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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to the sensing of gases dissolved in liquids and is
specifically adapted for the in vivo sensing of blood gases. Blood gases
have been sensed and analyzed by various prior art methods, one of which
being disclosed in U.S. Pat. Nos. 3,983,864 and 4,016,864. As shown in
those patents, a carrier gas is introduced into a special catheter probe,
and held in a chamber where the blood gases equilibrate through a gas
permeable membrane with the carrier gas and the carrier gas containing the
equilibrated blood gases is thereafter withdrawn and analyzed.
The catheter is introduced in vivo into the particular blood vessel sought
to be analyzed. An equilibration chamber is provided in the probe and
allows an equilibration between the carrier gas passing through the probe
and the blood gases contained in the blood. Equilibration occurs through a
gas permeable membrane that surrounds the equilibration chamber and has
its outside surface in direct contact with the blood to be analyzed. The
blood gases pass into the equilibration chamber through the gas permeable
material until the partial pressures within the chamber achieve blood
levels.
The carrier gas remains in the equilibration chamber for a specific period
of time to insure equilibration is completed, at which time, the carrier
gas containing that bolus of carrier gas with the equilibrated blood gases
is removed and its content determined by an analyzer such as a gas
chromatograph.
A vacuum means is used to transport the bolus of equilibrated gases to the
analyzer through various valving means.
One of the difficulties with such present systems is that a certain finite
time is needed for the sample gas to fully equilibrate with the blood
gases and thus, the number of samples one can take within any specific
period of time is limited. That time is partially dependent upon the ratio
of area of the equilibration chamber that directly receives the blood
gases through the permeable material to the volume of the equilibration
chamber as well as other factors such as the gas permeability of the
membrane. To be effective and rapid, the aforementioned ratio should be
high, that is, there needs to be a large surface area through which the
blood gases pass into the equilibration chamber in relation to the volume
of carrier gas in the chamber.
The equilibration chamber itself is normally located at the distal end of
the catheter and it comprises an active length of the catheter at that
distal end. Present catheters have a relatively long active length in
order to include a sufficiently large equilibration chamber and therefore
a technician utilizing the catheter may be unable to pinpoint the exact
location in the blood vessel where the blood gases are being analyzed. It
is therefore desirable that the active length of the catheter be
minimized.
A further disadvantage of prior art catheters is in the carrier gas passing
through the catheter through dissimilar pathway areas, that is, an
internal mixing takes place when the pathway of the gas through the
catheter passes from one cross-sectional area to another that is
significantly dissimilar in size. A mixing occurs between the equilibrated
bolus and its edges that are surrounded by the carrier gas so that the
defined edges of the bolus itself are disrupted.
SUMMARY OF THE INVENTION
The catheter probe of the present invention comprises an annular inlet
capillary that receives the carrier gas from the gas source and forms a
path for that carrier gas to the distal end of the probe. At the distal
end, the carrier gas enters the equilibration chamber that is surrounded
by a gas permeable membrane and which is in contact with the liquid
containing the dissolved gases to be analyzed. In the preferred
embodiment, that gas permeable membrane is introduced directly into the
patient's bloodstream.
The equilibration chamber comprises the active length of the catheter probe
and comprises an elongated, tortuous path through which the carrier gas
passes. As will be noted, the elongated, tortuous path is preferably a
spiral path formed directly under the inner surface of the permeable
membrane so that the equilibration chamber volume is constrained to be a
thin layer (or shell) at the outer surface of the probe. As the carrier
gas winds its way along the spiral equilibration chamber, its outside
surface area is in contact with the gas permeable membrane and is
relatively large in respect to the volume of carrier gas passing through
the equilibration chamber.
Thus, the ratio of active surface area per unit of equilibration volume is
high, in the order of 50:1 so that the rapidity of the equilibration
process is enhanced.
In addition, since the carrier gas passes through an elongated, tortuous
path as it passes through the active length of the catheter probe,
considerably more equilibration surface area is made available to carry
out the equilibration process per unit of linear catheter length. The
active length of the catheter probe may be thus reduced, thereby allowing
the user to more accurately ascertain the position within the patient's
blood stream that is being analyzed. The length of the tortuous spiral
path is important since it delineates the length of the blood gas bolus
which enters the centrally disposed capillary outlet tube, (the transport
capillary).
This bolus will be eroded by convection and diffusion effects which cause
mixing with the carrier gas during transport through the outlet tube. The
mixing effect is evident initially at the leading and trailing edges of
the gas bolus; a minimum length of bolus (l) for initial partial pressure
information to be preserved within the bolus center can be calculated from
the relation:
l={4/3*a.sup.2 *U*L/D}.sup.1/2
where
a=radius of the outlet tube;
L=length of the outlet tube;
U=carrier gas velocity;
D=carrier/bolus coefficient of interdiffusion.
