Patent Publication Number: US-2022211930-A1

Title: Coaxial cannula for use with extracorporeal membrane oxygenation systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to provisional application Ser. No. 63/133,995, filed Jan. 5, 2021, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE DISCLOSURE 
     Field of the Disclosure 
     The present disclosure relates generally to extracorporeal membrane oxygenation systems, and more specifically, to a coaxial cannula for two-way blood flow for use in to extracorporeal membrane oxygenation systems. 
     Background 
     Many types of cardiac assist devices have been developed for applications in which a patient&#39;s heart is incapable of providing adequate circulation, commonly referred to as heart failure or congestive heart failure. For example, a patient suffering from chronic heart failure may use a ventricular assist device or VAD that is implanted in the patient while awaiting a heart transplant or as a long term destination therapy. As another example, a patient suffering from acute heart failure may use an extracorporeal pump or circulatory support system that pumps blood out and back into a patient&#39;s body. Extracorporeal circulatory support systems may also be used perioperatively, for example, to direct blood through a patient while surgery is performed on the heart. 
     At least some extracorporeal circulatory support systems temporarily replace a patient&#39;s heart and lung functions by pumping blood around or bypassing the patient&#39;s heart and lungs. Such extracorporeal circulatory support systems will typically include an oxygenator, such as an extracorporeal membrane oxygenator or ECMO, to provide oxygen to the blood passing through extracorporeal circulatory support system. 
     At least some known extracorporeal circulatory support systems use cannulae that include single lumen cannulae at multiple insertion sites, high volume circuits and cannulae that are not capable of long term use. Multiple sites increase the risk of bleeding, vessel damage, infection, as well as pain and discomfort to the patient. Additionally, at least some known extracorporeal circulatory support system cannulae have a tendency to kink, or may cause the blood flow therethrough to result in blood damage. 
     Accordingly, a need exists for extracorporeal circulatory support systems that provide a coaxial lumen cannula with improved blood flow. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure is directed to a coaxial cannula assembly. The coaxial cannula assembly includes an infusion tube defining a return lumen and having a proximal end and a distal end, wherein the distal end of the infusion tube includes a plurality of infusion openings. The coaxial cannula also includes a drainage tube co-axially aligned with the infusion tube and having a proximal end and a distal end, wherein the distal end of the drainage tube includes a plurality of drainage openings and wherein a length of the drainage tube is less than a length of the infusion tube. A drainage lumen is defined by a space between the infusion tube and the drainage tube. The coaxial cannula also includes a flow router extending into and attached with the proximal end of the drainage tube and having a reservoir for receiving a fluid from the proximal end of the drainage tube. The infusion tube extends through the flow router, and wherein the diameter of the infusion tube is constant across the flow router as the infusion tube extends through the flow router from a proximal end of the flow router to a distal end of the flow router, and further wherein the infusion tube gradually tapers in diameter proximal the flow router. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of an extracorporeal circulatory support system connected to a patient&#39;s body. 
         FIG. 2  is a top perspective view of one embodiment of a coaxial cannula of the present invention. 
         FIG. 3A  is a cross-sectional view of a portion of the coaxial cannula shown in  FIG. 2  illustrating a flow router connected to various tubes. 
         FIG. 3B  is an enlarged view of a distal end of the flow router shown in  FIG. 3A . 
         FIG. 3C  is an enlarged view of a proximal end of the flow router shown in  FIG. 3A . 
         FIG. 4  is a cross-sectional end view of the flow router shown in  FIG. 3A  illustrating an infusion tube within a drainage tube. 
         FIG. 5A  is a perspective cross-sectional view of the flow router illustrating one embodiment of a stator within the flow router. 
         FIG. 5B  is a perspective cross-sectional view of the flow router illustrating another embodiment of a stator within the flow router. 
         FIG. 6  is a top view of a bend relief member circumscribing the infusion tube. 
         FIG. 7A  is a top view of the coaxial cannula of  FIG. 2  illustrating one embodiment of a stabilizing cuff proximal of the flow router. 
         FIG. 7B  is a top view of the coaxial cannula of  FIG. 2  illustrating a second embodiment of a stabilizing cuff proximal of the flow router. 
         FIG. 7C  is a top view of the coaxial cannula of  FIG. 2  illustrating a third embodiment of a stabilizing cuff proximal of the flow router. 
         FIG. 8  is a perspective view of one embodiment of the distal end of the infusion tube. 
         FIG. 9  is a perspective view of a second embodiment of the distal end of the infusion tube. 
