guide wire to access the inlet tip  36  for selective positioning or repositioning of the cannula  22  in the heart. 
       FIGS. 25-30  illustrate an example of an introducer  14  in greater detail according to one embodiment. By way of example only, the introducer  14  includes an introducer hub  140  locating at the proximal end of the introducer  14  and a flexible, thin-walled tubular sheath  142  extending distally from the hub  140 . The introducer hub  140  comprises a first hemostasis valve  144 , a second hemostasis valve  146 , and a lumen port  148 . The sheath  142  includes a proximal portion  150  and a distal portion  152 . The proximal portion comprises a central lumen  154  and a side lumen  156 , and a side lumen aperture  158  formed therein that provides access to the side lumen  156 . The distal portion  152  includes a distal lumen  160 , a tapered distal tip  162  designed for percutaneous insertion into a peripheral vessel, and an occlusion balloon  164  configured for sealing blood flow from the femoral artery around the introducer  14  to prevent bleeding at the introducer access site when the bypass catheter balloon  172  is inflated. By way of example only, the first hemostasis valve  144  may be of rotating collet or self-sealing duckbill-type valve for slidable sealing of the drive cable sheath  34  during selective positioning or repositioning of the cannula  22  in the heart. The second hemostasis valve  146  enables access to the proximal side lumen  156  of the sheath  142 . The lumen port  148  provides access to the central lumen  154  of the sheath  142 . 
     Referring now to  FIG. 30 , the side lumen aperture  158  forms a passage between the distal lumen  160  and the side lumen  156  for insertion of a distal perfusion cannula  166  to bypass blood under systemic pressure around the insertion site of the introducer sheath  142 . The circuit allows blood flow through an annulus formed by the outer diameter of the drive cable sheath  34  and the inner diameter of the introducer sheath distal lumen  160 , through the introducer sheath proximal central lumen  154  alongside the drive cable sheath  34 , through the introducer sheath central lumen port  148 , through removably connected distal perfusion cannula hub  168 , through flexible tubing of the distal perfusion cannula  166  which has distal segment placed through the annulus formed by the outer diameter of the drive cable sheath  34  and the inner diameter of the introducer sheath distal lumen  160  then through the introducer sheath side lumen aperture  158  whereby the tip  170  of the distal perfusion cannula  166  is placed for flow distal to the insertion site of the introducer sheath  142  in the patient. Optionally, to isolate the insertion site from blood leakage from the patient, the introducer sheath  142  has occlusion balloon  162  connected to its outer diameter. The occlusion balloon  162  may be selectively inflated using saline injected through another side lumen of the introducer sheath  142  via side-arm (not shown) of introducer hub  140 . Additionally, the distal perfusion cannula  166  may have a distal perfusion occlusion balloon  172  connected to its outer diameter, which may also be selectively inflated using saline injected through side-lumen (not shown) of distal perfusion cannula  166  via side-arm (not shown) of distal perfusion cannula hub  168 . 
       FIGS. 31-38  illustrate several examples of an inlet tip  36  that may be provided on the distal portion  42  of the cannula  22 . By way of example,  FIGS. 31-32  illustrate the inlet tip  36  example described above, comprising a tapered shape with an axial aperture  48  at the distal tip of the cannula  22  and a plurality of large side apertures  50  on the tapered portion and small side apertures  51  spaced about the distal portion  42  of the cannula proximal of the tapered portion. The axial aperture  48  and side apertures  50 ,  51  are configured to allow for sufficient blood flow into the cannula  22  to the first and second pump assemblies  24 ,  26 . 
       FIGS. 33-34  illustrate an example of an inlet tip  36  comprising a duckbill shape similar to the taper shape of  FIGS. 31-32  but with a section of the inlet tip  36  removed to create an extra large axial aperture  48  at the distal tip of the cannula  22 . The inlet tip  36  also includes a plurality of large side apertures  50  on the tapered portion and small side apertures  51  spaced about the distal portion  42  of the cannula proximal of the tapered portion. The axial aperture  48  and side apertures  50 ,  51  are configured to allow for sufficient blood flow into the cannula  22  to the first and second pump assemblies  24 ,  26 . 
       FIGS. 35-36  illustrate an example of an inlet tip  36  comprising a taper shape similar to the taper shape of  FIGS. 31-32  with an expandable balloon element  174  provided thereon. The inlet tip  36  includes an axial aperture  48 , a plurality of large side apertures  50  on the tapered portion, and a plurality of small side apertures  51  spaced about the distal portion  42  of the cannula proximal of the tapered portion. The axial aperture  48  and side apertures  50 ,  51  are configured to allow for sufficient blood flow into the cannula  22  to the first and second pump assemblies  24 ,  26 . The expandable balloon element  174  is positioned about the side apertures  51  so that upon expansion of the balloon element  174  the side apertures  51  are protected from obstruction by tissue. The expandable balloon element  174  may be selectively inflated with saline after insertion into the patient by injecting saline through a side-port (not shown) in drive hub  28  which is connected to at least one side-lumen (not shown) in drive cable sheath  34 . 
       FIGS. 37-38  illustrate an example of an inlet tip  36  comprising a taper shape similar to the taper shape of  FIGS. 31-32  with an expandable mesh element  176  provided thereon. The inlet tip  36  includes an axial aperture  48 , a plurality of large side apertures  50  on the tapered portion, and a plurality of small side apertures  51  spaced about the distal portion  42  of the cannula proximal of the tapered portion. The axial aperture  48  and side apertures  50 ,  51  are configured to allow for sufficient blood flow into the cannula  22  to the first and second pump assemblies  24 ,  26 . The expandable mesh element  176  is positioned about the side apertures  51  so that upon expansion of the mesh element  176  the side apertures  51  are protected from obstruction by tissue. The expandable mesh element  176  may be made from elastic metal or plastic and is elastically collapsed inside of the introducer sheath  142  during insertion and then expands when the cannula  22  is slid outside of the introducer sheath  142  by pushing on the drive cable sheath  34 . For removal from the patient, the mesh element  176  is slid back inside the introducer sheath  142  by pulling on the drive cable sheath  34 . 
