the present invention relates to methods and apparatus for renal neuromodulation and / or other neuromodulation . more particularly , the present invention relates to methods and apparatus for renal neuromodulation using a pulsed electric field to effectuate electroporation or electrofusion . as used herein , electroporation and electropermeabilization are methods of manipulating the cell membrane or intracellular apparatus . for example , short high - energy pulses cause pores to open in cell membranes . the extent of porosity in the cell membrane ( e . g ., size and number of pores ) and the duration of the pores ( e . g ., temporary or permanent ) are a function of the field strength , pulse width , duty cycle , field orientation , cell type and other parameters . in general , pores will generally close spontaneously upon termination of lower strength fields or shorter pulse widths ( herein defined as “ reversible electroporation ”). each cell type has a critical threshold above which pores do not close such that pore formation is no longer reversible ; this result is defined as “ irreversible electroporation ,” “ irreversible breakdown ” or “ irreversible damage .” at this point , the cell membrane ruptures and / or irreversible chemical imbalances caused by the high porosity occur . such high porosity can be the result of a single large hole and / or a plurality of smaller holes . certain types of electroporation energy parameters also appropriate for use in renal neuromodulation are high voltage pulses with a duration in the sub - microsecond range ( nanosecond pulsed electric fields , or nspef ) which may leave the cellular membrane intact , but alter the intracellular apparatus or function of the cell in ways which cause cell death or disruption . certain applications of nspef have been shown to cause cell death by inducing apoptosis , or programmed cell death , rather than acute cell death . also , the term “ comprising ” is used throughout to mean including at least the recited feature such that any greater number of the same feature and / or additional types features are not precluded . several embodiments of the present invention provide intravascular devices for inducing renal neuromodulation , such as a temporary change in target nerves that dissipates over time , continuous control over neural function , and / or denervation . the apparatus and methods described herein may utilize any suitable electrical signal or field parameters , e . g ., any electric field , that will achieve the desired neuromodulation ( e . g ., electroporative effect ). to better understand the structures of the intravascular devices and the methods of using these devices for neuromodulation , it is useful to understand the renal anatomy in humans . with reference now to fig1 , the human renal anatomy includes kidneys k that are supplied with oxygenated blood by renal arteries ra , which are connected to the heart by the abdominal aorta aa . deoxygenated blood flows from the kidneys to the heart via renal veins rv and the inferior vena cava ivc . fig2 illustrates a portion of the renal anatomy in greater detail . more specifically , the renal anatomy also includes renal nerves rn extending longitudinally along the lengthwise dimension l of renal artery ra generally within the adventitia of the artery . the renal artery ra has smooth muscle cells smc that surround the arterial circumference spiral around the angular axis θ of the artery , i . e ., around the circumference of the artery . the smooth muscle cells of the renal artery accordingly have a lengthwise or longer dimension extending transverse ( i . e ., non - parallel ) to the lengthwise dimension of the renal artery . the misalignment of the lengthwise dimensions of the renal nerves and the smooth muscle cells is defined as “ cellular misalignment .” referring to fig3 , the cellular misalignment of the renal nerves and the smooth muscle cells may be exploited to selectively affect renal nerve cells with reduced effect on smooth muscle cells . more specifically , because larger cells require less energy to exceed the irreversibility threshold of electroporation , several embodiments of electrodes of the present invention are configured to align at least a portion of an electric field generated by the electrodes with or near the longer dimensions of the cells to be affected . in specific embodiments , the intravascular device has electrodes configured to create an electrical field aligned with or near the lengthwise dimension of the renal artery ra to affect renal nerves rn . by aligning an electric field so that the field preferentially affects the lengthwise aspect of the cell rather than the diametric or radial aspect of the cell , lower field strengths may be used to necrose cells . as mentioned above , this is expected to reduce power consumption and mitigate effects on non - target cells in the electric field . similarly , the lengthwise or longer dimensions of tissues overlying or underlying the target nerve are orthogonal or otherwise off - axis ( e . g ., transverse ) with respect to the longer dimensions of the nerve cells . thus , in addition to aligning the pef with the lengthwise or longer dimensions of the target cells , the pef may propagate along the lateral or shorter dimensions of the non - target cells ( i . e . such that the pef propagates at least partially out of alignment with non - target smooth muscle cells smc ). therefore , as seen in fig3 , applying a pef with propagation lines li generally aligned with the longitudinal dimension l of the renal artery ra is expected to preferentially cause electroporation , electrofusion , denervation or other neuromodulation in cells of the target renal nerves rn without unduly affecting the non - target arterial smooth muscle cells smc . the pulsed electric field may propagate in a single plane along the longitudinal axis of the renal artery , or may propagate in the longitudinal direction along any angular segment θ through a range of 0 °- 360 °. embodiments of the method shown in fig3 may have particular application with the intravascular methods and apparatus of the present invention . for instance , a pef catheter placed within the renal artery may propagate an electric field having a longitudinal portion that is aligned to run with the longitudinal dimension of the artery in