Hollow core coaxial catheter

A hollow core coaxial catheter supporting a heatable probe or balloon transmits electrical power, such as RF energy, to the ohmically resistive probe or balloon. The hollow core may accommodate passage of a guide wire, fiber optics for imaging or laser, fluid flow for perfusion and/or a lumen for fluid inflation and pressurization of a balloon. Inner and outer electrical conductors are disposed on radially opposed sides of dielectric tubing with an outer covering of dielectric material mechanically shielding and electrically insulating the outer conductor. A similar inner covering of dielectric material may be disposed radially inwardly of the inner conductor to mechanically and electrically insulate the inner conductor. An ohmically resistive load, in the event RF energy is the power source, interconnects the inner and outer conductors to form the probe. An inflatable balloon of predetermined expanded configuration and inflatable via the lumen may be formed proximate or as a part of the probe to expand arterial plaque heated by the probe or to heat and expand the arterial plaque to a predetermined configuration. The RF energy, if used, will heat the probe or balloon and monitor and manage the temperature of the probe or the balloon.

RELATED PATENT APPLICATIONS 
The present application describes an invention related to the subject 
matter described in a copending patent application entitled "RF ENERGIZED 
AND TEMPERATURE MONITORED AND MANAGED CATHETER MOUNTED PROBE", assigned 
Ser. No. 337,903, filed on Apr. 13, 1989, and assigned to the present 
Assignee. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to revascularization of coronary and 
peripheral arteries through catheter apparatus for treating coronary 
stenosis and for reestablishing and maintaining coronary circulation and, 
more particularly, to a highly flexible hollow core coaxial cable for 
supporting and energizing an electrically or radiant energy heated probe 
or balloon or fluid inflated balloon, while simultaneously maintaining 
blood flow and fiber optics provisions. 
2. Description of the Prior Art 
Cardiovascular restriction or occlusion due to coronary artery disease and 
peripheral vascular disease may be addressed by any of a number of medical 
procedures. Pharmacological approaches for inducing dilation of the blood 
vessels are of a temporary nature and may have undesirable secondary 
results. Surgical techniques include coronary bypass surgery involving 
implantation of substitute blood vessels to bypass blood flow around the 
blockage; as with any major surgery, substantial risks are involved. 
Recent improvements in laser technology have resulted in development of 
the capability to medically insert a laser delivery fiber optic close to 
the blockage to permit lasing the blockage. Such lasing may vaporize, 
segment or otherwise disengage plaque from the artery. An inflatable 
balloon may be used to maintain the laser emitting fiber optic end close 
to the blockage and to widen the artery. Such coronary laser angioplasty 
procedures and equipment suffer from several significant drawbacks. The 
particulate matter disengaged will become suspended in the blood stream 
and may become relocated elsewhere. The extraordinarily high and 
uncontrolled heat from the laser beam or laser heated tip may permanently 
damage the artery wall or nearby tissue. The disposable and non disposable 
parts of the apparatus are very expensive. Danger also exists from laser 
perforation of the blood vessel wall. 
Radio frequency (RF) energy has been discharged from the discharge end of a 
catheter to electro abrade arterial plaque. The lack of control of the 
amount of energy radiated poses a serious threat to cardiovascular 
integrity and to damage of even surrounding tissue. Alternatively, RF has 
been used to heat the tip of a catheter, which heated catheter is used to 
thermally mold and displace the plaque. To be effective, sufficient power 
must be applied to overcome the damping effects of blood flow rate, the 
distance between the source of RF radiation or hot tip and the plaque, the 
thickness of the plaque, the extent of fatty tissue, etc.; where these 
damping factors are minimal, cardiovascular damage is probable. 
Alternatively, a two electrode device for transmitting RF (discharging) 
energy therebetween has been used at a specific location adjacent one of 
the electrodes. The high uncontrolled concentration of heat poses a 
serious threat of cardiovascular damage. 
Ultrasonic techniques have been used to emulsify or fragment arterial 
plaque. In conjunction therewith, an aspiration tube may be employed to 
remove the fragmented plaque. 
Recent techniques suggest the use of heating liquids adjacent arterial 
plaque by a chemical exothermic reaction. 