       FIGS. 39-42  illustrate an example of a distal end portion  16  of the blood pump system  10 , comprising an inlet cannula  178  with multiple impeller blood pumps according to one embodiment of the disclosure. By way of example only, the inlet cannula  178  of the present example comprises a radial multi-lumen cannula  178  having an inlet tip  180 , an outlet  182 , a plurality of pump assemblies  184  spaced along the inside of the cannula  178 , and a plurality of inlet lumens  186  and outlet lumens  188  separated by lumen partitions  190 . Each pump assembly  184  includes an impeller  192  contained within a dedicated pump housing  193 , which includes an inlet port  194 , an outlet port  196 , and a shaft bearing  198  (similar to the pump arrangements described above). Each pump assembly  184  has a dedicated inlet lumen  186  supplying blood flow to the impeller  192  and a dedicated outlet lumen  188  for the blood to flow away from the impeller  192  to the outlet  182  where the blood enters the aorta (for example). Rotational energy from the drive cable (not shown) connected to an impeller  192  and supported by pump shaft bearing  198  creates a pressure difference between the inlet port  194  and the outlet port  196 . The pressure at the outlet port  196  is higher than the pressure at the inlet port  194  resulting in flow from the inlet tip  180  to the outlet  182  due to the lumen partition  190 . More specifically, blood flow travels from the inlet tip  180  into an inlet lumen  186  of the radial multi-lumen cannula  178 , through an inlet port  194  of one of the several pump assemblies  184 , through an outlet port  196  and into an outlet lumen  188  on the other side of a lumen partition  190  where it travels to the cannula outlet  182 . The inlet port  194  and outlet port  196  may be arranged in the same angular plane or in different angular planes with respect to the central axis of the pump housing  193 . 
     Each inlet lumen  186  and corresponding outlet lumen  188  (e.g. that are separated by a lumen partition  190 ) together form a radial channel  200 . The radial channels  200  in the radial multi-lumen cannula  178  may be arranged in a linear orientation with respect to the central axis of the pump housing  193  or alternatively in a spiral orientation. In some embodiments, the lumen partition  190  may be of self-sealing type having construction that allows through passage of a tubular or wire structure such as a catheter or guide wire  202  and seals against retrograde flow from the proximal side when the tubular or wire structure is removed. In some embodiments, the lumen partition  190  may be an elastically expandable orifice or other type of hemostasis valve. Alternatively, the lumen partition  190  may be constructed to allow a guide wire to remain in place while allowing the guide wire or catheter tip to selectively be positioned. The radial multi-lumen cannula  178  may be of expandable/collapsible construction in which it is inserted in the patient constrained within a smaller diameter introducer sheath  142  (for example) and then self-expands by way of elastic support members in the wall of the tubing when selectively positioned outside the introducer sheath  142  by pushing on the drive cable sheath  34 . Removal from the patient may be by way of selectively withdrawing the drive cable sheath  34  to position radial multi-lumen cannula  178  inside a smaller diameter introducer sheath  142  causing elastic support members used in construction of radial multi-lumen cannula  178  to collapse. As shown in  FIG. 42 , the plurality of pump assemblies  184  are arranged in series and separated by pump assembly couplers  204 , but operate in parallel as with the pump assemblies described above. 
       FIGS. 43-45  illustrate an example of a percutaneous blood pump system  210  according to another embodiment of the disclosure. By way of example only, the blood pump system  210  of the present example includes a sheath  212 , a catheter  214 , a pump subsystem  216 , and an obturator  218 . Generally, the sheath  212  is configured to receive the catheter  214  therein and constrain the expandable cannula  272  in a collapsed configuration during insertion while sealing the catheter shaft  268  for positioning in an unsheathed state in the body. The catheter  214  functions as a conduit for blood flow from the heart chamber into the body, and also seals the guide wire  202  and pump drive shaft  334  for insertion into the body. The pump subsystem  216  creates a pressure difference between the inlet and outlet apertures of the cannula to drive blood flow into the body. 
     The blood pump system  210  of the present example is similar to the blood pump system  10  of the previously described example in that the blood pump system  210  is a multiple impeller pump system having a plurality of pump assemblies arranged in a linear or tandem arrangement but operating in parallel, in that blood (or any other fluid) pumped through one pump assembly will not pass through any other pump assemblies. However, the blood pump system  210  of the present example differs from the blood pump system  10  described above in at least two aspects: first, the blood pump system  210  of the present example employs a single lumen cannula that supplies all of the pump assemblies with intake blood, and second, the pump subsystem  216  of the present example is removable/replaceable and is inserted after initial placement of the catheter and removal of the guide wire  202  and obturator  218 . This enables the use of a smaller diameter catheter than may be otherwise needed. 
     The blood pump system  210  of the present example is scalable to meet the needs of any particular patient. For example, the number of pump assemblies may be increased or decreased depending on flow requirements without affecting hemolysis efficiency. If a smaller catheter is needed (for example due to partial blockage, other anatomical limitations, or to reduce access site bleeding complications), then additional pump assemblies may be added to increase flow with the same hemolysis index (mg plasma free hemoglobin per liter blood pumped). If lower hemolysis index is needed, then additional pump assemblies may be added and the pump speed of each reduced, resulting in the same flow with lower hemolysis index. 
       FIGS. 46-48  illustrate an example of a sheath  212  in greater detail according to one example embodiment. Continued reference to  FIGS. 43-45  may be made to understand how the various components of the sheath  212 , catheter  214 , and pump subsystem  216  interact with one another. By way of example only, the sheath  212  includes a proximal end  220 , a distal end  222 , and a shaft  224  extending between the proximal and distal ends  220 ,  222 . The proximal end  220  may include a sterile sleeve  226 , a hemostasis valve  228 , and a fluid line  230 . The sterile sleeve  226  of the instant example comprises a clear, thin-walled plastic sleeve having a distal seal  232  configured to fluidly seal the hemostasis valve  228  proximal of the fluid line  230 , a distal chamber  234  configured to contain the valve handle  244  within a sterile environment while allowing the valve handle  244  rotational freedom within the distal chamber  234 , an expandable chamber  236  having bellowed folds  238  configured to enable the expandable chamber  236  to expand proximally to cover a length of the catheter shaft  268  (see e.g.  FIGS. 72-73 ), and a proximal seal  240  configured to fluidly seal the catheter shaft  268  during use. The sterile sleeve  226  is configured to maintain the sterility of the hemostasis valve  228  and the catheter shaft  268  while repositioning the catheter  214  relative to the sheath  212  after insertion into the patient during an initial sterile procedure. 
