Gas column pressure monitoring catheters

• US 5,573,007 A
• Filed: 08/08/1994
• Issued: 11/12/1996
• Est. Priority Date: 08/08/1994
• Status: Expired due to Term

• Current Assignee: IRRAS USA, Inc. (Irras AB)
• Original Assignee: Innerspace
• Inventors: Bobo, Donald E. Sr.
• Primary Examiner(s): HINDENBURG, MAX F
• Application Number:
  US08/287,195
• Time in Patent Office: 827 Days
• Field of Search: 128/748, 128/774, 128/775, 128/778, 128/780, 128/673
• US Class Current: 600/561
• CPC Class Codes: A61B 5/0215 by means inserted into the body catheters A61M25/00
• CPC Class Codes: A61B 5/031 Intracranial pressure
• CPC Class Codes: A61B 5/035 Intra-uterine probes therefor
• CPC Class Codes: A61M 2025/0003 having an additional lumen transmitting fluid pressure to the outside for measurement

Abstract

A gas-column pressure measuring catheter which is insertable into a mammalian body for purposes of transmitting pressure changes from a location within the mammalian body to a pressure sensor which is either incorporated into, or connected to, the catheter. In a preferred embodiment, the gas-column pressure measuring catheter comprises an elongate catheter body having a gas-filled lumen extending longitudinally therethrough, and a gas-filled membrane-walled chamber positioned on the catheter body in communication with the gas-filled lumen such that pressure changes exerted against the outer surface of the membrane-walled chamber will result in the transmission of pressure changes through the gas-filled catheter lumen. The gas-filled membrane-walled chamber may be located on the side wall of the catheter body, or may be located on the distal end of the catheter body. The invention includes methods for measuring various intracorporeal pressures using gas-column pressure measuring catheters, including methods for measuring blood pressure, intracranial pressure, intrauterine pressure, pulmonary artery pressures, and other intravascular or intracomparmental pressures. The invention also provides means and methods for replenishing gas lost by diffusion out of the membrane-walled chamber,

Claims

1. A gas-column catheter for monitoring intravascular pressure within the vein or artery of a mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface and being of a size and shape to fit within the vein or artery of said mammalian body;

a gas-filled lumen extending longitudinally through at least a portion of said catheter body;

a gas-filled membrane-wailed chamber at a fast location on said catheter body, said chamber being in gaseous communication with said lumen and being of a size and shape to fit within the vein or artery of said mammalian body and to monitor said intravascular pressure, said chamber being defined at least in pan by a flaccid membrane having an inner surface and an outer surface, said membrane being configured and positioned such that;

an increase in pressure against the outer surface of said membrane will cause said membrane to move inwardly, thereby increasing the pressure of gas within said chamber and said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to move outwardly, thereby decreasing the pressure of gas within said chamber and said lumen;

said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in intravascular pressure exerted within the vein or artery and against the outer surface of said flaccid membrane; and

a catheter-inflating connector apparatus connectable to said catheter, said apparatus being constructed such that connection of said catheter thereto will automatically cause a prescribed volume of gas to be entrapped within said catheter.

2. The catheter of claim 1 wherein said gas-filled membrane-walled chamber is located on the distal end of said catheter body.

3. The catheter of claim 1 wherein said gas-filled membrane-walled chamber is located on the outer surface of said catheter body, at a location between the proximal and distal ends thereof.

4. The catheter of claim 2 wherein:

the flaccid membrane of said membrane walled chamber located on the distal end of the catheter body is a generally bulbous membrane formed of non-elastic material which, when filled in gas of sufficient pressure to provide the desired pressure monitoring function, will protrude laterally no more than approximately two millimeters beyond the adjacent outer surface of the distal end of said catheter body.

5. The catheter of claim 2 wherein said generally bulbous distal membrane, when in its fully gas-filled operative configuration, extends distally beyond the distal tip of said catheter body by a distance of no more than eight times the diameter of the distal end of said catheter body.

