Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF FABRICATING A HEATER COIL FOR A CATHETER
Field of the Invention
The present invention generally pertains to a catheter used to monitor the
rate at which blood is pumped from the heart, and more specifically, to a
heater
that is used on such a catheter for heating the blood in the heart so that the
cardiac
output can be determined.
Background of the Invention
Various techniques are used for determining the volumetric output of a
patient's heart, i.e., the rate at which blood is pumped from the heart. One
technique that is used requires that a bolus of chilled saline solution be
injected into
the heart through an intracardial catheter. By measuring the change in the
temperature of the blood leaving the heart, the -volumetric flow rate (cardiac
output) can be determined.
It will be apparent that the preceding technique for determining cardiac
output cannot be used on a continuous basis. An alternative technique, which
enables continuous monitoring of cardiac output, involves heating the blood in
a
chamber of the heart and then monitoring the temperature of the blood
downstream of the chamber. The preferred method for heating the blood is with
an electrical heating element that is disposed on the outer surface of a
catheter,
near its distal end. The catheter is threaded through the patient's
cardiovascular
system, and the heater section of the catheter is positioned in the desired
cardiac
chamber. Small gauge leads disposed in one of the lumens of the catheter
convey
electrical current from an external power source to the heating element. This
heating element warms the blood in the chamber when the heater is energized,
so
that cardiac output can be monitored on a continuous basis.
Because the catheter must be threaded through a patient's cardiovascular
system, the heater element formed on the outer surface of the catheter must be
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relatively compact; it cannot add significantly to the diameter of the
catheter.
Furthermore, the heater element must have a smooth outer surface so that it
can '
pass freely through blood vessels.
To form a heating element on the outer surface of the catheter, the '
previous technique provided for drawing a bifilar wire from which the heating
element is formed through a syringe that contains a thermal-set polyurethane
resin.
The bifilar wire comprises two adjoined insulated copper conductors, each
provided with nylon (or other plastic) insulation jackets by which they are
adjoined. When drawn from the syringe, the bifilar wire is covered with liquid
polyurethane and is coiled around the catheter while wet with this coating.
Each
of the conductors comprising the coiled bifilar wire is electrically joined to
one of
two leads; these leads are extended to the proximal end of the catheter
through one
of its lumens. The connections between the leads and the bifilar wire
conductors
are separately insulated and forced back inside the catheter lumen.
The polyurethane coating is thermally cured at an elevated temperature for
about 48 hours. To minimize the diameter of the heating element, the bifilar
wire
is wound in a single layer with the two conductors comprising the wire running
side-by-side. The distal ends of the bifilar conductors are connected together
to
form a series circuit. Current flows in opposite directions in adjacent
conductors
of the wire comprising the heater element. Any magnetic field caused by
current
flowing in one of the two conductors of the bifilar wire cancels that caused
by the
current flowing in the other conductor.
There are several problems in fabricating a heater element for a constant
cardiac catheter in the manner described above. The process requires skilled
hand
labor and is therefore expensive. In' addition, during the relatively long
time (48
hours) required for the thermal-set polyurethane to cure, the resin, although
viscous, remains su~ciently fluid to flow around the catheter, forming a
thickened
slump on the lower side of the catheter. Consequently, the polyurethane
coating
on the heater coils is non-uniform in thickness. Before the resin sets, it is
very
tacky and tends to pickup dust motes and other undesirable pyrogens from the
environment. In addition, irregularities and bumps in the resin can cause the
surface to be too rough. Accordingly, it will be evident that a more efficient
technique is desired to fabricate the heater element and bond it to the
catheter.
The new method should produce a consistently smooth 'and regular surface on
the
exterior of the heater element. Furthermore, the new method should enable the
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heater element to be produced with a minimum of hand Iabor and should
eliminate
the long cure time that is presently required.