     The hemostasis valve  228  of the present example embodiment is a clear, rigid polymer valve assembly with an elastomeric seal and a rotating locking handle that seals blood inside the patient while also allowing axial translation of the catheter shaft  268  within the sheath  212 . The hemostasis valve  228  includes (by way of example only) an inner lumen  242  extending axially therethrough, a proximally-located rotating valve handle  244  and a fluid port  246  fluidly connected to and extending laterally from the inner lumen  242 . The lumen  242  is sized and configured to allow passage of a number of instrument and components therethrough, including but not limited to the catheter shaft  268 , obturator  218 , pump drive shaft  334 , and the like, and is also configured to allow the flow of fluids therethrough. The valve handle  244  may be generally cylindrical in shape and have a friction element  248  (e.g. grooves, ridges, etc.) to enable a user to grip and rotate the valve handle  244  through the sleeve  226  to selective close and open the hemostasis valve  228 . The fluid port  246  fluidly connects to the outlet opening  252  of the fluid line  230 . By way of example only, the fluid line  230  is a clear flexible polymer tube having a proximal inlet opening  250 , distal outlet opening  252 , and a stopcock valve  254 . The fluid line  230  may be configured to allow de-airing and flushing of the hemostasis valve  228  and sheath  212  with anticoagulant fluid (for example). 
     By way of example, the distal end  222  comprises a tip tube  256  and a tip funnel  258 . The tip tube  256  is a thin-walled rigid tube positioned within the tip funnel  258  and transmits forces applied to the catheter  214  for sheathing and unsheathing of the expandable cannula  272 . The tip funnel  258  guides the expandable cannula  272  into the shaft  224  and may include an outwardly-flared edge  260  that flexes and collapses when inserted into a patient&#39;s vasculature. The shaft  224  is a generally cylindrical flexible tube having an inner lumen  262  extending therethrough. The shaft  224  may be sized and configured such that the outer diameter of the proximal end fits snugly within the inner lumen  242  of the hemostasis valve  282  so as to fluidly seal the interface between the outer shaft  224  and hemostasis valve  228 . The inner lumen  262  is sized and configured to allow passage of a number of instruments and components therethrough, including but not limited to the catheter shaft  268 , obturator  218 , pump drive cable assembly  332 , and the like, and is also configured to allow the flow of fluids therethrough. 
       FIGS. 49-51  illustrate an example of a catheter  214  in greater detail according to one example embodiment. Continued reference to  FIGS. 43-45  may be made to understand how the various components of the sheath  212 , catheter  214 , and pump subsystem  216  interact with one another. By way of example only, the catheter  214  comprises a hemostasis valve  264 , a fluid line  266 , a shaft  268 , a proximal shroud  270 , an expandable cannula  272 , a catheter tip housing  274 , and an atraumatic tip  276 . The hemostasis valve  264  of the present example embodiment is a clear, rigid polymer valve assembly with an elastomeric seal and a pump latch that seals blood inside the patient while also providing a conduit for and allowing axial translation of the guide wire  202  and pump shaft  334  within the catheter  214 . The hemostasis valve  264  includes (by way of example only) an inner lumen  278  extending axially therethrough, a pump latch  280 , and a fluid port  282  fluidly connected to and extending laterally from the inner lumen  278 . The lumen  278  is sized and configured to allow passage of a number of instrument and components therethrough, including but not limited to the obturator  218 , guide wire  202 , pump shaft  334 , and the like, and is also configured to allow the flow of fluids therethrough. The pump latch  280  is configured to interact with a corresponding latching element of the pump motor assembly  330  and includes a locking element  284  (e.g. snap-fit, etc.) to securely connect the catheter hemostasis valve  264  to the pump  216  while simultaneously opening the hemostasis valve  264 . In some embodiments, the pump latch  280  may be configured to provide visual, audible, and/or tactile feedback to the user to indicate a successful association has been made. The fluid port  282  fluidly connects to the outlet opening  288  of the fluid line  266 . By way of example only, the fluid line  266  is a clear flexible polymer tube having a proximal inlet opening  286 , distal outlet opening  288 , and a stopcock valve  290 . The fluid line  266  may be configured to allow de-airing and flushing of the hemostasis valve  264  and catheter  214  with anticoagulant fluid (for example). The fluid line  266  also provides an access portal for connecting a pressure monitoring system (not shown) to measure the patient blood pressure on the outside of the catheter  214  near the proximal shroud  270 . 
     The shaft  268  by way of example only comprises an elongated thin-walled, flexible tubular member extending between the hemostasis valve  264  and the proximal shroud  270 . The shaft  268  has an outer diameter configured for snug interaction within the lumen  278  of the hemostasis valve  264  so as to provide a sealed interface between the hemostasis valve  264  and the catheter shaft  268 . The shaft  268  further includes an inner lumen  292  sized and configured to allow passage of a number of instruments and components therethrough, including but not limited to the obturator  218 , guide wire  202 , pump shaft  334 , and the like, and is also configured to allow the flow of fluids therethrough. The shaft  268  also includes at least one distal opening  294  positioned near the interface with the proximal shroud  270 , the distal opening  294  configured to enable a pressure monitoring system to measure the patient blood pressure on the outside of the catheter  214  near the proximal shroud  270 . 
     The proximal shroud  270  by way of example only is a generally cylindrical tubular member of rigid construction having an inner lumen  296  extending axially therethrough and one or more flow ports  298  formed therein. The proximal shroud  270  is positioned between the distal end of the catheter shaft  268  and the proximal end of the expandable cannula  272 , and serves as a housing for the proximal pump impeller  356  and thus the flow ports  298  serve as inlet ports or outlet ports for impeller flow depending on flow direction. The inner lumen  296  is sized and configured to allow passage of a number of instruments and components therethrough, including but not limited to the obturator  218 , guide wire  202 , one or more pump assemblies  336 ,  338 ,  340 , and the like, and is also configured to allow the flow of fluids therethrough. 
     The catheter tip housing  274  by way of example only is a generally cylindrical rigid member having a inner lumen  300  extending axially therethrough, a tapered distal tip  302 , and a plurality of flow ports  304  formed therein. The inner lumen  300  is sized and configured to allow passage of a number of instruments and components therethrough, including but not limited to the obturator  218 , guide wire  202 , and the like, and is also configured to allow the flow of fluids therethrough. The tapered distal tip  302  provides a tapered transition into the patient&#39;s vasculature. The flow ports  304  serve as inlet ports or outlet ports for blood flow to or from the cannula  272  depending on flow direction. The flow ports  304  may be curved to prevent blockage by anatomical structures inside the heart. 
