Patent Publication Number: US-11026707-B2

Title: Shock wave device with polarity switching

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. application Ser. No. 15/138,147, filed Apr. 25, 2016, also entitled SHOCK WAVE DEVICE WITH POLARITY SWITCHING, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     FIELD 
     The current invention relates to devices and methods for producing shock waves. The devices and methods may be used for angioplasty and/or valvuloplasty procedures. 
     BACKGROUND 
     Currently, angioplasty balloons are used to open calcified lesions in the wall of an artery. However, as an angioplasty balloon is inflated to expand the lesion in the vascular wall, the inflation pressure stores a tremendous amount of energy in the balloon until the calcified lesion breaks or cracks. That stored energy is then released and may stress and injure the wall of the blood vessel. 
     Electrohydraulic lithotripsy has been typically used for breaking calcified deposits or “stones” in the urinary or biliary track. Lithotripsy electrodes may similarly be useful for breaking calcified plaques in the wall of a vascular structure. Shock waves generated by lithotripsy electrodes may be used to selectively fracture a calcified lesion to help prevent sudden stress and injury to the vessel or valve wall when it is dilated using a balloon. It may therefore be useful to find improved ways to form shock waves in a balloon. 
     BRIEF SUMMARY 
     Described here are devices and methods for forming a shock wave in an angioplasty or valvuloplasty procedure. Generally, a shock wave device described here comprises an axially extending elongate member. The elongate member may comprise a first electrode pair comprising a first electrode and a second electrode. The electrode pair may be positioned within a conductive fluid. A controller may be coupled to the first electrode pair and may be configured to deliver a series of individual pulses to the first electrode pair such that each of the pulses creates a shock wave in the conductive fluid. The controller may cause current to flow through the electrode pair in a first direction for some of the pulses in the series and in a second direction opposite the first direction for the remaining pulses in the series. In some variations, the current may flow in the second direction for between twenty five percent and fifty percent of the pulses in the series. The shock wave devices and methods described herein may help facilitate the uniform and consistent delivery of energy to the electrodes, which may enhance the durability and performance of the electrodes. 
     In some variations, the controller may cause the current to flow in the second direction for between one third and half of the pulses in the series. In other variations, the controller may cause the current to flow in the second direction for at least about half of the pulses in the series. 
     In some variations, the controller may comprise a voltage polarity switch to switch a polarity of the electrodes between positive and negative. The electrodes may have opposite polarities. In other variations, a first surface area of a first conductive region of the first electrode may be smaller than a second surface area of a second conductive region of the second electrode. 
     In some variations, the controller may comprise a voltage source. A first wire may connect the first electrode to a first terminal of the voltage source, and a second wire may connect the second electrode to a second terminal of the voltage source. In some instances, the first terminal is positive and the second terminal is negative in the first direction of current flow, and the first terminal is negative and the second terminal is positive in the second direction. 
     In some variations, a second electrode pair may be provided and the controller may further comprise a multiplexer configured to selectively deliver the series of pulses to the first electrode pair and the second electrode pair. In other variations, the device may further comprise a fluid enclosure surrounding the electrode pair. The fluid enclosure may comprise a balloon surrounding a portion of the elongate member. The balloon may be configured to be filled with a conductive fluid, and the first electrode pair may be enclosed within and spaced from the balloon. 
     In yet other variations, the shock wave devices described here may comprise an axially extending elongate member. The elongate member may comprise a first electrode assembly comprising a first electrode pair and a second electrode pair. The first electrode assembly may be positioned within a conductive fluid. A controller may be coupled to the first electrode assembly and configured to deliver a series of individual pulses to the first electrode assembly such that each of the pulses creates a shock wave in the conductive fluid. The controller may cause current to flow through the electrode assembly in a first direction for some of the pulses in the series and in a second direction opposite the first direction for the remaining pulses in the series. In some instances, the current flows in the second direction for between twenty five percent and fifty percent of the pulses in the series. 
     In some variations, the first electrode assembly may comprise a first electrode, a second electrode, and a common electrode. The first electrode pair may comprise the first electrode and the common electrode and the second electrode pair may comprise the second electrode and the common electrode. In some instances, the controller may comprise a voltage polarity switch to switch a polarity of the first electrode and the second electrode between positive and negative. The first electrode and the second electrode may have opposite polarities. In other instances, a first surface area of a first conductive region of the first electrode and a second surface area of a second conductive region of the second electrode may be different than a third surface area of a third conductive region of the common electrode. In some instances, the controller may comprise a voltage source where a first wire may connect the first electrode to a first terminal of the voltage source, and a second wire may connect the second electrode to a second terminal of the voltage source. In other instances, the controller may comprise a voltage source where a first wire may connect the first electrode to a first terminal of the voltage source, a second wire may connect the second electrode to a second terminal of the voltage source, and a third wire may connect the common electrode to a third terminal of the voltage source. 
     In some variations, a second electrode assembly may be coupled in series to the first electrode assembly. In some instances, the controller may comprise a voltage source where a first wire may connect the first electrode assembly to a first terminal of the voltage source, a second wire may connect the first electrode assembly to the second electrode assembly, and a third wire may connect the second electrode assembly to a second terminal of the voltage source. 
     In other variations, the device may comprise a second electrode assembly. The controller may further comprise a multiplexer that selectively delivers the series of pulses to the first electrode assembly and the second electrode assembly. In yet other variations, the device may further comprise a fluid enclosure surrounding the first electrode assembly. The fluid enclosure may comprise a balloon surrounding a portion of the elongate member. The balloon may be configured to be filled with a conductive fluid, and the first electrode assembly may be enclosed within and spaced from the balloon. 
     In some variations, methods of forming shock waves described here may comprise advancing a shock wave device into a blood vessel. The shock wave device may comprise an axially extending elongate member. The elongate member may comprise a first electrode pair comprising a first electrode and a second electrode. The first electrode pair may be positioned within a conductive fluid. A series of individual pulses may be delivered to the first electrode pair to create shock waves in the conductive fluid to cause current to flow through the electrode pair in a first direction for some of the pulses in the series and in a second direction opposite the first direction for the remaining pulses in the series. In some variations, the current may flow in the second direction for between twenty five percent and fifty percent of the pulses in the series. 
     In some variations, the current may flow in the second direction for between one third and half of the pulses in the series. In other variations, the current may flow in the second direction for at least about half of the pulses in the series. In some variations, a voltage pulse width may be measured to monitor a condition of the shock wave device. In some of these variations, the percentage of pulses that cause current to flow in the second direction may be adjusted according to the measured voltage pulse width. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1D  are illustrative depictions of a variation of an electrode assembly.  FIGS. 1A and 1D  are block diagrams of variations of a controller coupled to an electrode assembly.  FIGS. 1B and 1C  are illustrative depictions of variations of a flattened electrode assembly. 
         FIGS. 2A-2B  are illustrative depictions of a variation of a series of electrode assemblies.  FIG. 2A  is a block diagram of a variation of a controller coupled to the series of electrode assemblies.  FIG. 2B  is an illustrative depiction of a variation of flattened electrode assemblies. 
         FIG. 3  is an illustrative block diagram of a variation of a shock wave system comprising electrode assemblies, a controller, and a voltage source. 
         FIG. 4  is an illustrative timing diagram of a variation of a shock wave system. 