Summary of the Invention
In accordance with the present invention, a method is defined for making a
S heater for a catheter that is employed to monitor cardiac output. The method
includes the steps of providing a catheter having at least one Iumen adapted
to
accept a heater lead. A wire that will be used for forming the heater is
coated with
a thermoplastic material to form a coated lead. The coated lead is wound
around
the catheter to form a heater coil. When the heater coil is formed, it
includes gaps
defined between adjacent coils of the coated lead. The temperature of the
heater
coil is then elevated, melting the thermoplastic material, causing the
material to
flow into and fill the gaps between the adjacent coils of the coated lead and
bonding the heater coil to the catheter.
In one preferred form of the invention, the adjacent coils of the coated lead
do not contact each other when the heater coil is wound. When the
thermoplastic
material melts, the material flows into and fills the gaps to form a generally
smooth
outer surface on the heater coil.
In another embodiment, the adjacent coils of coated lead contact each other
when the heater coil is wound. The gaps each comprise notches disposed
adjacent
the surface of the catheter and outwardly of each point where the coils of
coated
lead contact each other. When the thermoplastic material melts, the material
flows
into the notches to form a generally smooth outer surface on the heater coil.
In one embodiment, the coated lead has a generally circular cross-sectional
shape. In another embodiment, the coated lead has a generally quadrilateral
cross
sectional shape. Preferably, the wire comprises a bifilar wire with two
adjacent
conductors.
Brief Description of the Drawing Fi ures
The foregoing aspects and many of the attendant advantages of this
invention will become more readily appreciated as the same becomes better
understood by reference to the following detailed description, when taken in
conjunction with the accompanying drawings, wherein:
- FIGURE 1 is a block diagram that shows a catheter having a heater
element fabricated in accord with the present invention and a control and
power
source for the heater element;
FIGURE 2 is an elevational view showing a section of the catheter
. illustrated in FIGURE 1 on which the heater element is disposed;
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FIGURE 3 is a cross-sectional view of a bifilar wire of the type used in the
present invention;
FIGURE 4 is a cross-sectional view of several adjacent coated bifilar wires
on an outer surface of a catheter, before heat is applied to the wire to melt
the
coating;
FIGURE 5 is a cross-sectional view like that of FIGURE 3, showing the
coated bifilar wires after heat is applied to melt the coating;
FIGURE 6 shows a portion of the catheter helically wound with bifilar wire
and covered with a helically wound tape (or film) that is melted to coat the
wire;
FIGURE 7 is a cross-sectional view of another embodiment in which the
coating applied to the bifilar wires has a generally quadrilateral cross-
sectional
shape;
FIGURE 8 is a cross-sectional view of the embodiment shown in
FIGURE 7, after heat has been applied to melt the coating;
FIGURE 9 is a cross-sectional view of another embodiment in which a heat
shrinkable tube is fitted over the helically coiled bifilar wire on the
catheter and
then heated to cause the tubing to shrink;
FIGURE 10 is a cross-sectional view of the embodiments of FIGURE 6 or
FIGURE 9, showing how the wire is coated after the tape or film of the
embodiment of FIGURE 6 is melted, or after the heat shrinkable tubing has been
shrunk around the catheter; and
FIGURE 11 is an isometric view showing two portions of a die (split apart)
in which the section of the catheter on which the coated bifilar winding is
coifed is
held when the coating is heated.
Description of the Preferred Embodiment
Referring to FIGURE 1, a catheter 10 of the type used for monitoring
constant cardiac output, and a control and power source 14 for the catheter
are
schematically shown. Catheter 10 includes a heater 12 that is fabricated in
accord
with the present invention. At the distal end of catheter 10 is disposed a
balloon 15, which can be inflated after the catheter has been positioned
within a
patient's heart to ensure that the distal end of the catheter is carried out
of the right
ventricle, into the pulmonary artery. When preparing to continuously monitor a
patient's cardiac output, catheter 10 is typically inserted into a patient's
body
through a slit in an appropriate artery, threaded through the cardiovascular
system ,
into the right atrium of the heart, and passed into the right ventricle.