the region of the renal nerves rn and the smooth muscle cell smc of the vessel wall so that the wall of the artery remains at least substantially intact while the outer nerve cells are destroyed . fig4 shows one embodiment of an intravascular pulsed electric field apparatus 200 in accordance with the present invention that includes one or more electrodes configured to physically contact a target region within the renal vasculature and deliver a pulsed electric field across a wall of the vasculature . the apparatus 200 is shown within a patient &# 39 ; s renal artery ra , but it can be positioned in other intravascular locations ( e . g ., the renal vein ). this embodiment of the apparatus 200 comprises an intravascular catheter 210 having a proximal section 211 a , a distal section 211 b , and a plurality of distal electrodes 212 at the distal section 211 b . the proximal section 211 a generally has an electrical connector to couple the catheter 210 to a pulse generator , and the distal section 211 b in this embodiment has a helical configuration . the apparatus 200 is electrically coupled to a pulsed electric field generator 100 located proximal and external to the patient ; the electrodes 212 are electrically coupled to the generator via catheter 210 . the generator 100 may be utilized with any embodiment of the present invention described hereinafter for delivery of a pef with desired field parameters . it should be understood that electrodes of embodiments described hereinafter may be connected to the generator , even if the generator is not explicitly shown or described with each variation . the helical distal section 211 b of catheter 210 is configured to appose the vessel wall and bring electrodes 212 into close proximity to extra - vascular neural structures . the pitch of the helix can be varied to provide a longer treatment zone , or to minimize circumferential overlap of adjacent treatments zones in order to reduce a risk of stenosis formation . this pitch change can be achieved by combining a plurality of catheters of different pitches to form catheter 210 , or by adjusting the pitch of catheter 210 through the use of internal pull wires , adjusting mandrels inserted into the catheter , shaping sheaths placed over the catheter , or by any other suitable means for changing the pitch either in - situ or before introduction into the body . the electrodes 212 along the length of the pitch can be individual electrodes , a common but segmented electrode , or a common and continuous electrode . a common and continuous electrode may , for example , comprise a conductive coil formed into or placed over the helical portion of catheter 210 . a common but segmented electrode may , for example , be formed by providing a slotted tube fitted onto or into the helical portion of the catheter , or by electrically connecting a series of individual electrodes . individual electrodes or groups of electrodes 212 may be configured to provide a bipolar signal , or all or a subset of the electrodes may be used together in conjunction with a separate external patient ground for monopolar use ( the ground pad may , for example , be placed on the patient &# 39 ; s leg ). electrodes 212 may be dynamically assignable to facilitate monopolar and / or bipolar energy delivery between any of the electrodes and / or between any of the electrodes and an external ground . catheter 210 may be delivered to renal artery ra in a low profile delivery configuration within sheath 150 . once positioned within the artery , the catheter may self - expand or may be expanded actively , e . g ., via a pull wire or a balloon , into contact with an interior wall of the artery . a pulsed electric field then may be generated by the pef generator 100 , transferred through catheter 210 to electrodes 212 , and delivered via the electrodes 212 across the wall of the artery . in many applications , the electrodes are arranged so that the pulsed electric field is aligned with the longitudinal dimension of the artery to modulate the neural activity along the renal nerves ( e . g ., denervation ). this may be achieved , for example , via irreversible electroporation , electrofusion and / or inducement of apoptosis in the nerve cells . fig5 illustrates an apparatus 220 for neural modulation in accordance with another embodiment of the invention . the apparatus 220 includes a pair of catheters 222 a and 222 b having expandable distal sections 223 a and 223 b with helical electrodes 224 a and 224 b , respectively . the helical electrodes 224 a and 224 b are spaced apart from each other by a desired distance within a patient &# 39 ; s renal vasculature . electrodes 224 a - b may be actuated in a bipolar fashion such that one electrode is an active electrode and the other is a return electrode . the distance between the electrodes may be altered as desired to change the field strength and / or the length of nerve segment modulated by the electrodes . the expandable helical electrodes may comprise shape - memory properties that facilitate self - expansion , e . g ., after passage through sheath 150 , or the electrodes may be actively expanded into contact with the vessel wall , e . g ., via an inflatable balloon or via pull wires , etc . the catheters 222 a - b preferably are electrically insulated in areas other than the distal helices of electrodes 224 a - b . fig6 illustrates an apparatus 230 comprising a balloon catheter 232 having expandable balloon 234 , a helical electrode 236 arranged about the balloon 234 , and a shaft electrode 238 on the shaft of catheter 232 . the shaft electrode 238 can be located proximal of expandable balloon 234 as shown , or the shaft electrode 238 can be located distal of the expandable balloon 234 . when the apparatus 230 is delivered to a target vessel , e . g ., within renal artery ra , the expandable balloon 234 and the helical electrode 236 are arranged in a low profile delivery configuration . as seen in fig6 , once the apparatus has been positioned as desired , expandable balloon 234 may be inflated to drive the helical electrode 236 into physical contact with the wall of the vessel . in this embodiment , the shaft electrode 238 does not physically contact the vessel wall . it is well known in the art of both traditional thermal rf energy delivery and of relatively non - thermal pulsed rf energy delivery