6. The catheter of claim 3 wherein said membrane-walled chamber comprises an annular chamber extending about the outer surface of said catheter body and wherein said membrane comprises a generally annular membrane mounted about the outer surface of said catheter body.

7. The catheter of claim 3 wherein said membrane-walled chamber located on the outer surface of said catheter body extends no more than 3 millimeters laterally outboard of said outer surface of said catheter body.

8. The catheter of claim 1 further comprising:

means for absorbing moisture which may accumulate within said gas-filled lumen.

9. The catheter of claim 1 further comprising means for preventing condensation of moisture within at least a portion of said gas-filled lumen.

10. The catheter of claim 1 wherein said gas-filled lumen is connectable to a separate pressure sensor.

11. The catheter of claim 1 further comprising a pressure sensor incorporated into said catheter, in communication with said gas-filled lumen.

12. The catheter of claim 1 wherein said flaccid membrane is formed of material selected from the group of materials consisting of:

a) polyurethane;

b) polyvinyl chloride;

c) polyvinylidene; and

d) combinations thereof.

13. The catheter of claim 1 further comprising:

an apparatus for passing make-up gas into said gas filled lumen for replenishing gas which is lost from said catheter.

14. The catheter of claim 1 further comprising:

at least one working lumen extending longitudinally through said catheter and terminating in at least one aperture formed in said catheter body, said working lumen being usable to infuse a fluid or withdraw blood through said catheter.

15. The central venous catheter of claim 14 wherein said at least one fluid infusion lumen comprises:

a first working lumen extending longitudinally through said catheter body from the proximal end thereof and opening through a first aperture formed at a first location thereon,

a second working lumen extending longitudinally through said catheter body from the proximal end thereof and opening through a second aperture at a second location thereon.

16. The catheter of claim 1 wherein said membrane-walled chamber comprises a chamber formed on the outer surface of said catheter body at a first location between the proximal and distal ends of the catheter body.

17. The catheter of claim 1 wherein said membrane-walled chamber comprises a distal tip chamber formed on the distal end of said catheter body, and wherein said flaccid membrane comprises a generally bulbous distal membrane.

18. The catheter of claim 17 in combination with a fixed volume of gas entrapped within said lumen and within said distal tip chamber, said fixed volume of gas being sufficient to enable said bulbous flaccid membrane to move inwardly and outwardly in response to changes in said central venous pressure, and further such that said membrane protrudes laterally no more than approximately 2 mm beyond the outer surface of the distal end of the catheter body.

19. The catheter of claim 16 wherein said membrane-walled chamber formed on the outer surface of said catheter body comprises an annular chamber having an annular membrane disposed around said catheter body.

20. The catheter of claim 16 in combination with a fixed volume of gas entrapped within said lumen and within said membrane-walled chamber formed on the outer surface of said catheter body, said fixed volume of gas being sufficient to enable said membrane to move inwardly and outwardly in response to changes in said central venous pressure, and further such that said membrane protrudes laterally no more than approximately 3 mm beyond the adjacent outer surface of the catheter body.

21. The catheter of claim 1 wherein said catheter body and said flaccid membrane, when filled with sufficient gas to monitor central venous pressure, will remain smaller than the cardiovascular anatomical passageways in which said catheter is positioned so as not to occlude said passageways.

22. A gas-column catheter insertable into the vein or artery of a mammalian body for monitoring intravascular pressure changes within said mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface having an outer diameter and being of a size and shape to fit within the vein or artery of said mammalian body;

a gas-filled lumen extending longitudinally through said catheter body and terminating distally in an aperture at the distal end of said catheter body, said lumen being sized to contain therein a fixed volume of gas;

a first gas-filled membrane-walled chamber located on the distal end of said catheter body and in gaseous communication with said lumen through said aperture, said first membrane-wailed chamber being defined at least in pan by a bulbous membrane mounted on and extending distally from the distal end of said catheter body and being of a size and shape to fit within the vein or artery of said mammalian body and to monitor said intravascular pressure, said membrane having an outer surface and an inner surface, said membrane being flaccid and substantially non-elastic when in its fully gas-filled operative configuration, such that;