     The atraumatic tip  276  by way of example only is a flexible member having an inner lumen  306  extending from the distal end of the catheter  214 . The inner lumen  306  is sized and configured to allow passage of a number of instruments and components therethrough, including but not limited to the obturator  218 , and guide wire  202 . When configured for left-ventricular support (for example), the atraumatic tip  276  prevents trauma to heart by flexing and distributing axial load along larger area. The atraumatic tip  276  also positions the catheter tip housing  274  away from structures in heart that may impede blood flow into the cannula  214 , and provides conduit for tracking the catheter over the guide wire  202  for positioning in the heart. 
       FIGS. 52-57  illustrate an example of an expandable cannula  272  in greater detail according to one embodiment. Continued reference to  FIGS. 43-45  may be made to understand how the various components of the sheath  212 , catheter  214 , and pump subsystem  216  interact with one another. The cannula  272  of the present example comprises a thin-wall, self-expanding, re-collapsible, cylindrical tube providing a conduit for blood flow out of heart (for example). The cannula  272  is inserted in collapsed configuration (see, e.g.  FIG. 74 ) and expands to an expanded operating configuration upon emergence from the sheath  212 . The expanded cross-section is configured to be suitable for the desired length of the cannula  214 , number of tandem pumps operating, and desired maximum pressure drop from the cannula inlet to the pump inlets at the desired maximum system flow rate. 
     By way of example only, the expandable cannula  272  comprises an expandable body  308 , a distal end  310 , a proximal end  312 , inner lumen  314 , and one or more flow port(s)  316  formed in the bottom side of the body  308  (by way of example). The expandable body  308  comprises a thin-walled, self-expanding, re-collapsible tube made from flexible polymer and reinforcing frame, and includes an inner lumen  314  extending axially therethrough and a proximal taper  320  on the outer proximal surface of the expandable body  308 . The inner lumen  314  is sized and configured to allow passage of a number of instrument and components therethrough, including but not limited to the obturator  218 , guide wire  202 , and the like, and is also configured to allow the flow of fluids therethrough. 
     The inner lumen  314  also houses one or more middle and/or distal pump assemblies  338 ,  340  that are inserted into the cannula  272  after expansion of the expandable body  308 . A flow port  316  is provided for each middle and/or distal pump assembly  338 ,  340  within the cannula  272  to allow the fluid to flow through tandem arranged impellers (e.g. impellers  374 ,  396 ) in parallel through the cannula  272 . To ensure that the pump assemblies are properly aligned with the flow ports  316  upon insertion into the cannula  272 , the inner lumen may further include one or more pump alignment features, including but not limited to (and by way of example only) a laterally-oriented pump stop  322  and/or an axially oriented pump guide  324 . By way of example only, the pump stop  322  may be a physical barrier to prevent advancement of the pump assemblies once the shroud flow ports  384  are laterally aligned with the cannula flow ports  316 . The pump guide  324  of the present example comprises an elongated axially-oriented tongue or rail in the inner lumen  314  that is configured to slidably mate with a complementary alignment feature  386  (e.g. a corresponding axially-aligned groove or track) formed on the outer surface of the middle and/or distal pump shroud(s)  372  to ensure rotational alignment of the flow ports  316 ,  384 . 
     The proximal taper  320  facilitates collapsing of the expandable body  308  for removal from the body. More specifically, to remove the expandable cannula  272  from the body, a user exerts an axial force in the proximal direction to pull the catheter back through the sheath  212 . As the proximal taper  320  encounters the tip funnel  258  of the sheath  212 , the proximal taper  320  translates the axial force applied to the cannula body  308  by the tip tube  256  (due to its rigidity) into inward radial force to collapse the expandable body  308  for removal through the sheath  212 . 
     The distal end  310  is configured with a plurality of apertures  326  formed in a distal taper element  328  of the expandable body  308 . The apertures  326  may function as ingress or egress apertures (depending of flow direction) to the cannula  272 , augmenting the cross-sectional area of the catheter tip housing  274 . The cannula  272  also provides a conduit for tracking the catheter  214  over the guide wire  202  for positioning in the heart. 
       FIGS. 58-67  illustrate the pump subsystem  216  in greater detail according to one embodiment. Continued reference to  FIGS. 43-45  may be made to understand how the various components of the sheath  212 , catheter  214 , and pump subsystem  216  interact with one another. By way of example only, the pump subsystem  216  comprises a motor assembly  300 , a drive cable assembly  332 , a drive shaft, and a plurality of impeller pump assemblies arranged in a linear or tandem fashion, for example a first or proximal pump assembly  336 , a second or middle pump assembly  338 , and a third or distal pump assembly  340 . For the purpose of illustration, the embodiment described herein by way of example includes three pump assemblies, however it should be understood that the number of pump assemblies employed is scalable depending upon the specific needs of the patient, so long as there is a minimum of two impeller pumps present (e.g. a first or proximal pump assembly  336  and a second or distal pump assembly  340 ). By way of example, the motor assembly  330  includes an electric motor, a drive cable assembly coupler, and a purge tubing manifold, and is configured to transmit rotational energy to the and purge fluid to the drive cable assembly  332 . The motor assembly  330  is connected to a control unit (not shown) by way of a cable  342  and cable connector  344 . The cable  342  is an electrical and hydraulic cord that conducts electrical power from the control unit (via cable connector  344 ) to the motor assembly  330 , and transmits purge fluid to/from the cable connector  344  to/from the motor assembly  330 . 
     By way of example, the drive cable assembly  332  may be a flexible torque cable having an outer drive sheath  346  and an inner drive sheath  348 . The drive cable assembly  332  may be configured to transmit rotational energy to the drive shaft  334  and purge fluid power to the proximal pump assembly  336 . The drive shaft  334  may be a hollow shaft having rigid segments  350 , flexible segments  352 , and tension spring segments  354 . For example, the rigid segments  350  support pump impellers, the flexible segments  352  allow flex between impellers for insertion into patient anatomy, and tension spring segments  354  provide axial compression force for hydrodynamic bearings. The drive shaft  334  may also provide a conduit for purge fluid from the proximal pump assembly  336  to the middle pump assembly  338  and/or the distal pump assembly  340 . 
     An example of the first or proximal pump assembly  336  will now be described with particular reference to  FIGS. 59, 65, and 70 . By way of example, the proximal pump assembly  336  includes an impeller  356 , a bearing  358 , a bearing housing  360 , and a drive shaft collar  362 . The proximal pump assembly  336  does not have a housing or shroud in this example embodiment because upon insertion the proximal pump assembly  336  is positioned within the proximal shroud  270  of the catheter  214  such that the impeller  356  aligns with the flow ports  298  of the shroud  270  to enable blood flow out of the proximal pump assembly  336  (or into the proximal pump assembly  336  depending upon the flow direction). The impeller  356  has a proximal base  364 , a distal end  338 , and a plurality of blades  368  (e.g. straight or curved) extending along the hub  370  from the base  364  to the distal end  366 . The impeller  356  further includes an axial lumen extending therethrough configured to receive the drive shaft  334  therein, thereby coupling the drive shaft  334  to the impeller  356  so that the drive shaft  334  may transfer rotational energy from the motor assembly  330  to the proximal pump impeller  356  to draw blood flow through the proximal pump assembly  336 . 
