Abstract:
A system that breaks calcium in a liquid includes a catheter including first and second electrodes arranged to receive there-across a high electrical voltage at an initial low current. The high electrical voltage causes an electrical arc to form across the electrodes creating a gas bubble within the liquid, a high current to flow through the electrodes, and a mechanical shock wave. A power source provides the electrodes with the high electrical voltage at the initial current and terminates the high electrical voltage in response to the high current flow through the electrodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of U.S. application Ser. No. 13/615,107, entitled SHOCKWAVE CATHETER SYSTEM WITH ENERGY CONTROL, filed Sep. 13, 2012, which is hereby incorporated by reference in its entirety and for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a treatment system for percutaneous coronary angioplasty or peripheral angioplasty in which a dilation catheter is used to cross a lesion in order to dilate the lesion and restore normal blood flow in the artery. It is particularly useful when the lesion is a calcified lesion in the wall of the artery. Calcified lesions require high pressures (sometimes as high as 10-15 or even 30 atmospheres) to break the calcified plaque and push it back into the vessel wall. With such pressures comes trauma to the vessel wall which can contribute to vessel rebound, dissection, thrombus formation, and a high level of restenosis. Non-concentric calcified lesions can result in undue stress to the free wall of the vessel when exposed to high pressures. An angioplasty balloon when inflated to high pressures can have a specific maximum diameter to which it will expand but the opening in the vessel under a concentric lesion will typically be much smaller. As the pressure is increased to open the passage way for blood the balloon will be confined to the size of the opening in the calcified lesion (before it is broken open). As the pressure builds a tremendous amount of energy is stored in the balloon until the calcified lesion breaks or cracks. That energy is then released and results in the rapid expansion of the balloon to its maximum dimension and may stress and injure the vessel walls. 
         [0003]    Recently, a new system and method has been contemplated for breaking up calcium deposits in, for example, arteries and veins. Such a system is described, for example in U.S. Patent Publication No. 2009/0312768, Published Dec. 17, 2009. Embodiments described therein include a catheter having balloon, such as an angioplasty balloon, at the distal end thereof arranged to be inflated with a fluid. Disposed within the balloon is a shock wave generator that may take the form of, for example, a pair of electrodes, which are coupled to a high voltage source at the proximal end of the catheter through a connector. When the balloon is placed adjacent a calcified region of a vein or artery and a high voltage pulse is applied across the electrodes, a shock wave is formed that propagates through the fluid and impinges upon the wall of the balloon and the calcified region. Repeated pulses break up the calcium without damaging surrounding soft tissue. 
         [0004]    Each high voltage pulse causes an arc to form across the electrodes. The arc in turn causes a steam bubble to form. Each steam bubble has the potential of producing two shock waves, a leading edge shock wave as a result of bubble expansion and a trailing edge shock wave as a result of bubble collapse. The trailing edge shock waves exhibit highly variable energy levels and generally, much greater energy levels than the leading edge shock waves. The energy levels of the trailing edge shock waves are substantially dependent on the uniformity of the bubble collapse. The uniform collapse of spherical bubbles to a point appears to create the highest shock wave energies. Unfortunately, spherical bubble configuration requires a substantially larger space than is available in a balloon that must fit into a calcified vein or artery or even a ureter. In fact, the trailing edge shock wave can be substantially eliminated by confining the bubble to an irregular shape. As a result, for angioplasty or other cardiac and non-cardiac applications of shock waves, the trailing edge shock wave cannot be reliably relied upon to produce consistent results. 
         [0005]    However, the leading edge shock waves formed by bubble expansion are a different matter. While exhibiting generally lower energies, they are more consistent in energy level. As a result, leading edge shock waves are good candidates for use in medical procedures such, for example, angioplasty or valvuloplasty. 