an increase in pressure against the outer surface of said membrane will compress said membrane causing a decrease in the volume of air within said chamber and a corresponding increase in the pressure of air within said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to decompress, thereby causing an increase in the volume of air within said chamber and a corresponding decrease in the pressure of air within said lumen;

said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in intravascular pressure exerted within the vein or artery and against the outer surface of said flaccid membrane; and

a catheter-inflating connector apparatus connectable to said catheter, said apparatus being constructed such that connection of said catheter thereto will automatically cause a prescribed volume of gas to be entrapped within said catheter.

23. The catheter of claim 22 wherein:

said bulbous distal membrane is formed of material selected from the group of materials consisting of;

A. polyurethane

B. polyvinyl chloride

C. polyvinylidene, and

D. combinations thereof.

24. The catheter of claim 22 wherein said bulbous distal membrane is configured such that, when the catheter is charged with sufficient volume of gas to carry out its intended function, said bulbous membrane will not protrude laterally more than approximately 2 mm beyond the outer surface of the catheter body at the distal end thereof.

25. The catheter of claim 22 wherein:

the catheter body has an outer diameter of less than 5 mm and said generally bulbous distal membrane has an outer diameter of less than 9mm when in its fully distended configuration.

26. The catheter of claim 22 wherein said generally bulbous distal membrane, when said catheter is charged with a volume of gas sufficient for its intended function, extends distally beyond the distal tip of said catheter body by a distance of no more than eight times the diameter of the distal end of said catheter body.

27. The catheter of claim 22 further comprising:

a second gas-filled lumen extending longitudinally through said catheter body to a second location proximal to said first membrane-walled chamber on the distal end of said catheter body;

a second gas-filled membrane-walled chamber located at said second location and in gaseous communication with said second lumen, said second gas filled chamber being defined at least in part by a second flaccid membrane mounted on the outer surface of said catheter body and having an inner surface and an outer surface such that;

i. an increase in pressure against the outer surface of said second membrane will cause said second membrane to move inwardly thereby increasing the pressure of gas within said second chamber and second lumen; and

ii. a decrease in pressure against the outer surface of said second membrane will cause said second membrane to move outwardly, thereby decreasing the pressure of gas within said second chamber and said second lumen;

said second lumen being communicable with a second pressure sensing apparatus operative to sense changes in pressure within said second lumen.

28. The catheter of claim 27 wherein said second gas-filled membrane-walled chamber is configured and constructed such that, when said second gas-filled lumen and said second membrane-walled chamber are charged with sufficient volume of gas for their intended pressure monitoring function, said second flaccid membrane will protrude laterally no more than approximately 3 mm beyond the adjacent side wall of the catheter body.

29. The catheter of claim 27 wherein said second membrane-walled chamber comprises an annular chamber extending about the outer surface of said catheter body and wherein said second membrane comprises a generally annular membrane mounted about the outer surface of said catheter body.

30. The catheter of claim 22 further comprising:

means for absorbing moisture from said gas-filled lumen.

31. The catheter of claim 22 further comprising means for preventing condensation of moisture within at least a portion of said gas-filled lumen.

32. The catheter of claim 22 wherein said catheter inflating connector apparatus is further configured and constructed such that:

upon initial connection of said catheter to said catheter-inflating connector apparatus will cause a first volume of gas to be contained within said catheter; and

,

upon subsequent disconnection and reconnection of said catheter to said pressure sensing apparatus, said coupling apparatus will cause a second volume of gas to be contained within said catheter.

33. The catheter of claim 22 further comprising:

an apparatus for passing make-up gas into said gas-filled lumen for replenishing gas which is lost from said catheter.