SUMMARY OF THE INVENTION 
A highly flexible hollow core coaxial catheter includes electrical 
conductors disposed on the inner and outer surfaces of flexible dielectric 
tubing. An inner sheath is interiorly juxtaposed with the inner conductor 
to shield it against mechanical abuse and to provide electrical 
insulation. An outer dielectric shield encircles the outer conductor to 
shield it against abuse and to electrically insulate it. An electrical 
ohmically resistive load interconnects the inner and outer conductors at 
the distal end of the catheter in response to current flow through the 
conductors from a source of electrical energy. The source of electrical 
energy may be an RF generator, including monitoring and managing circuitry 
for regulating the applied RF energy to maintain the ohmically resistive 
load at a predetermined and adjustable temperature. A hollow expandable 
balloon, connected to a source of fluid, may be disposed at the distal end 
of the catheter. The hollow balloon defines a predetermined configuration 
in its expanded state to accommodate molding of heated arterial plaque 
into a predetermined configuration. In one embodiment of the expanded 
balloon, an ohmically resistive load electrically interconnected between 
the inner and outer conductors of the hollow coaxial catheter is disposed 
upon the balloon to heat the arterial plaque to a predetermined 
temperature simultaneously with application of an expansion force to mold 
the heated arterial plaque to a configuration predetermined by the shape 
of the expanded balloon. Aside from accommodating the presence of a lumen 
to inflate the balloon, the hollow core of the catheter may support a 
guide wire for insertion and manipulation of the catheter within the 
vascular system. Furthermore, the hollow core catheter may be used for the 
purpose of accommodating blood flow during an angioplasty procedure. 
Furthermore, the hollow core catheter may be used for insertion of fiber 
optics, for optical image monitoring the procedure or for additional 
heating from a laser source. 
It is therefore a primary object of the present invention to provide a 
highly flexible hollow core coaxial catheter for transmitting electrical 
power to a heatable probe. 
Another object of the present invention is to provide a hollow core coaxial 
catheter for guiding and housing the catheter over a guide wire while 
transmitting electrical power to a probe. 
Yet another object of the present invention is to provide a hollow core 
coaxial catheter for housing a lumen to fluid inflate a balloon while 
transmitting electrical power to heat an element associated with the 
balloon. 
Still another object of the present invention is to provide a hollow core 
coaxial catheter for transmitting electrical power to an ohmically 
resistive load disposed intermediate the conductors at the distal end of 
the catheter. 
A further object of the present invention is to provide and heat an 
inflatable balloon disposed at the distal end of a hollow core coaxial 
catheter, which catheter transmits RF energy to an ohmically resistive 
load associated with heating the balloon. 
A yet further object of the present invention is to provide a hollow core 
coaxial catheter for supporting at the distal end an inflatable balloon of 
predetermined inflated configuration to conform encircling arterial plaque 
with such configuration upon heating of the balloon with RF energy 
transmitted through the conductors of the coaxial catheter. 
A still further object of the present invention is to provide an inflatable 
balloon of predetermined inflated configuration mounted at the distal end 
of a hollow core coaxial catheter which catheter transmits electrical 
power to a heat responsive load associated with the balloon and directs 
fluid into and out of the balloon from a lumen while accommodating a guide 
wire disposed within the hollow core to position the catheter pre and post 
an angioplasty procedure. 
A still further object of the present invention is to provide an RF energy 
responsive probe mounted at the distal end of a hollow core coaxial 
catheter. 
A still further object of the present invention is to provide a hollow core 
coaxial catheter for transmitting RF energy to an RF responsive inflatable 
balloon disposed at the distal end. 
A still further object of the present invention is to provide a method for 
applying heat to arterial plaque and for simultaneously expanding the 
arterial plaque during an angioplasty procedure. 
A still further object of the present invention is to provide a method for 
radially heating and expanding arterial plaque during an angioplasty 
procedure by application of RF energy transmitted through a hollow core 
coaxial conductor. 
A still further object of the present invention is to provide a method for 
transmitting electrical energy through a catheter. 
A still further object of the present invention is to provide a method for 
transmitting RF energy though a catheter during an angioplasty procedure. 