that energy may be conducted to tissue to be treated from a short distance away from the tissue itself . thus , it may be appreciated that “ nerve contact ” comprises both physical contact of a system element with a nerve , as well as electrical contact alone without physical contact , or a combination of the two . a centering element optionally may be provided to place electrodes in a central region of the vessel . the centering element may comprise , for example , an expandable balloon , such as balloon 234 of apparatus 230 , or an expandable basket as described hereinafter . one or more electrodes may be positioned on a central shaft of the centering element — either longitudinally aligned with the element or positioned on one or both sides of the element — as is shaft electrode 238 of apparatus 230 . when utilizing a balloon catheter such as catheter 232 , the inflated balloon may act as an insulator of increased impedance for directing a pulsed electric field along a desired electric flow path . as will be apparent , alternative insulators may be utilized . as seen in fig6 , when the helical electrode 236 physically contacts the wall of renal artery ra , the generator 100 may generate a pef such that current passes between the helical electrode 236 and the shaft electrode 238 in a bipolar fashion . the pef travels between the electrodes along lines li that generally extend along the longitudinal dimension of the artery . the balloon 234 locally insulates and / or increases the impedance within the patient &# 39 ; s vessel such that the pef travels through the wall of the vessel between the helical and shaft electrodes . this focuses the energy to enhance denervation and / or other neuromodulation of the patient &# 39 ; s renal nerves , e . g ., via irreversible electroporation . fig7 illustrates an apparatus 240 similar to those shown in fig4 - 6 in accordance with another embodiment of the invention . the apparatus 240 comprises a balloon catheter 242 having an expandable balloon 244 and a shaft electrode 246 located proximal of the expandable balloon 244 . the apparatus 240 further comprises an expandable helical electrode 248 configured for delivery through a guidewire lumen 243 of the catheter 242 . the helical electrode 248 shown in fig7 is self - expanding . as seen in fig7 , after positioning the catheter 242 in a target vessel ( e . g . renal artery ra ), the balloon 244 is inflated until it contacts the wall of the vessel to hold the shaft electrode 246 at a desired location within the vessel and to insulate or increase the impedance of the interior of the vessel . the balloon 244 is generally configured to also center the shaft electrode 246 within the vessel or otherwise space the shaft electrode apart from the vessel wall by a desired distance . after inflating the balloon 244 , the helical electrode 248 is pushed through lumen 243 until the helical electrode 248 extends beyond the catheter shaft ; the electrode 248 then expands or otherwise moves into the helical configuration to physically contact the vessel wall . a bipolar pulsed electric field may then be delivered between the helical electrode 248 and the shaft electrode 246 along lines li . for example , the helical electrode 248 may comprise the active electrode and the shaft electrode 246 may comprise the return electrode , or vice versa . with reference now to fig8 , apparatus comprising an expandable basket having a plurality of electrodes that may be expanded into contact with the vessel wall is described . apparatus 250 comprises catheter 252 having expandable distal basket 254 formed from a plurality of circumferential struts or members . a plurality of electrodes 256 are formed along the members of basket 254 . each member of the basket illustratively comprises a bipolar electrode pair configured to contact a wall of renal artery ra or another desired blood vessel . basket 254 may be fabricated , for example , from a plurality of shape - memory wires or ribbons , such as nitinol , spring steel or eigiloy wires or ribbons , that form basket members 253 . when the basket members comprise ribbons , the ribbons may be moved such that a surface area contacting the vessel wall is increased . basket members 253 are coupled to catheter 252 at proximal and distal connections 255 a and 255 b , respectively . in such a configuration , the basket may be collapsed for delivery within sheath 150 , and may self - expand into contact with the wall of the artery upon removal from the sheath . proximal and / or distal connection 255 optionally may be configured to translate along the shaft of catheter 252 for a specified or unspecified distance in order to facilitate expansion and collapse of the basket . basket 254 alternatively may be formed from a slotted and / or laser - cut hypotube . in such a configuration , catheter 252 may , for example , comprise inner and outer shafts that are moveable relative to one another . distal connection 255 b of basket 254 may be coupled to the inner shaft and proximal connection 255 a of the basket may be coupled to the outer shaft . basket 254 may be expanded from a collapsed delivery configuration to the deployed configuration of fig8 by approximating the inner and outer shafts of catheter 252 , thereby approximating the proximal and distal connections 255 of the basket and expanding the basket . likewise , the basket may be collapsed by separating the inner and outer shafts of the catheter . as seen in fig9 , individual electrodes may be arranged along a basket strut or member 253 . in one embodiment , the strut is formed from a conductive material coated with a dielectric material , and the electrodes 256 may be formed by removing regions of the dielectric coating . the insulation optionally may be removed only along a radially outer surface of the member such that electrodes 256 remain insulated on their radially interior surfaces ; it is expected that this will direct the current flow outward into the vessel wall . in addition , or as an alternative , to the fabrication technique of fig9 , the electrodes may be affixed to the inside surface , outside surface or embedded within the struts or members of basket 254 . the electrodes placed along each strut or member may comprise individual electrodes , a common but segmented electrode , or a common and continuous electrode . individual