Cannula Having Reverse Flow Tip

1. Field of the Invention

This invention relates to single- and multi-lumen cannulas and, more particularly, to a reinforced cannula having staggered lumen fluid outlets spaced along the length of the cannula and adapted to redirect the flow of fluid, making the cannula ideally suited for use in a variety of cardiac surgical procedures.

2. Description of the Related Art

Cannulas have long been used in a variety of applications to inject or withdraw fluid from the body. It is known to create a single-lumen cannula having wire reinforcement integrated in the body of the cannula to provide enhanced rigidity and avoid kinking of the cannula. It is also known to create a cannula having multiple lumens provided therein for delivering a variety of fluids and medications into and out of the body. However, one significant problem which exists in the art is the creation of a single cannula having multiple lumens formed therein wherein each lumen is independently reinforced so that the cannula can accommodate dramatically different fluid pressures in the adjacent cannulas without risk of collapsing the septum separating the different lumens.

Cannulas are often used in cardiac surgical procedures to conduct fluid to and from the various chambers of the heart and vessels conducting fluid to and from the heart. One desirable goal of a cardiac surgical procedure is to minimize the number of incisions which are formed in the heart and the vessels leading to and from the heart. It is especially important to minimize the number of incisions in the aorta in view of the significant fluid pressures which are experienced by this vessel during normal beating of the heart.

Another desirable goal is to minimize trauma to the heart and vessels leading to and from the heart. Pressurized fluid flowing through the cannula typically exits the cannula at a high velocity and may damage the heart, vessels or valves. In addition, the fluid flow may dislodge plaque from the walls surrounding the cannula, increasing the risk of embolic complications. Accordingly, it is important to reduce trauma to the body during a cardiac surgical procedure.

The cannula according to the invention overcomes the problems of the prior art by providing a multi-lumen cannula with independently reinforced lumens so that dramatically different fluid pressures can be accommodated in the same multi-lumen cannula. In addition, the cannula according to the invention achieves some of the desirable goals of cardiac surgery by minimizing the number of incisions which must be created in the heart and vessels leading to and from the heart as well as by reducing trauma to the body through redirection of the fluid flow as it exits the cannula.

In accordance with one embodiment of the invention, a cannula for conducting fluid to a body is provided. The cannula includes a cannula body and a cannula tip. The cannula body has proximal and distal ends, and a lumen extending therebetween. The lumen is adapted to receive the fluid flowing therethrough. The cannula tip is coupled to the distal end of the cannula body and includes a side wall having a plurality of fluid outlets formed therein. Each fluid outlet is directed toward the proximal end of the cannula body to reverse the flow of fluid exiting the cannula by an obtuse angle from the original direction of flow.

In another embodiment of the invention, a multi-lumen cannula assembly includes first and second sub-cannulas. Each sub-cannula has proximal and distal ends, and a lumen extending therebetween. The second sub-cannula is adhered to a portion of the first sub-cannula to create a septum separating the lumens of the first and second sub-cannulas. The first sub-cannula also includes a reinforcement to resist radial deflection of the lumen, and a cannula tip, located at the distal end. The cannula tip has a plurality of fluid apertures formed therein. The reinforcement of the first sub-cannula resists deflection of the septum as a result of differing fluid pressure levels inside the lumens. The second sub-cannula, which is adhered to a portion of the first sub-cannula, includes a cannula tip located at a distal end. The cannula tip of the second sub-cannula has a plurality of fluid outlets formed therein, each fluid outlet being directed toward the proximal end of the second sub-cannula. The fluid outlets reverse the flow of fluid exiting the second sub-cannula by an obtuse angle from the original direction of flow.

The invention is also directed to a method of steering a cannula. The method includes the step of providing a cannula including a cannula tip. The cannula tip includes a side wall having at least one fluid outlet formed therein and disposed about a sector of the cannula tip, the sector being less than the entire cannula tip. The method also includes the step of conducting a fluid through the lumen of the cannula. The fluid exits the cannula through the at least one fluid outlet and guides the cannula tip in a direction away from the at least one fluid outlet.

The invention is further directed to a cannula for conducting fluid to a body. The cannula includes a cannula body and a cannula tip coupled to a distal end of the cannula body. The cannula body includes a mid-section having a plurality of fluid outlets formed therein. Each of the plurality of fluid outlets is directed toward a proximal end of the cannula body to reverse the flow of fluid exiting the cannula by an obtuse angle from the original direction of flow.