A still further object of the present invention is to provide a hollow core 
coaxial catheter for accommodating transmission of electrical power to 
heat a probe, transmitting probe temperature monitoring signals and 
heating the probe. 
A still further object of the present invention is to provide a method 
using a hollow core coaxial catheter for transmitting electrical power and 
control signals, accommodating fluid flow to an inflatable balloon, 
supporting a guide wire and permitting continuing blood flow during an 
angioplasty procedure. 
It is a still further object of the present invention to provide a method 
for transmitting power, transmitting control and monitoring signals, 
transmitting fluid for actuating an inflatable balloon, supporting a guide 
wire and accommodating blood flow during an angioplasty procedure 
involving heating and radial expansion of arterial plaque. 
These and other objects of the present invention will become apparent to 
those skilled in the art as the description thereof proceeds.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Catheters useful in heat assisted angioplasty procedures must meet three 
basic parameters. There must be a source of energy to effect heating; 
because of the potential hazard of an electrical energy source, a low 
voltage direct current source is preferable. Such source, if a battery, 
also provides the freedom of portability. A second parameter relates to 
the transmission media for transmitting the energy from the source to the 
remote point of application. Because the energy source is preferably low 
voltage, low current, the transmission media must satisfy certain 
electrical parameters to avoid preventable losses. As an example, wires of 
extremely small diameter have electric resistance of sufficient magnitude 
to generate heat even when conducting small electrical currents. 
Furthermore, the resistance of the wires will present a voltage drop as a 
function of Ohm's Law. Preferably, a coaxial cable (coax) should be used 
for transmitting RF energy to avoid losses from and the effects of 
spurious radiation along the transmission media. The third parameter 
relates to the load for receiving the transmitted energy and converting 
the energy to heat. Preferably, the load is configured or configurable to 
be capable of molding the heated arterial plaque to a predetermined 
configuration to open and maintain open the artery after cooling of the 
arterial plaque. Because the catheter is inserted within a blood vessel 
and advanced therealong to the location of a restriction or an occlusion, 
cross sectional size must be at a minimum while providing the capability 
for flexing and bending to accommodate the normally tortuous path to the 
restriction or occlusion. 
Where an inflatable balloon is used in conjunction with a heatable probe or 
as the heatable probe, means are formed or disposed within the catheter to 
permit fluid flow to and from the balloon for inflation and deflation 
purposes. To assist in manipulating and advancing the catheter, a guide 
wire extends through and protrudes beyond the distal end of the catheter. 
Real time diagnostic imaging can be accomplished by locating within the 
hollow core fiber optic elements necessary to illuminate and view the 
vascular site. Preferred procedures suggest the necessity of maintaining 
blood flow, rather than blood blockage, at the site of an angioplasty 
procedure. Such flow or perfusion can be accommodated by channeling the 
blood flow through the hollow core of the catheter. 
To provide an overview of the power source for heating the load, whether 
disposed as a probe at the distal end of the catheter or as an inflatable 
balloon at the distal end of the catheter, certain considerations 
attendant transmission of the radio frequency (RF) energy through the 
catheter will be discussed. The RF energy is generated by an energy 
source, such as that described in the above referenced application, Ser. 
No. 337,903, and transmitted via the catheter to the probe or balloon. The 
electrical resistance of the load at the probe or balloon will change as a 
function of temperature. By measuring this resistance change, it is 
possible to determine the temperature or to correlate the temperature with 
a change in resistance. By providing a constant current source for a DC 
current or low frequency AC current (multiplexed or filtered to segregate 
it from the RF generator), a change in electrical resistance of the load 
will produce a voltage responsive to the change in resistance. This 
voltage change can be sensed and the change is used to regulate the power 
of the RF energy applied to the load. Because the electrical resistance of 
the load must be extremely low, the transmission media through the 
catheter may have a significant effect upon the overall resistance. 
Moreover, the resistance of the transmission media will produce heat. It 
is therefore of significant import to employ electrical conductors within 
the catheter which are compatible with transmission of RF energy and which 
provide a low resistive path relative to the load. By using the below 
described coaxial conductors, spurious RF radiation along the catheter 
will be minimized and the resistance of the conductors will be 
sufficiently low to avoid heat generation and a voltage drop of any 
significance along the catheter. 