electrodes or groups of electrodes may be configured to provide a bipolar signal , or all or a subset of the electrodes may be actuated together in conjunction with an external patient ground for monopolar use . one advantage of having electrodes 256 contact the vessel wall as shown in the embodiment of fig8 is that it may reduce the need for an insulating element , such as an expandable balloon , to achieve renal denervation or other neuromodulation . however , it should be understood that such an insulating element may be provided and , for example , expanded within the basket . furthermore , having the electrodes contact the wall may provide improved field geometry , i . e ., may provide an electric field more aligned with the longitudinal axis of the vessel . such contacting electrodes also may facilitate stimulation of the renal nerves before , during or after neuromodulation to better position the catheter 252 before treatment or for monitoring the effectiveness of treatment . in a variation of apparatus 250 , electrodes 256 may be disposed along the central shaft of catheter 252 , and basket 254 may simply center the electrodes within the vessel to facilitate more precise delivery of energy across the vessel wall . this configuration may be well suited to precise targeting of vascular or extra - vascular tissue , such as the renal nerves surrounding the renal artery . correctly sizing the basket or other centering element to the artery provides a known distance between the centered electrodes and the arterial wall that may be utilized to direct and / or focus the electric field as desired . this configuration may be utilized in high - intensity focused ultrasound or microwave applications , but also may be adapted for use with any other energy modality as desired . referring now to fig1 , it is expected that electrodes forming a circumferential contact with the wall of the renal artery may provide for more complete renal denervation or renal neuromodulation . in fig1 , a variation of the present invention comprising ring electrodes is described . apparatus 260 comprises catheter 262 having expandable ring electrodes 264 a and 264 b configured to contact the wall of the vessel . the electrodes may be attached to the shaft of catheter 262 via struts 266 , and catheter 262 may be configured for delivery to renal artery ra through sheath 150 in a low profile configuration . struts 266 may self - expanding or may be actively or mechanically expanded . catheter 262 comprises guidewire lumen 263 for advancement over a guidewire . catheter 262 also comprises optional inflatable balloon 268 that may act as an insulating element of increased impedance for preferentially directing current flow that is traveling between electrodes 264 across the wall of the artery . fig1 a - 11c illustrate various embodiments of windings for ring electrodes 264 . as shown , the ring electrodes may , for example , be wound in a coil ( fig1 a ), a zigzag ( fig1 b ) or a serpentine configuration ( fig1 c ). the periodicity of the winding may be specified , as desired . furthermore , the type of winding , the periodicity , etc ., may vary along the circumference of the electrodes . with reference to fig1 , a variation of apparatus 260 is described comprising ring electrodes 264 ′ having a sinusoidal winding in one embodiment of the serpentine winding shown in fig1 c . struts 266 illustratively are attached to apexes of the sinusoid . the winding of electrodes 264 ′ may provide for greater contact area along the vessel wall than do electrodes 264 , while still facilitating sheathing of apparatus 260 within sheath 150 for delivery and retrieval . fig1 illustrates another variation of apparatus 260 comprising a proximal ring electrode 264 a , and further comprising a distal electrode 270 delivered through guidewire lumen 263 of catheter 262 . the distal electrode 270 is non - expanding and is centered within the vessel via catheter 262 . the distal electrode 270 may be a standard guide wire which is connected to the pulsed electric field generator and used as an electrode . however , it should be understood that electrode 270 alternatively may be configured for expansion into contact with the vessel wall , e . g ., may comprise a ring or helical electrode . delivering the distal electrode through the lumen of catheter 262 may reduce a delivery profile of apparatus 260 and / or may improve flexibility of the device . furthermore , delivery of the distal electrode through the guidewire lumen may serve as a safety feature that ensures that the medical practitioner removes any guidewire disposed within lumen 263 prior to delivery of a pef . it also allows for customization of treatment length , as well as for treatment in side branches , as described hereinafter . ring electrodes 264 and 264 ′ optionally may be electrically insulated along their radially inner surfaces , while their radially outer surfaces that contact the vessel wall are exposed . this may reduce a risk of thrombus formation and also may improve or enhance the directionality of the electric field along the longitudinal axis of the vessel . this also may facilitate a reduction of field voltage necessary to disrupt neural fibers . materials utilized to at least partially insulate the ring electrodes may comprise , for example , ptfe , eptfe , fep , chronoprene , silicone , urethane , pebax , etc . with reference to fig1 , another variation of apparatus 260 is described , wherein the ring electrodes have been replaced with point electrodes 272 disposed at the ends of struts 266 . the point electrodes may be collapsed with struts 266 for delivery through sheath 150 and may self - expand with the struts into contact with the vessel wall . in fig1 , catheter 262 illustratively comprises four point electrodes 272 on either side of balloon 268 . however , it should be understood that any desired number of struts and point electrodes may be provided around the circumference of catheter 262 . in fig1 , apparatus 260 illustratively comprises four struts 266 and four point electrodes 272 on either side of balloon 268 . by utilizing all of the distally disposed electrodes 272 b as active electrodes and all proximal electrodes 272 a as return electrodes , or vice versa , lines li along which the electric field propagates may be aligned with the longitudinal axis of