     The bearing  358  is positioned proximal of the impeller  356  and comprises a generally cylindrical rotary hydrodynamic shaft bushing, that constrains the drive shaft for rotational axial alignment. The bearing  358  also transmits purge fluid to the proximal impeller  356 . The bearing housing  360  is a generally cylindrical rigid tubular member configured to contain the proximal bearing  336  therein. The drive shaft collar  362  is positioned proximal of the bearing  358  and comprises a rigid element attached to the drive shaft  334 . The drive shaft collar  362  reacts the axial tension spring force from the drive shaft  334  on the proximal end of the bearing  358  creating a hydrodynamic seal with the bearing  358 . 
     An example of the second or middle pump assembly  338  will now be described with particular reference to  FIGS. 60, 64, and 70 . By way of example, the middle pump assembly  338  includes a shroud  372 , an impeller  374 , a bearing  376 , a drive shaft collar  378 , and a drive shaft sleeve  380 . The shroud  372  in this case is necessary because the middle pump assembly  338  is positioned within the lumen  314  of the expandable cannula  272 . By way of example only, the shroud  372  is generally cylindrical and includes an inner cavity  382 , at least one flow port  384 , and an alignment feature  386 . The inner cavity is sized and configured to contain the impeller  374  and a substantial portion of the bearing  376  therein. The at least one flow port  384  is configured to align with a flow port  316  in the cannula  272  to allow the fluid to flow into or out of the middle pump assembly  338  (depending on flow direction). The alignment feature  386  is configured to interact with a corresponding feature on the cannula  272  described above. The alignment feature  386  of the present example comprises an elongated axially-oriented groove or track configured to slidably mate with a complementary alignment feature in the cannula  272  (e.g. a corresponding axially-aligned tongue or rail in the inner lumen  314  described above) to ensure rotational alignment of the flow ports  316 ,  384 . 
     The impeller  374  has a proximal base  388 , a distal end  390 , and a plurality of blades  392  (e.g. straight or curved) extending along the hub  394  from the base  388  to the distal end  390 . The impeller  374  further includes an axial lumen extending therethrough configured to receive the drive shaft  334  therein, thereby coupling the drive shaft  334  to the impeller  374  so that the drive shaft  334  may transfer rotational energy from the motor assembly  330  to the proximal pump impeller  374  to draw blood flow through the middle pump assembly  338 . 
     The bearing  376  is positioned proximal of the impeller  374  and comprises a generally cylindrical rotary hydrodynamic shaft bushing, that constrains the drive shaft  334  for rotational axial alignment. The bearing  376  also transmits purge fluid to the middle impeller  374 . The drive shaft collar  378  is positioned proximal of the bearing  376  and comprises a rigid element attached to the drive shaft  334 . The drive shaft collar  378  reacts the axial tension spring force from the drive shaft  334  on the proximal end of the bearing  376  creating a hydrodynamic seal with the bearing  376 . The drive shaft sleeve  380  by way of example only is a flexible tube attached and sealed to the drive shaft  334  to constrain purge fluid within the drive shaft  332 . 
     An example of the third or distal pump assembly  340  will now be described with particular reference to  FIGS. 61, 66, 67, and 70 . By way of example, the distal pump assembly  340  includes a shroud  372 , an impeller  396 , a bearing  376 , a drive shaft collar  378 , and a drive shaft sleeve  380 . The distal pump assembly  340  is substantially similar to the middle pump assembly  338  described above, and in fact several components including the shroud  372 , bearing  376 , drive shaft collar  378 , and drive shaft sleeve  380  identical in form and function, and a repeat discussion is not necessary. The impeller  374  has a proximal base  388 , a distal end  390 , and a plurality of blades  392  (e.g. straight or curved) extending along the hub  394  from the base  388  to the distal end  390 . The impeller  374  further includes an axial lumen extending therethrough configured to receive the drive shaft  334  therein, thereby coupling the drive shaft  334  to the impeller  374  so that the drive shaft  334  may transfer rotational energy from the motor assembly  330  to the proximal pump impeller  374  to draw blood flow through the middle pump assembly  338 . The distal impeller  396  differs from the middle impeller  374  in that the distal impeller  396  also includes a distal cap  398  configured to seal the distal end of the axial lumen. 
       FIGS. 68-70  illustrate by way of example only the positioning of the pump assemblies  336 ,  338 ,  340  within the catheter  214  according to one embodiment, and as described above. 
       FIG. 71  illustrates an example of an obturator  218  according to one embodiment. By way of example, the obturator of the present example includes a flexible tube  400  configured to receive a guide wire therein, and a guide wire hemostasis valve  402  configured to seal blood inside the patient and allow for axial translation of the catheter  214  over the guide wire  202 . 
       FIGS. 72-74  illustrate the percutaneous blood pump assembly  210  configured for insertion into a patient according to one embodiment. By way of example only, and as shown in  FIG. 72 , the obturator  218  is inserted into the catheter  214 , which in turn is inserted into the sheath  212 . As shown in  FIG. 73 , a guide wire  202  may be inserted through the obturator  218 . As shown in  FIG. 74 , the self-expanding cannula  272  is held in a collapsed state for insertion by the sheath shaft  224 . Notably, the expandable body  308  in the collapsed state occupies space that will be occupied by one or more pump assemblies (e.g. second pump assembly  338  and/or third pump assembly  340 , and so on) upon expansion of the expandable cannula  272 . This enables a cannula  272  with a smaller (collapsed) diameter to be inserted through the body, improving the ease of access. 