Other advantages of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only, since, from this detailed description, various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.

The preferred exemplary embodiments of the present invention will hereinafter be described in conjunction with the accompanying drawings, wherein like numerals denote like elements and:

FIG. 1 is a top, plan view of a multi-lumen cannula according to the invention;

FIG. 2 is a cross-sectional view of the multi-lumen cannula taken generally along the line 2--2 of FIG. 1;

FIG. 3 is an exploded, top, plan view of the multi-lumen cannula of FIG. 1;

FIG. 4 is a sectional view of a die assembly used in the manufacture of the multi-lumen cannula, the die assembly being adapted to deform the sub-cannula;

FIG. 5 is a sectional view of the die assembly of FIG. 4 showing the die assembly in the closed position;

FIG. 6 is a schematic view of the multi-lumen cannula in a first operative position positioned in the human heart passing through the aortic valve;

FIG. 7 is a schematic view of the multi-lumen cannula in a second operative position positioned in the human heart passing through the aortic valve;

FIG. 8 is a schematic view of the multi-lumen cannula in a third operative position positioned in the human heart passing through the aortic valve;

FIG. 9 is a schematic view of the multi-lumen cannula in a fourth operative position positioned in the human heart passing through the aortic valve;

FIG. 10 is a schematic view of the multi-lumen cannula in a fifth operative position positioned in the human heart passing through the pulmonary valve;

FIG. 11 is a schematic view of the multi-lumen cannula in a sixth operative position positioned in the human heart passing through both the tricuspid valve and pulmonary valve;

FIG. 12 is a partial, top plan view of a second embodiment of a multi-lumen cannula showing a second sub-cannula with a reverse flow tip;

FIG. 13 is a cross-sectional view of taken generally along the line 13--13 of FIG. 12;

FIG. 14 is sectional view taken generally along the line 14--14 of FIG. 13;

FIG. 15 is a partial top plan view of a single-lumen cannula, according to the invention, having a reverse flow tip;

FIG. 16 is a cross-sectional view taken generally along the line 16--16 of FIG. 15;

FIG. 17 is an alternate embodiment of the cannula reverse flow tip, showing fluid outlets that are non-parallel to a longitudinal axis of the cannula tip;

FIG. 18 is a partial top plan view of a single-lumen cannula including a plurality of reverse flow fluid outlets disposed about a mid-section of the cannula; and

FIG. 19 is an alternate embodiment of the cannula of FIG. 19, showing reverse flow fluid outlets that are non-parallel to a longitudinal axis of the cannula.

Referring now to the drawings and to FIGS. 1-3 in particular, a multi-lumen cannula 12 according to the invention is shown. The multi-lumen cannula 12 comprises a first sub-cannula 14 and a second sub-cannula 16 which are mounted to one another to create the multi-lumen cannula 12. The first sub-cannula 14 comprises a wire reinforced body portion 18 having a proximal end 20 and a distal end 22. A lumen 24 extends from a proximal lumen aperture 26 provided on the proximal end 20 to at least one distal lumen aperture 28 formed in a cannula tip 29 at the distal end 22. The body portion 18 is formed of a flexible material acceptable for use inside the human body, preferably polyvinyl chloride. Preferably, the body portion 18 includes some form of reinforcing to provide radial rigidity to the cannula and to prevent kinking of the cannula during deformation. The preferred means of reinforcement comprises a conventional helical wire 30 imbedded in the body portion 18.

The structural elements of the second sub-cannula 16 are substantially identical to those of the first sub-cannula and include a body portion 34, a proximal end 36, a distal end 38, a lumen 40, a proximal lumen aperture 42, at least one distal lumen aperture 44 formed in a cannula tip 45, and a reinforcing wire 46.

In the preferred embodiment, the first sub-cannula 14 is longer than the second sub-cannula 16, and the second sub-cannula 16 is secured to the first sub-cannula 14 such that the proximal ends 20, 36 are immediately adjacent one another and the distal ends 22, 38 are spaced from one another. Further, the distal end 22 of the second sub-cannula 16 and a portion of the body immediately adjacent the distal end 22 is securely adhered to the body 18 of the first sub-cannula 14. The adhered portion 50 of the two sub-cannulas is dimensioned and designed to avoid any sharp corners or contours and provide a smooth transition along the exterior surface, without adversely affecting the flow rate through the lumens of the two sub-cannulas. These features are achieved by deforming portions of the sub-cannulas utilizing a die similar to that seen in FIGS. 4 and 5.