a vessel . a degree of line li overlap along the rotational axis of the vessel may be specified by specifying the angular placement and density of point electrodes 272 about the circumference of the catheter , as well as by specifying parameters of the pef . with reference now to fig1 , another variation of an intravascular pef catheter is described . apparatus 280 comprises catheter 282 having optional inflatable balloon or centering element 284 , shaft electrodes 286 a and 286 b disposed along the shaft of the catheter on either side of the balloon , as well as optional radiopaque markers 288 disposed along the shaft of the catheter , illustratively in line with the balloon . balloon 284 serves as both a centering element for electrodes 286 and as an electrical insulator for directing the electric field , as described previously . apparatus 280 may be particularly well - suited for achieving precise targeting of desired arterial or extra - arterial tissue , since properly sizing balloon 284 to the target artery sets a known distance between centered electrodes 286 and the arterial wall that may be utilized when specifying parameters of the pef . electrodes 286 alternatively may be attached to balloon 284 rather than to the central shaft of catheter 282 such that they contact the wall of the artery . in such a variation , the electrodes may be affixed to the inside surface , outside surface or embedded within the wall of the balloon . electrodes 286 arranged along the length of catheter 282 can be individual electrodes , a common but segmented electrode , or a common and continuous electrode . furthermore , electrodes 286 may be configured to provide a bipolar signal , or electrodes 286 may be used together or individually in conjunction with a separate patient ground for monopolar use . referring now to fig1 , a method of using apparatus 280 to achieve renal denervation is described . as seen in fig1 a , catheter 282 may be disposed at a desired location within renal artery ra , balloon or centering element 284 may be expanded to center electrodes 286 and to optionally provide electrical insulation , and a pef may be delivered , e . g ., in a bipolar fashion between the proximal and distal electrodes 286 . it is expected that the pef will achieve renal denervation and / or neuromodulation along treatment zone one t 1 . if it is desired to modulate neural activity in other parts of the renal artery , balloon 284 may be at least partially deflated , and the catheter may be positioned at a second desired treatment zone t 2 , as in fig1 b . the medical practitioner optionally may utilize fluoroscopic imaging of radiopaque markers 288 to orient catheter 282 at desired locations for treatment . for example , the medical practitioner may use the markers to ensure a region of overlap o between treatment zones t 1 and t 2 , as shown . with reference to fig1 , a variation of apparatus 280 is described comprising a plurality of dynamically controllable electrodes 286 disposed on the proximal side of balloon 284 . in one variation , any one of proximal electrodes 286 a may be energized in a bipolar fashion with distal electrode 286 b to provide dynamic control of the longitudinal distance between the active and return electrodes . this alters the size and shape of the zone of treatment . in another variation , any subset of proximal electrodes 286 a may be energized together as the active or return electrodes of a bipolar electric field established between the proximal electrodes and distal electrode 286 b . although the apparatus 280 shown in fig1 has three proximal electrodes 286 a , it should be understood that the apparatus 280 can have any alternative number of proximal electrodes . furthermore , the apparatus 280 can have a plurality of distal electrodes 286 b in addition , or as an alternative , to multiple proximal electrodes . additionally , one electrode of a pair may be coupled to the catheter 282 , and the other electrode may be delivered through a lumen of the catheter , e . g ., through a guidewire lumen . the catheter and endoluminally - delivered electrode may be repositioned relative to one another to alter a separation distance between the electrodes . such a variation also may facilitate treatment of a variety of renal vasculature anatomies . in the variations of apparatus 280 described thus far , distal electrode 286 b is coupled to the shaft of catheter 282 distal of balloon 284 . the distal electrode may utilize a lumen within catheter 282 , e . g ., for routing of a lead wire that acts as ground . additionally , the portion of catheter 282 distal of balloon 284 is long enough to accommodate the distal electrode . catheters commonly are delivered over metallic and / or conductive guidewires . in many interventional therapies involving catheters , guidewires are not removed during treatment . as apparatus 280 is configured for delivery of a pulsed electric field , if the guidewire is not removed , there may be a risk of electric shock to anyone in contact with the guidewire during energy delivery . this risk may be reduced by using polymer - coated guidewires . with reference to fig1 , another variation of apparatus 280 is described wherein distal electrode 286 b of fig1 and 17 has been replaced with a distal electrode 270 configured to be moved through a lumen of the catheter as described previously with respect to fig1 . as will be apparent , proximal electrode 286 a alternatively may be replaced with the luminally - delivered electrode , such that electrodes 286 b and 270 form a bipolar electrode pair . electrode 270 does not utilize an additional lumen within catheter 282 , which may reduce profile . additionally , the length of the catheter distal of the balloon need not account for the length of the distal electrode , which may enhance flexibility . furthermore , the guidewire must be exchanged for electrode 270 prior to treatment , which reduces a risk of inadvertent electrical shock . in one variation , electrode 270 optionally may be used as the guidewire over which catheter 282 is advanced into position prior to delivery of the pef , thereby obviating a need for exchange of the guidewire for the electrode . alternatively , a standard metallic guidewire may be used as the electrode 270 simply by connecting the standard guidewire to the pulsed electric field generator . the distal electrode 270 may be extended any desired distance beyond the distal end of catheter 282 . this may provide for dynamic alteration of the length of a treatment zone . furthermore , this might facilitate treatment within distal vasculature of reduced diameter . with reference to fig1 , it might be desirable to perform treatments within one or more vascular branches that extend from a main vessel , for example , to perform treatments within the branches of the renal artery in the vicinity of the renal hilum . furthermore , it might be desirable to perform treatments within abnormal or less common branchings of the renal vasculature , which are observed in a minority of patients . as seen in fig1 a , distal electrode 270 may be placed in such a branch of renal artery ra , while catheter 282 is positioned within the main branch of the artery . as seen in fig1 b , multiple distal electrodes 270 might be provided and placed in various common or uncommon branches of the renal artery , while the catheter remains in the main arterial branch . referring to fig2 , yet another variation of an intravascular pef catheter is described . apparatus 290 comprises catheter 292 having a plurality of shaft electrodes 294 disposed in line with centering element 296 . centering element 296 illustratively comprises an expandable basket , such as previously described expandable basket 254 of fig8 . however , it should be understood that the centering element alternatively may comprise a balloon or any other centering element . electrodes 294 may be utilized in a bipolar or a monopolar fashion . referring now to fig2 , another variation of the invention is described comprising electrodes configured for dynamic radial repositioning of one or more of the electrodes relative to a vessel wall , thereby facilitating focusing of a pulsed electric field delivered by the electrodes . apparatus 300 comprises catheter 302 having electrodes 304 disposed in line with nested expandable elements . the nested expandable elements comprise an inner expandable element 306 and an outer expandable centering element 308 . electrodes 304 are disposed along the inner expandable element , while the outer expandable centering element is configured to center and stabilize catheter 302 within the vessel . inner element 306 may be expanded to varying degrees , as desired by a medical practitioner , to dynamically alter the radial positions of electrodes 304 . this dynamic radial repositioning may be utilized to focus energy delivered by electrodes 304 such that it is delivered to target tissue . nested elements 306 and 308 may comprise a balloon - in - balloon arrangement , a basket - in - basket arrangement , some combination of a balloon and a basket , or any other expandable nested structure . in fig2 , inner expandable element 306 illustratively comprises an expandable basket , while outer expandable centering element 308 illustratively comprises an expandable balloon . electrodes 302 are positioned along the surface of inner balloon 306 . any of the variations of the present invention described herein optionally may be configured for infusion of agents into the treatment area before , during or after energy application , for example , to enhance or modify the neurodestructive or neuromodulatory effect of the energy , to protect or temporarily displace non - target cells , and / or to facilitate visualization . additional applications for infused agents will be apparent . if desired , uptake of infused agents by cells may be enhanced via initiation of reversible electroporation in the cells in the presence of the infused agents . infusion may be especially desirable when a balloon centering element is utilized . the infusate may comprise , for example , saline or heparinized saline , protective agents , such as poloxamer - 188 , or anti - proliferative agents . variations of the present invention additionally or alternatively may be configured for aspiration . for example , infusion ports or outlets may be provided on a catheter shaft adjacent a centering device , the centering device may be porous ( for instance , a “ weeping ” balloon ), or basket struts may be made of hollow hypotubes and slotted or perforated to allow infusion or aspiration . with reference to fig2 , a variation of the present invention comprising an infusion / aspiration pef catheter is described . apparatus 310 comprises catheter 312 having proximal and distal inflatable balloons 314 a and 314 b , respectively . proximal shaft electrode 316 a is disposed between the balloons along the shaft of catheter 312 , while distal electrode 316 b is disposed distal of the balloons along the catheter shaft . one or more infusion or aspiration holes 318 are disposed along the shaft of catheter 312 between the balloons in proximity to proximal electrode 316 a . apparatus 310 may be used in a variety of ways . in a first method of use , catheter 312 is disposed within the target vessel , such as renal artery ra , at a desired location . one or both balloons 314 are inflated , and a protective agent or other infusate is infused through hole ( s ) 318 between the balloons in proximity to electrode 316 a . a pef suitable for initiation of reversible electroporation is delivered across electrodes 316 to facilitate uptake of the infusate by non - target cells within the vessel wall . delivery of the protective agent may be enhanced by first inflating distal balloon 314 b , then infusing the protective agent , which displaces blood , then inflating proximal balloon 314 a . remaining infusate optionally may be aspirated such that it is unavailable during subsequent pef application when irreversible electroporation of nerve cells is initiated . aspiration may be achieved by at least partially deflating one balloon during aspiration . alternatively , aspiration may be achieved with both balloons inflated , for example , by infusing saline in conjunction with the aspiration to flush out the vessel segment between the inflated balloons . such blood flushing may reduce a risk of clot formation along proximal electrode 316 a during pef application . furthermore , flushing during energy application may cool the electrode and / or cells of the wall of the artery . such cooling of the wall cells might protect the cells from irreversible electroporative damage , possibly reducing a need