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SUMMARY OF THE INVENTION
It is known, e.g, Scholander, et al, Negative Pressure in the Interstitial
Fluid of Animals, Science, 161:321-328, 7-26-68, to insert a catheter
carrying a wick at the end in order to measure the interstitial pressure.
The Scholander wick consists of a piece of loose cotton pulled into the
end of the catheter tubing by means of a loop which is removed once the
wick is in position. The present invention employs a wick consisting of
two short lengths of multifilament, substantially uniform, synthetic
plastic braid, doubled back on itself, with the bight drawn into the
intratissue end of the catheter. The braid wick is drawn into the catheter
by means of a monofilament tether having one end tied securely to the
bight of the wick, the tether then extending through the catheter to the
other end of the tube. The tether is left in place after the wick is
positioned so that should the wick work itself loose, it may be retrieved
by virtue of its fixed securement to the tether, the outer end of the
tether being readily accessible at the outer end of the catheter.
The present invention also contemplates a method of inserting the catheter
into a desired interstitial region within the tissue, with minimal tissue
damage and edema.
A preferred form of the present invention, both apparatus and method, will
now be described with reference to the accompanying drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a illustrates a catheter tube, and a monofilament tether with a
braided wick at the end, which is adapted to be drawn into the tube prior
to use.
FIG. 1b is a detail view to show the braided wick at the end of the tether.
FIG. 2 illustrates the tether and wick in position inside the catheter
tube. The outer end of the catheter is fitted with a standard adapter for
conventional, clinical and hospital pressure measuring apparatus.
FIG. 3 is a further view of the catheter showing schematically the
attachment of the adapter to a capillary tube which transmits the pressure
to a manometer or pressure transducer.
FIG. 4 illustrates a first step in the process or method of inserting the
wick into the desired tissue region.
FIG. 5 illustrates a second step in the insertion process.
FIG. 6 illustrates a third step in the insertion process; and
FIG. 7 illustrates the next step, showing the catheter tube inserted
through the sheath. In the final position of the parts (not shown) the
sheath is removed from the body and slid back along the catheter tube.
FIG. 8 is a schematic view corresponding to FIG. 7, illustrating an
alternative use of the apparatus for withdrawing interstitial fluid from a
desired tissue region.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, 10 is an epidural catheter with a length, for example,
of 20 cm, an outside diameter of 1.02 mm, and an inside diameter of 0.62
mm. Numeral 12 is a nylon or polypropylene monofilament tether having a
uniform diameter of approximately 80-100 micrometers i.e., microns and a
length of approximately 25 cm. The left end of the tether 12 is tied to
the mid-point or bight of a wick 14 made of soluble braided polyglycolic
acid suture having a uniform fiber diameter of approximately 20
micrometers. Typically, two parallel braided lengths, each approximately
2.5 cm long, may be tied at their common bights to the nylon monofilament
12.
The right end of the monofilament 12 is passed from left to right through
the catheter 12 and the wick 14 is drawn approximately half of its length
into the left end 16 of the catheter 10, leaving approximately 0.6 cm of
the wick outside the end 16 of the catheter tube 10. The catheter tube is
then fitted with an adapter 17 so that it may be connected to suitable
pressure measuring equipment. This is shown in FIG. 3, where the tube 18
fitted to the outer end of the adapter 17 may lead to a manometer or
transducer for measuring interstitial pressure as detected by the fluids
picked up through microchannels of the wick 14. A suitable pressure
measuring technique is disclosed in the aforementioned Scholander article.
At an expedient point in this procedure, the catheter 10, adapter 17, and
tubing 18 are filled with sterile heparinized saline solution to transmit
fluid pressure built up in the wick, and the pressure measuring equipment
is calibrated. The catheter is now ready for insertion into the body of
the patient.
This is accomplished by apparatus and method shown in FIGS. 4 through 7.
FIG. 4 illustrates a metal catheter placement needle 20 sheathed by a
plastic sheath 22, with the pentrating point 24 of the needle 20 extending
slightly beyond the sheath 22. With the parts positioned as shown in FIG.
4, the assembly is inserted through the skin 26 of the patient and through
the outer layer of subcutaneous tissue (fascia) which is typically of
relatively high penetration resistance. After this high-resistance tissue
has been penetrated, the needle 20 is partially retracted within the
sheath 22, as shown in FIG. 5, so that the tip 24 of the needle 20 lies
within the end of the sheath 22, leaving the tip 26 of the sheath 22 to
serve as the penetrating point for further and deeper tissue insertion.
The plastic sheath or tubing 22, being of softer material, is appreciably
less damaging to the internal tissue as it proceeds inward, than is the
metal point 24. Because of the easier penetration, it is able to
satisfactorily penetrate further into a desired placement region inside
the tissue. During this second phase of the insertion, the metal needle 22
remains in position to rigidify the sheath 22. When the tip 26 of the
sheath 22 has been inserted to a point of desired placement, the metal
needle 20 is completely withdrawn from the sheath 22 and laid aside, as
shown in FIG. 6. Thereafter, the catheter, with tether and wick, as shown
in FIG. 2, is inserted through the sheath 22, wick first, until the wick
14 clears the end 26 of the sheath 22 and resides at the desired placement
region, as shown in FIG. 7. The operator or technician senses this
placement by the fact that resistance to further insertion of the catheter
10 is encountered when the wick 14 clears the end 26 of the sheath 22.
To ascertain that the apparatus is correctly inserted and positioned, the
doctor presses inward or squeezes at the region where the measurement is
desired. This should produce a marked transient excursion in the manometer
reading. If it does not, then the wick should be cleared by flushing with
a small amount of saline solution.
After placement of the catheter 10, the sheath 22 is pulled out of the body
and slid along the catheter 10 out of the way. The apparatus is then ready
to make interstitial pressure measurements in accordance with known
techniques as taught, for example, in Scholander.
The wick is removed by pulling out the catheter slowly until the tip has
cleared the skin. If the wick has slipped out, it may be easily retrieved
by the tether 12.
In addition to being useful for measuring interstitial fluid pressure, the
present apparatus and method are also readily adaptable for the withdrawal
of interstitial fluids. In this case, the same apparatus and techniques
are employed except that the catheter is left empty instead of being
filled with saline solution. Thus, when the wick 14 is in position in the
desired interstitial tissue region, the capillary action of the wick 14
draws tissue fluids into the end of the catheter 10 and then into the bore
of the catheter 10. In the case of edema, the fluid pressure within the
tissue itself is usually enough to drive a sufficient amount of tissue
fluid into the catheter, which may then be pulled from the muscle or other
tissue, and the fluid removed for appropriate examination. If desired, a
vacuum may be applied at 30 to the exterior end of the catheter, as shown
in FIG. 8, to further amplify the withdrawal of fluid from the tissue via
the wick 14 into the catheter 10.
For a further understanding of the clinical implications of the present
invention, as well as a bibliography of additional references, reference
is made to (1) Mubarak, et al., "The Wick Catheter Technique for
Measurement of Intramuscular Pressure", The Journal of Bone and Joint
Surgery, Volume 58-A, No. 7, pages 1016-1020, October 1976, and (2)
Hargens, et al., "Interstitial Fluid Pressure on Muscle and Compartment
Syndromes in Man", Microvascular Research, Volume 14, pages 1-10, October
1977.
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