Balloon 15 is
then inflated, carrying the distal end of the catheter out of the heart and
into the
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pulmonary artery. The distal end of catheter 10 includes a thermister (not
shown)
or other temperature sensor for monitoring the change in the temperature of
the
blood leaving the heart as it is heated inside the right ventricle by heater
12. Based
upon the temperature change of the blood and the amount of heat added, the
S volumetric rate of blood flow from the patient's heart (cardiac output) can
be
continuously monitored. Control and power source 14 supplies the electrical
power to heater 12 and monitors the change in temperature of the blood to
determine the cardiac output. Since the method employed in using catheter 10
to
determine cardiac output is not the subject of this invention, there is no
need for a
detailed explanation of the process.
A sheath 16 encloses the proximal end of catheter 10 at a point where a
plurality of electrical leads 19 emerge to connect to terminals 18 on control
and
power source 14. Two of electrical leads 19 convey electrical current to
heater 12
through a lumen (not shown) in catheter 10, from control and power source 14;
1 S the remaining leads, which are conveyed through other lumens of the
catheter, are
used in connection with monitoring the temperature of blood within the heart
and
within the pulmonary artery. A fluid line 20 extends from the proximal end of
catheter 10 and can be coupled to a source of pressurized fluid for inflating
balloon 1 S. Additional fluid lines coupled to other lumens of catheter 10 can
be
included to convey fluids into the heart through openings within the catheter
adjacent its distal end.
FIGURE 2 illustrates a portion of catheter 10 upon which heater 12 is
disposed and shows further details of its construction. Although heater 12
appears
to substantially increase the diameter of catheter IO as illustrated in FIGURE
1 and
2S 2, in reality, the heater adds very little to the cross-sectional size of
the catheter.
With reference to FIGURE 2, it will be noted that the two leads, which convey
electrical current to heater 12, are separately connected to the heater at
insulated
junctions 37. The junctions are then pushed back into the lumen that conveys
the
leads, through an opening 36.. At the distal end of the heater, a termination
38 of
the leads comprising the heater is similarly forced .into an opening 36 within
the
lumen. Openings 36 are subsequently sealed. In the preferred embodiment of
heater 12 in FIGURE 2, the outer surface of the heater comprises a heat fusion
bonded polyvinyl chloride (PVC) coating 34',_ which is relatively free of
bumps or
other surface irregularities. A small cut-away section 41 shows the conductors
3S comprising the heater. A more detailed view of this embodiment is shown in
FIGURE 8.
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Heater 12 is preferably fabricated using a bifilar wire 21 comprising two
side-by-side conductors 31, as shown in FIGURE 3. Each of conductors 31
comprises type CI 10 copper and is 39 AWG (about 0.0035 inches in diameter). A
polyester insulation layer 30 surrounds each of conductors 31 and is
approximately
0.008 inches thick (measured in the radial direction). Surrounding the
polyester
insulation layer and bonding conductors 31 together in the side-by-side
configuration is a film of NYLONTM 28 that is approximately 0.000025 inches
thick (also measured in the radial direction). Based upon the dimensions
recited
above, it should be apparent that bifilar wire 21 is remarkably small in
cross-sectional size. The greatly enlarged cross section of the bifilar wire
shown in
FIGURE 3 is therefore somewhat misleading.
To fabricate heater 12, bifilar wire 21 is processed through an extruder to
provide it with a PVC coating 26, forming a coated lead 24, as shown for an
embodiment 22 in FIGURE 4. A generally conventional extrusion process like
that
used to apply an insulating coating to electrical wire is used to produce the
coated
lead. In one preferred embodiment, PVC coating 26 comprises the same type of
PVC as the material from which catheter 10 is fabricated. Thus, the PVC
comprising catheter,l0 and PVC coating 26 have identical characteristics, so
that
heat fusion bonding is enhanced and biocompatability is assured. In another
preferred embodiment, the melt temperature of PVC coating 26 is less than the
melt temperature of catheter 10. However, bonding between the catheter and the
PVC coating still occurs.