         FIG. 10  is a perspective view of a third embodiment of the distal end of the infusion tube. 
         FIG. 11  is a perspective view of a fourth embodiment of the distal end of the infusion tube. 
         FIG. 12A  is a perspective view of the coaxial cannula of  FIG. 2  illustrating drainage slots at the distal end of the drainage tube. 
         FIG. 12B  is a cross-sectional end view of the drainage slots shown in  FIG. 12A . 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Referring now to the drawings,  FIG. 1  is an illustration of an extracorporeal mechanical circulatory support system  10  connected a patient&#39;s  12  vasculature. The extracorporeal mechanical circulatory support system  10  includes a blood pump assembly  14 , an inflow or first conduit  16 , an outflow or second conduit  18 , a coaxial tube cannula  20 , a controller (not shown), and a power supply (not shown). 
     The blood pump assembly  14  includes a blood pump  24 , an extracorporeal membrane oxygenator (ECMO)  26 , and an inlet  28  and an outlet  30  for connection of flexible conduits thereto. The blood pump assembly  14  may include any suitable type of pump that enables the blood pump assembly  14  to function as described herein, including, for example and without limitation, an axial rotary pump and a centrifugal rotary pump. The ECMO  26  includes an oxygenator membrane (not shown) configured to increase the oxygen concentration and/or decrease the carbon dioxide concentration of blood pumped through the blood pump assembly  14 . The oxygenator membrane may include any suitable type of oxygenator membrane that enables the blood pump assembly  14  to function as described herein including, for example and without limitation, fiber bundles. In some embodiments, the extracorporeal mechanical circulatory support system  10  also includes a purge valve (not shown) to release air or other gasses present within the extracorporeal mechanical circulatory support system  10 . The purge valve can be connected, for example, to the outflow conduit  18 , or may be integrated within the ECMO  26  (e.g., at an outlet of the ECMO  26 ). 
     The blood pump assembly  14  is connected to the patient&#39;s vasculature through the inflow conduit  16  and the outflow conduit  18 . More specifically, the inlet  28  of the blood pump assembly  14  is connected to the inflow conduit  16 , and the outlet  30  of the blood pump assembly  14  is connected to the outflow conduit  18 . Furthermore, the inflow conduit  16  is connected to an outlet  32  of coaxial cannula  20 , and the outflow conduit  18  is connected to an inlet  34  of coaxial cannula  20 . The coaxial cannula  20  has substantial placement flexibility that allows the coaxial cannula  20  to be placed in a patient at various vascular insertion sites and depths. The coaxial cannula  20  is designed for insertion into the internal jugular vein  36  and placement above or below the right atrium and therefore will not typically cross the heart. Accordingly, the coaxial cannula  20  is less intrusive than cannulae that cross the heart. However, the coaxial cannula  20  can be used to cross the heart in certain applications. The coaxial cannula  20  is adapted for use with an introducer (not shown) that extends through the cannula and helps the user to properly place the coaxial cannula  20  in the correct position and depth in the body of the patient  12 . 
     It will be understood that the illustrated connections to the patient&#39;s vasculature are for illustrative purposes only, and that the blood pump assembly  14  may be connected to the patient&#39;s vasculature in any other suitable manner that enables that extracorporeal mechanical circulatory support system  10  to function as described herein, including, for example and without limitation, veno-venous (VV) connections and veno-arterial (VA) connections. 
     The controller is communicatively coupled to the blood pump assembly  14 , and is configured to control operation thereof. For example, the controller is configured to control operation (e.g., a speed) of the blood pump  24 . The controller can generally include any suitable computer and/or other processing unit, including any suitable combination of computers, processing units and/or the like that may be communicatively coupled to one another (e.g., controller can form all or part of a controller network). Thus, controller can include one or more processor(s) and associated memory device(s) configured to perform a variety of computer-implemented functions (e.g., performing the methods, steps, calculations and/or the like disclosed herein). As used herein, the term “processor” refers not only to integrated circuits referred to in the art as being included in a computer, but also refers to a controller, a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit (ASIC), a digital signal processor (DSP), a field programmable gate array (FPGA), and other programmable circuits. Additionally, the memory device(s) of the controller may generally include memory element(s) including, but not limited to, non-transitory computer readable medium (e.g., random access memory (RAM)), computer readable non-volatile medium (e.g., a flash memory), a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc (DVD) and/or other suitable memory elements. Such memory device(s) can generally be configured to store suitable computer-readable instructions that, when implemented by the processor(s), configure the controller to perform various functions including, but not limited to, controlling components of the blood pump assembly  14  as described herein. 