         FIG. 5  is a perspective view of another variation of a shock wave device. 
         FIGS. 6A-6C  are illustrative depictions of another variation of an electrode assembly.  FIG. 6A  is a top view and  FIG. 6B  is a bottom view of a variation of the electrode assembly.  FIG. 6C  is a perspective view of a variation of a common electrode. 
         FIGS. 7A-7B  are illustrative depictions of a variation of a series of electrode assemblies.  FIG. 7A  is a top view and  FIG. 7B  is a bottom view of a variation of the series of electrode assemblies. 
         FIG. 8A  is an illustrative graph comparing the voltage drop as a function of pulse number between switching and non-switching energy pulses.  FIGS. 8B and 8C  are illustrative graphs comparing voltage drop as a function of pulse width for a non-switching electrode assembly and a switching electrode assembly, respectively. 
         FIG. 9A  is an illustrative graph of pulse number as a function of voltage polarity.  FIG. 9B  is an illustrative graph of energy delivered at the electrode assembly as a function of pulse number between switching and non-switching energy pulses. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein are devices and systems that comprise one or more shock wave electrodes that may be suitable for use in angioplasty and/or valvuloplasty procedures. Generally, shock wave electrodes are provided along an axially extending elongate member (e.g., catheter) and may be attached to a source of high voltage pulses, ranging from 0.1 kV to 10 kV for various pulse durations. In some variations, the electrodes may be surrounded by an enclosure carrying a conductive fluid (e.g., saline). In some variations, the enclosure may comprise a balloon surrounding a portion of the elongate member and configured to be filled with a conductive fluid, where the electrodes may be enclosed within and spaced from the balloon walls. 
     A controller may be coupled to the first electrode pair to deliver a series of energy pulses to produce shock waves. The shock waves generated may disrupt calcified obstructions in an artery or a valve. One mechanism for shock wave formation is described below. When a high voltage is applied across a pair of electrodes located within the conductive fluid, a plasma arc may form between them, giving rise to a steam bubble in the fluid. A first shock wave may occur when the steam bubble first forms and a second shock wave may occur as the steam bubble collapses. Shock waves may be mechanically conducted through the fluid to apply mechanical force or pressure to break apart any calcified plaques on, or in, the vasculature walls. 
     The size, rate of expansion, and collapse of the bubble (and therefore, the magnitude, duration, and distribution of the mechanical force) may vary based on the magnitude and duration of the voltage pulse. Furthermore, the timing and size of the bubble, along with the sonic output and propagation direction of the resultant shock waves, depend at least in part on the location, geometry, size, condition, and distances between the electrodes (electrode gap distance). For example, an increase in electrode gap distance decreases the corresponding sonic output. The size and arrangement of the electrodes may also impact the types of vascular structures that may be accessed and treated by a shock wave device. Shock wave electrodes may be made of materials that can withstand high voltage levels and intense mechanical forces (e.g., about 300-3000 psi or 20-200 ATM in a few microseconds) that may be generated during use. For example, the electrodes may be made of stainless steel, tungsten, nickel, iron, steel, and the like. 
     Generally, current flowing between a pair of electrodes in a conductive fluid causes movement of metal from the positive terminal to the negative terminal, eventually depleting the positive terminal electrode of material, and may be referred to as one-sided erosion when a direction of current flow is fixed. Shock wave electrodes may experience a high rate of wear and erosion with every pulse applied due to the necessarily high current (e.g., hundreds of amps) flowing through the fluid, heat generated by the plasma arc, and mechanical shock wave forces. 
     The devices, systems, and methods described herein may help to reduce the rate of electrode wear to enhance electrode durability and shock wave consistency. The longevity of an electrode pair may depend on at least one of the following: polarity of a voltage pulse, length of a voltage pulse, magnitude of a voltage pulse, material properties, fluid conductivity, electrode gap distance between conductive regions of each of the electrodes in an electrode pair, and/or the surface area of the conductive region(s) of the electrodes in the pair. A longer pulse may increase the wear/erosion of an electrode pair as compared to a shorter pulse. In some variations where the electrode pair has different sized electrodes where one electrode has a smaller conductive region surface area than the other electrode, the electrode with the smaller conductive region may be more susceptible to erosion than the electrode with a larger conductive region. That is, the electrode with the smaller conductive region may erode at a greater rate than the electrode with the larger conductive region. The devices and methods described herein may help to even out the rate of erosion between the electrodes in an electrode pair, so that the electrodes erode at approximately the same rate. This may enhance the durability of the overall electrode pair, and may also facilitate uniform energy delivery to the electrode pair over a greater number of pulses. 
     It should be appreciated that a shock wave device generates the strongest shock waves when the electrodes are dissimilar in size where the smaller electrode has a positive polarity and the larger electrode has a negative polarity. Thus, while an electrode pair having a small, positive terminal electrode and a large, negative terminal electrode may form the strongest shock wave, this combination of size and polarity may shorten the lifetime of the electrode pair. The problem of a short lifespan may not be a simple matter of increasing electrode size since the size of a shock wave device and electrodes may be limited by the size of the vasculature through which it is advanced. However, voltage polarity switching, as described in further detail below, may facilitate electrode longevity while maintaining electrode size such that the electrode can be navigated through vasculature. Additionally or alternatively, voltage polarity switching may facilitate a reduction in electrode size with a similar electrode longevity relative to a non-polarity switching device. 
     Furthermore, the devices, systems, and methods described herein may facilitate the uniformity of shock wave intensity formed at different sites along a shock wave device. In some variations, a shock wave device may comprise a plurality of spaced apart electrode pairs connected in series. The shock waves generated by the electrode pairs may vary in strength even if the size and shape of the electrode pairs are the same. For instance, identical electrode pairs in series positioned 180 degrees apart from each other may form shock waves of varying strength from opposing sides of the device. This difference may be negligible for any single pulse. However, over a series of pulses, one side of the shock wave device may be more effective at cracking calcium deposits than the other side of the device. Voltage polarity switching, as described in further detail below, may facilitate the uniformity of shock wave intensity formed in different electrode pairs. 
     For example, a controller may cause current to flow through an electrode pair in a first direction for some pulses and in a second direction that is opposite to the first direction for other pulses. As an example, the direction of current flow may vary pulse to pulse or every second pulse, and is not particularly limited. It should be noted that the pulses are outputted discretely such that there is an interval of time between pulses when current does not flow through a shock wave device. The duration of the interval of time may be pre-selected according to, for example, a desired rate or frequency of shock wave generation. Furthermore, each pulse has a single direction of current flow and does not switch within the pulse. For instance, a voltage polarity switch may switch only when current is not flowing to the shock wave device (i.e., the voltage polarity switch may only occur in the interval between voltage pulses, and not during a voltage pulse). 
     Furthermore, the direction of current flow may vary randomly for each pulse so long as the total number of pulses maintain a predetermined current flow direction ratio. For example, for a set of 50 pulses being split evenly in the first direction and the second direction, the direction of current flow need not switch every pulse. As an illustrative example, 20 pulses in the first direction may be followed by 10 pulses in the second direction, then 3 pulses in the first direction, 15 pulses in the second direction, and 2 pulses in the first direction. Accordingly, while the total number of pulses is split evenly between the first and second direction, the number of switches in current flow does not necessarily correspond to the current flow ratio direction. 