         [0006]    Another consideration is the amount of energy represented by the high voltage applied to the electrodes. Each high voltage pulse removes a portion of the electrode material. Since the size of the electrodes must be small in order to fit into the calcified vein or artery, they are only capable of sustaining a limited numbers of high voltage pulses sufficient to form the shock wave resulting electrical arc. 
         [0007]    Also, it has been learned that to sustain a leading edge shock wave, it is not necessary to sustain the high voltage throughout the shock wave. Sustaining the high voltage beyond some point after the initial arc does not lead to shock waves of any greater intensity. Further, since the bubbles are formed of steam, the steam produces heat which can increase the temperature of adjacent soft tissue. Just a two degree Celsius elevation in temperature above body temperature can result in tissue damage. 
         [0008]    A still further important aspect of prior art attempts to use shock waves from electrical arcs for therapeutic purposes is that from the time the high voltage is first applied to the electrodes to the time in which the arc occurs there is a dwell time (Td) that is highly variable from one high voltage application to the next. To account for the dwell times that are long, prior art strategies have relied upon high voltage applications where all high voltage pulse durations or pulse widths are of the same length and of a length sufficient to extend through the longest of the anticipated dwell times plus the associated arc and steam bubble. As a result, when the dwell times are shorter than the maximum, the high voltage application durations are longer than necessary and can unnecessarily extend the arc and the steam bubble well beyond a time required to produce a shock wave of maximum intensity. The result is wasted energy, extended electrode erosion, and unnecessary heating of the adjoining tissue. 
         [0009]    Hence, there is a need in the art to be able to control the energy applied to the electrodes of an electrical arc shock wave generator. More particularly, there is a need to control the applied energy to assure appropriate bubble and shock wave formation while at the same time conserving electrode material and assuring tissue safety. The present invention addresses these and other issues. 
       SUMMARY OF THE INVENTION 
       [0010]    In one embodiment, a system includes a catheter including an elongated carrier and a balloon about the carrier in sealed relation thereto. The balloon is arranged to receive a fluid therein that inflates the balloon. The catheter further includes first and second electrodes within the balloon arranged to receive there-across a high electrical voltage at an initial low current. The high electrical voltage causes an electrical arc to form across the first and second electrodes within the balloon. The electrical arc creates a gas bubble within the liquid, a high current to flow through the first and second electrodes, and a mechanical shock wave within the balloon. The system further includes a power source that provides the first and second electrodes with the high electrical voltage at the initial current and that terminates the high electrical voltage in response to the high current flow through the first and second electrodes. 
         [0011]    The power source includes a current sensor that senses current flowing through the first and second electrodes. The current sensor causes the power source to terminate the high electrical voltage when the current flowing through the first and second electrodes reaches a predetermined limit. The predetermined limit may be on the order of fifty amperes. 
         [0012]    The system may further include a temperature sensor within the balloon that senses temperature of the fluid within the balloon. The power source may be further responsive to the temperature sensor. 
         [0013]    The temperature sensor may cause the power source to decrease energy applied to the first and second electrodes responsive to the temperature of the fluid within the balloon increasing to control the temperature of the fluid. The temperature sensor may cause the power source to decrease energy applied to the first and second electrodes responsive to the temperature of the fluid within the balloon increasing to above two degrees Celsius above ambient temperature. 
         [0014]    Each pulse of the serial electrical high voltage pulses has an amplitude. The temperature sensor may cause the power source to decrease the energy applied to the first and second electrodes by decreasing the amplitude of the serial electrical high voltage pulses. Alternatively, the temperature sensor may cause the power source to decrease the energy applied to the first and second electrodes by temporarily terminating the serial electrical high voltage pulses. 
         [0015]    The serial electrical high voltage pulses have a pulse rate. The temperature sensor may cause the power source to decrease the energy applied to the first and second electrodes by decreasing the pulse rate of the serial electrical high voltage pulses. 
         [0016]    The balloon may be a dilation balloon. The dilation balloon may be an angioplasty balloon. In some applications, such as lithotripsy, a balloon may not be required. 
