U.S. patent number 3,658,053 [Application Number 04/853,784] was granted by the patent office on 1972-04-25 for catheter for use in detecting dissolved gas in fluids such as blood.
This patent grant is currently assigned to Scientific Research Instruments Corporation. Invention is credited to Gordon J. Fergusson, Austin L. Wahrhaftig.
United States Patent |
3,658,053 |
Fergusson , et al. |
April 25, 1972 |
CATHETER FOR USE IN DETECTING DISSOLVED GAS IN FLUIDS SUCH AS
BLOOD
Abstract
A blood catheter including a cannula covered with a thin layer
of silicone rubber or other material permeable to one or more of
the gases that are or might be found in blood and wherein the
cannula preferably includes a helical arrangement of apertures for
enabling the diffusion of gas through the membrane and into the
center portion of the cannula. The helical pattern of apertures
around the periphery of the cannula enables the catheter to contact
the interior wall of a blood vessel without restricting blood flow
past more than a small fraction of the total number of apertures.
Other hole configurations can be used, for example, when a
plurality of holes are located at spaced axial locations along the
cannula and at spaced intervals around the circumference of the
cannula at the various axial locations.
Inventors: |
Fergusson; Gordon J.
(Lutherville, MD), Wahrhaftig; Austin L. (Salt Lake City,
UT) |
Assignee: |
Scientific Research Instruments
Corporation (Baltimore, MD)
|
Family
ID: |
25316892 |
Appl.
No.: |
04/853,784 |
Filed: |
August 28, 1969 |
Current U.S.
Class: |
600/364 |
Current CPC
Class: |
A61M
25/0069 (20130101); A61B 5/145 (20130101) |
Current International
Class: |
A61B
5/00 (20060101); A61M 25/00 (20060101); A61b
005/00 () |
Field of
Search: |
;128/2,2.05,2.1,344,348
;73/23 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Truluck; Dalton L.
Claims
What is claimed is:
1. A catheter operable with a detecting instrument to provide a
continuous in vivo measurement of blood gases by mass spectrometry
or the like comprising an elongated tubular structure having a
blood vessel entering portion terminating in a distal end, said
portion including an inner cannula and an outer membrane extending
thereover throughout the longitudinal extent thereof, said cannula
and membrane being operable to be inserted and maintained in a
blood vessel in an operative position wherein the longitudinal
extent thereof is generally aligned with the longitudinal extent of
the blood vessel, the transverse size and longitudinal extent of
said cannula and membrane being such that when disposed in said
operative position blood will flow through the longitudinal
coextensive portion of the vessel with a surface layer of
substantial longitudinal extent in contact with the exterior
surface of said membrane, said cannula defining a longitudinally
elongated exterior periphery and a substantially longitudinally
coextensive interior gas passage adapted to be communicated with a
means on the detecting instrument for inducing a gas flow in a
direction away from said gas passage and toward the detecting
instrument, said membrane being in intimate contact with said
exterior periphery and being formed of a material impervious to
blood but pervious to the blood gases to be continuously measured,
said cannula including a material substantially impervious to the
blood gases to be continuously measured disposed between said
exterior periphery and said gas passage throughout the longitudinal
extent thereof, said cannula having apertures extending from said
exterior periphery into communication with said interior gas
passage in spaced relation throughout substantially the entire
longitudinal extent thereof defining with the portions of said
membrane extending thereover a plurality of spaced discrete blood
impervious gas diffusion paths extending transversely from the
exterior of said membrane to said gas passage, said gas diffusion
paths having a total area providing for a continuous flow of blood
gases to the detecting instrument adequate to insure continuous
measurement thereof, each aperture being sufficiently small in
longitudinal dimension so that the blood in said surface layer
flowing over the gas diffusion path associated therewith will not
be excessively depleted in gas content during the time of passage
thereby, the spacing between apertures in the direction of blood
flow being such that blood which has been partially depleted of gas
content by passing over one gas diffusion path will have its gas
content substantially restored to that of the bulk blood by
turbulent mixing and/or by diffusion of gas from the bulk flow
before passing over another gas diffusion path.
2. A catheter as defined in claim 1 wherein said apertures are
spaced with respect to each other both longitudinally and
circumferentially about said exterior periphery of said cannula and
are provided in a sufficient number to enable substantially the
same amount of gas to pass through the sum of all of the gas
diffusion paths per unit time irrespective of the blockage of
certain of said gas diffusion paths resulting from local contact of
the exterior periphery of said membrane with the blood vessel walls
when said cannula and said membrane are disposed in said operative
position therein.
3. A catheter as defined in claim 2 wherein the ratio of the
longitudinal spacing between apertures aligned longitudinally to
the longitudinal dimension of each aperture is substantially five
to one.
4. A catheter as defined in claim 3 wherein the longitudinal
dimension of each of said apertures is in the range of from 0.006
inch to 0.012 inch.