for infusion of a protective agent . after infusion and optional aspiration , a pef suitable for initiation of irreversible electroporation in target nerve cells may be delivered across electrodes 316 to denervate or to modulate neural activity . in an alternative method , infusion of a protective agent may be performed during or after initiation of irreversible electroporation in order to protect non - target cells . the protective agent may , for example , plug or fill pores formed in the non - target cells via the irreversible electroporation . in another alternative method , a solution of chilled ( i . e ., less than body temperature ) heparinized saline may be simultaneously infused and aspirated between the inflated balloons to flush the region between the balloons and decrease the sensitivity of vessel wall cells to electroporation . this is expected to further protect the cells during application of the pef suitable for initiation of irreversible electroporation . such flushing optionally may be continuous throughout application of the pulsed electric field . a thermocouple or other temperature sensor optionally may be positioned between the balloons such that a rate of chilled infusate infusion may be adjusted to maintain a desired temperature . the chilled infusate preferably does not cool the target tissue , e . g ., the renal nerves . a protective agent , such as poloxamer - 188 , optionally may be infused post - treatment as an added safety measure . infusion alternatively may be achieved via a weeping balloon catheter . further still , a cryoballoon catheter having at least one electrode may be utilized . the cryoballoon may be inflated within a vessel segment to locally reduce the temperature of the vessel segment , for example , to protect the segment and / or to induce thermal apoptosis of the vessel wall during delivery of an electric field . the electric field may , for example , comprise a pef or a thermal , non - pulsed electric field , such as a thermal rf field . referring now to fig2 , a variation of a pef catheter configured for passage of electrode ( s ) at least partially across the vessel wall is described . for example , the electrode ( s ) may be positioned within the renal vein and then passed across the wall of the renal vein such that they are disposed in gerota &# 39 ; s or renal fascia and near or at least partially around the renal artery . in this manner , the electrode ( s ) may be positioned in close proximity to target renal nerve fibers prior to delivery of a pulsed electric field . as seen in fig2 a , apparatus 320 comprises catheter 322 having needle ports 324 and centering element 326 , illustratively an inflatable balloon . catheter 322 also optionally may comprise radiopaque markers 328 . needle ports 324 are configured for passage of needles 330 therethrough , while needles 330 are configured for passage of electrodes 340 . renal vein rv runs parallel to renal artery ra . an imaging modality , such as intravascular ultrasound , may be used to identify the position of the renal artery relative to the renal vein . for example , intravascular ultrasound elements optionally may be integrated into catheter 322 . catheter 322 may be positioned within renal vein rv using well - known percutaneous techniques , and centering element 326 may be expanded to stabilize the catheter within the vein . needles 330 then may be passed through catheter 322 and out through needle ports 324 in a manner whereby the needles penetrate the wall of the renal vein and enter into gerota &# 39 ; s or renal fascia f . radiopaque markers 328 may be visualized with fluoroscopy to properly orient needle ports 324 prior to deployment of needles 330 . electrodes 340 are deployed through needles 330 to at least partially encircle renal artery ra , as in fig2 a and 23b . continued advancement of the electrodes may further encircle the artery , as in fig2 c . with the electrodes deployed , stimulation and / or pef electroporation waveforms may be applied to denervate or modulate the renal nerves . needles 330 optionally may be partially or completely retracted prior to treatment such that electrodes 340 encircle a greater portion of the renal artery . additionally , a single electrode 340 may be provided and / or actuated in order to provide a monopolar pef . infusate optionally may be infused from needles 330 into fascia f to facilitate placement of electrodes 340 by creating a space for placement of the electrodes . the infusate may comprise , for example , fluids , heated or chilled fluids , air , co 2 , saline , contrast agents , gels , conductive fluids or any other space - occupying material — be it gas , solid or liquid . heparinized saline also may be injected . saline or hypertonic saline may enhance conductivity between electrodes 340 . additionally or alternatively , drugs and / or drug delivery elements may be infused or placed into the fascia through the needles . after treatment , electrodes 340 may be retracted within needles 330 , and needles 330 may be retracted within catheter 322 via needle ports 324 . needles 330 preferably are small enough that minimal bleeding occurs and hemostasis is achieved fairly quickly . balloon centering element 326 optionally may remain inflated for some time after retrieval of needles 330 in order to block blood flow and facilitate the clotting process . alternatively , a balloon catheter may be advanced into the renal vein and inflated after removal of apparatus 320 . referring to fig2 , variations of the invention comprising detectors or other elements for measuring or monitoring treatment efficacy are described . variations of the invention may be configured to deliver stimulation electric fields , in addition to denervating or modulating pefs . these stimulation fields may be utilized to properly position the apparatus for treatment and / or to monitor the effectiveness of treatment in modulating neural activity . this may be achieved by monitoring the responses of physiologic parameters known to be affected by stimulation of the renal nerves . such parameters comprise , for example , renin levels , sodium levels , renal blood flow and blood pressure . stimulation also may be used to challenge the denervation for monitoring of treatment efficacy : upon denervation of the renal nerves , the known physiologic responses to stimulation