         [0017]    The system may further include a timer that times a delay time in response to the high current flow through the first and second electrodes and the power source may terminate the high electrical voltage after the delay time is timed. The power source may include a current sensor that senses current flowing through the first and second electrodes and the current sensor may cause the timer to time the delay time when the current flowing through the first and second electrodes reaches a predetermined limit. The predetermined limit may be on the order of fifty amperes. 
         [0018]    In another embodiment, a system includes a catheter including an elongated carrier having a guide wire lumen and a balloon having an inner surface about the carrier in sealed relation thereto. The balloon forms a channel with the carrier. The channel is arranged to receive a fluid that inflates the balloon. The catheter further includes first and second electrodes within the balloon, between the carrier and the inner surface of the balloon, arranged to receive there-across a high electrical voltage at an initial low current to cause an electrical arc to form across the first and second electrodes within the balloon. The electrical arc creates a gas bubble within the liquid, a high current to flow through the first and second electrodes, and a mechanical shock wave within the balloon. The system further includes a power source that provides the first and second electrodes with the high electrical voltage at the initial current and that terminates the high electrical voltage in response to the high current flow through the first and second electrodes. 
         [0019]    In a further embodiment, a system includes a catheter including an elongated carrier and a balloon about the carrier in sealed relation thereto. The balloon is arranged to receive a fluid therein that inflates the balloon. The catheter further includes first and second electrodes within the balloon arranged to receive there-across a high electrical voltage at an initial low current to cause an electrical arc to form across the first and second electrodes within the balloon. The electrical arc creates a steam bubble within the liquid, a high current to flow through the first and second electrodes, and a mechanical shock wave within the balloon. The steam bubble increases the temperature of the fluid. The system further includes a temperature sensor within the balloon that senses temperature of the fluid within the balloon and a power source that provides the first and second electrodes with the high electrical voltage at the initial current and that controls energy provided by the high electrical voltage in response to the sensed temperature of the fluid within the balloon. 
         [0020]    The temperature sensor causes the power source to decrease energy applied to the first and second electrodes responsive to the temperature of the fluid within the balloon increasing to control the temperature of the fluid. The temperature sensor causes the power source to decrease energy applied to the first and second electrodes responsive to the temperature of the fluid within the balloon increasing to about two degrees Celsius above ambient temperature. 
         [0021]    Each pulse of the serial electrical high voltage pulses has an amplitude. The temperature sensor may alternatively cause the power source to decrease the energy applied to the first and second electrodes by decreasing the amplitude of the serial electrical high voltage pulses. The temperature sensor may alternatively cause the power source to decrease the energy applied to the first and second electrodes by temporarily terminating the serial electrical high voltage pulses. 
         [0022]    The serial electrical high voltage pulses have a pulse rate. The temperature sensor may alternatively cause the power source to decrease the energy applied to the first and second electrodes by decreasing the pulse rate of the serial electrical high voltage pulses. 
         [0023]    The carrier of the catheter may have a guide wire lumen. The balloon has an inner surface that with the carrier, forms a channel arranged to receive the fluid that inflates the balloon. The first and second electrodes may be disposed between the carrier and the inner surface of the balloon. 
         [0024]    According to a further embodiment, the invention provides a method that includes the steps of providing a catheter including an elongated carrier, a balloon about the carrier in sealed relation thereto, the balloon being arranged to receive a fluid therein that inflates the balloon, and first and second electrodes within the balloon. The method further includes introducing the fluid into the balloon to inflate the balloon, applying an electrical voltage across the first and second electrodes to form an electrical arc across the first and second electrodes, sensing current flow through the first and second electrodes, and varying the application of the electrical voltage across the first and second electrodes in response to sensed current flow through the first and second electrodes after the electrical arc is formed across the first and second electrodes. 
         [0025]    The varying step may include terminating the application of the electrical voltage across the first and second electrodes. The high electrical voltage may be terminated when the current flowing through the first and second electrodes reaches a predetermined limit. The predetermined limit may be on the order of fifty amperes. 