     In some variations, a single pulse may be provided in the second direction with the remaining pulses in the first direction, and vice-versa. This allows the electrode pair to produce shock waves with a greater number of pulses before failure relative to an electrode pair receiving pulses with a constant polarity. Durability may thus be improved by distributing the electrode wear of the positive pulse over both electrodes. 
     In one variation, the current may flow in a first direction for about half of the pulses in the series and in the second, opposite direction for about the other half of the pulses in the series. In doing so, each electrode is set to be the positive terminal for about half of the pulses, thereby distributing the number of high erosion positive pulses experienced by any one of the electrodes about equally between the electrodes in the pair. The direction of current flow may be switched one or more times. In some cases, the electrode pair longevity may be about doubled relative to electrodes having a single direction of current flow, allowing more shock waves to be formed and/or the electrode pair to be formed smaller relative to electrodes having a single direction of current flow. Therefore, the shock wave devices described herein may be particularly useful in small arteries such as coronary arteries. Moreover, a shock wave device comprising a plurality of electrode pairs having pulses with current flow in both directions may facilitate the uniformity of shock wave intensity generated by the electrode pairs. 
     I. Devices 
     Generally described here are shock wave devices for angioplasty and/or valvuloplasty procedures. The devices and methods described here may use one or more devices or elements described in U.S. Pat. No. 8,888,788 and titled “LOW PROFILE ELECTRODES FOR AN ANGIOPLASTY SHOCK WAVE CATHETER,” and/or one or more devices or elements described in U.S. Pat. No. 9,011,463 and titled “SHOCK WAVE BALLOON CATHETER WITH MULTIPLE SHOCK WAVE SOURCES,” each of which is hereby incorporated by reference in its entirety. 
       FIG. 1A  is a block diagram of a controller  120  coupled to an electrode assembly  100 . Electrode assembly  100  may comprise a first electrode  102 , a second electrode  104 , and a third electrode  106 . The first electrode  102  may be connected to a first voltage output terminal V 01  of a voltage source of the controller  120  by first wire  108 , the third electrode  106  may be connected to a second voltage output terminal V 02  of a voltage source of the controller  120  by a second wire  110 , and the second or common electrode  104  may be provided in series between the first electrode  102  and third electrode  106 . Upon application of a sufficient voltage pulse, a first plasma arc may form between the first electrode  102  and the second electrode  104  (i.e., a first electrode pair), and a second plasma arc may form between the second electrode  104  and the third electrode  106  (i.e., a second electrode pair). The first and second electrode pairs are connected in series, where the second electrode  104  is shared between the first and second electrode pairs. Although electrode assembly  100  is described above as comprising three electrodes that form two electrode pairs, some variations of an electrode assembly may comprise two electrodes that form one electrode pair. 
     A first direction of current flow  112  of an energy pulse may be delivered to the electrode assembly  100  by a voltage source  122  of the controller  120 . The controller  120  may cause other pulses delivered to the electrode assembly  100  to have a second direction  116  of current flow that is the opposite direction of the first direction  112 . The controller  120  may select a direction of current flow, and thus the voltage polarity of the electrodes, for each pulse delivered to the electrode assembly  100 . In order to select a direction of current flow, the controller  120  may comprise a voltage polarity switch  124  to switch a polarity of the electrodes  102 ,  106  between positive and negative where the electrodes  102 ,  106  have opposite polarities. 
     In some variations, for a series of pulses, the controller may cause current to flow through the electrode pair in a first direction for some of the pulses in the series and in a second direction opposite the first direction for the remaining pulses in the series. In one variation, a first direction of current flow may be provided for at least one of the pulses. In another variation, a first direction of current flow may be provided for at least about 5% of the pulses. In another variation, a first direction of current flow may be provided for at least about 10% of the pulses. In another variation, a first direction of current flow may be provided for at least about 15% of the pulses. In another variation, a first direction of current flow may be provided for at least about 20% of the pulses. In another variation, a first direction of current flow may be provided for at least about 25% of the pulses. In another variation, a first direction of current flow may be provided for at least about 30% of the pulses. In yet another variation, a first direction of current flow may be provided for at least about a third of the pulses. In another variation, a first direction of current flow may be provided for at least about 40% of the pulses. In another variation, a first direction of current flow may be provided for at least about 45% of the pulses. In still another variation, a first direction of current flow may be provided for at least about half of the pulses. 
     In still other variations, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:6. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 5:6. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 3:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 5:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 7:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 2:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 4:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 5:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 7:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 8:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:12. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:16. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:32. 
     It should be appreciated that these examples are non-limiting. For example, the controller may provide current delivery in a first direction for every pulse except for one pulse provided in the second direction, and vice versa. The number of pulses in each direction of current flow (e.g., ratio of current flow of first direction to second direction) may be determined based on a desired longevity of the shock wave device, shock wave uniformity, shock wave energy, and so forth. In some variations, a first direction of current flow provided for about half of the pulses may maximize the longevity of the shock wave device. 
     The number of transitions between current flow directions is not particularly limited. In some instances, the direction of current flow may be switched according to the ratio of pulses in the first direction to the second direction. For example, the direction of current flow may transition every pulse when there are an equal number of positive and negative pulses. However, the direction of current flow may also vary randomly for each pulse so long as the total number of pulses maintains a predetermined ratio of current flow direction. Accordingly, the direction of current flow need not switch for every pulse even if the number of pulses in each direction is equal. As another example, alternating on average the direction of current flow of the pulses may about double the durability of the smaller electrodes, and thus the lifetime of the electrode assembly. Even when the electrodes in an electrode pair are of equal size, alternating the direction of current flow so that each electrode receives about the same number of positive pulses will distribute the wear over two electrodes so as to about double the durability of the electrode pair relative to electrodes receiving a single direction of current. It should be noted that polarity switching of any number of pulses aids durability (e.g., the number of shock waves the electrode generates before electrode failure). 
     Moreover, shock waves output from different electrode pairs may facilitate uniformity of shock wave forces between the shock wave sites on average as polarity switching allows each electrode pair to receive the positive pulse. This allows more predictable results with a higher average shock wave strength delivered at each shock wave site. For example,  FIG. 9B  is an illustrative graph of energy delivered as a function of pulse number for a static (constant polarity) shock wave device and a switching (alternating polarity) shock wave device. In  FIG. 9B , the electrical energy delivered by the polarity-switching shock wave device is higher on average per pulse and decays less than the energy delivered to the constant polarity device. The electrical energy delivered may be positively correlated with shock wave strength. 
     Next, the second electrode  104  illustrated in  FIGS. 1B and 1C  may have a cylindrical or ring shape, similar to that depicted in  FIG. 6C  as discussed in further detail below. However, for the ease of explanation,  FIGS. 1B and 1C  depict a flattened second electrode  104  to illustrate the different voltage polarities that may be applied to the electrode assembly  100 . In  FIG. 1B , the controller  120  may output one or more positive pulses in a first direction  112  of current flow where the first wire  108  is coupled to a positive terminal of a voltage source  122  of the controller  120  and the first electrode  102 , and the second wire  110  is coupled to a negative terminal of a voltage source of the controller  120  and the third electrode  106 . In use, the application of the voltage pulse creates a plasma in the fluid that extends across the electrode pairs and permits conduction of the current. The current then flows from the first electrode  102  to second electrode  104 , and then to third electrode  106 . Plasma formation thus creates two electrode pairs connected in series. As discussed above, the positive terminal first electrode  102  may experience a higher rate of wear than the negative terminal third electrode  106  when receiving a positive pulse from the controller  120 . 