       FIGS. 75-95  illustrate an example of a pump subsystem  410  configured for use with the blood pump system  10  disclosed herein above according to one embodiment of the disclosure. By way of example only, the pump subsystem  410  comprises a motor assembly  412 , a drive cable assembly  414 , a drive cable  416  (similar to drive cable  32 ), and a plurality of impeller pump assemblies arranged in a linear or tandem fashion, for example a first or proximal pump assembly  418  and a second or distal pump assembly  420 . For the purpose of illustration, the embodiment described herein by way of example includes two pump assemblies, however it should be understood that the number of pump assemblies employed is scalable depending upon the specific needs of the patient, so long as there is a minimum of two impeller pumps present (e.g. a first or proximal pump assembly  418  and a second or distal pump assembly  420 ). By way of example, the motor assembly  412  includes an electric motor, a drive cable  416  coupler, and a purge tubing/drive sheath manifold, and is configured to transmit rotational energy to the drive cable  416  and purge fluid to/from the drive cable assembly  414 . The motor assembly  412  is connected to a control unit (not shown) by way of a cable  422  and cable connector  424 . The cable  422  is an electrical and hydraulic cord that conducts electrical power from the control unit (via cable connector  424 ) to the motor assembly  412 , and transmits purge fluid to/from the cable connector  424  and to/from the motor assembly  412 . 
     By way of example, the drive cable assembly  414  may include a drive cable  416 , an outer drive sheath  426 , and an inner drive sheath  428 . The drive cable assembly  414  may be configured to transmit rotational energy to the drive cable  416  and purge fluid pressure and flow to the proximal pump assembly  418  for operation of hydrodynamic bearings. Fresh purge fluid is transmitted to the proximal pump assembly  418  via the outer drive sheath  426  which is coaxially arranged outside the inner drive sheath  428 . The inner drive sheath  428  houses the drive cable  416  and waste purge fluid that flushes the wear particles outside the patient. The drive cable  416  is made from multiple wires (filars) and layers for torque transmission and flexibility suitable for the anatomical route required. The drive cable assembly  414  is connected to the bearing assembly  438  by way of a sheath adapter  580  and cable adapter  486 . By way of example only, the sheath adapter  580  includes a distal post  582  sized and configured to nest within the inner lumen  488  of the bearing housing  478 , and may be secured to the bearing housing  478  by any suitable mechanism (e.g. threaded connection, adhesives, etc.). The sheath adapter  580  has an inner lumen  584  configured to bond the outer drive sheath  426  and seat the inner drive sheath  428 . The inner lumen  584  has axial grooves  586  formed therein to allow for the passage of purge fluid from the outer sheath to the proximal pump assembly. 
       FIGS. 77-87  illustrate an example of a first or proximal pump assembly  418  according to one example embodiment. By way of example, the first or proximal pump assembly  418  includes an impeller assembly  436  and a bearing assembly  438  each contained with in a housing  440 . The housing  440  of the instant example comprises a generally cylindrical tubular member having an inner lumen  442  sized and configured to contain the impeller assembly  436  and the bearing assembly  438  therein. The housing  440  further comprises a plurality of flow apertures  444  configured to align with the impeller  448  upon assembly to facilitate ingress into or egress from the inner lumen  442  (for example depending on the flow direction) and a plurality of axial slots  446  in the inner lumen wall at the distal end of the housing  440 . Each axial slot  446  is configured to receive one radial support strut  468  of the tip bearing  450 . Notably, in the instant example embodiment the proximal pump housing  440  is a part of the first pump assembly  418  and not catheter assembly (for example as described above), and as such both the proximal and distal pump assemblies  418 ,  420  have attached pump housings and impellers of the same or closely similar diameter. 
     The impeller assembly  436  includes an proximal pump impeller  448 , a tip bearing  450 , an drive shaft  452 , and a collar  454 . The proximal pump impeller  448  has a proximal base  456 , impeller fulcrum  458 , a plurality of blades  460  (e.g. straight or curved) extending along the hub  462  from the base  456  to the impeller fulcrum  458 , and a proximal shaft  464  extending proximally from the base  456  and configured to engage the bearing assembly  438  as described below. The proximal pump impeller  448  further includes an axial lumen extending proximally therethrough configured to receive the cable adapter  486  therein, thereby coupling the drive cable  416  to the proximal pump impeller  448  so that drive cable  416  may transfer rotational energy from the motor assembly to the proximal pump assembly  418 . The proximal pump impeller  448  further includes an axial lumen extending distally therethrough configured to receive the drive shaft  452  therein, thereby coupling the drive shaft  452  to the proximal pump impeller  448  so that the drive shaft  452  may transfer rotational energy from proximal pump impeller  448  to the distal pump assembly  420 . 
     The tip bearing  450  has a base  466 , a plurality of radial struts  468 , and a central aperture  470  extending axially through the base. The radial struts  468  extend radially outward from the base  466  and are sized to span the distance between the base  466  and the axial slots  446  of the housing  440  so that the tip bearing  450  may be relatively constrained within the axial slots  446 . The radial struts  468  may be straight or curved to form an inducer to precondition the fluid flow path to minimize hydraulic instability (e.g. flow separation, cavitation, vortices). When radial struts  468  are curved to form an inducer, the outer ends are configured straight for axial alignment with slots  446 . The central aperture  470  is sized and configured to rotatably receive the drive shaft  416  therethrough and allow the drive shaft  416  and therefore the proximal pump impeller  448  to rotate at high speed while maintaining axial alignment of the proximal pump impeller  448  to ensure coaxial rotation. Although shown in  FIG. 78  by way of example only as having three radial struts  468 , the tip bearing  450  may have any number of radial struts  468  without departing from the scope of the disclosure. 
     The tip bearing  450  of the present example is positioned centrally in the pump housing  440  to align the impeller distal end or fulcrum  458  to centerline and allow torque transmission from the proximal pump assembly  418  to the distal pump assembly  420  without deflection of the proximal pump impeller  448  which may cause the tips of the impeller blades  460  to rub against the pump housing  440 . The tip bearing  450  is self-aligning in an axial direction due to the axial slots  446  of the housing  440  having longer lengths than the axial length of each radial support strut  468 . This allows hydrodynamic bearings on both ends (proximal and distal) of the proximal pump impeller  448  to function without negative effect from component axial manufacturing tolerance stack up. 
     By way of example, the drive shaft  452  comprises a hollow shaft with tension spring segments  472  for loading hydrodynamic bearings (e.g. tip bearing  450  and distal pump  420 ) and a middle flexible segment  474  for bending during pump insertion through torturous anatomy. The drive shaft  452  may be sealed with a flexible jacket or drive shaft cover  476  for transporting the purge fluid to one or more distal pump assemblies  420 . The drive shaft  452  may be constructed of a single piece (e.g. laser cut hypo tube) or of multiple pieces (e.g. solid hollow shaft for rigid segments, flexible drive cable for middle flexible segments  474 , and laser cut thin-wall tube or single-wire coiled tension spring for tension spring segments  472 , or any combination therein). 