         [0026]    The method may include the further step of sensing temperature of the fluid within the balloon and the varying step may include varying the application of the electrical voltage across the first and second electrodes in response to sensed temperature of the fluid. 
         [0027]    The varying step may include decreasing energy applied to the first and second electrodes responsive to the temperature of the fluid within the balloon increasing to control the temperature of the fluid. The energy applied to the first and second electrodes may be decreased responsive to the temperature of the fluid within the balloon increasing to above two degrees Celsius above ambient temperature. 
         [0028]    The applying step may include applying energy in the form of serial electrical high voltage pulses and the varying step may further include decreasing the energy applied to the first and second electrodes by temporarily terminating the serial electrical high voltage pulses. 
         [0029]    The serial electrical high voltage pulses have a pulse rate. Alternatively, the varying step may further include decreasing the energy applied to the first and second electrodes by decreasing the pulse rate of the serial electrical high voltage pulses. 
         [0030]    The method may include the further step of timing a delay time in response to sensed current flow through the first and second electrodes and the varying step may include terminating the application of the electrical voltage across the first and second electrodes after timing the delay time. The delay time may be timed when the current flowing through the first and second electrodes reaches a predetermined limit. The predetermined limit may be on the order of fifty amperes. 
         [0031]    According to another embodiment, a method includes the steps of providing a catheter including an elongated carrier, a balloon about the carrier in sealed relation thereto, the balloon being arranged to receive a fluid therein that inflates the balloon, and first and second electrodes within the balloon. The method further includes the steps of introducing the fluid into the balloon to inflate the balloon, applying energy in the form of an electrical voltage across the first and second electrodes to form an electrical arc across the first and second electrodes, sensing temperature of the fluid within the balloon, and varying the application of the energy across the first and second electrodes in response to sensed temperature of the fluid within the balloon. 
         [0032]    The varying step may include decreasing the energy applied to the first and second electrodes responsive to the temperature of the fluid within the balloon increasing. The varying step may include decreasing the energy applied to the first and second electrodes responsive to the temperature of the fluid within the balloon increasing to about two degrees Celsius above ambient temperature. 
         [0033]    Each pulse of the serial electrical high voltage pulses has an amplitude. The varying step may include decreasing the energy applied to the first and second electrodes by decreasing the amplitude of the serial electrical high voltage pulses. 
         [0034]    The applying step may include applying energy in the form of serial electrical high voltage pulses and the varying step may further include decreasing the energy applied to the first and second electrodes by temporarily terminating the serial electrical high voltage pulses. 
         [0035]    The applying step may include applying energy in the form of serial electrical high voltage pulses, wherein the serial electrical high voltage pulses have a pulse rate. The varying step may further include decreasing the energy applied to the first and second electrodes by decreasing the pulse rate of the serial electrical high voltage pulses. 
         [0036]    In a still further embodiment, a system treats obstructions within bodily fluid and includes a catheter including first and second electrodes arranged to receive there-across a high electrical voltage at an initial low current. The high electrical voltage causes an electrical arc to form across the first and second electrodes. The electrical arc creates a gas bubble within the bodily fluid, a high current to flow through the first and second electrodes, and a mechanical shock wave within the bodily fluid. The system further includes a power source that provides the first and second electrodes with the high electrical voltage at the initial current and that terminates the high electrical voltage in response to the high current flow through the first and second electrodes. 
         [0037]    The energy applied by the power source may be in the form of serial electrical high voltage pulses. Each pulse of the serial electrical high voltage pulses has an amplitude. The power source may control the energy applied to the first and second electrodes by varying the amplitude of the serial electrical high voltage pulses. 
         [0038]    The serial electrical high voltage pulses have a pulse rate. Alternatively, the power source may vary the energy applied to the first and second electrodes by varying the pulse rate of the serial electrical high voltage pulses. 