34. The catheter of claim 33 wherein said apparatus for passing make-up gas into said gas-filled lumen comprises:

a diffusive make-up gas system coupled to said gas-filled lumen for replenishing gas which is lost from said catheter through outward diffusion, said diffusive make up gas system comprising;

i. a gas-filled reservoir containing gas at a pressure which exceeds the pressure of gas within the catheter lumen;

ii. a gas flow pathway through which gas may flow from said reservoir into said catheter lumen; and

iii. a membranous diffusion barrier disposed within said gas flow pathway such that gas flowing from said reservoir into said lumen must pass through said membranous barrier.

35. The catheter of claim 34 wherein the gas-filled reservoir of said diffusive make-up gas system further comprises:

a gas compressing apparatus for compressing gas within said reservoir.

36. The catheter of claim 34 wherein the rate of gas diffusion through the membranous diffusion barrier of the diffusive make-up gas system is matched to the diffusion characteristics of the flaccid membrane such that, at the time of connection, the rate at which make-up gas will pass into the catheter lumen is substantially the same as the rate at which gas is lost from said catheter due to diffusion through said flaccid membrane.

37. The catheter of claim 34 wherein said diffusive make-up gas system is incorporated into a coupler to which said catheter may be coupled, said coupler being further constructed such that:

the act of coupling said catheter to said coupler will automatically result in entrapment of a preselected volume of gas within said catheter.

38. The catheter of claim 34 wherein said diffusive make-up gas system is further constructed such that:

the act of coupling said catheter to said coupler will entrap and compress a quantity of gas within the gas-filled reservoir of said diffusive make up gas system.

39. A gas-column catheter for monitoring pressure within a mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface;

a gas-filled lumen extending longitudinally through at least a portion of said catheter body;

a gas-filled membrane-walled chamber at a fast location on said catheter body, said chamber being in gaseous communication with said lumen, said chamber being defined at least in part by a flaccid membrane having an inner surface and an outer surface, said membrane being configured and positioned such that;

an increase in pressure against the outer surface of said membrane will cause said membrane to move inwardly, thereby increasing the pressure of gas within said chamber and said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to move outwardly, thereby decreasing the pressure of gas within said chamber and said lumen;

said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in pressure exerted against the outer surface of said flaccid membrane; and

said catheter further including means for absorbing moisture which may accumulate within said gas-filled lumen.

40. The catheter of claim 39 wherein said means for absorbing moisture comprises a moisture absorbing material disposed within said lumen.

41. The catheter of claim 40 wherein said moisture absorbing material is polyacrylamide.

42. A gas-column catheter insertable into a mammalian body for monitoring pressure changes within said mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface having an outer diameter;

a gas-filled lumen extending longitudinally through said catheter body and terminating distally in an aperture at the distal end of said catheter body, said lumen being sized to contain therein a fixed volume of gas;

a fast gas-filled membrane-walled chamber located on the distal end of said catheter body and in gaseous communication with said lumen through said aperture, said fast membrane-walled chamber being defined at least in part by a bulbous membrane mounted on and extending distally from the distal end of said catheter body, said membrane having an outer surface and an inner surface, said membrane being flaccid and substantially non-elastic when in its fully gas-filled operative configuration, such that;

an increase in pressure against the outer surface of said membrane will compress said membrane causing a decrease in the volume of air within said chamber and a corresponding increase in the pressure of air within said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to decompress, thereby causing an increase in the volume of air within said chamber and a corresponding decrease in the pressure of air within said lumen;

and said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in pressure exerted against the outer surface of said flaccid membrane; and

said catheter further including means for absorbing moisture from said gas filled lumen.

43. The catheter of claim 42 wherein said means for absorbing moisture comprises a moisture absorbing material within said lumen.