A further centrally disposed capillary outlet tube having an open end
adjacent the distal end of the catheter probe receives the equilibrated
carrier gas and through that capillary outlet tube, the equilibrated
carrier gas bolus is withdrawn and directed to an analyzer.
As a further feature, the cross-sectional areas of the various paths
through which the carrier gas passes as it progresses through the catheter
probe are designed to be substantially equal, thus inadvertent mixing of
the equilibrated carrier gas with the non-equilibrated carrier gas is
minimized. More specifically, the cross-section area of the annular inlet
capillary; the cross-section area of the elongated, tortuous path of the
equilibration chamber and the cross-section area of the capillary outlet
are substantially equal.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing the flow of carrier gas and its control
for the system utilizing the present invention;
FIG. 2 is a cross-sectional view of the catheter probe of the invention
showing, in particular, the active length thereof; and
FIG. 3 is an isometric view, partly cut away, showing the components of the
catheter probe constructed in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1, there is shown a schematic view of the overall
gas analysis system for which the catheter probe of the present invention
is useable.
In FIG. 1, catheter probe 10 is shown in schematic form, partially cut
away, and having a distal end 12 that is inserted into the patient's blood
stream to be analyzed. The catheter probe 10 comprises an annular
capillary inlet 16 through which carrier gas is delivered to the catheter
probe 10, as will be explained. An outlet capillary 18 having its open end
20 at or adjacent to the distal end 12 of the catheter probe 10 is
provided and which withdraws carrier gas from the catheter probe 10.
A gas permeable membrane 22 makes up the exterior of the catheter probe 10
and, as will be explained, allows the blood gases to pass there through
into the equilibration chamber 24, formed therein. As will be noted, the
equilibration chamber 24 receives carrier gas from annular capillary inlet
16 and that carrier gas thereafter passes through the equilibration
chamber 24 before entering the open end 20 of outlet capillary 18.
Equilibration chamber 24 is formed directly against the inner surface of
gas permeable membrane 22 for reasons which will become apparent. A wire
25 is wound around the central conduit 26 in which outlet capillary 18 is
formed. That part of the catheter probe 10 in which the equilibration
chamber 24 is located is the active length and is indicated in FIG. 1 by
the dimension A.
A valve 28 is used to control the introduction of gas to and remove gas
from the catheter probe 10. A suitable valve for use with the present
system is shown and described in U.S. Pat. No. 4,706,700 of Jumeau and
therefore is described herein schematically only.
Valve 28 comprises an outer housing 30 forming therein a main chamber 32.
An inlet 34 is provided in outer housing 30 and receives carrier gas from
a suitable source (not shown). The preferred carrier gas is argon,
however, other carrier gases, such as helium, may be employed. As noted,
the carrier gas is thus introduced into main chamber 32 though inlet 34.
The main chamber 32 also communicates directly with annular capillary inlet
16 as further illustrated by reference to arrows B; annular capillary
inlet 16 being formed between the outer diameter of central conduit 26 and
the inner diameter of tubing 33 and, as noted, forms a path for the
carrier gas to the equilibration chamber 24.
Valve 28 further includes an inner chamber 36 within main chamber 32 and
inner chamber 36 is formed by fixed walls 38 and a moveable wall 40. As
shown in the solid line position of FIG. 1, moveable wall 40 is displaced
with respect to fixed walls 38 so that inner chamber 36 is open and
readily communicates with main chamber 32.
As also noted, central conduit 26 is sealed to fixed walls 38 so that
outlet capillay 18 communicates with the interior of inner chamber 36. A
further capillary conduit 42 also communicates with the interior of inner
chamber 36 and forms a path to an outlet conduit 44 leading to the
exterior of main chamber 32 and, as will be explained communicates with an
analyzer such as a mass spectrometer for analyzing gases.
A solenoid 46 is electrically energizable and de-energizable to move
moveable wall 40 between its dotted line position seated against fixed
walls 38, and its solid line position displaced from fixed walls 38 thus
selectively closing or opening inner chamber 36 with respect to main
chamber 32. A bleed vent 48 is also formed in outer housing 30.
Briefly then, the operation of the system can be described as follows. When
the solenoid 46 is suitably energized, the moveable wall 40 is moved to
its dotted line position closing inner chamber 36. In this position,
carrier gas is continually drawn through the system, including catheter
probe 10, by a slight vacuum drawn by the mass spectrometer acting at the
outlet conduit 44. That vacuum draws the carrier gas from its source into
main chamber 32 of valve 28 and thereafter sequentially through annular
capillary inlet 16, equilibration chamber 24, outlet capillary 18, through
inner chamber 36 and out through capillary conduit 42 to outlet conduit
44.