         [0039]    The system may further include a timer that times a delay time in response to the high current flow through the first and second electrodes and the power source may terminate the high electrical voltage after the delay time is timed. The power source may include a current sensor that senses current flowing through the first and second electrodes and the current sensor may cause the timer to time the delay time when the current flowing through the first and second electrodes reaches a predetermined limit. The predetermined limit may be on the order of fifty amperes. 
         [0040]    In still a further embodiment, a method includes the steps of providing a catheter including first and second electrodes, applying an electrical voltage across the first and second electrodes to form an electrical arc across the first and second electrodes, sensing current flow through the first and second electrodes, and varying the application of the electrical voltage across the first and second electrodes in response to sensed current flow through the first and second electrodes after the electrical arc is formed across the first and second electrodes. 
         [0041]    The applying step may includes applying energy in the form of serial electrical high voltage pulses, the serial electrical high voltage pulses having a pulse rate, and wherein the varying step further includes controlling the energy applied to the first and second electrodes by varying the pulse rate of the serial electrical high voltage pulses. 
         [0042]    The serial high voltage pulses have amplitudes. The varying step may alternatively or in addition include controlling the energy applied to the first and second electrodes by varying the amplitude of the serial electrical high voltage pulses. 
         [0043]    The method may include the further step of timing a delay time in response to sensed current flow through the first and second electrodes and the varying step may include terminating the application of the electrical voltage across the first and second electrodes after timing the delay time. The delay time may be timed when the current flowing through the first and second electrodes reaches a predetermined limit. The predetermined limit may be on the order of fifty amperes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further features and advantages thereof, may best be understood by making reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify identical elements, and wherein: 
           [0045]      FIG. 1  is a simplified side view of an angioplasty balloon catheter of the type that may utilize various embodiments of the invention to advantage; 
           [0046]      FIG. 2  is a simplified side view of an electrode structure that may be employed in the catheter of  FIG. 1  coupled to a source of high voltage pulses according to one embodiment of the invention; 
           [0047]      FIG. 3  is a front plan view of the electrode structure of  FIG. 2 ; 
           [0048]      FIG. 4  is a simplified equivalent circuit diagram of a system according to an embodiment of the invention; 
           [0049]      FIG. 5  is a graph illustrating a high voltage pulse applied to a pair of electrical arc shock wave producing electrodes and the resulting current flow through the electrodes in accordance with an embodiment of the invention; 
           [0050]      FIG. 6  is a schematic diagram of a power source for use in an angioplasty electrical arc shock wave angioplasty catheter according to an embodiment of the invention; 
           [0051]      FIG. 7  is a side view of a dilating catheter with an electrical arc producing electrode structure and a temperature probe therein according to aspects of the invention; 
           [0052]      FIG. 8  is a schematic diagram of an angioplasty catheter system according to further embodiments of the invention; 
           [0053]      FIG. 9  is a simplified side view, partly in section, of a further embodiment wherein a balloon is not required; and 
           [0054]      FIG. 10  is a flow diagram illustrating a further embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0055]      FIG. 1  is a simplified side view of an angioplasty balloon catheter  20  of the type that may utilize various embodiments of the invention to advantage. The catheter  20  includes an elongated carrier, such as a hollow sheath  21 , a dilating balloon  26  formed about the sheath  21  in sealed relation thereto and a guide wire member  28  to which the balloon is sealed at a seal  23 . The guide wire member has a longitudinal lumen  29  through which a guide wire (not shown) may be received for directing the catheter  20  to a desired location within a vein or artery, for example. 
         [0056]    The sheath  21  forms with the guide wire member  28  a channel  27  through which fluid, such as saline, may be admitted into the balloon to inflate the balloon. The channel  27  further permits the balloon  26  to be provided with an electrode pair  25  including electrodes  22  and  24  within the fluid filled balloon  26 . 
