Variable energy shock wave production

An arrangement for the comminution of concrements of various kinds in the body of living being, uses an underwater or submerged arc gap with spark discharge, and an electrical energizing circuitry for that arc gap, a plurality of electrode assemblies from which one is selected to be used in the equipment is connected to the energizing circuit; the electrode assemblies are basically similarly configured and each has an ohmic resistor which is connected directly in series with one of the electrodes, the resistance value for the resistors in the electrode assemblies of the plurality are different, and in each instance much lower than the resistance of the unignited gap, but of comparable magnitude with the resistance of a fully developed plasma channel in the gap following ignition by the energizing circuit.

BACKGROUND OF THE INVENTION 
The present invention relates to medical shock wave treatment and here 
particularly to the contactless comminution of concrements in the body of 
a living being under utilization of a submerged arc discharge serving as a 
shock wave generator, there being an appropriate switching and operating 
structure for the generator. 
Concerning the technology generally and the comminution of concrements in 
particular, reference is made to German patent 23 51 247 corresponding to 
U.S. Pat. No. 3,942,531, and other patents of common assignee. Typical arc 
discharge devices and units in conjunction with which the invention can be 
practiced with advantage are shown e.g. in U.S. Pat. Nos. 4,608,983 and 
4,809,682. Positioning electrode exchange units are shown e.g. in U.S. 
Pat. No. 4,040,050. Moreover, specific reference is made to a brochure by 
Ch. Chaussy ed., "Extracorporeal Shock wave Lithotripsy", giving an 
extensive background treatment on the subject mater including the 
comminution process itself and the generation of shock waves in 
particular. 
Broadly speaking therapeutic shock wave generation uses a spark gap 
produced by a discharge between electrodes the discharge through the gap 
between the electrodes results from the discharge of an electric 
capacitor. That capacitor is discharged in certain instants. The capacitor 
is recharged thereafter, requiring a certain period of time for such a 
restoration but the discharge itself is a phenomenon of very short 
duration. Since the electrodes are submerged, on discharge a shock wave is 
produced in the water. A rotational ellipsoid partially encloses the spark 
gap which is situated in one of the two focal points of that rotational 
ellipsoid. As shock waves are produced by the gap in that one focal point 
they are reflected by the rotational ellipsoid and are refocused in the 
second focal point for comminuting concrement thereat. 
The acoustic energy that can be concentrated in the body of the person, 
depends on the efficiency of coupling the waves to the body. From a 
primary point of view the discharge depends on the amount of electrical 
energy that is fed into the discharge device. Specific relevant parameters 
include the voltage and the capacitance of the capacitor. Generally 
speaking, one needs some form of control over the intensity and energy of 
the shock wave and, therefore, it is desirable to provide for a certain, 
possibly even large variability of the shock wave energy. Depending on the 
medical specifics; a rather broad energy spectrum is quite desirable. 
Gall stones require usually a higher energy level for their comminution 
than is necessary for the destruction of kidney stones. It is obvious that 
the same equipment should be suited for both kinds of treatment, 
comminution of kidney stones, as well as comminution of gall stones. 
Consequently this requirement of multiple use carries with it the 
requirement of making available a fairly wide range of energy. 
Shock wave energies can easily be varied through variation of the voltage 
that is applied to the capacitor for charging it. Variation of that 
voltage controls the amount of charge on the capacitor. However, the 
ignition properties of the submerged arc limit the range of variability of 
the voltage that can be accommodated. Another factor to be considered is 
that multiple use of the same electrodes entails certain burn-off which 
inherently increases the voltage minimum for obtaining any ignition at 
all. The required energy thus increases with the frequency of use and 
burn-off of the electrodes. Hence, the discharge and shock producing 
energy cannot be reduced to any level below that minimal level needed for 
the ignition process. Another way of controlling the shock wave energy, of 
course, is the variation of the capacitance. While such an approach is 
feasible in principle it requires high voltage and/or high current 
switches which are mechanically bulky and expensive. 
DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a new and improved 
technique for varying the operating range for the comminution of 
concrements whereby on one hand the ignition in the submerged arc device 
will not vary as far as ignition voltage is concerned while the 
capacitance is likewise kept constant but still, the amount of energy used 
can be reduced. 
In accordance with the preferred embodiment of the present invention it is 
suggested to provide a set of different electrode assemblies to the user 
and to connect an ohmic resistance directly in series with these 
electrodes. The ohmic resistance is included as a component in the 
electrode structure as far as mounting is concerned, and in the case of 
coaxial feed system it is the inner conductor which contains the resistor 
or even constitutes by itself the ohmic resistor. The resistors are 
preferably constructed of stainless steel and the resistance values differ 
from each other in a range from 0.1 to 1 ohm. Any heat is conducted out of 
the system in the normal fashion. 
It can thus be seen that it is essential for the invention to provide a 
supplemental ohmic resistance structure between the shock wave energy 
source, e.g. the capacitor and the arc discharge gap. This ohmic load is 
selectively interconnected, and exchanged for different comminution tasks. 
These ohmic resistors will convert into heat some of the energy that is 
not needed for any specific instance of arc generation. It is essential 
that this series resistor is much lower than the resistance of the 
underwater gap between the electrodes so that it does not interfere with 
the arc ignition. The underwater gap has typically a resistance in the 
kiloohm range. On the other hand, the ohmic resistance is comparable with 
the resistance of a fully developed plasma channel between these 
electrodes. This resistance has a conductivity is typically several ohms. 
For this reason, the ignition voltage has to remain high and constant. 
Only after a low impedance plasma channel has developed in the underwater 
discharge gap and after the capacitor actually begins to discharge, the 
low ohmic impedance will show its effect. As stated, the resistor will 
generally be above about 1.10 ohm but not more than about 1 ohm so that 
the resistor will not interfere with the breakdown mechanism mentioned 
above and during which operation the gap resistance is in the kiloohm 
range. 
The electrodes are usually mounted in a particular structure which, as a 
whole, is exchangeable for ease of refurnishing or the like, and it is of 
advantage to provide the ohmic resistor as an inner conductor or part of 
the inner conductor in a coaxial conductor system being a part of an 
exchangeable electrode structure. The inner conductor is normally made of 
a highly conductive material and is normally comprised, e.g. of silver 
plated brass, just as is the outer conductor. The resistance now is 
included in a series circuit connection, particularly int he path of the 
inner conductor, in that at least a portion of the inner conductor is made 
to be of a fairly poorly conducting medium. 
The principle of variability concerning the ohmic resistance in the circuit 
can be extended to include the electrode tips themselves, e.g. one or the 
other of the electrodes or both can be made of a fairly poor electrical 
conductor. 
The electric power dissipated as heat in such a resistor when between 0.1 
and 1 ohm, is in the order of 30W maximum for the lower ohm value. It was 
found that natural thermal conduction suffices for removing the heat. It 
is preferred to use coaxial transmission. In this case the inner conductor 
portion may be exchangeable within a given electrode-plus-conductor 
assembly or one used fixed assemblies of that kind with the inner 
conductors having different resistances. Still alternatively, a resistance 
change can be made to take place, e.g. through temperature control or 
pressure/tension application, though switching actions should be avoided.

