Cannula having reduced flow resistance

1 . A multilumen cannula for reducing resistance in blood flowing therethrough, the multilumen cannula comprising:

a first elongate body comprising a proximal portion, a transition portion having a diameter at a distal end greater than a diameter at a proximal end, a distal portion, and a first lumen that extends therethrough, the first lumen having a first cross-sectional area within the proximal portion and a second cross-sectional area within the distal portion, the second cross-sectional area being greater than the first cross-sectional area; and

a second elongate body defining a second lumen, the second elongate body extending along the proximal portion of the first elongate body.

2 . The multilumen cannula of claim 1 , wherein the proximal portion of the first elongate body extends into the second lumen.

3 . The multilumen cannula of claim 2 , wherein the longitudinal axes of the first and the second lumens are parallel.

4 . The multilumen cannula of claim 3 , wherein the first lumen and the second lumen are co-axial.

5 . The multilumen cannula of claim 1 , wherein the proximal portion extends alongside the second lumen.

6 . The multilumen cannula of claim 5 , wherein the first and second lumens are substantially D-shaped lumens.

7 . A multilumen cannula for reducing resistance in blood flowing therethrough, the multilumen cannula comprising:

a first elongate portion defining a first lumen that extends between a first proximal end and a first distal end, the inner cross-section of the first lumen being greater proximate the first distal end than proximate the first proximal end; and

a second elongate portion defining a second lumen that extends between a second proximal end and a second distal end.

8 . The multilumen cannula of claim 7 , wherein the first elongate portion and the second elongate portion are axially movable relative to each other.

9 . The multilumen cannula of claim 7 , wherein at least a portion of the inner cross-section of the first lumen is circular.

10 . The multilumen cannula of claim 7 , wherein a portion of the first elongate portion extends through the second lumen.

11 . The multilumen cannula of claim 10 , wherein the first elongate portion and the second elongate portion are translatable relative to each other.

12 . The multilumen cannula of claim 10 , wherein an outer dimension of the second elongate portion is no greater than an outer dimension of the first elongate portion.

13 . The multilumen cannula of claim 7 , wherein each of the first elongate portion and the second elongate portion comprise a D-shaped cross-section proximate the second proximal end.

14 . The multilumen cannula of claim 7 , wherein the first proximal end of the first elongate portion has a D-shaped cross-section and the first distal end thereof has a circular cross-section.

15 . The multilumen cannula of claim 7 , further comprising means for creating substantially vortex flow.

16 . The multilumen cannula of claim 15 , wherein the creating means comprises an inwardly protruding ridge oriented in a helical arrangement in the first lumen.

17 . A multilumen cannula for reducing resistance in blood flowing therethrough, the multilumen cannula comprising:

a first elongate portion defining a first lumen that extends between a first proximal end and a first distal end; and

a second elongate portion defining a vessel wall engagement portion and a second lumen that extends between a second proximal end and a second distal end,

wherein the inner cross-section of the first lumen is greater at the first distal end than at a location corresponding to the vascular wall engagement portion.

18 . A method for accessing the vascular system of a patient, comprising:

providing a cannula comprising a first elongate portion and a second elongate portion, the first elongate portion defining a transition portion and a first lumen that extends between a first proximal end and a first distal end, the second elongate portion defining a second lumen that extends between a second proximal end and a second distal end, wherein the inner cross-section of the first lumen is greater proximate the first distal end than proximate a vessel wall engagement portion of said second elongate portion;

translating a portion of said cannula to a first position wherein said second distal end is located distal to at least a portion of said transition portion of said first elongate portion;

inserting said cannula into a blood vessel through a vascular access site such that said vessel wall engagement portion of said second elongate portion is adjacent said vessel wall;

translating a portion of said cannula to a second position wherein the second distal end is proximal to at least a portion of said transition portion.

19 . The method of claim 18 , wherein when said cannula is in said first position, the second distal end is located distal substantially all of said transition portion.

20 . The method of claim 18 , wherein when said cannula is in said second position, the second distal end is located proximal substantially all of said transition portion.

21 . The method of claim 18 , further comprising the step of coupling the proximal ends of the first and second elongated portions to a pump

22 . The method of claim 18 , wherein the first elongate portion extends axially through at least a portion of the second elongate portion

23 . The method of claim 22 , wherein both the first and second elongate portions have substantially circular cross-sections.

24 . The method of claim 18 , wherein the cannula provides a substantially constant outer cross-sectional profile when the second elongate portion is in the first position.