     The collar  454  is attached to the drive shaft  452  distal to the tip bearing  450 . The collar  454  puts the tension spring segment  472  of coupling drive shaft under tensile load when attached (e.g. laser welded), reacting load to impeller fulcrum  458 . This squeezes the tip bearing  450  ends for hydrodynamic effect whereby thin film of pressurized purge fluid (e.g. saline solution, dextrose solution) leaks out of rotating interface at end faces (e.g. proximal and distal) of tip bearing  450  resulting in a “hydroplaning” effect that minimizes the temperature increase from rotational friction while maintaining axial alignment of the proximal pump impeller  448  and impeller housing  440 . Excessive heat from rotational friction is known to activate the clotting cascade which poses risk of vascular embolism to the patient. Excessive impeller runout can cause flow disturbances within the impeller flow region reducing pump efficiency, cause blood damage or activate platelets. 
     The bearing assembly  438  of the instant example embodiment includes a bearing housing  478 , distal bushing  480 , proximal bushing  482 , compression spring  484 , and threaded cable adapter  486 . By way of example, the bearing housing  478  comprises a generally cylindrical tubular member having an inner lumen  488  sized and configured to house the distal bushing  480 , proximal bushing  482 , compression spring  484 , threaded cable adaptor  486 , and impeller proximal shaft  464  therein, and has a smooth outer surface  490  configured for attachment to the housing inner lumen  442 . The distal bushing  480  is fixed to the bearing housing  478  and includes axial grooves  492  on an inner diameter to transport purge fluid along the impeller shaft  464  to a proximal-facing hydrodynamic bearing surface  494  at the impeller base  456 . Alternatively, bearing housing  478  may be integrated into impeller housing  440 . Alternatively, bearing housing  478  and distal bushing  480  may be integrated into impeller housing  440 . The compression spring  484  applies force to the proximal bushing  482  that is slip-fit to the bearing housing  478  in an axial “floating” manner. The proximal bushing  482  has axial grooves  496  on an inner diameter for purge flow, and proximal grooves  498  on a proximal face for purge flow from sheath (not shown but see description above) into the proximal pump assembly  418 . 
     The threaded cable adapter  486  has a proximal flange  500 , and a distal-extending post  502 . The proximal flange  500  reacts the force that the compression spring  484  applies to the proximal bushing  482 . The distal-extending post  502  has a distal threaded coupler  504  and an inner cavity  506  sized and configured to receive at least a portion of the drive cable  416  therein. The inner cavity  506  also includes a thin-wall crimping element  508  configured to crimp the drive cable  416  onto a pin mandrel  510  inside distal end of drive cable  416  to securely connect the cable adapter  486  to the drive cable  416 . 
     The proximal impeller shaft  464  and cable adapter  486  may have side-holes  512  formed therein to allow purge flow into the central lumens of the cable adapter  486 , proximal impeller  448 , and drive shaft  452  to supply purge fluid to the distal pump(s)  420 . 
     After crimp connection of the drive cable  416  to the cable adapter  486  (e.g. by way of pin mandrel  510 ), the drive cable  416  is essentially threaded to impeller shaft  464  (e.g. by way of a threaded engagement between the threaded coupler  504  of the cable adaptor  486  and a threaded cavity  514  of the impeller shaft  464 . The proximal bushing  482 , distal bushing  480 , and bearing housing  478  fitted to compress the compression spring  484 , connected to proximal pump impeller  448  and proximal pump housing  440  form the proximal pump assembly  418 . 
     By way of example,  FIG. 85  provides an axial cross-section view of the proximal pump assembly  418  with the section cut along line T-T of  FIG. 84  (e.g. through the proximal bushing  482 ).  FIG. 86  provides an axial cross-section view of the proximal pump assembly  418  with the section cut along line U-U of  FIG. 84  (e.g. through the distal bushing  480 ).  FIG. 87  provides an axial cross-section view of the proximal pump assembly  418  with the section cut taken along line V-V of  FIG. 84  (e.g. through the tip bearing  450 ). 
       FIGS. 88-95  illustrate an example of a second or distal pump assembly  420  according to one example embodiment. By way of example, the distal pump assembly  420  includes a housing  516 , bushing  518 , impeller  524 , proximal collar  522 , and a proximal end cap  524 . The housing  516  of the instant example comprises a generally cylindrical tubular member having an inner lumen  526  sized and configured to contain the various components described herein. The housing  516  further comprises a plurality of flow apertures  528  configured to align with the impeller  520  upon assembly to facilitate ingress into or egress from the inner lumen  526  (for example depending on the flow direction). The bushing  518  is press-fit or bonded into the inner lumen  526  of the housing  516  and includes axial grooves  530  on an inner diameter to transport purge fluid to the proximal-facing hydrodynamic bearing surface  542  at the impeller base  532  and distal-facing hydrodynamic bearing surface  546  on the base  544  of the proximal collar  522 . Alternatively, bushing  518  may be integrated into housing  516 . 
     The impeller  520  has a proximal base  532 , a distal end  534 , a plurality of blades  536  (e.g. straight or curved) extending along the hub  538  from the base  532  to the distal end  534 , and a proximal shaft  540  extending proximally from the base  532  and configured to engage the bushing  518 . The impeller  520  further includes a proximal-facing hydrodynamic bearing surface  542  configured to hydrodynamically engage a distal-facing outer surface  544  of the bushing  518 , and an axial lumen extending therethrough configured to receive the drive shaft  452  therein, thereby coupling the drive cable  416  to the impeller  520  (by way of drive shaft  452 , proximal pump impeller  448 , and threaded cable adapter  486  as described above so that the drive shaft  416  may transfer rotational energy from the motor assembly  412  to the distal pump impeller  520  to draw blood flow through the distal pump assembly  420 . 
     By way of example, the distal pump assembly  420  is shown with a hydrodynamic bearing arrangement similar to the tip bearing  520  of the proximal pump assembly  418  described above, where the drive shaft tension spring segment  472  is stretched during assembly and fixed by the attachment (e.g. by welding) to the proximal collar  522 . The proximal collar  522  includes a generally cylindrical base  544  having a planar distal-facing hydrodynamic bearing surface  546 , and a distal shaft  550  having an inner lumen extending therethrough. The distal shaft  550  is sized and configured to be received within the inner lumen of the bushing  518  while the outer diameter of the base  544  is sized and configured for rotational clearance with the inner lumen  526  of the housing  516 . The proximal end cap  524  generally cylindrical distal shaft  552  sized and configured for press-fit or bonding into the housing  516 . The proximal end cap  524  may have a shaped proximal end  554  having a generally concave surface (for example) shaped to fill blood stasis volume outside the high velocity flow streams to prevent thrombus formation. Alternatively, at least one radial blade (not shown) may be attached to the outer surface of the drive shaft cover  476  near the end cap  524  to induce turbulence that washes the volume and prevents fluid stasis. 