Proceeding now to the detailed description of the drawings, FIG. 1 
illustrates a circuit 2 which shows connectors 3 to be connected to a 
suitable power supply source. The circuit further includes a capacitor 4 
which may be charged through a high ohmic structure, not shown, and that 
may be included in the circuit to which circuit 2 is connected by means of 
the connectors 3 and being part of the charge circuit. Reference numeral 8 
refers to a spark gap composed of and defined by electrodes 8a and 8b, 
placed in a certain distance from each other. Detailed configurations will 
be shown and explained more fully below. 
One of these electrodes (8b) is connected directly to the grounded side of 
the capacitor 4, and it is usually that side which is grounded. The other 
electrode (8a) is connected to a resistor 10 which is part of the 
invention and is incorporated in the circuit and device in a manner to be 
described more fully below. A gap 6 is connected in series with the 
resistor 10, which, in this case, is a switching spark gap and which will 
be ignited in case the voltage on the capacitor exceeds a certain limit. 
In lieu of this particular spark gap 6, one could provide a switch that is 
manually or electronically operated or the like. Structure is provided in 
the circuit for interrupting or permitting current flow by providing a 
very low ohmic connection. 
The electrode structure is shown in greater detail in FIG. 2 and 3. It 
includes a sleeve serving as a holder 12 into which the electrode 
arrangement 14 is plugged. The electrode assembly includes the outer 
electrode cage element 15 for holding electrode 8b, and the inner 
electrode 8a, which is continued in a tip element, the front of which 
faces the tip of the electrode 8b. The various applications and patents 
referred to above show details of this particular structure which, as far 
as the electrode proper is concerned, is directly incorporated and 
includes in particular the cage element 15 on which the tip element is 
mounted for the grounded electrode 8b. 
The cage 15 is connected to and extends from the outer conductor 22, and 
the inner electrode 8a is connected in a series with the inner conductor 
18. The inner conductor 18 is separated from the outer conductor 22 by 
means of an insulation 24. The element shown in FIG. 3 is the one that is 
been plugged-in into the sleeve arrangement 12. 
As far as the invention is concerned, a plurality of these devices shown in 
FIG. 3 is suggested. It is essential that as far as overall geometry is 
concerned, the overall positioning of all these electrodes and their 
respective assembly are the same. They differ in the effective (numerical) 
electrical ohmic resistance of the inner conductor 18. That inner 
conductor 18 is different in the different configurations in that the 
cross section (FIG. 4) and/or material composition (FIG. 5) and/or 
effective length and/or the length of a portion being reduced in 
cross-section, to obtain the requisite variety of resistances. 
Typically, a resistance of not lower than 100 millohms is needed while the 
upper limit for practical purposes is in the order of 1 ohm. The 
variability covers, therefore, a range that extends over, roughly one 
order of magnitude. In the case of a specific lithotripter whenever a 
specific task requires a particular shock wave range and particular 
impedance and resistance within the electrode assembly, one can simply 
exchange the electrode structure for the one that is specifically adapted 
to the task at hand in terms of internal resistance in series with the 
submerged spark gap. The adaptation merely requires a proper selection of 
the ohmic resistances within the spark gap circuit. 
The invention is not limited to the embodiments described above but all 
changes and modifications thereof, not constituting departures from the 
spirit and scope of the invention, are intended to be included.