Because of the small physical size constraints imposed upon the probe, the 
balloon and catheter to permit them to be inserted within and advanced 
through an affected section of the cardiovascular system having a 
restriction or an occlusion, flexible components very small in diameter 
must be used. Where the catheter must perform multiple functions, it is 
mandatory that one or more elements be capable of performing more than one 
function in order to meet the preferred size constraints. Preferably, the 
overall diameter of the catheter is on the order of 0.025 to 0.030 inches 
and it will readily negotiate 0.5 inch radius turns. 
Referring to FIG. 1, there is illustrated a hollow core coaxial catheter 
10. The hollow center or core 12 is approximately 0.020 inches in 
diameter. An inner cylindrical layer 14 mechanically defines core 12; it 
is of dielectric material to provide electrical insulation. Preferably, 
inner layer 14 is of polyimide or PTFE; the former is sold under the 
trademark Kapton by the Dupont company and the latter is sold under the 
trademark Teflon by the Dupont company. The wall thickness of inner 
insulative layer 14 may be in the range of 0.5 to 1.0 mils. An inner 
conductor 16, which may be of copper, generally defines a cylindrical 
shape concentric with inner layer 14. The thickness of the inner conductor 
may be in the range of 0.5 to 2.0 mils. A cylinder 18 is disposed radially 
outwardly concentric with inner conductor 16. The cylinder is of 
dielectric material, such as polyimide or PTFE. The wall thickness of 
cylinder 18 may be in the range 4.0 to 10.0 mils. An outer conductor 20, 
which may be of copper, is disposed about cylinder 18. It may have a wall 
thickness in the range of 0.5 to 2.0 mils. 
Layer 14 and cylinder 18 are illustrated as circular in cross section; such 
configuration is not mandatory and other cross sectional configurations 
can be employed. The dielectric material of either or both of layer 14 and 
cylinder 18, in addition to the above noted materials, may be a glass 
ceramic compound or an alumina silica compound of the type available from 
Galileo Electro-Optics Corp. of Sturbridge, Mass. These compounds, used in 
the configuration and sizes discussed below, have a flexural modulus which 
permits the below described flexibility and bending radius. 
As RF energy is transmitted through the inner and outer conductors, it is 
necessary that the wall thickness of cylinder 18 be sufficient to minimize 
losses between the conductors. Accordingly, both the power levels and the 
frequency of the RF energy will impose certain constraints upon the wall 
thickness of cylinder 18. A outer layer 22 encapsulates outer conductor 20 
to electrically insulate it and to physically shield it against abuse and 
damage. 
The preferred use of polyimide material is based upon certain of the 
properties of the material. It has high dielectric properties whereby the 
distance between the electrical conductors can be reduced without fear of 
voltage breakdown as compared to other materials. It can be heated to a 
relatively high temperature without melting, deformation or other damage; 
typically, it will withstand continuous heat of over 250.degree. C. and 
heat up to 500.degree. C. for short periods of time. It has certain 
structural, torsional and flexural properties which are beneficial. It has 
great tensile strength and is relatively inelastic. 
In one embodiment, the inner and outer conductors may be deposited upon the 
corresponding surfaces of cylinder 18. Such deposition could fracture and 
produce open circuits in the event the underlying material 
expands/contracts in response to the stresses and strains imposed. The 
deposited material may be fine grain or pure copper to assure maximum 
flexibility and elasticity. Because one of the properties of polyimide is 
that of relative inelasticity, expansion will not occur and fracturing of 
the deposited conductors is unlikely to occur. 
Referring jointly to FIGS. 2 and 3, there is shown an alternate 
configuration of the inner and/or outer conductors (16,20). By wrapping 
cylinder 18 with a thin strip of conductive material (30), such as copper, 
the coax becomes highly flexible and the possibility of fracturing a 
deposition applied conductor will be eliminated. Such wrapping will also 
augment the structural integrity of the catheter. Moreover, inner 
conductor 16 can be similarly wrapped within cylinder 18; such wrapping 
may be within the interior surface of the cylinder or about inner layer 
14. In the latter configuration, cylinder 18 may be formed about layer 14 
wrapped by conductor 16 or slid thereonto. 