5. A catheter as defined in claim 4 wherein the longitudinal
dimension of each of said apertures is approximately 0.008
inch.
6. A catheter as defined in claim 2 wherein said elongated tubular
structure comprises a unitary cylindrical tube of metal having a
closed distal end formed in the portion thereof defining said
cannula.
7. A catheter as defined in claim 6 wherein said apertures are
formed by a coping saw to a depth substantially one-third the
outside diameter of said cannula to provide adequate opening into
said gas passage consistent with minimum structural weakening of
said cannula.
8. A catheter as defined in claim 7 wherein said apertures are
arranged sequentially in a generally helical pattern throughout the
exterior periphery of said cannula, adjacent apertures being
angularly spaced with respect to each other approximately
120.degree. and longitudinally spaced from one another a distance
generally five times the longitudinal dimension of each
aperture.
9. A catheter as defined in claim 8 wherein the longitudinal
dimension of each of said apertures is approximately 0.008 inch and
the number of apertures is approximately 14.
10. A catheter as defined in claim 9 wherein said membrane extends
throughout the longitudinal extent of said cylindrical tube.
11. A catheter as defined in claim 10 wherein said cylindrical tube
is formed of stainless steel and said membrane is comprised of
silicone rubber.
Description
The present invention relates to a catheter and more particularly
to a blood catheter which may be used in enabling the determination
of the amount and type of dissolved gas in blood.
Those concerned with the development of blood catheters have long
been faced with the problem of providing a catheter which enables
the sampling of dissolved gas in the blood stream with minimal
error due to local depletion of the gas by diffusion into the
catheter and for providing a catheter which minimizes the effect of
contact of the permeable area thereof with the wall of the blood
vessel while not significantly increasing the time-constant for the
response of the membrane-cannula system.
Accordingly, the general purpose of this invention is to provide a
catheter which embraces all the advantages of similarly employed
catheters and possesses none of the aforedescribed disadvantages.
To attain this, the present invention contemplates a unique cannula
structure wherein the cannula includes a plurality of apertures
arranged sequentially in a generally helical pattern around the
cannula and wherein the cannula is covered by a gas-permeable
membrane to enable the diffusion of gas through the membrane and
into the cannula via the apertures therein.
An object of the present invention is the provision of a catheter
which permits sampling of dissolved gas in the blood stream by
diffusion into the cannula of the catheter with minimal error due
to local depletion of the gas.
Another object is to provide a blood catheter which minimizes the
effect of contact of the permeable area of the cannula with the
wall of the blood vessel while not significantly increasing the
time-constant for the response of the membrane-cannula system.
Other objects and features of the invention will become apparent to
those of ordinary skill in the art as the disclosure is made in the
following description of a preferred embodiment of the invention as
illustrated in the accompanying sheet of drawing in which:
FIG. 1 is a perspective view of a preferred embodiment of the
invention;
FIG. 2 is an enlarged fragmentary sectional view of the catheter of
this invention;
FIG. 3 is a section of the catheter taken on the line 3--3 of FIG.
2 looking in the direction of the arrows; and
FIG. 4 is a perspective view of another embodiment of the
invention.
With reference now to the drawings, wherein like reference
characters designate like or corresponding parts throughout the
several views, there is shown in FIGS. 1-3 a catheter 10 embodying
in continuous and unitary structural relationship a tube member 14
and a cannula 16, both of which may be comprised of stainless steel
or appropriate plastic material, for example. The cannula 16 has a
plurality of apertures 18 therein preferably arranged sequentially
in a generally helical pattern around the cannula. The apertures
are preferably but not necessarily spaced at 120.degree. around the
cannula. The catheter also includes a gas-permeable membrane 20 of
silicone rubber or other material, e.g., Teflon, permeable to one
or more of the gases that are to be diffused into the catheter. The
membrane 20 is fitted over the end of the cannula 16 and extends
the entire length of the catheter over the tube 14. The silastic
membrane can be affixed to the tubular structure of the tube 14 and
cannula 16 by solvent expansion and evaporation for example. This
is easily accomplished by first soaking the silastic material in a
solvent such as diethylether, placing the wetted membrane over the
tubular structure and then permitting the solvent to evaporate so
that the membrane shrinks and tightly embraces the tubular
structure. In the event materials such as Teflon are used for the
membrane, the membrane can be affixed to the tubular structure by
heat shrinkage.
In the event that throw-away catheter types are to be used, an
alternative arrangement may be utilized (as shown in FIG. 4)
wherein the cannula 16 is coupled in a conventional manner by
fitting 12 to tube member 14 and then the membrane is fitted over
the cannula and a ligature 22 of surgical silk, for example, is
used to make a leak proof seal at the end of the cannula adjacent
to the tube at fitting 12. In view of the above, the term "cannula"
as used herein refers to the end portion of the tubular structure
of the catheter, excluding the membrane, within which the apertures
are formed.