     Conversely, in  FIG. 1C , a negative terminal first electrode  102  may deplete less material than the positive terminal third electrode  106  when receiving a negative pulse in a second direction  116  of current flow from a voltage source  122  of the controller  120 . In order to distribute the wear between the first electrode  102  and third electrode  106  more evenly, the controller  120  may cause a current to flow in a first direction  112  for some of the pulses ( FIG. 1B ) and in a second direction  116  opposite the first direction  112  for the other pulses ( FIG. 1C ). As a consequence, the electrode assembly  100  may form a greater number of shock waves with improved consistency before one or both of the smaller electrodes ( 102 ,  106 ) are depleted and the electrode assembly  100  fails. 
     Furthermore, as shown in  FIGS. 1B and 1C , the first electrode  102  and the third electrode  106  will have opposite voltage polarities no matter the direction of current flow. Therefore, the strength of the shock waves formed by the first electrode pair and the second electrode pair will differ for every pulse. In the illustrated embodiment, the conductive region of the first electrode  102  and the third electrode  106  may be smaller than the conductive region of the second electrode  104 . Accordingly, the first electrode pair receiving the positive pulse  112  ( FIG. 1B ) may generate a stronger shock wave than the second electrode pair. Similarly, the first electrode pair receiving the negative pulse  116  may generate a weaker shock wave than the second electrode pair. 
     However, by alternating positive and negative pulses to the electrode assembly  100 , the average shock wave strength output by the first electrode pair and the second electrode pair may be more uniform to reduce variability. This may provide more consistent and predictable treatment by the shock wave device such that a practitioner may not need to align the shock wave device in vasculature based on differences shock wave strength between electrode pairs. 
       FIG. 1D  is a block diagram of another variation of a controller  120  coupled to the electrode assembly  100 . The electrode assembly  100  may comprise a first electrode  102 , second electrode  104 , and a third electrode  106 . The first electrode  102  and second or common electrode  104  form a first electrode pair, and the third electrode  106  and the second electrode  104  form a second electrode pair. A first direction of current flow  112  of an energy pulse may be delivered to the electrode assembly  100  by a voltage source  122  of the controller  120 . The controller  120  may cause other pulses delivered to the electrode assembly  100  to have a second direction of current flow  116  opposite the first direction  112  through the electrode assembly  100 . The voltage polarity switch  124  of the controller  120  may select a direction of current flow, and thus the voltage polarity of the electrodes, for each pulse delivered to the electrode assembly  100 . 
     In  FIG. 1D , the first electrode  102  may be connected to a first voltage output terminal V 01  of a voltage source  122  of the controller  120  by first wire  108 , the third electrode  106  may be connected to a second voltage output terminal V 02  of a voltage source  122  of the controller  120  by a second wire  110 , and the second electrode  104  may be connected to a third voltage output terminal V 03  (ground channel) of a voltage source  122  of the controller  120  by a third wire  114 . In some variations, the first voltage output terminal V 01  and the second voltage output terminal V 02  may be positive channels while the third voltage output terminal V 03  may be a negative channel for some of the pulses. The controller  120  may also set the first voltage output terminal V 01  and the second voltage output terminal V 02  to be negative channels while the third voltage output terminal V 03  may be a positive channel for the remaining pulses. 
     During a high voltage pulse on the first and/or second voltage output terminals V 01 , V 02 , current may flow in the first direction  112  or the second direction  116  over the first wire  108  and/or second wire  110  to respective first electrode  102  and third electrode  106 . The voltage source  122  of controller  120  may apply a positive or negative pulse on output terminal V 01  such that the potential difference between the first electrode  102  and the second electrode  104  is large enough to form a plasma arc between them, generating a bubble that gives rise to a shock wave. Similarly, the voltage source of the controller  120  may simultaneously or sequentially apply a positive or negative energy pulse on output terminal V 02  such that the potential difference between the third electrode  106  and the second electrode  104  is large enough to form a plasma arc between them, generating a bubble that gives rise to a different shock wave. In some variations, when energy pulses are applied to output terminals V 01  and V 02  simultaneously, a first shock wave formed between the first electrode  102  and the second electrode  104  and a second shock wave formed between the third electrode  106  and the second electrode  104  may be formed simultaneously. 
     Where the first electrode  102  and third electrode  106  are located circumferentially opposite to each other (e.g., 180 degrees apart from each other around the circumference of the elongate member), the shock waves generated by the first and second electrodes pairs may propagate in opposite directions, extending outward from the sides of a shock wave device. The current that traverses the bubble from the first electrode  102  and/or the third electrode  106  to the second electrode  104  may return via third wire  114  to voltage output terminal V 03  (which may be a ground channel). Voltage output terminals V 01  and V 02  may be independently addressed (e.g., voltage and current may be applied to one output but not necessarily the other), or may not be independently addressed (e.g., activating one output necessarily activates the other). 
     In another variation,  FIG. 2A  is a block diagram of a controller  220  coupled to the first and second electrode assemblies  200 ,  250 . The first electrode  202  and the first common electrode  206  form a first electrode pair that may generate a first shock wave, and the second electrode  204  and the first common electrode  206  form a second electrode pair that may generate a second shock wave. Likewise, the third electrode  252  and the second common electrode  256  form a third electrode pair that may generate a third shock wave, and the fourth electrode  254  and the second common electrode  256  form a fourth electrode pair that may generate a fourth shock wave. 
     The first, second, third, and fourth electrode pairs may be connected in a series configuration and receive a series of pulses. A first direction of current flow  214  of some of the pulses in the series may be delivered to the first and second electrode assemblies  200 ,  250  by a voltage source  222  of the controller  220 . The controller  220  may cause the remaining pulses in the series that are delivered to the first and second electrode assemblies  200 ,  250  to have a second direction  216  of current flow through the electrode assemblies  200 ,  250 . A voltage polarity switch  224  of the controller  220  may select a direction of current flow, and thus the voltage polarity of the electrodes, for each pulse delivered to the electrode assemblies  200 ,  250 . For instance, the voltage polarity switch  224  may switch a polarity of the first electrode  202  and fourth electrode  254  between positive and negative, where the first electrode  202  and fourth electrode  254  have opposite polarities. 
     In some variations, for a series of pulses, the controller may cause current to flow through the electrode pair in a first direction for some of the pulses in the series and in a second direction opposite the first direction for the remaining pulses in the series. In one variation, a first direction of current flow may be provided for at least one of the pulses. In another variation, a first direction of current flow may be provided for at least about 5% of the pulses. In another variation, a first direction of current flow may be provided for at least about 10% of the pulses. In another variation, a first direction of current flow may be provided for at least about 15% of the pulses. In another variation, a first direction of current flow may be provided for at least about 20% of the pulses. In another variation, a first direction of current flow may be provided for at least about 25% of the pulses. In another variation, a first direction of current flow may be provided for at least about 30% of the pulses. In yet another variation, a first direction of current flow may be provided for at least about a third of the pulses. In another variation, a first direction of current flow may be provided for at least about 40% of the pulses. In another variation, a first direction of current flow may be provided for at least about 45% of the pulses. In still another variation, a first direction of current flow may be provided for at least about half of the pulses. 