     The power supply provides power to the blood pump  24 , controller, and other electrical components of the blood pump assembly  14 , and may generally include any suitable power supply that enables the extracorporeal mechanical circulatory support system  10  to function as described herein. The controller and power supply may be external to the blood pump assembly  14 , or all or part of the controller and/or the power supply  22  may be incorporated within the blood pump assembly  14  in other embodiments. 
       FIG. 2  is a top perspective view of one embodiment of a coaxial cannula  20 .  FIG. 3A  is a cross-sectional view of a portion of the coaxial cannula  20  illustrating a flow router  62  connected to various tubes.  FIG. 3B  is an enlarged view of a distal end of the flow router  62 .  FIG. 3C  is an enlarged view of a proximal end of the flow router  62 .  FIG. 4  is a cross-sectional end view of the flow router  62  illustrating an infusion tube  40  within a drainage tube  52 .  FIG. 5A  is a perspective cross-sectional view of the flow router  62  illustrating one embodiment of a stator  78  within the flow router  62 .  FIG. 5B  is a perspective cross-sectional view of the flow router  62  illustrating another embodiment of a stator  78  within the flow router  62 . 
     The coaxial cannula  20  includes an infusion tube  40  having a proximal end  42 , a distal end  44 , and defining an internal or return lumen  46  extending between ends  42  and  44 . The distal end  44  includes an end return aperture  48  and a plurality of infusion openings  50  defined through the infusion tube  40  and in flow communication with the return lumen  46 . The coaxial cannula  20  also includes a drainage tube  52  that is coaxial with the infusion tube  40  and includes a proximal end  54 , a distal end  56 , and a drainage lumen  58  extending between ends  54  and  56 . The distal end  56  of the drainage tube  52  is fixedly attached to the infusion tube  40  and includes a plurality of drainage openings  60  defined through the drainage tube  52  and in flow communication with the drainage lumen  58 . As best shown in  FIG. 3A , the infusion tube  40  is located within the drainage lumen  58  of the drainage tube  52  such that blood flowing through the drainage lumen  58  flows around the infusion tube  40 . Specifically, the return lumen  46  includes a first cross-sectional diameter that is smaller than the drainage lumen  58  such that a predetermined ratio of the return lumen  46  to the drainage lumen  58  is defined. More specifically, the ratio is designed to reduce the pressure inside the drainage lumen  58  by increasing the cross-sectional area of the drainage lumen  58  with respect to the return lumen  46  as compared to at least some known coaxial cannulae. 
     A flow router  62  includes a distal end  64 , a drainage proximal end  66 , and an infusion proximal end  68 . The distal end  64  is attached to the proximal end  54  of the drainage tube  52 , the proximal drainage end  66  is connected to a proximal drainage tube  70 , and the infusion tube  40  extends through the flow router  62  and exits the infusion proximal end  68 . The flow router  62  also includes a reservoir  72  for receiving fluid from the proximal end  54  of the drainage tube  52 , wherein the infusion tube  40  extends through the flow router  62  and does not connect with the drainage tube  52  at the flow router  62 , and wherein the infusion tube  40  remains substantially coaxial with the drainage tube  52  throughout the length of the drainage tube  52 . 
     Referring now to  FIGS. 3B and 3C , the distal end  64  of the flow router  62  includes a transition feature  74  at the transition from the drainage tube  52  to the flow router  62 . Similarly, the drainage proximal end  66  includes a transition feature  76  at the transition from the flow router  62  to the proximal drainage tube  70 . The transition features  74  and  76  improve hemodynamics and hemocompatibility by allowing for a smooth transition between components that is free of a stepped transition, which may cause turbulence within the blood flow. As illustrated, transition features  74  and  76  are rounded or slanted surfaces that taper the thickness of the flow router  62  at the distal end  64  and the drainage proximal end  66 . 
     Still referring to  FIGS. 3A and 3C , it can be seen that infusion tube  40  includes a constant cross-sectional diameter throughout the length of the drainage tube  52  and throughout the length of the flow router  62 . When the infusion tube  40  exits the flow router  62 , the infusion tube  40  begins to taper outward into a larger diameter in order to match the diameter of the outflow conduit  18 . 