         [0057]    As may be seen in  FIG. 2 , the electrodes  22  and  24  are attached to a source  40  of high voltage pulses. As may be seen in  FIG. 3 , the electrodes  22  and  24  are coaxially disposed with electrode  22  being a center electrode and electrode  24  being a ring shaped electrode about electrode  22 . The center electrode  22  is coupled to a positive terminal  44  of source  40  and the ring electrode  24  is coupled to a negative terminal  46  of the source  40 . The electrodes  22  and  24  are formed of metal, such as stainless steel, and are maintained a controlled distance apart to allow a reproducible arc to form for a given applied voltage and current. 
         [0058]    The electrical arcs between electrodes  22  and  24  in the fluid are used to generate shock waves in the fluid. Each pulse of high voltage applied to the electrodes  22  and  24  forms an arc across the electrodes. The voltage pulses may have amplitudes as low as 500 volts, but preferably, the voltage amplitudes are in the range of 1000 volts to 10,000 volts. The balloon  26  may be filled with water or saline in order to gently fix the balloon in the walls of the artery or vein, for example, in direct proximity with the calcified lesion. The fluid may also contain an x-ray contrast to permit fluoroscopic viewing of the catheter during use. Once the catheter  20  is positioned with the guide wire (not shown), the physician or operator can start applying the high voltage pulses to the electrodes to form the shock waves that crack the calcified plaque. Such shockwaves will be conducted through the fluid, through the balloon, through the blood and vessel wall to the calcified lesion where the energy will break the hardened plaque without the application of excessive pressure by the balloon on the walls of the artery. 
         [0059]      FIG. 4  is a simplified equivalent circuit diagram of a system according to an embodiment of the invention. Here it may be seen that a capacitance stores a high voltage. When a switch  60  is closed, the voltage drop across the electrodes  22  and  24  begins to quickly rise at an initially low current level. After a dwell time, when the voltage across the electrodes reaches the breakdown voltage of the fluid between the electrodes, an electrical arc occurs across the electrodes. The arc causes a steam bubble to form between the electrodes and a relatively high current to flow through the electrodes. The expansion of the bubble forms a first or leading edge shock wave. After a time, the steam bubble cools and condenses causing the bubble to collapse. The collapsing bubble has the potential for forming a second or trailing edge shock wave. As previously mentioned, the trailing edge shock wave is relatively unreliable exhibiting inconsistent intensities from shock wave to shock wave. Hence, it is the leading edge shock wave that holds the most promise for reliable therapy. 
         [0060]    It has been found that effective shock wave intensity may be accomplished without holding the high voltage pulses on during the entire extent of their corresponding steam bubbles. Moreover, terminating the application of the high voltage before steam bubble collapse can serve to preserve electrode material, permitting a pair of electrodes to last for an increased number of applied high voltage pulses. Still further, as will be seen subsequently, early termination of the high voltage can also be used to advantage in controlling the temperature within the balloon fluid. 
         [0061]      FIG. 5  is a graph illustrating a high voltage pulse applied to a pair of electrical arc shock wave producing electrodes and the resulting current flow through the electrodes in accordance with an embodiment of the invention. When the switch  60  ( FIG. 4 ) is first closed, the voltage across the electrodes quickly rises to a level  70 . During this time, as shown by dashed lines  72 , the current through the electrodes is relatively low. After a dwell time (Td), the arc occurs between the electrodes. At this time the steam bubble begins to form and a high current begins to flow through the electrodes. In accordance with embodiments of the invention, responsive to the current through the electrodes, the application of the high voltage is terminated. This conserves energy applied to the electrodes, causing the electrodes to remain useful for a greater number of pulses than otherwise would be the case if the high voltage were applied longer or sustained throughout the bubble existence. The advantages of controlling the applied energy in this manner are obtained without adversely affecting the intensity of the leading edge shock waves produced. 