     In still other variations, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:6. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 5:6. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 3:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 5:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 7:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 2:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 4:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 5:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 7:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 8:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:12. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:16. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:32. 
     In some variations, the number of pulses in each direction of current flow (e.g., ratio of current flow of first direction to second direction) may be determined based on a desired longevity of the shock wave device, shock wave uniformity, shock wave energy, material properties, electrode gap distance, fluid conductivity, and so forth. In some instances, the direction of current flow may be switched according to the ratio of pulses in the first direction to the second direction. In other instances, the direction of current flow may vary randomly for each pulse so long as the total number of pulses maintains a predetermined ratio of current flow direction. 
     Furthermore, shock waves output from the first through fourth electrode pairs may have more uniform strength on average as polarity switching allows each electrode pair to receive positive pulses. This allows more predictable results, with a greater amount of electrical energy delivered to each electrode pair, which may facilitate the generation of stronger shock waves. Thus, a shock wave device may be able to more uniformly apply mechanical forces/pressures regardless of its orientation within the vasculature. 
     The first and second common electrodes  206 ,  256  illustrated in  FIG. 2A  may in some variations have a cylindrical or ring shape, similar to that depicted in  FIG. 6C  as discussed in further detail below. However, for the ease of explanation,  FIG. 2B  depicts flattened first and second common electrodes  206 ,  256  to illustrate the different voltage polarities that may be applied to the first and second electrode assemblies  200 ,  250 . A voltage source  222  of the controller  220  may output one or more pulses where the first wire  208  is coupled to a positive terminal V 01  of controller  220  and the third wire  212  is coupled to a negative terminal V 02  of controller  220 . The second electrode  204  may be connected to the third electrode  252  via a second wire  210  (e.g., an interconnect wire). In this configuration, the first and second electrode assemblies  200 ,  250  receive a positive pulse where the first and third electrode pairs generate stronger shock waves than the second and fourth electrode pairs. 
     Conversely, a voltage source  222  of the controller  220  may output one or more pulses where the first wire  208  is coupled to a negative terminal V 01  of controller  220  and the third wire  212  is coupled to a positive terminal V 02  of controller  220 . In this configuration, the first and second electrode assemblies  200 ,  250  receive a negative pulse where the second and fourth electrode pairs generate the stronger shock waves. Therefore, in order to distribute the wear between the electrodes of the first and second electrode assemblies  200 ,  250  more evenly, the voltage polarity switch  224  of the controller  220  may cause a current to flow in a first direction for some of the pulses in a series of pulses and in a second direction opposite the first direction for the remaining pulses in the series. As a consequence, the electrode assemblies  200 ,  250  may form a greater number of shock waves before one or more of the smaller inner electrodes ( 202 ,  204 ,  252 ,  254 ) are depleted, as well as more uniform shock waves propagated on average from the electrode assemblies ( 200 ,  250 ). 
       FIG. 3  is an illustrative block diagram of a variation of a shock wave system  300  comprising a first electrode assembly  302 , a second electrode assembly  303 , a third electrode assembly  304 , fourth electrode assembly  306 , and fifth electrode assembly  307 . The first electrode assembly  302  may comprise a first electrode  302   a , second electrode  302   b , and a third electrode  302   c  having a structure analogous to first electrode  102 , second electrode  104 , and third electrode  106 , respectively, as depicted in  FIGS. 1A and 1B . As denoted symbolically in  FIG. 3 , the conductive surface areas of the first electrode  302   a  and third electrode  302   c  may be smaller relative to the conductive surface areas of the second electrode  302   b . In other variations, the larger electrode  302   b  may comprise individual electrodes connected by, for example, an interconnect wire. The second through fifth electrode assemblies  303 ,  304 ,  306 , and  307  may comprise a similar configuration of electrodes as first electrode assembly  302 . 
     The first and second electrode assemblies  302  and  303  are connected in series. The fourth and fifth electrode assemblies  306 ,  307  are connected in series. As shown in  FIG. 3 , the electrode assemblies  302 ,  303 ,  304 ,  306 , and  307  are switchably connected in parallel to a controller  310 . The controller  310  may comprise a voltage source  312  to deliver pulses to the electrode assemblies  302 ,  303 ,  304 ,  306 , and  307 . A multiplexer  316  of the controller  310  may selectively activate first and second electrode assemblies  302  and  303 , third electrode assembly  304 , and fourth and fifth electrode assemblies  306  and  307 . The multiplexer  316  may be configured to selectively connect the voltage source  312  across the parallel electrode assembly lines individually, one at a time, or in any combination. The controller  310  may further comprise a voltage polarity switch  314  configured to provide a first direction of current flow corresponding to a first switch position  318  and a second direction of current flow opposite the first direction, the second direction corresponding to a second switch position  320 . 
     For example, the voltage source  312  outputs a predetermined voltage pulse to the voltage polarity switch  314 . In the switch  314 , a direction of current flow is selected between a first direction of current flow and a second direction opposite the first direction. The multiplexer  316  may receive the energy pulse in either the first direction or the second direction, and then selectively deliver a series of pulses, having the current flow direction selected by the voltage polarity switch  314 , to the electrode assemblies  302 ,  303 ,  304 ,  306 , and  307  as illustrated in the timing diagram of  FIG. 4 . 
       FIG. 4  is an illustrative timing diagram of a variation of a shock wave system  300  for selectively coupling electrodes to a power source with a selectively delivered direction of current. For example, the controller  310  may activate the different sets of electrode assemblies sequentially (e.g., one at a time) at a first voltage polarity and then activate the different sets of electrode assemblies sequentially at a second voltage polarity. This reserves all of the high voltage for each shock wave source to thus form shock waves of maximum strength to be applied to the calcium deposits all along the vasculature. Reversing the polarity in the subsequent pulse for each of the electrode assemblies  302 ,  303 ,  304 ,  306 , and  307  may distribute electrode wear and shock wave strength more evenly within the electrode pairs of the electrode assemblies, thus increasing the longevity and consistency of the shock wave device. In other examples, the voltage polarity of an electrode assembly may vary pulse to pulse, every second pulse, every third pulse, and is not particularly limited. Furthermore, the voltage polarity may vary randomly. For example, for a set of 50 pulses being split evenly between a first voltage polarity (e.g., positive pulse) and a second voltage polarity (e.g., negative pulse), the voltage polarity need not switch every pulse. As an illustrative example, 8 pulses at the first voltage polarity may be followed by 5 pulses at the second voltage polarity, then 7 pulses at the first voltage polarity, 5 pulses at the second voltage polarity, 10 pulses at the first voltage polarity, and 15 pulses at the second voltage polarity. Accordingly, while the total number of pulses is split evenly between the first and second voltage polarities, the number of switches in polarity does not necessarily depend on the current flow ratio. 
     In some variations, a multiplexer may be coupled to one or more of the electrode assemblies  302 ,  303 ,  304 ,  306 , and  307 , as depicted in  FIG. 3 . For example, any of the voltage polarity switching sequences discussed herein may be incorporated with the multiplexer  316 . In some variations, the selection of voltage polarity may be independent of the electrode assembly selected by a multiplexer. Alternating polarity and timing may provide the dual benefits of distributing positive pulse wear over multiple electrodes and increasing rest time for the electrode assemblies. 
