Method and device for removing concretions within human ducts
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and device for fragmenting concretions in human ducts. 
     2. The Related Art 
     The technique of using high voltage electrical discharge in a fluid medium to create a shockwave capable of fragmenting rocks was developed in the Soviet Union as an industrial technique over 35 years ago. In the early 1960&#39;s, the Soviets successfully applied the principles of electrohydraulic lithotripsy (EHL) in the treatment of bladder calculi. As early as 1973, attempts were made to apply this technique to the treatment of ureteral calculi. 
     U.S. Pat. No. 4,027,674 and U.S. Pat. No. 4,030,505 are typical of the known art. Therein is described a method and device for catheterization within human ducts so as to remove concretions, plaques and sclerotic clots. A series of high voltage pulses of sufficiently low amperage are generated in this method. These pulses are then dispatched through a flexible conduit containing a positive electrode extending coaxially within the conduit embedded in an electrically insulated material and a negative electrode peripherally embedded in the conduit encircling and coextensive with the positive electrode. 
     Although the use of EHL in treatment of bladder calculi has proved to be a reliable and essentially safe procedure, treatment of ureteral calculi and biliary calculi with this technique is not widely accepted. There are risks associated with the use of existing EHL electrodes in small body lumens such as the ureter and the biliary tree. Moreover, there are difficulties with manipulation and positioning of the probe electrodes in proper proximity to a target calculus. These miniature probes are also of inadequate reliability and have an undesirably short useful life. 
     Ultra high speed photographic analysis has been used to document the phenomena that actually occurs during the electrohydraulic lithotripsy of a target calculus. When the high voltage discharge occurs across the two electrodes at the tip of the EHL probe within a fluid medium, a small amount of the fluid is vaporized and super heated, turning into a rapidly expanding gas plasma bubble. The rapid expansion of this gas plasma bubble creates a hydraulic shock wave. A portion of the energy in this shock wave can impact a target calculus in close proximity to the tip of the probe, and can cause the calculus to crack into fragments. 
     Unfortunately, the only portion of the expanding gas bubble that is effective in transferring energy from existing probes to the target calculus is the small percentage travelling along the axis from the surface of these electrode probes to the closest portion of the target calculus. Since existing probes are generally flat across the electrode surface, whether the electrodes are parallel or coaxial, the largest portion of the energy of the expanding gas plasma bubble is dissipated radially away from this axis to the target, and does not contribute to the fragmentation of the calculus. 
     The inefficiency of existing EHL probes requires that clinical users employ high total energy levels to have adequate energy transfer to the target calculus. If the tips of existing probes are placed too close or in direct contact with the calculus, insufficient fluid medium may be present to allow formation of an effective gas plasma bubble and shock wave. The heat created by high voltage discharge under these circumstances only serves to heat the materials of the probe and surrounding body tissue. Within confined body structures, like the lumen of the ureter or the biliary tree, it has been well documented through clinical experience that the excessive amounts of total energy delivered via existing probes can cause damage to these delicate body tissues. Furthermore, the materials selected for construction of existing probes do not tolerate these high energy discharges creating temperatures in excess of 500° F. without structural damage to these probes. 
     The insulation materials used to separate the electrodes on existing probes is typically of the thermoplastic type routinely used in the electronics industry as general purpose electrical insulation. These materials tend to melt and shrink due to the instantaneous high temperatures created by high voltage discharge across the probe electrodes. The electrode materials used in existing probes are typically of the types chosen for ease of handling and assembly in the electronics industry, i.e., copper, copper alloys like beryllium copper, and phosphorous bronze. These metals tend to erode easily, and to actually transfer material between electrodes due to the high voltage discharges across the probe electrodes. 
     These changes to the insulation separating the probe electrodes, and to the electrodes themselves, further decreases the efficiency of the lithotripsy effect obtainable from existing probes. This requires the clinical user to further increase the intensity of the total energy level delivered in an attempt to maintain usable electrohydraulic lithotripsy effect. This, in turn, increases the risk of damage to sensitive surrounding body tissue. The useful life of existing probes is often insufficient to allow completion of the desired lithotripsy treatment. The replacement of damaged electrode probes unnecessarily extends the time required for completion of the clinical procedure. 
     Accordingly, it is an object of the present invention to provide a device and method for use in electrohydraulic lithotripsy which achieves a more efficient fragmentation of target calculi and requires a lower total energy level. 
     Another object of the present invention is to provide a device and method for use in electrohydraulic lithotripsy that delivers less energy to surrounding sensitive tissue surfaces and thereby minimizes damage to such surfaces. 
     A still further object of the present invention is to provide a device and method for use in electrohydraulic lithotripsy that resists the damaging effects of high temperatures created by high voltage discharge across the electrodes and that resists the transfer of materials between electrodes. 
     A still further object of the present invention is to provide a device and method for use in electrohydraulic lithotripsy that includes a probe having an increased useful life and have a construction of smaller diameter that nevertheless is equivalent in effectiveness to known larger diameter probes. 
     A still further object of the present invention is to provide a device and method for use in electrohydraulic lithotripsy which does not require positioning of the probe tip in precise distance from a target calculus surface and that achieves effective EHL when in direct contact with the surface of a target calculus. 