         [0062]      FIG. 6  is a schematic diagram of a power source  80  for use in an electrical arc shock wave angioplasty catheter according to an embodiment of the invention. The power source  80  has an output terminal  82  that may be coupled to electrode  22  of  FIG. 1  and an output terminal  84  that may be coupled to electrode  24  of  FIG. 1 . A switch circuit  86  selectively applies a high voltage on line  88  across the electrodes. A microprocessor  90 , or other similar control circuitry, such as a gate array, controls the overall operation of the source  80 . A Field Programmable Gate Array (FPGA) may also be substituted for the microprocessor in a manner know in the art. The microprocessor  90  is coupled to the switch  86  by an optical driver  92 . The switch includes a current sensor  94  that includes a current sensing resistor  96  that generates a signal that is applied to an optical isolator  98  when the current flowing through the electrodes reaches a predetermined limit, such as, for example, fifty (50) amperes. 
         [0063]    In operation, the microprocessor  90  through the optical driver  92 , causes the switch  86  to apply the high voltage to the electrodes  22  and  24 . The current sensed through resister  96  is monitored by the microprocessor  90  through the optical isolator  98 . When the current flowing through the electrodes reaches a predetermined limit, as for example 50 amperes, the microprocessor  90  causes the application of the high voltage to be terminated. The forgoing occurs for each high voltage pulse applied to the electrodes  22  and  24 . Each pulse creates a shock wave of consistent and useful intensity. Further, because the application of the high voltage is terminated early, the electrode material is preserved to lengthen the useful life of the electrodes. 
         [0064]      FIG. 7  is a side view of a dilating catheter with an electrical arc producing electrode structure and a temperature probe therein according to aspects of the invention. The catheter  20  of  FIG. 7  may be the same catheter as shown in  FIG. 1 . Here however, the catheter  20  further includes a temperature probe or sensor  100 . The temperature sensor may be employed for sensing the temperature of the fluid within the balloon. Preferably, the temperature of the fluid within the balloon  26  should not be permitted to rise more than two degrees Celsius above the ambient body temperature. If this were to occur, soft tissue damage may result. 
         [0065]      FIG. 8  is a schematic diagram of an angioplasty catheter system  110  according to further embodiments of the invention which includes the catheter  20  and temperature probe  100 . Here the system also includes the microprocessor  90 , the switch  86 , optical driver  92  and optical isolator  98 . All of these elements may function as previously described. In addition, the temperature sensor  100  conveys a temperature signal through another optical isolator  120  indicative of the temperature of the fluid within the balloon  26 . If the temperature within the balloon  26  rises to more than a certain temperature, for example to more than two degrees Celsius above ambient body temperature, the energy applied to the electrodes is decreased. This will decrease the size and duration of the steam bubbles produced by the electrodes to maintain the temperature of the fluid within the balloon to within safe limits. The microprocessor  90  may cause the switch  86  to decrease the pulse amplitude of the applied high voltage pulses or the pulse rate of the applied high voltage pulse. It could alternatively temporarily terminate the application of the pulses. 
         [0066]      FIG. 9  is a simplified side view, partly in section, of a further embodiment wherein a balloon is not required. In this embodiment, a system  134 , according to further aspects of the invention, is shown treating an obstruction, more particularly, a kidney stone  131 . The system includes a catheter  133  that terminates at its distal end with an electrode pair  132  similar to electrode pair  25  of  FIGS. 1 and 2 . The system further includes a power source  140 . The power source has a positive output terminal  142  and a negative output terminal  144 . The center electrode of the electrode pair  132  may be coupled to the positive terminal  142  of source  140  and the ring electrode of the electrode pair  132  may be coupled to the negative terminal  144  of the source  140 . The electrodes of the electrode pair  132  may be formed of metal, such as stainless steel, and are maintained a controlled distance apart to allow a reproducible arc to form for a given applied voltage and current. 