       FIGS. 96-102  illustrate an example of an expandable cannula  560  forming part of the percutaneous blood pump system  210 , according to one embodiment. The cannula  560  of the present example comprises a thin-wall, self-expanding, re-collapsible, cylindrical tube providing a conduit for blood flow out of heart (for example). The cannula  560  is inserted in collapsed configuration (see, e.g.  FIG. 74 ) and expands to an expanded operating configuration upon emergence from the sheath  212 . The expanded cross-section is configured to be suitable for the desired length of the cannula  560 , number of tandem pumps operating, and desired maximum pressure drop from the cannula inlet to the pump inlets at the desired maximum system flow rate. The cannula  560  of the present example is substantially similar to the cannula  272  described above such that description of like features will not be repeated, and those features that are the same as above will be referenced with the same numbers used above, however features that are new or different will be assigned new reference numbers and described accordingly. 
     By way of example only, the expandable cannula  560  comprises a single inner lumen  314 , and one or more flow port(s)  562  formed in the body  308  (by way of example). Unlike the flow ports  316  on the cannula  272  above, the flow ports  562  of the instant example may be formed not only on the “bottom” of the cannula  560  but also partially on the lateral sides. The reason for this is that the cannula  560  has an alignment feature in the form of a tubular pump guide  564 . By way of example, the tubular pump guide  564  may be a form-fitting cover that blocks flow from any ports that may be facing the tubular pump guide  564  upon insertion of the pumps such as by way of example distal proximal or distal pumps  418 ,  420  of shown in  FIG. 76  into the cannula  560 . Thus, the tubular pump guide  564  enables the use of a distal pump shroud with a full 360° array of ports so a user does not have to align rotationally to ports  562  in the cannula  560 . 
     The cannula  560  of the present example is configured for use with a proximal pump housing as part of the pump assembly (for example like the proximal pump assembly  418  described above) instead of having the housing part of the catheter (for example like the proximal pump assembly  336  described above). The cannula  560  and proximal guide shaft may be all one piece back to the hemostasis valve, or of two or more pieces, for example proximal and middle with unobstructed 360° ports, and a distal expandable segment as described above. 
       FIGS. 103-115  illustrate a method of using the percutaneous blood pump system  210  described above, according to one example embodiment. The first step is to prime the system  210  for use. To accomplish this, as shown in  FIG. 103 , a non-sterile technician may connect the cable connector  344  to a control console  570 , and also set up the control console  570  with a purge fluid bag  572  and purge fluid waste bag  574 . The technician then activates the console  570  to prime the drive cable assembly  332  with purge fluid (e.g. heparnized 5% dextrose solution). 
     The next step is to establish femoral artery access and track the guide wire  202  into the left ventricle of the heart. At this point the blood pump system  210  is configured for initial insertion, namely the obturator  218  is inserted into the catheter  214 , which is inserted into the sheath  212 . The user first hydrates the lubricious coating of the self-expanding cannula  272  in a bowl of sterile saline  576  (e.g.  FIG. 104 ). The user may then sheathes the self-expanding cannula  272  by holding the sheath  212  and pulling the catheter  214  until the tip housing  274  is seated against the distal end of the sheath shaft  224  (e.g.  FIG. 105 ). The next step is to secure the sheath hemostasis valve  228  onto the catheter  214  by rotating the valve handle  244  (e.g.  FIG. 106 ). Next, the user may insert the guide wire  202  by backloading the guide wire  202  through the atraumatic tip  276  of the catheter  214  until the guide wire  202  emerges from the obturator  218  at the proximal end. The user may then close the obturator guide wire hemostatis valve  402  over the guide wire (e.g.  FIG. 107 ). The user may then track the sheath  212  and catheter  214  over the guide wire  202  into the descending aorta. The user then loosens the sheath hemostasis valve  228  by rotation the valve handle  244  in the opposite direction (e.g.  FIG. 108 ). The user then unsheathes the self-expanding cannula  272  by holding the sheath  212  and pushing the catheter  214  until the proximal shroud  270  is distal of the distal end of the sheath shaft  224  (e.g.  FIG. 109 ). Expanding the cannula  272  opens the space to be occupied by one or more of the pump assemblies of the pump subsystem. The user then tracks the catheter tip housing  274  over the guide wire  202  and into the left ventricle. At this point the cannula  272  is seated in the desired intra-valvular position (e.g. with the distal end  310  of the cannula  272  positioned in the left ventricle and the proximal end  312  of the cannula  272  positioned in the aorta, as shown in  FIG. 110 ). 
     To insert the pump system  216 , the user must first remove the guide wire  202  and obturator  218  from the catheter  214 . To accomplish this, the user secures the sheath hemostasis valve  228  onto the catheter by rotating the valve handle  244  (e.g.  FIG. 111 ). The user may then remove the obturator  218  and guide wire  202  by pulling each proximally from the catheter  214  (e.g.  FIG. 112 ). The pump subsystem  216  is then introduced by inserting through the catheter hemostasis valve  264  (e.g.  FIG. 113 ). The user then tracks the pump subsystem  216  into the catheter  214  until the pump motor assembly  330  connects with the catheter hemostasis valve  264 , clicking to secure (e.g.  FIG. 114 ). At this point, the proximal pump assembly will be located in the catheter immediately proximal of the cannula  272 , and the distal pump assembly will be located inside the cannula in the space previously occupied by the collapsed cannula prior to expansion. The user may then verify the catheter position and suture the sheath to the patient. The percutaneous blood pump system  210  may now be used to pump blood from the left ventricle  578  of the heart, across the aortic valve  580  and into the aorta  582 , as shown in  FIG. 115 . 
     Any of the features or attributes of the above the above described embodiments and variations can be used in combination with any of the other features and attributes of the above described embodiments and variations as desired. 
     From the foregoing disclosure and detailed description of certain preferred embodiments, it is also apparent that various modifications, additions and other alternative embodiments are possible without departing from the true scope and spirit. The embodiments discussed were chosen and described to provide the best illustration of the principles of the present invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by any and all claims deriving from this disclosure when interpreted in accordance with the benefit to which they are fairly, legally, and equitably entitled.