The arrangement of the apertures 18 in a generally helical pattern
around the periphery of the cannula 16 permits sampling of the
dissolved gas in the fluid or blood passing over the catheter by
diffusion thereof into the cannula and with minimal error due to
local depletion of the gas since the surface layer of the fluid
which is partially depleted of gas in passage over one aperture
will have its gas content substantially restored to that of the
bulk fluid or blood passing by the catheter either by turbulent
mixing or by diffusion of gas from the bulk flow during passage
from one aperture to the next.
In addition, the arrangement of apertures in the generally helical
pattern minimizes the effect of blockage of certain of the
apertures by the wall of the blood vessel. For example, since the
apertures are located around the entire periphery of the cannula
with only a small number of apertures being located in linear
alignment along the length thereof the amount of gas which is
diffused through the permeable membrane is not significantly
reduced when the catheter is placed into contact with the vessel
wall. Furthermore, as a result of this configuration the
time-constant for response of the membrane-cannula system is not
significantly increased so that substantially the same amount of
gas is diffused through the membrane and into the cannula over a
predetermined time period when the catheter is in contact with the
wall of a blood vessel as is diffused through the membrane when the
catheter is not in contact with the blood vessel wall for the same
time period.
Alternative geometric configurations of apertures are also possible
although not as efficient as the helical pattern. For example, the
apertures can be spaced circumferentially around the catheter with
groups of these circumferentially spaced apertures being located at
spaced axial locations along the catheter as shown in FIG. 4.
In designing the catheter of this invention the dimensions of the
various portions thereof are important to the successful operation
of the catheter in the manner desired. For example, the apertures
18 must be sufficiently small in dimension in the direction of
blood flow so that the surface layers of blood flowing over the
permeable membrane 20 will not be excessively depleted in gas
content during time of passage over one aperture. In addition, the
apertures must be sufficiently well separated along the length of
the cannula in the direction of blood flow so that the surface
layer of blood which has been partially depleted of gas in passage
over one aperture will have its gas content substantially restored
to that of the bulk blood either by turbulent mixing or by
diffusion of gas from the bulk flow during passage from one
aperture to the next. Furthermore, the total number of apertures
must be adequate to give the gas flow required by the detecting
instrument, e.g., a mass spectrometer. The choice of the size of
the apertures and of the spacing between the apertures is also
influenced by the ease of manufacture and resultant strength of the
apertured cannula.
The catheter of this invention preferably utilizes apertures having
dimensions along the direction of flow of the fluid in the range of
from 0.006 inch to 0.012 inch with apertures having a linear
dimension of 0.008 inch being a particularly preferable dimension
since they can be easily cut with coping saws. The depth of the
apertures are preferably approximately one-third of the outside
diameter of the cannula 16 so as to give adequate opening into the
center space of the cannula consistent with minimum weakening
thereof. The apertures 18 are also preferably spaced a distance
apart from one another such that the ratio of the distance between
apertures to the linear dimension of each of the apertures is five
to one. Accordingly, with an aperture linear dimension of 0.008
inch the apertures are typically spaced 0.040 inch apart with the
apertures preferably being arranged sequentially at 120.degree.
around the cannula 16. The number of apertures used may vary
considerably with the number preferably in the range of from six to
24 but generally 14 apertures are utilized in the blood catheter.
The gas-permeable membrane 20 is typically a medical grade silastic
tubing having a thickness in the range of from 0.0085 inch to
0.0105 inch.
The catheter of this invention thus provides for a unique
arrangement of apertures in a cannula whereby the apertures are
helically arranged around the cannula to permit sampling of the
dissolved gas in a fluid with minimal error due to local depletion
of the gas by diffusion into the cannula. The catheter also
minimizes the effect of contact of the catheter with the wall of a
blood vessel, for example, while not significantly increasing the
time-constant for the response of the membrane-cannula system due
to the helical arrangement of the apertures around the cannula
since the blood flow is restricted only past a small fraction of
the total number of apertures. Furthermore, this invention provides
for important dimensional relationships between the apertures, the
spacings therebetween and the gas-permeable membrane wherein the
linear dimension of the apertures in the direction of fluid flow is
sufficiently small such that the surface layers of fluid flowing
over the permeable membrane covering the apertures will not be
excessively depleted of gas content during time of passage over one
aperture. The apertures are also sufficiently well separated along
the length of the cannula in the direction of fluid flow so that
the surface layer of fluid partially depleted of gas in passage
over one aperture will have its gas content essentially restored
either by turbulent mixing or by diffusion of gas from the bulk
flow of fluid during passage from one aperture to the next
aperture.
Obviously many modifications and variations of the present
invention are possible in light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims the invention may be practiced otherwise than as
specifically described.
* * * * *