     Referring now to  FIGS. 4, 5A, and 5B , the flow router  62  includes a stator  78  that is used to smoothly direct blood flow from the drainage tube  52  around the infusion tube  40  within the reservoir  72  of the flow router  62 . Specifically, the flow router  62  includes an interior surface  80  that defines reservoir  72  and the stator  78  extends from the interior surface  80  into the reservoir  72 . The stator  78  includes a top surface  82 , a distal tip  84 , and a pair of sidewalls  86  that taper outward in a proximal direction from the tip  84 . The sidewalls  86  may have any constant or varying height that enables operation of the stator  78  as described herein. Further, the sidewalls  86  meet at the tip  84  at any angle that enables operation of the stator  78  as described herein. 
     In operation, the infusion tube  40  is fixed to the top surface  82  and the tip  84  and the sidewalls  86  guide the blood flow around the infusion tube  40  and into the proximal drainage tube  70 . Additionally, although only a single stator  78  is shown, the flow router  62  may include a plurality of stators  78  circumferentially spaced about interior surface  80 . As such, the stator/s  78  guide the blood flow around the infusion tube  40  and into the proximal drainage tube  70  to reduce recirculation within reservoir  72 . Furthermore, the stator/s  78  are positioned within the flow router  62  where the infusion tube  40  diverts to the side and the cross-section of the reservoir  72  available for drainage flow increases. As such, the stator/s  78  occupy space within the reservoir  72  to maintain a relatively constant pressure.  FIG. 5A  illustrates one embodiment of a stator  78   a  where the distal portions of the sidewalls  86   a  and top surface  82   a  blend into the interior surface  80  of the flow router  62 .  FIG. 5B  illustrates an embodiment of a stator  78   b  where the sidewalls  86   b  and top surface  82   b  do not blend into the interior surface  80 . 
       FIG. 6  is a top view of a bend relief member  90  circumscribing the infusion tube  40 . The infusion tube  40  includes a distal portion  92  having a first diameter, a proximal portion  94  having a second diameter larger than the first diameter, and a tapered portion  96  between the proximal portion  94  and the distal portion  92 . As described herein, the distal portion  92  extends through the flow router  62  and exits out the infusion proximal end  68  of the flow router  62  such that the tapered portion  96  begins proximal of the flow router  62 . A bend relief member  90  may be positioned around at least the tapered portion  96  of the infusion tube  40  to prevent or reduce bending and kinking (a reduction in cross-sectional area) of the infusion tube  40  at the transition between the distal portion  92  and the tapered portion  96 . The bend relief member  90  is a flexible material that may include metallic braiding for additional structural support. Additionally, the infusion tube  40  is shown in  FIG. 6  as including braiding  98  for additional structural support. Such braiding  98  is not limited to embodiments that include the bend relief member  90 , and may be included on all embodiments described herein. Furthermore, a distal end  100  of the bend relief member  90  may be connected to the infusion proximal end  68  of the flow router  62  to ensure the bend relief member covers the transition between the distal portion  92  and the tapered portion  96  of the infusion tube  40 . The infusion proximal end  68  of the flow router  62  may include an engagement feature (not shown) to secure the bend relief member  90  in place. 
       FIG. 7A  is a top view of the coaxial cannula  20  illustrating one embodiment of a stabilizing cuff  102   a  proximal of the flow router  62 . Stabilizing cuff  102   a  includes a first cuff  104   a  positioned around infusion tube  40  at the transition from the tapered portion  96  to the proximal portion  94 . Stabilizing cuff  102   a  also includes a second cuff  106   a  positioned around proximal drainage tube  70  and spaced from flow router  62 . A bridge member  108   a  extends between cuffs  104   a  and  106   a  and limits movement of the infusion tube  40  relative to the proximal drainage tube  70 . Stabilizing cuff  102   a  provides support to the infusion tube  40  proximal the flow router  62  and may be used instead of bend relief member  90 . Specifically, stabilizing cuff  102   a  is made from a stiff material and prevents or reduces bending and kinking (a reduction in cross-sectional area) of the infusion tube  40 . Furthermore, the stabilizing cuff  102   a  may be used to maintain a predetermined angle between the infusion tube  40  and the proximal drainage tube  70  proximal the flow router  62 . 