Referring to FIG. 2, there is shown a ribbon conductor 30 wrapped about the 
exterior surface of cylinder 18. As depicted by dashed lines 32, the strip 
conductor may be overlapped by 50% to provide a double thickness for 
continuous electrical conduction and shielding and an essentially smooth 
exterior and interior surface. 
FIG. 3 illustrates, in representative form, a copper strip conductor 30. 
This conductor may be on the order of 0.250 inches wide and 0.0013 inches 
thick. Where dictated by manufacturing and/or operational requirements, 
strip 30 may be encapsulated polyimide ribbon 31 within copper coating 34. 
Referring to FIGS. 4A and 4B, there is shown an inflatable balloon 42 
secured to the distal end of a hollow core coaxial catheter 10. The 
following detailed description of the structural considerations attendant 
the coaxial catheter and the balloon will be commenced generally from the 
distal end. Core 12 of catheter 10 is defined by the protruding part of 
inner layer 14. Inner conductor 16, which is depicted as a wrap of a 
ribbon of conductive material terminates short of the distal end. This 
conductor may be dimensioned as discussed with respect to FIGS. 2 and 3 or 
it may be in the manner of a flat wire 20 mils wide and 1 mil thick which 
has been double wrapped to provide a 2 mil thickness. Cylinder 18, being 
distally beveled by bevel 40, as shown, terminates proximally of the end 
of inner conductor 16. An annular balloon 42 defining an inflatable 
annular cavity 44 is supported by and disposed about the exposed length of 
cylinder 18. Exterior surface 46 of the balloon is coated or electro 
deposited with a coat 48 of ohmically resistive material that serves as a 
heatable load thermally responsive to application of RF energy. The distal 
end of balloon 42 includes an annular section 50 circumscribing cylinder 
18 and extending distally from cavity 44 to approximately the terminal end 
of cylinder 18. An electro deposited layer 52 of electrically conductive 
material extends from about the exposed portion of conductor 16 along 
bevel 40 and about a proximal segment of the exposed area of cylinder 18. 
A ring 54 of electrically conductive material electrically interconnects 
substrate 52 with electrically conductive coat 48. Accordingly, conductor 
16 is electrically connected to coat 48. A covering 56 of electrically 
insulating material is disposed about catheter 10 distally of balloon 42 
and about the balloon. This covering electrically insulates the electrical 
elements attendant the distal end of the catheter and the balloon. 
Moreover, the covering serves a structural function of maintaining the 
electrical conductors in their respective positions and prevents slippage 
between the balloon and the encircled catheter. 
The following description will relate primarily to the structure attendant 
the proximal end of balloon 42. Outer conductor 20 terminates at end 24 
distally of the balloon. Surface 46 of balloon 42 extends proximally to 
form an annular section 60 in circumscribing relationship with cylinder 
18. The proximal end of the annular section may abut end 24, as shown. 
Coat 48 of electrically conductive material and disposed upon exterior 
surface 46 extends to and circumscribes annular section 60. A ring 62 of 
electrically conductive material overlies and is in electrical contact 
with the distal end of outer conductor 20 and the proximal end of coat 42. 
Thereby, an electrical path is established between outer conductor 20 and 
coat 48 disposed about balloon 42. 
Covering 56 extends proximally from balloon 42 and encloses annular section 
60 and ring 62. The covering may extend proximally along catheter 10 to 
physically protect and electrically shield outer conductor 20. 
From the above description, it will become apparent that coat 48 of 
electrically conductive material forms a load interconnected between outer 
conductor 20 and inner conductor 16. This coat, if made of ohmically 
resistive material, may be heated upon application of RF energy 
transmitted via the inner and outer conductors. 
Inflation and deflation of balloon 42 may be accomplished by a lumen 
disposed within core 12 and extending into cavity 44 of the balloon. Such 
extension into the cavity may be effected by penetration of inner layer 
14, inner conductor 16 and cylinder 18. Alternatively, and as illustrated 
in FIG. 4B, a plurality of passageways 66 may be formed at the proximal 
end of balloon 42 through covering 56, coat 48 and exterior surface 46. 