The portions of the first and second sub-cannulas 14, 16 which comprise the adhered portion 50 are preferably D-shaped in cross-section and secured to one another so that the flat portions of the D-shape are adjacent one another, thereby creating a substantially circular, assembled cross-section as seen in FIG. 2. The D-shaped contour is formed by inserting a portion of the sub-cannulas between a pair of opposed dies and then closing the dies to deform the sub-cannulas and create the desired shape. The preferred embodiment of the dies are seen in FIGS. 4 and 5 and comprise an upper, convex die 52 and a lower, concave die 54. As seen in FIG. 4, a portion of the sub-cannula 14 is positioned between the opposed dies 52, 54. Next, the dies 52, 54 are closed, as seen in FIG. 5. The molding surface 60 of the concave lower die 54 is substantially complementary to the contour of the undeformed sub-cannula 14. Therefore, the portion of the sub-cannula received in this portion of the die retains substantially the same contour as prior to deformation. The molding surface 62 of the upper die 52 is preferably convex and deforms a portion of the reinforced sub-cannula 14 to deflect inwardly into the lumen 24 of the sub-cannula. The memory of the wire 30 utilized in the sub-cannula 14 is such that the convex portion 52 of the deformed sub-cannula will spring back to a substantially planar condition as seen in FIG. 2. In other words, in order to achieve the D-shaped contour of the sub-cannula 14 as seen in FIG. 2, it is preferred to use a convex die 52 to overcome the memory of the materials forming the reinforced sub-cannula. While experimentation has shown that it is preferred to use the combination of a convex and concave die, it may be possible to utilize a concave die and a substantially planar die, depending upon the selection of materials and the response of the materials to the deforming operation.

During conventional manufacturing operations, substantially the entire length of the first and second sub-cannulas are circular in cross-section prior to deformation in the dies 52, 54. As seen in FIG. 1, it is preferred that only an intermediate portion 64 of the first sub-cannula 14 is deformed into the D-shaped configuration, a body portion 65 distally from the D-shaped portion 64 remains circular in cross-section as does the body portion 66 proximately from the D-shaped portion 64. It is preferred that only a portion of the body adjacent the distal portion of the second sub-cannula 18 be deformed into the D-shaped configuration. The two deformed D-shaped portions are secured to one another by a conventional adhesive 56 to create the adhered portion 50 of the multi-lumen cannula 12. It is preferred that an adhesive be used which will fill any gaps in the adhered portion 50 to create a substantially smooth, exterior surface for the adhered portion 50.

The preferred embodiment of the adhesive used is known as Dymax 191 M adhesive which is manufactured by Dymax Corporation located in Torrington, Conn. This is a light curing adhesive which fills the gaps to create a smooth contour on the exterior surface of the multi-lumen cannula. Any adhesive which is essentially non-shrinking and preferably uses little or no evaporating solvents can be used.

The preferred embodiment of the multi-lumen cannula spaces the distal lumen apertures of the first and second sub-cannulas from one another. With this configuration, the multi-lumen cannula is ideally suited for use in a variety of cardiac surgical procedures which will be described in greater detail below. However, it is to be understood that the multi-lumen cannula 12 according to the invention can be adapted for a wide variety of applications and uses such that the proximal lumen apertures of the two sub-cannulas can be positioned immediately adjacent one another or spaced a variety of distances from one another. In addition, the multi-lumen cannula 12 according to the invention utilizes a pair of sub-cannulas which are D-shaped in cross-section. It is understood by persons skilled in the art that the multi-lumen cannula according to the invention can incorporate more than two sub-cannulas merely by changing the contour of the die and the adhered portion. For example, three or more wedge- or pie-shaped sub-cannulas can be mounted to one another according to the invention.

One key aspect of the preferred embodiment of the multi-lumen cannula according to the invention is the fact that reinforcement is provided in the septum 58 which separates the lumens of the two sub-cannulas. This reinforced septum provides significant advantages over previous multi-lumen cannulas because the cannula 12 can now accommodate dramatically different fluid pressure levels inside the immediately adjacent lumens. For example, significant positive fluid pressure can be created in one of the lumens while a significant negative fluid pressure is created in the adjacent lumen. Prior multi-lumen cannulas could not accommodate such varying pressure levels in adjacent lumens because the septum would deflect and quickly pinch closed the lower pressure lumen. The reinforced septum of the multi-lumen cannula according to the invention is a significant improvement in the art. In the preferred embodiment, each of the flat portions of the sub-cannulas are independently reinforced, resulting in a septum 58 which can resist deflection despite dramatically different fluid pressure levels in the adjacent lumens. However, it is understood that a multi-lumen cannula according to the invention could be constructed wherein only one of the several sub-cannulas is reinforced.