     The polarity switching and multiplexing described above may be applied to any of the shock wave devices described herein, including the illustrative variations of  FIGS. 5-7B  as described in detail below. In one variation, a shockwave device having a plurality of electrode assemblies is described. In particular,  FIG. 5  depicts the distal portions of a shock wave device having two electrode assemblies  506 ,  508 . In particular,  FIG. 5  depicts one variation of a shock wave device  500  comprising an elongate member  502 , a first electrode assembly  506  at a first location along the length of the elongate member  502 , a second electrode assembly  508  at a second location along the length of the elongate member  502 , and optionally, an enclosure  504  configured to be fillable with a conductive fluid to sealably enclose the first and second electrode assemblies  506 ,  508 . In some variations, the enclosure may comprise a membrane and/or a balloon may be made of an electrically insulating material that may be non-compliant (e.g., PET, etc.), semi-compliant (e.g., PBAX, nylon, PEBA, polyethylene, etc.), and/or compliant (e.g., polyurethane, silicone, etc.). The enclosure  504  may enclose any number of electrode assemblies. 
     The shock wave device  500  may comprise a fluid lumen (not shown) that is in communication with a fluid source that introduces a conductive fluid into the enclosure  506 . A voltage source (not shown) having a first terminal  510  and a second terminal  512  may be coupled to the shock wave device  500 . As discussed above, the polarity of the terminals  510 ,  512  may vary per pulse or in a predetermined sequence. An energy pulse may be applied to the electrode pairs  506 ,  508 , thereby generating one or more shock waves that may propagate through the fluid to impinge on a calcified obstruction. Although the shock wave device  500  in  FIG. 5  is depicted as having two electrode pairs  506 ,  508 , it should be understood that other variations may have a different number of electrode pairs (e.g., 3, 4, 5, 6 or more electrode pairs). 
     In some variations, the electrode assemblies  506 ,  508  each may comprise two inner electrodes that are positioned circumferentially opposite each other, an insulating sheath with two openings aligned over the two inner electrodes, and an outer common electrode with two openings that are coaxially aligned with the two openings of the insulating sheath.  FIGS. 6A-6C  illustrate one variation of an electrode assembly in this configuration including two inner electrodes and an outer common electrode. Each of the electrode assemblies  506 ,  508  may be configured to generate a pair of directed shock waves, where the shock waves resulting from a high voltage pulse to the first inner electrode propagate in a direction that is opposite to the direction of the shock waves resulting from a high voltage pulse to the second inner electrode. In some variations, the electrode assemblies  506 ,  508  may generate shock waves that propagate outward from different locations around the circumference of elongate member  502 . For example, the electrode assembly  506  may generate shock waves that propagate from the left and right longitudinal side of the elongate member, while the electrode assembly  508  may generate shock waves that propagate from the top and bottom longitudinal side of the elongate member  502 . 
     In other variations, the electrode assembly  506  may generate a pair of shock waves that propagate outward from positions at 0 degrees and 180 degrees around the circumference of the elongate member  502 , while the electrode assembly  508  may generate a pair of shock waves that propagate outward from positions at 60 degrees and 240 degrees around the circumference of the elongate member. In still other variations, electrode assemblies  506 ,  508  may each generate a pair of shock waves that propagate outward at the same locations around the circumference of the elongate member, but from different locations along the length of the elongate member. Optionally, one or more radiopaque marker bands may be provided along the length of the elongate member to allow a practitioner to identify the location and/or orientation of the shock wave device  500  as it is inserted through the vasculature of a patient. 
     It should be appreciated that shock wave devices with a plurality of electrode assemblies distributed along the length of a catheter may be suitable for use in angioplasty procedures to break up calcified plaques that may be located along a length of a vessel. Shock wave devices with a plurality of electrode assemblies along the length of a curved elongate member may be suitable for use in valvuloplasty procedures to break up calcified plaques that may be located around the circumference of a valve (e.g., at or around the leaflets of a valve). It should be noted that the circuit diagram of  FIG. 2A  and the simplified layout of  FIG. 2B  correspond electrically to  FIG. 5  embodiment, when the electrode assemblies  506 ,  508  each include two electrode pairs as shown in  FIG. 6 . 
       FIGS. 6A-6B  depict top and bottom views, respectively, of one variation of a shock wave device having an electrode assembly  600  that may be configured to generate shock waves in opposite directions.  FIG. 6A  is a top view of the electrode assembly  600  where the first inner electrode  602  is depicted and  FIG. 6B  is a bottom view of the electrode assembly  600  where the second inner electrode  604  is depicted. The first and second inner electrodes share a common electrode  606  and are located circumferentially opposite each other (i.e., 180 degrees apart). The first electrode  602  may be connected to a first voltage output terminal V 01  of a voltage source of a controller (not shown in  FIGS. 6A-6B ) by a first wire  608  and the second electrode  604  may be connected to a second voltage output terminal V 02  of a voltage source of the controller by a second wire  610 . The first electrode  602  and the common electrode  606  form a first electrode pair that may generate a first shock wave that propagates outwards in a first shock wave direction, and the second electrode  604  and the common electrode  606  form a second electrode pair that may generate a second shock wave that propagates outwards in a second shock wave direction that is opposite to the first shock wave direction. For a positive pulse provided in a first current direction  616 , current flows from the first electrode pair to the second electrode pair. Likewise, for a negative pulse provided in a second current direction  618  opposite the first current direction  616 , current flows from the second electrode pair to the first electrode pair. 
     A difference in surface area of conductive regions within an electrode pair may be provided to generate larger shock waves. For instance, the surface area of the edges of the first electrode  602  and the second electrode  604  may serve as conductive regions, and may be the portions of the electrodes which are most likely to wear due to high energy pulses. An electrode such as the common electrode  606  may form two conductive regions for each of the first and second electrodes  602 ,  604  having different surface areas. In some variations, a surface area of a conductive region of the first electrode  602  and second electrode  604  may be smaller in surface area relative to the common electrode  606 . Therefore, the longevity of the electrode assembly  600  may depend on the rate of depletion of the smaller electrodes  602 ,  604 . 
     As energy pulses are applied to the electrodes pairs to form shock waves, erosion of electrode material from the inner and outer electrodes may increase a distance between the electrodes in the electrode pair until a plasma arc is no longer to likely form. At this point, the electrode pair has failed and reached the end of its lifespan. As discussed in further detail below, the degree of erosion and wear may be determined by measuring one or more of voltage drop, voltage pulse width, low voltage analogs of voltage pulse width, and/or any signals that indicate (or are correlated with) the duration of a high-voltage pulse across an electrode pair. 
       FIG. 6C  depicts a perspective view of the outer common electrode  606 . As depicted there, first opening  612  may be located directly across from second opening  614 . The outer common electrode  606  may have the second opening  614  coaxially aligned over the second inner electrode, and the first inner electrode  604  may be coaxially aligned with the first opening  612  of the outer common electrode  606 . This configuration may generate a first shock wave that propagates outwards in a first direction and a second shock wave that propagates outwards in a second direction that is opposite to the first direction. 