44. The catheter of claim 43 wherein said moisture absorbing material is polyacrylamide.

45. A gas-column catheter insertable into a mammalian body for monitoring pressure changes within said mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface having an outer diameter;

,

a gas-filled lumen extending longitudinally through said catheter body and terminating distally in an aperture at the distal end of said catheter body, said lumen being sized to contain therein a fixed volume of gas;

a fast gas-filled membrane-walled chamber located on the distal end of said catheter body and in gaseous communication with said lumen through said aperture, said fast membrane-walled chamber being defined at least in pan by a bulbous membrane mounted on and extending distally from the distal end of said catheter body, said membrane having an outer surface and an inner surface, said membrane being flaccid and substantially non-elastic when in its fully gas-filled operative configuration, such that;

an increase in pressure against the outer surface of said membrane will compress said membrane causing a decrease in the volume of air within said chamber and a corresponding increase in the pressure of air within said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to decompress, thereby causing an increase in the volume of air within said chamber and a corresponding decrease in the pressure of air within said lumen;

and said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in pressure exerted against the outer surface of said flaccid membrane; and

said catheter further including means for means for preventing condensation of moisture within at least a portion of said gas-filled lumen.

46. A gas-column catheter for monitoring pressure within a mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface;

a gas filled lumen extending longitudinally through at least a portion of said catheter body;

a gas-filled membrane-walled chamber at a first location on said catheter body, said chamber being in gaseous communication with said lumen, said chamber being defined at least in pan by a flaccid membrane having an inner surface and an outer surface, said membrane being configured and positioned such that;

an increase in pressure against the outer surface of said membrane will cause said membrane to move inwardly, thereby increasing the pressure of gas within said chamber and said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to move outwardly, thereby decreasing the pressure of gas within said chamber and said lumen;

said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in pressure exerted against the outer surface of said flaccid membrane; and

said catheter further including means for means for preventing condensation of moisture within at least a portion of said gas-filled lumen.

47. A gas-colunm catheter for monitoring pressure within a mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface;

a gas-filled lumen extending longitudinally through at least a portion of said catheter body;

a gas-filled membrane-walled chamber at a first location on said catheter body, said chamber being in gaseous communication with said lumen, said chamber being defined at least in pan by a flaccid membrane having an inner surface and an outer surface, said membrane being configured and positioned such that;

an increase in pressure against the outer surface of said membrane will cause said membrane to move inwardly, thereby increasing the pressure of gas within said chamber and said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to move outwardly, thereby decreasing the pressure of gas within said chamber and said lumen;

said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in pressure exerted against the outer surface of said flaccid membrane; and

said catheter further including a catheter-inflating connector apparatus connectable to said catheter, said apparatus being constructed such that connection of said catheter thereto will automatically cause a prescribed volume of gas to be entrapped within said catheter.

48. The catheter of claim 47 wherein said catheter inflating connector apparatus is further configured and constructed such that:

upon initial connection of said catheter to said pressure sensing apparatus, said apparatus will cause a first volume of gas to be contained within said catheter; and

,

upon subsequent disconnection and reconnection of said catheter to said pressure sensing apparatus, said coupling apparatus will cause a second volume of gas to be contained within said catheter.

49. A gas-column catheter for monitoring pressure within a mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface;

a gas-filled lumen extending longitudinally through at least a portion of said catheter body;

a gas-filled membrane-walled chamber at a fast location on said catheter body, said chamber being in gaseous communication with said lumen, said chamber being defined at least in part by a flaccid membrane having an inner surface and an outer surface, said membrane being configured and positioned such that;

an increase in pressure against the outer surface of said membrane will cause said membrane to move inwardly, thereby increasing the pressure of gas within said chamber and said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to move outwardly, thereby decreasing the pressure of gas within said chamber and said lumen;

said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in pressure exerted against the outer surface of said flaccid membrane; and

said catheter further including a catheter-inflating apparatus for passing make-up gas into said gas filled lumen for replenishing gas which is lost from said catheter said apparatus including a diffusive make-up gas system coupled to said gas-filled lumen for replenishing gas which is lost from said catheter through outward diffusion, said diffusive make up gas system comprising a gas-filled reservoir containing gas at a pressure which exceeds the pressure of gas within the catheter lumen;

a gas flow pathway through which gas may flow from said reservoir into said catheter lumen; and

a membranous diffusion barrier disposed with said gas flow pathway such that gas flowing from said reservoir into said lumen must pass through said membranous barrier.