The multi-lumen cannula 12 according to the invention, as seen in FIG. 1, can be used in a variety of applications to conduct fluid into and out of the human body. However, the lumen 12 is ideally suited for use in cardiac surgical procedures. As seen in FIG. 6, the cannula 12 can be used to conduct blood in a human vascular system across the aortic valve 70. In this first operative position, the distal lumen apertures 28 of the first sub-cannula 14 are positioned in the left ventricle 72 of the heart 74, and the distal lumen apertures 44 of the second sub-cannula 16 are positioned in the aorta 76 downstream from the aortic valve 70. With this structure and position, a single incision 78 is formed in the aorta to accommodate the fluid flow paths for withdrawing blood from the left ventricle and simultaneously supplying blood to the aorta 76. Previously, two incisions were required to accomplish both of these functions. The fluid withdrawn from the left ventricle through the first sub-cannula 14 will likely be at a lower fluid pressure than the blood supplied to the aorta 76 through the second sub-cannula 16. The reinforced septum 58 can accommodate these differing fluid pressures without deforming and altering the cross-sectional area of the lumens 24, 40.

In the first operative position as seen in FIG. 6, the proximal end 20 of the first sub-cannula 14 is fluidly connected to a pump or some other mechanism for withdrawing blood from the left ventricle 72, and the proximal end 36 of the second sub-cannula 16 is fluidly connected to the outlet of this pump.

FIG. 7 shows the multi-lumen cannula 12 according to the invention in a second operative position. Similar to the positioning shown in FIG. 6, the distal lumen apertures 28, 42 of the sub-cannulas 14, 16 are positioned on opposite sides of the aortic valve 70. However, the incision 78 has been repositioned on the aorta 76 closer to the valve 70.

FIG. 8 is a schematic drawing of a third operative position for the multi-lumen cannula 12 according to the invention. Similar to the positions seen in FIGS. 6 and 7, the distal lumen apertures 28,42 are positioned on opposite sides of the aortic valve and the incision 78 has been positioned further downstream from the earlier applications.

FIG. 9 shows the multi-lumen cannula 12 in a fourth operative position which, similar to the earlier operative positions, shows the distal lumen apertures 28, 42 on opposite sides of the aortic valve 70. However, in this position, the cannula 12 is inserted through an incision 80 formed directly in the wall of the left ventricle 72. In this position, blood is withdrawn from the left ventricle 72 through the distal lumen aperture 44 of the second sub-cannula 16, and the blood is discharged into the aorta 76 through the distal lumen aperture 28 of the first sub-cannula 14. As is evident from the various applications seen in FIGS. 6-9, the preferred embodiment of the multi-lumen cannula according to the invention is ideally suited to provide left-heart assist.

FIGS. 10 and 11 show use of the multi-lumen cannula 12 according to the invention for right-heart assist procedures. As seen in FIG. 10, the multi-lumen cannula 12 has been inserted into the heart 74 through an incision 82 formed in the wall of the right ventricle 84. The distal lumen apertures 44 of the second sub-canula 16 arc positioned in the right ventricle 84 while the distal lumen apertures 28 of the first sub-cannula 14 are positioned on the other side of the pulmonary valve 86 in the pulmonary artery 88. In this operative position, the distal end 38 of the second sub-cannula 16 is fluidly connected to the inlet of a pump, and the distal end 22 of the first sub-cannula 14 is fluidly connected to the outlet of the pump (not shown).

FIG. 11 shows use of the multi-lumen cannula 12 according to the invention in a sixth operative position. In this position, the cannula is inserted through an incision (not shown) formed in one of the right atrium 90 or the vena cava 92. The cannula is inserted past the tricuspid valve 94, through the right ventricle 84, past the pulmonary valve 86, and into the pulmonary artery 88. In this position, the blood is withdrawn from the right atrium 90 and vena cava 92 through the distal lumen apertures 44 of the second sub-cannula 16 and the blood is discharged into the pulmonary artery 88 through the distal lumen apertures 28 of the first sub-cannula 14. The distal ends of the first and second sub-cannulas 14, 16 are preferably connected to a fluid pump (not shown) to accommodate this movement of the blood in the vascular system.