These passageways are in fluid communication with an annular cavity 68 
disposed about catheter 10. The annular cavity may be developed by a 
sheath 70 concentric with the catheter. A flared or cone section 72 of the 
sheath may be employed to sealingly engage the proximal end of balloon 42, 
as illustrated. Necessarily, the junction between the cone section and the 
balloon must be sealed and remain sealed during both inflation and 
deflation of the balloon. 
To provide perspective to the size of catheter 10 and balloon 42, 
representative dimensions of the various components, shown in FIGS. 4A and 
4B, are listed below. 
Core 12 - 20 mils diameter 
Layer 14 - 0.5 to 2 mils thick 
Inner conductor 16 - 0.1 to 2 mils thick 
Cylinder 18 - 6 to 10 mils thick 
Outer conductor 20 - 0.1 to 2 mils thick 
Rings 54,62.fwdarw.0.5 mils thick 
Covering 56 - 0.3 to 2 mils thick 
Surface 46 - 0.5 mils thick 
Ohmically resistive coat 48 - 500 .ANG. to 2 mils thick 
Sheath 70 - 1 to 3 mils thick 
To enlarge the scope of utility of the catheter for use in angioplasty 
procedures, it is preferrable that the catheter be not only very flexible 
but also small in cross sectional area. The dimensions of the various 
components listed above permit fabrication of a catheter having a cross 
section of not more than 0.0030 square inches. Through judicious selection 
of component dimensions, the cross sectional area can be less than 0.0020 
square inches. 
The expanded size and configuration of balloon 42 will vary as a function 
of the medical procedure to be performed; its selection is made by the 
physician. 
Referring to FIG. 5, core 14 of catheter 10 may be employed to house 
several elements useful in the analysis, performance and evaluation of an 
angioplasty procedure. A guide wire 80 is disposed within the catheter and 
extends from the distal end of the catheter. Such a guide wire is 
manipulatable by a physician to thread the distal end of the catheter to a 
particular vascular site where the angioplasty procedure is to be 
performed. To assist in evaluating and diagnosing a vascular site, as well 
as other physiological conditions, a fiber optic element 82 may be 
disposed within the hollow core and protrude from the distal end. As is 
well known, such fiber optic element is capable of imaging a site of 
interest. To provide visible light, for illuminating a site of interest 
for imaging purposes, a source of light 84 may be housed within core 14 to 
illuminate the site of interest proximate the distal end of the catheter. 
A lumen 86 may also be disposed within hollow core 14 to serve as a 
conduit for fluid flowing into and out of balloon 42. 
During certain angioplasty procedures and for other reasons, it would be 
preferable not to impede blood flow during an angioplasty procedure at a 
vascular site. As shown in FIG. 6, hollow core 14 can accommodate such 
perfusion. A plurality of apertures 90 extend through wall 92 of catheter 
10. These apertures will have minimal disruptive effect upon the RF energy 
transmitted through conductors 16 and 20. Apertures 90, in combination 
with opening 94 defined by hollow core 14 at the distal end of the 
catheter, accommodate perfusion through core 14 past the interior of 
balloon 42. Accordingly, catheter 10 can accommodate perfusion during 
inflation of the balloon. 
As shown in FIG. 7, balloon 42 may be inflated and deflated by flow of 
fluid through lumen 86. The lumen is generally housed within hollow core 
14. The proximal end of the lumen must necessarily be connected to a 
source of fluid for controlling the amount, rate and direction of fluid 
flow. Generally, the lumen and balloon are under pressure from a source of 
fluid under pressure and the pressure is adjusted for inflation and 
deflation. Terminal end 88 of the lumen is penetrably engaged with wall 92 
via a passageway 94. Preferably, the lumen is in sealed engagement with 
the passageway. Terminal end 88 terminates within the envelope defined by 
balloon 42 and includes an opening 96 to establish fluid communication 
between the interior of the balloon and the lumen. 
It is to be understood that certain or all of the optic related and 
hydraulic conduits may be integrated within the inner and outer surfaces 
of cylinder 18. 
While the principles of the invention have now been made clear in an 
illustrative embodiment, there will be immediately obvious to those 
skilled in the art many modifications of structure, arrangement, 
proportions, elements, materials and components used in the practice of 
the invention which are particularly adapted for specific environments and 
operating requirements without departing from those principles.