       FIG. 7B  is a top view of the coaxial cannula  20  illustrating a second embodiment of a stabilizing cuff  102   b  proximal of the flow router  62 . Stabilizing cuff  102   b  includes a first cuff  104   b  positioned around infusion tube  40  at the transition from the tapered portion  96  to the proximal portion  94 . Stabilizing cuff  102   b  also includes a second cuff  106   b  positioned around flow router  62  and/or a portion of proximal drainage tube  70  adjacent to flow router  62 . As such, the first cuff  104   b  and the second cuff  106   b  are offset from each other. A bridge member  108   b  extends between cuffs  104   b  and  106   b  and limits movement of the infusion tube  40  relative to the proximal drainage tube  70 . Stabilizing cuff  102   b  provides support to the infusion tube  40  proximal the flow router  62  and may be used instead of bend relief member  90 . Specifically, stabilizing cuff  102   b  is made from a stiff material and prevents or reduces bending and kinking (a reduction in cross-sectional area) of the infusion tube  40 . Furthermore, the stabilizing cuff  102   b  may be used to maintain a predetermined angle between the infusion tube  40  and the proximal drainage tube  70  proximal the flow router  62 . 
       FIG. 7C  is a top view of the coaxial cannula  20  illustrating a third embodiment of a stabilizing cuff  120   c  proximal of the flow router  62 . Stabilizing cuff  102   c  includes a first cuff  104   c  positioned around infusion tube  40  at the transition from the tapered portion  96  to the proximal portion  94 . Stabilizing cuff  102   c  also includes a second cuff  106   c  positioned around flow router  62  and/or a portion of proximal drainage tube  70  adjacent to flow router  62 . As such, the first cuff  104   c  and the second cuff  106   c  are offset from each other. Furthermore, the second cuff  106   c  is longer in length than the first cuff  104   c  to provide additional support. A bridge member  108   c  extends between cuffs  104   c  and  106   c  and limits movement of the infusion tube  40  relative to the proximal drainage tube  70 . Stabilizing cuff  102   c  provides support to the infusion tube  40  proximal the flow router  62  and may be used instead of bend relief member  90 . Specifically, stabilizing cuff  102   c  is made from a stiff material and prevents or reduces bending and kinking (a reduction in cross-sectional area) of the infusion tube  40 . Furthermore, the stabilizing cuff  102   c  may be used to maintain a predetermined angle between the infusion tube  40  and the proximal drainage tube  70  proximal the flow router  62 . 
     In another embodiment, a web (not shown) extends between the infusion tube  40  and the proximal drainage tube  70 . The web may be an extension of the flow router  62  that is integral to the flow router  62  and secures around both the infusion tube  40  and the proximal drainage tube  70 . Alternatively, the web is made from the same material as the infusion tube  40  and the proximal drainage tube  70 . 
       FIG. 8  is a perspective view of one embodiment of the distal end  44  of the infusion tube  40 . As described herein, the infusion tube  40  is designed to allow blood to be infused into the main pulmonary artery of the patient  12 . The distal end  44  of the infusion tube  40  includes the plurality of infusion openings  50  and the end return aperture  48 . More specifically, the distal end  44  includes an end cap  110  positioned distal of the infusion openings  50  and in which end return aperture  48  is defined. In the illustrated embodiment, the distal end  44 , including the end cap  110 , includes constant inner and outer diameters. In another embodiment, the outer diameter of the end cap  110  is tapered for ease of insertion, but the inner diameter is constant. The end cap  110  is formed from a softer material than the infusion tube  40  for atraumatic insertion. 
       FIG. 9  is a perspective view of a second embodiment of the distal end  44  of the infusion tube  40 . As described herein, the infusion tube  40  is designed to allow blood to be infused into the main pulmonary artery of the patient  12 . The distal end  44  of the infusion tube  40  includes the plurality of infusion openings  50 , a tapered portion  112  distal to the infusion openings  50 , and the end return aperture  48 . The tapered portion  112  allows a smooth transition between the cannula tip and the introducer (not shown). The tapered portion  112  also allows for more flexibility around the distal end  44  of the cannula  20  for tracking into position. Furthermore, the distal end  44  includes an end cap  110  positioned distal of the tapered portion  112  and in which end return aperture  48  is defined. In the illustrated embodiment, the distal end  44 , including the end cap  110  and the tapered portion  112 , includes a constant inner diameter. In another embodiment, the outer diameter of the end cap  110  is tapered for ease of insertion, but the inner diameter remains constant. The end cap  110  is formed from a softer material than the infusion tube  40  for atraumatic insertion. 