FIGS. 10 and 11 show two alternative positions of the multi-lumen cannula 12 according to the invention to provide right-heart assist. It is to be understood that two different multi-lumen cannulas 12 according to the invention can be used simultaneously to provide both right and left-heart assist functions. For example, a first multi-lumen cannula can be positioned in one of the positions seen in FIGS. 6-9 to provide left-heart assist while a second multi-lumen cannula according to the invention can be positioned as seen in one of FIGS. 10 and 11 to provide right-heart assist. Each use of the multi-lumen cannula 12 according to the invention as described above provides an advantage over the prior art by eliminating an incision in the vasculature system which, in view of the significant fluid pressures inside the system, can be a dramatic improvement over the past procedures. In addition, a single, multi-lumen cannula can be used to accommodate dramatically different fluid pressures within each of the lumens without undesirable deforming or altering the cross-sectional area of the lumens.

Referring now to FIGS. 12-14, a second embodiment of a multi-lumen cannula 12' is shown. The multi-lumen cannula 12' is similar to the multi-lumen cannula 12 of FIG. 1. For purposes of simplicity, like parts of the multi-lumen cannulas 12, 12' will be identified by like reference numerals. The multi-lumen cannula 12' includes a first sub-cannula 14' and a second sub-cannula 16'. Each of the first and second sub-cannulas 14', 16' includes a body portion 18', 34' having a proximal end (not shown), a distal end 22', 38', and a cannula tip 29', 45' coupled to the distal end 22', 38'. In the first sub-canula 14', a lumen 24' extends from a proximal lumen aperture (not shown) provided in the proximal end to at least one distal lumen aperture 28' formed in the cannula tip 29' at the distal end 22'.

The second sub-cannula 16' includes a lumen 40' which extends from a proximal lumen aperture (not shown) to at least one fluid outlet 100 formed in a side wall 102 of the cannula tip 45'. The cannula tip 45' is a reverse flow tip which preferably has a plurality of fluid outlets 100 and a substantially closed distal end 104. The fluid outlets 100 are formed in a sector of the cannula tip 45'. The fluid outlets 100 have a configuration that is different from the distal lumen apertures 44 of the multi-lumen cannula 12. The fluid outlets 100 extend toward the proximal end and are configured to reverse the flow of fluid exiting the second sub-cannula 16'. Each fluid outlet 100 includes an opening 106, formed in an exterior surface 108 of the side wall 102, and an opening 110 formed in an interior surface 112 of the side wall 102. The exterior surface opening 106 of each fluid outlet 100 is located proximally of the interior surface opening 110. As will be described in greater detail below, the fluid, which enters the second sub-cannula 16' through the proximal lumen aperture 42' and flows through the lumen 40', will exit the cannula tip 45' in a direction generally opposite to the original direction of flow.

The fluid outlets 100 of cannula tip 45' are advantageous, as they minimize trauma to the heart and vessels leading to and from the heart caused by pressurized fluid exiting the cannula tip 45'. For example, in a cardiac surgical procedure such as that shown in FIG. 6, the fluid outlets 100 of the second sub-cannula 16', the tip of which is inserted downstream of the aorta 76, will direct fluid exiting the second sub-cannula 16' away from the aortic valve 70 as well as away from the walls of the aorta 76. Since the fluid outlets 100 reverse the flow of fluid and direct it backwards, the fluid outlets 100 decrease the fluid pressure on the aortic valve 70 and walls of the aorta 76. The decreased fluid pressure on the vessel walls reduces the possibility that plaque or blood clots may become dislodged from the vessel walls, thereby minimizing the risk of embolic complications.

As illustrated in FIGS. 15 and 16, a cannula tip with a plurality of reverse flow fluid outlets, such as the cannula tip 45', may also be provided on a single-lumen cannula 114. The cannula 114 has a proximal end 116, a distal end 118, and a cannula tip 45' coupled to the distal end 118 and including a substantially closed distal end 104. A lumen 124 extends between a proximal lumen aperture 126 provided in the proximal end 116 and at least one fluid outlet 100 formed in a side wall 102 of the cannula tip 45'. The fluid outlets 100 extend toward the proximal end 116 and reverse the flow of fluid exiting from the cannula tip 45'.