When it is desired to utilize the catheter probe 10 to take a sample of
blood gases, solenoid 46 is activated to move moveable wall 40 to its
solid line position opening inner chamber 36 to main chamber 32. Since
inner chamber 36 is now fully open to main chamber 32, the vacuum drawn by
the mass spectrometer draws carrier gas directly through inlet 34 and
through capillary conduit 42 to outlet conduit 44, thereby by-passing any
flow to catheter probe 10.
The carrier gas thus contained within capillary probe 10 is retained at a
standstill and equilibration occurs through blood gases passing from the
blood through gas permeable membrane 22 and into carrier gas in the
equilibration chamber 24. After a sufficient period of time has elapsed to
insure equilibration has been completed, the solenoid 46 returns the
moveable wall 40 to its dotted line position, closing inner chamber 36 and
again causing flow through the catheter probe 10. This time however, that
flow of carrier gas from catheter probe 10 contains a bolus of carrier gas
into which the various blood gases have equilibrated from the patient's
blood. That bolus continues to the mass spectrometer where it is detected
and the blood gases analyzed.
Turning now to FIGS. 2 and 3, the details of the construction of the
catheter probe 10 of the present invention can be explained.
Annular capillary inlet 16 is formed between the inner diameter of tubing
33 and the outer diameter of central conduit 26. Tubing 33 is preferably a
commercially available polymer tubing, one example of which is suitable is
crystaline polytrifluorochloro-ethylene available under the trademark
Kel-F. The requirements of tubing 33 are that it provides some stiffening
to the catheter probe 10 and yet is sufficiently flexible so as to be
introducible along the interior of a patient's blood vessel. In the
preferred embodiment, and to illustrate the size of the various
components, tubing 33 has an outer diameter of about 0.250 millimeters
(mm) and an inner diameter of about 0.160 (mm).
Central conduit 26 is preferably vitreous silica coated with several layers
of a polyamide for mechanical strength and is commercially available from
the Scientific Glass Engineering Co., of Australia. The material is
utilized commercially for various fiber optic applications and is
preferably of an outer diameter of about 0.150 mm, and an inner diameter
of between 0.025 to 0.050 mm. The inner diameter of the central conduit 26
thus forms outlet capillary 18 and in the illustrative embodiment
therefore has a cross-sectional area of about 2.0.times.10.sup.-3
mm.sup.2. The annular capillary inlet 16 dimensionally therefore has an
outside diameter of 0.160 mm and an inside diameter of about 0.152 mm and
thus can be calculated to also have a cross-sectional area of about
2.0.times.10.sup.-3 mm.sup.2.
The wire 25 is used to form the elongated tortuous path of the
equilibration chamber 24. Preferably that wire 25 is wound spirally around
the outside diameter of central conduit 26 beginning at the terminal end
52 of tubing 33 and continuing to the open end 30 of central conduit 26
adjacent the distal end 12 of catheter probe 10. Wire 25 is preferably of
molybdenum and has a coating that inhibits the condensation of water such
as gold. In the dimensions of the preferred embodiment, wire 25 has a
diameter of about 0.05 mm and its pitch is about 0.10 mm. In construction,
the wire 25 is tension wound about and is embedded to some extent in the
polyamide coating on central conduit 26. As such, the cross sectional area
of the elongated, tortuous path of the equilibration chamber 24 is about
2.5.times.10.sup.-3 mm.sup.2.
Around the outside of equilibration chamber 24 is secured the gas permeable
membrane 22. The gas permeable member 22 may readily be made of silicone
rubber having a thickness of about 50 microns.
As can now be seen, equilibration chamber 24 is, in effect, an elongated,
tortuous path that leads from annular capillary inlet 16 to the outlet
capillary 18 and along which the carrier gas is passed. Since the
equilibration chamber 24 is in direct contact with the inner surface of
gas permeable membrane 22, a considerable surface area is provided within
the active length, dimension A, of the catheter probe. Thus the ratio of
surface area to unit volume of carrier gas sample is relatively high; with
the present given dimensions, that ratio is about 50:1. The active length
dimension can thus be less than 5 centimeters, preferably less than about
2 centimeters and yet there is sufficient path length along the
equilibration chamber 24 to fully effect the desired equilibration.
Also, in accordance with the preferred dimensions, the cross-sectional area
of the annular capillary inlet 16, the elongated, tortuous path of the
equilibration chamber 24, and the outlet capillary are substantially the
same, accordingly the cross-sectional areas of the entire path for the
carrier gas through the catheter probe 10 are substantially the same, thus
mixing of the bolus containing the equilibrated sample and the carrier gas
is reduced.
While the present invention has been set forth in terms of a specific
embodiment, it will be understood that the blood gas analyzing probe
herein disclosed may be modified or altered by those skilled in the art to
other configurations. Accordingly, the invention is to be broadly
construed and limited only by the scope and spirit of the claims appended
hereto.
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