should no longer occur in response to such stimulation . efferent nerve stimulation waveforms may , for example , comprise frequencies of about 1 - 10 hz , while afferent nerve stimulation waveforms may , for example , comprise frequencies of up to about 50 hz . waveform amplitudes may , for example , range up to about 50v , while pulse durations may , for example , range up to about 20 milliseconds . when the nerve stimulation waveforms are delivered intravascularly , as in several embodiments of the present invention , field parameters such as frequency , amplitude and pulse duration may be modulated to facilitate passage of the waveforms through the wall of the vessel for delivery to target nerves . furthermore , although exemplary parameters for stimulation waveforms have been described , it should be understood that any alternative parameters may be utilized as desired . the electrodes used to deliver pefs in any of the previously described variations of the present invention also may be used to deliver stimulation waveforms to the renal vasculature . alternatively , the variations may comprise independent electrodes configured for stimulation . as another alternative , a separate stimulation apparatus may be provided . one way to use stimulation to identify renal nerves is to stimulate the nerves such that renal blood flow is affected — or would be affected if the renal nerves had not been denervated or modulated . stimulation acts to reduce renal blood flow , and this response may be attenuated or abolished with denervation . thus , stimulation prior to neural modulation would be expected to reduce blood flow , while stimulation after neural modulation would not be expected to reduce blood flow to the same degree when utilizing similar stimulation parameters and location ( s ) as prior to neural modulation . this phenomenon may be utilized to quantify an extent of renal neuromodulation . variations of the present invention may comprise elements for monitoring renal blood flow or for monitoring any of the other physiological parameters known to be affected by renal stimulation . in fig2 a , a variation of apparatus 280 of fig1 is described having an element for monitoring of renal blood flow . guidewire 350 having doppler ultrasound sensor 352 has been advanced through the lumen of catheter 282 for monitoring blood flow within renal artery ra . doppler ultrasound sensor 352 is configured to measure the velocity of flow through the artery . a flow rate then may be calculated according to the formula : where q equals flow rate , v equals flow velocity and a equals cross - sectional area . a baseline of renal blood flow may be determined via measurements from sensor 352 prior to delivery of a stimulation waveform , then stimulation may be delivered between electrodes 286 , preferably with balloon 284 deflated . alteration of renal blood flow from the baseline , or lack thereof , may be monitored with sensor 352 to identify optimal locations for neuromodulation and / or denervation of the renal nerves . fig2 b illustrates a variation of the apparatus of fig2 a , wherein doppler ultrasound sensor 352 is coupled to the shaft of catheter 282 . sensor 352 illustratively is disposed proximal of balloon 284 , but it should be understood that the sensor alternatively may be disposed distal of the balloon . in addition or as an alternative to intravascular monitoring of renal blood flow via doppler ultrasound , such monitoring optionally may be performed from external to the patient whereby renal blood flow is visualized through the skin ( e . g ., using an ultrasound transducer ). in another variation , one or more intravascular pressure transducers may be used to sense local changes in pressure that may be indicative of renal blood flow . as yet another alternative , blood velocity may be determined , for example , via thermodilution by measuring the time lag for an intravascular temperature input to travel between points of known separation distance . for example , a thermocouple may be incorporated into , or provided in proximity to , each electrode 286 , and chilled ( i . e ., lower than body temperature ) fluid or saline may be infused proximally of the thermocouple ( s ). a time lag for the temperature decrease to register between the thermocouple ( s ) may be used to quantify flow characteristic ( s ). a baseline estimate of the flow characteristic ( s ) of interest may be determined prior to stimulation of the renal nerves and may be compared with a second estimate of the characteristic ( s ) determined after stimulation . commercially available devices optionally may be utilized to monitor treatment . such devices include , for example , the smartwire ™, flowire ™ and wavewire ™ devices available from volcano ™ therapeutics inc ., of rancho cordova , calif ., as well as the pressurewire ® device available from radi medical systems ab of uppsala , sweden . additional commercially available devices will be apparent . an extent of electroporation additionally or alternatively may be monitored directly using electrical impedance tomography (“ eit ”) or other electrical impedance measurements , such as an electrical impedance index . although preferred illustrative variations of the present invention are described above , it will be apparent to those skilled in the art that various changes and modifications may be made thereto without departing from the invention . for example , although the variations primarily have been described for use in combination with pulsed electric fields , it should be understood that any other electric field may be delivered as desired . it is intended in the appended claims to cover all such changes and modifications that fall within the true spirit and scope of the invention .
methods and apparatus are provided for renal neuromodulation using a pulsed electric field to effectuate electroporation or electrofusion . it is expected that renal neuromodulation may , among other things , reduce expansion of an acute myocardial infarction , reduce or prevent the onset of morphological changes that are affiliated with congestive heart failure , and / or be efficacious in the treatment of end stage renal disease . embodiments of the present invention are configured for percutaneous intravascular delivery of pulsed electric fields to achieve such neuromodulation .