         [0067]    The catheter  133  of system  134  is shown in a ureter  130 . The ureter has a kidney stone  131  requiring treatment. According to this embodiment, voltage pulses are applied to the electrode pair  132  to produce leading edge shock waves as previously described. The shock waves propagate through the fluid within the ureter and impinge directly on the kidney stone  131 . In a manner as previously described, the power source may be operated to maintain the energy applied to the electrode pair within limits to assure that the steam bubbles produced by the generated arcs do not harm the ureter. To that end, the amplitude or pulse rate of the applied voltages may be controlled. Hence, by controlling the energy of the current during the produced arc, such as by controlling the on time of the current, barotrauma to the ureter may be minimized even though a balloon is not employed as in previous embodiments. Of course, the system of  FIG. 9  may be used in other body organs as well, such as the bile duct, for example. 
         [0068]      FIG. 10  is a flow diagram illustrating the process of a further embodiment of the invention. The embodiment of  FIG. 10  takes into account the time it takes for a high voltage switch, such as switch  86  ( FIG. 6 ), to turn off (the turn off time) and the rise time of the current flowing through the electrodes once the electrical arc starts. The current through the electrodes can eventually reach one-hundred amperes or more, at which point the maximum intensity shock wave will be formed. In order to permit the maximum current to be reached and to account for the turn off time of the switch  86 , a delay is timed extending from when the current flowing through the electrodes is at a fixed threshold known to be below the maximum current, to the turn off time of the switch before the expected current maximum. For example, the current threshold may be fifty amperes. When the current through the electrodes equals fifty amperes, the delay timing is begun by the starting of a delay timer within the microprocessor  90 . If the current is expected to be at a maximum 200 nanoseconds after the current reaches fifty amperes, and if it takes 100 nanoseconds for the high voltage switch to actually turn off after receiving a turn off signal, a delay of 100 nanoseconds should be timed from the 50 ampere sensing before a turn off signal is applied to the high voltage switch. Hence, a total time of 200 nanoseconds will pass after the current reaches 50 amperes and, as a result, will reach its maximum. As the current reaches its maximum, or shortly thereafter, the voltage applied to the electrodes will be terminated. 
         [0069]    Referring now to the flow diagram  200  of  FIG. 10 , and also with reference to  FIG. 6 , the process begins with activity step  202  wherein the high voltage is applied to the output terminals  82  and  84  for application to the electrodes, for example, electrodes  22  and  24  ( FIG. 1 ). At first, the current initially flowing through the electrodes is relatively low. However, after a dwell time, the applied high voltage causes an electrical arc to begin to form between the electrodes, the current through the electrodes is sensed, and the current rapidly rises. The current through the electrodes is sensed as previously described. At decision block  204 , the microprocessor  90  determines if the sensed current has reached fifty amperes. When the current reaches fifty amperes, the process advances to activity block  206  where the timing of the aforementioned delay time (x) is started. Next, in decision block  208 , it is determined when the delay time has been timed. In accordance with this embodiment, the delay time (x) may be 100 nanoseconds. When the delay time of 100 nanoseconds is timed, the process advances to activity block  210  wherein the process completes with a turn off signal being applied by the microprocessor  90  to the high voltage switch  86 . The switch  86  will actually turn of a turn of time after the turn off signal is applied to the switch  86 . Since it takes 100 nanoseconds for the switch to turn off and since 100 nanoseconds are timed before the turn off signal is applied to the switch, 200 nanoseconds form the 50 ampere current sensing will pass before the applied voltage to the electrodes is actually terminated. That provides sufficient time for the current to reach its maximum to generate the maximum intensity shock wave. The voltage application will terminated as the current reaches maximum, or shortly thereafter. 
         [0070]    As a result of the foregoing, a maximum intensity shock wave is formed without wasting energy, without unduly eroding the electrodes, and without generating unnecessary heat. As may be appreciated, the delay timing may be employed to advantage in each of the embodiments disclosed herein including the embodiment of  FIG. 9  which does not require a balloon. 
         [0071]    While particular embodiments of the present invention have been shown and described, modifications may be made. It is therefore intended in the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the invention as defined by those claims.