     Turning back to  FIGS. 6A and 6B , the wires  608 ,  610  may be electrically insulated along their length (e.g., by an insulating coating or sheath made of, for example, polyimide, PEBA, PET, FEP, PTFE, etc.) except for one or more regions where the electrically conductive core of the wire is exposed to contact a portion of the inner and/or outer electrode. The wires  608 ,  610  may be made of any conductive material, for example, free oxygen copper or copper or silver. 
       FIGS. 7A-7B  depict top and bottom views of one variation of a shock wave device having a first electrode assembly  700  and a second electrode assembly  750  that may be configured to generate shock waves along a length of the shock wave device. The electrode assemblies  700 ,  750  may be connected in series with respect to each other such that activating a first electrode assembly  700  also activates a second electrode assembly  750 . In some variations, it may be desirable to have multiple shock wave sources without as many wires and using fewer terminals on a controller. For example, connecting two electrode assemblies in series may allow the shock wave device to simultaneously generate up to four different shock waves using just two voltage output terminals (e.g., one positive channel and one negative channel). In addition, reducing the number of wires that extend along the length of the elongate member may allow the elongate member to bend and flex more freely as it is advanced through the vasculature of a patient (e.g., may allow for a tighter turning radius). 
       FIG. 7A  is a top view of the electrode assemblies  700 ,  750  where the first inner electrode  706  and the fourth inner electrode  754  are depicted.  FIG. 7B  is a bottom view of the electrode assemblies  700 ,  750  where the second inner electrode  704  and the third inner electrode  752  are depicted. The first and second inner electrodes  702 ,  704  share a first common electrode  706  and are located circumferentially opposite each other (i.e., 180 degrees apart). The third and fourth inner electrodes  752 ,  754  share a second common electrode  756  and are also located circumferentially opposite each other. Alternatively, the inner electrodes and electrode assemblies may be offset from each other in some other manner as described above. 
     The first electrode  702  and the first common electrode  706  form a first electrode pair that may generate a first shock wave that propagates outwards in a first direction, and the second electrode  704  and the first common electrode  706  form a second electrode pair that may generate a second shock wave that propagates outwards in a second direction. Likewise, the third electrode  752  and the second common electrode  756  form a third electrode pair that may generate a third shock wave that propagates outwards in a third direction, and the fourth electrode  754  and the second common electrode  756  form a fourth electrode pair that may generate a fourth shock wave that propagates outwards in a fourth direction. 
     The first and second electrode assemblies  700 ,  750  in  FIGS. 7A-7B  may be connected in series. The first electrode  702  may be connected to a first voltage output terminal V 01  of a voltage source of a controller (not shown in  FIGS. 7A-7B ) by a first wire  708 . The second electrode  704  may be connected to the third electrode  752  via a second wire  710  (e.g., an interconnect wire). The fourth electrode  754  may be connected to a second voltage output terminal V 02  of the voltage source of the controller by a third wire  712 . Therefore, for a positive pulse provided in a first current direction  714 , current flows (in ascending order) from the first electrode pair to the fourth electrode pair. Likewise, for a negative pulse provided in a second current direction  716  opposite the first current direction  714 , current flows (in descending order) from the fourth electrode pair to the first electrode pair. Each of the first through fourth electrodes  702 ,  704 ,  752 ,  754  may be smaller in size relative to the first and second common electrodes  706 ,  756 . In some variations, size may refer to the total size of the electrode and/or a surface area of a conductive region of the electrode. Therefore, the longevity of the electrode assemblies  700 ,  750  may depend on the rate of depletion of the smaller electrodes  702 ,  704 ,  752 ,  754 . 
     Any of the shock wave devices described herein may be provided in a shock wave system suitable for use in an angioplasty or valvuloplasty procedure. A shock wave system (not shown) may include a shock wave device, catheter, a high voltage pulse generator (e.g., voltage source), and/or an enclosure configured to be fillable with a conductive fluid. The catheter may have a guide wire lumen therethrough. In some variations, the high voltage pulse generator may be a 0.1 kV to 10 kV pulsed power supply, for example, a 2 kV to 6 kV pulsed supply. 
     II. Methods 
     Generally described here are methods for forming shock waves. Any of the shock wave devices described herein may be used in an angioplasty and/or valvuloplasty procedure. The methods described here may include advancing a guide wire from an entry site on a patient (e.g., an artery in the groin area of the leg) to a target region of a vessel (e.g., a region having calcified plaques that need to be broken up. A shock wave device may comprise an axially extending elongate member with a guide wire lumen, an electrode pair comprising a first electrode and a second electrode and/or an electrode assembly provided along a length of the elongate member. The electrode pair and/or electrode assembly may be any of the electrodes described herein. 
     In some variations, a balloon may be collapsed over the elongate member while the device is advanced through the vasculature. In some variations, the location of the shock wave device may be determined by x-ray imaging and/or fluoroscopy. When the shock wave device reaches the target region, the balloon may be filled with a conductive fluid (e.g., saline and/or saline mixed with an image contrast agent). A series of pulses may be delivered to the first electrode pair and/or electrode assembly to create shock waves that may break up calcified plaque, for example. 
     In some variations, current flows through the electrode pair and/or electrode assembly in a first direction for some of the pulses in the series and in a second direction that is opposite the first direction for the remaining pulses in the series. In one variation, a first direction of current flow may be provided for at least one of the pulses. In another variation, a first direction of current flow may be provided for at least about 5% of the pulses. In another variation, a first direction of current flow may be provided for at least about 10% of the pulses. In another variation, a first direction of current flow may be provided for at least about 15% of the pulses. In another variation, a first direction of current flow may be provided for at least about 20% of the pulses. In another variation, a first direction of current flow may be provided for at least about 25% of the pulses. In another variation, a first direction of current flow may be provided for at least about 30% of the pulses. In yet another variation, a first direction of current flow may be provided for at least about a third of the pulses. In another variation, a first direction of current flow may be provided for at least about 40% of the pulses. In another variation, a first direction of current flow may be provided for at least about 45% of the pulses. In still another variation, a first direction of current flow may be provided for at least about half of the pulses. 
     In still other variations, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:6. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 5:6. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 3:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 5:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 7:8. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 2:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 4:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 5:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 7:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 8:9. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:12. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:16. In another variation, the ratio of current flow of the pulses in the first direction to the second direction may be about 1:32. The number of pulses in each direction of current flow (e.g., ratio of current flow of first direction to second direction) may be determined based on a desired longevity of the shock wave device, shock wave uniformity, shock wave energy, and so forth. 
     The number of transitions of current flow directions is not particularly limited. In some instances, the direction of current flow may be switched according to the ratio of pulses in the first direction to the second direction. In other instances, the direction of current flow may vary randomly for each pulse so long as the total number of pulses maintains a predetermined ratio of current flow direction. Accordingly, the current flow direction need not switch for every pulse even if the number of pulses in each direction is equal. Alternating on average the direction of current flow of the pulses may about double the durability of the smaller electrodes, and thus the lifetime of the electrode assembly. Even when the electrodes in an electrode pair are of equal size, alternating the direction of current flow so that each electrode receives about the same number of positive pulses will distribute the wear over two electrodes so as to about double the electrode pair durability. 
     Furthermore, shock waves output from different electrode pairs may have more uniform strength on average as polarity switching allows each electrode pair to receive the positive pulse. This allows more predictable shock waves with less variance in shock wave strength. 