       FIG. 10  is a perspective view of a third embodiment of the distal end  44  of the infusion tube  40 . The distal end  44  includes the plurality of infusion openings  50  and a plurality of elongated flexible members  114  at the distal tip of the distal end  44  such that the distal tip is crenulated. Specifically, the distal end  44  illustrated in  FIG. 10  includes elongate members  114  that define the end return aperture  48  rather than the end cap  110  shown in  FIGS. 8 and 9 . The elongate members  114  are circumferentially spaced about the perimeter of the infusion tube  40  and are separated by a plurality of infusion flow slots  116 . More specifically, adjacent elongate members  114  are separated by an infusion flow slot  116 . The infusion flow slots  116  are open-ended such that the distal tips  118  of the elongate members  114  are separated by the infusion flow slots  116 . The configuration of the slots  116  and the elongate members  114  allows for blood to exit the distal end  44  even if the distal tip is abutted against a blood vessel or other structure within the patient  12 . In operation, the infusion flow slots  116  allow for increased blood flow through slots  116  and through end return aperture  48 , which allows for the infusion openings  50  to be smaller in size or fewer in number than if the distal end  44  did not include the infusion flow slots  116 . More specifically, with the use of the infusion flow slots  116 , the total cross-sectional area of the infusion openings  50  is less than the cross-sectional area of the infusion tube  40 . 
     Additionally, as shown in  FIG. 10 , the distal tips  118  of the elongate members  114  taper toward the distal end. Specifically, each elongate member  114  includes a pair of circumferential sidewalls  120  that taper towards each other to form the distal tip  118 . It is important to note that the tapering is circumferential such that the cross-sectional diameter of the elongate members  114  is constant. Such tapering allows the elongate members  114  to bend or flex inward during insertion of the coaxial cannula  20 . During insertion, the distal tips  118  may engage a wall of the patient&#39;s vasculature and cause the engaging elongate member  114  to bend inward to facilitate an atraumatic insertion. 
       FIG. 11  is a perspective view of a fourth embodiment of the distal end  44  of the infusion tube  40 . The illustrated distal end  44  includes the plurality of infusion openings  50  and a flow restriction member  122  positioned between the infusion openings  50  and the end return aperture  48 . The flow restriction member  122  defines a lumen  124  with a cross-sectional area smaller than the cross-sectional area of the end return aperture  48 . As such, in operation, an area of higher pressure will build up proximal of the flow restriction member  122 . This high pressure area forces more blood flow through the infusion openings  50  than is the flow restriction member  122  were not present. The higher pressure and higher flow rate allow for the infusion openings  50  to be smaller in size or fewer in number than if the distal end  44  did not include the flow restriction member  122 . More specifically, with the use of the flow restriction member  122 , the total cross-sectional area of the infusion openings  50  is less than the cross-sectional area of the infusion tube  40 . 
       FIG. 12A  is a perspective view of the coaxial cannula  20  illustrating the distal end  56  of the drainage tube  52 .  FIG. 12B  is a cross-sectional end view of the distal end  56  of the drainage tube  52 . The distal end  56  includes a tapered portion  126  that is connected to an exterior surface of the infusion tube  40 . The tapered portion  126  is positioned distal to the plurality of drainage openings  60  formed in drainage tube  52 . Furthermore, the tapered portion  126  includes a plurality of circumferentially-spaced slots  128  that enable the flow of blood therethrough. Although only four slots  128  are shown, tapered portion  126  may include any number of slots  128  that facilitates operation of cannula  20  as described herein. As shown in  FIGS. 12 a    and  12 B, the slots  128  are oriented parallel to the axis of drainage tube  52  such that blood enters through the slots  128  in an axial direction rather than in a radial direction as does blood flow through the openings  60 . The axially-oriented slots  128  allow for more efficient blood flow therethrough because the blood is not required to change direction as it is when flowing through the drainage openings  60 . As such, the higher efficiency leads to a higher flow rate through the slots, which allows for the drainage openings  60  to be smaller in size and/or fewer in number than if the distal end  56  did not include the slots  128 . More specifically, with the use of the slots  128 , the total cross-sectional area of the drainage openings  60  plus the cross-sectional area of the slots  128  is less than the cross-sectional area of the drainage tube  52 . 
     Although the embodiments and examples disclosed herein have been described with reference to particular embodiments, it is to be understood that these embodiments and examples are merely illustrative of the principles and applications of the present disclosure. It is therefore to be understood that numerous modifications can be made to the illustrative embodiments and examples and that other arrangements can be devised without departing from the spirit and scope of the present disclosure as defined by the claims. Thus, it is intended that the present application cover the modifications and variations of these embodiments and their equivalents. 
     This written description uses examples to disclose the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.