In the next step in the process of fabricating heater 12, coated lead 24 is
helically wrapped around an exterior surface of catheter 10 at the point where
the
heater is to be formed; the successive wraps of the coated lead thus produce a
closely spaced helical coil around the catheter. Only a small portion of the
heater,
along one side of the catheter is shown in FIGURE 4. When the coated lead is
wrapped around the catheter, a small gap 23 may be provided between the facing
surfaces of adjacent coils. The dimension of gap 23 is carefizlly controlled
to
ensure that when PVC coating 26 is heated above its characteristic melting
temperature, the molten PVC coating comprising the radially outer surface of
coated leads 24 will flow into and completely fill the void between the
adjacent
coils, with a minimum movement of material. The required size of the gap is
determined by calculating the total void area between adjacent coils of coated
'
lead 24, which varies with the size of gap 23, and selecting a gap size that
yields a
void area approximately equal to the area of PVC coating 26 that radially
overlies
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the outer surface of bifilar wire 21. For example, in
FIGURE 4, the void area that
must be filled between two adjacent coils of coated lead
is shown by the closely
spaced horizontal lines, and the PVC coating that is available
to fill the void area is
disposed above the horizontal dash-dot-dash line that
extends over this void ar
ea.
Gap 23 is maintained at this selected value between successive
coils of coated
lead 24 so that the cross-sectional area of void between
the coils is equal (or
slightly less than) the area of the PVC coating covering
the radially outer portion
of the coated leads.
FIGURE 5 shows how embodiment 22 appears after the PVC
coating of
the coated lead is heated above its characteristic melting
temperature and forced to
flow into gap 23. As shown in this Figure, most of the
PVC coating on the top
surface (radially outer surface) of the coated lead has
melted and flowed into the
void between adjacent coils of the coated lead, completely
filling that space and
substantially reducing the total thickness of heater 12.
The outer surface of PVC
coating 26' in embodiment 22 is relatively smooth, having
only minor ripples
formed over each bifilar wire coil. When PVC coating 26
melts, it flows into the
voids between adjacent wraps of coated wire 24 and bonds
with the outer surface
of catheter 10. The outer surface thus formed on heater
12 is dependent upon the
surface finish of a channel 48 in a die 40 (shown in FIGURE
11 and discussed
below) in which the coating is heated and is substantially
smoother and much more
regular than that produced by the prior art technique
used to form a heater from a
wire coated with a thermal-set polyurethane coating. In
the prior art technique, the
polyurethane coating was subject to slumping due in part
to the long cure time and
the effects of gravity flow, producing a bumpy surface,
thin on one side and thick
on the other. In contrast, the PVC coating used in the
present invention is
reflowed, bonded, and cooled within minutes, yielding
a relatively smooth surface
that is of substantially constant thickness.
Two very different techniques can be employed to melt
PVC coating 26 on
coated lead 24 and bond the coated lead to the catheter.
The f rst technique uses
the inherent resistance of the coil assembly in combination
with electrical current
flow through it to heat the PVC coating above its melting
temperature.
- Conductors 31 are electrically coupled together at the
distal end of the heater.
Next, a source of electrical current is coupled to the
proximal end of the
_ conductors (or to leads 19 if junction.36 has already
been fabricated). In the
preferred embodiment, a direct current (DC) voltage is
applied to the heater at a
level to cause approximately four amps to flow through
conductors 31 for ten to
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fifteen seconds. Because of the relatively small gauge of conductors 31 in
bifilar
wire 21, the current quickly heats the PVC coating above its melting point,
causing '
it to flow into the voids between adjacent wraps of the coated lead.
Alternatively,
a high-frequency alternating current (AC) of approximately one amp at 200 kHz
can be applied to the heater for ten to fifteen seconds. Less current is
required for
heating at this high frequency due to the higher effective resistance of the
heater.
At this frequency, the AC tends to flow along the outer surface of conductors
31
due to the "skin effect." Heating of the coating with electrical current
flowing
through the wire is more efficient than heating with an external heat source.