     SUMMARY OF THE INVENTION 
     A method and device for removing concretions from human ducts is provided. The device includes a flexible probe having first and second ends at opposite termini formed of inert material extendible within the ducts to a situs of the concretions. The probe includes a flexible first and second electrode. The flexible first electrode extends coaxially along a length of the probe and is embedded in a solid electrically insulative material. The flexible second electrode is peripherally arranged radially outward from the first electrode as a continuous electrically conductive surface encircling and coextensive with the first electrode. The first and second electrodes terminate at the first end of the probe while the electrically insulative material terminates short of the first end. Also included within the device is a generator of electrical pulses connected at a second end of the conduit to the positive electrode such that electrical discharges radiate from the first electrode outwardly to the encircling second electrode across a surface of the concretions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above features, advantages and objectives of this invention will become more readily apparent from the following specific description, reference being made to the accompanying diagrammatic drawing in which: 
     FIG. 1 is a partial cross-sectional view of a prior art probe with parallel electrodes; 
     FIG. 2 is a cross-sectional view along lines 2--2 of the probe shown in FIG. 1; 
     FIG. 3 is a cross-sectional view of a prior art probe with coaxial electrodes; 
     FIG. 4 is a cross-sectional view taken along line 4--4 of the probe shown in FIG. 3; 
     FIG. 5 is a partial elevational view showing a distal portion of a preferred embodiment of the invention; 
     FIG. 6 is a cross-sectional view taken along line 6--6 of the distal portion shown in FIG. 5; and 
     FIG. 7 is a cross-sectional view of the distal portion of the probe shown in FIG. 5 contacting a target calculus. 
    
    
     DETAILED DESCRIPTION 
     The present invention is best understood when compared to the prior art devices shown in FIGS. 1-4. In one known device illustrated in FIGS. 1-2, the electrodes are a pair of wires oriented parallel to one another. Another known configuration is depicted in FIGS. 3-4 where a central electrode wire is coaxially encircled by a second electrode sheath, each of these electrodes being separated by an electrically insulative material. 
     The present invention improves upon the known art through a probe having a special electrode tip arrangement shown in FIG. 5. The improved EHL coaxial electrode probe is in the form of a flexible conduit 10 as is shown in FIG. 5. Conduit 10 is formed with a flexible first electrode 12 (e.g. positive electrode) extending coaxially within the conduit and being embedded in a solid electrically insulative material 14. Additionally, the conduit includes a flexible second electrode 16 (e.g. negative electrode) peripherally embedded in the conduit radially outward from the first electrode 12 and provides a continuous electrically conductive surface encircling and being coextensive with the first electrode. 
     The material of the first electrode is chosen for its high temperature melting point and resistance to erosion under high voltage sparking conditions. Tungsten is the preferred material. The electrically insulative material is also chosen for its high temperature melting point, resistance to erosion under high voltage sparking conditions, and further for its ability to maintain the electrical insulating characteristics at high temperatures. Ceramics are the materials of choice for this application. The second or outer electrode is also chosen for its high temperature melting point, high tensile strength, and resistance to erosion under high voltage sparking conditions. Stainless steel is here preferred. These dissimilar conductive substances, e.g. different metals, are chosen for the two electrodes to eliminate any affinity for exchange of material from one electrode to another during high voltage discharge. 
     The key improvement of the present invention lies with the arrangement at the tip end of conduit 10. FIG. 6 shows that the solid electrically insulative material 14 terminates in a surface 15 that is retracted a distance R from the most distal leading edge 17 of the second or outer electrode 16. Retracted surface 15 defines along with an inner surface of the electrode 18 a recess 19 which is devoid of any solid insulative material. Distance R may range anywhere from about 0.0001 to about 10 mm, preferably from about 0.01 to 3 mm. Recess 19 can receive a fluid medium 20 to be captured therewithin when the tip end of the EHL electrode is close to, or in direct contact with, a surface of a target calculi 21 as illustrated in FIG. 7. The fluid medium may either be a bodily fluid within the human duct or a flushing fluid medium delivered exteriorally, such as delivered through an outer annulus of flexible conduit as described in U.S. Pat. No. 4,027,674 herein incorporated by reference. 
     Besides the probe in the form of a flexible conduit, the device of the present invention will also include a source for generating an electrical pulse to which the conduit is attached. Exemplative of a pulse generator 22 is an apparatus that includes a 3-amp slow-blow dual pole switch, a time delay, a foot switch, a 47 K 1/4 W or similar resistor, employed together with a NE 51 H glow lamp, a 120 volt 2.25 amps variable transformer, meter rectifier assembly, and high voltage assembly. 
     The method for removing concretions within human ducts according to the present invention involves the following steps. Conduit 10 is inserted through a duct such as the ureter so that a tip thereof can be juxtaposed against a concretion in the area with the kidney or bladder. A series of high voltage pulses of amperage of sufficiently short duration, i.e. pulse width to avoid harm to human tissues is generated in the electrical pulse generator 22. Pulses therefrom are directed from one end to another of flexible conduit 10 along a first or inner positive electrode 12 to the situs of the concretion at the tip end of the conduit. Pulses are then selectively discharged radially outwardly across recess 19 between positive and negative electrodes. A liquid within or adjacent recess 19 receives the discharging pulses which cause the fluid to vaporize and have a hydroelectric impact against the concretions. 
     While the invention has been described in combination with the embodiments thereof, it is evident that many alternative modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.