     The progress of the plaque break-up may be monitored by x-ray and/or fluoroscopy. The shockwave device may be moved along the length of the vessel if the calcified region is longer than the length of the elongate member with the electrode assemblies, and/or if the calcified region is too far away from the electrode assemblies to receive the full force of the generated shock waves. For example, the shockwave device may be stepped along the length of a calcified vessel region to sequentially break up the plaque. 
     Voltage and current measurements may be taken at the load (in this case, the electrode assembly) to determine the condition of the shock wave device as the pulses are delivered. As discussed in further detail below, measurements including voltage drop, voltage pulse width, and current flow may be used to determine a condition/longevity of the shock wave device. 
     In some variations, polarity switching of the pulses may be based on the condition of the electrodes in a shock wave device. For example, the direction of current flow of pulses in a series may be determined using measurements correlated to voltage drop, such as voltage pulse width, which may be correlated to the condition of the electrodes. For instance, a first voltage pulse width across the electrode pair may be measured for a first direction of current flow and a second voltage pulse width across the electrode pair may be measured for a second direction of current flow. A difference (if any) between the first voltage pulse width and the second voltage pulse width may indicate a difference in electrode wear. In some cases, differences in the degree of electrode erosion between the electrodes in a pair may lead to a shorter lifespan of the overall shock wave device. By switching the direction of current flow based on the measured voltage pulse widths to balance the wear between the first and second electrodes, the lifetime of a shock wave device may be prolonged. 
     In some variations, the direction of current flow for pulses in a series (e.g., the polarity of each of the voltage pulses in a series) may be selected according to the measured voltage pulse widths. For instance, the direction of current flow may be selected such that a difference between the voltage pulse width across the electrode pair when the current flows in a first direction and the voltage pulse width when the current flows in a second direction are within a predetermined threshold. In other variations, the direction of current flow may be selected such that the voltage pulse width across the electrode pair when the current flows in a first direction is substantially equal to the voltage pulse width across the electrode pair when the current flows in a second direction. The ratio of current flow of the pulses in a first direction to a second (opposite) direction (i.e., polarity ratio) may be determined at least in part by the measured voltage pulse width. For example, if the measured voltage pulse width in the first direction of current flow (i.e., first polarity) meets or exceeds a predetermined threshold, the controller may determine that the first electrode of the electrode pair has a greater degree of erosion than the second electrode of the electrode pair. The controller may then adjust the polarity ratio so that the second direction of current flow (i.e., second polarity) is increased relative to the first direction of current flow. This may help reduce or stabilize the erosion rate of the first electrode so that the overall electrode pair may be more durable (e.g., longer lifespan) as compared to a shock wave system where the polarity is not switched and/or polarity ratio is not adjusted. 
     The electrode assemblies of the shock wave device may be connected in series, and may be activated simultaneously and/or sequentially, as described above. For example, a pair of electrode assemblies may be connected in series and activated simultaneously or sequentially. 
     Once the calcified region has been sufficiently treated, the balloon may be inflated further or deflated, and the shock wave device and guide wire may be withdrawn from the patient. Wear of the shock wave device may be evaluated through visual inspection by microscopy. 
     III. Examples 
     In the following examples, test results are provided for electrodes to compare electrical energy delivery to a set of electrodes receiving using a constant polarity (static device) for every pulse and alternating polarity as described above (switching device). Electrical current, voltage, energy measurements, and visual inspection as a function of pulses were taken of an 8 gap copper coil emitter to determine erosion, electrode integrity, and electrode lifetime. 
     After pulsing the devices, bright field microscopy evaluation was performed to visually inspect the wear of the electrodes. It was determined that the positive terminals in the static device had more consumed conductor than the negative terminals. The switching device had more even wear than the static device. In  FIGS. 8 and 9A-9B  discussed below, the shock wave devices were pulsed at 0.5 Hz or slower with a pulse width of about 6 μs. 
       FIG. 8A  is an illustrative graph of voltage drop (kV) as a function of pulse number. Voltage drop is defined as the difference between the highest and lowest voltage prior to current flow (representing voltage losses which are correlated to electrode wear). As shown in  FIG. 8A , voltage drop increases as a function of pulse number. However, static energy delivery has a much larger rate of voltage drop compared to switching energy delivery indicating more electrode wear.  FIG. 8A  illustrates the rate of voltage drop (slope a) having a linear best fit (R). From the linear best fits, the voltage losses represented by the voltage drop (kV) increase at a much faster rate  800  for static energy compared to the rate  802  for switching energy. It should be noted that the static shock wave device fails  804  at about 40 pulses before the switching shock wave device fails at about 70 pulses. 
     Although voltage drop is plotted in  FIG. 8A , voltage pulse width correlates strongly with voltage drop such that measurements of voltage pulse width may be used to monitor the lifetime and/or condition of a shock wave device from pulse to pulse. For example,  FIGS. 8B and 8C  are illustrative graphs comparing voltage drop as a function of pulse width for a non-switching electrode assembly and a switching electrode assembly, respectively. In  FIG. 8B , line  808  represents the high correlation fit between voltage drop and pulse width for a non-switching shock wave device. Similarly, line  810  illustrates a high correlation fit between voltage drop and pulse width for a polarity switching shock wave device. The width of each pulse (e.g., voltage, current) is defined herein as the first time and the last time the pulse reaches 20% of the maximum voltage peak value. Similar to voltage drop (kV), the voltage pulse width increases as a function of pulse number and is an indicator of electrode degradation. In other words, measurement of voltage pulse width may be used as a proxy for voltage drop to monitor the condition (e.g., degradation) of an electrode pair. That is, the longer the measured voltage pulse width, the more the electrode with the positive polarity has eroded or degraded. Alternatively, measurement of an internal signal that indicates the onset and offset of a high voltage pulse across an electrode pair may be used to monitor the condition of the electrodes. For example, the internal signal may be a trigger TTL output signal of the high voltage pulse generator. 
       FIG. 9A  is an illustrative graph of pulse number as a function of voltage polarity. As discussed above,  FIG. 9B  is an illustrative graph of energy delivered as a function of pulse number. Since more pulses may be provided to a switching device than a static device, the lifetime of the switching device may be about double that of the static device. In  FIG. 9B , it is apparent that the number of pulses and the average amount of energy delivered by the switching device is significantly more than the static device due to polarity switching. For instance, the static device provides about 40 pulses before static device failure  900  while the switching device provides about 70 pulses before switching device failure  902 . 
     The test results indicate that switching current flow direction may increase the lifetime and energy delivered of an 8 gap shock wave device over a constant current flow direction 8 gap shock wave device. Visual inspection also shows biased wear of the electrodes when a single direction of current flow is applied compared to alternating current flow. Experimental testing has shown that the voltage pulse width is directly proportional to energy voltage drop prior to current flow. Therefore, a static device has more voltage loss compared to a switching device. The voltage pulse width may be used as a metrology tool to monitor the lifetime and/or condition of a shock wave device from pulse to pulse. Thus, switching the direction of current flow may improve the efficiency and/or consistency of shock waves output by a shock wave device. 
     It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications, alterations and combinations can be made by those skilled in the art without departing from the scope and spirit of the invention. Any of the variations of the various shock wave devices disclosed herein can include features described by any other shock wave devices or combination of devices disclosed. Accordingly, it is not intended that the invention be limited, except as by the appended claims. For all of the variations described above, the steps of the methods need not be performed sequentially.