Coated lead 24 can also be heated using externally applied heat.
Preferably, die 40, which is shown in FIGURE 8, is used in heating PVC
coating 26 above its melting temperature. Die 40, includes two mating die
sections 42 and 44. The section of catheter 10 wrapped with coated lead 24 is
positioned within a channel 48 that extends longitudinally along the upper
surface
1 S of die section 44. Die 40 is sufficiently long so that the section of
catheter 10 on
which coated lead 24 is wrapped fits between the ends of the channel. Die
section 42 includes a corresponding channel 46 running longitudinally along
its
lower surface. On the upper surface of die section 44 are disposed dowel pins
50
and 52. The dowel pins are slightly inset from the outer edges of the upper
surface
and are inset from the ends of the die section. Dowel pin 52, which is larger
in
diameter than dowel pin 50, is sized to fit within an orifice 56 that extends
through
die section 42. Likewise, dowel pin SO is sized to fit within a corresponding
orifice 54 formed in die section 42.
Die section 42 is fitted into mating engagement with die section 44, locking
the section of catheter 10 on which the heater is wound within channels 46 and
48.
An upright post 58 attached to the upper surface of die section 44, adjacent
one
end, supports a rotatable leaf spring clip 60. Similarly, an upright post 62,
which is
disposed on the upper surface of die section 42, adjacent its other end,
supports a
rotatable leaf spring clip 64. After the two die sections are coupled together
around the catheter, leaf spring clips 60 and 64 are rotated to overlie the
upper
surface of die section 42, applying a force that clamps die section 42 against
die
section 44.
Once the section of catheter 10 on which coated lead 24 is wrapped is thus
clamped in place within die 40, two heated blocks 70 and 72 having pockets 76 -
formed to fit over die 40 from opposite sides are brought together to enclose
the
die 40. The heated blocks include passages 74 into which cartridge heaters
(not
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shown) are inserted. The cartridge heaters, which have
previously been energized
for some time, heat the two-to-three pound mass of aluminum
comprising the
heated blocks to approximately 140C. Heated blocks 70
and 72 are clamped in
w
contact with die 40 for approximately ninety seconds,
during which time heat is
transferred to the die and thus to the PVC coating on
the coated lead wrapped
around the catheter to raise the PVC coating above its
melting point. Immediately
thereafter, the two heated blocks are removed, and two
cooled blocks (not shown)
of approximately the same size and mass and having identical
pockets to
accommodate die 40 are clamped around the die. The cooled
blocks, which have
previously been cooled to approximately 0C by passing
a chilled fluid through
passages within the blocks, draw heat from die 40, quickly
cooling and solidifying
the melted PVC coating.
The PVC coating on the coated lead that was melted by
heat transferred
from the heated blocks will thus be bonded to the outer
surface of catheter 10 and
will have flowed into the gaps or notches between adjacent
wraps of the coated
lead. In a production run, the blocks used to heat and
cool die 40 at spaced-apart
points on an assembly line will likely be moved to enclose
die 40 by hydraulic or
mechanical rams, substantially automating the process.
Instead of using a coated lead, bifilar wire 21 can be
helically coiled around
catheter 10 and then covered with a PVC tape 29, which
is helically coiled over the
bifilar wire. _ Alternatively, a PVC film (not shown)
can be wrapped around the
helical coils of the bifilar wire. The section of the
catheter around which the coiled
wire and PVC tape/film has been applied is then heated
above the melting point of
the PVC material using either of the two methods described
above, causing the
PVC tape/film to melt, flow into the voids between the
coils of bifilar wire 21, and
bond to the outer surface of catheter 10. The result appears
similar to an
embodiment 78 shown in FIGURE 10 (although formed in a
different manner). A
smoother outer surface can be produced by coiling the
bifllar wire so that the
adjacent coils are in contact with each other, substantially
eliminating the gap
between adjacent coils. In this manner, the PVC tape/film
will bond to the bifilar
wire when melted, forming a smooth coating around the
coils of the bifilar
i
w
re
and bonding to the catheter at each end of the helical
coil to hold the wire in place
on the catheter.