50. The catheter of claim 49 wherein the gas-filled reservoir of said diffusive make-up gas system further comprises:

a gas compressing apparatus for compressing gas within said reservoir.

51. The catheter of claim 49 wherein the rate of gas diffusion through the membranous diffusion barrier of the diffusive make-up gas system is matched to the diffusion characteristics of the flaccid membrane such that, at the time of connection the rate at which make-up gas will pass into the catheter lumen is substantially the same as the rate at which gas is lost from said catheter due to diffusion through said flaccid membrane.

52. The catheter of claim 49 wherein said diffusive make-up gas system is incorporated into a coupler to which said catheter may be coupled, said coupler being further constructed such that:

the act of coupling said catheter to said coupler will automatically result in entrapment of a preselected volume of gas within said catheter.

53. The catheter of claim 52 wherein said diffusive make-up gas system is further constructed such that:

the act of coupling said catheter to said coupler sensor will entrap and compress a quantity of gas within the gas-filled reservoir of said diffusive make up gas system.

54. A gas-column catheter insertable into a mammalian body for monitoring pressure changes within said mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface having an outer diameter,

a gas-filled lumen extending longitudinally through said catheter body and terminating distally in an aperture at the distal end of said catheter body, said lumen being sized to contain therein a fixed volume of gas;

a first gas-filled membrane-walled chamber located on the distal end of said catheter body and in gaseous communication with said lumen through said aperture, said first membrane-walled chamber being defined at least in part by a bulbous membrane mounted on and extending distally from the distal end of said catheter body, said membrane having an outer surface and an inner surface, said membrane being flaccid and substantially non-elastic when in its fully gas-filled operative configuration, such that;

an increase in pressure against the outer surface of said membrane will compress said membrane causing a decrease in the volume of air within said chamber and a corresponding increase in the pressure of air within said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to decompress, thereby causing an increase in the volume of air within said chamber and a corresponding decrease in the pressure of air within said lumen;

and said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in pressure exerted against the outer surface of said flaccid membrane; and

a catheter-inflating connector apparatus connectable to said catheter, said apparatus being constructed such that connection of said catheter thereto will automatically cause a prescribed volume of gas to be entrapped within said catheter.

55. A gas-column catheter insertable into a mammalian body for monitoring pressure changes within said mammalian body, said catheter comprising:

a catheter body having a proximal end, a distal end, and an outer surface having an outer diameter;

a gas-filled lumen extending longitudinally through said catheter body and terminating distally in an aperture at the distal end of said catheter body, said lumen being sized to contain therein a fixed volume of gas;

a fast gas-fried membrane-walled chamber located on the distal end of said catheter body and in gaseous communication with said lumen through said aperture, said first membrane-walled chamber being defined at least in part by a bulbous membrane mounted on and extending distally from the distal end of said catheter body, said membrane having an outer surface and an inner surface, said membrane being flaccid and substantially non-elastic when in its fully gas-filled operative configuration, such that;

an increase in pressure against the outer surface of said membrane will compress said membrane causing a decrease in the volume of air within said chamber and a corresponding increase in the pressure of air within said lumen;

and a decrease in pressure against the outer surface of said membrane will allow said membrane to decompress, thereby causing an increase in the volume of air within said chamber and a corresponding decrease in the pressure of air within said lumen;

and said gas filled lumen being thereby operable to transmit, in the proximal direction, changes in pressure exerted against the outer surface of said flaccid membrane; and

an apparatus for passing make-up gas into said gas-filled lumen for replenishing gas which is lost from said catheter, said apparatus including it diffusive make-up gas system coupled to said gas-filled lumen for replenishing gas which is lost from said catheter through outward diffusion, said diffusive make up gas system comprising a gas-filled reservoir containing gas at a pressure which exceeds the pressure of gas within the catheter lumen;

a gas flow pathway through which gas may flow from said reservoir into said catheter lumen; and

a membranous diffusion barrier disposed within said gas flow pathway such that gas flowing from said reservoir into said lumen must pass through said membranous barrier.