The details of the fluid outlets 100 for the multi-lumen cannula 12' and the single-lumen cannula 114 are best shown in FIGS. 13, 14 and 16. Each fluid outlet 100 includes an opening 106 formed in an exterior surface 108 of the side wall 102 and an opening 110 formed in an interior surface 112 of the side wall 102, with the exterior surface opening 106 located proximally of the interior surface opening 110. Each fluid outlet 100 is positioned at an obtuse angle .THETA. with respect to a longitudinal axis 130 of the cannula tip 45'. The optimal angle .THETA. for each fluid outlet 100 is preferably between 135.degree. and 165.degree., however, any angle between 105.degree. and 175.degree. may be selected.

In operation, fluid flows through the lumen of the multi- or single-lumen cannula 12', 114 from the proximal end to the cannula tip 45' at the distal end 22', 118. When the fluid reaches the substantially closed distal end 104 of the cannula tip 45', it is re-directed toward the proximal end and exits through the fluid outlets 100 at the angle .THETA. into the receiving chamber of the heart or vessel. While the substantially closed distal end 104 of the cannula tip 45' prevents fluid from exiting therethrough, a tiny aperture is provided to prevent air from becoming entrapped in the distal end of the cannula tip 45'.

The multi- and single-lumen cannulas 12', 114 with the reverse flow fluid outlets 100 are primarily applicable to arterial cannulas or cannulas used for retrograde cardioplegia solution perfusion. The number and size of the fluid outlets 100 provided on the cannula tip 45' is dependent on the desired flow rate and fluid pressure. Given the angle .THETA., the fluid outlets 100 are preferably disposed about the cannula tip 45' such that the exterior surface opening 106 of a first fluid outlet 100 does not overlap the interior surface opening 110 of any adjacent fluid outlet 100. However, it is also foreseeable that the exterior surface opening 106 of one fluid outlet 100 may overlap the interior surface opening 110 of an adjacent fluid outlet 100 by as much as 50%.

In the preferred embodiment of the cannulas 12', 114 shown in FIGS. 12-16, the fluid outlets 100 extend along and are substantially parallel to the longitudinal axis 130 of the cannula tip 45'. One skilled in the art will appreciate that it is also within the scope of the invention for at least a portion of the fluid outlets 100 to be non-parallel to the longitudinal axis 130. As illustrated in FIGS. 17 and 18, the fluid outlets 100 may spiral about or twist with respect to the longitudinal axis 130.

The fluid outlets 100 are spaced about at least a sector of the cannula tip 45'. The single-lumen cannula 114 has fluid outlets 100 disposed equally about the entire cannula tip 45'. However, the fluid outlets 100 may be disposed in only a sector less than the entire cannula tip 45'. As illustrated in the second sub-cannula 16' of the multi-lumen cannula 12', the fluid outlets 100 are disposed about a 180.degree. sector, or half, of the cannula tip 45'. It is understood by those skilled in the art that the sector about which the fluid outlets 100 are disposed may be increased or decreased, depending on the desired need.

One inherent result achieved by providing fluid outlets 100 on a sector of the cannula tip 45', which is less than the entire tip, is steerability of the single-lumen cannula 114. For example, in a cannula having fluid outlets disposed about half (i.e., a 180.degree. sector), or less than half, of the cannula tip, fluid flowing through the lumen of the cannula and exiting the cannula tip will steer the cannula tip in a direction away from the side with the fluid outlets. It should be noted that steerability of the single-lumen cannula 114 may also be accomplished by providing an unbalanced amount of fluid outlets 100 about the cannula tip 45', thereby concentrating a number of the fluid outlets 100 in a sector of the cannula tip 45'.

While it is preferred that the fluid outlets 100 are provided in the cannula tip 45', the fluid outlets 100 may also be provided elsewhere along the cannula. As illustrated in FIGS. 18 and 19, the single-lumen cannula 114 has a cannula tip 29', identical to that of the second sub-cannulas 16, 16' of multi-lumen cannulas 12, 12' and including normal distal lumen apertures 28'. The cannula 114 also includes a plurality of fluid outlets 100 formed in a mid-section 134 of the cannula. The fluid outlets 100 in the mid-section 134 reverse the flow of fluid which exits the lumen 124. The fluid outlets 100 may be substantially parallel to a longitudinal axis of the cannula 114 (FIG. 18) or may spiral or twist thereabout (FIG. 19).

Reasonable variation and modification are possible within the spirit of the foregoing specification and drawings without departing from the scope of the invention.