Another embodiment 39 of heater 12 is illustrated in FIGURES
7 and 8. In
FIGURE 7, embodiment 39 is shown before a PVC coating
34 applied to bifilar
wire 21 is melted, and in FIGURE 8, is shown after the
PVC coating has been
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melted, forming a smooth PVC coating 34'. To produce embodiment 39, a coated
lead 32 is formed for fabricating heater 12 using an extrusion process that
yields a
quadrilateral or rectangular shape coating with rounded corners, as shown in
FIGURE 7. This embodiment has an advantage over embodiment 22, since the
gap between adjacent wraps of coated lead 32 comprises only the relatively
small
V-shaped notches formed at the rounded corners. The "V-shaped" notches formed
adjacent the outer surface of catheter 10 and at the radially outer corners of
two
adjacent wraps of coated lead 32 (indicated by the horizontal cross-hatch
lines in
the Figure) have substantially less volume than the void between adjacent
wraps of
the coated lead in the first embodiment. As a result, when PVC coating 34 is
heated above its melting temperature, the melted PVC has only a small volume
to
fill when it flows and bonds to the catheter, producing a relatively smoother
outer
surface, as shown in FIGURE 8, compared to the outer surface of PVC
coating 34'. The rectangular shape of coated lead 32 has another advantage,
since
it is easier to wrap the rectangularly-shaped lead around catheter 10 and
maintain
the side-by-side relationship of conductors 31. It will be apparent that the
rectangular shape of coated lead 32 is more readily retained in flat contact
with the
outer surface of catheter 10 during the fabrication and coiling process.
However,
it is more difficult to extrude PVC coating 34 to produce coated lead 32 with
a
rectangular shape than with the oblate spheroid shape of coated lead 24, which
is
used in the first embodiment.
Once coated lead 32 is wrapped around the exterior surface of catheter 10
to form heater 12 of the required length, heat is applied to melt PVC coating
34
using either of the two techniques described above. PVC coating flows into the
notches between adjacent wraps and bonds to catheter 10, just as described
above
in connection with the first embodiment.
Yet another embodiment 78 is illustrated in FIGURES 9 and 10. This
embodiment also is produced by coiling bifilar wire 21 helically around
catheter 10,
just as shown in the left side of FIGURE 6. However, in embodiment 78, the
helical coils of wire are covered by a section of a heat-shrink tube 80, which
is
sufficiently long to completely cover the helical wire coil. The diameter of
the
section of heat-shrink tube 80 is selected so that the tube readily slides
over the
external diameter of the helically coiled wire. Once positioned over the wire
coils,
heat is applied to cause the diameter of the heat-shrink tubing to decrease,
so that
it fills the voids between the bifilar wire coils as shown in FIGURE 10. In
this
Figure, the heat-shrink tubing is identified by the reference number 80' to
indicate
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that a change in state has occurred relative to the condition of the tubing in
FIGURE 9. The change in state occurs when the heat-shrink tubing shrinks
around the helical winding. Heat is applied to achieve the shrinkage with a
hot air
source or an infra-red source (neither shown). The shrinkage of the heat-
shrink
tube around the bifilar wire coils is su~cient to completely seal the wire
coils
relative to the outer surface of the catheter and hold them securely in place.
By
winding the bifilar wire around the catheter so that adjacent coils of the
wire are in
contact, a much smoother outer surface can be provided than that of heat-
shrink
tubing 80' in FIGURE 9, since the heat-shrink tubing will generally conform to
the
radially outer surface of the helical coils of the bifilar wire wrapped around
the
catheter.
Although the present invention has been described in connection with the
preferred form of practicing it, it will be understood by those of ordinary
skill in
the art that many modifications can be made thereto within the scope of the
claims
that follow. Accordingly, it is not intended that the scope of the invention
in any
way be limited by the above description, but that it be determined entirely by
reference to the claims that follow.
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